Petrogenesis of Mount Rainier Andesite: Magma Flux and Geologic

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Petrogenesis of Mount Rainier andesite: Magma ﬂ ux and geologic controls on the contrasting differentiation styles at stratovolcanoes of the southern Washington Cascades

T.W. Sisson1,†, V.J.M. Salters2,†, and P.B. Larson3,† 1U.S. Geological Survey, 345 Middleﬁ eld Road, Menlo Park, California 94025, USA 2National High Magnetic Field Laboratory, Isotope Geochemistry, and Department of Geological Sciences, Florida State University, 1800 E. Paul Dirac Drive, Tallahassee, Florida 32310, USA 3Department of Geology, Washington State University, Pullman, Washington 99164, USA

ABSTRACT eral saturation. Subsequent magmas ascend- routinely carry features indicative of magma ing through the intrusive plexus entrain and mingling and mixing, and that defi ne scattered Quaternary Mount Rainier (Washington, mix with the residual liquids and low-degree whole-rock compositional arrays permissive of USA) of the Cascades magmatic arc consists re-melts of those antecedent intrusions, pro- many explanations. Radiogenic and stable iso- of porphyritic calc-alkaline andesites and ducing hybrid andesites and dacites. Mount tope measurements can be more discriminating, subordinate dacites, with common evidence St. Helens volcanic rocks have geochemical allowing for quantitative estimates of the mag- for mingling and mixing with less evolved similarities to those at Mount Rainier, and nitudes of different processes and components, magmas encompassing andesites, basaltic may also result from in situ differentiation but their successful use depends on suffi ciently andesites, and rarely, basalts. Basaltic ande- and mixing due to low and intermittent long- large and representative sample suites, com- sites and amphibole andesites (spessartites) term magma supply, accompanied by modest prehensive major and trace element analyses, that erupted from vents at the north foot crustal assimilation. Andesites and dacites of adequately precise isotopic measurements, and of the volcano represent some of Mount Mount Adams isotopically overlap the least well-understood and well-characterized geo- Rainier’s immediate parents and overlap in contaminated Mount Rainier magmas and logic settings. composition with regional basalts and basal- derive from similar parental magma types, Here we present results of a combined geo- tic andesites. Geochemical (major and trace but have trace element variations more con- chemical, isotopic, and geologic study of the elements) and isotopic (Sr, Nd, Pb, O) com- sistent with progressive crystallization-dif- origins of andesite series magmas from Mount positions of Mount Rainier andesites and da- ferentiation, probably due to higher magma Rainier, Washington State, in the Cascades cites are consistent with modest assimilation fl uxes leading to slower crystallization of magmatic arc of western North America. The (typically ≤20 wt%) of evolved sediment or large magma batches, allowing time for pro- study benefi ts from abundant samples col- sediment partial melt. Sandstones and shales gressive separation of minerals from melt. lected during geologic mapping of the volcano, of the Eocene Puget Group, derived from the Mount Adams also sits atop the southern sparser sampling of small-volume Quaternary continental interior, are exposed in regional projection of a regional anticlinorium, so Eo- mafi c volcanic rocks erupted across southwest anticlines fl anking the volcano, and prob- cene sediments are absent, or are at shallow Washington, as well as representative samples ably underlie it in the middle to lower crust, crustal levels, and so are cold and diffi cult to of pre-Quaternary basement rocks. The study accounting for their assimilation. Mesozoic assimilate. Differences between southwest also benefi ts from improvements in the ease and Cenozoic igneous basement rocks are Washington stratovolcanoes highlight some and precision of isotopic analyses that allow unsuitable as assimilants due to their high ways that crustal geology and magma fl ux effi cient characterization of chemically ordinary 143Nd/144Nd, diverse 206Pb/204Pb, and gener- are primary factors in andesite generation. and atypical samples. Our new data indicate that ally high δ18O. Mount Rainier’s magmas incorporated small The dominant cause of magmatic evolu- INTRODUCTION but variable amounts (typically ≤20 wt%, but tion at Mount Rainier, however, is inferred up to 30 wt%) of evolved sedimentary rocks, or to be a version of in situ crystallization-dif- Andesite series magmas have been explained their partial melts, known to be present in the ferentiation and mixing (Langmuir, 1989) as products of basaltic crystallization-differ- middle or lower crust, but that the predominant wherein small magma batches stall as crustal entiation, as partial melts of the deep crust or cause of magmatic diversity is multi-stage in intrusions and solidify extensively, yielding subducting slabs, and by composite scenarios situ crystallization-differentiation and mixing. silicic residual liquids with trace element involving crystallization, assimilation, mix- This process involves magmatic replenish- concentrations infl uenced by accessory min- ing, and in certain cases, reaction with or direct ments incorporating advanced differentiates or derivation from mantle peridotite (Gill, 1981; low-degree partial melts from earlier magmatic †E-mail: [email protected] (Sisson, corresponding); DePaolo, 1981; Kelemen, 1990; Grove et al., pulses that stalled and nearly or completely [email protected] (Salters); [email protected] 2005). The multiplicity of interpretations stems, solidifi ed. Because geology and structure infl u- (Larson). in part, from the complexity of the rocks that ence the course of magmatic evolution at Mount

GSA Bulletin; January/February 2014; v. 126; no. 1/2; p. 122–144; doi:10.1130/B30852.1; 9 ﬁ gures; 8 tables; Data Repository item 2014027.

Rainier, and the other major volcanoes of the 126°W 124° 122° 120° region, we fi rst summarize the tectonic setting and geologic development of the southern pre-Cenozoic Washington Cascades. basement 48°N Pacific Ocean GP TECTONIC SETTING Olympic accretion S Active volcanoes of the Cascade Range are complex products of northeasterly directed subduction of the oceanic Juan de Fuca plate beneath North America. The Juan de Fuca spreading ridge T lies only 250–450 km from the volcanic-arc 47° axis, along the direction of convergence, with the result that the subducting slab is one of the MR youngest and hottest worldwide (Hyndman and Columbia River Wang, 1993; Syracuse et al., 2010). In its south- flood basalts ern portion, in northern California and southern MA Oregon, the arc is impinged upon from the east 46° by the Basin and Range extensional province. In MSH the north, westward defl ection of the continental Juan de Siletzia ca Plate margin along Vancouver Island, British Colum- Fu basaltic bia (Canada), arches the slab beneath central terrain P and northern Washington State. Consequently, 50 km ~4.3 cm/y MH cross-arc strain (McCaffrey et al., 2007) passes from neutral or slightly extensional in the south Figure 1. Regional geologic setting of Mount Rainier volcano, simplifi ed from Schuster to increasingly convergent moving northward, (2005) and Walker and MacLeod (1991). Geologic units are: chiefl y Mesozoic and Paleozoic culminating with arc-normal convergence atop igneous and metamorphic rocks (light blue; Tertiary plutons omitted for clarity); Paleo- the arch in the subducting slab, as marked by cene–middle Eocene submarine basalts of the Siletzia terrain (purple); middle Eocene and uplift of the Olympic and North Cascades moun- younger sandstones and shales (light yellow); Eocene–Miocene marine sedimentary rocks tains. Volcanic output tracks these changes, of the Olympic accretionary complex (gray); chiefl y Oligocene–Miocene arc igneous rocks diminishing northward as the hanging wall of (green); Miocene fl ood basalts (brown); Quaternary arc volcanic rocks (red); and Quater- the arc becomes increasingly compressional. nary sediments, chiefl y glacial (stippled). Medium and heavy black lines are faults. Triangles Mount Rainier is situated within the transition show the locations of Mount Rainier (MR), Mount Adams (MA), Mount St. Helens (MSH), from widespread diffuse mafi c volcanism in the Mount Hood (MH), and Glacier Peak (GP); Mount Baker plots outside the map area to south to widespread basement uplift and negli- the north. Circles show the cities of Portland, Oregon (P), and Tacoma (T) and Seattle (S), gible mafi c volcanism in the north. Washington. Blue dash-dot line is Columbia River. Yellow lines show the axes of the St. Helens and west Rainier seismic zones, coincident with anticlinal exposures of Eocene sedi- GEOLOGIC SETTING mentary rocks that may mark the buried eastern margin of the Siletzia terrain. Arrow in the Pacifi c Ocean shows the convergence direction and velocity of the Juan de Fuca plate Regional Cascades Geology relative to the North American plate interior (McCaffrey et al., 2007).

The geologic framework of the U.S. Pacifi c Northwest is important for understanding Cas- line core to the north of Mount Rainier (Frizzell pecosh Formation, overlain unconformably in cades arc magmatism due to potential assimi- et al., 1987). Quartz-rich sands and silts from the Mount Rainier region by silicic ash-fl ow lation and crustal melting. Regional crustal the North American continental interior spread tuffs of the 25 Ma rhyolitic Stevens Ridge For- domains (Fig. 1) include the Siletzia Paleocene– across present-day southern Washington and mation and the 22 Ma rhyodacitic tuff of Clear early Eocene submarine basalt province that northern to central Oregon during the Eocene West Peak caldera (Vance et al., 1987; Tabor fl oors the forearc basin west of the active arc and partly buried the mélange and Siletzia et al., 2000). These late Oligocene and early from southernmost Vancouver Island southward basalts with fl uvial, deltaic, and shallow-marine Miocene ash-fl ow tuffs are largely overlain by, through Oregon’s Willamette Valley (Duncan, sediments. These continent-derived sedimen- but also interfi nger with, andesitic volcanic 1982). Younger rocks conceal Siletzia’s east- tary rocks form the Puget Group in southwest rocks of the chiefl y Miocene Fifes Peak For- ern margin, but the terrain boundary has been Washington, and the widespread Tyee Forma- mation. Granodioritic plutons then intruded the inferred from seismic velocity sections to under- tion, among others, in Oregon (Buckovic, 1979; Tertiary volcanic section, represented at Mount lie the St. Helens seismic zone (Parsons et al., Heller et al., 1985). Rainier by the 19–14 Ma Tatoosh intrusive suite 1998), which passes beneath Mount St. Helens. Cascades arc volcanism commenced inter- and Carbon River stock (Mattinson, 1977; du In Washington, the unexposed deep basement mittently and locally in the middle Eocene Bray et al., 2010). Rapid uplift of the Washing- abutting Siletzia on the east probably correlates and became extensive and voluminous in the ton Cascade and Olympic ranges commenced with Mesozoic tectonite mélange of the West- Oligocene and Miocene (Armentrout, 1987). ca. 10–12 Ma (Reiners et al., 2002), shallowly ern and Eastern mélange belts that border the Oligocene volcanism in southwest Washington unroofi ng the plutons, tilting the 16 Ma Colum- western margin of the North Cascades crystal- is manifest as the widespread andesitic Ohana- bia River fl ood basalts that onlap the eastern

Geological Society of America Bulletin, January/February 2014 123 Sisson et al. margin of the Cascade range, and incising the 122°W deep unconformity upon which the Quaternary N Flank Miocene vents & flows volcanoes grew. 75 CRB 47°N Puget 5 Local Geology of the Mount Rainier Area lowland Miocene plutons Mount Rainier sits atop a broad, north-northwest–trending, internally folded synclinorium Eocene of Tertiary arc volcanic rocks, intruded by the 50 Tatoosh plutonic suite, and is fl anked on the west sandstone & shale and southeast by steep-sided north-northwest– 40 50 striking anticlinoria (Fig. 2). To the west is the anticlinoriumWhite Pass Carbon River anticlinorium cored by middle Carbon River Eocene Puget Group arkoses, carbonaceous anticlinorium black shales, and local coal seams that dip steeply 40 Oligocene - Miocene 70 beneath, and fl oor, the Tertiary arc volcanic sec- arc volcanics tion. The base of the Puget Group is not exposed Morton N KJ in the Carbon River area, but Stanley et al. (1994) 60 anticlinorium Eocene basalt melange presented geologic and geophysical evidence for 25 km 3 a deformed thickness of 10–20 km, including postulated underlying marine shales that together Figure 2. Local geologic setting of Mount Rainier volcano, simplifi ed from Fiske et al. create the southern Washington Cascades (elec- (1963), Tabor et al. (2000), Schasse (1987), and Miller (1989); Quaternary volcanic rocks of trical) conductor (SWCC). The Carbon River the White Pass area are omitted for clarity. Colors as for Figure 1, except: Miocene plutons anticlinorium coincides with the west Rainier are shown (light red); Mesozoic tectonite mélange is subdivided into sandstones (light blue), seismic zone, and seismic velocity sections greenstone (black), orthogneiss (gray), and chert (medium blue); Quaternary lava fl ows (Stanley et al., 1999) show that it probably over- (dark blue) that erupted from vents (stars) to the north of Mount Rainier are distinguished lies the buried eastern margin of Siletzia, similar from lavas that erupted through the axial vent system (red), and from the small lava dome to the Morton anticlinorium and St. Helens seis- and vent at St. Paul Lookout (orange) to the northwest of Mount Rainier; and ice on Mt. mic zone to the south (Parson et al., 1998). Rainier shown (white). CRB—Columbia River Basalt. To the southeast is the White Pass anticlinorium (Fig. 2) which is cored by the Late Jurassic–Early Cretaceous Rimrock Lake inlier cades projecting north across the Columbia andesite with a lamprophyric texture defi ned by of tectonite mélange composed of sheared dirty River to andesitic Mount Adams, and diminish- phenocrysts of amphibole and with plagioclase arkoses to graywackes, greenstone, minor chert, ing progressively northward from there through restricted to the groundmass [Rock, 1987]). The and tectonically bounded blocks and belts of the inactive Goat Rocks volcanic center, ending next vent north along the segment is at Windy intrusive rocks ranging from hornblende dio- with small basalt, basaltic andesite, and dacite Gap, 12 km from Mount Rainier’s summit, ritic orthogneiss to cataclastically deformed vents and fl ows between White Pass and Bump- which erupted the amphibole-olivine porphyritic biotite granodiorite (Miller, 1989). Mesozoic ing Lake (Fig. 1). The major stratovolcanoes spessartite lava fl ow of Bee Flat. Farthest north mélange rocks are overlain unconformably on of Mounts St. Helens and Rainier defi ne an arc (outside the area of Fig. 2) are the basalt–basaltic the west limb of the anticlinorium by the steeply segment that is stepped to the west and aligned andesite center of Canyon Creek (also known as west-dipping Eocene Summit Basalt, which is diagonal to the Oregon Cascades–Mount Adams the basalt of Three Sisters [Reiners et al., 2000]) overlain by the arkosic Eocene Summit Sand- segment, with Mount St. Helens displaced and the basaltic center of Dalles Ridge, respec- stone, and then Oligocene andesitic rocks of the 55 km west of Mount Adams but Mount Rainier tively 27 km north-northwest and 31 km north- Ohanapecosh Formation (Vance et al., 1987). only 25–35 km west of Quaternary vents near northeast of Mount Rainier’s summit (Tabor Thus, Eocene continent-derived sedimentary Bumping Lake. et al., 2000). Quaternary volcanic products are rocks dip steeply beneath Mount Rainier both Quaternary vents are absent directly between unknown for another 105–110 km northward from the west and the southeast (Fig. 2) and are Mounts St. Helens and Rainier, but minor vents until reaching small basalt and basaltic andesite probably continuous beneath the volcano in the and fl ows of basalt, basaltic andesite, and horn- fl ows shortly south of Glacier Peak stratovol- middle and perhaps lower crust, possibly under- blende-rich andesite continue up to 30 km north cano. An outlier to the Mount St. Helens–Mount lain by Eocene–Paleocene non-arc basalts, all of Mount Rainier as a diffuse extension of the Rainier arc segment is an isolated Pleistocene fl oored by Mesozoic sedimentary and igneous Mount St. Helens–Mount Rainier arc segment. dacite dome at St. Paul Lookout 26 km west- tectonite mélange, and disrupted by Tertiary and The closest of these vents to Mount Rainier sit northwest of Mount Rainier on the west limb of Quaternary igneous intrusions. at the volcano’s foot in the vicinity of Echo and the Carbon River anticlinorium (Fig. 2). Observation Rocks (Fig. 2), only 6–7 km north- Distribution of Quaternary Volcanism in northwest of the summit. This north-fl ank vent SAMPLE SELECTION AND Southwest Washington cluster erupted the olivine basaltic andesites of ANALYTICAL METHODS Spray Park, as well as distinctly amphibole-por- Quaternary arc volcanism is geographically phyritic spessartite andesite of the Russell Gla- About 1100 Quaternary rock and tephra segmented in southwest Washington, with the cier, and spessartite–basaltic andesite hybrid lava samples were collected from Mount Rainier and main stratovolcano chain of the Oregon Cas- of the Flett Glacier (spessartite is calc-alkaline the surrounding region during geologic map-

124 Geological Society of America Bulletin, January/February 2014 Petrogenesis of Mount Rainier andesite

ping of the volcano. About 800 of these were dard (87Sr/86Sr = 0.708000). Neodymium and Mayeda, 1963; Sharp, 1990), and the released powdered in alumina or agate shatterboxes and Pb isotope ratios were measured on a Thermo- oxygen was passed successively over cold traps analyzed for major and trace elements by X-ray Finnigan NEPTUNE MC-ICPMS (multicollec- and cleaned with KBr. The δ18O values were fl uorescence (XRF) at the U.S. Geological Sur- tor ICPMS). 143Nd/144Nd ratios are corrected for measured with a Finnigan Delta S isotope ratio vey (USGS), Denver, and later at Washington fractionation to 146Nd/144Nd = 0.7219. Repeated mass spectrometer. Isotopic compositions are State University, Pullman (WSU), to support measurements of Nd standard La Jolla yielded expressed in the δ notation as the relative dif- the mapping effort. Sixty-seven Quaternary an average value of 143Nd/144Nd = 0.511839 ± ference in the isotope values between the sam- volcanic rocks were selected from the greater 0.000018 (n = 110, 2σ). 143Nd/144Nd ratios of ple and the Vienna standard mean ocean water sample suite for more comprehensive isotopic the samples are reported relative to the accepted (VSMOW) standard in parts per thousand (‰). and geochemical study (Table 1). Samples cho- ratio of the La Jolla standard (143Nd/144Nd = The measured δ18O values of the samples were sen for this additional analysis typify the main 0.511850). corrected by repeated analyses of the UWG-2 compositional trend of Mount Rainier’s erup- Lead isotope ratios were measured using a garnet standard (δ18O = ~5.8‰) (Valley et al., tives, or are samples with atypically high or low Tl spike (Pb/Tl ~6) to account for mass frac- 1995), with analytical precision of 0.05‰ to concentrations of one or more elements (K, Sr, tionation and are corrected for fractionation 0.22‰ (1σ). Ba, Zr). Regional basalts and basaltic andesites to 203Tl/205Tl = 0.4188. Long-term averages To further investigate potential crustal mag- were included to characterize potential paren- of the Pb standard NBS-981 are 206Pb/204Pb = matic sources, oxygen isotope ratios of Meso- tal magmas, and sixteen samples of Mesozoic 16.9294 ± 0.0023, 207Pb/204Pb = 15.4824 ± zoic and Cenozoic basement whole rocks were and Cenozoic basement rocks were analyzed to 0.0025, 208Pb/204Pb = 36.6698 ± 0.0077 (n = 100, also determined (USGS, Denver) by conven- σ investigate crustal infl uences on magma produc- 2 ). Lead isotope ratios are reported relative to tional BrF5 fl uoridation in Ni reaction vessels tion (Table 2). Basement samples were collected the accepted values of NBS-981 (206Pb/204Pb = (Clayton and Mayeda, 1963). Upon conver- 207 204 208 204 to be representative of preserved materials and 16.9356, Pb/ Pb = 15.4891, Pb/ Pb = sion to CO2, oxygen isotopic values (Table 7) therefore contain secondary carbonate, clay, and 36.7006) (Todt et al., 1996). were measured on a Finnigan MAT 251 mass silica phases. Oxygen isotopes give some of the clearest spectrometer, yielding an average value of Concentrations of a broad suite of trace ele- indications of interaction with the hydrosphere 9.6‰ (VSMOW) for quartz standard NBS-28. ments were measured in these samples and in and atmosphere, and therefore whether a magma Whole-rock measurements were not replicated, many of the larger mapping-support sample has incorporated supra-crustal rocks. To investi- but typical reproducibility for the Denver facil- suite by instrumental neutron activation analysis gate the possibility of crustal interaction, plagio- ity is ±0.1‰–0.15‰ (1σ). (INAA) mainly at the USGS, Denver, and later clase phenocrysts were separated from Mount Pumice deposited in subalpine meadows by inductively coupled plasma mass spectrom- Rainier samples by magnetic and density meth- around Mount Rainier is commonly hydrated and etry (ICPMS) at WSU (Tables 3 and 4). Meth- ods, including a diamagnetic step to remove any partly converted to authigenic clays by reaction ods, accuracy, and precision of USGS analyses vapor-phase or groundmass quartz, followed by of glass with organic acid–rich pore waters. Such are similar to those of Baedecker (1987) and removal of any adhering glass and groundmass samples are excluded from consideration based Bacon and Druitt (1988), and those for WSU by etching in dilute HF (7.5%) for 10 min while on low whole-rock analytical totals (<98 wt%), analyses are available at http://www .sees .wsu ultrasonicating, and then rinsing. The magnetic except for a hydrated rhyodacite pumice deposit .edu/Geolab /note .html. Major and trace element separation methods, in particular, select strongly that is the sole biotite phenocrystic product concentrations were also determined for fi ve against grains with mineral and melt inclusions, known from Mount Rainier; its plotted major representative pumice bombs and for their sepa- so the analyzed plagioclase separates are biased oxide composition (analysis 95SR446* in rated glasses to assess the effects of crystal-melt toward grains with simple textures and zoning Table 1) was reconstructed from point-counted separation (Table 5). These pumices consist of that may be true phenocrysts and micropheno- modes coupled with electron microprobe analy- pyroxene andesites and hornblende-pyroxene crysts. Also analyzed were olivine phenocrysts ses of minerals and glass (Kirn, 1995). dacites from Mount Rainier, and a zircon-bear- separated from two samples and cleaned by ing hornblende rhyodacite bomb from Glacier similar methods, and one atypically large quartz RESULTS Peak, included because it is fresher and larger xenocryst from a regional basaltic andesite. than similarly evolved tephra samples from Mineral separates were analyzed for oxygen General Character of Mount Mount Rainier (Table 5). isotopes by laser fl uorination at WSU (Table 6). Rainier’s Magmas Isotopic values of Sr, Nd, and Pb (Tables 6 Oxygen isotope values were measured in dupli- and 7) were measured on 25 mg splits of whole- cate or triplicate on ~2 mg splits loaded with Most of Mount Rainier’s magmas ascended rock powders at the National High Magnetic standards in an 18-hole puck (6–8 standards, through an axial magmatic system underlying Field Laboratory, Tallahassee, Florida. Lead, Sr, 12–10 unknowns). Before applying laser heat- the main edifi ce, and erupted from the sum- and Nd were separated from the same aliquot ing (Sharp, 1990), two or three brief (~1.5 min) mit or from upper- and mid-fl ank vents fed by following the techniques outlined in Stracke pre-fl uorinations were conducted to remove radial dikes. The axial products are predomi- et al. (2003). Strontium isotope ratios were mea- water molecules on mineral surfaces and on nantly (77%) calc-alkaline andesites (57–63

sured by thermal ionization mass spectrometry the internal walls of the sample chamber and wt% SiO2), with lesser (23%) dacites (63–68

(TIMS) in dynamic mode on a Finnigan MAT the vacuum system. Tests have shown that no wt% SiO2), and a sole rhyodacite pumice fall 262 RPQ mass spectrometer, with 87Sr/86Sr ratios measurable oxygen was extracted during each deposit (<<1%) (Fig. 3). No known ﬂ ows or 86 88 corrected for fractionation to Sr/ Sr = 0.1194. short pre-ﬂ uorination. Also, reproducibility of tephras of basaltic andesite (52–57 wt% SiO2)

Long-term average of Sr standard E&A yields samples and standards was higher with the short or basalt (<52 wt% SiO2) erupted through the an 87Sr/86Sr value of 0.708004 ± 0.000013 (n = pre-ﬂ uorinations. Each sample or standard was axial magmatic system. Porphyritic andesites σ 87 86 25, 2 ), and Sr/ Sr of the samples are reported then heated with a 20 W CO2 laser, oxygen was and dacites are the norm, dominated by pheno- relative to the accepted ratio of the E&A stan- liberated by reaction with BrF5 (Clayton and crysts of, in decreasing abundance, plagio-

Geological Society of America Bulletin, January/February 2014 125 Sisson et al.

TABLE 1. MAJOR OXIDE CONCENTRATIONS (WT%) OF MOUNT RAINIER–REGION QUATERNARY VOLCANIC ROCKS

Sample no. SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2OK2OP2O5 Total Latitude Longitude Axial magmatic system JVDMAZ 57.75 0.98 18.85 6.30 0.11 3.42 6.97 3.95 1.29 0.37 99.39 46.7802 –121.7300 95RE462 58.38 1.03 16.89 6.18 0.11 5.11 6.75 3.74 1.57 0.25 99.39 46.7857 –121.7489 99RE777 58.42 1.01 17.12 6.09 0.11 5.06 6.79 3.59 1.54 0.27 99.95 46.8133 –121.7350 99ML770 58.74 1.01 16.71 6.05 0.10 5.04 6.76 3.81 1.46 0.31 99.41 46.8817 –121.8583 93RE41 58.98 1.13 17.29 5.91 0.10 4.22 6.40 4.03 1.67 0.27 99.37 46.8418 –121.7270 97RE614 59.02 1.04 17.79 6.10 0.10 3.76 6.63 4.05 1.30 0.22 100.46 46.8183 –121.7317 96RE528 59.39 1.10 16.49 5.99 0.10 3.65 6.41 4.11 2.33 0.43 99.05 46.8217 –121.6850 94ML318 59.49 0.95 17.29 5.56 0.10 4.08 6.73 4.03 1.53 0.25 98.95 46.8813 –121.7832 96RE539 59.64 1.11 16.85 5.92 0.10 3.58 6.29 3.87 2.25 0.40 100.20 46.8300 –121.6917 95SR514 59.68 0.94 17.69 5.75 0.10 3.61 6.56 3.96 1.45 0.25 99.65 46.9017 –121.6262 93RE4 59.72 0.96 16.89 5.78 0.10 4.46 6.27 3.86 1.71 0.23 99.79 46.7991 –121.7336 99ML764 60.05 0.85 17.56 5.38 0.10 3.69 6.33 4.10 1.62 0.32 99.68 46.9181 –121.7768 98RE692P1 60.32 0.95 17.51 5.48 0.09 3.83 6.02 3.97 1.59 0.24 99.41 46.8646 –121.6611 95RE464 60.69 0.97 17.04 5.36 0.09 2.93 6.14 3.94 2.46 0.39 99.40 46.8121 –121.7075 93RE26 60.71 0.92 17.31 5.28 0.09 3.55 6.12 4.07 1.69 0.26 99.51 46.8355 –121.7298 93MW68 60.76 0.94 17.41 5.61 0.10 3.09 6.09 4.14 1.59 0.29 99.44 46.8082 –121.8936 93MW72 60.63 0.94 17.46 5.61 0.10 3.11 6.12 4.14 1.60 0.29 99.28 46.8079 –121.8948 95RE494 60.80 0.83 17.96 5.18 0.09 3.07 6.26 4.19 1.39 0.23 99.63 46.8656 –121.6916 93RE197 60.91 0.81 17.89 5.15 0.09 3.12 6.24 4.07 1.56 0.17 99.57 46.7958 –121.7253 96RW581 60.93 0.94 17.11 5.41 0.09 3.35 5.99 4.07 1.83 0.26 100.31 46.8250 –121.7933 95RE506 61.15 0.86 17.64 5.40 0.10 2.71 5.94 4.18 1.72 0.31 99.66 46.8596 –121.7252 JV506CL3 60.70 0.87 17.69 5.58 0.10 2.82 6.17 4.11 1.62 0.33 100.12 46.8645 –121.6619 93RW3 61.58 0.85 18.18 4.73 0.08 2.42 5.96 4.18 1.80 0.23 100.11 46.7706 –121.7793 93RW177 61.68 0.88 17.19 5.34 0.10 2.90 5.86 4.09 1.69 0.27 98.97 46.8522 –121.7536 94ML329 61.80 0.80 18.02 4.85 0.08 2.75 5.87 4.17 1.50 0.18 99.89 46.8833 –121.7871 99GL769 62.12 0.85 17.10 5.17 0.09 3.23 5.48 3.92 1.78 0.26 99.41 46.8788 –121.8785 93RE53 62.28 0.89 16.69 4.99 0.08 2.78 5.57 3.94 2.45 0.33 99.46 46.8037 –121.6951 96RW555 62.31 0.87 17.07 4.88 0.08 3.04 5.43 4.06 2.00 0.26 99.64 46.8567 –121.8467 96RE530 62.64 0.93 16.48 5.03 0.09 2.83 5.26 4.08 2.32 0.32 100.03 46.8267 –121.6750 94RW275 62.81 0.74 17.84 4.51 0.08 2.44 5.80 4.08 1.53 0.17 99.69 46.8635 –121.8115 96RW570 62.89 0.86 16.77 5.04 0.09 2.90 5.27 4.09 1.85 0.24 100.09 46.8317 –121.7850 00RW821 62.93 0.81 17.27 4.49 0.08 2.44 5.45 4.12 2.15 0.24 99.49 46.8044 –121.7878 93RE39 63.20 0.79 16.86 4.60 0.08 2.78 5.30 4.18 1.98 0.24 99.58 46.8430 –121.7295 94RW288 63.48 0.62 17.66 4.12 0.07 2.54 5.74 4.09 1.53 0.14 99.54 46.8645 –121.8190 94RE379 63.59 0.72 17.05 4.67 0.09 2.29 5.24 4.08 2.02 0.26 98.48 46.8453 –121.7432 00RE849 63.94 0.81 16.37 4.59 0.09 2.60 4.89 4.12 2.29 0.30 98.88 46.8556 –121.7024 93RE193 63.95 0.72 16.78 4.51 0.08 2.72 4.98 4.29 1.79 0.18 99.62 46.7889 –121.7304 00RE801 64.14 0.71 16.99 4.27 0.08 2.46 5.07 4.04 2.02 0.22 99.95 46.8287 –121.7245 93RE120 64.14 0.76 16.63 4.59 0.08 2.46 4.98 4.04 2.11 0.21 98.81 46.8057 –121.7468 97RE629 64.44 0.63 16.93 4.15 0.07 2.61 5.00 4.31 1.68 0.17 99.86 46.8100 –121.6900 01RW894 65.33 0.64 16.41 4.01 0.07 2.48 4.63 4.07 2.18 0.18 99.79 46.8197 –121.7592 93RW87 66.67 0.60 16.12 3.58 0.07 1.91 4.30 3.82 2.76 0.18 96.53 46.8600 –121.7809 96RW544 66.72 0.63 16.27 3.73 0.07 1.84 4.04 4.28 2.27 0.16 100.31 46.8617 –121.8583 93RW100 66.83 0.57 16.59 3.54 0.06 1.70 4.15 4.21 2.18 0.17 99.15 46.7987 –121.7771 94RW282 67.89 0.53 15.89 3.33 0.06 1.71 3.84 4.16 2.45 0.14 99.20 46.8737 –121.7923 95SR446 69.07 0.38 19.40 2.20 0.05 0.42 1.56 3.88 2.93 0.11 91.71 46.9188 –121.6451 95SR446* 72.71 0.39 14.61 2.01 0.06 0.74 1.68 4.41 3.29 0.11 100.00 46.9188 –121.6451 Quenched magmatic inclusions 96RE532 52.74 1.39 17.11 8.41 0.15 7.54 8.34 3.55 0.45 0.33 99.96 47.8233 –122.6667 93RE191 55.48 0.93 15.69 7.99 0.15 8.26 7.93 2.70 0.69 0.18 99.77 46.7943 –121.7427 93RE58 55.82 1.22 17.23 7.03 0.12 5.57 8.07 3.55 0.99 0.40 99.15 46.8133 –121.7445 94ML314 56.87 1.18 16.65 6.27 0.12 6.32 7.79 3.22 1.42 0.15 98.00 46.8901 –121.7775 98RE675 58.06 1.07 18.21 6.90 0.11 3.83 6.52 3.89 1.18 0.23 99.11 46.8275 –121.6667 96RW576 58.79 1.11 17.64 6.39 0.10 3.77 6.39 4.17 1.35 0.29 100.02 46.8300 –121.7850 Gabbronorite inclusions 93RE16 56.08 1.11 15.99 7.95 0.13 6.88 7.55 2.79 1.34 0.19 99.14 46.7951 –121.7112 01SR878 57.83 1.39 16.38 6.23 0.10 5.50 7.06 3.72 1.46 0.33 99.59 46.9076 –121.6351 North-ﬂ ank vents 99ML762 55.05 1.06 16.70 6.95 0.12 6.46 8.59 3.39 1.32 0.37 99.59 46.9125 –121.8063 93ML214 55.24 1.19 17.37 7.43 0.13 6.06 7.52 3.75 1.00 0.30 99.84 46.9034 –121.8075 94ML444 57.26 1.09 17.53 6.65 0.11 4.71 7.20 3.90 1.30 0.25 99.41 46.9023 –121.8184 97ML657 58.45 1.13 16.76 5.92 0.10 4.01 6.86 3.79 2.49 0.48 99.71 46.9047 –121.7916 97ML656 59.15 1.04 16.95 5.78 0.09 4.01 6.57 3.96 2.06 0.39 100.01 46.9245 –121.8053 93ML98 61.26 0.86 16.62 5.02 0.09 4.10 5.61 4.14 2.03 0.27 99.29 46.9699 –121.7935 Regional Quaternary basalts and basaltic andesites 01SB872 49.87 1.25 17.02 9.28 0.16 8.67 10.01 3.02 0.49 0.22 100.90 46.2292 –121.9933 00ECR836 49.88 1.30 16.66 8.58 0.15 8.75 9.95 3.05 1.23 0.44 99.98 46.3392 –121.7414 00BL834 50.23 1.35 16.58 8.51 0.15 8.61 9.69 3.12 1.30 0.46 100.48 46.3973 –121.7279 00OHS831 50.70 1.45 16.09 9.01 0.15 9.30 8.84 3.03 1.00 0.44 99.81 46.6876 –121.5414 00WP830 50.92 1.28 16.80 9.42 0.16 7.85 9.62 3.15 0.57 0.23 100.42 46.6887 –121.4672 97BM664 53.51 1.13 15.64 7.67 0.13 8.94 8.04 3.30 1.30 0.34 100.13 47.0917 –121.8515 01MCP874 56.39 1.13 17.25 7.09 0.12 4.62 7.58 3.95 1.39 0.47 99.91 46.3477 –121.8607 Note: Concentrations are normalized to 100% with all Fe as FeO; total gives original sum. Analysis 95SR446* gives oxide concentrations for weathered pumice sample SR446 reconstructed from modes and electron-microprobe analyses of glass and minerals. Locations are referenced to the NAD27 datum.

126 Geological Society of America Bulletin, January/February 2014 Petrogenesis of Mount Rainier andesite

TABLE 2. MAJOR OXIDE CONCENTRATIONS (WT%) OF SOUTHWEST WASHINGTON PRE-PLEISTOCENE ROCKS

Sample no. Unit SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2OK2OP2O5 Total Latitude Longitude Miocene 93T59 Tatoosh Granodiorite 66.6 0.79 15.1 4.38 0.07 1.89 3.89 3.77 3.40 0.15 98.9 46.7663 –121.7318 203004 Tatoosh Granodiorite 65.4 0.71 15.6 4.49 0.07 2.53 4.83 3.92 2.34 0.13 99.5 46.7774 –121.7343 203080 Tatoosh Granodiorite 58.5 1.11 17.2 7.04 0.14 3.55 6.52 3.88 1.82 0.23 99.4 46.7954 –121.8093 203085 Tatoosh Granodiorite 69.3 0.53 15.2 3.54 0.06 1.31 2.94 3.61 3.47 0.10 99.6 46.7633 –121.7195 Oligocene 08MW1016 Ohanapecosh volcaniclastic 64.9 0.78 17.9 5.51 0.09 1.31 7.11 1.65 0.68 0.12 91.9 46.7962 –121.8879 Eocene–Paleocene 08W1012 Puget Group arkose 84.5 0.17 8.7 0.80 0.02 0.34 1.14 2.46 1.85 0.03 97.6 46.1047 –122.0297 08GL1015 Puget Group arkose 81.5 0.28 10.8 1.22 0.03 0.50 1.05 2.34 2.22 0.06 96.7 46.9512 –121.9828 08GL1014 Puget Group shale 62.7 1.09 27.0 2.89 0.01 1.16 0.15 0.43 4.03 0.18 83.3 46.9438 –121.9722 08WP1008 Summit Basalt 51.1 2.36 16.3 14.66 0.15 3.22 9.17 2.09 0.56 0.34 89.6 46.6666 –121.4838 08CR1010 Crescent/Siletzia basalt 49.9 3.46 14.2 12.73 0.20 5.21 9.89 3.15 0.56 0.70 97.2 46.3506 –122.9381 08KS1011 Crescent/Siletzia basalt 47.4 1.41 15.1 10.29 0.18 9.83 12.56 2.03 0.83 0.38 95.9 46.1562 –122.9146 Mesozoic 08RR1001 Russell Ranch arkose 71.9 0.54 14.2 4.19 0.06 1.92 1.95 3.57 1.55 0.12 97.8 46.6463 –121.1735 08SB1005 Russell Ranch arkose 69.8 0.74 13.9 6.13 0.08 2.36 1.70 3.38 1.78 0.17 96.7 46.6290 –121.2946 08RR998 Sheared granodiorite 67.0 0.48 16.2 4.92 0.10 2.18 4.23 3.48 1.29 0.11 98.2 46.6469 –121.1461 08RR999 Greenstone 48.8 2.04 17.6 9.78 0.14 11.22 5.59 2.95 1.49 0.32 92.5 46.6445 –121.1545 08SB1003 Orthogneiss 60.8 0.50 16.5 7.72 0.16 2.95 8.45 2.57 0.20 0.10 98.1 46.6650 –121.2824 Unknown 93RE63 Metavolcanic xenolith 49.0 0.39 27.4 13.08 0.22 2.07 2.58 3.03 1.94 0.24 98.8 46.8147 –121.7351 Note: Concentrations are normalized to 100% with all Fe as FeO; total gives original sum. Tatoosh Granodiorite and 93RE63 xenolith samples were analyzed at the U.S. Geological Survey, the rest at Washington State University. Locations are referenced to the NAD27 datum.

clase, orthopyroxene, and clinopyroxene, with Mount Rainier eruptives contain sieve-textured laterally to its side, whereas basaltic andesites microphenocrysts of FeTi oxides and apatite. phenocrysts and phenocrysts with overgrown ascending through the axial magmatic system Amphibole is absent to minor in most ande- internal resorption surfaces, as are common- only reach the surface as QMIs after having

sites, becoming abundant in relatively high-K2O place in calc-alkaline andesitic magmas world- mingled with the andesites and dacites that andesites, and also in the higher-SiO2 dacites wide, recording magma mingling and mixing dominate the axial system. Six to seven kilome- and rhyodacite. Olivine is seen irregularly in events (Sakuyama, 1981). Accompanying the ters is therefore a maximum limiting estimate of hand sample and thin section in rocks with SiO2 complex grains in variable proportions, how- the radius of the axial andesitic system. North- as great as 59 wt%, but sparse rounded olivine ever, are idiomorphic phenocrysts and micro- fl ank basaltic andesite compositions also over- grains are routinely recovered in mineral sepa- phenocrysts with few resorption features. At the lap the high-SiO2 portion of the fi eld of regional rates from higher-SiO2 samples, probably as extreme are rare lava fl ows of non-porphyritic basalts and basaltic andesites of the southern relicts from magma mingling. Other minor andesite in which texturally simple non-sieved Washington Cascades. mineral phases include a Cu-Fe magmatic sul- mineral grains range continuously from ground- fi de (typically preserved only where included mass up to ~0.5 mm. These fl ows result from Major Oxide Compositions of in phenocrysts), minute zircon grains in the infrequent eruptions of phenocryst-free ande- Mount Rainier–Region Quaternary more evolved dacites and rhyodacite, biotite sitic liquids, as opposed to crystal-laden ande- Volcanic Rocks phenocrysts solely in the rhyodacite pumice- sitic magmas whose phenocrysts betray their fall deposit but as a trace groundmass phase complex origins. Additional magmatic products Major oxide trends of Mount Rainier’s mag- in thick and well-crystallized andesite-dacite are medium-grained inclusions of vuggy gab- mas (Fig. 3) are familiar from calc-alkaline lava fl ows, and traces of resorbed phenocrystic bronorite to quartz diorite, up to 20 cm across, andesitic suites worldwide (Gill, 1981; Thorpe, quartz in an evolved dacite. that characterize some lava fl ows; U-Pb ages 1982), with simple, near-linear arrays of Evidence for magma mingling in the form of zircons from these inclusions indicate that de creasing concentrations of MgO, CaO, FeO*,

of ﬁ ne-grained quenched magmatic inclu- they are Pleistocene plutonic products of Mount and TiO2 with increasing SiO2. Considering only sions (enclaves; Bacon, 1986) is widespread in Rainier’s magmatic system (Sisson et al., 2009). eruptives from Mount Rainier’s axial magmatic

Mount Rainier’s eruptive products. Quenched True xenoliths of pre-Quaternary rocks are system, concentrations of K2O increase continu- magmatic inclusions (QMIs) are commonly exceptionally uncommon. ously from basaltic andesite QMIs to andesites

4–8 cm across, ﬁ ne grained, sparsely vesicular Vents on the north ﬂ ank of Mount Rainier to dacites, but with greater K2O diversity at

in their interiors, and contain <10% pheno- (Fig. 2) erupted magmas distinct from those of higher SiO2. Deﬁ ning its calc-alkaline character, crysts of olivine (or orthopyroxene or amphi- the axial magmatic system, consisting of olivine FeO*/MgO of the Mount Rainier suite is low and

bole in some higher-SiO2 QMIs), commonly basaltic andesites and amphibole-phenocrystic increases only modestly with increasing SiO2. accompanied by traces of resorbed phenocrysts spessartite, as well as basaltic andesite–spes- Although most of the suite has Mg# [100 • molar entrained from the host magma. Compositions sartite hybrids. North-fl ank basaltic andesite Mg/(Mg + ΣFe)] from 65 to 50, absolute MgO of QMIs overlap those of Mount Rainier’s lavas compositions plot overlapping the fi eld of QMIs concentrations are generally <6 wt%, dissimilar and tephras, but extend the suite to lower SiO2 (Fig. 3). These basaltic andesites result from to high-Mg andesite localities of the Cascades, concentrations (Fig. 3), consisting of andesites eruptions of some types of mafi c magmas that such as Mount Shasta (California) (Grove et al.,

(77%) and basaltic andesites (22%), with only replenish Mount Rainier’s axial magmatic sys- 2005). Plots of Al2O3 and P2O5 versus SiO2 (Figs. one basaltic QMI having been found. Nearly all tem, but that bypassed it by ascending 6–7 km 3 and 4) deﬁ ne fan-shaped ﬁ elds with diverse

Geological Society of America Bulletin, January/February 2014 127 Sisson et al.

SiO2 (wt%) SiO2 (wt%) 46 50 54 58 62 6670 74 46 50 54 58 62 6670 74 3.5 4.0 MA MA 3.0 high-K 3.5 MSH SWC B/BA med-K tholeiitic 2.5 3.0 Mg# 40 2.0 2.5 SWC B/BA 1.5 2.0 O (wt%) 50 2

K 1.0 low-K 1.5 60 0.5 FeO*/MgO (wt%) 1.0 calc-alkaline MSH 70 0.0 0.5 2.5 20 edifice lava & tephra quenched magmatic inclusions 19 2.0 north flank vents regional mafic (circled points this study) 18 1.5 (wt%)

(wt%) 17 2 1.0 3 O 2

TiO 16 SWC B/BA Al 0.5 15 SWC B/BA 0.0 14 10 12

10 8 8 6 6 4 SWC B/BA 4 SWC B/BA CaO (wt%) MgO (wt%) 2 2

0 0 46 50 54 58 62 6670 74 46 50 54 58 62 6670 74 SiO2 (wt%) SiO2 (wt%)

Figure 3. Major oxide variation diagrams for magmas that erupted through Mount Rainier’s axial magmatic system as lavas and tephras (both orange) and as quenched magmatic inclusions (yellow), and from its north-fl ank vents (dark blue). Fields show Quaternary andesites and dacites of Mount St. Helens (MSH, light blue) and Mount Adams (MA, pink), and regional basalts and basaltic andesites of the southern Washington Cascades (SWC B/BA, gray). Lines delimit the high-, medium-, and low-K fi elds of Gill (1981), and the tholeiitic versus calc- alkaline series of Miyashiro (1974) [Mg# is 100 Mg/(ΣFe + Mg)]. Circled symbols are analyses reported in this study; other Mount Rainier compositions (uncircled) are from Stockstill et al. (2002), McKenna (1994), Sisson and Vallance (2009), and Sisson (unpublished data). Southern Washington Cascades, Mount Adams, and Mount St. Helens compositions are from Hammond and Korosec (1983), Leeman et al. (1990, 2004, 2005), Conrey et al. (1997), Bacon et al. (1997), Reiners et al. (2000), Smith and Leeman (1987, 1993, 2005), Jicha et al. (2009), Mullineaux (1996), Clynne et al. (2008), Gardner et al. (1995), and Halliday et al. (1983). Mount St. Helens fi elds include some glass compositions.

Figure 4 (on following page). Trace element variation diagrams for magmas that erupted through Mount Rainier’s axial magmatic system as lavas and tephra (orange) and as quenched magmatic inclusions (yellow), and from its north-ﬂ ank vents (dark blue). Fields show Quater- nary andesites and dacites of Mount St. Helens (MSH, light blue) and Mount Adams (MA, pink), and regional basalts and basaltic andesites of the southern Washington Cascades (SWC B/BA, gray) (data sources as for Fig. 3). Red and green diamonds with tie lines are whole-rock and glass compositions from Mount Rainier and Glacier Peak pumices (Table 5). Dashed lines with temperatures are isotherms for saturation of melt with zircon and apatite (Watson and Harrison, 1983; Harrison and Watson, 1984). Circled symbols are analyses reported in this study. Other data sources are reported in the caption for Figure 3. Eu/Eu* values for U.S. Geological Survey (USGS) analyses (background points) are raised by ~10% relative to account for Washington State University (WSU)/USGS laboratory bias.

128 Geological Society of America Bulletin, January/February 2014 Petrogenesis of Mount Rainier andesite

SiO2 (wt%) SiO2 (wt%) 46 50 54 58 62 6670 74 78 46 50 54 58 62 6670 74 78 125 1250 edifice lava & tephra MA quenched magmatic inclusions SWC B/BA 100 north flank vents 1000 regional mafic pumice ( )-glass ( ) pairs 75 (circled points this study) 750 MSH SWC B/BA 50 500 Rb (ppm) Sr (ppm)

25 MSH 250 MA 0 0 50 1.25 MA 40 SWC B/BA 1.00 30 Eu/Eu*

Y (ppm) Y 20 0.75 SWC B/BA 10 MSH 5 0.50 450 0.6 400 MA 850 ° 0.5 SWC B/BA 350 C SWC B/BA 0.4 300 MA (wt%)

250 5 0.3

800 °C O 1000 200 2 °C Zr (ppm) P 0.2 150 950 °C 0.1 100 750 MSH °C MSH 900 °C 50 0.0 40 1200 35 SWC B/BA 1000 30 MA 800 25 MA 20 600 SWC B/BA 15 Ba (ppm)

Nb (ppm) 400 10 200 MSH 5 MSH 0 0 20 7 18 6 SWC B/BA 16 14 SWC B/BA 5 12 4 10

Ba/Zr 3 Th/Ta 8 6 2 4 1 2 MA MSH 0 0 46 50 54 58 62 6670 74 78 46 50 54 58 62 6670 74 78 SiO2 (wt%) SiO2 (wt%)

Figure 4.

Geological Society of America Bulletin, January/February 2014 129 Sisson et al. ) – .7 2.3 9.6 18.9 15.8 13.3 13.6 6 19.9 6 19.4 0 21.2 8 17.1 8 17.3 4 20.9 5 13.4 923.7 126.3 3 14.6 715.1 5 16.3 912.4 515.5 613.1 313.3 91.8 19.5 .2 22.5 .0 19.4 .0.0 16.1 16.1 .5 12.8 .8 15.5 .0 15.5 .6 17.8 8.3 18.1 7.0 38.1 7.2 21.2 0.6 17.3 7.56.6 – 24.4 8.41.8 15.7 14.1 9.9 14.4 6.7 20.6 2.6 16.4 0.1 13.1 continued ( 10.6 19.6 10.4 13.4 5 8.5 27.9 4 5.4 25.0 4 10.2 20.0 1 15.5 31.4 1.30 0.17 – 110 2.61 – 0.54 18.7 – 27.1 ICPMS/INAA TABLE 3. TRACE ELEMENT CONCENTRATIONS (PPM) OF MOUNT RAINIER–REGION QUATERNARY VOLCANIC ROCKS RAINIER–REGION QUATERNARY (PPM) OF MOUNT CONCENTRATIONS TRACE ELEMENT 3. TABLE XRF 7 506 21 172 16.4 274 117 59 87 245 179 18 8 5 509 0.3 274 2.83 0.88 20.3 42.3 5.36 21.8 4.80 1.65 4.65 0.74 4.21 0.80 2.11 0.29 1.78 0.27 21 171 4.00 15.8 1.03 22.3 29 720 18 166 8.5 339 15 34 77 6 125 22 7 28 734 1.3 340 4.60 1.45 21.6 47.3 6.09 24.3 4.84 1.51 4.15 0.60 3.48 0.68 1.73 0.25 1.52 0.24 17 167 4.24 7.7 0.50 13.4 7.1 29 678 10 109 4.1 284 13 18 61 21 90 21 12 27 686 1.0 282 2.56 1.32 10.0 21.8 2.86 12.3 2.83 0.96 2.56 0.37 2.05 0.38 0.97 0.13 0.86 0.13 10 107 3.00 3.5 0.26 10.5 10 41 464 21 162 11.3 389 50 24 69 172 132 19 7 39 458 2.0 391 5.23 1.77 19.7 41.1 5.11 20.3 4.34 1.33 4.11 0.65 3.86 0.77 2.06 0.29 1.81 0.29 20 160 4.13 9.6 0.71 19.0 56 470 17 167 10.3 469 27 22 65 50 94 20 10 54 474 2.9 477 7.06 2.50 22.2 44.8 5.40 20.7 4.19 1.25 3.72 0.55 3.15 0.61 1.58 0.22 1.38 0.21 15 167 4.40 10.0 0.78 11.1 34 563 15 143 8.8 378 28 30 71 50 128 21 8 33 563 1.4 376 4.19 1.44 18.1 37.4 4.67 18.5 4.02 1.25 3.52 0.55 3.13 0.61 1.56 0.23 1.37 0.22 15 141 3.76 8.1 0.56 15.1 7. 49 587 18 172 9.5 469 21 19 69 54 119 21 9 46 590 1.8 467 6.33 2.14 24.8 51.4 6.42 25.2 5.02 1.48 4.25 0.64 3.56 0.69 1.76 0.25 1.54 0.24 17 170 4.41 9.2 0.66 14.1 8. 46 443 20 162 9.4 416 41 2440 68 54539 137 18 123 555 170 2031 10.1 19 448 644 164 8 10.8 15 15 435 139 44 22 13 7.5 446 319 76 24 1.8 18 28 75 121 419 26 20 26 5.87 121 2.03 68 8 20 20.5 15 41.7 7 112 39 22 5.12 557 20.1 39 7 4.14 1.7 566 1.29 3.92 446 29 1.7 0.62 5.12 648 3.68 1.64 443 0.73 22.4 5.16 1.1 1.98 1.63 46.0 0.27 317 22.5 1.77 4.04 5.68 46.2 0.27 1.25 22.3 19 15.8 4.63 5.68 162 1.35 22.5 33.7 4.21 4.04 4.67 4.31 0.61 1.42 9.9 17.4 3.49 4.07 0.75 3.68 0.68 0.61 16.6 1.22 1.80 3.48 3.26 0.26 0.67 8 0.50 1.58 1.78 2.79 0.24 0.26 17 0.54 1.57 170 1.41 0.24 4.32 17 0.20 164 1.24 9.8 4.23 0.19 0.67 14 9.8 138 14.2 0.67 3.64 13.8 8 6.7 0.46 8 12.2 6. 30 489 26 160 13.2 328 87 63 75 129 160 18 9 29 491 1.1 335 4.07 1.46 19.0 43.5 6.04 26.1 6.44 1.83 6.10 0.93 5.39 1.02 2.54 0.35 2.07 0.32 25 161 4.47 12.6 0.78 22. 28 558 16 159 11.1 361 37 48 84 67 122 21 7 27 564 1.0 355 3.92 1.31 19.2 40.4 5.09 20.7 4.46 1.47 3.97 0.60 3.28 0.61 1.52 0.21 1.27 0.20 15 158 3.96 10.5 0.71 13.5 25 860 14 124 <10 300 49 80 72 140 – – – 24 944 1.5 290 3.38 1.00 19.5 40.3 – 21.0 4.40 1.31 – 0.48 – – – – 30 524 18 131 8.1 345 34 31 71 52 136 20 5 28 524 0.5 344 3.49 1.33 15.3 32.0 4.13 17.0 3.82 1.28 3.57 0.55 3.26 0.64 1.69 0.24 1.43 0.23 16 130 3.37 7.4 0.51 15.9 5. 51 883 20 22650 12.5 718 817 20 34 271 12.3 36 678 87 33 47 35 133 21 82 11 58 136 48 20 893 9 1.8 717 48 11.0 817 2.87 46.7 1.3 97.0 676 9.39 12.1 2.52 47.5 37.9 8.62 2.36 79.9 6.34 10.2 0.86 39.4 4.42 7.39 0.79 2.02 1.94 5.59 0.27 0.77 1.62 4.17 0.25 20 0.77 224 1.94 5.66 0.26 11.8 1.65 0.78 0.24 20 14. 266 6.98 10.650 0.72 458 15.4 17 171 10.8 454 27 25 69 54 95 20 9 49 467 1.8 457 6.37 2.21 22.4 45.4 5.51 21.3 4.50 1.29 3.86 0.58 3.28 0.63 1.63 0.24 1.46 0.23 16 173 4.48 10.3 0.77 11.6 7 37 565 14 137 6.9 356 11 31 65 12 105 21 13 36 567 0.8 360 4.77 1.64 16.3 32.8 4.04 16.0 3.47 1.12 3.10 0.47 2.63 0.51 1.39 0.19 1.22 0.18 13 136 3.58 6.3 0.49 11.8 1 36 654 17 165 8.7 390 45 41 7960 76 829 12945 17 20 513 224 8 17 9.8 165 722 10.4 34 419 20 658 41 3338 1.3 27 77 504 394 68 16 28 5.27 145 125 1.56 72 20 22.2 113 7.5 11 21 358 46.0 9 18 5.74 57 22.6 23 4.66 836 43 1.41 64 1.8 4.00 517 0.59 47 726 2.0 3.34 114 11.9 0.63 19 3.46 420 1.67 42.8 5.69 9 0.24 1.95 87.3 1.49 21.2 0.22 10.8 36 16 43.1 41.7 163 506 7.53 5.32 4.24 2.06 21.2 1.9 9.1 5.44 4.42 0.72 1.36 358 0.62 3.72 3.91 4.55 15.7 0.67 0.59 1.66 1.68 3.35 16.0 8. 0.23 0.66 33.6 1.40 1.70 0.21 0.24 4.29 17 1.48 17.0 219 0.22 3.83 5.79 17 1.16 165 3.42 9.1 4.26 0.54 10.1 0.67 3.14 0.75 13.0 0.61 13.3 1.62 1 0.23 1.48 0.22 16 142 3.83 6.9 0.51 14.6 7. 23 461 16 146 9.9 314 35 38 88 57 130 21 5 22 465 1.0 318 2.94 1.08 14.6 32.7 4.30 17.8 4.13 1.42 3.90 0.60 3.32 0.62 1.57 0.21 1.26 0.20 15 140 3.69 8.4 0.57 14.0 5. 36 505 18 111 6.8 287 137 33 98 247 188 20 17 34 507 2.6 287 3.87 1.39 14.2 30.8 4.12 17.8 4.45 1.33 4.22 0.63 3.54 0.66 1.71 0.24 1.44 0.21 17 109 3.23 6.1 0.46 26. 22 447 21 142 8.6 296 38 48 78 67 159 19 6 21 450 0.9 290 3.26 1.10 16.4 34.5 4.39 18.2 4.21 1.33 4.03 0.64 3.82 0.77 2.05 0.29 1.81 0.27 20 139 3.54 8.4 0.57 21.4 5. 41 483 19 169 13.0 401 54 32 69 81 129 19 8 40 494 1.7 401 4.87 1.70 19.7 41.1 5.12 20.7 4.54 1.37 4.10 0.65 3.74 0.72 1.89 0.27 1.64 0.25 18 171 4.30 12.8 0.88 15.0 41 464 20 158 10.2 391 48 25 69 171 129 19 7 39 466 0.9 392 5.10 1.75 19.7 40.7 4.99 19.7 4.28 1.33 3.94 0.64 3.74 0.73 1.99 0.2932 1.76 0.28 849 1947 157 18 499 181 3.98 17 9.0 9.5 156 431 0.69 9.9 18.1 11 417 13 8 23 77 59 11 113 24 22 124 23 6 7 31 45 857 498 0.7 1.1 433 6.15 415 1.69 5.94 25.9 2.15 19.4 55.4 39.3 7.05 27.4 4.81 5.19 18.8 1.54 4.00 4.20 1.28 0.61 3.65 3.46 0.56 0.68 3.26 1.79 0.64 0.25 1.68 1.59 0.24 0.24 1.53 17 0.23 178 16 4.71 154 4.11 8.2 9.2 0.54 0.70 10.6 11.6 6.2 6. 40 497 17 162 10.4 402 58 28 68 78 112 20 8 38 495 1.7 401 4.91 1.63 18.5 39.0 4.87 19.5 4.13 1.23 3.71 0.57 3.31 0.64 1.66 0.24 1.45 0.24 16 159 4.12 9.7 0.68 13.1 8 36 796 18 183 9.240 429 548 1039 19 488 22 173 10.7 15 77 426 132 20 12 7.6 102 373 24 23 14 76 8 18 29 104 34 68 21 804 20 7 10561 1.3 20 499 433 39 8 17 6.74 168 1.80 558 26.1 9.4 38 1.7 516 55.1 495 430 15 6.85 5.35 0.9 26.8 1.73 19 5.16 369 22.5 1.50 4.10 57 46.6 4.02 1.72 0.58 30 15.5 5.75 3.31 102 22.7 31.8 0.65 19 4.53 1.69 3.84 1.37 9 0.25 15.6 4.01 1.53 3.52 0.62 0.24 1.09 3.50 59 17 3.19 0.66 183 497 0.49 1.75 4.73 2.82 0.25 3.4 0.54 1.58 8.3 1.42 0.24 509 0.56 17 0.20 8.39 10.1 172 1.27 2.89 4.40 0.20 23.5 8. 14 9.7 133 47.5 0.69 3.45 5.70 12.6 21.7 6.9 4.27 0.51 7 1.26 11.8 3.63 0.56 6. 3.16 0.62 1.66 0.24 1.47 0.23 16 168 4.40 8.7 0.71 12.1 8. 35 818 15 148 7.6 455 32 35 73 50 122 22 6 33 827 0.6 465 5.58 1.69 26.5 56.2 7.11 28.0 5.30 1.52 3.95 0.56 3.02 0.58 1.52 0.21 1.32 0.20 15 148 3.85 7.1 0.48 13.1 6. 37 450 20 134 <10 345 88 50 83 152 – – – 35 480 1.1 270 4.27 1.30 15.4 29.8 – 16.0 3.72 0.95 – 0.54 – – – – 1.40 0.20 – 170 3.10 – 0.59 20.8 – 34.6 57 556 18 213 10.3 581 20 20 70 35 93 19 11 55 562 2.7 587 8.92 2.78 30.6 62.9 7.71 29.6 5.64 1.52 4.40 0.64 3.56 0.67 1.74 0.25 1.56 0.24 17 212 5.52 10.1 0.75 11.1 46 432 15 156 8.9 433 25 18 65 56 83 19 9 45 438 2.5 433 5.32 2.03 18.3 37.4 4.54 17.7 3.80 1.12 3.33 0.49 2.76 0.51 1.34 0.20 1.19 0.18 13 155 4.09 8.2 0.62 10.7 9.1 37 543 16 141 11.3 349 55 39 70 133 139 20 6 35 543 0.8 350 4.79 1.70 18.8 38.4 4.78 18.8 3.95 1.26 3.55 0.55 3.16 0.62 1.61 0.22 1.38 0.21 16 139 3.63 10.7 0.73 17. 56 661 19 245 11.4 649 23 26 71 39 110 22 8 53 674 1.1 648 9.10 2.72 34.3 71.4 8.87 34.4 6.32 1.69 5.06 0.71 3.84 0.72 1.86 0.27 1.62 0.25 19 243 6.29 10.7 0.77 12.7 51 640 18 181 10.0 530 30 33 69 53 97 21 9 49 638 1.1 529 7.65 2.40 31.1 60.6 7.76 29.8 5.56 1.49 4.46 0.63 3.47 0.66 1.74 0.24 1.50 0.22 17 181 4.71 9.3 0.68 12.5 8. 49 685 16 173 9.359 486 434 9 16 158 18 9.3 84 474 21 13 25 88 21 61 10 35 47 88 697 20 2.3 8 488 7.43 57 2.27 441 26.4 1.9 54.2 6.62 485 25.2 7.71 4.71 2.74 1.33 23.5 3.77 44.7 0.55 3.14 5.38 0.61 20.4 1.62 4.03 0.24 1.16 1.49 3.54 0.24 0.53 16 3.01 173 0.60 4.62 1.57 0.22 8.6 1.38 0.65 0.22 16 8.7 158 4.23 9.6 8.6 0.72 1 10.9 9.0 63 397 15 169 9.6 528 18 17 55 25 71 19 10 59 382 1.5 522 8.27 3.08 22.4 43.7 5.06 19.1 3.75 1.02 3.21 0.49 2.86 0.55 1.46 0.21 1.30 0.21 14 164 4.43 9.1 0.77 8.9 9.5 60 696 17 199 9.7 655 25 28 69 40 115 19 12 58 703 1.4 656 10.8 3.36 36.2 74.1 9.02 34.8 6.49 1.78 4.78 0.66 3.44 0.64 1.63 0.22 1.39 0.22 16 197 5.21 9.0 0.70 12.3 1 62 429 15 160 8.7 532 21 11 57 51 81 19 10 61 437 1.9 527 7.66 2.71 22.0 43.7 5.14 19.3 3.89 1.05 3.24 0.50 2.92 0.55 1.45 0.21 1.34 0.21 14 159 4.19 8.1 0.66 10.0 9. 76 253 21 272 16.1 870 4 13 54 4 10 20 21 74 255 4.4 869 16.6 5.52 34.9 57.2 7.72 26.6 4.74 1.07 3.67 0.57 3.34 0.67 1.85 0.29 1.91 0.31 19 271 7.41 15.2 1.31 2.1 18. 78 366 16 16665 9.3 393 532 12 149 15 7.7 495 8 51 9 28 10 68 53 19 13 18 57 76 19 10 376 4.6 62 534 398 11.6 4.13 2.1 24.2 498 46.6 7.38 5.34 2.75 19.7 19.5 3.87 38.6 1.02 3.24 4.42 0.49 16.6 2.89 3.27 0.57 0.94 1.50 2.79 0.22 0.44 1.39 2.46 0.21 0.48 14 1.25 165 0.18 4.53 1.14 0.17 9.3 12 0.89 147 3.95 7.9 7.4 12.0 0.64 7.6 9.5 8 46 498 16 175 10.3 459 35 28 72 68 116 20 8 43 500 1.7 453 5.84 2.08 21.4 44.1 5.40 21.1 4.32 1.26 3.78 0.58 3.23 0.63 1.63 0.23 1.46 0.22 16 170 4.40 9.2 0.67 13.0 7 43 455 12 125 6.6 391 30 14 61 63 80 19 10 42 458 1.4 391 4.25 1.87 14.9 30.5 3.77 14.8 3.29 1.03 2.80 0.42 2.30 0.46 1.19 0.17 1.02 0.17 12 126 3.37 6.2 0.48 10.4 8. 67 388 14 158 9.2 547 15 15 54 30 58 19 12 65 388 2.6 550 9.03 3.42 22.5 43.7 5.04 18.8 3.65 0.98 2.91 0.44 2.46 0.47 1.25 0.18 1.14 0.18 12 154 4.24 8.7 0.78 7.2 11. 58 450 16 175 10.1 485 18 21 66 32 94 20 10 56 456 3.1 486 7.41 2.65 22.5 45.5 5.44 20.9 4.31 1.21 3.64 0.55 3.11 0.59 1.55 0.22 1.35 0.21 15 176 4.64 9.7 0.76 10.8 1 98RE692P1 99ML764 93RE16 95SR514 93RE4 93MW68 93MW72 95RE494 Gabbronorite inclusions 96RE532 93RE191 97RE614 96RE528 96RE539 96RW570 Axial conduit system 94ML318 95RE464 93RE26 01SR878 01RW894 93RE193 Quenched magmatic inclusions 93RE41 95RE462 99RE777 99ML770 JV506CL3 93RW3 99GL769 JVDMAZ 95RE506 93RW177 94ML329 94RW275 94RW288 Sample no. Rb Sr Y Zr Nb Ba Ni Cu Zn Cr V Ga Pb Rb Sr Cs Ba Th U La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y Zr Hf Nb Ta Sc Pb Co 94RE379 00RE801 96RE530 93RE197 96RW581 96RW555 93RE120 97RE629 00RE849 93RW87 93RW100 98RE675 94ML314 00RW821 93RE39 96RW544 96RW576 93RE58 93RE53 94RW282 95SR446

130 Geological Society of America Bulletin, January/February 2014 Petrogenesis of Mount Rainier andesite 5.7 1– .7 – .4 – 1.0 – 0.4 – 2.2 43.5 5.0 30.9 5.0 26.2 4 4.5 39.6 0 6.3 18.9 .8 6.6 25.2 .5 9.8 19.9 .3 4.8 30.8 7.3 3.1 43.5 3.5 4.5 42.8 4.7 3.4 43.1 21 16.5 23.6 – 9 16.1 14.1 21.5 ) continued ctrometry (ICPMS) at the U.S. Geological Survey, the rest by XRF and ctrometry (ICPMS) at the U.S. Geological Survey, al Survey (USGS), Denver; analyses with complete rare earth elements (REE) ICPMS/INAA ICPMS/INAA ion analysis (INAA) at USGS, Denver. TABLE 4. TRACE ELEMENT CONCENTRATIONS (PPM) OF SOUTHWEST WASHINGTON PRE-PLEISTOCENE ROCKS PRE-PLEISTOCENE WASHINGTON (PPM) OF SOUTHWEST CONCENTRATIONS TRACE ELEMENT 4. TABLE XRF TABLE 3. TRACE ELEMENT CONCENTRATIONS (PPM) OF MOUNT RAINIER–REGION QUATERNARY VOLCANIC ROCKS ( RAINIER–REGION QUATERNARY (PPM) OF MOUNT CONCENTRATIONS TRACE ELEMENT 3. TABLE XRF 2 297 15 39 <1 182 1 30 64 4 218 15 2 1 301 0.6 181 0.07 0.03 3.2 7.2 1.15 5.8 1.85 0.71 2.27 0.39 2.61 0.54 1.51 0.23 1.47 0.23 14 34 1.00 0.4 0.02 30.5 0.9 – 7 366 22 96 9.4 122 157 47 74 345 204 17 2 6 367 0.06 118 0.88 0.30 8.7 19.8 2.77 12.4 3.31 1.23 3.70 0.64 4.04 0.82 2.23 0.32 1.99 0.31 21 94 2.35 8.4 0.53 32.3 1.8 4 22 591 27 126 5 309 142 140 84 385 306 16 2 22 597 0.2 302 3.48 0.90 29.5 67.9 9.76 43.6 9.88 2.94 8.27 1.11 5.96 1.10 2.75 0.36 2.12 0.33 27 119 3.21 5.6 0.36 37.4 2 12 458 37 243 36 207 31 57 133 45 281 23 1 12 469 0.2 200 3.06 0.88 34.5 77.8 10.3 45.1 10.5 3.72 10.4 1.52 8.60 1.59 3.86 0.50 2.89 0.42 38 232 6.20 37.8 2.57 27.7 1 18 210 25 141 20 318 181 46 70 339 241 18 1 18 213 0.7 314 1.63 0.57 15.5 33.1 4.24 18.1 4.48 1.60 4.71 0.80 5.02 1.03 2.70 0.38 2.31 0.36 25 134 3.28 21.0 1.47 30.2 63 198 14 110 7 514 10 3 29 22 30 12 13 64 201 1.3 520 6.85 1.52 26.8 50.8 5.72 20.5 3.71 0.91 2.95 0.46 2.69 0.53 1.46 0.21 1.32 0.21 14 109 3.06 7.7 0.61 4.7 12.6 – 11 192 42 168 13 109 10 71 135 6 185 20 1 11 197 1.6 105 1.29 0.41 12.4 28.1 3.98 18.7 5.60 2.06 7.04 1.26 8.23 1.74 4.75 0.68 4.31 0.66 43 161 4.37 13.3 0.92 29.5 1. 59 385 21 89 9 389 47 26 68 50 156 18 8 59 385 4.0 389 3.80 1.49 19.2 40.4 5.17 20.8 4.50 1.19 4.54 0.65 4.01 0.75 2.26 0.29 2.00 0.26 21 89 3 9 0.6 16 8 19.9 51 300 20 96 <10 590 32 <10 620 87 – – – 46 320 3.3 560 8.40 1.40 33.9 67.5 – 29.0 5.31 1.07 – 0.52 – – – – 1.60 0.22 – 85 2.17 – 0.35 25.1 – 16.0 11 243 43 251 11 1527 3 26 90 12 61 19 10 12 24547 3.7 189 154045 15 193 112 6.6222 18 3.06 398 138 6 25.5 16 54.7 573 6 98 7.18 22 545 29.7 1 18 23 7.16 1.69 69 436 28 58 7.29 91 102 1.24 14 3 7.81 66 152 10 1.63 14 8 4.56 52 0.67 7 4.16 48 8 121 191 0.66 15 2.2 43 45 254 193 2 6.82 1.7 579 11.1 5.31 538 0.79 22 1.87 399 18.4 4.60 17.2 1 0.5 1.83 32.6 15.8 437 4.00 30.9 15.5 1.35 3.95 0.56 3.36 15.8 0.97 8.1 3.61 2.97 1.05 0.48 17.7 2.85 3.29 2.50 0.54 0.58 10.7 1.55 3.36 0.23 0.68 2.62 0.91 1.83 1.44 0.28 0.23 2.64 1.77 0.44 15 109 0.29 2.83 3.20 18 0.60 136 1.68 3.74 6.2 0.26 1.77 0.48 7.0 0.29 12.0 0.51 15 8.8 14.3 94 2.77 8.0 – 1.0 – 0.08 15.9 2.2 – 45 214 10 71 4 535 1 1 17 19 22 8 9 46 215 0.7 537 4.87 0.96 20.7 39.0 4.43 15.8 2.86 0.70 2.20 0.35 1.92 0.38 1.00 0.15 0.92 0.15 9 73 2.12 4.4 0.37 3.2 8.9 – 60 371 15 66 7 482 7 39 62 39 87 17 9 60 371 3.0 482 7.00 1.54 24.4 48.0 5.97 22.1 4.30 0.99 4.21 0.64 3.09 0.63 1.66 0.26 1.60 0.26 15 66 2 7 0.8 11 9 14.1 19 880 19 192 13.6 422 77 57 99 106 150 23 6 18 893 0.37 415 5.83 1.42 36.8 77.0 9.66 38.1 7.00 2.04 5.44 0.77 4.03 0.74 1.80 0.25 1.51 0.22 19 191 4.67 12.8 0.75 16 26 768 21 163 9.7 370 149 25 77 392 164 20 6 24 765 0.40 368 6.58 1.52 27.3 57.8 7.38 29.4 5.86 1.73 4.85 0.73 4.24 0.84 2.26 0.32 1.98 0.31 21 160 4.13 8.6 0.53 24. 34 490 23 202 15.5 347 202 62 88 415 201 19 3 33 492 0.50 344 2.13 0.75 19.5 44.0 5.89 25.0 5.53 1.79 5.24 0.82 4.84 0.93 2.41 0.34 2.07 0.32 24 200 4.98 14.8 0.91 2 12 321 25 115 8.2 165 90 47 80 250 192 17 3 12 321 0.26 162 1.53 0.48 11.1 24.3 3.41 14.8 3.88 1.37 4.36 0.75 4.79 0.97 2.67 0.38 2.33 0.37 25 114 2.81 7.3 0.47 31.4 50 616 17 173 12.8 476 78 31 58 110 106 20 7 48 621 1.1 476 7.84 2.70 25.5 50.8 6.15 23.6 4.61 1.32 3.87 0.59 3.34 0.62 1.64 0.24 1.49 0.24 16 170 4.52 11.4 0.85 12. 48 1139 1941 294 11.9 875 768 19 233 38 11.4 569 94 39 87 58 57 152 81 24 14 70 141 23 46 10 1137 0.61 39 765 13.3 873 3.25 0.49 52.9 565 110.8 9.31 13.8 2.45 53.4 37.6 9.50 2.54 78.2 6.61 9.76 0.85 37.8 4.32 7.01 0.77 1.96 1.91 5.28 0.26 0.72 1.58 3.86 0.24 0.70 20 1.82 290 7.56 0.25 10.9 1.55 0.6 0.24 18 232 6.02 10.6 0.70 15 29 741 18 154 9.8 467 60 28 73 259 169 20 6 28 740 0.38 461 5.67 1.53 29.1 61.1 7.71 30.9 6.03 1.75 4.79 0.69 3.82 0.74 1.89 0.27 1.65 0.26 19 154 4.02 8.5 0.53 24.1 32 511 20 146 11.3 333 43 31 75 93 144 20 4 30 517 0.72 333 4.31 1.42 18.0 37.5 4.80 19.6 4.19 1.38 4.03 0.65 3.72 0.72 1.89 0.28 1.71 0.27 19 144 3.81 9.9 0.67 19.2 22 493 21 141 11.9 267 99 38 79 185 146 20 5 21 494 0.86 266 2.89 0.96 15.7 33.9 4.40 18.5 4.27 1.46 4.18 0.67 3.97 0.80 2.10 0.30 1.85 0.28 20 139 3.53 10.4 0.70 21 24 745 24 178 14.8 391 158 89 78 287 181 18 4 22 751 0.31 386 4.06 1.16 28.6 62.9 8.27 33.5 6.68 2.03 5.50 0.80 4.69 0.91 2.41 0.34 2.12 0.32 23 176 4.31 14.0 0.84 3 23 794 23 176 11.7 391 153 71 74 287 189 17 3 22 793 0.17 389 4.02 1.17 28.8 65.2 8.60 35.6 7.07 2.08 5.75 0.83 4.82 0.95 2.52 0.35 2.21 0.34 23 173 4.26 11.0 0.67 3 Rb Sr Y Zr Nb Ba Ni Cu Zn Cr V Ga Pb Rb Sr Cs Ba Th U La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y Zr Hf Nb Ta Sc Pb Co 143 118 34 172 13 1084 16 43 41 68 136 28 13 146 123 14.5 1158 16.55 6.11 45.0 98.0 11.77 45.3 9.31 2.11 7.78 1.19 6.98 1.34 3.64 0.54 3.37 0.51 35 178 5.22 14.6 1. 103 282 17 80 8 758 <5 16 32 <10 45 16 10 103 282 4.1 758 9.80 2.20 26.7 48.3 5.71 21.8 4.10 1.00 3.92 0.52 3.17 0.58 1.83 0.24 1.70 0.21 17 80 2 8 0.6 7 10 9.3 115 283 24 145 10 674 12 61 61 20 73 19 13 115 283 4.1 674 12.1 3.34 31.3 63.0 7.81 30.5 5.70 1.28 5.54 0.87 4.52 0.92 2.91 0.35 2.20 0.34 24 145 4 10 0.8 10 13 11.1 Miocene Tatoosh pluton samples and xenolith 93RE63 analyzed by X-ray fluorescence (XRF) and inductively coupled plasma mass spe Tatoosh Miocene Analyses by X-ray fluorescence (XRF) at Washington State University (WSU), except for samples reporting Nb <10 at U.S. Geologic Analyses by X-ray fluorescence (XRF) at Washington ank vents Note: Note: 08GL1014 08WP1008 08SB1003 Eocene–Paleocene 08CR1010 08SB1005 08RR998 08RR999 08MW1016 Sample no. 08W1012 08GL1015 08KS1011 Oligocene Mesozoic 203080 93T59 203004 93RE63 08RR1001 203085 Miocene Unknown ICPMS at Washington State University. See Table 2 for rock types. Table See State University. ICPMS at Washington 93ML98 by inductively coupled plasma mass spectrometry (ICPMS) at WSU, all Co and partial REE analyses instrumental neutron activat 01MCP874 97BM664 00OHS831 00BL834 00WP830 01SB872 Regional Quaternary basalts and basaltic andesites 93ML214 97ML657 97ML656 99ML762 Sample no. Rb Sr Y Zr Nb Ba Ni Cu Zn Cr V Ga Pb Rb Sr Cs Ba Th U La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y Zr Hf Nb Ta Sc Pb Co 94ML444 00ECR836 North-fl

Geological Society of America Bulletin, January/February 2014 131 Sisson et al.

TABLE 5. MAJOR OXIDE (WT%) AND TRACE ELEMENT (PPM) ANALYSES OF PUMICE AND GLASS SEPARATES Sample 05KI921-1 05KI921-1 gl 05KI921-2 05KI921-2 gl 08RE1034A 08RE1034A gl 08RE1034B 08RE1034B gl 01GPM1 01GPM1 gl

SiO2 65.90 74.66 66.55 74.59 61.67 69.23 61.50 69.27 67.61 77.61 TiO2 0.62 0.29 0.61 0.30 0.85 0.67 0.87 0.63 0.50 0.19 Al2O3 16.40 13.66 16.14 13.72 17.65 15.52 17.86 15.72 16.04 12.46 FeO* 3.39 1.38 3.30 1.39 4.43 2.65 4.45 2.52 3.29 0.93 MnO 0.07 0.03 0.07 0.03 0.09 0.05 0.09 0.05 0.07 0.03 MgO 2.09 0.28 1.93 0.29 2.90 0.88 2.85 0.73 1.71 0.17 CaO 4.47 1.49 4.21 1.52 5.89 3.18 5.84 3.19 4.06 1.14

Na2O 3.95 3.96 4.00 3.92 4.08 4.34 4.09 4.42 4.27 3.65 K2O 2.59 4.03 2.68 4.01 1.74 2.97 1.74 2.99 2.32 3.78 P2O5 0.15 0.07 0.15 0.07 0.20 0.22 0.21 0.19 0.13 0.05 Total 96.17 96.69 96.56 97.24 98.17 98.45 98.53 98.31 100.00 96.68 Rb (XRF) 73 117 76 116 47 85 47 83 43 73 Sr 399 164 385 166 499 344 496 341 453 146 Ba 513 695 534 697 426 653 430 645 553 735 Y 14191618172418251412 Zr 161 205 165 200 160 268 162 261 141 113 Nb 8.6 11.2 8.8 11.3 8.3 13.1 8.1 13.6 4.9 6.5 Ni11210<211811310<2 Cu7–10–21–28–4– Zn 55 48 52 50 66 53 69 52 52 27 Cr 35 2 25 3 37 3 34 3 11 2 V 7017692110252101456410 La 21 30 24 33 19 30 21 31 16 22 Ce 45 57 45 61 44 65 45 59 35 37 Nd 20 22 21 23 20 28 22 28 16 14 Ga 17 16 17 16 20 18 20 19 15 13 Pb 13 18 12 19 9 16 8 17 9 17 Cs (ICPMS) 4.4 7.3 4.6 7.3 2.4 4.3 2.4 4.4 1.87 3.3 Rb 71 121 76 121 47 82 47 85 43 76 Sr 390 170 377 174 508 339 503 342 449 152 Ba 521 732 539 725 429 645 438 652 549 773 Y 14.4 17.5 14.7 17.5 16.9 23.5 17.6 23.9 13.4 10.8 Zr 156 223 162 219 163 271 164 271 130 123 Hf 4.51 6.28 4.68 6.13 4.22 7.15 4.29 7.06 3.54 3.59 Nb 9.2 11.3 9.5 11.2 8.7 12.5 8.7 12.3 4.81 5.4 Ta0.841.150.881.140.650.950.670.940.410.56 Th 10.9 18.3 11.6 18.3 5.9 10.3 6.1 10.4 6.28 10.8 U 3.92 6.46 4.15 6.47 2.04 3.59 2.14 3.68 2.22 3.89 La 23.5 31.0 24.3 31.2 20.1 30.0 20.5 30.4 17.36 20.6 Ce 45.9 59.6 47.2 59.6 41.6 62.5 42.6 62.9 34.55 39.2 Pr 5.33 6.6 5.45 6.6 5.2 7.7 5.3 7.7 4.07 4.2 Nd 19.6 23.4 19.9 23.3 20.3 29.2 20.8 29.3 15.10 14.1 Sm 3.93 4.47 3.91 4.35 4.27 6.04 4.42 6.08 2.93 2.36 Eu 1.07 0.84 1.07 0.81 1.21 1.24 1.30 1.28 0.85 0.44 Gd 3.22 3.63 3.30 3.61 3.88 5.15 4.07 5.19 2.63 1.82 Tb0.500.580.510.570.580.800.620.810.410.30 Dy 2.87 3.28 2.89 3.28 3.39 4.71 3.59 4.69 2.50 1.79 Ho 0.56 0.67 0.57 0.65 0.65 0.91 0.70 0.92 0.52 0.37 Er 1.50 1.82 1.52 1.78 1.68 2.40 1.82 2.45 1.38 1.11 Tm 0.22 0.27 0.22 0.27 0.24 0.34 0.26 0.35 0.21 0.18 Yb 1.38 1.73 1.40 1.71 1.53 2.14 1.64 2.17 1.38 1.25 Lu 0.22 0.28 0.22 0.27 0.24 0.33 0.25 0.34 0.23 0.22 Sc 8.1 4.2 8.0 4.2 13.6 8.2 13.3 7.7 8.1 2.1 Pb 11.7 17.4 12.2 17.5 9.1 15.1 9.3 14.5 10.0 15.7 Latitude 47.1472 47.1472 47.1472 47.1472 46.8449 46.8449 46.8449 46.8449 48.1735 48.1735 Longitude –122.6377 –122.6377 –122.6377 –122.6377 –121.7421 –121.7421 –121.7421 –121.7421 –121.3343 –121.3343 Note: Major oxide concentrations are normalized to 100% with all Fe as FeO; total gives original sum. Cu is not reported for glass separates (– in table) due to heavy liquid contamination. Locations are referenced to the NAD27 datum. XRF (labeled and following elements)—X-ray ﬂuorescence; ICPMS (labeled and following elements)— inductively coupled plasma mass spectrometry.

concentrations at low SiO2, converging to low K north-ﬂ ank spessartite lavas have the highest concentrations of Nb and Ta, and somewhat and restricted values at high SiO2. Regional P2O5 concentrations of magmas erupted in the less depleted in Zr, Hf, and Ti, relative to Ba, basalts and basaltic andesites, including north- immediate vicinity of Mount Rainier. Rb, K, U, and Th (McKenna, 1994; Stockstill

fl ank basaltic andesites, overlap the low-SiO2, et al., 2002; Sisson and Vallance, 2009). Con- low-Al2O3 corner of the Mount Rainier fi eld, Trace Element Features of Quaternary centrations of strongly incompatible trace ele- together defi ning a broadly arched Al2O3-SiO2 Volcanic Rocks ments Rb, Ba, U, and Th increase broadly with array. Concentrations of P2O5 are diverse in whole-rock SiO2, with trends that are paralleled regional basalts and basaltic andesites, attain- Trace element features of Mount Rainier by pumice-glass tie lines (Rb and Ba are illus- ing the greatest values in volumetrically minor eruptives are also familiar from convergent- trated in Fig. 4). Concentrations of plagioclase- K-rich shoshonites and absarokites (Leeman margin calc-alkaline suites elsewhere in the compatible Sr and Eu decrease with increasing et al., 1990, 2005; Conrey et al., 1997; Smith and Cascades and worldwide, being enriched in whole-rock SiO2, also parallel to pumice-glass Leeman, 2005). Similarly, the relatively highlight rare earth elements (LREEs), with low tie lines. Strontium concentrations are diverse in

132 Geological Society of America Bulletin, January/February 2014 Petrogenesis of Mount Rainier andesite

TABLE 6. ISOTOPIC ANALYSES OF MOUNT RAINIER–REGION QUATERNARY VOLCANIC ROCKS AND MINERALS 143 144 87 86 206 204 207 204 208 204 δ18 δ18 δ18 δ18 Sample no. Nd/ Nd 2sem Sr/ Sr 2sem Pb/ Pb Pb/ Pb Pb/ Pb OPL OOL OWR OMAGMA Axial conduit system JVDMAZ 0.512942 ± 5 0.703569 ± 7 18.9449 15.5872 38.6298 – – – – 95RE462 0.512889 ± 4 0.703783 ± 9 18.9599 15.5926 38.6737 – – – – 99RE777 0.512895 ± 3 0.703690 ± 8 18.9482 15.5928 38.6576 6.36 – – 6.3 99ML770 0.512909 ± 5 0.703578 ± 9 18.7594 15.5413 38.2150 – – – – 93RE41 0.512914 ± 4 0.703660 ± 10 18.9855 15.5856 38.6579 – – – – 97RE614 0.512876 ± 6 0.703792 ± 9 18.9408 15.5882 38.6390 6.12 – – 6.1 96RE528 0.512868 ± 6 0.703920 ± 8 18.9736 15.5986 38.6854 6.29 – – 6.3 94ML318 0.512889 ± 7 0.703678 ± 8 18.9610 15.5932 38.6703 6.46 – – 6.4 96RE539 0.512922 ± 5 0.703815 ± 9 18.9549 15.5951 38.6579 6.63 – – 6.6 95SR514 0.512874 ± 6 0.703698 ± 6 18.9578 15.5961 38.6710 6.39 – – 6.4 93RE4 0.512896 ± 7 0.703710 ± 7 18.9593 15.5948 38.6742 6.21 – – 6.2 99ML764 0.512869 ± 8 0.703750 ± 12 18.9406 15.5905 38.6512 6.61 – – 6.6 98RE692 0.512887 ± 5 0.703766 ± 9 18.9430 15.5883 38.6408 – – – – 95RE464 0.512871 ± 7 0.703968 ± 10 18.9582 15.5932 38.6612 6.26 – – 6.2 93RE26 0.512884 ± 4 0.703715 ± 8 18.9553 15.5948 38.6656 6.24 – – 6.2 93MW68 0.512878 ± 7 0.703814 ± 10 18.9630 15.5952 38.6645 6.15 – 7.3 6.1 93MW72 – – – – – 5.79 – 7.6 5.8 95RE494 0.512878 ± 5 0.703486 ± 7 18.8963 15.5812 38.5744 6.16 – – 6.2 93RE197 0.512866 ± 7 0.703720 ± 9 18.9238 15.5880 38.6255 6.51 – – 6.5 96RW581 0.512880 ± 5 0.703772 ± 7 18.9618 15.5979 38.6866 6.41 – – 6.4 95RE506 0.512924 ± 6 0.703597 ± 10 18.9165 15.5854 38.6009 6.33 – – 6.3 JV506CL3 0.512917 ± 3 0.703561 ± 8 18.9035 15.5840 38.5842 5.69 – – 5.7 93RW3 0.512881 ± 13 0.703708 ± 6 18.9439 15.5855 38.6447 6.34 – – 6.4 93RW177 0.512895 ± 7 0.703677 ± 6 18.9380 15.5904 38.6327 5.88 – – 5.9 94ML329 0.512849 ± 7 0.703891 ± 13 18.9656 15.5957 38.6903 6.81 – – 6.8 99GL769 0.512874 ± 6 0.703824 ± 7 18.9602 15.5942 38.6695 6.41 – – 6.4 93RE53 0.512871 ± 3 0.703904 ± 6 18.9626 15.5963 38.6711 6.00 – – 6.0 96RW555 0.512859 ± 5 0.703917 ± 9 18.9802 15.6024 38.7200 6.37 – – 6.4 96RE530 0.512913 ± 3 0.703792 ± 9 18.9521 15.5942 38.6585 6.54 – – 6.6 94RW275 0.512898 ± 3 0.703518 ± 8 18.9254 15.5907 38.6255 6.26 – – 6.3 96RW570 0.512835 ± 7 0.703790 ± 7 18.9618 15.5962 38.6761 6.25 – – 6.3 00RW821 0.512874 ± 2 0.703754 ± 6 18.9675 15.6003 38.6958 6.33 – – 6.4 93RE39 0.512896 ± 8 0.703759 ± 8 18.9636 15.5975 38.6830 6.36 – – 6.4 94RW288 0.512911 ± 8 0.703434 ± 9 18.8395 15.5701 38.4917 6.41 – – 6.5 94RE379 0.512906 ± 5 0.703696 ± 19 18.9294 15.5879 38.6224 6.52 – – 6.6 00RE849 0.512903 ± 11 0.703823 ± 7 18.9552 15.5942 38.6603 7.01 – – 7.1 93RE193 0.512841 ± 7 0.703992 ± 9 18.9533 15.5900 38.6584 6.26 – – 6.3 00RE801 0.512864 ± 2 0.703780 ± 6 18.9619 15.5951 38.6759 6.48 – – 6.5 93RE120 0.512855 ± 4 0.703860 ± 9 18.9499 15.5919 38.6632 6.23 – – 6.3 97RE629 0.512838 ± 5 0.703999 ± 9 18.9370 15.5930 38.6592 6.51 – – 6.6 01RW894 0.512847 ± 7 0.703977 ± 7 18.9763 15.6007 38.7083 6.73 – – 6.8 93RW87 0.512878 ± 6 0.703757 ± 7 18.9647 15.5985 38.6836 6.49 – – 6.6 96RW544 0.512868 ± 4 0.703852 ± 8 18.9796 15.5976 38.6858 – – – – 93RW100 0.512824 ± 3 0.704025 ± 8 18.9679 15.5990 38.6973 – – – – 94RW282 0.512845 ± 5 0.703894 ± 7 18.9627 15.5998 38.6942 7.16 – – 7.3 95SR446 0.512875 ± 7 0.703797 ± 8 18.9480 15.5965 38.6620 6.36 – – 6.7 Quenched magmatic inclusions 96RE532 0.512977 ± 3 0.703258 ± 7 18.9336 15.5903 38.6281 – – 7.3 – 93RE191 0.512911 ± 7 0.703560 ± 11 18.9501 15.5858 38.6197 – – – – 93RE58 0.512878 ± 7 0.703700 ± 8 18.9630 15.5952 38.6644 – – – – 94ML314 0.512879 ± 4 0.703758 ± 8 18.9519 15.5899 38.6545 – – – – 98RE675 0.512901 ± 4 0.703625 ± 7 18.9257 15.5884 38.6192 – – – – 96RW576 0.512893 ± 5 0.703750 ± 7 18.9521 15.5955 38.6617 6.74 – – 6.7 Gabbronorite inclusions 93RE16 0.512893 ± 12 0.703686 ± 7 18.9363 15.5921 38.6339 – – – – 01SR878 – – – – – 5.39 – – – North-ﬂ ank vents 99ML762 0.512872 ± 7 0.703865 ± 6 18.9739 15.5926 38.6792 6.17 5.58 – 6.0 93ML214 0.512914 ± 8 0.703546 ± 10 18.9249 15.5815 38.5912 6.47 5.41 – 6.4 94ML444 0.512905 ± 8 0.703669 ± 7 18.9325 15.5840 38.6155 6.49 – 6.4 97ML657 0.512912 ± 3 0.703733 ± 7 18.9440 15.5902 38.6355 – – 7.1 – 97ML656 0.512906 ± 6 0.703745 ± 6 18.9490 15.5909 38.6440 6.96 – – 6.9 93ML98 0.512895 ± 6 0.703591 ± 7 18.9665 15.5969 38.6822 6.25 – – 6.2 Regional Quaternary basalts and basaltic andesites 01SB872 0.513005 ± 6 0.703201 ± 21 18.8614 15.5611 38.4511 – – – – 00ECR836 0.512965 ± 4 0.703523 ± 9 18.9323 15.5947 38.6041 – – – – 00BL834 0.512953 ± 3 0.703525 ± 11 18.9443 15.5843 38.6094 – – – – 00OHS831 0.512919 ± 4 0.703550 ± 9 18.9321 15.5788 38.5481 – – – – 00WP830 0.512963 ± 4 0.703273 ± 8 18.9638 15.5857 38.5886 – – – – Duplicate 830 – – 18.9651 15.5861 38.5902 – – – – 97BM664 0.512884 ± 3 0.703675 ± 9 18.9485 15.5885 38.6360 – – – – 01MCP874 0.512902 ± 4 0.703704 ± 8 18.9155 15.5801 38.5753 – – – – Note: Radiogenic isotope analyses at National High Magnetic Field Laboratory, Tallahassee, Florida. Oxygen isotope analyses at Washington State University, except whole δ18 rocks at U.S. Geological Survey, Denver. OMAGMA calculated from CIPW norms assuming: An75, 1100°C for SiO2 52–57 wt%; An65, 1025°C for SiO2 57–60 wt%; An55, 975°C for SiO2 60–64 wt%; and An45, 925°C for SiO2 >64 wt%.

Geological Society of America Bulletin, January/February 2014 133 Sisson et al.

TABLE 7. ISOTOPIC ANALYSES OF SOUTHWEST WASHINGTON PRE-PLEISTOCENE ROCKS 143 144 87 86 206 204 207 204 208 204 δ18 Sample no. Unit Nd/ Nd 2sem Sr/ Sr 2sem Pb/ Pb Pb/ Pb Pb/ Pb OWR Miocene 93T59 Tatoosh Granodiorite 0.512897 ± 4 0.703870 ± 7 18.9474 15.5883 38.6381 7.4 203004 Tatoosh Granodiorite 0.512909 ± 10 0.703719 ± 9 18.9482 15.5795 38.5932 7.8 203080 Tatoosh Granodiorite 0.512874 ± 3 0.703905 ± 9 18.9792 15.5850 38.6537 11.0 203085 Tatoosh Granodiorite 0.512898 ± 5 0.703933 ± 8 19.0027 15.5870 38.6813 0.4 Oligocene 08MW1016 Ohanapecosh volcaniclastic 0.512833 ± 3 0.704135 ± 8 18.9112 15.5795 38.5843 8.5 Eocene–Paleocene 08W1012 Puget Group arkose 0.512325 ± 3 0.709087 ± 9 19.1070 15.6177 38.8042 16.8 08GL1015 Puget Group arkose 0.512123 ± 5 0.711182 ± 9 19.2367 15.6687 39.3583 10.4 08WP1008 Summit Basalt 0.513008 ± 6 0.703424 ± 8 19.5091 15.5846 39.1837 11.6 08CR1010 Crescent/Siletzia basalt 0.512909 ± 4 0.703414 ± 9 18.8587 15.5374 38.4852 8.3 08KS1011 Crescent/Siletzia basalt 0.513000 ± 3 0.703478 ± 9 19.0998 15.6338 39.0264 8.1 Mesozoic 08RR1001 Russell Ranch arkose 0.512517 ± 3 0.706469 ± 7 19.2442 15.6533 38.9388 13.7 08SB1005 Russell Ranch arkose 0.512626 ± 4 0.706126 ± 8 19.1456 15.6320 38.7418 14.6 08RR998 Sheared granodiorite 0.513009 ± 3 0.703349 ± 8 19.6569 15.5888 38.9218 9.2 08RR999 Greenstone 0.512951 ± 3 0.704107 ± 8 18.4369 15.5526 38.1270 11.9 08SB1003 Orthogneiss 0.513005 ± 6 0.704022 ± 9 19.0420 15.5389 38.5670 9.7 Xenolith/xenocryst 93RE63 Metavolcanic xenolith 0.512890 ± 4 0.703707 ± 9 18.9569 15.5873 38.6456 Quartz xenocryst (97BM662) 11.7 Note: Radiogenic isotope analyses at National High Magnetic Field Laboratory, Tallahassee, Florida. Oxygen isotope analyses at U.S. Geological Survey, Denver, except quartz xenocryst at Washington State University.

206 204 87 86 206 204 the lower-SiO2 samples, defi ning a fan-shaped increasing Pb/ Pb and Sr/ Sr, and of jointly Pb/ Pb, unlike Mounts Rainier and Adams 206 204 207 204 array like that of P2O5 versus SiO2, with the increasing Pb/ Pb and Pb/ Pb (Fig. 5). and regional calc-alkaline mafi c lavas. Mount highest Sr concentrations in spessartite and in The sole basaltic QMI (sample 96RE532) lies Rainier andesite and dacite samples selected regional K-rich mafi c lavas. on the projection of these trends but at distinctly for their anomalously low K2O concentrations Concentrations of mafi c mineral–compatible higher 143Nd/144Nd and lower 87Sr/86Sr than other also have anomalously low 206Pb/204Pb and plot elements Ni, Cr, Sc, and V decrease with increas- Mount Rainier samples. With increasing mag- displaced toward a fi eld defi ned by similarly 143 144 ing SiO2 and decreasing MgO (not illustrated). matic evolution (whole-rock SiO2), Nd/ Nd low-K2O andesites and dacites from Mount St. As with MgO, the concentrations of these com- decreases broadly, but Nd isotopic values scat- Helens, although the Mount Rainier sample 206 204 patible trace elements do not attain the high val- ter at any SiO2 value, well outside of analytical with the lowest Pb/ Pb is not low in K2O ues of primitive high-Mg andesitic suites. uncertainty (Fig. 6). Likewise, more evolved (99ML770). Trace elements of intermediate compatibil- Mount Rainier samples have scattered, but Oxygen isotope results for Mount Rainier pla- ity, or whose compatibility varies strongly with overall increasing 206Pb/204Pb and 87Sr/86Sr (not gioclase phenocrysts range from 5.7‰ to 7.2‰, mineral assemblage, such as Y, Zr, and Nb, plot illustrated). averaging 6.40‰ ± 0.28‰ (Fig. 6). Plagioclase as broad, gently sloping fi elds versus SiO2, with Quaternary mafi c lavas of the Cascades was also separated from a Quaternary gab- concentrations in Mount Rainier samples gener- are divisible into “calc-alkaline” types with bronorite inclusion, yielding a relatively low ally intermediate between those of Mounts St. moderate to strong subduction trace element δ18O value of 5.4‰, and olivine phenocrysts Helens and Adams (Fig. 4). Concentrations of signatures, “low-K olivine tholeiites” that separated from two north-fl ank basaltic ande- Y, Zr, and Nb increase steeply and monotoni- resemble many back-arc-basin basalts in their sites have δ18O values of 5.4‰ and 5.6‰, simi- cally with SiO2 for Mount Adams samples, but low concentrations of incompatible elements lar to olivine phenocrysts from Mount Adams decrease with SiO2, overall, at Mount St. Helens, (also known as “high-alumina olivine tholei- and from southern Washington Cascades calc- albeit with scatter (Fig. 4). Pumice-glass tie lines ites”), and “within plate”–type basalts that are alkaline basalts (Fig. 6). Bulk-magma δ18O val- correlate with mineral assemblage and overall chemically similar to basalts erupted at intra- ues calculated either by the CIPW normative degree of pumice evolution, with glasses from plate ocean islands (Leeman et al., 1990, 2005; mineral approximation (Eiler, 2001; Bindeman pyroxene andesite and hornblende-pyroxene Hart et al., 1984; Bacon et al., 1997; Conrey et al., 2004) or from measured or estimated dacite being enriched in Zr, Nb, Y, and SiO2 rela- et al., 1997). Calc-alkaline basalts and basaltic phase compositions (Zhao and Zheng, 2003) tive to their bulk pumices, similar to the Mount andesites of southern Washington plot over- give effectively identical results, with bulk- Adams suite, but glass in zircon-bearing horn- lapping the high-143Nd/144Nd (generally more magma estimates slightly less than measured blende rhyodacite being depleted in Zr and Y, mafi c) end of the Mount Rainier array, with plagioclase values for the most mafi c samples, enrichment retarded in Nb, and strongly enriched discernible elongation along that array (Fig. 5). and slightly greater than measured plagioclase in SiO2, similar to Mount St. Helens eruptives. Andesites and dacites from the Mount Adams for the most evolved samples (Fig. 6). Measured volcanic fi eld also plot overlapping the high- plagioclase and estimated bulk-magma δ18O val- Radiogenic and Oxygen Isotope Variations 143Nd/144Nd end of the Mount Rainier array, but ues increase modestly with degree of magmatic of Quaternary Volcanic Rocks except for a few outliers, defi ne a non-elon- evolution (SiO2), albeit with signifi cant scatter. gate cluster (not shown separately). Nearly all Linear regression through the array yields a char- Mount Rainier samples have a limited range low-K tholeiitic and within plate–type basalts acteristic increase for bulk magmas of ~0.05‰ 143 144 δ18 of radiogenic isotope values (Table 6), defi n- of southern Washington have high Nd/ Nd per wt% SiO2, from a O value of ~6.0‰ at 143 144 87 86 ing an array of decreasing Nd/ Nd versus and low Sr/ Sr, but are substantially diverse in 55 wt% SiO2, and with outliers of up to ±0.7‰.

134 Geological Society of America Bulletin, January/February 2014

Petrogenesis of Mount Rainier andesite

Nd/ )

al 144 143

Pb Pb/

204 207 long dashed 0.51305 0.51300 0.51295 0.51290 0.51285 0.51280 15.59 15.58 15.60 15.57 15.56 15.55 15.54 15.53 15.61 20 CAB ), Mesozoic and Tertiary Tertiary ), Mesozoic and ) of 0.5, and bulk mineral/ 10 20 r 20 IGB (inclusive) red itted. Fields include literature itted. Fields include literature 10 30 10 L-Sr/Y 20 lled circles are regional basalts and regional are lled circles L-Sr/Y CAB ) and high-Sr/Y (H-Sr/Y, (H-Sr/Y, ) and high-Sr/Y 10 MA MA ). Unﬁ Pb Pb MA 204 204 ank vents and quenched magmatic inclusions, pink H-Sr/Y H-Sr/Y LKT-WPB LKT -WPB short dashed Pb/ Pb/ 206 206 (Tertiary arc) MSH ), calc-alkaline basalts (CAB, MSH IGB dark gray (Mesozoic-Eocene) ) and Mount Adams (MA, ) and Mount

IGB light blue 18.65 18.70 18.75 18.80 18.85 18.90 18.95 19.00 19.05 18.65 18.70 18.75 18.80 18.85 18.90 18.95 19.00 19.05 20 MSH 30 10 IGB IGB (Tertiary arc) (Mesozoic-Eocene) 20 30 20 20 IGB (Mesozoic-Eocene) MA 10 L-Sr/Y Pb H-Sr/Y 10 Sr 204 CAB H-Sr/Y 86 CAB IGB are mixing chords to Puget Group arkose samples from low-Sr/Y (L-Sr/Y, (L-Sr/Y, low-Sr/Y arkose samples from mixing chords to Puget Group are at 10 wt% intervals; mixing chords to model in situ rhyolite are coincident and are omitted for clarity. Assimilation–fraction clarity. omitted for coincident and are at 10 wt% intervals; mixing chords to model in situ rhyolite are Pb/ Sr/ (Tertiary arc) 87 206 ) for model parents assimilating mean Puget Group arkose assume a ratio of assimilant gain to crystal loss ( assimilating mean Puget Group model parents ) for L-Sr/Y cross-ties MSH LKT-WPB elds for low-K tholeiites and within plate–type basalts (LKT-WPB, low-K tholeiites and within plate–type basalts (LKT-WPB, elds for Dashed lines ), and andesites dacites of Mount St. Helens (MSH, MA green lines ), and ﬁ high Sr/Y - Puget Group mixing lines Group high Sr/Y - Puget mixing lines Group Sr/Y - Puget low arkose Group Puget AFC curves average to light gray LKT -WPB cients (Ds) of Sr of 1.8, Nd of 0.38, and Pb of 0.03. AFC lines are identical to mixing lines in the Pb-Pb plot, and so are om identical to mixing lines in the Pb-Pb plot, and so are AFC lines are of 1.8, Nd 0.38, and Pb 0.03. cients (Ds) of Sr regional mafic lavas (this study) mafic lavas regional Mt. Rainier inclusive: lavas & tephras, Mt. Rainier lavas inclusive: & north inclusions, vents quenched magmatic flank orange circles 18.65 18.70 18.75 18.80 18.85 18.90 18.95 19.00 19.05 0.7028 0.7030 0.7032 0.7034 0.7036 0.7038 0.7040 0.7042

0.7040 0.7038

0.7042 0.7036 0.7034 0.7032 0.7030 0.7028

Sr 0.51305 0.51300 0.51295 0.51290 0.51285 0.51280 Sr/

Nd Nd/

86 87 144 143 model basaltic parents (Table 8), with (Table model basaltic parents ( crystallization (AFC) trends melt partition coefﬁ Leeman et al. (1990, 2004), Bacon (1997), Jicha (2009), and Halliday (1983). data from igneous basement (IGB, this study. basaltic andesites from Figure 5. Isotope-isotope variation diagrams for Mount Rainier Quaternary volcanic rocks (all samples, including from north-ﬂ (all samples, including from Quaternary volcanic rocks Mount Rainier 5. Isotope-isotope variation diagrams for Figure shown as are

Geological Society of America Bulletin, January/February 2014 135 Sisson et al.

SiO (wt%) Geochemical and Isotopic Results from 2 Basement Rock Samples 45 50 55 60 65 70 75 7.5 The diversity of southwest Washington base- IGB ment rock types is reﬂ ected in their wide-ranging 20 70 7.0 chemical and isotopic compositions (Tables 2, 50 60 4, and 7). Most evolved are samples of Eocene LKT-WPB crystal- Puget Group arkoses with high SiO (>80 wt%) ) oliv 2 6.5 fractionation trend and with Nd, Sr, Pb, and O isotopic values consistent with a continental interior provenance that 10 includes abundant Precambrian materials. These

per mil 6.0 rocks are similar to Eocene sandstones elsewhere 20 in the U.S. Paciﬁ c Northwest (Heller et al., 1985) O ( and to continental interior sediments supplied 18 MA oliv to the Cascadia accretionary complex via the δ 5.5 Columbia River (Prytulak et al., 2006). Eocene

Puget Group shale has lower SiO2 (63 wt%), but 5.0 higher concentrations of many petrogenetically IGB (0.4) indicative trace elements (Rb, Cs, Ba, Zr, etc.). CAB oliv Tectonized Mesozoic arkoses to graywackes 4.5 exposed in the core of the White Pass anticlinorium are similar to the Eocene sandstones, 0.51305 but are somewhat less evolved chemically and high Sr/Y - ave. Puget Group mixing line LKT-WPB low Sr/Y - ave. Puget Group mixing line isotopically. Least evolved are Paleocene–Eocene IGB mixing lines to model hybrid rhyolite basalts of Siletzia and the Summit Basalt, as well 0.51300 as Mesozoic greenstone from the White Pass anticlinorium. These basaltic basement rocks have generally high and restricted 143Nd/144Nd, MA 206 204 10 but range widely in Pb/ Pb, simi lar to the Nd 0.51295 Quaternary Cascades low-K olivine tholeiites

144 CAB 20 and within plate–type basalts. Unlike Quaternary regional maﬁ c lavas, the Mesozoic and Cenozoic

Nd/ 10 mafi c basement samples have consistently high 0.51290 10 MSH bulk-rock δ18O values (8‰–12‰) due to low- 143 temperature alteration. Mesozoic intermediate- composition igneous basement rocks (ortho- 0.51285 20 gneiss, granodiorite) also have high and restricted 70 143Nd/144Nd, but have 206Pb/204Pb greater to much 60 30 greater than Quaternary vol canic rocks of Mount 30 70 80 Rainier or elsewhere in the southern Washington 0.51280 Cascades, and have high bulk-rock δ18O values 45 50 55 60 65 70 75 (9‰–10‰). The subduction-generated Miocene SiO2 (wt%) Tatoosh Granodiorite and Oligocene Ohana- pecosh Formation samples from the immediate 18 143 144 Figure 6. Plots of whole-rock SiO2 concentration versus δ O and Nd/ Nd. Oxygen isotope Mount Rainier area are similar to Mount Rain- plot (upper plot) shows Mount Rainier plagioclase phenocryst δ18O values (orange circles) ier eruptives in Sr, Nd, and Pb isotopes (Table connected to calculated bulk magma values (vertical lines) where the difference exceeds 7), albeit with greater scatter around the Mount the symbol size; north-fl ank basaltic andesite olivine pheno crysts (orange diamonds); and Rainier array (Fig. 5). They are distinct, however, gabbronorite plagioclase (orange square). Fields show δ18O values of olivines from regional in having bulk-rock δ18O values greater to much calc-alkaline basalts and basaltic andesites (CAB, red), low-K tholeiites and within plate– greater (7.4‰–11‰), and in one instance much type basalts (LKT-WPB, dark gray), and Mount Adams andesites and dacites (MA, pink). less (0.4‰), than Mount Rainier plagioclase or Mesozoic and Cenozoic igneous basement whole-rock samples (IGB, gray) plot mainly off inferred bulk-magma values (Fig. 6). the graph area at high, and in one instance, low, δ18O. Dashed and solid lines are mixing Remaining basement samples are an atypi- chords to average Puget Group arkose and to model in situ rhyolite, respectively, from cally large (~1.5 cm) quartz xenocryst with a representative calc-alkaline basaltic parents (Table 8), with cross-ticks at 10 wt% intervals. high δ18O value of 11.7‰ erupted in a regional Literature data sources are as for previous fi gures. Heavy line shows expected trajectory of calc-alkaline basaltic andesite (Canyon Creek progressive crystallization-differentiation from an average Mount Rainier basaltic andesite locality), and a xenolith of hydrothermally (Supplemental Item 2 [see footnote 1]). Neodymium isotope plot (lower plot) shows whole- altered volcanic rock of unknown protolith age rock determinations for Mount Rainier samples (orange circles), fi elds for other Quaternary from a Mount Rainier lava fl ow, with Sr, Nd, volcanic and basement samples, and mixing chords, as defi ned for the upper plot. and Pb isotopic values similar to Quaternary

136 Geological Society of America Bulletin, January/February 2014 Petrogenesis of Mount Rainier andesite

Mount Rainier eruptives. Small quartz xeno- of the calc-alkaline basalt fi eld (Fig. 5), but with 9.4‰–10.2‰ in the most assimilant-modifi ed crysts (to ~2 mm) are sparse but widespread in trace element abundances designed, in part, to magmas, greatly exceeding estimated Mount calc-alkaline basaltic andesites of the southern bracket the Mount Rainier suite (e.g., high-Sr/Y, Rainier bulk-magma values. Exposed south- Washington Cascades. Fig. 7). The fi rst assimilant considered is simply west Washington igneous basement rocks are Eocene Puget Group arkose. also unsuitable as major assimilants. Paleo- INTERPRETATIONS Mixing lines from both prospective mafi c par- cene–Eocene basalts and Mesozoic greenstone, ents to Eocene Puget Group arkoses trend along orthogneiss, and granitoid have 143Nd/144Nd too Isotopic Evidence for an Evolved the axes of the Mount Rainier 143Nd/144Nd– high, and 206Pb/204Pb too diverse, to produce the Crustal Assimilant 206Pb/204Pb–87Sr/86Sr–207Pb/204Pb arrays (Fig. 5). narrow and simple Mount Rainier Sr-Nd-Pb iso- For the low-Sr/Y parent, the majority of Mount topic arrays. The Mount Rainier Sr-Nd-Pb isotopic arrays Rainier samples would require ~10–20 wt% Oligocene and Miocene arc rocks from the are elongate toward the values of Mesozoic and arkose assimilation to match their Sr-Nd-Pb iso- local Mount Rainier area plot along or fl ank Eocene sedimentary basement rocks and signal topic values, with the maximum assimilation of the Mount Rainier Sr-Nd-Pb isotopic arrays the incorporation of a component or compo- ~30 wt%. The high-Sr/Y parental end member (Fig. 5). Assimilation of such materials would nents derived from the old continental interior. has higher (model) concentrations of Sr, Nd, accordingly be diffi cult to recognize from radio- Subduction of evolved sediments and their and Pb, so its isotopic values are less sensitive genic isotopes, but the Oligocene and Miocene processing through primary magmas is another to assimilation, but those isotopic values are samples are generally not more evolved isotopi- possibility, but the δ18O values of 6‰–7‰ cal- also more similar to those of the Mount Rainier cally than the Mount Rainier array, so their pro- culated for Mount Rainier andesites and dacites suite. These factors result in a wider range of gressive assimilation did not cause the Mount indicate assimilation in the crust, as does the estimated amounts of assimilation, from none Rainier isotopic trend. The Oligocene and Mio- rough correspondence between isotopic values up to nearly 30 wt% (Fig. 5), but with most of cene samples also have high, and in one instance ≤ δ18 and magmatic evolution (SiO2). the samples requiring 20 wt% assimilation. very low, O (7.4‰–11‰, and 0.4‰), so their Estimating amounts of crustal-level assimila- Thus, using either parent, typical amounts of large-degree assimilation would produce mag- tion requires specifying representative assimi- assimilation would be inferred as ~10–15 wt%, mas with δ18O generally greater than Mount lant and unmodifi ed parental magma charac- with maximum amounts of assimilation of less Rainier’s andesites and dacites, and with sub- teristics. Two primary (unmodifi ed) magma than 30 wt%. Oxygen isotope results give simi- stantial scatter. The Oligocene and Miocene approximations employed herein are calc-alka- lar estimates for the common extent of assimila- rocks are products of subduction-zone mag- line basalts with weak and strong subduction tion. Assimilation of 9–20 wt% arkose would be matism subsequent to deposition of the Puget trace-element signatures, referred to for conve- required to produce the average Mount Rainier Group, so their trend toward low 143Nd/144Nd, nience as low-Sr/Y and high-Sr/Y, respectively andesite-dacite δ18O of 6.4‰, as deduced from high 86Sr/87Sr, and slightly high 206Pb/204Pb prob- (Table 8). These compositions are generalized their plagioclase phenocrysts, from parental ably also results from assimilation of evolved after many analyses of Cascades basalts and basalt with δ18O of 5.4‰ and measured δ18O of sediments derived from the continental interior, basaltic andesites (data sources in captions of Puget Group sandstones of 10.4‰–16.8‰. as inferred for the Quaternary magmas. Figs. 3 and 5), and are intended to be illustra- Other basement materials are less successful as tive, not unique; the magmatic system is prob- predominant assimilants. Tectonized Mesozoic Crystallization-Differentiation ably fed by a continuum of primitive magmas sandstones from the White Pass anti clinorium between and beyond these model parents. In are not as evolved isotopically as Eocene Puget Although more evolved Mount Rainier isotopic and trace element characteristics, the Group rocks, so greater amounts of their assimi- samples generally have lower 143Nd/144Nd and low-Sr/Y parent plots in the region of overlap lation would be required. For the same paren- higher δ18O, consistent with greater extents of between calc-alkaline basalts and the combined tal basalts, ~35 wt% assimilation of Mesozoic assimilation, whole-rock SiO2 concentrations fi eld of low-K tholeiites and within-plate basalts sandstone would be required to produce the increase too much, relative to isotopic values, to (Figs. 5 and 7); it has isotopic values similar to median 143Nd/144Nd of Mount Rainier samples, have resulted predominantly from bulk assimi- the least radiogenic (and sole basaltic) Mount and 45–55 wt% for the lowest 143Nd/144Nd lation of sediment or sediment partial melt into Rainier QMI. The high-Sr/Y model parent has rock, but assimilation of those amounts would basalt (Fig. 6). Bulk assimilation of Puget Group isotopic characteristics similar to the mid-point also yield δ18O of 8.3‰–8.9‰ in typical, and rocks also would fail to match observed arrays

TABLE 8. MIXING END-MEMBER ISOTOPIC VALUES AND COMPOSITIONS High Sr/Y basalt Low Sr/Y basalt Puget Group sandstone 08W1012 Puget Group sandstone 08GL1015 Model hybrid rhyolite 87Sr/86Sr 0.70356 0.70330 0.70909 0.71118 0.70500 143Nd/144Nd 0.51294 0.51297 0.51233 0.51212 0.51275 206Pb/204Pb 18.900 18.930 19.110 19.240 19.025 207Pb/204Pb 15.584 15.580 15.620 15.670 15.610 δ18O (per mil) 5.4 5.4 16.8 10.4 8.0 Sr (ppm) 1000 600 215 200 200 Nd 30 20 16 21 20 Pb 8.4 7 9 13 10 Ba 500 200 535 515 800 Zr 190 125 70 110 200 La 40 14 21 27 28 Y 14 25 10 14 7

SiO2 (wt%) 52 51 85 82 76 Note: ppm applies to noted and following elements.

Geological Society of America Bulletin, January/February 2014 137 Sisson et al.

0.51305 0.51305 LKT-WPB LKT-WPB 0.51300 IGB 0.51300 MA MA Nd CAB 0.51295 10 0.51295 MSH Nd 144 20 MSH 20 CAB 144

Nd/ 30

0.51290 IGB 0.51290 Nd/

143 40

10 143 0.51285 60 AFC 0.51285 20 70 70 sediment 80 mixing lines AFC 30 0.51280 AFC 0.51280 010203040506070800.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Sr/Y Ba/Zr 0.7042 13 30 30 12 0.7040 IGB 20 60 60 11 0.7038 CAB MSH 10 0.7036 40 IGB

Sr 9

86 10 8 0.7034 MA 20 O (per mil)

Sr/ 70 18

87 LKT-WPB mixing lines to 70 40 7 δ 0.7032 model hybrid rhyolite 20 CAB 6 20 0.7030 LKT-WPB IGB 5 MA 0.7028 4 0 5 10 15 20 25 30 35 40 0.51280 0.51285 0.512900.51295 0.51300 0.51305 Ba/La 143Nd/144Nd

Figure 7. Plots of whole-rock trace element concentration ratios, and δ18O, versus radiogenic isotopic values, showing Mount Rainier samples (all as orange circles) and fi elds for Quaternary volcanic rocks and igneous basement, as defi ned for previous fi gures. Dashed and solid lines are mixing chords to Puget Group arkose samples, and to model in situ rhyolite, respectively, from representative calc-alkaline basaltic parents (white circles, Table 8), with cross-ties or ticks at 10 wt% intervals. Assimilation–fractional crystallization (AFC) trends (green lines) are as in Figure 5, with additional bulk partition coeffi cients (Ds) of Ba of 0.16, La of 0.04, Y of 0.28, and Zr of 0.01. Literature data sources as for previous fi gures. See Figure 6 for abbreviation defi nitions.

of isotopic values versus various trace element modestly and irregularly (Zr, Nb), or diminish that are, in other respects, too primitive. As ratios (Fig. 7). Instead, crystal-melt segregation (Y), passing from basaltic andesites to dacites originally proposed, in situ differentiation was processes within the Mount Rainier magmatic and rhyodacites. A version of in situ crystalli- applied to a single magma reservoir crystalliz- system are likely to have caused much of the zation-differentiation in the crustal roots of the ing along its margins, but herein it is generalized magmatic compositional diversity, but trace ele- magmatic systems, accompanied by mixing to encompass a complex magmatic system. For ment variations preclude predominantly classi- between the resulting evolved liquids and the long-lived intermittently fed magmatic systems, cal progressive crystal fractionation. Progressive replenishing mafi c to intermediate injections, like Mount Rainier’s, small magma batches growth and separation of pyroxenes and plagio- may account for this trace element behavior, that stall in the crust can solidify quickly to clase from mafi c parents, producing andesitic and may be the dominant magmatic differentia- advanced degrees, potentially too fast for pro- to dacitic daughter liquids, would be expected tion process at Mount Rainier, and possibly also gressive separation of crystals from melt. If to increase concentrations of Y, Zr, and Nb in Mount St. Helens. such intrusions are in the middle and deep crust, residual liquids, and would cause their incom- For in situ crystallization-differentiation, however, they may remain close to the solidus patible trace element ratios (e.g., Ba/Zr) to hold portions of a magmatic system solidify to high due to high ambient temperatures, as well as nearly constant. Andesitic pumice-glass pairs degrees and then deliver evolved liquids to other from heat supplied from earlier and subsequent behave in this fashion, as do basaltic andesites less solidifi ed and evolved parts of the system intrusions. Thermal models of small, deep- through dacites from Mount Adams (Fig. 4). At (Langmuir, 1989). The primary signature of crustal intrusions indicates that evolved silicic Mount St. Helens and Mount Rainier, however, in situ differentiation is that chemical effects liquids residual from advanced crystallization concentrations of those elements increase only of advanced crystallization appear in magmas persist for long times, and thus, have the high-

138 Geological Society of America Bulletin, January/February 2014 Petrogenesis of Mount Rainier andesite

est probabilities of extraction (Dufek and Bach- con fractionation is seen in a plot of P/K ver- through the Mount Rainier andesite-dacite suite mann, 2010). Pumice-glass pairs show (Fig. 4, sus Zr/K (Fig. 8). Fractionation of apatite, (Fig. 5). Puget Group AFC would be unsuccess- Table 5) that at high degrees of melt evolution, along with plagioclase, maﬁ c silicates, and ful, however, in matching trends of Ba/La and Zr becomes compatible due to zircon saturation, FeTi oxides, drives liquids to low P/K, with Ba/Zr (Fig. 7) due to the broadly similar values Y and the middle to heavy REEs become com- only modest reduction in Zr/K (due to Zr being of those element ratios in calc-alkaline parents, patible probably due to amphibole and apatite slightly less incompatible than K). Andesites Puget Group sedimentary rocks, and the low and saturation, and Nb becomes only weakly incom- and dacites from Mount Adams behave in this similar mineral/melt partitioning for those ele- patible probably due to incorporation by FeTi manner, consistent with progressive crystalliza- ments during plagioclase + pyroxene ± olivine oxides (see Supplemental Item 1 in the GSA tion-differentiation, as do andesite pumice-glass crystallization. Data Repository1 for trace element partition- pairs. Zircon fractionation drives liquids to low Smith and Leeman (1993) showed that ing values estimated from pumice-glass pairs). Zr/K. Arc andesites and dacites routinely con- similar AFC calculations do not match Ba/La Residual liquids at advanced extents of solidiﬁ - tain apatite but rarely contain zircon due to high and K/La trends for Mount St. Helens basalts

cation therefore have high SiO2 concentrations, temperatures and insuffi cient Zr concentrations through dacites, but that two-component mix- but can have low concentrations of Zr and Y, (Fig. 4). Nevertheless, Mount Rainier andesites ing between mafi c and evolved St. Helens and only weak enrichments of Nb. Subsequent and dacites defi ne an array of jointly decreas- magmas is more successful. Numerically, two- magma replenishments transiting such mushy ing Zr/K and P/K, trending toward rhyolitic component mixing can be considered a special or wholly solidifi ed antecedent intrusions can glasses that have undergone zircon and apatite case of AFC wherein the mass assimilation rate preferentially incorporate their highly differen- crystallization (Fig. 8). Incorporation of such an (Ṁa) greatly exceeds the mass crystallization tiated liquids, encompassing both residua from evolved component thus appears to have been rate (Ṁc), or r = Ṁa /Ṁc 1 (DePaolo, 1981). advanced solidifi cation and low-degree partial widespread. Because latent heats of crystallization and of re-melts, leading to the overall trace element melting are similar, r 1 would seem to vio- trends observed at Mount Rainier and prob- Assimilation–Fractional late conservation of energy; however, for in situ ably also Mount St. Helens. At times, larger Crystallization (AFC) differentiation, the material being assimilated intrusions may form and undergo progressive is molten, carrying heat residual from its par- crystal-fractionation due to slow cooling rates, As magma assimilates country rock, heat is ent magma(s), so the latent heat of fusion costs enriching their evolved magmas in Y, Zr, and consumed by increasing the temperature of the are diminished or absent, reducing the extents Nb (e.g., biotite rhyodacite sample 95SR446), assimilated material and by fusing its minerals. to which the replenishing magmas must crystal- introducing scatter in concentration plots. Pro- Magmatic crystallization chiefl y provides that lize. Compositional and isotopic variations can gressive crystallization-differentiation may also heat (Bowen, 1928), inspiring chemical-iso- therefore more closely approach simple mix- account for the P compositional distribution that topic-thermodynamic models of joint assimila- ing arrays. peaks at intermediate SiO2, with a well-defi ned tion–fractional crystallization (AFC) (DePaolo,

upper SiO2 limit interpretable as an apatite satu- 1981; Bohrson and Spera, 2007). Although Combined In Situ Crystallization- ration surface (Fig. 4). widely used, such models require many choices Differentiation and Assimilation Smith and Leeman (1993) proposed a similar of assimilant composition(s), crystallizing pro- “braided stream” confi guration for the inter- portions, mineral/melt element partitioning, and A refi ned model wherein evolved residual mittently replenished Mount St. Helens mag- accessory mineral saturation, making it diffi cult melts mix with replenishing mafi c parents pro- matic system, with andesites forming chiefl y to assess the robustness of results. A conserva- duces a closer chemical and isotopic match to as mixtures between dacite and basalt or basal- tive AFC model assimilating average Puget the Mount Rainier magmatic suite (Fig. 7). In tic andesite. A difference, however, is that they Group arkose into the generalized parental nature, such residual liquids might span from interpret the silicic component as due to ana- calc-alkaline basalts produces Sr-Nd-Pb isotope silicic dacite or rhyodacite to rhyolite, depend- texis of amphibolites or mid-ocean ridge basalt arrays little different from the two-component ing on local circumstances, but for modeling (MORB)–like rocks in the deep crust, accompa- (Puget Group–basalt) mixing lines that trend and illustrative purposes we consider rhyolite. nied by subordinate continent-derived sediments (Smith and Leeman, 1987), whereas here the evolved component is identifi ed as mainly resid- 0.020 ual liquids from earlier Mount Rainier magmas that stalled and solidifi ed to advanced degrees. MA Such evolved residual liquids are the melt com- 0.015 plements to antecrysts increasingly recognized Figure 8. Plot of P/K versus SWC B/BA in magmatic systems (Reid et al., 1997; Hildreth, MSH Zr/K for Mount Rainier igne- 2001; Charlier et al., 2005) including Mount St. ous rocks, pumice-glass pairs, Helens (Claiborne et al., 2010). 0.010 and fi elds for other Quaternary The widespread gain (or loss) of an evolved

volcanic centers of southwest Zr/K (ppm) component that has undergone apatite and zir- apatite-saturated Washington. Symbols and ﬁ elds fractionation 0.005 are as in Figures 3 and 4. 1GSA Data Repository item 2014027, Oxygen iso- gain or loss of apatite & zircon tope fractionation during magmatic differentiation, -saturated evolved liquids and trace element partitioning estimated from pumice-glass pairs, is available at http://www .geosociety 0 .org/pubs /ft2014 .htm or by request to editing@ 0 0.005 0.0100.015 0.020 geosociety.org. P/K (ppm)

Geological Society of America Bulletin, January/February 2014 139 Sisson et al.

Chemical and isotopic features of this evolved and Vallance, 2009; Sisson, unpublished data), component, including high Ba, intermediate and δ18O of 6.4‰. Mass balance for basalt end

Zr, and low Sr and Y concentrations, as well members with SiO2 of 50–52 wt%, and a rhyo- as isotopic values intermediate between paren- lite component with SiO2 of 75 wt% consisting tal basalts and continental sediments (Table 8), of one-quarter to one-third sediment, indicates were estimated by projecting trace element 10–15 wt% sediment-derived material in the and isotopic trends of Mount Rainier rocks to a average Mount Rainier magma. Similarly for rhyolitic SiO2 concentration (76 wt%), by plot- oxygen isotopes, parent basalts in the range Oligo-Miocene ting trace element/Sr versus 87Sr/86Sr and trace 5.4‰–5.6‰, and a rhyolite component of 8‰ volcanics element/Nd versus 143Nd/144Nd, and by trial- consisting of one-quarter to one-third sediment, 4 and-error simulations of mixing arrays. Because give 8–13 wt% sediment in the average Mount this rhyolite’s composition is estimated, not Rainier magma. sampled and analyzed, we refrain from more These estimates indicate that, while the complex AFC models and focus instead on extents of crustal-level interaction are large for simple mixing. Isotopic values inferred for Mount Rainier andesites, most of this interac- the model rhyolite would indicate that mod- tion is with earlier intrusive products of the Eocene est assimilation of sediment or sediment par- magmatic system, and the amounts of direct 5 sediments tial melt generally accompanies production of incorporation of old continental materials are 4 in situ liquids. With δ18O of 8‰, this average low. Extensive interaction with Mesozoic and admixed rhyolite could consist of 30 ± 3 wt% pre-arc Eocene igneous basement would pro- 3 sediment or sediment partial melt, with the duce diverse 206Pb/204Pb and high 143Nd/144Nd, remainder being cognate residual liquid from not seen in Mount Rainier magmas, and thus solidified mafi c parents (calculated with sediment δ18O of appears to be minor. Instead, sedimentary rocks intrusions 14‰ and residual liquid δ18O of 5.8‰–5.4‰). or their melts are the predominant old crustal A similar percentage of sediment or sediment material assimilated at Mount Rainier, albeit partial melt (27 wt%) in the rhyolitic end mem- in small amounts, probably due to the great ber is inferred from Nd isotopes, assuming that thickness of Puget Group sediments (Stanley the sediment and cognate residual components et al., 1994), their structural displacement to have equal Nd concentrations, the 143Nd/144Nd middle-to-lower crustal depths, the melt fer- mafic deep crust of the sediment component matches average tility of shales, and possibly the low density 2 mantle Puget Group arkose, and the 143Nd/144Nd of the of sediments retarding the ascent of transiting cognate residual component is similar to the small magma batches (Fig. 9). Antecedent deep 1 model calc-alkaline basaltic parents (0.51295). intrusions from the Quaternary Mount Rainier Figure 9. Schematic cross-section through the Thus, for an in situ differentiation scenario, the system are, however, the chief source of evolved Mount Rainier magmatic system. 1—Basaltic rhyolitic component could consist of roughly components because those intrusions are the infl ux from the mantle. 2—Ponding near the one-quarter to one-third material derived from hottest materials in the area, and because they base of the crust with high-pressure differ- continental sediments, with three-quarters to are situated along the pathways of subsequent entiation creating spessartites and basaltic two-thirds derived by advanced differentiation ascending magmas. andesites. 3—Assimilation of continental- or low-degree partial melting of mafi c intrusions interior sediments, or their partial melts, similar to Quaternary calc-alkaline basalts near Minimum Intrusive to perhaps with magmas ponding at the base Mount Rainier. Extrusive Proportions of the sedimentary section (Vp < 6.5 km/s). The majority of Mount Rainier samples 4—Mixing with evolved interstitial liquids plot in the range of 20–40 wt% admixed in The above estimates, while admittedly broad, from highly solidifi ed earlier intrusions. situ rhyolite component, with maximum val- show that roughly one-third to one-half of the 5—Ascent to the side of the axial magmatic ues slightly less than 70 wt% (Figs. 6 and 7). average Mount Rainier magma could be silicic system delivering basaltic andesites and Many samples plot outside the compositional liquid entrained from deep intrusions, and this spessartites to the surface. space bracketed by the model mixing lines, allows estimates of the minimum intrusive to probably due in some cases to progressive extrusive proportions. Based on experiments, crystallization-differentiation, to atypically high arc basalts that solidify or re-melt in the deep or low amounts of sediment assimilation, or to crust can yield up to 15–25 wt% rhyolitic to from comparing the K2O concentration of distinct magma sources (low 206Pb/204Pb Mount rhyodacitic liquid, with amounts varying with 1.76 wt% for average Mount Rainier magma,

St. Helens–like samples). Although the in situ basalt bulk composition and oxidation state with K2O of 0.52 wt% for an average of differentiation scenario implies large amounts (Sisson et al., 2005). From these mixing pro- ~400 Quaternary Cascades basalts with MgO ≥ of admixed evolved liquid, the amount of portions and melt-yield fractions, the intrusive 8 wt%. Treating K2O as perfectly incompati- sediment-derived component is generally low, portion of the Mount Rainier magmatic system ble, and ignoring contributions from old crustal consistent with earlier estimates of bulk sedi- can be estimated as at least 0.7–2.8 times the sources quantifi ed as small, gives a minimum ment assimilation. The average magma erupted eruptive volume. Assigning a 10 wt% yield of intrusive to extrusive ratio of 2.4. Thus, the geo- through Mount Rainier’s axial magmatic system silicic liquid from deep-crustal mafi c intrusions chemically estimated intrusions are equal to, or has a SiO2 concentration of 61.7 wt% (Table 1; increases the minimum intrusive-to-extrusive up to as much as nearly fi ve times, the erupted McKenna, 1994; Stockstill et al., 2002; Sisson ratio to 3–4.5. An independent estimate comes mass. True intrusive-to-extrusive proportions

140 Geological Society of America Bulletin, January/February 2014 Petrogenesis of Mount Rainier andesite

206 204 will be greater than these estimates because are also distinguished by low Pb/ Pb and that at any SiO2 value, Mount Rainier’s higher- intrusions will not completely expel their inter- approach Mount St. Helens Sr-Nd-Pb isotopic K rocks are more likely to have amphibole stitial liquids, and because part or all of some values. The buried eastern margin of Siletzia lies phenocrysts and to be relatively enriched in Sr, magma batches can solidify underground with- ~15 km west of Mount Rainier beneath the west consistent with spessartites feeding into and out supplying fractionated liquids to magmas Rainier seismic zone and the Carbon River anti- mixing with axial magmas. that erupt. clinorium (Stanley et al., 1995), whereas Mount St. Helens straddles the St. Helens seismic zone A Common Southern Washington Cascades Crustal-Level Composition of the identifi ed as overlying Siletzia’s buried eastern Isotopically Primitive Arc End Member Mount Rainier Magmatic System margin (Parsons et al., 1998), consistent with lower contributions from Siletzia basement to A further notable fi nding of this study is that As modeled, the net magmatic fl ux into the the Mount Rainier magmatic system. Mount there is a common and restricted Sr-Nd-Pb crust would be basaltic, and the average compo- Rainier is also farther from a possible offset isotopic fi eld for Quaternary volcanic rocks of sition of the magmatic system would be basaltic in the subducted Juan de Fuca plate imaged at Mounts Adams and Rainier, southwest Wash- andesite due to modest assimilation of sediment 90 km depth by S-wave tomography (Schmandt ington calc-alkaline mafi c lavas, and Oligo- or sediment partial melt. Widespread eruption and Humphreys, 2010), so subordinate infl u- cene and Miocene arc igneous rocks (Fig. 5). of basalt across the southern Washington Cas- ence from slab-edge melts is also possible. The The isotopically primitive (high 143Nd/144Nd, cades, albeit in small volumes (Hammond and Mount Rainier sample with the lowest measured low 87Sr/86Sr) end of this fi eld is distinct from Korosec, 1983; Leeman et al., 1990), as well as 206Pb/204Pb (andesite 99ML770) is not, however, Juan de Fuca MORBs (off scale in Fig. 5), but

eruptions of basalt through the mainly dacitic low in K2O, indicating additional origins for iso- coincides with the center of the isotopic fi eld Mount St. Helens system (Smith and Leeman, topic outliers. defi ned by Quaternary low-K olivine tholeiites 1993) are evidence that basalt is supplied to and within plate–type basalts from southwest the roots of stratovolcano systems in southern A Deep, Progressive Fractionation Origin Washington and northwest Oregon. Low-K Washington. For Mount Rainier, however, evi- of Mount Rainier’s Spessartites olivine tholeiites and within plate–type basalts dence is weak for a net basaltic (sensu stricto) of the Cascades have, at most, weak subduction fl ux into the crust, as shown by the absence of Spessartites that erupted from the north fl ank contributions (Leeman et al., 1990), so their basalt erupted from proximal vents, and the near of Mount Rainier are distinctive texturally due isotopic values can be taken as representative absence of basalt as QMIs. Instead, primary to their prominent phenocrysts of amphibole of the ambient upper mantle beneath the U.S. basalts may arrest in the deep crust or upper but not plagioclase, and are distinctive chemi- Pacifi c Northwest. Voluminous basalts of the mantle beneath the mushy andesitic crustal cally by their high concentrations of Sr, Ba, Rb, Paleocene–early Eocene Siletzia terrain share magmatic system where they undergo modest K, Th, P, Zr, LREE, LREE/HREE (ratio of light this isotopic fi eld (D. Pyle, 2008, personal com- progressive crystallization, producing basaltic to heavy rare earth elements), and Sr/Y for their mun.), showing that shallow mantle with such

andesite differentiates (Fig. 9). Coupled with intermediate SiO2. Their high Sr/Y (55–70) characteristics underlay the region at least as assimilation, this process indicates that the and La/Yb (~30) are reminiscent of adakites early as the inception of arc magmatism in the mean composition of the crustal magmatic sys- (Defant and Drummond, 1990), but their high Eocene. The Pb isotopic diversity of the U.S. tem may therefore be andesitic. K, Rb, and Ba concentrations are inconsistent Paciﬁ c Northwest subarc mantle, as indicated with melts from subducted MORB crust. A by Quaternary low-K tholeiites and within Mount St. Helens–Type Magmas in spessartite (sample 97ML657) and a spessar- plate–type basalts, and of Siletzia contrasts the Mount Rainier System tite-basaltic andesite hybrid (97ML656) are with the uniformity of the isotopically primi- also indistinguishable isotopically from ordi- tive Mount Rainier–Mount Adams subduction- Andesites and dacites from Mount St. Helens nary amphibole-poor or amphibole-free Mount related end member, and a resolution may be

have distinctly lower K2O concentrations than Rainier andesites (Table 6). Deep progres- that the primitive subduction-related end mem- those from Mounts Rainier and Adams, and sive fractionation of parental Mount Rainier ber is an average of the ambient upper mantle are also distinguished by their low 206Pb/204Pb magmas, involving garnet and little or no beneath the region. Sediment subduction may

(<18.9). The origin of the low-K2O Mount St. plagioclase, may account for the spessartites, then account for the displacement of some calc- Helens suite is unclear, with options including as interpreted for some adakite-like magmas alkaline basalts toward continental isotopic partial melting of the buried eastern margin elsewhere (Macpherson et al., 2006; Rodríguez compositions, but this is difﬁ cult to distinguish of the Siletzia terrane, or by partial melting et al., 2007). The Mount Rainier spessartites from crustal-level assimilation of Eocene sand- of MORB-source or MORB-like material in are not strongly depleted in Y or Yb, so some stones and shales, recorded by widespread trace the mantle, subducting slab, or hidden in the additional mineral phase, probably pyroxene, quartz xenocrysts. crust, in all instances modiﬁ ed by incorpo- accompanied garnet; nor are the spessartites ration of sediment components to lower the enriched in Ti, Ta, and Nb commensurate with CONCLUDING REMARKS magmas’ 143Nd/144Nd, increase their 87Sr/87Sr, their enrichments in K, Ba, Rb, Sr, P, and Th, so and slightly increase their 206Pb/204Pb (Halliday some Ti-rich phase, such as ilmenite or rutile, Mounts Rainier, Adams, and St. Helens each et al., 1983; Smith and Leeman, 1987, 1993). would also have to have crystallized. Deep frac- produce andesite-dacite series magmas, but by

Resolving among these possibilities is beyond tionation enriched the liquids in H2O, as well somewhat different processes. Mount Rainier’s the scope of this study, but a notable ﬁ nding is as incompatible trace elements, explaining the magmas show isotopic evidence for variable that small amounts of Mount St. Helens–like absence of plagioclase phenocrysts and abun- but modest assimilation of isotopically highly magmas contribute to the Mount Rainier mag- dance of amphibole phenocrysts. Spessartites evolved crustal materials. Magmatic evolution matic system. Mount Rainier samples selected have not erupted through Mount Rainier’s axial chieﬂ y takes place, however, by in situ fraction-

for their lower-than-typical K2O concentrations magmatic system, but a general observation is ation and mixing, wherein magma batches stall

Geological Society of America Bulletin, January/February 2014 141 Sisson et al.

in the crust along an intrusive plexus beneath andesites also signal mixing between more and for calculating trace element and isotope variations of open-system magmatic systems: Geochemistry the volcano where they crystallize largely or less evolved materials (Kent et al., 2010), con- Geophysics Geosystems, v. 8, Q11003, doi: 10.1029 completely; subsequent magmas that ascend sistent with an in situ differentiation and mix- /2007GC001781. through these antecedent intrusions entrain and ing process. Bowen, N.L., 1928, The Evolution of the Igneous Rocks: Princeton, New Jersey, Princeton University Press, 334 p. mix with their residual liquids and partial melts. These geologic, geochemical, and isotopic Buckovic, W.A., 1979, The Eocene deltaic system of west- This intrusive plexus transects a synclinorium observations highlight some ways that geologic central Washington, in Armentrout, J.M., Cole, M.R., where Puget Group sandstones and shales are setting and magma supply infl uence the genera- and TerBest, H., eds., Cenozoic Paleogeography of the Western United States: Society of Economic Paleon- at middle or deep crustal levels, and therefore tion of andesite-dacite magmas. High and sus- tologists and Mineralogists, Pacifi c Coast Paleogeog- are hot, accounting for their assimilation. Inter- tained magma supply, and refractory basement raphy Symposium 3, p. 147–163. Chacko, T., Cole, D.R., and Horita, J., 2001, Equilibrium mittent, modest magma supply over long time rocks, can generate andesite and dacites by oxygen, hydrogen, and carbon isotope fractionation periods leads to the in situ differentiation and straightforward progressive crystallization-dif- factors applicable to geologic systems, in Valley, J.W., mixing style of magmatic evolution. ferentiation (Mount Adams). A similar parent, and Cole, D.R., eds., Stable Isotope Geochemistry: Re- views in Mineralogy, v. 43, p. 1–62. Mount Adams magmas, in contrast, have but with lower magma supply and the presence Charlier, B.L.A., Wilson, C.J.N., Lowenstern, J.B., Blake, little Sr-Nd-Pb isotopic evidence for crustal of fertile rocks in the mid-deep crust promotes S., Van Calsteren, P.W., and Davidson, J.P., 2005, assimilation, although Os results do record modest and variable assimilation and generates Magma generation at a large, hyperactive silicic volcano (Taupo, New Zealand) revealed by U-Th and crustal interaction not readily distinguished by andesite and dacites mainly though in situ differ- U-Pb systematics in zircons: Journal of Petrology, other measurements (Jicha et al., 2009). Mount entiation and mixing processes (Mount Rainier). v. 46, p. 3–32, doi: 10.1093 /petrology /egh060. Claiborne, L.L., Miller, C.F., Flanagan, D.M., Clynne, M.A., Adams sits atop the southern projection of the An even lower and less sustained magma supply and Wooden, J.L., 2010, Zircon reveals protracted White Pass anticlinorium, so Eocene continen- leads to an even stronger in situ differentiation magma storage and recycling beneath Mount St. Helens: tal interior–derived sedimentary rocks are pres- and mixing signal (Mount St. Helens). Geology, v. 38, p. 1011–1014, doi: 10.1130 /G31285.1. ent only in the shallow crust, and so are cold and Clayton, R.N., and Mayeda, T.K., 1963, The use of bromine ACKNOWLEDGMENTS pentafl uoride in the extraction of oxygen from oxides resistant to assimilation, or have been eroded and silicates for isotopic analysis: Geochimica et Cos- away entirely. Mount Adams magmas lack the Chemical analyses of Mount Rainier whole-rocks mochimica Acta, v. 27, p. 43–52, doi: 10.1016 /0016 -7037 (63)90071-1. 206 204 δ18 were performed by the late Dave Siems (USGS–XRF), broad diversity in Pb/ Pb and high O of Clynne, M.A., Calvert, A.T., Wolfe, E.W., Evarts, R.C., Mesozoic (meta-)igneous rocks that core the by Jim Budahn (USGS–INAA), and by Diane John- Fleck, R.J., and Lanphere, M.A., 2008, The Pleisto- son, Rick Conrey, and Charles Knapp (WSU–XRF White Pass anticlinorium; Mesozoic mélange cene eruptive history of Mount St. Helens, Washing- and ICPMS). Bob Rye provided whole-rock oxy gen ton, from 300,000 to 12,800 years before present, in basement therefore does not appear to be assim- isotope analyses, David Pyle shared unpublished iso- Sherrod, D.R., Scott, W.E., and Stauffer, P.H., eds., A ilated in signifi cant amounts. Mount Adams topic results for the Siletzia terrain, James Vallance Volcano Rekindled: The Renewed Eruption of Mount magmas also have a different differentiation provided some Mount Rainier samples, and Ilya Bin- St. Helens, 2004–2009: U.S. Geological Survey Pro- deman provided suggestions on modeling oxygen fessional Paper 1750, p. 593–628. style in that they become enriched in Zr, Y, and isotope fractionation. Eric Bard, David Zimbelman, Conrey, R.M., Sherrod, D.R., Hooper, P.R., and Swanson, Nb with increasing SiO , consistent with pro- D.A., 1997, Diverse magmas in the Cascade arc, north- 2 Liz Schermer, David Lewis, and Steven Sherostsky ern Oregon and southern Washington: Canadian Min- gressive fractional crystallization of plagioclase accompanied Tom Sisson in the fi eld. Manuscript eralogist, v. 35, p. 367–396. and mafi c silicates. Mount Adams is the largest reviews by Michelle Coombs, Bill Leeman, Mike Defant, M.J., and Drummond, M.S., 1990, Derivation of late Pleistocene–Holocene volcanic fi eld along Dungan, and an anonymous reviewer, and additional some modern arc magmas by melting of young sub- comments and editorial assistance by Wes Hildreth, ducted lithosphere: Nature, v. 347, p. 662–665, doi: the Cascades arc axis (Hildreth and Lanphere, Nancy Riggs, and Michael Ort, are appreciated. 10.1038 /347662a0. 1994), so it may be fed by magma batches that DePaolo, D.J., 1981, Trace element and isotopic effects of REFERENCES CITED combined wallrock assimilation and fractional crystal- are suffi ciently large or frequent that their intru- lization: Earth and Planetary Science Letters, v. 53, sions cool slowly, allowing gradual and progres- Armentrout, J.M., 1987, Cenozoic sequence stratigraphy, p. 189–202, doi: 10.1016 /0012-821X (81)90153-9. sive separation of melt from crystals. unconformity-bounded sequences, and tectonic history du Bray, E.A., Bacon, C.R., John, D.A., Wooden, J.L., and Mazdab, F.K., 2010, Episodic intrusion, internal differ- Finally, though highly active over the last of southwest Washington, in Schuster, J.E., ed., Se- lected Papers on the Geology of Washington: Washing- entiation, and hydrothermal alteration of the Miocene ~4000 years, Mount St. Helens is a small vol- ton Division of Geology and Earth Resources Bulletin Tatoosh intrusive suite south of Mount Rainier, Wash- canic system with prolonged spans of quies- 77, p. 291–320. ington: Geological Society of America Bulletin, v. 123, Bacon, C.R., 1986, Magmatic inclusions in silicic and p. 534–561, doi: 10.2230 /B30095.1. cence (Mullineaux, 1996; Sherrod, 1990; intermediate volcanic rocks: Journal of Geophysi- Dufek, J., and Bachmann, O., 2010, Quantum magmatism: Clynne et al., 2008). 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