Petrogenesis of Mount Rainier andesite: Magma ﬂ …122 Petrogenesis of Mount Rainier andesite: Magma ﬂ ux and geologic controls on the contrasting differentiation styles at stratovolcanoes

122

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, USA2National High Magnetic Field Laboratory, Isotope Geochemistry, and Department of Geological Sciences, Florida State University, 1800 E. Paul Dirac Drive, Tallahassee, Florida 32310, USA3Department of Geology, Washington State University, Pullman, Washington 99164, USA

ABSTRACT

Quaternary Mount Rainier (Washington, USA) of the Cascades magmatic arc consists of porphyritic calc-alkaline andesites and subordinate dacites, with common evidence for mingling and mixing with less evolved magmas encompassing andesites, basaltic andesites, and rarely, basalts. Basaltic ande-sites and amphibole andesites (spessartites) that erupted from vents at the north foot of the volcano represent some of Mount Rainier’s immediate parents and overlap in composition with regional basalts and basal-tic andesites. Geochemical (major and trace elements) and isotopic (Sr, Nd, Pb, O) com-positions of Mount Rainier andesites and da-cites are consistent with modest assimilation (typically ≤20 wt%) of evolved sediment or sediment partial melt. Sandstones and shales of the Eocene Puget Group, derived from the continental interior, are exposed in regional anticlines fl anking the volcano, and prob-ably underlie it in the middle to lower crust, accounting for their assimilation. Mesozoic and Cenozoic igneous basement rocks are unsuitable as assimilants due to their high 143Nd/144Nd, diverse 206Pb/204Pb, and gener-ally high δ18O.

The dominant cause of magmatic evolu-tion at Mount Rainier, however, is inferred to be a version of in situ crystallization-dif-ferentiation and mixing (Langmuir, 1989) wherein small magma batches stall as crustal intrusions and solidify extensively, yielding silicic residual liquids with trace element concentrations infl uenced by accessory min-

eral saturation. Subsequent magmas ascend-ing through the intrusive plexus entrain and mix with the residual liquids and low-degree re-melts of those antecedent intrusions, pro-ducing hybrid andesites and dacites. Mount St. Helens volcanic rocks have geochemical similarities to those at Mount Rainier, and may also result from in situ differentiation and mixing due to low and intermittent long-term magma supply, accompanied by modest crustal assimilation. Andesites and dacites of Mount Adams isotopically overlap the least contaminated Mount Rainier magmas and derive from similar parental magma types, but have trace element variations more con-sistent with progressive crystallization-dif-ferentiation, probably due to higher magma fl uxes leading to slower crystallization of large magma batches, allowing time for pro-gressive separation of minerals from melt. Mount Adams also sits atop the southern projection of a regional anticlinorium, so Eo-cene sediments are absent, or are at shallow crustal levels, and so are cold and diffi cult to assimilate. Differences between southwest Washington stratovolcanoes highlight some ways that crustal geology and magma fl ux are primary factors in andesite generation.

INTRODUCTION

Andesite series magmas have been explained as products of basaltic crystallization-differ-entiation, as partial melts of the deep crust or subducting slabs, and by composite scenarios involving crystallization, assimilation, mix-ing, and in certain cases, reaction with or direct derivation from mantle peridotite (Gill, 1981; DePaolo, 1981; Kelemen, 1990; Grove et al., 2005). The multiplicity of interpretations stems, in part, from the complexity of the rocks that

routinely carry features indicative of magma mingling and mixing, and that defi ne scattered whole-rock compositional arrays permissive of many explanations. Radiogenic and stable iso-tope measurements can be more discriminating, allowing for quantitative estimates of the mag-nitudes of different processes and components, but their successful use depends on suffi ciently large and representative sample suites, com-prehensive major and trace element analyses, adequately precise isotopic measurements, and well-understood and well-characterized geo-logic settings.

Here we present results of a combined geo-chemical, isotopic, and geologic study of the origins of andesite series magmas from Mount Rainier, Washington State, in the Cascades magmatic arc of western North America. The study benefi ts from abundant samples col-lected during geologic mapping of the volcano, sparser sampling of small-volume Quaternary mafi c volcanic rocks erupted across southwest Washington, as well as representative samples of pre-Quaternary basement rocks. The study also benefi ts from improvements in the ease and precision of isotopic analyses that allow effi cient characterization of chemically ordinary and atypical samples. Our new data indicate that Mount Rainier’s magmas incorporated small but variable amounts (typically ≤20 wt%, but up to 30 wt%) of evolved sedimentary rocks, or their partial melts, known to be present in the middle or lower crust, but that the predominant cause of magmatic diversity is multi-stage in situ crystallization-differentiation and mixing. This process involves magmatic replenish-ments incorporating advanced differentiates or low-degree partial melts from earlier magmatic pulses that stalled and nearly or completely solidifi ed. Because geology and structure infl u-ence the course of magmatic evolution at Mount

For permission to copy, contact [email protected]© 2013 Geological Society of America

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.

†E-mail: [email protected] (Sisson, corresponding); [email protected] (Salters); [email protected] (Larson).

Petrogenesis of Mount Rainier andesite

Geological Society of America Bulletin, January/February 2014 123

Rainier, and the other major volcanoes of the region, we fi rst summarize the tectonic set-ting and geologic development of the southern Washington Cascades.

TECTONIC SETTING

Active volcanoes of the Cascade Range are products of northeasterly directed subduction of the oceanic Juan de Fuca plate beneath North America. The Juan de Fuca spreading ridge lies only 250–450 km from the volcanic-arc axis, along the direction of convergence, with the result that the subducting slab is one of the youngest and hottest worldwide (Hyndman and Wang, 1993; Syracuse et al., 2010). In its south-ern portion, in northern California and southern Oregon, the arc is impinged upon from the east by the Basin and Range extensional province. In the north, westward defl ection of the continental margin along Vancouver Island, British Colum-bia (Canada), arches the slab beneath central and northern Washington State. Consequently, cross-arc strain (McCaffrey et al., 2007) passes from neutral or slightly extensional in the south to increasingly convergent moving northward, culminating with arc-normal convergence atop the arch in the subducting slab, as marked by uplift of the Olympic and North Cascades moun-tains. Volcanic output tracks these changes, diminishing northward as the hanging wall of the arc becomes increasingly compressional. Mount Rainier is situated within the transition from widespread diffuse mafi c volcanism in the south to widespread basement uplift and negli-gible mafi c volcanism in the north.

GEOLOGIC SETTING

Regional Cascades Geology

The geologic framework of the U.S. Pacifi c Northwest is important for understanding Cas-cades arc magmatism due to potential assimi-lation and crustal melting. Regional crustal domains (Fig. 1) include the Siletzia Paleocene–early Eocene submarine basalt province that fl oors the forearc basin west of the active arc from southernmost Vancouver Island southward through Oregon’s Willamette Valley (Duncan, 1982). Younger rocks conceal Siletzia’s east-ern margin, but the terrain boundary has been inferred from seismic velocity sections to under-lie the St. Helens seismic zone (Parsons et al., 1998), which passes beneath Mount St. Helens. In Washington, the unexposed deep basement abutting Siletzia on the east probably correlates with Mesozoic tectonite mélange of the West-ern and Eastern mélange belts that border the western margin of the North Cascades crystal-

line core to the north of Mount Rainier (Frizzell et al., 1987). Quartz-rich sands and silts from the North American continental interior spread across present-day southern Washington and northern to central Oregon during the Eocene and partly buried the mélange and Siletzia basalts with fl uvial, deltaic, and shallow-marine sediments. These continent-derived sedimen-tary rocks form the Puget Group in southwest Washington, and the widespread Tyee Forma-tion, among others, in Oregon (Buckovic, 1979; Heller et al., 1985).

Cascades arc volcanism commenced inter-mittently and locally in the middle Eocene and became extensive and voluminous in the Oligocene and Miocene (Armentrout, 1987). Oligocene volcanism in southwest Washington is manifest as the widespread andesitic Ohana-

pecosh Formation, overlain unconformably in the Mount Rainier region by silicic ash-fl ow tuffs of the 25 Ma rhyolitic Stevens Ridge For-mation and the 22 Ma rhyodacitic tuff of Clear West Peak caldera (Vance et al., 1987; Tabor et al., 2000). These late Oligocene and early Miocene ash-fl ow tuffs are largely overlain by, but also interfi nger with, andesitic volcanic rocks of the chiefl y Miocene Fifes Peak For-mation. Granodioritic plutons then intruded the Tertiary volcanic section, represented at Mount Rainier by the 19–14 Ma Tatoosh intrusive suite and Carbon River stock (Mattinson, 1977; du Bray et al., 2010). Rapid uplift of the Washing-ton Cascade and Olympic ranges commenced ca. 10–12 Ma (Reiners et al., 2002), shallowly unroofi ng the plutons, tilting the 16 Ma Colum-bia River fl ood basalts that onlap the eastern

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Figure 1. Regional geologic setting of Mount Rainier volcano, simplifi ed from Schuster (2005) and Walker and MacLeod (1991). Geologic units are: chiefl y Mesozoic and Paleozoic igneous and metamorphic rocks (light blue; Tertiary plutons omitted for clarity); Paleo-cene–middle Eocene submarine basalts of the Siletzia terrain (purple); middle Eocene and younger sandstones and shales (light yellow); Eocene–Miocene marine sedimentary rocks of the Olympic accretionary complex (gray); chiefl y Oligocene–Miocene arc igneous rocks (green); Miocene fl ood basalts (brown); Quaternary arc volcanic rocks (red); and Quater-nary sediments, chiefl y glacial (stippled). Medium and heavy black lines are faults. Triangles show the locations of Mount Rainier (MR), Mount Adams (MA), Mount St. Helens (MSH), Mount Hood (MH), and Glacier Peak (GP); Mount Baker plots outside the map area to the north. Circles show the cities of Portland, Oregon (P), and Tacoma (T) and Seattle (S), 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-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 relative to the North American plate interior (McCaffrey et al., 2007).

Sisson et al.

124 Geological Society of America Bulletin, January/February 2014

margin of the Cascade range, and incising the deep unconformity upon which the Quaternary volcanoes grew.

Local Geology of the Mount Rainier Area

Mount Rainier sits atop a broad, north-north-west–trending, internally folded synclinorium of Tertiary arc volcanic rocks, intruded by the Tatoosh plutonic suite, and is fl anked on the west and southeast by steep-sided north-northwest–striking anticlinoria (Fig. 2). To the west is the Carbon River anticlinorium cored by middle Eocene Puget Group arkoses, carbonaceous black shales, and local coal seams that dip steeply beneath, and fl oor, the Tertiary arc volcanic sec-tion. The base of the Puget Group is not exposed in the Carbon River area, but Stanley et al. (1994) presented geologic and geophysical evidence for a deformed thickness of 10–20 km, including postulated underlying marine shales that together create the southern Washington Cascades (elec-trical) conductor (SWCC). The Carbon River anticlinorium coincides with the west Rainier seismic zone, and seismic velocity sections (Stanley et al., 1999) show that it probably over-lies the buried eastern margin of Siletzia, similar to the Morton anticlinorium and St. Helens seis-mic zone to the south (Parson et al., 1998).

To the southeast is the White Pass anti-clinorium (Fig. 2) which is cored by the Late Jurassic–Early Cretaceous Rimrock Lake inlier of tectonite mélange composed of sheared dirty arkoses to graywackes, greenstone, minor chert, and tectonically bounded blocks and belts of intrusive rocks ranging from hornblende dio-ritic orthogneiss to cataclastically deformed biotite granodiorite (Miller, 1989). Mesozoic mélange rocks are overlain unconformably on the west limb of the anticlinorium by the steeply west-dipping Eocene Summit Basalt, which is overlain by the arkosic Eocene Summit Sand-stone, and then Oligocene andesitic rocks of the Ohanapecosh Formation (Vance et al., 1987). Thus, Eocene continent-derived sedimentary rocks dip steeply beneath Mount Rainier both from the west and the southeast (Fig. 2) and are probably continuous beneath the volcano in the middle and perhaps lower crust, possibly under-lain by Eocene–Paleocene non-arc basalts, all fl oored by Mesozoic sedimentary and igneous tectonite mélange, and disrupted by Tertiary and Quaternary igneous intrusions.

Distribution of Quaternary Volcanism in Southwest Washington

Quaternary arc volcanism is geographically segmented in southwest Washington, with the main stratovolcano chain of the Oregon Cas-

cades projecting north across the Columbia River to andesitic Mount Adams, and diminish-ing progressively northward from there through the inactive Goat Rocks volcanic center, ending with small basalt, basaltic andesite, and dacite vents and fl ows between White Pass and Bump-ing Lake (Fig. 1). The major stratovolcanoes of Mounts St. Helens and Rainier defi ne an arc segment that is stepped to the west and aligned diagonal to the Oregon Cascades–Mount Adams segment, with Mount St. Helens displaced 55 km west of Mount Adams but Mount Rainier only 25–35 km west of Quaternary vents near Bumping Lake.

Quaternary vents are absent directly between Mounts St. Helens and Rainier, but minor vents and fl ows of basalt, basaltic andesite, and horn-blende-rich andesite continue up to 30 km north of Mount Rainier as a diffuse extension of the Mount St. Helens–Mount Rainier arc segment. The closest of these vents to Mount Rainier sit at the volcano’s foot in the vicinity of Echo and Observation Rocks (Fig. 2), only 6–7 km north-northwest of the summit. This north-fl ank vent cluster erupted the olivine basaltic andesites of Spray Park, as well as distinctly amphibole-por-phyritic spessartite andesite of the Russell Gla-cier, and spessartite–basaltic andesite hybrid lava of the Flett Glacier (spessartite is calc-alkaline

andesite with a lamprophyric texture defi ned by phenocrysts of amphibole and with plagioclase restricted to the groundmass [Rock, 1987]). The next vent north along the segment is at Windy Gap, 12 km from Mount Rainier’s summit, which erupted the amphibole-olivine porphyritic spessartite lava fl ow of Bee Flat. Farthest north (outside the area of Fig. 2) are the basalt–basaltic andesite center of Canyon Creek (also known as the basalt of Three Sisters [Reiners et al., 2000]) and the basaltic center of Dalles Ridge, respec-tively 27 km north-northwest and 31 km north-northeast of Mount Rainier’s summit (Tabor et al., 2000). Quaternary volcanic products are unknown for another 105–110 km northward until reaching small basalt and basaltic andesite fl ows shortly south of Glacier Peak stratovol-cano. An outlier to the Mount St. Helens–Mount Rainier arc segment is an isolated Pleistocene dacite dome at St. Paul Lookout 26 km west-northwest of Mount Rainier on the west limb of the Carbon River anticlinorium (Fig. 2).

SAMPLE SELECTION AND ANALYTICAL METHODS

About 1100 Quaternary rock and tephra samples were collected from Mount Rainier and the surrounding region during geologic map-

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Figure 2. Local geologic setting of Mount Rainier volcano, simplifi ed from Fiske et al. (1963), Tabor et al. (2000), Schasse (1987), and Miller (1989); Quaternary volcanic rocks of the White Pass area are omitted for clarity. Colors as for Figure 1, except: Miocene plutons are shown (light red); Mesozoic tectonite mélange is subdivided into sandstones (light blue), greenstone (black), orthogneiss (gray), and chert (medium blue); Quaternary lava fl ows (dark blue) that erupted from vents (stars) to the north of Mount Rainier are distinguished from lavas that erupted through the axial vent system (red), and from the small lava dome and vent at St. Paul Lookout (orange) to the northwest of Mount Rainier; and ice on Mt. Rainier shown (white). CRB—Columbia River Basalt.



ping of the volcano. About 800 of these were powdered in alumina or agate shatterboxes and analyzed for major and trace elements by X-ray fl uorescence (XRF) at the U.S. Geological Sur-vey (USGS), Denver, and later at Washington State University, Pullman (WSU), to support the mapping effort. Sixty-seven Quaternary volcanic rocks were selected from the greater sample suite for more comprehensive isotopic and geochemical study (Table 1). Samples cho-sen for this additional analysis typify the main compositional trend of Mount Rainier’s erup-tives, or are samples with atypically high or low concentrations of one or more elements (K, Sr, Ba, Zr). Regional basalts and basaltic andesites were included to characterize potential paren-tal magmas, and sixteen samples of Mesozoic and Cenozoic basement rocks were analyzed to investigate crustal infl uences on magma produc-tion (Table 2). Basement samples were collected to be representative of preserved materials and therefore contain secondary carbonate, clay, and silica phases.

Concentrations of a broad suite of trace ele-ments were measured in these samples and in many of the larger mapping-support sample suite by instrumental neutron activation analysis (INAA) mainly at the USGS, Denver, and later by inductively coupled plasma mass spectrom-etry (ICPMS) at WSU (Tables 3 and 4). Meth-ods, accuracy, and precision of USGS analyses are similar to those of Baedecker (1987) and Bacon and Druitt (1988), and those for WSU analyses are available at http:// www .sees .wsu .edu /Geolab /note .html. Major and trace element concentrations were also determined for fi ve representative pumice bombs and for their sepa-rated glasses to assess the effects of crystal-melt separation (Table 5). These pumices consist of pyroxene andesites and hornblende-pyroxene dacites from Mount Rainier, and a zircon-bear-ing hornblende rhyodacite bomb from Glacier Peak, included because it is fresher and larger than similarly evolved tephra samples from Mount Rainier (Table 5).

Isotopic values of Sr, Nd, and Pb (Tables 6 and 7) were measured on 25 mg splits of whole-rock powders at the National High Magnetic Field Laboratory, Tallahassee, Florida. Lead, Sr, and Nd were separated from the same aliquot following the techniques outlined in Stracke et al. (2003). Strontium isotope ratios were mea-sured by thermal ionization mass spectrometry (TIMS) in dynamic mode on a Finnigan MAT 262 RPQ mass spectrometer, with 87Sr/86Sr ratios corrected for fractionation to 86Sr/88Sr = 0.1194. Long-term average of Sr standard E&A yields an 87Sr/86Sr value of 0.708004 ± 0.000013 (n = 25, 2σ), and 87Sr/86Sr of the samples are reported relative to the accepted ratio of the E&A stan-

dard (87Sr/86Sr = 0.708000). Neodymium and Pb isotope ratios were measured on a Thermo-Finnigan NEPTUNE MC-ICPMS (multicollec-tor ICPMS). 143Nd/144Nd ratios are corrected for fractionation to 146Nd/144Nd = 0.7219. Repeated measurements of Nd standard La Jolla yielded an average value of 143Nd/144Nd = 0.511839 ± 0.000018 (n = 110, 2σ). 143Nd/144Nd ratios of the samples are reported relative to the accepted ratio of the La Jolla standard (143Nd/144Nd = 0.511850).

Lead isotope ratios were measured using a Tl spike (Pb/Tl ~6) to account for mass frac-tionation and are corrected for fractionation to 203Tl/205Tl = 0.4188. Long-term averages of the Pb standard NBS-981 are 206Pb/204Pb = 16.9294 ± 0.0023, 207Pb/204Pb = 15.4824 ± 0.0025, 208Pb/204Pb = 36.6698 ± 0.0077 (n = 100, 2σ). Lead isotope ratios are reported relative to the accepted values of NBS-981 (206Pb/204Pb = 16.9356, 207Pb/204Pb = 15.4891, 208Pb/204Pb = 36.7006) (Todt et al., 1996).

Oxygen isotopes give some of the clearest indications of interaction with the hydrosphere and atmosphere, and therefore whether a magma has incorporated supra-crustal rocks. To investi-gate the possibility of crustal interaction, plagio-clase phenocrysts were separated from Mount Rainier samples by magnetic and density meth-ods, including a diamagnetic step to remove any vapor-phase or groundmass quartz, followed by removal of any adhering glass and groundmass by etching in dilute HF (7.5%) for 10 min while ultrasonicating, and then rinsing. The magnetic separation methods, in particular, select strongly against grains with mineral and melt inclusions, so the analyzed plagioclase separates are biased toward grains with simple textures and zoning that may be true phenocrysts and micropheno-crysts. Also analyzed were olivine phenocrysts separated from two samples and cleaned by similar methods, and one atypically large quartz xenocryst from a regional basaltic andesite.

Mineral separates were analyzed for oxygen isotopes by laser fl uorination at WSU (Table 6). Oxygen isotope values were measured in dupli-cate or triplicate on ~2 mg splits loaded with standards in an 18-hole puck (6–8 standards, 12–10 unknowns). Before applying laser heat-ing (Sharp, 1990), two or three brief (~1.5 min) pre-fl uorinations were conducted to remove water molecules on mineral surfaces and on the internal walls of the sample chamber and the vacuum system. Tests have shown that no measurable oxygen was extracted during each short pre-fl uorination. Also, reproducibility of samples and standards was higher with the short pre-fl uorinations. Each sample or standard was then heated with a 20 W CO2 laser, oxygen was liberated by reaction with BrF5 (Clayton and

Mayeda, 1963; Sharp, 1990), and the released oxygen was passed successively over cold traps and cleaned with KBr. The δ18O values were measured with a Finnigan Delta S isotope ratio mass spectrometer. Isotopic compositions are expressed in the δ notation as the relative dif-ference in the isotope values between the sam-ple and the Vienna standard mean ocean water (VSMOW) standard in parts per thousand (‰). The measured δ18O values of the samples were corrected by repeated analyses of the UWG-2 garnet standard (δ18O = ~5.8‰) (Valley et al., 1995), with analytical precision of 0.05‰ to 0.22‰ (1σ).

To further investigate potential crustal mag-matic sources, oxygen isotope ratios of Meso-zoic and Cenozoic basement whole rocks were also determined (USGS, Denver) by conven-tional BrF5 fl uoridation in Ni reaction vessels (Clayton and Mayeda, 1963). Upon conver-sion to CO2, oxygen isotopic values (Table 7) were measured on a Finnigan MAT 251 mass spectrometer, yielding an average value of 9.6‰ (VSMOW) for quartz standard NBS-28. Whole-rock measurements were not replicated, but typical reproducibility for the Denver facil-ity is ±0.1‰–0.15‰ (1σ).

Pumice deposited in subalpine meadows around Mount Rainier is commonly hydrated and partly converted to authigenic clays by reaction of glass with organic acid–rich pore waters. Such samples are excluded from consideration based on low whole-rock analytical totals (<98 wt%), except for a hydrated rhyodacite pumice deposit that is the sole biotite phenocrystic product known from Mount Rainier; its plotted major oxide composition (analysis 95SR446* in Table 1) was reconstructed from point-counted modes coupled with electron microprobe analy-ses of minerals and glass (Kirn, 1995).

RESULTS

General Character of Mount Rainier’s Magmas

Most of Mount Rainier’s magmas ascended through an axial magmatic system underlying the main edifi ce, and erupted from the sum-mit or from upper- and mid-fl ank vents fed by radial dikes. The axial products are predomi-nantly (77%) calc-alkaline andesites (57–63 wt% SiO2), with lesser (23%) dacites (63–68 wt% SiO2), and a sole rhyodacite pumice fall deposit (<<1%) (Fig. 3). No known fl ows or tephras of basaltic andesite (52–57 wt% SiO2) or basalt (<52 wt% SiO2) erupted through the axial magmatic system. Porphyritic andesites and dacites are the norm, dominated by pheno-crysts of, in decreasing abundance, plagio-

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 Na2O K2O P2O5 Total Latitude LongitudeAxial magmatic systemJVDMAZ 57.75 0.98 18.85 6.30 0.11 3.42 6.97 3.95 1.29 0.37 99.39 46.7802 –121.730095RE462 58.38 1.03 16.89 6.18 0.11 5.11 6.75 3.74 1.57 0.25 99.39 46.7857 –121.748999RE777 58.42 1.01 17.12 6.09 0.11 5.06 6.79 3.59 1.54 0.27 99.95 46.8133 –121.735099ML770 58.74 1.01 16.71 6.05 0.10 5.04 6.76 3.81 1.46 0.31 99.41 46.8817 –121.858393RE41 58.98 1.13 17.29 5.91 0.10 4.22 6.40 4.03 1.67 0.27 99.37 46.8418 –121.727097RE614 59.02 1.04 17.79 6.10 0.10 3.76 6.63 4.05 1.30 0.22 100.46 46.8183 –121.731796RE528 59.39 1.10 16.49 5.99 0.10 3.65 6.41 4.11 2.33 0.43 99.05 46.8217 –121.685094ML318 59.49 0.95 17.29 5.56 0.10 4.08 6.73 4.03 1.53 0.25 98.95 46.8813 –121.783296RE539 59.64 1.11 16.85 5.92 0.10 3.58 6.29 3.87 2.25 0.40 100.20 46.8300 –121.691795SR514 59.68 0.94 17.69 5.75 0.10 3.61 6.56 3.96 1.45 0.25 99.65 46.9017 –121.626293RE4 59.72 0.96 16.89 5.78 0.10 4.46 6.27 3.86 1.71 0.23 99.79 46.7991 –121.733699ML764 60.05 0.85 17.56 5.38 0.10 3.69 6.33 4.10 1.62 0.32 99.68 46.9181 –121.776898RE692P1 60.32 0.95 17.51 5.48 0.09 3.83 6.02 3.97 1.59 0.24 99.41 46.8646 –121.661195RE464 60.69 0.97 17.04 5.36 0.09 2.93 6.14 3.94 2.46 0.39 99.40 46.8121 –121.707593RE26 60.71 0.92 17.31 5.28 0.09 3.55 6.12 4.07 1.69 0.26 99.51 46.8355 –121.729893MW68 60.76 0.94 17.41 5.61 0.10 3.09 6.09 4.14 1.59 0.29 99.44 46.8082 –121.893693MW72 60.63 0.94 17.46 5.61 0.10 3.11 6.12 4.14 1.60 0.29 99.28 46.8079 –121.894895RE494 60.80 0.83 17.96 5.18 0.09 3.07 6.26 4.19 1.39 0.23 99.63 46.8656 –121.691693RE197 60.91 0.81 17.89 5.15 0.09 3.12 6.24 4.07 1.56 0.17 99.57 46.7958 –121.725396RW581 60.93 0.94 17.11 5.41 0.09 3.35 5.99 4.07 1.83 0.26 100.31 46.8250 –121.793395RE506 61.15 0.86 17.64 5.40 0.10 2.71 5.94 4.18 1.72 0.31 99.66 46.8596 –121.7252JV506CL3 60.70 0.87 17.69 5.58 0.10 2.82 6.17 4.11 1.62 0.33 100.12 46.8645 –121.661993RW3 61.58 0.85 18.18 4.73 0.08 2.42 5.96 4.18 1.80 0.23 100.11 46.7706 –121.779393RW177 61.68 0.88 17.19 5.34 0.10 2.90 5.86 4.09 1.69 0.27 98.97 46.8522 –121.753694ML329 61.80 0.80 18.02 4.85 0.08 2.75 5.87 4.17 1.50 0.18 99.89 46.8833 –121.787199GL769 62.12 0.85 17.10 5.17 0.09 3.23 5.48 3.92 1.78 0.26 99.41 46.8788 –121.878593RE53 62.28 0.89 16.69 4.99 0.08 2.78 5.57 3.94 2.45 0.33 99.46 46.8037 –121.695196RW555 62.31 0.87 17.07 4.88 0.08 3.04 5.43 4.06 2.00 0.26 99.64 46.8567 –121.846796RE530 62.64 0.93 16.48 5.03 0.09 2.83 5.26 4.08 2.32 0.32 100.03 46.8267 –121.675094RW275 62.81 0.74 17.84 4.51 0.08 2.44 5.80 4.08 1.53 0.17 99.69 46.8635 –121.811596RW570 62.89 0.86 16.77 5.04 0.09 2.90 5.27 4.09 1.85 0.24 100.09 46.8317 –121.785000RW821 62.93 0.81 17.27 4.49 0.08 2.44 5.45 4.12 2.15 0.24 99.49 46.8044 –121.787893RE39 63.20 0.79 16.86 4.60 0.08 2.78 5.30 4.18 1.98 0.24 99.58 46.8430 –121.729594RW288 63.48 0.62 17.66 4.12 0.07 2.54 5.74 4.09 1.53 0.14 99.54 46.8645 –121.819094RE379 63.59 0.72 17.05 4.67 0.09 2.29 5.24 4.08 2.02 0.26 98.48 46.8453 –121.743200RE849 63.94 0.81 16.37 4.59 0.09 2.60 4.89 4.12 2.29 0.30 98.88 46.8556 –121.702493RE193 63.95 0.72 16.78 4.51 0.08 2.72 4.98 4.29 1.79 0.18 99.62 46.7889 –121.730400RE801 64.14 0.71 16.99 4.27 0.08 2.46 5.07 4.04 2.02 0.22 99.95 46.8287 –121.724593RE120 64.14 0.76 16.63 4.59 0.08 2.46 4.98 4.04 2.11 0.21 98.81 46.8057 –121.746897RE629 64.44 0.63 16.93 4.15 0.07 2.61 5.00 4.31 1.68 0.17 99.86 46.8100 –121.690001RW894 65.33 0.64 16.41 4.01 0.07 2.48 4.63 4.07 2.18 0.18 99.79 46.8197 –121.759293RW87 66.67 0.60 16.12 3.58 0.07 1.91 4.30 3.82 2.76 0.18 96.53 46.8600 –121.780996RW544 66.72 0.63 16.27 3.73 0.07 1.84 4.04 4.28 2.27 0.16 100.31 46.8617 –121.858393RW100 66.83 0.57 16.59 3.54 0.06 1.70 4.15 4.21 2.18 0.17 99.15 46.7987 –121.777194RW282 67.89 0.53 15.89 3.33 0.06 1.71 3.84 4.16 2.45 0.14 99.20 46.8737 –121.792395SR446 69.07 0.38 19.40 2.20 0.05 0.42 1.56 3.88 2.93 0.11 91.71 46.9188 –121.645195SR446* 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 inclusions96RE532 52.74 1.39 17.11 8.41 0.15 7.54 8.34 3.55 0.45 0.33 99.96 47.8233 –122.666793RE191 55.48 0.93 15.69 7.99 0.15 8.26 7.93 2.70 0.69 0.18 99.77 46.7943 –121.742793RE58 55.82 1.22 17.23 7.03 0.12 5.57 8.07 3.55 0.99 0.40 99.15 46.8133 –121.744594ML314 56.87 1.18 16.65 6.27 0.12 6.32 7.79 3.22 1.42 0.15 98.00 46.8901 –121.777598RE675 58.06 1.07 18.21 6.90 0.11 3.83 6.52 3.89 1.18 0.23 99.11 46.8275 –121.666796RW576 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 inclusions93RE16 56.08 1.11 15.99 7.95 0.13 6.88 7.55 2.79 1.34 0.19 99.14 46.7951 –121.711201SR878 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 vents99ML762 55.05 1.06 16.70 6.95 0.12 6.46 8.59 3.39 1.32 0.37 99.59 46.9125 –121.806393ML214 55.24 1.19 17.37 7.43 0.13 6.06 7.52 3.75 1.00 0.30 99.84 46.9034 –121.807594ML444 57.26 1.09 17.53 6.65 0.11 4.71 7.20 3.90 1.30 0.25 99.41 46.9023 –121.818497ML657 58.45 1.13 16.76 5.92 0.10 4.01 6.86 3.79 2.49 0.48 99.71 46.9047 –121.791697ML656 59.15 1.04 16.95 5.78 0.09 4.01 6.57 3.96 2.06 0.39 100.01 46.9245 –121.805393ML98 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 andesites01SB872 49.87 1.25 17.02 9.28 0.16 8.67 10.01 3.02 0.49 0.22 100.90 46.2292 –121.993300ECR836 49.88 1.30 16.66 8.58 0.15 8.75 9.95 3.05 1.23 0.44 99.98 46.3392 –121.741400BL834 50.23 1.35 16.58 8.51 0.15 8.61 9.69 3.12 1.30 0.46 100.48 46.3973 –121.727900OHS831 50.70 1.45 16.09 9.01 0.15 9.30 8.84 3.03 1.00 0.44 99.81 46.6876 –121.541400WP830 50.92 1.28 16.80 9.42 0.16 7.85 9.62 3.15 0.57 0.23 100.42 46.6887 –121.467297BM664 53.51 1.13 15.64 7.67 0.13 8.94 8.04 3.30 1.30 0.34 100.13 47.0917 –121.851501MCP874 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.



clase, orthopyroxene, and clinopyroxene, with microphenocrysts of FeTi oxides and apatite. Amphibole is absent to minor in most ande-sites, becoming abundant in relatively high-K2O andesites, and also in the higher-SiO2 dacites and rhyodacite. Olivine is seen irregularly in hand sample and thin section in rocks with SiO2 as great as 59 wt%, but sparse rounded olivine grains are routinely recovered in mineral sepa-rates from higher-SiO2 samples, probably as relicts from magma mingling. Other minor mineral phases include a Cu-Fe magmatic sul-fi de (typically preserved only where included in phenocrysts), minute zircon grains in the more evolved dacites and rhyodacite, biotite phenocrysts solely in the rhyodacite pumice-fall deposit but as a trace groundmass phase in thick and well-crystallized andesite-dacite lava fl ows, and traces of resorbed phenocrystic quartz in an evolved dacite.

Evidence for magma mingling in the form of fi ne-grained quenched magmatic inclu-sions (enclaves; Bacon, 1986) is widespread in Mount Rainier’s eruptive products. Quenched magmatic inclusions (QMIs) are commonly 4–8 cm across, fi ne grained, sparsely vesicular in their interiors, and contain <10% pheno-crysts of olivine (or orthopyroxene or amphi-bole in some higher-SiO2 QMIs), commonly accompanied by traces of resorbed phenocrysts entrained from the host magma. Compositions of QMIs overlap those of Mount Rainier’s lavas and tephras, but extend the suite to lower SiO2 concentrations (Fig. 3), consisting of andesites (77%) and basaltic andesites (22%), with only one basaltic QMI having been found. Nearly all

Mount Rainier eruptives contain sieve-textured phenocrysts and phenocrysts with overgrown internal resorption surfaces, as are common-place in calc-alkaline andesitic magmas world-wide, recording magma mingling and mixing events (Sakuyama, 1981). Accompanying the complex grains in variable proportions, how-ever, are idiomorphic phenocrysts and micro-phenocrysts with few resorption features. At the extreme are rare lava fl ows of non-porphyritic andesite in which texturally simple non-sieved mineral grains range continuously from ground-mass up to ~0.5 mm. These fl ows result from infrequent eruptions of phenocryst-free ande-sitic liquids, as opposed to crystal-laden ande-sitic magmas whose phenocrysts betray their complex origins. Additional magmatic products are medium-grained inclusions of vuggy gab-bronorite to quartz diorite, up to 20 cm across, that characterize some lava fl ows; U-Pb ages of zircons from these inclusions indicate that they are Pleistocene plutonic products of Mount Rainier’s magmatic system (Sisson et al., 2009). True xenoliths of pre-Quaternary rocks are exceptionally uncommon.

Vents on the north fl ank of Mount Rainier (Fig. 2) erupted magmas distinct from those of the axial magmatic system, consisting of olivine basaltic andesites and amphibole-phenocrystic spessartite, as well as basaltic andesite–spes-sartite hybrids. North-fl ank basaltic andesite compositions plot overlapping the fi eld of QMIs (Fig. 3). These basaltic andesites result from eruptions of some types of mafi c magmas that replenish Mount Rainier’s axial magmatic sys-tem, but that bypassed it by ascending 6–7 km

laterally to its side, whereas basaltic andesites ascending through the axial magmatic system only reach the surface as QMIs after having mingled with the andesites and dacites that dominate the axial system. Six to seven kilome-ters is therefore a maximum limiting estimate of the radius of the axial andesitic system. North-fl ank basaltic andesite compositions also over-lap the high-SiO2 portion of the fi eld of regional basalts and basaltic andesites of the southern Washington Cascades.

Major Oxide Compositions of Mount Rainier–Region Quaternary Volcanic Rocks

Major oxide trends of Mount Rainier’s mag-mas (Fig. 3) are familiar from calc-alkaline andesitic suites worldwide (Gill, 1981; Thorpe, 1982), with simple, near-linear arrays of de creasing concentrations of MgO, CaO, FeO*, and TiO2 with increasing SiO2. Considering only eruptives from Mount Rainier’s axial magmatic system, concentrations of K2O increase continu-ously from basaltic andesite QMIs to andesites to dacites, but with greater K2O diversity at higher SiO2. Defi ning its calc-alkaline character, FeO*/MgO of the Mount Rainier suite is low and increases only modestly with increasing SiO2. Although most of the suite has Mg# [100 • molar Mg/(Mg + ΣFe)] from 65 to 50, absolute MgO concentrations are generally <6 wt%, dissimilar to high-Mg andesite localities of the Cascades, such as Mount Shasta (California) (Grove et al., 2005). Plots of Al2O3 and P2O5 versus SiO2 (Figs. 3 and 4) defi ne fan-shaped fi elds with diverse

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

OiStinU.onelpmaS 2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 Total Latitude LongitudeMiocene93T59 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 –121.7318203004 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 –121.7343203080 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 –121.8093203085 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

Oligocene08MW1016 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–Paleocene08W1012 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 –122.029708GL1015 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 –121.982808GL1014 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 –121.972208WP1008 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 –121.483808CR1010 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 –122.938108KS1011 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

Mesozoic08RR1001 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 –121.173508SB1005 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 –121.294608RR998 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 –121.146108RR999 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 –121.154508SB1003 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

Unknown93RE63 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.

Sisson et al.


SWC B/BA

MA

MSH

46 50 54 58 62 66 7470SiO2 (wt%)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

K2O

(wt%

)

high-K

med-K

low-K

SWC B/BA

MAMSH

46 50 54 58 62 66 7470SiO2 (wt%)

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

FeO

*/M

gO (w

t%)

70

60

50

40

Mg#tholeiitic

calc-alkaline

SWC B/BA

0.0

0.5

1.0

1.5

2.0

2.5

TiO

2 (w

t%)

edifice lava & tephraquenched magmatic inclusionsnorth flank ventsregional mafic(circled points this study)

SWC B/BA14

15

16

17

18

19

20

Al 2O

3 (w

t%)

SWC B/BA

46 50 54 58 62 66 7470SiO2 (wt%)

0

2

4

6

8

10

MgO

(wt%

)

SWC B/BA

46 50 54 58 62 66 7470SiO2 (wt%)

0

2

4

6

8

10

12

CaO

(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 satura-tion 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.



SWC B/BA

MA

MSH

46 50 54 58 62 66 74 7870SiO2 (wt%)

0

25

50

75

100

125

Rb

(ppm

)

edifice lava & tephraquenched magmatic inclusionsnorth flank ventsregional mafic

(circled points this study)pumice ( )-glass ( ) pairs

SWC B/BA

MSH

MA

46 50 54 58 62 66 74 7870SiO2 (wt%)

0

250

500

750

1000

1250

Sr (

ppm

)

SWC B/BA

0.50

0.75

1.00

1.25

Eu/

Eu*

SWC B/BA

MA

MSH5

10

30

20

50

40

Y (p

pm)

SWC B/BA

MA

MSH

MA

MSH0.0

0.1

0.2

0.3

0.4

0.5

0.6P

2O5

(wt%

)

1000 °C950 °C

900 °C

SWC B/BA

MA

MSH50

150200

100

250300350400450

Zr (p

pm)

850 °C

800 °C

750 °C

SWC B/BA

46 50 54 58 62 66 74 7870SiO2 (wt%)

86420

1210

1614

Th/T

a

1820

SWC B/BA

MA MSH

46 50 54 58 62 66 74 7870SiO2 (wt%)

0

1

3

2

5

4

Ba/

Zr

6

7

SWC B/BAMA

MSH0

1520253035

40

Nb

(ppm

)

10

5

SWC B/BA

0

200

400

600

800

1000

1200

Ba

(ppm

)

Figure 4.

Sisson et al.


TAB

LE 3

. TR

AC

E E

LEM

EN

T C

ON

CE

NT

RA

TIO

NS

(P

PM

) O

F M

OU

NT

RA

INIE

R–R

EG

ION

QU

AT

ER

NA

RY

VO

LCA

NIC

RO

CK

S

XR

FIC

PM

S/IN

AA

Sam

ple

no.

Rb

Sr

YZ

rN

bB

aN

iC

uZ

nC

rV

Ga

Pb

Rb

Sr

Cs

Ba

Th

ULa

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

LuY

Zr

Hf

Nb

TaS

cP

bC

oA

xial

con

duit

syst

emJV

DM

AZ

2972

018

166

8.5

339

1534

776

125

227

2873

41.

334

04.

601.

4521

.647

.36.

0924

.34.

841.

514.

150.

603.

480.

681.

730.

251.

520.

2417

167

4.24

7.7

0.50

13.4

7.1

18.9

95R

E46

241

464

2116

211

.338

950

2469

172

132

197

3945

82.

039

15.

231.

7719

.741

.15.

1120

.34.

341.

334.

110.

653.

860.

772.

060.

291.

810.

2920

160

4.13

9.6

0.71

19.0

7.5

–99

RE

777

4146

420

158

10.2

391

4825

6917

112

919

739

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

0.29

1.76

0.28

1915

73.

989.

50.

6918

.16.

624

.499

ML7

7037

543

1614

111

.334

955

3970

133

139

206

3554

30.

835

04.

791.

7018

.838

.44.

7818

.83.

951.

263.

550.

553.

160.

621.

610.

221.

380.

2116

139

3.63

10.7

0.73

17.4

5.4

25.0

93R

E41

4148

319

169

13.0

401

5432

6981

129

198

4049

41.

740

14.

871.

7019

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)



TAB

LE 3

. TR

AC

E E

LEM

EN

T C

ON

CE

NT

RA

TIO

NS

(P

PM

) O

F M

OU

NT

RA

INIE

R–R

EG

ION

QU

AT

ER

NA

RY

VO

LCA

NIC

RO

CK

S ( c

ontin

ued

)

XR

FIC

PM

S/IN

AA

Sam

ple

no.

Rb

Sr

YZ

rN

bB

aN

iC

uZ

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Sisson et al.


concentrations at low SiO2, converging to low and restricted values at high SiO2. Regional basalts and basaltic andesites, including north-fl ank basaltic andesites, overlap the low-SiO2, low-Al2O3 corner of the Mount Rainier fi eld, together defi ning a broadly arched Al2O3-SiO2 array. Concentrations of P2O5 are diverse in regional basalts and basaltic andesites, attain-ing the greatest values in volumetrically minor K-rich shoshonites and absarokites (Leeman et al., 1990, 2005; Conrey et al., 1997; Smith and Leeman, 2005). Similarly, the relatively high-

K north-fl ank spessartite lavas have the highest P2O5 concentrations of magmas erupted in the immediate vicinity of Mount Rainier.

Trace Element Features of Quaternary Volcanic Rocks

Trace element features of Mount Rainier eruptives are also familiar from convergent-margin calc-alkaline suites elsewhere in the Cascades and worldwide, being enriched in light rare earth elements (LREEs), with low

concentrations of Nb and Ta, and somewhat less depleted in Zr, Hf, and Ti, relative to Ba, Rb, K, U, and Th (McKenna, 1994; Stockstill et al., 2002; Sisson and Vallance, 2009). Con-centrations of strongly incompatible trace ele-ments Rb, Ba, U, and Th increase broadly with whole-rock SiO2, with trends that are paralleled by pumice-glass tie lines (Rb and Ba are illus-trated in Fig. 4). Concentrations of plagioclase-compatible Sr and Eu decrease with increasing whole-rock SiO2, also parallel to pumice-glass tie lines. Strontium concentrations are diverse in

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 glSiO2 65.90 74.66 66.55 74.59 61.67 69.23 61.50 69.27 67.61 77.61TiO2 0.62 0.29 0.61 0.30 0.85 0.67 0.87 0.63 0.50 0.19Al2O3 16.40 13.66 16.14 13.72 17.65 15.52 17.86 15.72 16.04 12.46FeO* 3.39 1.38 3.30 1.39 4.43 2.65 4.45 2.52 3.29 0.93MnO 0.07 0.03 0.07 0.03 0.09 0.05 0.09 0.05 0.07 0.03MgO 2.09 0.28 1.93 0.29 2.90 0.88 2.85 0.73 1.71 0.17CaO 4.47 1.49 4.21 1.52 5.89 3.18 5.84 3.19 4.06 1.14Na2O 3.95 3.96 4.00 3.92 4.08 4.34 4.09 4.42 4.27 3.65K2O 2.59 4.03 2.68 4.01 1.74 2.97 1.74 2.99 2.32 3.78P2O5 0.15 0.07 0.15 0.07 0.20 0.22 0.21 0.19 0.13 0.05Total 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 73Sr 399 164 385 166 499 344 496 341 453 146Ba 513 695 534 697 426 653 430 645 553 735Y 14 19 16 18 17 24 18 25 14 12Zr 161 205 165 200 160 268 162 261 141 113Nb 8.6 11.2 8.8 11.3 8.3 13.1 8.1 13.6 4.9 6.5Ni 11 2 10 <2 11 8 11 3 10 <2Cu 7 – 10 – 21 – 28 – 4 –Zn 55 48 52 50 66 53 69 52 52 27Cr 35 2 25 3 37 3 34 3 11 2V 70 17 69 21 102 52 101 45 64 10La 21 30 24 33 19 30 21 31 16 22Ce 45 57 45 61 44 65 45 59 35 37Nd 20 22 21 23 20 28 22 28 16 14Ga 17 16 17 16 20 18 20 19 15 13Pb 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 3.3Rb 71 121 76 121 47 82 47 85 43 76Sr 390 170 377 174 508 339 503 342 449 152Ba 521 732 539 725 429 645 438 652 549 773Y 14.4 17.5 14.7 17.5 16.9 23.5 17.6 23.9 13.4 10.8Zr 156 223 162 219 163 271 164 271 130 123Hf 4.51 6.28 4.68 6.13 4.22 7.15 4.29 7.06 3.54 3.59Nb 9.2 11.3 9.5 11.2 8.7 12.5 8.7 12.3 4.81 5.4Ta 0.84 1.15 0.88 1.14 0.65 0.95 0.67 0.94 0.41 0.56Th 10.9 18.3 11.6 18.3 5.9 10.3 6.1 10.4 6.28 10.8U 3.92 6.46 4.15 6.47 2.04 3.59 2.14 3.68 2.22 3.89La 23.5 31.0 24.3 31.2 20.1 30.0 20.5 30.4 17.36 20.6Ce 45.9 59.6 47.2 59.6 41.6 62.5 42.6 62.9 34.55 39.2Pr 5.33 6.6 5.45 6.6 5.2 7.7 5.3 7.7 4.07 4.2Nd 19.6 23.4 19.9 23.3 20.3 29.2 20.8 29.3 15.10 14.1Sm 3.93 4.47 3.91 4.35 4.27 6.04 4.42 6.08 2.93 2.36Eu 1.07 0.84 1.07 0.81 1.21 1.24 1.30 1.28 0.85 0.44Gd 3.22 3.63 3.30 3.61 3.88 5.15 4.07 5.19 2.63 1.82Tb 0.50 0.58 0.51 0.57 0.58 0.80 0.62 0.81 0.41 0.30Dy 2.87 3.28 2.89 3.28 3.39 4.71 3.59 4.69 2.50 1.79Ho 0.56 0.67 0.57 0.65 0.65 0.91 0.70 0.92 0.52 0.37Er 1.50 1.82 1.52 1.78 1.68 2.40 1.82 2.45 1.38 1.11Tm 0.22 0.27 0.22 0.27 0.24 0.34 0.26 0.35 0.21 0.18Yb 1.38 1.73 1.40 1.71 1.53 2.14 1.64 2.17 1.38 1.25Lu 0.22 0.28 0.22 0.27 0.24 0.33 0.25 0.34 0.23 0.22Sc 8.1 4.2 8.0 4.2 13.6 8.2 13.3 7.7 8.1 2.1Pb 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 48.1735Longitude –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.



TABLE 6. ISOTOPIC ANALYSES OF MOUNT RAINIER–REGION QUATERNARY VOLCANIC ROCKS AND MINERALS

Sample no. 143Nd/144Nd 2sem87Sr/86Sr 2sem

206Pb/204Pb 207Pb/204Pb 208Pb/204Pb δ18OPL δ18OOL δ18OWR δ18OMAGMA

Axial conduit systemJVDMAZ 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 – – 6.399ML770 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 – – 6.196RE528 0.512868 ± 6 0.703920 ± 8 18.9736 15.5986 38.6854 6.29 – – 6.394ML318 0.512889 ± 7 0.703678 ± 8 18.9610 15.5932 38.6703 6.46 – – 6.496RE539 0.512922 ± 5 0.703815 ± 9 18.9549 15.5951 38.6579 6.63 – – 6.695SR514 0.512874 ± 6 0.703698 ± 6 18.9578 15.5961 38.6710 6.39 – – 6.493RE4 0.512896 ± 7 0.703710 ± 7 18.9593 15.5948 38.6742 6.21 – – 6.299ML764 0.512869 ± 8 0.703750 ± 12 18.9406 15.5905 38.6512 6.61 – – 6.698RE692 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 – – 6.293RE26 0.512884 ± 4 0.703715 ± 8 18.9553 15.5948 38.6656 6.24 – – 6.293MW68 0.512878 ± 7 0.703814 ± 10 18.9630 15.5952 38.6645 6.15 – 7.3 6.1

8.56.7–97.5–––––27WM3995RE494 0.512878 ± 5 0.703486 ± 7 18.8963 15.5812 38.5744 6.16 – – 6.293RE197 0.512866 ± 7 0.703720 ± 9 18.9238 15.5880 38.6255 6.51 – – 6.596RW581 0.512880 ± 5 0.703772 ± 7 18.9618 15.5979 38.6866 6.41 – – 6.495RE506 0.512924 ± 6 0.703597 ± 10 18.9165 15.5854 38.6009 6.33 – – 6.3JV506CL3 0.512917 ± 3 0.703561 ± 8 18.9035 15.5840 38.5842 5.69 – – 5.793RW3 0.512881 ± 13 0.703708 ± 6 18.9439 15.5855 38.6447 6.34 – – 6.493RW177 0.512895 ± 7 0.703677 ± 6 18.9380 15.5904 38.6327 5.88 – – 5.994ML329 0.512849 ± 7 0.703891 ± 13 18.9656 15.5957 38.6903 6.81 – – 6.899GL769 0.512874 ± 6 0.703824 ± 7 18.9602 15.5942 38.6695 6.41 – – 6.493RE53 0.512871 ± 3 0.703904 ± 6 18.9626 15.5963 38.6711 6.00 – – 6.096RW555 0.512859 ± 5 0.703917 ± 9 18.9802 15.6024 38.7200 6.37 – – 6.496RE530 0.512913 ± 3 0.703792 ± 9 18.9521 15.5942 38.6585 6.54 – – 6.694RW275 0.512898 ± 3 0.703518 ± 8 18.9254 15.5907 38.6255 6.26 – – 6.396RW570 0.512835 ± 7 0.703790 ± 7 18.9618 15.5962 38.6761 6.25 – – 6.300RW821 0.512874 ± 2 0.703754 ± 6 18.9675 15.6003 38.6958 6.33 – – 6.493RE39 0.512896 ± 8 0.703759 ± 8 18.9636 15.5975 38.6830 6.36 – – 6.494RW288 0.512911 ± 8 0.703434 ± 9 18.8395 15.5701 38.4917 6.41 – – 6.594RE379 0.512906 ± 5 0.703696 ± 19 18.9294 15.5879 38.6224 6.52 – – 6.600RE849 0.512903 ± 11 0.703823 ± 7 18.9552 15.5942 38.6603 7.01 – – 7.193RE193 0.512841 ± 7 0.703992 ± 9 18.9533 15.5900 38.6584 6.26 – – 6.300RE801 0.512864 ± 2 0.703780 ± 6 18.9619 15.5951 38.6759 6.48 – – 6.593RE120 0.512855 ± 4 0.703860 ± 9 18.9499 15.5919 38.6632 6.23 – – 6.397RE629 0.512838 ± 5 0.703999 ± 9 18.9370 15.5930 38.6592 6.51 – – 6.601RW894 0.512847 ± 7 0.703977 ± 7 18.9763 15.6007 38.7083 6.73 – – 6.893RW87 0.512878 ± 6 0.703757 ± 7 18.9647 15.5985 38.6836 6.49 – – 6.696RW544 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 – – 7.395SR446 0.512875 ± 7 0.703797 ± 8 18.9480 15.5965 38.6620 6.36 – – 6.7

Quenched magmatic inclusions96RE532 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 inclusions93RE16 0.512893 ± 12 0.703686 ± 7 18.9363 15.5921 38.6339 – – – –

–––93.5–––––878RS10

North-fl ank vents99ML762 0.512872 ± 7 0.703865 ± 6 18.9739 15.5926 38.6792 6.17 5.58 – 6.093ML214 0.512914 ± 8 0.703546 ± 10 18.9249 15.5815 38.5912 6.47 5.41 – 6.4

4.6–94.65516.830485.515239.817±966307.08±509215.0444LM4997ML657 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 – – 6.993ML98 0.512895 ± 6 0.703591 ± 7 18.9665 15.5969 38.6822 6.25 – – 6.2

Regional Quaternary basalts and basaltic andesites01SB872 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 rocks at U.S. Geological Survey, Denver. δ18OMAGMA 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%.

Sisson et al.


the lower-SiO2 samples, defi ning a fan-shaped array like that of P2O5 versus SiO2, with the highest Sr concentrations in spessartite and in regional K-rich mafi c lavas.

Concentrations of mafi c mineral–compatible elements Ni, Cr, Sc, and V decrease with increas-ing SiO2 and decreasing MgO (not illustrated). As with MgO, the concentrations of these com-patible trace elements do not attain the high val-ues of primitive high-Mg andesitic suites.

Trace elements of intermediate compatibil-ity, or whose compatibility varies strongly with mineral assemblage, such as Y, Zr, and Nb, plot as broad, gently sloping fi elds versus SiO2, with concentrations in Mount Rainier samples gener-ally intermediate between those of Mounts St. Helens and Adams (Fig. 4). Concentrations of Y, Zr, and Nb increase steeply and monotoni-cally with SiO2 for Mount Adams samples, but decrease with SiO2, overall, at Mount St. Helens, albeit with scatter (Fig. 4). Pumice-glass tie lines correlate with mineral assemblage and overall degree of pumice evolution, with glasses from pyroxene andesite and hornblende-pyroxene dacite being enriched in Zr, Nb, Y, and SiO2 rela-tive to their bulk pumices, similar to the Mount Adams suite, but glass in zircon-bearing horn-blende rhyodacite being depleted in Zr and Y, enrichment retarded in Nb, and strongly enriched in SiO2, similar to Mount St. Helens eruptives.

Radiogenic and Oxygen Isotope Variations of Quaternary Volcanic Rocks

Mount Rainier samples have a limited range of radiogenic isotope values (Table 6), defi n-ing an array of decreasing 143Nd/144Nd versus

increasing 206Pb/204Pb and 87Sr/86Sr, and of jointly increasing 206Pb/204Pb and 207Pb/204Pb (Fig. 5). The sole basaltic QMI (sample 96RE532) lies on the projection of these trends but at distinctly higher 143Nd/144Nd and lower 87Sr/86Sr than other Mount Rainier samples. With increasing mag-matic evolution (whole-rock SiO2), 143Nd/144Nd decreases broadly, but Nd isotopic values scat-ter at any SiO2 value, well outside of analytical uncertainty (Fig. 6). Likewise, more evolved Mount Rainier samples have scattered, but overall increasing 206Pb/204Pb and 87Sr/86Sr (not illustrated).

Quaternary mafi c lavas of the Cascades are divisible into “calc-alkaline” types with moderate to strong subduction trace element signatures, “low-K olivine tholeiites” that resemble many back-arc-basin basalts in their low concentrations of incompatible elements (also known as “high-alumina olivine tholei-ites”), and “within plate”–type basalts that are chemically similar to basalts erupted at intra-plate ocean islands (Leeman et al., 1990, 2005; Hart et al., 1984; Bacon et al., 1997; Conrey et al., 1997). Calc-alkaline basalts and basaltic andesites of southern Washington plot over-lapping the high-143Nd/144Nd (generally more mafi c) end of the Mount Rainier array, with discernible elongation along that array (Fig. 5). Andesites and dacites from the Mount Adams volcanic fi eld also plot overlapping the high-143Nd/144Nd end of the Mount Rainier array, but except for a few outliers, defi ne a non-elon-gate cluster (not shown separately). Nearly all low-K tholeiitic and within plate–type basalts of southern Washington have high 143Nd/144Nd and low 87Sr/86Sr, but are substantially diverse in

206Pb/204Pb, unlike Mounts Rainier and Adams and regional calc-alkaline mafi c lavas. Mount Rainier andesite and dacite samples selected for their anomalously low K2O concentrations also have anomalously low 206Pb/204Pb and plot displaced toward a fi eld defi ned by similarly low-K2O andesites and dacites from Mount St. Helens, although the Mount Rainier sample with the lowest 206Pb/204Pb is not low in K2O (99ML770).

Oxygen isotope results for Mount Rainier pla-gioclase phenocrysts range from 5.7‰ to 7.2‰, averaging 6.40‰ ± 0.28‰ (Fig. 6). Plagio clase was also separated from a Quaternary gab-bronorite inclusion, yielding a relatively low δ18O value of 5.4‰, and olivine phenocrysts separated from two north-fl ank basaltic ande-sites have δ18O values of 5.4‰ and 5.6‰, simi-lar to olivine phenocrysts from Mount Adams and from southern Washington Cascades calc-alkaline basalts (Fig. 6). Bulk-magma δ18O val-ues calculated either by the CIPW normative mineral approximation (Eiler, 2001; Bindeman et al., 2004) or from measured or estimated phase compositions (Zhao and Zheng, 2003) give effectively identical results, with bulk-magma estimates slightly less than measured plagioclase values for the most mafi c samples, and slightly greater than measured plagioclase for the most evolved samples (Fig. 6). Measured plagioclase and estimated bulk-magma δ18O val-ues increase modestly with degree of magmatic evolution (SiO2), albeit with signifi cant scatter. Linear regression through the array yields a char-acteristic increase for bulk magmas of ~0.05‰ per wt% SiO2, from a δ18O value of ~6.0‰ at 55 wt% SiO2, and with outliers of up to ±0.7‰.

TABLE 7. ISOTOPIC ANALYSES OF SOUTHWEST WASHINGTON PRE-PLEISTOCENE ROCKS

Sample no. Unit 143Nd/144Nd 2sem87Sr/86Sr 2sem

206Pb/204Pb 207Pb/204Pb 208Pb/204Pb δ18OWR

Miocene93T59 Tatoosh Granodiorite 0.512897 ± 4 0.703870 ± 7 18.9474 15.5883 38.6381 7.4203004 Tatoosh Granodiorite 0.512909 ± 10 0.703719 ± 9 18.9482 15.5795 38.5932 7.8203080 Tatoosh Granodiorite 0.512874 ± 3 0.703905 ± 9 18.9792 15.5850 38.6537 11.0203085 Tatoosh Granodiorite 0.512898 ± 5 0.703933 ± 8 19.0027 15.5870 38.6813 0.4Oligocene08MW1016 Ohanapecosh volcaniclastic 0.512833 ± 3 0.704135 ± 8 18.9112 15.5795 38.5843 8.5

Eocene–Paleocene08W1012 Puget Group arkose 0.512325 ± 3 0.709087 ± 9 19.1070 15.6177 38.8042 16.808GL1015 Puget Group arkose 0.512123 ± 5 0.711182 ± 9 19.2367 15.6687 39.3583 10.408WP1008 Summit Basalt 0.513008 ± 6 0.703424 ± 8 19.5091 15.5846 39.1837 11.608CR1010 Crescent/Siletzia basalt 0.512909 ± 4 0.703414 ± 9 18.8587 15.5374 38.4852 8.308KS1011 Crescent/Siletzia basalt 0.513000 ± 3 0.703478 ± 9 19.0998 15.6338 39.0264 8.1

Mesozoic08RR1001 Russell Ranch arkose 0.512517 ± 3 0.706469 ± 7 19.2442 15.6533 38.9388 13.708SB1005 Russell Ranch arkose 0.512626 ± 4 0.706126 ± 8 19.1456 15.6320 38.7418 14.608RR998 Sheared granodiorite 0.513009 ± 3 0.703349 ± 8 19.6569 15.5888 38.9218 9.208RR999 Greenstone 0.512951 ± 3 0.704107 ± 8 18.4369 15.5526 38.1270 11.908SB1003 Orthogneiss 0.513005 ± 6 0.704022 ± 9 19.0420 15.5389 38.5670 9.7

Xenolith/xenocryst93RE63 Metavolcanic xenolith 0.512890 ± 4 0.703707 ± 9 18.9569 15.5873 38.6456Quartz 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.



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Sisson et al.


Geochemical and Isotopic Results from Basement Rock Samples

The diversity of southwest Washington base-ment rock types is refl ected in their wide-ranging chemical and isotopic compositions (Tables 2, 4, and 7). Most evolved are samples of Eocene Puget Group arkoses with high SiO2 (>80 wt%) and with Nd, Sr, Pb, and O isotopic values con-sistent with a continental interior provenance that includes abundant Precambrian materials. These rocks are similar to Eocene sandstones elsewhere in the U.S. Pacifi c Northwest (Heller et al., 1985) and to continental interior sediments supplied to the Cascadia accretionary complex via the Columbia River (Prytulak et al., 2006). Eocene Puget Group shale has lower SiO2 (63 wt%), but higher concentrations of many petrogenetically indicative trace elements (Rb, Cs, Ba, Zr, etc.). Tectonized Mesozoic arkoses to graywackes exposed in the core of the White Pass anti-clinorium are similar to the Eocene sandstones, but are somewhat less evolved chemically and isotopically. Least evolved are Paleocene–Eocene basalts of Siletzia and the Summit Basalt, as well as Mesozoic greenstone from the White Pass anticlinorium. These basaltic basement rocks have generally high and restricted 143Nd/144Nd, but range widely in 206Pb/204Pb, simi lar to the Quaternary Cascades low-K olivine tholeiites and within plate–type basalts. Unlike Quaternary regional mafi c lavas, the Mesozoic and Cenozoic mafi c basement samples have consistently high bulk-rock δ18O values (8‰–12‰) due to low-temperature alteration. Mesozoic intermediate-composition igneous basement rocks (ortho-gneiss, granodiorite) also have high and restricted 143Nd/144Nd, but have 206Pb/204Pb greater to much greater than Quaternary vol canic rocks of Mount Rainier or elsewhere in the southern Washington Cascades, and have high bulk-rock δ18O values (9‰–10‰). The subduction-generated Miocene Tatoosh Granodiorite and Oligocene Ohana-pecosh Formation samples from the immediate Mount Rainier area are similar to Mount Rain-ier eruptives in Sr, Nd, and Pb isotopes (Table 7), albeit with greater scatter around the Mount Rainier array (Fig. 5). They are distinct, however, in having bulk-rock δ18O values greater to much greater (7.4‰–11‰), and in one instance much less (0.4‰), than Mount Rainier plagioclase or inferred bulk-magma values (Fig. 6).

Remaining basement samples are an atypi-cally large (~1.5 cm) quartz xenocryst with a high δ18O value of 11.7‰ erupted in a regional calc-alkaline basaltic andesite (Canyon Creek locality), and a xenolith of hydrothermally altered volcanic rock of unknown protolith age from a Mount Rainier lava fl ow, with Sr, Nd, and Pb isotopic values similar to Quaternary

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crystal-

fractionation trend

Figure 6. Plots of whole-rock SiO2 concentration versus δ18O and 143Nd/144Nd. Oxygen isotope plot (upper plot) shows Mount Rainier plagioclase phenocryst δ18O values (orange circles ) connected to calculated bulk magma values (vertical lines) where the difference exceeds the symbol size; north-fl ank basaltic andesite olivine pheno crysts (orange diamonds); and gabbronorite plagioclase (orange square). Fields show δ18O values of olivines from regional calc-alkaline basalts and basaltic andesites (CAB, red), low-K tholeiites and within plate–type basalts (LKT-WPB, dark gray), and Mount Adams andesites and dacites (MA, pink). Mesozoic and Cenozoic igneous basement whole-rock samples (IGB, gray) plot mainly off the graph area at high, and in one instance, low, δ18O. Dashed and solid lines are mixing chords to average Puget Group arkose and to model in situ rhyolite, respectively, from representative calc-alkaline basaltic parents (Table 8), with cross-ticks at 10 wt% intervals. Literature data sources are as for previous fi gures. Heavy line shows expected trajectory of progressive crystallization-differentiation from an average Mount Rainier basaltic andesite (Supplemental Item 2 [see footnote 1]). Neodymium isotope plot (lower plot) shows whole-rock determinations for Mount Rainier samples (orange circles), fi elds for other Quaternary volcanic and basement samples, and mixing chords, as defi ned for the upper plot.



Mount Rainier eruptives. Small quartz xeno-crysts (to ~2 mm) are sparse but widespread in calc-alkaline basaltic andesites of the southern Washington Cascades.

INTERPRETATIONS

Isotopic Evidence for an Evolved Crustal Assimilant

The Mount Rainier Sr-Nd-Pb isotopic arrays are elongate toward the values of Mesozoic and Eocene sedimentary basement rocks and signal the incorporation of a component or compo-nents derived from the old continental interior. Subduction of evolved sediments and their processing through primary magmas is another possibility, but the δ18O values of 6‰–7‰ cal-culated for Mount Rainier andesites and dacites indicate assimilation in the crust, as does the rough correspondence between isotopic values and magmatic evolution (SiO2).

Estimating amounts of crustal-level assimila-tion requires specifying representative assimi-lant and unmodifi ed parental magma charac-teristics. Two primary (unmodifi ed) magma approximations employed herein are calc-alka-line basalts with weak and strong subduction trace-element signatures, referred to for conve-nience as low-Sr/Y and high-Sr/Y, respectively (Table 8). These compositions are generalized after many analyses of Cascades basalts and basaltic andesites (data sources in captions of Figs. 3 and 5), and are intended to be illustra-tive, not unique; the magmatic system is prob-ably fed by a continuum of primitive magmas between and beyond these model parents. In isotopic and trace element characteristics, the low-Sr/Y parent plots in the region of overlap between calc-alkaline basalts and the combined fi eld of low-K tholeiites and within-plate basalts (Figs. 5 and 7); it has isotopic values similar to the least radiogenic (and sole basaltic) Mount Rainier QMI. The high-Sr/Y model parent has isotopic characteristics similar to the mid-point

of the calc-alkaline basalt fi eld (Fig. 5), but with trace element abundances designed, in part, to bracket the Mount Rainier suite (e.g., high-Sr/Y, Fig. 7). The fi rst assimilant considered is simply Eocene Puget Group arkose.

Mixing lines from both prospective mafi c par-ents to Eocene Puget Group arkoses trend along the axes of the Mount Rainier 143Nd/144Nd–206Pb/204Pb–87Sr/86Sr–207Pb/204Pb arrays (Fig. 5). For the low-Sr/Y parent, the majority of Mount Rainier samples would require ~10–20 wt% arkose assimilation to match their Sr-Nd-Pb iso-topic values, with the maximum assimilation of ~30 wt%. The high-Sr/Y parental end member has higher (model) concentrations of Sr, Nd, and Pb, so its isotopic values are less sensitive to assimilation, but those isotopic values are also more similar to those of the Mount Rainier suite. These factors result in a wider range of estimated amounts of assimilation, from none up to nearly 30 wt% (Fig. 5), but with most of the samples requiring ≤20 wt% assimilation. Thus, using either parent, typical amounts of assimilation would be inferred as ~10–15 wt%, with maximum amounts of assimilation of less than 30 wt%. Oxygen isotope results give simi-lar estimates for the common extent of assimila-tion. Assimilation of 9–20 wt% arkose would be required to produce the average Mount Rainier andesite-dacite δ18O of 6.4‰, as deduced from their plagioclase phenocrysts, from parental basalt with δ18O of 5.4‰ and measured δ18O of Puget Group sandstones of 10.4‰–16.8‰.

Other basement materials are less successful as predominant assimilants. Tectonized Mesozoic sandstones from the White Pass anti clinorium are not as evolved isotopically as Eocene Puget Group rocks, so greater amounts of their assimi-lation would be required. For the same paren-tal basalts, ~35 wt% assimilation of Mesozoic sandstone would be required to produce the median 143Nd/144Nd of Mount Rainier samples, and 45–55 wt% for the lowest 143Nd/144Nd rock, but assimilation of those amounts would also yield δ18O of 8.3‰–8.9‰ in typical, and

9.4‰–10.2‰ in the most assimilant-modifi ed magmas, greatly exceeding estimated Mount Rainier bulk-magma values. Exposed south-west Washington igneous basement rocks are also unsuitable as major assimilants. Paleo-cene–Eocene basalts and Mesozoic greenstone, orthogneiss, and granitoid have 143Nd/144Nd too high, and 206Pb/204Pb too diverse, to produce the narrow and simple Mount Rainier Sr-Nd-Pb iso-topic arrays.

Oligocene and Miocene arc rocks from the local Mount Rainier area plot along or fl ank the Mount Rainier Sr-Nd-Pb isotopic arrays (Fig. 5). Assimilation of such materials would accordingly be diffi cult to recognize from radio-genic isotopes, but the Oligocene and Miocene samples are generally not more evolved isotopi-cally than the Mount Rainier array, so their pro-gressive assimilation did not cause the Mount Rainier isotopic trend. The Oligocene and Mio-cene samples also have high, and in one instance very low, δ18O (7.4‰–11‰, and 0.4‰), so their large-degree assimilation would produce mag-mas with δ18O generally greater than Mount Rainier’s andesites and dacites, and with sub-stantial scatter. The Oligocene and Miocene rocks are products of subduction-zone mag-matism subsequent to deposition of the Puget Group, so their trend toward low 143Nd/144Nd, high 86Sr/87Sr, and slightly high 206Pb/204Pb prob-ably also results from assimilation of evolved sediments derived from the continental interior, as inferred for the Quaternary magmas.

Crystallization-Differentiation

Although more evolved Mount Rainier samples generally have lower 143Nd/144Nd and higher δ18O, consistent with greater extents of assimilation, whole-rock SiO2 concentrations increase too much, relative to isotopic values, to have resulted predominantly from bulk assimi-lation of sediment or sediment partial melt into basalt (Fig. 6). Bulk assimilation of Puget Group 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 rhyolite87Sr/86 00507.081117.090907.003307.065307.0rS143Nd/144 57215.021215.033215.079215.049215.0dN206Pb/204 520.91042.91011.91039.81009.81bP207Pb/204 016.51076.51026.51085.51485.51bPδ18O (per mil) 0.84.018.614.54.5

0020025120060001)mpp(rS0212610203dN0131974.8bP008515535002005aB00201107521091rZ8272124104aL741015241Y

SiO2 6728581525)%tw(Note: ppm applies to noted and following elements.

Sisson et al.


of isotopic values versus various trace element ratios (Fig. 7). Instead, crystal-melt segregation processes within the Mount Rainier magmatic system are likely to have caused much of the magmatic compositional diversity, but trace ele-ment variations preclude predominantly classi-cal progressive crystal fractionation. Progressive growth and separation of pyroxenes and plagio-clase from mafi c parents, producing andesitic to dacitic daughter liquids, would be expected to increase concentrations of Y, Zr, and Nb in residual liquids, and would cause their incom-patible trace element ratios (e.g., Ba/Zr) to hold nearly constant. Andesitic pumice-glass pairs behave in this fashion, as do basaltic andesites through dacites from Mount Adams (Fig. 4). At Mount St. Helens and Mount Rainier, however, concentrations of those elements increase only

modestly and irregularly (Zr, Nb), or diminish (Y), passing from basaltic andesites to dacites and rhyodacites. A version of in situ crystalli-zation-differentiation in the crustal roots of the magmatic systems, accompanied by mixing between the resulting evolved liquids and the replenishing mafi c to intermediate injections, may account for this trace element behavior, and may be the dominant magmatic differentia-tion process at Mount Rainier, and possibly also Mount St. Helens.

For in situ crystallization-differentiation, portions of a magmatic system solidify to high degrees and then deliver evolved liquids to other less solidifi ed and evolved parts of the system (Langmuir, 1989). The primary signature of in situ differentiation is that chemical effects of advanced crystallization appear in magmas

that are, in other respects, too primitive. As originally proposed, in situ differentiation was applied to a single magma reservoir crystalliz-ing along its margins, but herein it is generalized to encompass a complex magmatic system. For long-lived intermittently fed magmatic systems, like Mount Rainier’s, small magma batches that stall in the crust can solidify quickly to advanced degrees, potentially too fast for pro-gressive separation of crystals from melt. If such intrusions are in the middle and deep crust, however, they may remain close to the solidus due to high ambient temperatures, as well as from heat supplied from earlier and subsequent intrusions. Thermal models of small, deep-crustal intrusions indicates that evolved silicic liquids residual from advanced crystallization persist for long times, and thus, have the high-

LKT-WPB

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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.



est probabilities of extraction (Dufek and Bach-mann, 2010). Pumice-glass pairs show (Fig. 4, Table 5) that at high degrees of melt evolution, Zr becomes compatible due to zircon saturation, Y and the middle to heavy REEs become com-patible probably due to amphibole and apatite saturation, and Nb becomes only weakly incom-patible probably due to incorporation by FeTi oxides (see Supplemental Item 1 in the GSA Data Repository1 for trace element partition-ing values estimated from pumice-glass pairs). Residual liquids at advanced extents of solidifi -cation therefore have high SiO2 concentrations, but can have low concentrations of Zr and Y, and only weak enrichments of Nb. Subsequent magma replenishments transiting such mushy or wholly solidifi ed antecedent intrusions can preferentially incorporate their highly differen-tiated liquids, encompassing both residua from advanced solidifi cation and low-degree partial re-melts, leading to the overall trace element trends observed at Mount Rainier and prob-ably also Mount St. Helens. At times, larger intrusions may form and undergo progressive crystal-fractionation due to slow cooling rates, enriching their evolved magmas in Y, Zr, and Nb (e.g., biotite rhyodacite sample 95SR446), introducing scatter in concentration plots. Pro-gressive crystallization-differentiation may also account for the P compositional distribution that peaks at intermediate SiO2, with a well-defi ned upper SiO2 limit interpretable as an apatite satu-ration surface (Fig. 4).

Smith and Leeman (1993) proposed a similar “braided stream” confi guration for the inter-mittently replenished Mount St. Helens mag-matic system, with andesites forming chiefl y as mixtures between dacite and basalt or basal-tic andesite. A difference, however, is that they interpret the silicic component as due to ana-texis of amphibolites or mid-ocean ridge basalt (MORB)–like rocks in the deep crust, accompa-nied by subordinate continent-derived sediments (Smith and Leeman, 1987), whereas here the evolved component is identifi ed as mainly resid-ual liquids from earlier Mount Rainier magmas that stalled and solidifi ed to advanced degrees. Such evolved residual liquids are the melt com-plements to antecrysts increasingly recognized in magmatic systems (Reid et al., 1997; Hildreth, 2001; Charlier et al., 2005) including Mount St. Helens (Claiborne et al., 2010).

The widespread gain (or loss) of an evolved component that has undergone apatite and zir-

con fractionation is seen in a plot of P/K ver-sus Zr/K (Fig. 8). Fractionation of apatite, along with plagioclase, mafi c silicates, and FeTi oxides, drives liquids to low P/K, with only modest reduction in Zr/K (due to Zr being slightly less incompatible than K). Andesites and dacites from Mount Adams behave in this manner, consistent with progressive crystalliza-tion-differentiation, as do andesite pumice-glass pairs. Zircon fractionation drives liquids to low Zr/K. Arc andesites and dacites routinely con-tain apatite but rarely contain zircon due to high temperatures and insuffi cient Zr concentrations (Fig. 4). Nevertheless, Mount Rainier andesites and dacites defi ne an array of jointly decreas-ing Zr/K and P/K, trending toward rhyolitic glasses that have undergone zircon and apatite crystallization (Fig. 8). Incorporation of such an evolved component thus appears to have been widespread.

Assimilation–Fractional Crystallization (AFC)

As magma assimilates country rock, heat is consumed by increasing the temperature of the assimilated material and by fusing its minerals. Magmatic crystallization chiefl y provides that heat (Bowen, 1928), inspiring chemical-iso-topic-thermodynamic models of joint assimila-tion–fractional crystallization (AFC) (DePaolo, 1981; Bohrson and Spera, 2007). Although widely used, such models require many choices of assimilant composition(s), crystallizing pro-portions, mineral/melt element partitioning, and accessory mineral saturation, making it diffi cult to assess the robustness of results. A conserva-tive AFC model assimilating average Puget Group arkose into the generalized parental calc-alkaline basalts produces Sr-Nd-Pb isotope arrays little different from the two-component (Puget Group–basalt) mixing lines that trend

through the Mount Rainier andesite-dacite suite (Fig. 5). Puget Group AFC would be unsuccess-ful, however, in matching trends of Ba/La and Ba/Zr (Fig. 7) due to the broadly similar values of those element ratios in calc-alkaline parents, Puget Group sedimentary rocks, and the low and similar mineral/melt partitioning for those ele-ments during plagioclase + pyroxene ± olivine crystallization.

Smith and Leeman (1993) showed that similar AFC calculations do not match Ba/La and K/La trends for Mount St. Helens basalts through dacites, but that two-component mix-ing between mafi c and evolved St. Helens magmas is more successful. Numerically, two-component mixing can be considered a special case of AFC wherein the mass assimilation rate (Ṁa) greatly exceeds the mass crystallization rate (Ṁc), or r = Ṁa/Ṁc 1 (DePaolo, 1981). Because latent heats of crystallization and of melting are similar, r 1 would seem to vio-late conservation of energy; however, for in situ differentiation, the material being assimilated is molten, carrying heat residual from its par-ent magma(s), so the latent heat of fusion costs are diminished or absent, reducing the extents to which the replenishing magmas must crystal-lize. Compositional and isotopic variations can therefore more closely approach simple mix-ing arrays.

Combined In Situ Crystallization-Differentiation and Assimilation

A refi ned model wherein evolved residual melts mix with replenishing mafi c parents pro-duces a closer chemical and isotopic match to the Mount Rainier magmatic suite (Fig. 7). In nature, such residual liquids might span from silicic dacite or rhyodacite to rhyolite, depend-ing on local circumstances, but for modeling and illustrative purposes we consider rhyolite.

1GSA Data Repository item 2014027, Oxygen iso-tope fractionation during magmatic differentiation, and trace element partitioning estimated from pum-ice-glass pairs, is available at http:// www .geosociety .org /pubs /ft2014 .htm or by request to editing@ geosociety.org.

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)

apatite-saturatedfractionation

gain or loss of apatite & zircon-saturated evolved liquids

Figure 8. Plot of P/K versus Zr/K for Mount Rainier igne-ous rocks, pumice-glass pairs, and fi elds for other Quaternary volcanic centers of southwest Washington. Symbols and fi elds are as in Figures 3 and 4.

Sisson et al.


Chemical and isotopic features of this evolved component, including high Ba, intermediate Zr, and low Sr and Y concentrations, as well as isotopic values intermediate between paren-tal basalts and continental sediments (Table 8), were estimated by projecting trace element and isotopic trends of Mount Rainier rocks to a rhyolitic SiO2 concentration (76 wt%), by plot-ting trace element/Sr versus 87Sr/86Sr and trace element/Nd versus 143Nd/144Nd, and by trial-and-error simulations of mixing arrays. Because this rhyolite’s composition is estimated, not sampled and analyzed, we refrain from more complex AFC models and focus instead on simple mixing. Isotopic values inferred for the model rhyolite would indicate that mod-est assimilation of sediment or sediment par-tial melt generally accompanies production of in situ liquids. With δ18O of 8‰, this average admixed rhyolite could consist of 30 ± 3 wt% sediment or sediment partial melt, with the remainder being cognate residual liquid from mafi c parents (calculated with sediment δ18O of 14‰ and residual liquid δ18O of 5.8‰–5.4‰). A similar percentage of sediment or sediment partial melt (27 wt%) in the rhyolitic end mem-ber is inferred from Nd isotopes, assuming that the sediment and cognate residual components have equal Nd concentrations, the 143Nd/144Nd of the sediment component matches average Puget Group arkose, and the 143Nd/144Nd of the cognate residual component is similar to the model calc-alkaline basaltic parents (0.51295). Thus, for an in situ differentiation scenario, the rhyolitic component could consist of roughly one-quarter to one-third material derived from continental sediments, with three-quarters to two-thirds derived by advanced differentiation or low-degree partial melting of mafi c intrusions similar to Quaternary calc-alkaline basalts near Mount Rainier.

The majority of Mount Rainier samples plot in the range of 20–40 wt% admixed in situ rhyolite component, with maximum val-ues slightly less than 70 wt% (Figs. 6 and 7). Many samples plot outside the compositional space bracketed by the model mixing lines, probably due in some cases to progressive crystallization-differentiation, to atypically high or low amounts of sediment assimilation, or to distinct magma sources (low 206Pb/204Pb Mount St. Helens–like samples). Although the in situ differentiation scenario implies large amounts of admixed evolved liquid, the amount of sediment-derived component is generally low, consistent with earlier estimates of bulk sedi-ment assimilation. The average magma erupted through Mount Rainier’s axial magmatic system has a SiO2 concentration of 61.7 wt% (Table 1; McKenna, 1994; Stockstill et al., 2002; Sisson

and Vallance, 2009; Sisson, unpublished data), and δ18O of 6.4‰. Mass balance for basalt end members with SiO2 of 50–52 wt%, and a rhyo-lite component with SiO2 of 75 wt% consisting of one-quarter to one-third sediment, indicates 10–15 wt% sediment-derived material in the average Mount Rainier magma. Similarly for oxygen isotopes, parent basalts in the range 5.4‰–5.6‰, and a rhyolite component of 8‰ consisting of one-quarter to one-third sediment, give 8–13 wt% sediment in the average Mount Rainier magma.

These estimates indicate that, while the extents of crustal-level interaction are large for Mount Rainier andesites, most of this interac-tion is with earlier intrusive products of the magmatic system, and the amounts of direct incorporation of old continental materials are low. Extensive interaction with Mesozoic and pre-arc Eocene igneous basement would pro-duce diverse 206Pb/204Pb and high 143Nd/144Nd, not seen in Mount Rainier magmas, and thus appears to be minor. Instead, sedimentary rocks or their melts are the predominant old crustal material assimilated at Mount Rainier, albeit in small amounts, probably due to the great thickness of Puget Group sediments (Stanley et al., 1994), their structural displacement to middle-to-lower crustal depths, the melt fer-tility of shales, and possibly the low density of sediments retarding the ascent of transiting small magma batches (Fig. 9). Antecedent deep intrusions from the Quaternary Mount Rainier system are, however, the chief source of evolved components because those intrusions are the hottest materials in the area, and because they are situated along the pathways of subsequent ascending magmas.

Minimum Intrusive to Extrusive Proportions

The above estimates, while admittedly broad, show that roughly one-third to one-half of the average Mount Rainier magma could be silicic liquid entrained from deep intrusions, and this allows estimates of the minimum intrusive to extrusive proportions. Based on experiments, arc basalts that solidify or re-melt in the deep crust can yield up to 15–25 wt% rhyolitic to rhyodacitic liquid, with amounts varying with basalt bulk composition and oxidation state (Sisson et al., 2005). From these mixing pro-portions and melt-yield fractions, the intrusive portion of the Mount Rainier magmatic system can be estimated as at least 0.7–2.8 times the eruptive volume. Assigning a 10 wt% yield of silicic liquid from deep-crustal mafi c intrusions increases the minimum intrusive-to-extrusive ratio to 3–4.5. An independent estimate comes

from comparing the K2O concentration of 1.76 wt% for average Mount Rainier magma, with K2O of 0.52 wt% for an average of ~400 Quaternary Cascades basalts with MgO ≥8 wt%. Treating K2O as perfectly incompati-ble, and ignoring contributions from old crustal sources quantifi ed as small, gives a minimum intrusive to extrusive ratio of 2.4. Thus, the geo-chemically estimated intrusions are equal to, or up to as much as nearly fi ve times, the erupted mass. True intrusive-to-extrusive proportions

mantle

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34

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4

Figure 9. Schematic cross-section through the Mount Rainier magmatic system. 1—Basaltic infl ux from the mantle. 2—Ponding near the base of the crust with high-pressure differ-entiation creating spessartites and basaltic andesites. 3—Assimilation of continental-interior sediments, or their partial melts, perhaps with magmas ponding at the base of the sedimentary section (Vp < 6.5 km/s). 4—Mixing with evolved interstitial liquids from highly solidifi ed earlier intrusions. 5—Ascent to the side of the axial magmatic system delivering basaltic andesites and spessartites to the surface.



will be greater than these estimates because intrusions will not completely expel their inter-stitial liquids, and because part or all of some magma batches can solidify underground with-out supplying fractionated liquids to magmas that erupt.

Crustal-Level Composition of the Mount Rainier Magmatic System

As modeled, the net magmatic fl ux into the crust would be basaltic, and the average compo-sition of the magmatic system would be basaltic andesite due to modest assimilation of sediment or sediment partial melt. Widespread eruption of basalt across the southern Washington Cas-cades, albeit in small volumes (Hammond and Korosec, 1983; Leeman et al., 1990), as well as eruptions of basalt through the mainly dacitic Mount St. Helens system (Smith and Leeman, 1993) are evidence that basalt is supplied to the roots of stratovolcano systems in southern Washington. For Mount Rainier, however, evi-dence is weak for a net basaltic (sensu stricto) fl ux into the crust, as shown by the absence of basalt erupted from proximal vents, and the near absence of basalt as QMIs. Instead, primary basalts may arrest in the deep crust or upper mantle beneath the mushy andesitic crustal magmatic system where they undergo modest progressive crystallization, producing basaltic andesite differentiates (Fig. 9). Coupled with assimilation, this process indicates that the mean composition of the crustal magmatic sys-tem may therefore be andesitic.

Mount St. Helens–Type Magmas in the Mount Rainier System

Andesites and dacites from Mount St. Helens have distinctly lower K2O concentrations than those from Mounts Rainier and Adams, and are also distinguished by their low 206Pb/204Pb (<18.9). The origin of the low-K2O Mount St. Helens suite is unclear, with options including partial melting of the buried eastern margin of the Siletzia terrane, or by partial melting of MORB-source or MORB-like material in the mantle, subducting slab, or hidden in the crust, in all instances modifi ed by incorpo-ration of sediment components to lower the magmas’ 143Nd/144Nd, increase their 87Sr/87Sr, and slightly increase their 206Pb/204Pb (Halliday et al., 1983; Smith and Leeman, 1987, 1993). Resolving among these possibilities is beyond the scope of this study, but a notable fi nding is that small amounts of Mount St. Helens–like magmas contribute to the Mount Rainier mag-matic system. Mount Rainier samples selected for their lower-than-typical K2O concentrations

are also distinguished by low 206Pb/204Pb and approach Mount St. Helens Sr-Nd-Pb isotopic values. The buried eastern margin of Siletzia lies ~15 km west of Mount Rainier beneath the west Rainier seismic zone and the Carbon River anti-clinorium (Stanley et al., 1995), whereas Mount St. Helens straddles the St. Helens seismic zone identifi ed as overlying Siletzia’s buried eastern margin (Parsons et al., 1998), consistent with lower contributions from Siletzia basement to the Mount Rainier magmatic system. Mount Rainier is also farther from a possible offset in the subducted Juan de Fuca plate imaged at 90 km depth by S-wave tomography (Schmandt and Humphreys, 2010), so subordinate infl u-ence from slab-edge melts is also possible. The Mount Rainier sample with the lowest measured 206Pb/204Pb (andesite 99ML770) is not, however, low in K2O, indicating additional origins for iso-topic outliers.

A Deep, Progressive Fractionation Origin of Mount Rainier’s Spessartites

Spessartites that erupted from the north fl ank of Mount Rainier are distinctive texturally due to their prominent phenocrysts of amphibole but not plagioclase, and are distinctive chemi-cally by their high concentrations of Sr, Ba, Rb, K, Th, P, Zr, LREE, LREE/HREE (ratio of light to heavy rare earth elements), and Sr/Y for their intermediate SiO2. Their high Sr/Y (55–70) and La/Yb (~30) are reminiscent of adakites (Defant and Drummond, 1990), but their high K, Rb, and Ba concentrations are inconsistent with melts from subducted MORB crust. A spessartite (sample 97ML657) and a spessar-tite-basaltic andesite hybrid (97ML656) are also indistinguishable isotopically from ordi-nary amphibole-poor or amphibole-free Mount Rainier andesites (Table 6). Deep progres-sive fractionation of parental Mount Rainier magmas, involving garnet and little or no plagioclase, may account for the spessartites, as interpreted for some adakite-like magmas elsewhere (Macpherson et al., 2006; Rodríguez et al., 2007). The Mount Rainier spessartites are not strongly depleted in Y or Yb, so some additional mineral phase, probably pyroxene, accompanied garnet; nor are the spessartites enriched in Ti, Ta, and Nb commensurate with their enrichments in K, Ba, Rb, Sr, P, and Th, so some Ti-rich phase, such as ilmenite or rutile, would also have to have crystallized. Deep frac-tionation enriched the liquids in H2O, as well as incompatible trace elements, explaining the absence of plagioclase phenocrysts and abun-dance of amphibole phenocrysts. Spessartites have not erupted through Mount Rainier’s axial magmatic system, but a general observation is

that at any SiO2 value, Mount Rainier’s higher-K rocks are more likely to have amphibole phenocrysts and to be relatively enriched in Sr, consistent with spessartites feeding into and mixing with axial magmas.

A Common Southern Washington Cascades Isotopically Primitive Arc End Member

A further notable fi nding of this study is that there is a common and restricted Sr-Nd-Pb isotopic fi eld for Quaternary volcanic rocks of Mounts Adams and Rainier, southwest Wash-ington calc-alkaline mafi c lavas, and Oligo-cene and Miocene arc igneous rocks (Fig. 5). The isotopically primitive (high 143Nd/144Nd, low 87Sr/86Sr) end of this fi eld is distinct from Juan de Fuca MORBs (off scale in Fig. 5), but coincides with the center of the isotopic fi eld defi ned by Quaternary low-K olivine tholeiites and within plate–type basalts from southwest Washington and northwest Oregon. Low-K olivine tholeiites and within plate–type basalts of the Cascades have, at most, weak subduction contributions (Leeman et al., 1990), so their isotopic values can be taken as representative of the ambient upper mantle beneath the U.S. Pacifi c Northwest. Voluminous basalts of the Paleocene–early Eocene Siletzia terrain share this isotopic fi eld (D. Pyle, 2008, personal com-mun.), showing that shallow mantle with such characteristics underlay the region at least as early as the inception of arc magmatism in the Eocene. The Pb isotopic diversity of the U.S. Pacifi c Northwest subarc mantle, as indicated by Quaternary low-K tholeiites and within plate–type basalts, and of Siletzia contrasts with the uniformity of the isotopically primi-tive Mount Rainier–Mount Adams subduction-related end member, and a resolution may be that the primitive subduction-related end mem-ber is an average of the ambient upper mantle beneath the region. Sediment subduction may then account for the displacement of some calc-alkaline basalts toward continental isotopic compositions, but this is diffi cult to distinguish from crustal-level assimilation of Eocene sand-stones and shales, recorded by widespread trace quartz xenocrysts.

CONCLUDING REMARKS

Mounts Rainier, Adams, and St. Helens each produce andesite-dacite series magmas, but by somewhat different processes. Mount Rainier’s magmas show isotopic evidence for variable but modest assimilation of isotopically highly evolved crustal materials. Magmatic evolution chiefl y takes place, however, by in situ fraction-ation and mixing, wherein magma batches stall

Sisson et al.


in the crust along an intrusive plexus beneath the volcano where they crystallize largely or completely; subsequent magmas that ascend through these antecedent intrusions entrain and mix with their residual liquids and partial melts. This intrusive plexus transects a synclinorium where Puget Group sandstones and shales are at middle or deep crustal levels, and therefore are hot, accounting for their assimilation. Inter-mittent, modest magma supply over long time periods leads to the in situ differentiation and mixing style of magmatic evolution.

Mount Adams magmas, in contrast, have little Sr-Nd-Pb isotopic evidence for crustal assimilation, although Os results do record crustal interaction not readily distinguished by other measurements (Jicha et al., 2009). Mount Adams sits atop the southern projection of the White Pass anticlinorium, so Eocene continen-tal interior–derived sedimentary rocks are pres-ent only in the shallow crust, and so are cold and resistant to assimilation, or have been eroded away entirely. Mount Adams magmas lack the broad diversity in 206Pb/204Pb and high δ18O of Mesozoic (meta-)igneous rocks that core the White Pass anticlinorium; Mesozoic mélange basement therefore does not appear to be assim-ilated in signifi cant amounts. Mount Adams magmas also have a different differentiation style in that they become enriched in Zr, Y, and Nb with increasing SiO2, consistent with pro-gressive fractional crystallization of plagioclase and mafi c silicates. Mount Adams is the largest late Pleistocene–Holocene volcanic fi eld along the Cascades arc axis (Hildreth and Lanphere, 1994), so it may be fed by magma batches that are suffi ciently large or frequent that their intru-sions cool slowly, allowing gradual and progres-sive separation of melt from crystals.

Finally, though highly active over the last ~4000 years, Mount St. Helens is a small vol-canic system with prolonged spans of quies-cence (Mullineaux, 1996; Sherrod, 1990; Clynne et al., 2008). With evolution, its mag-mas typically have even lower Nb, Y, and Zr than those from Mount Rainier, consistent with strongly in situ differentiation and mixing of small magma batches. Puget Group sedimentary rocks are exposed to the north of Mount St. Hel-ens in the Morton anticlinorium and are inferred in the subsurface geophysically (Stanley et al., 1994). Crustal interaction has long been recog-nized as accompanying the evolution of Mount St. Helens magmas (Halliday et al., 1983; Smith and Leeman, 1987, 1993). Assimilation of Puget Group sedimentary rocks may account for Mount St. Helens’s isotopic diversity, but its parental magmas also differ from the common parent magma type fueling Mounts Rainier and Adams. Mineral compositions for Mount Hood

andesites also signal mixing between more and less evolved materials (Kent et al., 2010), con-sistent with an in situ differentiation and mix-ing process.

These geologic, geochemical, and isotopic observations highlight some ways that geologic setting and magma supply infl uence the genera-tion of andesite-dacite magmas. High and sus-tained magma supply, and refractory basement rocks, can generate andesite and dacites by straightforward progressive crystallization-dif-ferentiation (Mount Adams). A similar parent, but with lower magma supply and the presence of fertile rocks in the mid-deep crust promotes modest and variable assimilation and generates andesite and dacites mainly though in situ differ-entiation and mixing processes (Mount Rainier ). An even lower and less sustained magma supply leads to an even stronger in situ differentiation and mixing signal (Mount St. Helens).

ACKNOWLEDGMENTS

Chemical analyses of Mount Rainier whole-rocks were performed by the late Dave Siems (USGS–XRF), by Jim Budahn (USGS–INAA), and by Diane John-son, Rick Conrey, and Charles Knapp (WSU–XRF and ICPMS). Bob Rye provided whole-rock oxy gen isotope analyses, David Pyle shared unpublished iso-topic results for the Siletzia terrain, James Vallance provided some Mount Rainier samples, and Ilya Bin-deman provided suggestions on modeling oxygen isotope fractionation. Eric Bard, David Zimbelman, Liz Schermer, David Lewis, and Steven Sherostsky accompanied Tom Sisson in the fi eld. Manuscript reviews by Michelle Coombs, Bill Leeman, Mike Dungan, and an anonymous reviewer, and additional comments and editorial assistance by Wes Hildreth, Nancy Riggs, and Michael Ort, are appreciated.

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ASSOCIATE EDITOR: MICHAEL ORT

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Petrogenesis of Mount Rainier andesite: Magma ﬂ …122 Petrogenesis of Mount Rainier andesite: Magma ﬂ ux and geologic controls on the contrasting differentiation styles at stratovolcanoes