Petrogenesis of Mount Rainier andesite: Magma fl ux and geologic controls on the contrasting differentiation styles at stratovolcanoes of the southern Washington Cascades T.W. Sisson1,†, V.J.M. Salters2,†, and P.B. Larson3,† 1U.S. Geological Survey, 345 Middlefi eld Road, Menlo Park, California 94025, USA 2National High Magnetic Field Laboratory, Isotope Geochemistry, and Department of Geological Sciences, Florida State University, 1800 E. Paul Dirac Drive, Tallahassee, Florida 32310, USA 3Department of Geology, Washington State University, Pullman, Washington 99164, USA ABSTRACT eral saturation. Subsequent magmas ascend- routinely carry features indicative of magma ing through the intrusive plexus entrain and mingling and mixing, and that defi ne scattered Quaternary Mount Rainier (Washington, mix with the residual liquids and low-degree whole-rock compositional arrays permissive of USA) of the Cascades magmatic arc consists re-melts of those antecedent intrusions, pro- many explanations. Radiogenic and stable iso- of porphyritic calc-alkaline andesites and ducing hybrid andesites and dacites. Mount tope measurements can be more discriminating, subordinate dacites, with common evidence St. Helens volcanic rocks have geochemical allowing for quantitative estimates of the mag- for mingling and mixing with less evolved similarities to those at Mount Rainier, and nitudes of different processes and components, magmas encompassing andesites, basaltic may also result from in situ differentiation but their successful use depends on suffi ciently andesites, and rarely, basalts. Basaltic ande- and mixing due to low and intermittent long- large and representative sample suites, com- sites and amphibole andesites (spessartites) term magma supply, accompanied by modest prehensive major and trace element analyses, that erupted from vents at the north foot crustal assimilation. Andesites and dacites of adequately precise isotopic measurements, and of the volcano represent some of Mount Mount Adams isotopically overlap the least well-understood and well-characterized geo- Rainier’s immediate parents and overlap in contaminated Mount Rainier magmas and logic settings. composition with regional basalts and basal- derive from similar parental magma types, Here we present results of a combined geo- tic andesites. Geochemical (major and trace but have trace element variations more con- chemical, isotopic, and geologic study of the elements) and isotopic (Sr, Nd, Pb, O) com- sistent with progressive crystallization-dif- origins of andesite series magmas from Mount positions of Mount Rainier andesites and da- ferentiation, probably due to higher magma Rainier, Washington State, in the Cascades cites are consistent with modest assimilation fl uxes leading to slower crystallization of magmatic arc of western North America. The (typically ≤20 wt%) of evolved sediment or large magma batches, allowing time for pro- study benefi ts from abundant samples col- sediment partial melt. Sandstones and shales gressive separation of minerals from melt. lected during geologic mapping of the volcano, of the Eocene Puget Group, derived from the Mount Adams also sits atop the southern sparser sampling of small-volume Quaternary continental interior, are exposed in regional projection of a regional anticlinorium, so Eo- mafi c volcanic rocks erupted across southwest anticlines fl anking the volcano, and prob- cene sediments are absent, or are at shallow Washington, as well as representative samples ably underlie it in the middle to lower crust, crustal levels, and so are cold and diffi cult to of pre-Quaternary basement rocks. The study accounting for their assimilation. Mesozoic assimilate. Differences between southwest also benefi ts from improvements in the ease and Cenozoic igneous basement rocks are Washington stratovolcanoes highlight some and precision of isotopic analyses that allow unsuitable as assimilants due to their high ways that crustal geology and magma fl ux effi cient characterization of chemically ordinary 143Nd/144Nd, diverse 206Pb/204Pb, and gener- are primary factors in andesite generation. and atypical samples. Our new data indicate that ally high δ18O. Mount Rainier’s magmas incorporated small The dominant cause of magmatic evolu- INTRODUCTION but variable amounts (typically ≤20 wt%, but tion at Mount Rainier, however, is inferred up to 30 wt%) of evolved sedimentary rocks, or to be a version of in situ crystallization-dif- Andesite series magmas have been explained their partial melts, known to be present in the ferentiation and mixing (Langmuir, 1989) as products of basaltic crystallization-differ- middle or lower crust, but that the predominant wherein small magma batches stall as crustal entiation, as partial melts of the deep crust or cause of magmatic diversity is multi-stage in intrusions and solidify extensively, yielding subducting slabs, and by composite scenarios situ crystallization-differentiation and mixing. silicic residual liquids with trace element involving crystallization, assimilation, mix- This process involves magmatic replenish- concentrations infl uenced by accessory min- ing, and in certain cases, reaction with or direct ments incorporating advanced differentiates or derivation from mantle peridotite (Gill, 1981; low-degree partial melts from earlier magmatic †E-mail: [email protected] (Sisson, corresponding); DePaolo, 1981; Kelemen, 1990; Grove et al., pulses that stalled and nearly or completely [email protected] (Salters); [email protected] 2005). The multiplicity of interpretations stems, solidifi ed. Because geology and structure infl u- (Larson). in part, from the complexity of the rocks that ence the course of magmatic evolution at Mount GSA Bulletin; January/February 2014; v. 126; no. 1/2; p. 122–144; doi:10.1130/B30852.1; 9 fi gures; 8 tables; Data Repository item 2014027. 122 For permission to copy, contact [email protected] © 2013 Geological Society of America Petrogenesis of Mount Rainier andesite Rainier, and the other major volcanoes of the 126°W 124° 122° 120° region, we fi rst summarize the tectonic set- ting and geologic development of the southern pre-Cenozoic Washington Cascades. basement 48°N Pacific Ocean GP TECTONIC SETTING Olympic accretion S Active volcanoes of the Cascade Range are complex products of northeasterly directed subduction of the oceanic Juan de Fuca plate beneath North America. The Juan de Fuca spreading ridge T lies only 250–450 km from the volcanic-arc 47° axis, along the direction of convergence, with the result that the subducting slab is one of the MR youngest and hottest worldwide (Hyndman and Columbia River Wang, 1993; Syracuse et al., 2010). In its south- flood basalts ern portion, in northern California and southern MA Oregon, the arc is impinged upon from the east 46° by the Basin and Range extensional province. In MSH the north, westward defl ection of the continental Juan de Siletzia ca Plate margin along Vancouver Island, British Colum- Fu basaltic bia (Canada), arches the slab beneath central terrain P and northern Washington State. Consequently, 50 km ~4.3 cm/y MH cross-arc strain (McCaffrey et al., 2007) passes from neutral or slightly extensional in the south Figure 1. Regional geologic setting of Mount Rainier volcano, simplifi ed from Schuster to increasingly convergent moving northward, (2005) and Walker and MacLeod (1991). Geologic units are: chiefl y Mesozoic and Paleozoic culminating with arc-normal convergence atop igneous and metamorphic rocks (light blue; Tertiary plutons omitted for clarity); Paleo- the arch in the subducting slab, as marked by cene–middle Eocene submarine basalts of the Siletzia terrain (purple); middle Eocene and uplift of the Olympic and North Cascades moun- younger sandstones and shales (light yellow); Eocene–Miocene marine sedimentary rocks tains. Volcanic output tracks these changes, of the Olympic accretionary complex (gray); chiefl y Oligocene–Miocene arc igneous rocks diminishing northward as the hanging wall of (green); Miocene fl ood basalts (brown); Quaternary arc volcanic rocks (red); and Quater- the arc becomes increasingly compressional. nary sediments, chiefl y glacial (stippled). Medium and heavy black lines are faults. Triangles Mount Rainier is situated within the transition show the locations of Mount Rainier (MR), Mount Adams (MA), Mount St. Helens (MSH), from widespread diffuse mafi c volcanism in the Mount Hood (MH), and Glacier Peak (GP); Mount Baker plots outside the map area to south to widespread basement uplift and negli- the north. Circles show the cities of Portland, Oregon (P), and Tacoma (T) and Seattle (S), gible mafi c volcanism in the north. Washington. Blue dash-dot line is Columbia River. Yellow lines show the axes of the St. Helens and west Rainier seismic zones, coincident with anticlinal exposures of Eocene sedi- GEOLOGIC SETTING mentary rocks that may mark the buried eastern margin of the Siletzia terrain. Arrow in the Pacifi c Ocean shows the convergence direction and velocity of the Juan de Fuca plate Regional Cascades Geology relative to the North American plate interior (McCaffrey et al., 2007). The geologic framework of the U.S. Pacifi c Northwest is important for understanding Cas- line core to the north of Mount Rainier (Frizzell pecosh Formation, overlain unconformably in cades arc magmatism due to potential assimi- et al., 1987). Quartz-rich sands and silts from the Mount Rainier region by silicic ash-fl ow lation and crustal melting. Regional crustal the North American continental interior spread tuffs