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

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 andesites 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 basaltic andesites. Geochemical (major and trace elements) and isotopic (Sr, Nd, Pb, O) compositions of Mount Rainier andesites and dacites 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 probably 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 143 Nd/ 144 Nd, diverse 206 Pb/ 204 Pb, and generally high δ 18 O.The dominant cause of magmatic evolution at Mount Rainier, however, is inferred to be a version of in situ crystallization-differentiation 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 ascending through the intrusive plexus entrain and mix with the residual liquids and low-degree re-melts of those antecedent intrusions, producing 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 longterm 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 consistent with progressive crystallization-differentiation, probably due to higher magma fl uxes leading to slower crystallization of large magma batches, allowing time for progressive separation of minerals from melt. Mount Adams also sits atop the southern projection of a regional anticlinorium, so Eocene 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. 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); Paleocene-middle Eocene submarine basalts of the Siletzia terrain (purple); middle Eocene and younger sandst...

Magmatic arc structure around Mount Rainier, WA, from the joint inversion of receiver functions and surface wave dispersion

“… Comparison between observed crustal S wave velocities and those predicted for the composition of rock samples from basement rocks collected around Mount Rainier and described in Sisson et al . []. For each group, black lines show the average, minimum, and maximum V S obtained from the joint inversion at each depth.…”

“…These seismic anomalies near the volcanic centers could be explained by a lower crust composed of a mushy andesitic crustal magmatic system and solidified intrusion along the volcanic conduits as suggested by Sisson et al . []. High lower‐crustal velocities (V S = 4.1 km/s) at station N120 are more enigmatic, as this station is located away from Rainier.…”

“…The main target of this study is Mount Rainier, a stratovolcano located at a transition between widespread diffuse mafic magmatism to the south and negligible mafic volcanism to the north [ Schmidt et al ., ]. Mount Rainier axial products are mostly calc‐alkaline andesite, and dacite to a lesser extent [ Hildreth , ; Sisson et al ., ]. Basaltic lavas were also documented up to 30 km to the north at the Three Sisters and Dalles Lakes vents [ Reiners et al ., ].…”

Shear Velocity Structure From Ambient Noise and Teleseismic Surface Wave Tomography in the Cascades Around Mount St. Helens

“…The basement east of Siletzia is an amalgam of older terranes of variable affinities (Coney et al, ), including mélange belts and roots of continental arcs with generally intermediate to felsic compositions (e.g., Valley et al, ). Contacts between Siletzia and these inboard terranes are partly buried by Eocene Puget Group sediments and Eocene‐to‐present Cascades volcanic and plutonic rocks (Sisson et al, ). As a result, the eastern boundary of Siletzia is not observed geologically; its crust is thought to terminate just west of MSH based on magnetic anomalies (Wells et al, ), although mantle tomography indicates that Siletzia mantle lithosphere may continue to eastern Washington (Schmandt & Humphreys, ).…”

“…White circles: seismic stations used in this study; black crosses: Quaternary vents (Hildreth, ); black line: outline of Siletzia (Wells et al, ); light blue lines: contours of subducting slab labeled in kilometers (McCrory et al, ); yellow boxes: areas of Vs averages in Figures a and b. Solid fields show simplified geology from Sisson et al (): (green) Tertiary volcanics, (tan) Eocene sandstones and silts, (purple) Siletz basalts and gabbros, (brown) Columbia River flood basalts, (red) Quaternary volcanics, (light blue) pre‐Cenozoic basement, (stippled white) Puget lowland. Other labeled features: Chehalis Basin (CB), Spud Mountain (SM) pluton, Spirit Lake (SL) pluton, and Silver Star (SS) pluton; Indian Heaven (IH) volcanic field.…”