The 3.1 Ma to 100 ka evolution of the Goat Rocks Volcanic Complex: Persistence and evolution of magmatism at a long-lived major andesite locus on the Cascades Arc

Wall, Kellie T.

PhD Dissertation Abstract: The imposing andesite stratovolcano is the characteristic expression of subduction zone magmatism, posing hazards to coastal populations and bearing insight into deep Earth processes. On a map of a typical volcanic arc, one can easily distinguish the approximately linear alignment and regular spacing of these major edifices that stand out from a diffuse distribution of mafic volcanoes (e.g. the Quaternary Cascades; Hildreth, 2007). The andesitic composite volcanoes have a reputation for being complex, open systems: crystal zoning “stratigraphies,” diverse crystal cargoes including antecrysts or xenocrysts, quenched magmatic inclusions, and variations in isotopic signatures are among the many lines of evidence that these systems involve a variety of igneous processes and melt sources. To investigate the development and evolution of such transcrustal magma factories, I have conducted a detailed temporal, spatial, and geochemical characterization of a long-lived arc volcanic center in the southern Washington Cascades, the Goat Rocks volcanic complex. Results from ⁴⁰Ar/³⁹Ar and U/Pb geochronology constrain the lifespan of the Goat Rocks volcanic complex from ~3.1 Ma to ~100 ka. During this time, four major composite volcanoes were built (as well as several smaller volcanoes). From oldest to youngest, these are Tieton Peak, Bear Creek Mountain, Lake Creek volcano, and Old Snowy Mountain. Four volcanic stages are defined based on the lifespans of these centers and distinct compositional changes that occur from one to the next: Tieton Peak stage (3.1-2.6 Ma), Bear Creek Mountain stage (1.6-1.1 Ma), Lake Creek stage (1.1 Ma to 456 ka), and Old Snowy Mountain stage (440 ka to 115 ka). Two lava flow remnants also have ages in the interim between Tieton Peak stage and Bear Creek Mountain stage (2.3 Ma and 2.1 Ma), and their sources are not yet identified. The ages of the Bear Creek Mountain and Lake Creek stages in fact overlap, and the gap between Lake Creek stage and Old Snowy Mountain stage is only on the order of 10⁴ years. Based on supporting compositional evidence, the Bear Creek Mountain, Lake Creek, and Old Snowy Mountain stage volcanoes are considered to be the migrating surface expressions of a continuous magmatic system that was active over at least ~1.5 million years. It remains uncertain whether the gaps between the Tieton Peak stage, scattered early Pleistocene andesites, and Bear Creek Mountain stage are due to incomplete exposure/sampling or real quiescent periods earlier in the development of the Goat Rocks volcanic complex. Throughout the construction of the andesitic complex, mafic volcanoes were active on its periphery. These include the Miriam Creek volcano (~3.6-3.1 Ma), Devils Washbasin volcano (3.0-2.7 Ma), Hogback Mountain (1.1 Ma – 891 ka), Lakeview Mountain (194 ka), and Walupt Lake volcano (65 ka). Two basalt and basaltic andesite units (Qob₁ and Qob₂, 1.4 and 1.3 Ma; Hammond, 2017) also erupted from the Goat Rocks area, likely an older incarnation of Hogback Mountain. The suite of mafic magmas erupted in this region are all calcalkaline basalt (or basaltic andesite; CAB), but two compositional groups emerge from the trace element and isotopic data. Group 1 is LILE and LREE-enriched, with higher ⁸⁷Sr/⁸⁶Sr isotopes, and includes compositions from Devils Washbasin, Lower Hogback Mountain, and Lakeview Mountain. Group 2 is less enriched in LILE and LREE and lower in ⁸⁷Sr/⁸⁶Sr, and includes the compositions of Miriam Creek, Qob1, Upper Hogback Mountain, Walupt Lake, and Coleman Weedpatch. The two groups are recurrent through time and with no geographic distinction; in fact, both types were tapped by the Hogback Mountain volcano. Together both of these groups, alongside CABs from Mount Adams and various basalts from Mount St. Helens, form a compositional array between the basalts of the High Cascades and the intraplate-type basalts (IPB) of Mount Adams and Simcoe volcanic field. These results lead to three conclusions. 1) Variably subduction-modified mantle is distributed across the region, perhaps either as stratified layers or a web-like network of fluid pathways amongst less metasomatized mantle. 2) Transitional compositions between the IPBs and typical “High Cascades” CAB/HAOT signature suggest a broader influence of the mantle domain that feeds IPBs—if asthenospheric mantle through a slab window, as suggested by Mullen et al. (2017), then perhaps it bleeds in smaller quantities over a broader area. This compositional trend solidifies the interpretation of the southern Washington Cascades as a unique and coherent “segment” of the arc (the Washington segment of Pitcher and Kent, 2019). 3) The recurrence of variable mafic magma types through time, and with no geographic boundaries, indicates that the compositional evolution of the Goat Rocks volcanic complex was not likely driven by a change in mafic input. Indeed, the Sr, Nd, Hf, and Pb isotope ratios of the intermediate to felsic suite are closely aligned with the local basalts and suggest a limited role of crustal assimilation. Importantly, several mineral thermometers (zircon, ilmenite-magnetite pairs, and amphibole) align in recording higher crystallization temperatures in Bear Creek Mountain to early Lake Creek time, a cooling trend through the Lake Creek stage, and a more diverse range of temperatures in the transition to Old Snowy Mountain stage. Thus, it is suggested that the compositional evolution at Goat Rocks represents a thermal cycle of waxing and waning magmatic flux: where the period of Bear Creek Mountain to early Lake Creek volcanism was the climactic phase of a vertically extensive magma homogenization factory, then the system waned and cooled, ultimately losing its ability to filter, homogenize, and enrich magmas.

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