Undergraduate Thesis Or Project

 

Constraining Upper Crustal Magmas in the North Chilean Andes Public Deposited

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https://ir.library.oregonstate.edu/concern/undergraduate_thesis_or_projects/t722hh22h

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  • The 4.59-4.18 Ma Caspana ignimbrite, found within the Altiplano-Puna Volcanic Complex (APVC), is the result of an ~8km3 eruption that took place during the Neogene Ignimbrite Flare-up in the N. Chilean Andes. While most of the eruptions during this flare up are large, monogenetic eruptions, the Caspana that is characterized by a compositional gap of ~16% SiO2 between the parental andesite and daughter rhyolite. The rhyolitic plinian fallout is distinct in composition from the majority rhyolite but is also the result of fractionation from andesite. The andesitic magma in the Caspana system are shown to represent some of the most mafic magma that was recharging the upper crustal chambers in the APVC, though all the andesites seem to share a similar signature. Depth estimates and spatial locality place the chamber at the edge of the large thermal anomaly known as the Altiplano-Puna Magma Body (APMB), where cooling would allow for dense solidification during the waning stages of mantle flux. During an initial period of cooling, the system rapidly fractionated along a shallow cotectic and produced the first of two rhyolites. The system was remobilized following recharge and renewed convection produced the second of two rhyolites and fractionation models are shown for both crystallization sequences. The bulk rhyolitic pumice contains fayalite, amongst other Fe-rich minerals, that indicate low Fe3+/Fe2+. Oxidation state in the rhyolite was likely due to the crystallization of orthopyroxene in the presence of magnetite in the parental andesite. With these two phases crystallizing together without clinopyroxene, the latter of which is common in other APVC magmas, the oxidation state in the daughter can be lowered without another method of reduction.
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