| Summary: | Master of Science Department of Geology Matthew E Brueseke The Maclaren River volcanic field (MRVF) of south-central Alaska consists of at least three monogenetic volcanoes that lie along the projection of the ca. ~1 Ma Yakutat slab tear. These volcanoes, which are <1 Ma. lie along a ~20 km south-north transect within the Denali Gap, a region between the Aleutian and Wrangell Arcs previously considered to be amagmatic. In this study, I present results from geologic mapping, thin section petrographic analysis, whole rock major and trace element geochemical analysis, and Sr-Nd-Pb-Mg-B isotopic analyses of lavas and pyroclastic deposits of three MRVF volcanoes to document their physical volcanology and petrogenesis. Maclaren River volcanic field eruptive products (e.g., lavas and near-vent pyroclastic deposits) range from alkali basalt to subalkaline basaltic andesite and transitional basaltic trachyandesite/trachyandesite (~46-57 wt% SiO2). The studied volcanoes are named #1, #2, #3, from north to south. Volcano #1 (ca. 958 ka) lavas are porphyritic and contain olivine phenocrysts in a groundmass of clinopyroxene + plagioclase ± sparse biotite microlites. Volcano #2 lavas and scoria are porphyritic and their mineralogy is indicative of a more hydrated mantle source (abundant phlogopite + amphibole + olivine + clinopyroxene phenocrysts) and show evidence of crustal interaction (granitic xenoliths). Volcano #3 is located ~9 km south of volcano #2, but erupted coevally at ca. 422 ka. At volcano #3, the rocks consist of olivine + clinopyroxene + plagioclase + opacitic amphibole and show evidence of crustal interaction (plagioclase + quartz xenocrysts). Sr-Nd-Pb isotopic analysis was conducted on a subset of samples from each MRVF volcano via thermal ionization mass spectrometry and multicollector inductively coupled plasma mass spectometry. 87Sr/86Sri values range from 0.70343-0.70401 and are similar to modern Aleutian and Wrangell Arc magmas and the Holocene, Buzzard Creek maars. This similarity is also ...
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