[S-VC25] International Volcanology

[SVC25-P04] The petrology and geochemistry of the high-Fo olivine-bearing mafic magma of the Conos de Licto, Northern Andean Volcanic Zone, Ecuador

*Derek James Weller¹, Chris Conway¹, Akihiko Tomiya¹, Kurumi Iwahashi¹, Nobuo Geshi¹, Yumiko Harigane¹ (1.National Institute of Advanced Industrial Science and Technology, Geologic Survey of Japan)

Keywords:Ecuador, olivine, mantle, diffusion chronometry, melt inclusions

The Conos de Licto are a group of three cones (Cerro Tulabug, Loma Bellavista, and Licto) in the southwestern sector of the inter-Andean valley of the Chimborazo Provence of Ecuador. Cerro Tulabug is the largest of the cones, and is composed of Quaternary scorias and associated lava flows that are predominately basaltic-andesite to andesite in composition. Samples of Tulabug were collected from a coarse interbedded and intercalated ash and scoria unit and a stratigraphically younger massive scoria. Both deposits contain abundant high-forsterite olivine phenocrysts with chromium-rich spinel and glassy melt inclusions. The groundmass matrix glass for each deposit contains microphenocrysts of plagioclase, olivine, and clinopyroxene. The whole-rock products of Tulabug are subalkaline basalts and basaltic-andesites while the olivine-hosted melt inclusions span a wider compositional range from basalt to andesite. The morphology of the olivines are typically euhedral to subhedral and they often have skeletal growth protrusions. The olivines have normally zoned or oscillatory zoned core-to-rim variations in forsterite and nickel concentrations. We have utilized the major and minor element compositions of the olivine phenocrysts and their hosted melt inclusions to infer mantle source characteristics in the subarc mantle beneath the southern portion of the Northern Andean Volcanic Zone. Diffusion chronometry techniques were applied to constrain magma ascent and magma mixing to eruption timescales. Mineral textures and major and minor element chemical maps of the phenocrysts have revealed their complex growth, ascent, and storage histories. The results of this work help further our understanding of small-volume mafic volcanic systems in a continental arc setting.