The explosive phonolitic volcanism of Petite-Terre (Mayotte, Western Indian Ocean)

Abstract

Abstract We describe four Quaternary volcanic phonolitic explosive edifices containing mantle xenoliths on Petite-Terre Island (Mayotte, Comoros Archipelago, Western Indian Ocean) to quantifying magma fragmentation processes and eruptive dynamics. Petite-Terre explosive volcanism is the westernmost subaerial expression of a 60 km volcanic chain, whose eastern submarine tip has been the site of the 2018–2021 sub-marine eruption which saw the birth of a new volcano, Fani Maoré. The scattered recent volcanic activity and the persistence of deep seismic activity along the volcanic chain requires to constrain the origin of past activity as a proxy of possible future volcanic activity on land. Through geomorphology, stratigraphy, grain size and componentry data we show that Petite-Terre tuff rings and tuff cones are likely formed by several closely spaced eruptions forming a monogenetic volcanic complex. The eruptive sequences are composed of few, relatively thin (cm-dm) coarse and lithic rich pumice fallout layers containing abundant ballistic clasts, and fine-ash rich deposits mostly emplaced by dilute pyroclastic density current (PDCs). All deposits are dominated by vesiculated, juvenile (pumice clasts, dense clasts, and obsidian) and non-juvenile clasts from older mafic scoria cones, coral reef and the volcanic shield of Mayotte as well as mantle xenoliths. We conclude that phonolitic magma ascended directly and rapidly from the mantle and first experienced a purely magmatic fragmentation at depth (≈ 1 km deep). The fragmented pyroclasts underwent a second shallower hydromagmatic, fragmentation where they interacted with liquid water, producing fine ash and building the tuff ring and tuff cone morphologies.