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

Abstract

We describe four Quaternary volcanic phonolitic explosive edi ces 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 ne-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 ma c 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 rst 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 ne ash and building the tuff ring and tuff cone morphologies.