Pliocene-to-Holocene volcano-tectonic activity on Mohéli Island (Comoros archipelago) constrained by new K[sbnd]Ar ages

Abstract

The volcanism of the Comoros archipelago (from west to east: Grande Comore, Mohéli, Anjouan, and Mayotte islands) has been under renewed scientific scrutiny since the eruption of a new submarine volcanic edifice (Fani Maoré) which begun in May 2018 east of Mayotte. West of Mayotte, Mohéli island has received little attention from the geoscience community, despite its largely uneroded volcanic landscapes suggesting a recent activity that has not been dated. Here we address this gap in knowledge by applying KAr geochronology on groundmass and major-trace element analyses to subaerial and submarine rocks, in an attempt to reconstruct the volcano-tectonic and geochemical evolution of Mohéli. Our results show that Mohéli's volcano-tectonic evolution proceeded in two constructional stages, including a primary edification along a N070°E axis from before ca. 3.8 Ma to ca. 3 Ma (Stage 1), and a second construction along a N110°E ridge shaping the present-day island since ca. 2 Ma (Stage 2). The two stages were separated by an increased subsidence (≥0.2 mm/yr) that drowned a large part of the N070°E primary edifice. The two identified volcano-tectonic stages are reflected also in the geochemical evolution of emitted magmas, which were moderately silica-undersaturated in Stage 1 and moderately to highly silica-undersaturated in Stage 2. Silica undersaturation increases with time up to the olivine melilitite field, together with enrichments in Ca, P, and incompatible trace elements, suggesting the increasing contribution of a metasomatized mantle in melts. The coeval changes of construction orientations and magmas compositions of Mohéli suggest a modification in the tectonics of the Comoros related to magmatism at about 2 Ma. The most recent volcanic morphology investigated is an olivine nephelinite lava flow on the north coast of the island dated at 8 ± 2 ka. This finding of Holocene volcanism on Mohéli implies that the entire Comoros archipelago should be considered as an active zone in any volcanic hazard assessment. The melilite-bearing rocks of Mohéli share geochemical similarities with those of Mayotte and of the Cenozoic volcanic provinces of Madagascar, further pointing to similar melting sources and magmatic processes over the Mozambique channel.