Abstract

We investigated the petrography of the Dalembert tuff, an assemblage of pyroclastic flows, in two sections named the Dalembert and Reneault sections. Tuffs in the Dalembert section consist of pumice, plagioclase crystals, and a polymict assemblage of lithic fragments probably derived from sub-aerial lavas. The size of plagioclase quench crystals varies somewhat across a thick (about 15 m) graded unit, proving eruption of hot pumice flows, but the tuffs are unwelded. The features suggest the eruption of overflowing glowing avalanches, and emplacement of the cold tuff under the sea.Plagioclase-phyric basaltic andesite, very similar to the pumice of the Dalembert section except for its poor vesiculation, comprises more than 95% of the tuffs in the Reneault section. A few dacite fragments are the accessory component. These features suggest derivation from collapsing domes or spines.The Dalembert section is representative of the Dalembert tuff as a whole, whereas the facies exposed at Reneault is a local variant. We suggest that the eruption began with formation of an overflowing glowing avalanche from a fissure on a volcanic island. Volatile content of the magma decreased and viscosity increased during the eruption. Spines and domes of viscous andesite lava extruded in the terminal stage and their collapse produced a local mantle of debris represented by the tuffs at Reneault. The feeding fissure is located close to and south of the Reneault section.

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