Re‐evaluation of the Roseau Tuff eruptive sequence and other ignimbrites in Dominica, Lesser Antilles

Abstract

ABSTRACTThe island of Dominica hosts several ignimbrites, including the Roseau Tuff, thought to represent the largest eruption in the Caribbean in the past 200 000 years. The volcanic stratigraphy of the island is poorly understood due to limited outcrops and a paucity of geochemical and geochronological data. The discovery of a new fully accessible exposure of three ignimbrites intercalated with paleosols provides an opportunity to re‐evaluate the current stratigraphic framework of ignimbrite‐forming eruptions on the island. Whole‐rock analyses of pumice clasts from Dominica ignimbrites are andesitic (61–66% SiO2) and in most cases are geochemically indistinguishable. Ignimbrites in the north of the island have less evolved glass compositions (73–75% SiO2) and more mafic orthopyroxene compositions (En > 56) than their southern counterparts (75–78% SiO2; En < 56). Pumice clasts from ignimbrites in southern Dominica have indistinguishable groundmass glass and mineral chemistry, making correlation of these deposits difficult. New (U–Th)/He eruption ages for the southern ignimbrites indicate that at least six separate explosive eruptions occurred between 24 and 61 ka. The non‐unique geochemistry of these deposits, together with the new (U−Th)/He ages, suggests that the large volume inferred for the Roseau Tuff eruption may actually be a composite of six smaller, geochemically homogeneous eruptions.