THE ANDESITIC IGNIMBRITES OF NORTHERN DOMINICA, CARIBBEAN

Dominica, a 750 km² island in the Lesser Antilles volcanic arc, has numerous volcanic centers. One of the more active volcanoes is Morne Diablotins, the proposed source of the Grand Savanne ignimbrite. The deposit is ~70 m thick and includes a block and ash flow overlain by a baked ash layer, and welded and unwelded tuff. Samples from each unit were characterized petrographically and geochemically to determine any variation within the eruptive sequence. The primary mineral assemblage is plagioclase, orthopyroxene, clinopyroxene, and oxides. The lower units are more crystalline (35-43%) and plagioclase-rich (23-36%) than the overlying unwelded tuff (18-25% phenocrysts and 15-19% plagioclase). All units are andesitic (57-60 wt% SiO₂) and virtually indistinguishable based on major, trace, and rare earth element chemistry. Reconnaissance geologic mapping suggests that several other pyroclastic deposits in northern Dominica erupted from Morne Diablotins (Smith and Roobol, 2013): Wesley ignimbrite, Bense ignimbrite, and Pointe Ronde ignimbrite. The Wesley ignimbrite is a ~27-m thick, layered deposit with a basal unit of 8-10 cm rounded hornblende-bearing pumice clasts, containing some blocks up to 0.5 m. The overlying layer contains pumice between 2-5 cm with some lithics up to 0.5 m in an ash-rich matrix. The uppermost unit is ash rich, with little pumice. The sharp contact between layers is likely representative of multiple events. The Bense ignimbrite is ~4 m of unconsolidated lithified crystal-rich ash, abundant in hornblende and angular hornblende andesite lithics (3-5cm), with no visible stratigraphic layering. The Pointe Ronde ignimbrite is a ~5-6 m thick pumice-rich unconsolidated deposit with soil developing on top. This deposit contains rounded hornblende pumice between 8-10 cm, with few lithics. Pumice clasts from each of these ignimbrites have been characterized and compared to the Grand Savanne ignimbrite to help determine if all the deposits are part of the same eruptive event. If all of the deposits are related, the total volume of material is tens of km³, which has implications for the size of the magma chamber beneath northern Dominica. If the deposits are disparate magma batches, then the hazard potential and likelihood of a future catastrophic explosive eruptive event in northern Dominica is smaller.