THE ROSEAU AND LAYOU IGNIMBRITES OF CENTRAL DOMINICA, LESSER ANTILLES

Dominica is a small island in the Lesser Antilles island arc. It has one of the highest concentrations of potentially active volcanoes in the world and features several large Pleistocene pyroclastic deposits that extend to the sea. Two of the ignimbrites emanate from central Dominica, with pyroclastic deposits filling the Layou and Roseau river valleys. Based on topography, the Layou Ignimbrite is believed to be from Morne Trois Pitons, whereas the Roseau Ignimbrite is derived from vents in the Wotten Waven region. On the coast in the village of Layou, the Layou Ignimbrite is 13 m thick with a basal large block and ash flow unit, with hornblende andesite clasts up to 0.5 m. This is overlain by a 10 cm pumice lapilli fall unit and a ~5 m thick unconsolidated horizon that contains pumice clasts that range from approximately from up to 14 cm and sparse 2-5 cm andesite lithics. There is no evidence of paleosol horizons. The basal Roseau Ignimbrite in Goodwill Quarry is 19 m thick and stratified, with pumice clasts that range from 3-8 cm. The outcrop does not contain a block and ash flow unit, but has multiple pyroclastic flow units and an air-fall pumice deposit, all of which are separated by paleosol horizons (Sigurdsson, 1972). Whole-rock major and trace element chemistry, phase assemblages, and vesicle sizes of pumice clasts are remarkably similar between the two ignimbrites. Both are dacitic, 60-63% SiO₂ for Roseau and 62-64% SiO₂ for Layou. Both ignimbrites have comparable major and trace element chemistry, typical of an island arc, with enrichment of LILE and depletion of HFSE. The ignimbrites are crystal-rich (19-35 vol%) and have a mineral assemblage of plagioclase + hornblende + orthopyroxene + oxides, but the abundance of hornblende is higher in Layou (1.1-3.1%) than in Roseau (<0.6%). Texturally, the distal pumices are comparable, suggesting a similar eruptive style and transport, with ~45% vesicularity and vesicle areas of .01-.05 mm². Although these two pyroclastic deposits appear to be from different vents, their similarities suggest that they may be tapping the same magma chamber. Additional analysis of the welded portions of these pyroclastic deposits upvalley are currently underway to determine if the deposits differ with proximity from their source vents.