HORNBLENDE IN DACITIC IGNIMBRITES FROM CENTRAL DOMINICA: IMPLICATIONS FOR MAGMA PLUMBING SYSTEMS

The 750 km² island of Dominica, located in the Lesser Antilles is the site of multiple young Pleistocene crystal-rich, andesite-dacite, lava domes and pyroclastic flows. Dominica has nine volcanic centers on the island, whereas the other islands of the Lesser Antilles have 0-3 centers. The abundance of volcanic centers and associated seismicity suggests the presence of an active magma chamber beneath the island and the potential for future explosive eruptions. Previous work identified three voluminous ignimbrite deposits (>50 km³), dated by ¹⁴C. These deposits include Grand Savanne (>22 ka), Roseau (~28-46 ka), and Grand Bay (~39 ka). In central Dominica, samples were collected in basal units in the Roseau and Layou Valleys. Pumice clasts from both locations are crystal-rich (19-35%) dacites (63-66 wt %) and include plagioclase + orthopyroxene + hornblende + ilmenite + magnetite + apatite. Point counts show hornblende crystals comprise 1.2-2.9% of phenocrysts in Layou and 0-0.5% of phenocrysts in Roseau. Whole-rock chemistry indicates that Layou is slightly more evolved, with 64-66 wt% SiO₂ and 5.3-5.7 wt% CaO compared to Roseau, which contains 63-65 wt% SiO₂ and 5.8-6.1 wt% CaO. Hornblende crystals were imaged using the back-scattered electron detector (BSE) and indicate the presence of four different textural types: zoned (Mg # 53-61), unzoned (Mg # 53-61), glomerocrysts (Mg # 51-83) and lithics (Mg # 68-75). The zoned and unzoned crystals tend to be higher in alkalies (0.03-0.41 vs. 0.03-0.29), as well as higher Al^IV and Al^VI than the hornblende that appears to be derived from a different melt. Glomerocrysts, lithics and the majority of zoned hornblendes are restricted to samples taken from the Layou ignimbrite. The lithics are likely incorporated from wallrock upon ascent. The higher SiO₂ content as well as the textural/chemical variations observed in hornblende crystals, suggests the ignimbrites in the Layou valley may be tapping into a different level of the magma chamber or be triggered by mingling with the injection of a new magma.