MONOTONOUS CRYSTAL-RICH DACITIC IGNIMBRITES FROM DOMINICA: IMPLICATIONS FOR MAGMA RECHARGE

The 750 km² island of Dominica in the Lesser Antilles is dominated by numerous young (<Pleistocene) crystal-rich andesitic-dacitic lava domes and associated pyroclastic flows. The young volcanism, shallow seismicity, and fumarolic activity suggest an active magma reservoir and potential for future eruptions, but only the southern Plat Pays volcanic complex has been studied extensively. Previous work has identified three ignimbrites >50 km³ in volume and provided broad age estimates based on C¹⁴ dating: Grand Savanne (>22 ka), Roseau (~28-46 ka) and Grand Bay (~39 ka). The Roseau Ignimbrite is the most voluminous (~58 km³) of these monotonous intermediate tuffs, with welded and unwelded deposits that underlie the capital city and span several river valleys, including Roseau and Layou. The pumices are homogeneous and crystal-rich (19-35 vol%) dacites (62-65 wt% SiO₂) with a phase assemblage of plagioclase + orthopyroxene + hornblende + clinopyroxene + ilmenite + magnetite + apatite + zircon. The presence of hornblende distinguishes Roseau from the other ignimbrites which likely erupted at higher temperatures. The plagioclase displays complex oscillatory zoning, An_50-70, but no evidence of resorption. The orthopyroxenes are unzoned, with compositions of En_46-65. Most of the hornblendes are also unzoned, with Mg# 52-61, but in some basal layers of the Layou Valley deposits, the hornblendes have 10-20 um rims, both more Al-rich and Al-poor than their cores. With the exception of an occasional Mg-rich glomerocryst (En₇₅) or calcic core of plagioclase (An₈₀), evidence for disequilibria and mafic inputs are limited. This contrasts with other monotonous intermediate ignimbrites such as the Fish Canyon Tuff, in which phenocrysts display varying degrees of dissolution and resorption. Therefore a model of thermal rejuvenation and mafic recharge need not be invoked for eruption, perhaps due to the smaller volume of the Roseau Ignimbrite. Ongoing investigation of trace elements in plagioclase and other phases may suggest an alternative trigger mechanism, such as silicic recharge.