The Volcanic Geology Of The Mid Arc Island Of Dominica PDF Download

Located in the centre of the Lesser Antilles intra-oceanic (island) arc, Dominica hosts three major stratovolcanic centres, a profusion of smaller lava dome complexes, and a series of pumiceous pyroclastic flow deposits (ignimbrites). Over the past 100 ka, Dominica has been the site of multiple silicic (andesitic-dacitic) eruptions including what is considered to be the largest eruption in the Caribbean in the past 200 ka, the Roseau Tuff eruption. This makes it a unique location in which to study the evolution of silicic volcanism in an island arc setting. Twenty new (U-Th)/He zircon and apatite ages obtained from 12 lava domes and 6 ignimbrites range from ~4 to ~744 ka. Combined with published 14C and K-Ar ages, these ages suggest that Dominica has experienced at least 28 eruptions in the past 100 ka. Pyroclastic deposits throughout the south of the island have overlapping whole-rock, mineral, and glass chemistry making fingerprinting of individual eruptions extremely difficult. Thus, the Roseau Tuff (with a proposed volume of~53 km3) likely represents material from at least six smaller eruptions, dated between 24 and 61 ka, which were erroneously correlated based on overlapping geochemical characteristics. Petrographic and geochemical analyses of lava and pumice samples reveal that Dominica's Late-Pleistocene to Recent volcanic deposits are almost entirely silicic (andesiticdacitic; ~57-67 wt.% SiO2) in composition and are characterized by wide ranges in mineral chemistry and a host of disequilibrium features. Mineral-mineral and mineral-melt equilibrium constraints indicate that three distinct components are involved in magma evolution and suggest that crystal recycling plays a major role in their petrogenesis. U-Th zircon rim ages show that most centres have experienced intermittent zircon crystallization over timescales of >350 ka. The preservation of variable zircon rim ages within individual samples implicates the existence of long-lived crystal mush beneath the island. Overlapping zircon age distributions within samples from centres up to 40 km apart on the surface suggest that this mush zone may be laterally extensive, reaching batholithic proportions. Geochemical and petrological constraints indicate that the mush is basaltic-andesitic in composition, and that the silicic composition of the erupted deposits results from entrainment of mafic antecrysts in interstitial rhyolitic melt. To the north, lavas additionally display evidence of assimilation, which is supported by the presence of Eocene zircons. Within Dominica's southernmost centre, exclusively young (