THE ZIRCON RECORD IN DOMINICA: FROM SOUTH AMERICA TO EXPLOSIVE ERUPTIONS

Within the Lesser Antilles volcanic arc on the island of Dominica, we have documented zircon phenocrysts, antecrysts, as well as xenocrysts that likely originated from South America and became incorporated into the magmatic system. Volcanism in Dominica began ~7 Ma, with extensive effusive and explosive eruptions (>58 km³) in the last 100 kyr. Recent U-Th analyses of >200 zircon rims from ignimbrite deposits across the island yield predominantly young ages, <200 ka, and display polymodal distributions of ages. The youngest zircons are within error, or younger than, (U-Th)/He eruption ages recorded by zircons in previous studies. The older zircons are likely antecrystic and may represent thermal pulses of enhanced crystallization. Some of the units appear to have shared crystallization histories peak, with crystallization episodes at 50-70 ka, ~100 ka, and ~200 ka, whereas others have distinct age patterns. Through U-Pb dating of zircons, we have also documented that <1% of the zircons analyzed are xenocrystic, or inherited detrital grains. Thirty-six zircons, extracted from both pumice clasts and lavas, can be grouped into five concordant age populations: 53-123 Ma (18 grains), 160-230 Ma (5 grains), 342-600 Ma (5 grains), 1057-1094 Ma (2 grains) and 1200-1726 Ma (6 grains). The youngest group contains 200-1600 ppm U, whereas the older zircons are all U-poor (<200 ppm). Epsilon Hf values were obtained from a sub-set of zircons and ranged from -26 to 15. The negative epsilon Hf values confirm that some of the zircons are Proterozoic. The inherited zircons have an age distribution similar to that of Barbadian sediments in the accretionary wedge, which are derived from the Proto-Orinoco River Basin and include zircons from the Eocene Caribbean arc (<100 Ma), a western Andean source from the Venezuelan Coastal Range, an older western Andean source, or the Guyana Shield (>1 Ga). These detrital zircons have survived 1000s of kilometers in transport from South America and were subsequently entrained in erupting magmas that have Fe-Ti oxide temperatures of 860-890˚C. Given that the zircon saturation temperature in these magmas is ~750 ˚C, the persistence of detrital zircon grains suggests that the magma chamber experienced higher temperatures only ephemerally.