LA-ICPMS U-PB ANALYSIS OF INHERITED ZIRCONS FROM DOMINICA, LESSER ANTILLES

Dominica is a volcanic island in the Lesser Antilles arc, created by the subduction of the North American Plate under the Caribbean Plate. Four periods of volcanism have been previously classified by K-Ar dating: Younger Pliocene – Recent, Older Pleistocene, Pliocene, and Miocene (~7 Ma). Older lavas are primarily effusive basalt-basaltic andesite whereas recent volcanism consists of andesite and dacite explosive deposits and lava domes. Recent U-Th and U-Pb studies of zircon allow for a more detailed interpretation of Dominica’s past. Most zircons yield young ages <200 ka and represent the age of eruption and/or crystallization. Within these young deposits, a few older inherited zircons (~54 and ~48 Ma) were identified in a previous study. This work presents a more extensive study of inherited zircon ages using LA-ICPMS U-Pb dating. Older regions of the island (1-7 Ma) were targeted for sampling. Most samples were dominated by igneous zircon, with ~1% older inherited zircons. Morphology was not useful for identification of older grains, but orange coloring and/or darker luminescence in CL was often an indication of an inherited grain. Results of the U-Pb dating produced four concordant age populations. The youngest is 52-90 Ma with 200-1600 ppm U. The older populations consisted of 342-607 Ma; 1057-1094 Ma; and 1222-1726 Ma and all contained <200 ppm U. The inherited zircons have potential sources to the west and south of the present arc. Sources could include 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). The age distribution of Dominican zircons requires that prior to assimilation by the magma, multiple sources were eroded and deposited on the sea floor, and possibly incorporated into the accretionary complex, as seen in Barbadian sediments. Zircons derived from South American sources likely followed an erosional path through the Proto-Orinoco River basin, located between the Andes and Guyana Shield, whereas zircons from the Eocene arc may have been part of airfall or pyroclastic deposits. Although the present data set is limited to 18 zircon ages, the age distribution has a surprisingly large contribution of 50-100 Ma dates, which appear to have a unique composition compared to both present day and inherited zircons.