Magmatic and rare Eocene to Precambrian xenocrystic zircons in Dominica reveal discrete crystallization episodes and multiple sediment sources

Abstract

Abstract The effects of sediment contribution to the Lesser Antilles island arc have been well explored with whole-rock trace-element chemistry and isotopic studies. To better understand the source of these sediments, we analyzed >400 zircons for U-Pb ages and trace-element chemistry in eight andesitic-dacitic ignimbrites and lavas younger than 100 ka from the island of Dominica in the central Lesser Antilles arc. The overwhelming majority of the zircons analyzed were magmatic in origin, with U-Pb ages younger than 10 Ma, but predominantly younger than 300 ka. Zircon trace-element chemistry is consistent with derivation from an oceanic-island arc, and positive εHf(t) values (+5 to +15) support a juvenile depleted mantle source. Rare Precambrian to Eocene xenocrystic zircons (36) were also found in the Dominican volcanics and record sediment dispersal from several different terranes as the Caribbean plate migrated eastward along the northern margin of South America. Although some previous detrital zircon studies in the region suggested zircons younger than 100 Ma were derived from the Great Arc of the Caribbean, the younger Dominican zircon xenocrysts (300–50 Ma) have elevated Th/Yb and U/Yb, as well as variable positive/negative εHf(t) values, inconsistent with an oceanic arc origin. These zircons in Dominica were most likely derived from the Eastern and Central Cordillera of the Andes, which experienced a flare-up in magmatism ca. 65–45 Ma. As the Great Arc of the Caribbean traversed along the South American margin, terrigenous sediments transported via river systems and turbidites accumulated in the forearc basin. Older zircons (1800–300 Ma) have a slightly different chemistry and equivocal source(s), including the Andes, northern Venezuelan coastal ranges, and/or the Guyana Shield.