Abstract
The latest Aptian to earliest Albian (~115 Ma) Water Island Fm in the Virgin Islands contains some of the oldest known arc-related strata in the Greater Antilles Island Arc. Hence, the unit is of considerable significance in tectonic reconstructions of initial subduction parameters along the long-lived destructive plate margin separating the North American and Caribbean Plates. Exposed Water Island strata are bimodal, consisting predominantly of altered dacite and rhyolite (originally called keratophyre; 65-85% SiO2 and subordinate degraded (spilite; 46-57% SiO2). TiO2 content of Water Island basalt averages approximately 0.5%, resembling borderline intermediate-Ti boninite basalts, consistent with low incompatible element abundances and low normalized light rare earth elements (LREE) with respect to Sm. Trace element patterns of the felsic suite, characterized by pronounced negative normalized anomalies for high field-strength elements (HFSE), low Sr/Y, and low absolute rare earth element (REE) abundances, and relatively flat normalized REE patterns, have analogues in plagiorhyolite suites from bimodal Cenozoic arcs, including the western Aleutians, Izu-Bonin, the Kermadecs, and South Sandwich. Relatively low incompatible element concentrations in plagiorhyolites and contrasting normalized incompatible trace element patterns in basalts preclude an origin of Water Island plagiorhyolite through MORB-type fractional crystallization. Compositions are consistent instead with melting models involving partial fusion of amphibole-bearing gabbro at low pressures (within the stability range of plagioclase) in response to introduction of heat and aqueous flux by arc-related basalt melts and associated hydrothermal fluids during transmission to the surface. Truncation of the basalt fractional crystallization trend at SiO2 = 57% indicates evolved island arc basalt (IAB) crystal fractionates were gradually displaced from crustal magma conduits by more buoyant plagiorhyolite melt, and trapped in underplated, sub-crustal magma chambers. Basalts have low (Ce/Ce*)N (average ~ 0.78), indicating the presence of significant pelagic sediment (0.5 to 1.5% Atlantic Cretaceous pelagic sediment, AKPS). One subunit of relatively high-HFSE plagiorhyolite has (Ce/Ce*)N near-expected values, but another with low-HFSE has slightly lower than expected (Ce/Ce*)N, consistent with a small sediment component. Absence of intermediate andesite from the Water Island Fm is inconsistent, however, with basaltrhyolite magma mixing processes. Consequently, incorporation of sediment by low-HFSE plagiorhyolite is inferred to have resulted from re-melting of arc-related gabbro.