Geochemical and Tectonic Evolution of Albian to Eocene Volcanic Strata In the Virgin Islands and Eastern and Central Puerto Rico

Early Cretaceous to Eocene volcanic strata in the Puerto Rican-Virgin Island (PR-VI) segment of the Greater Antilles island arc record a sequential geochemical change from predominantly island arc tholeiites (IAT) in volcanic phase I (Aptian-Early Albian), to intermediate calc-alkaline (CA) volcanic rocks (phase II, Late Albian), to high-K incompatible-element-enriched shoshonitic (SHO) basalts (phase III and IV, Cenomanian-Maastrichtian). Following an islands-wide eruptive hiatus, renewed eruption of CA basalts, andesites, and dacites (phase V) occurred in Eocene time. The geochemical evolution is marked by progressive increases in ratios of large-ion lithophile (LILE) and light rare earth (LREE) elements relative to high field strength (HFSE) and heavy rare earth (HREE) elements in the first four volcanic phases. Increasing radiogenic isotope content of the lavas mirrors the shift in trace element ratios. The transition in Aptian-Albian time from geochemically primitive IAT compositions to the more evolved CA series is probably the result of variations in subduction-related components rather than a major reversal in the polarity of subduction as evidenced by the absence of an extensive unconformity in the PR-VI region at this interval and the fact that the transition occurs at slightly different times in different parts of the region. Depleted incompatible element abundances of Jurassic pelagic chert from the Pacific basin are inconsistent with these sediments being part of the subduction component and with northeast-dipping subduction in this segment of the arc. Instead, the geochemical characteristics of all five volcanic phases (I-V) reflect long-term southwest-dipping subduction of Cretaceous pelagic sediments. Furthermore, trace element and isotopic data along this 450 km segment of the arc indicate that the sediment flux decreased dramatically from east to west in response to the changing tectonic and geographic position of the erupting volcanics along the subduction zone.