Abstract

Constraints on the polarity of Cretaceous subduction in the Greater Antilles are provided through geochemical comparison between the erupted island arc lavas in central Puerto Rico and potential pelagic sediment reservoirs in the flanking ocean basins. Early Jurassic to mid-Cretaceous (185- to 65-Ma) sediment from the open Pacific on the southwest is dominated by pelagic chert, which is highly refractory and depleted with respect to incompatible elements. In comparison, mid- to Late Cretaceous (ca. 112- to 65-Ma) sediment from the younger Atlantic basin on the northeast was dominated by mixtures of two end members. These include (1) biogenic clay and carbonates with elevated light rare-earth element (LREE) abundances, negative MORB-normalized, high field-strength element (HFSE) anomalies, and low Zr/Sm; and (2) turbiditic detritus of upper continental crust composition with high LREE, comparatively shallow HFSE anomalies, and high Zr/Sm. Compositions of Puerto Rican arc basalts are inconsistent with incorporation of Pacific pelagic chert. Instead, patterns characteristic of high-Fe island arc tholeiites are reproduced by incorporation of up to 4% of a low-Zr/Sm biogenic sediment component of Atlantic origin, whereas patterns of low-Fe lavas require, in addition to biogenic sediment, introduction of up to 2% of a high-Zr/Sm crustal turbidite component. The Atlantic origin of all the subducted sediments indicates the polarity of subduction throughout the Cretaceous in the northeast Antilles was persistently southwest dipping. This conclusion is supported by the presence of a low-Zr/Sm suprasubduction zone component of Atlantic origin in Caribbean plateau basalts (91–88 Ma) from southwest Puerto Rico, which were erupted within the broad back-arc region of the Greater Antilles during intermediate stages of arc development.

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