Abstract
Volcanic rocks from the Maimón Formation, metamorphosed under greenschist facies conditions, represent the most ancient arc magmatism recorded in the Cretaceous Caribbean island arc. From new geochemical data, and on the basis of immobile trace elements (high field strength elements - HFSE, rare earth elements - REE), a predominance of basaltic protoliths over intermediate and acid compositions has been determined. Geochemically, the basaltic protolith includes boninites, low-Ti LREE-depleted island arc tholeiites, normal low-Ti island arc tholeiites, and exiguous normal island arc tholeiites. The acid rocks present a clear boninitic and tholeiitic affinity and are of the M-type (mantle-derived). The geochemistry of low-Ti LREE-depleted island arc tholeiites permits their classification as fore-arc basalts (FAB), which is a term introduced recently to describe tholeiites with MORB (mid-ocean ridge basalt)-like features from the Izu-Bonin-Mariana fore-arc. The presence of FAB and their stratigraphic association with boninites indicate formation in a fore-arc environment during the earliest stage of island arc magmatism in an extensional setting that characterizes the onset of subduction and birth of an island arc. The Maimón Formation could represent the volcanic top of an ophiolite of the “subduction initiation” type.
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