Pre-arc basement complex and overlying early island arc strata, Southwestern Puerto Rico: overview, geologic evolution, and revised data bases

Abstract

The pre-arc basement complex in southwestern Puerto Rico consists of rocks exposed in the Bermeja complex. The oldest rocks are highly serpentinized peridotites that occur in three belts (Monte del Estado, Rio Guanajibo, and Sierra Bermeja). These serpentinites were emplaced into a sequence of Jurassic to mid-Cretaceous pelagic chert (Mariquita chert) that contains abundant rafts and blocks of N-MORB-type amphibolites (Las Palmas amphibolite) and tholeiite and associated trondhjemite fractionates (Lower Cajul MORB) also of N-MORB affinity. The rocks are apparently overlain by a younger sequence of pre-arc plateau basaltic and andesitic lava flows (Upper Cajul Formation) that occur in two distinct geographic sequences, one having E-MORB and the other OIB geochemical characteristics. Overlying these pre-arc rocks in western Puerto Rico are northwest-trending Late Cretaceous to Eocene (85 to 45Ma) island arc strata that chronologically overlap later volcanic phases in central Puerto Rico. These western Puerto Rico arc rocks have elevated incompatible element concentrations together with conspicuously shallow negative Nb-anomalies, slightly positive Zr-Hf anomalies, and exceedingly high OIB-like Nb/Zr, all indicative of enriched source compositions. Trace element patterns are reproduced by multiple component mixing models involving highly depleted spinel peridotite (RMM15 to 20) overprinted by small OIB-type (up to ~2%) and pelagic sediment components. Trace element abundances are too high to qualify Atlantic Cretaceous pelagic sediment as a potential contaminant, but mantle-melting models (f=0.25) are consistent with the incorporation of variable proportions of Caribbean Cretaceous pelagic sediment through northdipping subduction of the Caribbean basin. Anomalous two-pyroxene-bearing andesites with extraordinarily high SiO2/MgO compared with normal mantle basaltic compositions, also indicate the incorporation of Jurassic to Early Cretaceous pelagic chert from the Caribbean. The high degree of source enrichment in western Puerto Rico is inconsistent with regional within-plate plume tectonics. Instead, it is inferred that the younger north-dipping western Puerto Rico arc (dating from ~85Ma) sampled an upper mantle enrichment zone generated in the backarc region of the older (125 to ~85Ma) south-dipping arc system in central Puerto Rico