Petrologic characterization of the Paleogene arc, Costa Rica, Central America

Lava clasts in Late Cretaceous to Eocence debris flows are manifestations of the earliest Costa Rican volcanic arc. This study characterizes the petrography and geochemistry of volcaniclasts from four sample locations (Punta Samara, Isla Paloma, Quebrada Buenaventura, and Playa Soley) to determine the subduction parameters for the primitive arc. Geochemical compositions of the primitive arc lavas are compared to samples from the volcanic front from the modern arc, concentrating on the Costa Rican and Nicaraguan segments. Selected trace element ratios of these clasts are used to infer subduction parameters including mantle source (Zr/Nb), degree of mantle melting (La/Yb), slab input (Ba/La), and sediment addition (Ba/Th, U/Th). The majority of primitive arc lava clasts originated from a depleted, mantle source (MORB-like), and have higher Zr/Nb ratios than those from modern central Costa Rica (CCR). However, two lava clasts have Zr/Nb ratios similar to CCR volcanics, indicating a contribution from a more enriched mantle source. The degree of mantle melting increases (low La/Yb) with an increase in slab signal (high Ba/La) in the primitive arc lava clasts, and these values are more similar to what is observed in modern western Nicaragua than in modern Costa Rica. This may be a reflection of a steeper angle of subduction offshore Costa Rica during the Late Cretaceous to the Eocene, possibly because the Farallon Plate was older, colder, and denser than the modern day Cocos Plate. The primitive arc lava clasts have high Ba/Th and low U/Th ratios indicating that the sediment subducted in the primitive arc likely had a carbonate component (high Ba/Th), but lacked the hemipelagic sediment component (high U/Th) that is subducted in the modern arc.