THE ORIGIN OF EL HATO SILICIC IGNIMBRITE OF EL VALLE VOLCANO, PANAMA: EVIDENCE OF DEEP FRACTIONATION PROCESSES

The El Valle volcano in eastern Panama results from the eastward subduction of the Nazca plate underneath the Panama Isthmus. Previous workers (Defant et al. 1990) have shown that El Valle was dominated by typical calc-alkaline andesitic volcanism during the Miocene (~10-5 Ma) and silicic adakitic volcanism during the Quaternary. The silicic volcanism is best represented by the eruption of the El Hato ignimbrite that covers an area of ~300 km². The purpose of this study is to determine the chemical variation, age relations and source of the El Hato unit and associated volcanics by geochemical and ⁴⁰Ar/³⁹Ar analyses of a complete sample set from these deposits. New ⁴⁰Ar/³⁹Ar ages (~100 ka to ~30 ka) for the Quaternary El Hato silicic ignimbrite and related deposits are much younger than what had been previously reported. New geochemical data for the Quaternary units are used to characterize these deposits as adakites (e.g. depletion of HREEs, low Y and high Sr). Common to all models that are used to explain the origin of adakites is the equilibrium of a melt with a garnet-rich residue. Garnet stability is highly pressure dependant (greater than 8 kb) and is governed, in part, by water content (increasing dissolved H₂O enlarges the garnet stability field). Our preferred model for generation of adakites at El Valle volcano is closely related to the arrival of the Cocos and Coiba Ridges to the Panamanian Trench. The indentation of these overthickened buoyant sections of lithosphere to the trench resulted in increase compression within the Panamanian lithosphere (by enhanced coupling), causing melt separation at greater depth (within the garnet stability field). This indentation would have been the main cause of crustal shortening, variation in the stress condition in the overlying plate, and shutting off of the magma storage and fractionation in the mid-crust; all of these ultimately resulted in the transition from typical arc to adakitic volcanism. In this model the transition from “typical” arc magmatism in the Miocene to adakitic magmatism in the Quaternary at the El Valle Volcano marks the docking of the Coiba and Cocos Ridges with the Panama Isthmus and results in an abrupt increase in the depth of magmatic segregation.