The Panamanian arc exhibits a swift transition from calc-alkaline to adakitic magmatism during the Quaternary. This transition has important implications for magma generation processes in the region. Specifically, are adakite magmas derived from slab melting during subduction, or to volatile fluxing of the mantle wedge? At Baru and El Valle volcanoes, we have used a robust geochronological and geochemical dataset to examine processes that are responsible for the generation of magmas from the Miocene to present. Our results show that both of these volcanic edifices were constructed over timescales consistent with typical arc volcanoes - much faster than slab melt derived volcanic edifices. The magma production rates necessary to account for Quaternary magmatism in Panama are difficult to achieve via slab melting processes alone. Here we present a model whereby typical calc-alkaline magmas can acquire an adakitic signal through crystal fractionation processes in the lithosphere. The water content of arc magmas is a critical control of the fractionating assemblage. At moderate pressures, high magmatic water contents promote amphibole stability and fractionation paths that yield normal island arc magmas. With decreasing water contents garnet becomes stable at the expense of amphibole, yielding an adakitic magmatic signature. We have recorded such a drop in water content by using mass balance calculations and forward modeling of our geochemical dataset. Despite the lower modeled water contents of magmas during the Quaternary adakitic phase, these magma water contents are significantly elevated in comparison to magmas generated by slab melting processes. Our results support previous studies suggesting that some adakites may be produced by fractional crystallization of amphibole and garnet. This model is consistent with amphibole-rich cumulates observed in lavas throughout the region. We show that in arcs with moderate crustal thicknesses, variations in magma water content may be the primary variable controlling shifts in magma composition from typical calc-alkaline towards adakitic.
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