Oblique aseismic subduction below western Panama and southeastern Costa Rica has produced Recent arc-related volcanism. The aseismicity is probably related to the subduction of relatively hot oceanic lithosphere. The volcanism throughout this region over the past 2 Ma has been quite distinct, consisting of felsic magmas (andesites to rhyolites but mainly dacites) with geochemical signatures suggesting a metamorphosed basaltic source. It is believed that the subduction of young oceanic crust sets up conditions under which the slab melts rather than the overlying mantle wedge. Rocks with slab-melt geochemistries and associated with young subducted crust have been termed adakites elsewhere. The young adakite melts are sometimes associated with a few rare young high-Nb basalts, but there is no obvious genetic link between them through differentiation. High-Nb basalts may also be derived from the partial melting of the subducted oceanic crust. High-Nb basalt migmatites have been found with pegmatites of adakite compositions in the exposed subduction terrain of the Catalina Schist, California. Alternatively, the high-Nb basalts may be partial melts of phlogopite-rich mantle that has previously reacted with adakite magmas.Eruption of adakites and high-Nb basalts was preceded by a 2-3 Ma period of relative quiescence. Prior to this, there was a 7 Ma period of calc-alkaline volcanism typical of the present-day magmatism (associated with a distinct Benioff zone) found throughout the Central American arc. The abrupt transition in volcanism with time from an early calc-alkaline sequence to a later adakite-high-Nb basalt sequence may record a change in the tectonic setting of western Panama and southeastern Costa Rica over the past 12 Ma.

La Yeguada volcanic complex (LYVC) is one of many major volcanoes that represent the extension of the Central American arc in western Panama and that have resulted from current oblique subduction south of Panama. There are two major phases of calc-alkaline volcanic activity at LYVC based on mapping and K-Ar radiometric dates. The first phase began at approximately 13 Ma and ceased at approximately 7.5 Ma. This sequence, termed the old group, consists of basalts to rhyolites with typical arc mineralogies (OL, CPX, PL, MGT, and OPX). The samples have similar radiogenic Sr and Nd values and appear to be related by fractional crystallization with assimilation and/or magma mixing involved in the differentiation. The parental basalts were probably derived from the metasomatized mantle wedge via melting induced by fluids released from the subducted lithosphere.There was an apparent period of minor volcanic activity from 7.5 to 2.5 Ma (only one documented sample from this period). The second phase (< 2.5 Ma), termed the young group, consists only of dacites but with very different mineralogies (PL, MGT, AM, BI, with no PX) and geochemistries (e.g., high Sr and low Y and HREE) compared with the old-group dacites (and andesites and rhyolites). The dacites cannot be related to the old group by various petrogenetic modeling techniques. These high-Al dacites have the characteristics of magmas derived from the partial melting of the subducted oceanic lithosphere with a hornblende eclogite residuum. This has been substantiated by geochemical modeling.Samples similar to the young-group dacites in other arcs have been termed adakites and are associated with the subduction of young hot crust which may explain why the slab melts. The Panama basin has extremely high heat flow values, comparable with those of the Galapagos ridge system. The change from normal arc volcanism to adakites suggests that the subducted oceanic crust became hotter as time progressed. The subduction of an oceanic ridge or new ridge development along the Sandra Rift in the Panama basin can explain the change in volcanism with time but more geophysical data are needed.