DACITE GENESIS VIA BOTH SLAB MELTING AND DIFFERENTIATION - PETROGENESIS OF LA-YEGUADA VOLCANIC COMPLEX, PANAMA

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.