Origin of silicic volcanism in the Panamanian arc: evidence for a two-stage fractionation process at El Valle volcano
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ORIGINAL PAPER
Origin of silicic volcanism in the Panamanian arc: evidence for a two-stage fractionation process at El Valle volcano Paulo J. Hidalgo • Thomas A. Vogel • Tyrone O. Rooney • Ryan M. Currier Paul W. Layer
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Received: 26 October 2010 / Accepted: 25 April 2011 / Published online: 22 May 2011 Ó Springer-Verlag 2011
Abstract In the Central American Volcanic Arc, adakitelike volcanism has often been described as volumetrically insignificant. However, extensive silicic adakitic volcanism does occur in the Panamanian arc and provides an opportunity to evaluate the origin of this magma-type as well as to contrast its origin with other Central American silicic magmas. The Quaternary volcanic deposits of El Valle volcano are characterized by pronounced depletions in the heavy rare earth elements, low Y, high Sr, high Sr/Y, relatively high MgO, and low K2O/Na2O, when compared with other Quaternary Central American volcanics at similar SiO2. These chemical features are also diagnostic of adakitic signatures. Our new 40Ar/39Ar ages of lava flows and ash flows that compose the volcanic edifice of El Valle volcano illustrate that the eruptive volume of adakiticlike volcanism is substantial during the Quaternary (*120 km3). Adakitic-like magmas dominate the stratigraphic record. Common to all models for the origin of an
R. M. Currier Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD 21218, USA
adakite geochemical signature is the involvement of garnet, as a residual or fractionating phase. The stability of garnet in hydrous magmas has been recently reevaluated with important consequences; garnet is a stable primary igneous phase at pressure and temperature conditions expected for magma differentiation at the roots of a mature island arc. Moreover, adakite-like volcanism erupted at El Valle volcano displays the middle rare earth element depletion observed in other Panamanian volcanic centers that has been attributed to significant amphibole fractionation. Extensive amphibole fractionation may have occurred in two stages. The first stage of fractionation, garnet ? amphibole fractionation, occurs from hydrous basaltic– andesitic parental magmas that have ponded at the base of an overthickened crust. The second stage occurs at midlower crustal levels where abundant amphibole ? plagioclase and minor sphene crystallized from water-rich magmas. These two stages combined may have resulted in an amphibole-rich cumulate layer. This amphibole layer is likely the source of the abundant amphibole-rich cumulate enclaves and blobs found in volcanic products across the Panamanian arc. Stalling of water-rich magmas during this two-stage fractionation process could drive the interstitial liquids to the evolved compositions typical of continental crust, while leaving behind amphibole-rich cumulate rocks that may eventually be returned to the asthenosphere. Differentiation of H2O-rich magmas under the conditions appropriate for the roots of island arcs may therefore be a key process in developing a
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