Origin of silicic volcanism in the Panamanian arc: evidence for a two-stage fractionation process at El Valle volcano |
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| Authors: | Paulo J Hidalgo Thomas A Vogel Tyrone O Rooney Ryan M Currier Paul W Layer |
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| Institution: | (1) Department of Geological Sciences, Michigan State University, East Lansing, MI 48824, USA;(2) Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD 21218, USA;(3) Department of Geology and Geophysics and Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK 99775, USA |
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| Abstract: | In the Central American Volcanic Arc, adakite-like 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 adakitic-like volcanism is substantial during the Quaternary (~120 km3). Adakitic-like magmas dominate the stratigraphic record. Common to all models for the origin of an 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 mid-lower 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
better understanding of the generation of continental crust in island arc environments. |
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