Water-saturated magmas in the Panama Canal region: a precursor to adakite-like magma generation?
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Amphibole, while uncommon as a phenocryst in arc lavas, is increasingly recognized as a key constituent in the petrogenesis of arc magmas. Fractional crystallization of water-saturated arc magmas in the lower crust can yield substantial volumes of amphibole cumulates that, depending on the pressure of crystallization, may also contain garnet. Fractionation of this higher pressure assemblage has been invoked as a possible mechanism in the production of magmas that contain an adakitic signature. This study examines newly dated Late-Oligocene (25.37 ± 0.13 Ma) hypabyssal amphibole-rich andesites from Cerro Patacon in the Panama Canal region. These andesites contain nodules of amphibole cumulates that are ~4–6 cm in diameter and are almost entirely composed of 5–10-mm amphibole crystals (dominantly ferri-tschermakite). Geochemical variations, optical and chemical zoning of the Cerro Patacon amphiboles are consistent with their evolution in a crystal mush environment that had at least one recharge event prior to entrainment in the host andesite. Amphiboles hosted within the cumulate nodules differ from those hosted in the Cerro Patacon andesite and contain consistently higher values of Ti. We suggest these nodules represent the early stages of fractionation from a water-saturated magma. Cerro Patacon andesites have REE concentrations that plot at the most depleted end of Central American Arc magmas and exhibit a distinctive depletion in the middle REE. These geochemical and petrographic observations strongly support significant amphibole fractionation during formation of the Cerro Patacon andesite, consistent with the petrographic evidence. Fractionation of water-saturated magmas is a mechanism by which adakitic compositions may be produced, and the Cerro Patacon andesites do exhibit adakite-like geochemical characteristics (e.g., elevated Sr/Y; 28–34). However, the relatively elevated concentrations of Y and HREE indicate garnet was not stable in the fractionating assemblage during this early stage of arc development.
KeywordsAdakite Panama Canal Amphibole Water saturated Arc magma Andesite
This project was funded by the MSU Intramural Research Grants Program. Logistical support during fieldwork was generously provided by the Autoridad Del Canal De Panama. The authors wish to thank Carlos Rabat, president of the ECO-ROCA quarry for allowing access to the Cerro Patacon quarry. We wish to thank Chelsea Mack, Christian Briggs, Matt Parsons and Carl Henderson for sample preparation and analytical assistance. We are particularly grateful to Roberto Miranda for providing some the outline of volcanic edifices in the region based on new field mapping. Olivier Bachmann and Michael Krawczynski are thanked for thorough and insightful reviews which helped improve the manuscript. We thank Timothy Grove for editorial handling and for helpful suggestions.
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