THE GEOCHEMICAL CHARACTERISTICS OF PUMICE FROM THE LATE MIOCENE GATUN FORMATION WITHIN THE PANAMA CANAL: PROBING THE ROLE OF AMPHIBOLE IN EL VALLE VOLCANO

Within the Central American Arc, perhaps one of the greatest geochemical enigmas remains the origin of magmatism within the Panamanian portion of the arc. Early arc magmatism consisted of typical calc-alkaline compositions, but Quaternary volcanic activity extending from Baru to El Valle exhibits a distinctive depletion in the heavy REEs (rare earth elements) and elevated Sr/Y– a signature that has been linked to slab melting. Recent models have shown that fractionation of amphibole-rich cumulates with minor garnet is a more probable source of heavy REE depletion and Sr/Y enrichment in many arc magmas. Using evolved magmas, we can probe the magmatic plumbing system of the Panamanian arc and examine the temporal shift towards heavy REE depletion in arc lavas. Here we present major, trace element and petrographic observations of an exceptionally well-preserved volcanoclastic deposit exposed during the construction of the new locks along the Panama Canal. The Late Miocene Gatun formation alternates between siliciclastic and volcanoclastic horizons linked to El Valle volcano, and can be broadly divided into lower, middle and upper units. Geochemical analysis of 83 separate pumice clasts collected from throughout the formation reveals dominantly dacitic-rhyolitic peraluminous lava compositions. Clasts are typically crystal poor (<20%) containing feldspar and pyroxenes. Significant geochemical overlap exists between pumice clasts throughout the Gatun formation; however, the upper unit is distinguished by extending to higher values of SiO₂ and lower values of TiO₂ and Zr, implying temporal variation in the eruptive sequence. Common to most clasts analyzed is a distinctive depletion in the middle REEs, which we interpret as the cryptic fractionation of amphibole/allanite. The presence of amphibole within the fractionating assemblage of the Late Miocene Gatun pumice is consistent with existing amphibole-dominated fractionation models for Quaternary heavy REE depleted lavas in Panama. Our results highlight the continuity of amphibole within the fractionation assemblage of the magmatic plumbing system of El Valle volcano from the Late Miocene to Quaternary, but raise questions as to how garnet became a stable fractionating phase during the Quaternary.