Geochemical and Geochronological Characterisation of the Poas Stratovolcano Stratigraphy

In this chapter the stratigraphy of Poás volcano by using geological, petrographical, geochronological and geochemical analyses on the volcanic products erupted during the last 600 ka is defined. The northern flank of Poás consists of the following Units and Members: Río Sarapiquí, La Paz, Puente de Mulas Member (possibly from another source, though interdigitated with Poás products), Río Cuarto Lavas, Von Frantzius, Congo, Bosque Alegre and Laguna Kopper. The products on the southern flank are the Colima Formation, La Paz, Tiribí, Achiote, Poasito, Sabana Redonda and Poás Lapilli Tuff. The central part of the volcano is made up of the Poás Summit Unit, which includes the Main and Botos craters. Rock composition varies from basalts to dacites. During the last 600 ka the content of K₂O and other oxides (e.g. TiO₂ and P₂O₅) and trace elements (e.g. Zr, Ba) has varied significantly through time, implying the presence of two geochemical end-members since the beginning of the magmatic activity at Poás: (1) the Sabana Redonda Geochemical Component (TiO₂ > 1%) enriched in HSFE and other trace elements, present in La Paz Andesites, Lavas Río Cuarto, Poasito, Sabana Redonda, Poás Lapilli Tuff and Botos crater lavas and (2) The Von Frantzius Geochemical Component (TiO₂ < 0.8%) that was recognized in the lavas from the Main Crater, Von Frantzius, Achiote, Bosque Alegre, Congo and Botos. Lavas related to both magmatic components have coexisted over time as indicated by those found at Botos and the Main Crater. Units possibly related to a common vent, i.e. La Paz, Achiote and Main Crater, the percentages of K₂O and TiO₂ decreased through time.