Caracterización geoquímica y petrográfica de las unidades geológicas del macizo del volcán Poás, Costa Rica

Abstract

The present study defines the stratigraphy of Poás volcano by using geologic, petrographic, geochronologic and geochemical analyses made on the Poás units. The northern flank of the volcano is comprised of the following units: Río Sarapiquí, La Paz Andesites, Tiribí Formation (from Barva volcano, but interdigitated with Poás stratigraphy), Río Cuarto Lavas, Von Frantzius, Cerro Congo, Bosque Alegre and Laguna Kopper. The units on the southern flank are Colima Formation, La Paz Andesites, Tiribí, Achiote, Poasito, Sabana Redonda and Poás Lapilli Tuff. The central part of the volcano is made by the Poás Summit Unit, which includes the Main and Botos craters. The composition of the rocks spans the range from basalts to dacites. These units were geochemically correlated with two magmatic components: 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 some from Botos crater lavas. 2. The Von Frantzius Geochemical Component (TiO2 < 0.8 %) is present in lavas of the Main crater, Von Frantzius, Achiote, Bosque Alegre, Cerro Congo and some Botos crater lavas. During the last 600 ka the content of K2 O and other oxides (TiO2 and P2 O5 ) and traces (Zr, Ba) have varied significantly through time, suggesting the presence of these two geochemical end-members since the beginning of the magmatic activity of Poás. Within similar ranges of time, units with high and low values of these elements have coexisted; the latter is true for Botos lavas and the Main crater. For units that possibly shared a common vent, such as La Paz Andesites, Achiote and Main crater, the percentages of K2 O and TiO2 have decreased through time. Nevertheless, more geochronological data are needed to improve the interpretations given above.