Contribution to petrological and stratigraphical understanding of the Cordillera de Guanacaste lava flows, Costa Rica

Abstract

Geochemical data are presented for lavas from four pair Quaternary stratovolcanoes (Cordillera de Guanacaste), small basic outcrops trenchward of the volcanic front (VF), and paleovolcanic units (Pliocen- Pleistocene age) of northern Costa Rica. The more evolved lavas of the calc-alkaline series are from Rincón de la Vieja and Miravalles andesitic volcanoes, and from Cañas Dulces (dacite/rgyolite) and Bijagua (andesite) domes-field. They usually have high Zr, Ba and Rb contents. The Quaternary volcanoes grew over a regional basaltic to dacitic basement (Miocene-Pleistocene), called Pre-Neovolcanic lavas (PNL). The Intra-ignimbrite lava flows (IIL, Pliocene age) have a clearly different trend with respect to the volcanoes, especially in their high TiO₂ and P₂O₅ content. Both old lava units show a tholeiitic tendency, transitional to calc-alkaline series. The less evolved laves correspond to Orosi, Cacao, and Tenorio (Parcelas cone) volcanoes and also to small isolated basic lavas and cinder cones (IBL) located 20-26 krn in front of the VF. These rocks are high-alumina tholeiitic basalts/basaltic andesites or transitional to alkalic basalts. Crystal fractionation is the dominant mechanism involved in generating the range of magmatic compositions. The geochemistry suggests that Guanacaste lavas are cogenetic but some source heterogeneites are required to explain the observed petrological features.