The importance of mixing in the evolution of silicic magmas in northern Costa Rica

Silicic volcanism in the northern Costa Rican segment of the Central American volcanic arc was widespread from the Miocene through the Middle Pleistocene. Ignimbrites in the Bagaces formation are among the earliest high- silica products in this part of the arc (<10 Ma) and are thus important to understanding its chemical, temporal and spatial evolution. The large silicic eruptions are especially interesting in this part of the arc, since Costa Rica is built upon thick oceanic plateau of the Chorotega block, located on the western extent of the Caribbean Large Igneous Province (CLIP). A detailed study of two ignimbrites demonstrates that mixing is an important process in the evolution of large silicic systems in this part of the arc. The Papagayo and Pan de Azucar Tuffs outcrop north of the Nicoya Peninsula in northern Costa Rica. The Papagayo Tuff contains mingled pumice fragments. Petrography, whole-rock chemistry and microprobe data are consistent with the mingling and eruption of rhyolitic and andesitic magma batches. Dacitic pumice fragments from the Pan de Azucar unit are more homogeneous and chemically similar to intermediate bulk-rock compositions of mingled Papagayo pumice samples, supporting a model in which the Pan de Azucar magma was a homogenized part of the Papagayo magmas. However, small differences in composition require modification of the Pan de Azucar magma by some other process (e.g. magma mixing or assimilation). The data supporting mingling and mixing in these units provide an ideal test case for Polytopic Vector Analysis (PVA). Relatively new to igneous petrology, PVA is a multivariate statistical program that can incorporate all available geochemical analytes to simultaneously unmix samples, finding the number and composition of end members required to explain the variation within the population. For the Papagayo samples alone, PVA yields a three end member solution. One end member is andesitic (57 wt.% SiO2) and the other two are both rhyolitic (71 wt.% SiO2), but have different trace element compositions. When the Pan de Azucar samples are included with the Papagayo samples, PVA generates a four end member solution that indicates mixing among two rhyolitic end members (71 and 72 wt.% SiO2), a dacitic end member (66 wt.% SiO2), and a basaltic end member (52 wt.% SiO2). These solutions are mutually consistent and supported by petrographic and chemical data from the rocks.