PETROGENESIS OF DISTINCT SILICIC MAGMA TYPES FROM THE LOWER PLEISTOCENE GUACHIPELIN CALDERA, NW COSTA RICA: EXTENSIVE MAGMA MIXING AND PROTRACTED SUBVOLCANIC RESIDENCE
Polytopic vector analysis (PVA), a multivariate statistical method, was used to characterize the mixing relationship between individual units. The program defined four different end member (EM) magmas, which contributed to the generation of the seven units associated with the Guachipelin Caldera. These EM represent partial melts from chemically heterogeneous crystalline calc-alkaline andesites. Further evidence of discrete mixing relationships between magmas is based on some other geologic evidence (i.e. mineralogy, stratigraphy). In our model, we propose periodic pulses of magma produced by partial melting of the four chemically distinct intermediate sources and subsequent mixing in varying proportions produced the seven ash-flows. This requires extended periods when the EM magmas reside in subvolcanic zones prior to the inception of a particular eruptive event. Considering the temporal (≤0.5Ma) and spatial (single caldera) constraints of this sequence of eruptions, significant magma mixing has occurred on relatively short time scales.