PETROGENESIS OF DISTINCT SILICIC MAGMA TYPES FROM THE LOWER PLEISTOCENE GUACHIPELIN CALDERA, NW COSTA RICA: EXTENSIVE MAGMA MIXING AND PROTRACTED SUBVOLCANIC RESIDENCE

Lower Pleistocene pyroclastic ash-flow deposits in NW Costa Rica represent sequential eruptions of high-silica (69-79% SiO₂) magmas from the Guachipelin Caldera. Silicic magmatism such as this is uncommon in areas void of continental crust. However, the chemical variations within this suite of ash-flows are consistent with results from several different studies (i.e. Tatsumi and Tamura, 2002; Sisson et al., 2005) suggesting the partial melting of crystalline, calc-alkaline andesite could produce these silicic magmas. Chemical heterogeneities were discovered through evaluation of incompatible trace element ratios. Seven units were defined using the Nb/Ta ratio, which have a significantly wide range, from 3.8 to 29.4, considering their association with a single caldera. Ignimbrites from along the Central American arc, from Guatemala to Central Costa Rica, have Nb/Ta ratios ranging from 5 to 23, similar to the range in samples from the Guachipelin Caldera.

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.