Petrogenesis of distinct silicic magma types from the lower Pleistocene Guachipelin Caldera, NW Costa Rica: Extensive magma mixing and protracted subvolcanic residence.

摘要

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. Tamura and Tatsumi, 2002; Sisson et al., 2005) suggesting that partial melting of crystalline, calc-alkaline andesite or high-K basalts 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.; Polytopic vector analysis (PVA), a multivariate statistical method, was used to characterize the mixing relationship between individual units evident in the mineralogy and petrology of the pumice fragments. The program defined four different end member (EM) magmas, which contributed to the generation of the seven units associated with the Guachipelin Caldera. This requires extended periods when the EM melts 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 chemical variations of the magmas have occurred, which require processes to operate on relatively short time scales.