ORIGIN OF DISTINCT SILICIC MAGMA TYPES FROM THE GUACHIPELÍN CALDERA, NW COSTA RICA: EVIDENCE FOR MAGMA MIXING AND PROTRACTED SUBVOLCANIC RESIDENCE

Abstract

Lower Pleistocene pyroclastic deposits in NW Costa Rica represent sequential eruptions of distinct dacitic–rhyolitic (69–79% SiO2) magmas from the Guachipelín Caldera that were emplaced in a period of 0.75 myr. This southern segment of the Central American arc, the Chorotega block, evolved on the modified Caribbean Large igneous province, an overthickened oceanic plateau; silicic magmatism is uncommon in settings such as this, void of continental crust. Previously, models have proposed the generation of silicic magmas in this type of setting by the partial melting or extreme crystal fractionation of intermediate source rocks (e.g. [Tamura, Y., Tatsumi, Y., 2002. Remelting of an andesitic crust as a possible origin for rhyolite magma in oceanic arcs: an example from the Izu–Bonin arc. Journal of Petrology 43, 1029–1047]). In fact, recent melting experiments performed by Sisson et al. [Sisson, T.W., Ratajeski, K., Hankins, W.B., Glazner, A.F., 2005. Voluminous granitic magmas from common basaltic sources. Contributions to Mineralogy and Petrology 148 (5), 542–565] suggest that melting of andesite or high-K basalts, common along the Central American volcanic arc, could produce silicic melts with the characteristic alkali content of these types of eruptives.