Implications for the Petrogenesis of Distinct Silicic Magma Types from the Lower Pleistocene Guachipelin Caldera, NW Costa Rica

Lower Pleistocene pyroclastic ash-flow deposits in NW Costa Rica represent sequential eruptions of high-silica (69-79%\ SiO2) magmas from the Guachipelin Caldera. These high silica eruptions are not common in areas void of continental crust. The stratigraphic order of seven distinct units is identified by primary mineralogy and bulk chemical composition. Initial distinctions among separate stratigraphic units are defined based on pumice size, mineralogy, physical breaks, and color. First, six major units are identified based on field observations including mineralogy: glomerophyric plagioclase-amphibole (GPA), white biotite (WB), pink biotite (PB), amphibole (A), green unit rich in amphibole (GA), and plagioclase (PR). Further subdivisions are characterized by discrete chemical heterogeneities of trace elements within the macroscopic units. Most of the units identified in the field also have discrete ratios of trace elements (e.g. Nb/Ta): GPA (13.3-19.3), WB (7.6-14.6), PB (3.8-5.0), GA: (23.4-29.4); PR: (7.2-10.4). The amphibole unit (A) is the only one that presents two discrete ranges (6.5-9.5 and 11.5-13.0), which can be interpreted as an indication that the pumice fragments belong to two distinct units instead of one. These collective variations within the sequence provide the basis for petrogenetic interpretation. Differences in the incompatible element ratios behavior are consistent with partial melting (or melt segregation) of several different sources and/or partial melting of same source crust at varying degrees. Melt segregation (partial melting) from several different sources would require a complex plumbing system linking spatially distant crustal sources to a single shallow magma chamber or multiple magma chambers in the same area. In contrast, varying degrees of partial melting from a single crustal source could provide magma for recharge into a shallow chamber from a central conduit re-tapping the same source periodically. 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.