The basaltic plinian eruption of the ~6 ka San Antonio Tephra and formation of the Masaya caldera, Nicaragua

Abstract

Masaya is unusual for a basaltic caldera because it formed by piston-subsidence in response to large-volume magma withdrawal by highly explosive eruptions, i.e. in a fashion typical of silicic calderas. The first and most voluminous of the three explosive eruptions formed the 6 ka old basaltic San Antonio Tephra (SAT). This eruption is also unusual in that most of the 9 km3 DRE basaltic magma was discharged by a plinian eruption. The subsequent eruptions of the basaltic Masaya Triple Layer (MTL, 2.1 ka) and the Masaya Tuff/Ticuantepe Lapilli (MT-TIL, 1.9 ka) each discharged 2 km3 DRE magma and enlarged the Masaya caldera.The SAT consists of a lower sequence of alternating scoria lapilli and ash layers, interpreted as an alternation between more or less phreatomagmatically influenced fallout events. These are followed by two prominent well-sorted lapilli layers: the first one formed by a climactic plinian eruption whose column height reached 21–29 km and discharged most of the total erupted mass including about 35 Mt SO2. The second, lithic-rich lapilli layer probably formed by a phreatoplinian event when partial collapse of the magma chamber roof initiated increasing magma-water interaction which ultimately formed the upper sequence of phreatomagmatic cross-bedded surge deposits, accretionary lapilli-rich tuffs and a final fallout of dense lapilli. Phreatomagmatic activity may have been related to disruption of a hydrothermal system reflected in hydrothermally altered lithics, and/or by the caldera floor subsiding closer to the groundwater table.The bulk-rock chemical composition of the SAT is basaltic but the bimodal glass compositions demonstrate mixing of a basaltic with an andesitic melt probably in the conduit during eruption. The SAT basalt differentiated in a reservoir near the MOHO at 20 km depth by fractional crystallization of olivine, plagioclase, and minor clinopyroxene forming a tholeiitic fractionation trend. Minor intermediate-An plagioclase crystallized from the basaltic melt at H2O concentrations of about 2 wt% as measured by FTIR in melt inclusions. However, a key observation is that the melt inclusions are not in equilibrium with the high-An plagioclases hosting them. Re-equilibration of the inclusions requires initially higher water contents (about 5–6 wt%) which also fits the high Ba/La ~ 80 indicating input from the strongly hydrated subducting slab. Therefore, while the SAT magma evolved under hydrous conditions at depth, it was then stored at shallow level long enough to adjust to the low saturation pressure and to precipitate some intermediate-An plagioclase but still preserving its high temperature (around 1100 °C) and phenocryst-poor composition. Large overpressure due to connection to the deep-seated reservoir and water degassing during ascent limited the storage time at shallow level and drove the unusually intense and voluminous plinian-style eruption that facilitated piston-type collapse of the chamber roof.