Eruptive history and magmatic evolution of the 1.9 kyr Plinian dacitic Chiltepe Tephra from Apoyeque volcano in west-central Nicaragua

摘要

The youngest dacitic Plinian eruption in west-central Nicaragua, forming the 18 km3 Chiltepe Tephra (CT), occurred about nineteen hundred years ago at Apoyeque stratovolcano, which dominates the Chiltepe volcanic complex 15 km north of the capital Managua, where the CT is 2 m thick. We have traced the CT from its proximal facies at the crater rim, through the medial facies in the lowlands around Apoyeque, and to the distal facies up to 550 km offshore in the Pacific. While medial and distal facies consist of widespread Plinian fall deposits, the proximal facies reveals the complexity of this eruption, which we divide into four phases (I-IV). Interaction of rising magma with a pre-existing crater lake generated the phreatomagmatic opening phase I of the eruption, which produced ash fall with accretionary lapilli. Phase II marked a rapid change to persistent magmatic activity that yielded several large Plinian eruptions, declining through a period of unstable eruption conditions, followed by a short hiatus. Phase III began with unstable conditions, probably as a result of eastward migration and widening of the vent, leading to a second period of Plinian eruptions with three major events reaching magma discharge rates five times larger than those of phase II. Phase III again declined through unstable eruption conditions before magmatic activity terminated. Numerous explosions in the shallow hydrothermal system during the final phase IV resulted in the formation of a phreatic tuff ring on the rim of Apoyeque crater. The white, highly-vesicular, dacitic CT pumice contains plagioclase (An_(45-68)), orthopyroxene, clinopyroxene, and minor hornblende, apatite and titanomagnetite phenocrysts. A very subordinate fraction of gray pumice has the highest crystal content, the least evolved bulk-rock, but the most evolved matrix-glass composition. The CT dacite has two unusual compositional features: (1) all white dacite has the same melt (matrix-glass) composition such that variations in bulk-rock compositions (64-68 wt% SiO_2) simply reflect different phenocryst contents of 10-35%, interpreted as the result of gradual phenocryst settling in the magma chamber. (2) Abundant olivine crystals with a bimodal distribution in Mg# (modes at Mg# = 0.75 and Mg# = 0.8) are dispersed throughout the erupted dacite. These are clearly out of equilibrium with the dacitic melt and are interpreted as xenocrysts derived from the basaltic Nejapa-Miraflores volcanic lineament that intersects the Chiltepe volcanic complex and was contemporaneously active. Thermobarometric estimates place the dacitic CT magma reservoir in the upper crust (<250 MPa), with a temperature of about 890°C and about 5 wt% water dissolved in the melt. Comparing water and chlorine contents with respective solubility models suggests that volatile degassing began in the magma reservoir and triggered the CT eruption. From the vertical compositional variation pattern of the CT we deduce that the conduit tapped the magma chamber not at the top but from the side, at some deeper level, and that subsequent magma withdrawal was governed by both variations in discharge rate and possible upward migration and/or widening of the conduit entrance.