Kutterolf, Steffen , Freundt, Armin and Burkert, Cosima (2011) Eruptive history and magmatic evolution of the 1.9 ka Plinian dacitic Chiltepe Tephra from Apoyeque volcano in west-central Nicaragua. Bulletin of Volcanology, 73 (7). pp. 811-831. DOI 10.1007/s00445-011-0457-0.

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Abstract

The youngest dacitic Plinian eruption in westcentral
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 (An45–68), 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% SiO2) 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.

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