Holocene post-caldera magma evolution of Llaima volcano, Chile

Schindlbeck, Julie, Freundt, Armin, Kutterolf, Steffen and Strehlow, Karen (2012) Holocene post-caldera magma evolution of Llaima volcano, Chile [Poster] In: The Lübeck Retreat, Collaborative Research SFB 574 Volatiles and Fluids in Subduction Zones: Climate Feedback and Trigger Mechanisms for Natural Disasters, 23.-25.05.2012, Lübeck.

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Abstract

Llaima is a large active stratovolcano in the Southern Volcanic Zone in Chile. Field work in 2011
revised the postglacial stratigraphy after Naranjo&Moreno (1991) and led to the subdivision into units I
to V. Postglacial activity started 13.500 years ago with caldera-forming eruption of two mafic
ignimbrites (unit I). These are overlain by a sequence of three basaltic-andesitic to two dacitic lapilli
fallout deposits and reworked tuffaceous sediments (unit II). At ~8600 cal BC a large Plinian eruption
emplaced a compositionally zoned dacitic to andesitic fallout tephra (unit III) that became capped by
subsequent andesitic surge deposits (unit IV) when the eruption became unstable. The following unit
V represents a time interval of ~7000 years during which at least 30 basaltic to andesitic ash and
lapilli fallout deposits with intercalated tuffaceous sediments and paleosols were emplaced.
Bulk-rock, mineral and glass chemical analyses constrain the vertical compositional changes of
Llaima tephras. Tephra compositions switch between a calc-alkaline differentiation trend (unit I) and a
more tholeiitic trend (units II-IV), with samples of unit V varying between both trends, indicating a
strong control of f(O2) (and P(H2O)) on the relative timing of Fe-Ti oxide fractionation. Moreover, iron
rich fayalites that are in equilibrium with the glass composition occur in units II and III with calculated
T-f(O2) close to the FMQ suggesting that late-stage fayalite precipitation involved crossing of the FMQ
boundary. The younger unit V tephras and historical compositions define a second differentiation
trend relatively enriched in K2O, Rb, Ba and Zr; this is not the results of changing source conditions
but can be explained by a stronger early olivine fractionation in the respective magmas.
Thermobarometric calculations based on amph, cpx-liq, plag-liq, ol-liq and Fe-Ti-oxide compositions
constrain changing magma chamber positions over time. Storage depths were 14 - 19 km for unit I
andesite and and varied between 10 to 17 km for unit II andesites and dacites. The compositionally
zoned eruption of units III and IV withdrew dacite magma from ~10 km depth but andesite from a deeper level of 13-15 km. Storage depths of unit V andesitic magmas ranged from 6 to 15 km. Based
on temporally changing storage depths and differentiation paths, a 4-stage evolution of the postglacial
magmatic system of the Llaima volcanic complex is proposed.
Reference: Naranjo JA, Moreno H (1991) Actividad explosiva postglacial en el volcan Llaima, Andes del Sur (38°45’S).
Revista Geologica de Chile 18: 69-80

Document Type: Conference or Workshop Item (Poster)
Research affiliation: OceanRep > SFB 574 > C4
OceanRep > SFB 574
OceanRep > GEOMAR > FB4 Dynamics of the Ocean Floor > FB4-MUHS Magmatic and Hydrothermal Systems
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Date Deposited: 19 Sep 2012 10:29
Last Modified: 23 Dec 2016 10:29
URI: http://eprints.uni-kiel.de/id/eprint/15029

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