Gilbert, David, Freundt, Armin and Kutterolf, Steffen (2012) Post-glacial time series of explosive eruptions and associated changes in the magma plumbing system of Lonquimay Volcano, South Central Chile. [Talk] 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 . The Lübeck Retreat: final colloquium of SFB 574; May 23-25, 2012: program & abstracts. ; p. 7 .

Abstract

The Lonquimay Volcanic Complex (LVC) in the high Southern Andes comprises a stratocone and NEtrending
flank-cone alignments. Numerous effusive and explosive volcanic eruptions characterize its
post-glacial magmatic activity. Our tephrostratigraphic record, pre-dating the four historically
documented eruptions, comprises 22 dated pyroclastic deposits that are used to constrain repose
time distribution and eruption probability of the LVC magmatic system. Statistical examination of the
stratigraphy-based eruption time series yields probabilities of 20-50% for at least one explosive
(VEI"3) eruption within the next 100 years as of 2011.
The tephra deposits are subdivided into three petrographic groups: a felsic group (Lonquimay Colored
Pumice Tephra, LCPT), an intermediate population (Lonquimay Grey Pumice Tephra, LGPT) and a
mafic member (Lonquimay Dark Scoria Tephra, LDST). The distribution of these petrographic groups
through the LVC tephrostratigraphy is linked to the observed changes in repose times. LDST deposits
as well as deposits compositionally zoned from LCPT to LGPT dominate the lower part of the
stratigraphy for which recurrence times are short (RTmean=417±169a). Deposits younger than 6000
b2k (years before 2000 AD) have dominantly LCPT and minor LDST compositions, no longer contain
LGPT, and repose times are significantly longer (RTmean=1350±310a). We interpret the change in
eruption regime to result from a rearrangement in the magma storage and plumbing system.
Thermobarometric calculations based on cpx liquid equilibria and amphibole compositions reveal
three distinct magma storage levels: the mafic LDST derive from mid crustal storage (Pmean= 476±95
MPa, Tmean=1073±24°C), felsic LCPT mainly erupted from upper-crustal level (Pmean= 86±49 MPa ,
Tmean=936±24°C), whereas LGPT samples yield intermediate storage depths (Pmean= 239±100 MPa,
Tmean=1013±17°C). Magma contributions from this intermediate reservoir are restricted to >6000 b2k
when the Lonquimay plumbing system was in a regime of short repose times; disappearance of the
intermediate reservoir coincides with the change to longer repose times between eruptions.

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