The genetic relationship between andesites and dacites at Tungurahua volcano, Ecuador
Résumé
Volcanic eruptions of intermediary and silica-rich magmas (andesites, dacites and rhyolites) in convergent arc
settings generate voluminous and explosive eruptions that can strongly affect human activity and have signi
fi
-
cant environmental impacts. It is therefore crucial to understand how these magmas are generated in order to
anticipate their potential impact. At convergent margins, primitive magmas (primitive basalts and/or andesites)
are derived from the mantle wedge and they are progressively modi
fi
ed by physical and chemical processes op-
erating between the melting zone and the surface to produce silica-rich magmas.
In order to elucidate the relationship between andesites and dacites, we focus on Tungurahua volcano, located in
the Ecuadorian Andes. We collected a set of samples comprising such lithologies that were erupted during the
last 3000 year BP. This relatively short period of time allows us to assume that the geodynamic parameters re-
main constant. Petrology and major-trace element compositions of these lavas have already been examined,
and so we performed a complementary Pb-Sr isotope study in order to determine the nature and origin of the
components involved in andesite and dacite genesis. Sr isotopes range from 0.70417 to 0.70431, and Pb isotope
compositions range from 18.889 to 19.154 for
206
Pb/
204
Pb, from 15.658 to 15.696 for
207
Pb/
204
Pb, and from
38.752 to 38.918 for
208
Pb/
204
Pb. Dacites display a remarkably homogeneous Pb isotopic composition, with
higher
206
Pb/
204
Pb values for a given
207-208
Pb/
204
Pb compared to andesites. Andesites show notable
207
Pb/
206
Pb variations for a given SiO
2
content, whereas dacites have lower and homogenous
207
Pb/
206
Pb values.
Andesite and dacite altogether plot in a roughly triangular distribution, with dacitic magmas systematically plot-
ting at the high SiO
2
and
87
Sr/
86
Sr and low
207
Pb/
206
Pb
fi
elds. Based on our new dataset, we show that at least 3
different components are required to explain the Tungurahua compositional and isotope variation: one corre-
sponds to the mantle, the second has a deep origin (slab component or lower crust), and a mixture between
these two components explains andesite heterogeneity. The third component is derived from the underlying
upper continental crust. While andesites are derived from deep components, dacites are derived from the andes-
itic magmas that underwent an assimilation-fractional crystallization (AFC) process with incorporation of the
local metamorphic basement. Finally, we used the geochemical and isotopic data to produce a model of the mag-
matic plumbing system beneath Tungurahua, consistent with geophysical and experimental petrology con-
straints. We conclude that melt migration and storage in the upper crust appears to be a key parameter for
controlling volcanic behavior though time.