Evolution of silicic magmas in the Kos-Nisyros volcanic center,Greece: a petrological cycle associated with caldera collapse |
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| Authors: | Email author" target="_blank">Olivier?BachmannEmail author Chad?D?Deering Janina?S?Ruprecht Christian?Huber Alexandra?Skopelitis Cedric?Schnyder |
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| Institution: | 1.Department of Earth and Space Sciences,University of Washington,Seattle,USA;2.Department of Geology,University of Wisconsin,Oshkosh,USA;3.Lamont-Doherty Earth Observatory of Columbia University,Palisades,USA;4.School of Earth and Atmospheric Sciences, Georgia Institute of Technology,Atlanta,USA;5.Section des Sciences de la Terre et de l’Environnement,Université de Genève,Geneva,Switzerland;6.Muséum d’Histoire Naturelle de Genève,Geneva,Switzerland |
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| Abstract: | Multiple eruptions of silicic magma (dacite and rhyolites) occurred over the last ~3 My in the Kos-Nisyros volcanic center
(eastern Aegean sea). During this period, magmas have changed from hornblende-biotite-rich units with low eruption temperatures
(≤750–800°C; Kefalos and Kos dacites and rhyolites) to hotter, pyroxene-bearing units (>800–850°C; Nisyros rhyodacites) and
are transitioning back to cooler magmas (Yali rhyolites). New whole-rock compositions, mineral chemistry, and zircon Hf isotopes
show that these three types of silicic magmas followed the same differentiation trend: they all evolved by crystal fractionation
and minor crustal assimilation (AFC) from parents with intermediate compositions characterized by high Sr/Y and low Nb content,
following a wet, high oxygen fugacity liquid line of descent typical of subduction zones. As the transition between the Kos-Kefalos
and Nisyros-type magmas occurred immediately and abruptly after the major caldera collapse in the area (the 161 ka Kos Plateau
Tuff; KPT), we suggest that the efficient emptying of the magma chamber during the KPT drew out most of the eruptible, volatile-charged
magma and partly solidified the unerupted mush zone in the upper crust due to rapid unloading, decompression, and coincident
crystallization. Subsequently, the system reestablished a shallow silicic production zone from more mafic parents, recharged
from the mid to lower crust. The first silicic eruptions evolving from these parents after the caldera collapse (Nisyros units)
were hotter (up to >100°C) than the caldera-forming event and erupted from reservoirs characterized by different mineral proportions
(more plagioclase and less amphibole). We interpret such a change as a reflection of slightly drier conditions in the magmatic
column after the caldera collapse due to the decompression event. With time, the upper crustal intermediate mush progressively
transitioned into the cold-wet state that prevailed during the Kefalos-Kos stage. The recent eruptions of the high-SiO2 rhyolite on Yali Island, which are low temperature and hydrous phases (sanidine, quartz, biotite), suggest that another large,
potentially explosive magma chamber is presently building under the Kos-Nisyros volcanic center. |
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