Geochemical and petrographical studies of lavas and ignimbrites from the
Quaternary Nisyros-Yali volcanic system in the easternmost part of the
Hellenic arc (Greece) reveal insight into magma generating processes. A
compositional gap between 61 and 68 wt.% SiO2 is recognized that
coincides with the stratigraphic distinction between pre-caldera and
postcaldera volcanic units. Trace element systematics support the
subdivision of Nisyros and Yali volcanic units into two distinct suites
of rocks. The variation of Nd and Hf present day isotope data and the
fact that they are distinct from the isotope compositions of MORB rule
out an origin by pure differentiation and require assimilation of a
crustal component. Lead isotope ratios of Nisyros and Yali volcanic
rocks support mixing of mantle material with a lower crust equivalent.
However, Sr-87/Sr-86 ratios of 0.7036-0.7048 are incompatible with a
simple binary mixing scenario and give low depleted mantle extraction
ages (< 0.1 Ga), in contrast with Pb model ages of 0.3 Ga and Hf and Nd
model ages of ca. 0.8 Ga. The budget of fluid-mobile elements Sr and Pb
is likely to be dominated by abundant hydrous fluids characterised by
mantle-like Sr isotope ratios. Late stage fluids probably were enriched
in CO2, needed to explain the high Th concentrations. The occurrence of
hydrated minerals (e.g., amphibole) in the first post-caldera unit with
the lowermost Sr-87/Sr-86 ratio of 0.7036 +/- 2 can be interpreted as
the result of the increased water activity in the source. The presence
of two different plagioclase phenocryst generations in the first lava
subsequent to the caldera-causing event is indicative for a longer
storage time of this magma at a shallower level. A model capable of
explaining these observations involves three evolutionary stages. First
stage, assimilation of lower crustal material by a primitive magma of
mantle origin (as modelled by Nd-Hf isotope systematics). This stage
ended by an interruption in replenishment that led to an increase of
crystallization and, hence, an increase in viscosity, suppressing
eruption. During this time gap, differentiation by fractional
crystallization led to enrichment of incompatible species, especially
aqueous fluids, to silica depolymerisation and to a decrease in
viscosity, finally enabling eruption again in the third stage. (c) 2005
Elsevier B.V. All rights reserved.