Abstract
The Campanian Ignimbrite (CI) is a large-volume trachytic tuff erupted
at 37 ka from the Campi Flegrei and composed of a fallout deposit
overlain by ignimbrite. The ignimbrite was spread over an area of
about 30,000 km2 including the Campanian Plain and the Apennine Mountains,
with ridges over 1000 m a.s.l. The pumice fragments of the CI range
in composition from trachyte to phonolitic-trachyte (DI = 75-90).
They do not show any systematic compositional variation with stratigraphic
height, but the analyzed sections can be divided into three groups
on the basis of chemical composition of pumices. Least-evolved pumices
(DI = 75-83) occur in the ignimbrite in the central sector of the
Campanian Plain up to 30 km from the vent, while the most-differentiated
pumices (DI = 88-90) characterize the cogenetic fallout deposit and
the ignimbrite in the western sector of the Campanian Plain, on the
Tyrrhenian scarp of the Apennines between Caserta and Mt. Maggiore,
on Roccamonfina volcano, and on the Sorrento Peninsula, up to 50
km from the source. Pumice fragments of intermediate composition
(DI = 84-87) occur in the ignimbrite on the Apennine Mountains and
Roccamonfina volcano, up to 65 km from the vent. In one exposure
at Mondragone, at the base of a calcareous ridge, an ignimbrite with
pumices of most-evolved composition is overlain by an ignimbrite
with pumices of intermediate composition.The observed compositional
variation between most- and least-evolved ignimbrite was generated
in part by crystal-liquid fractionation, although other magmatic
processes such as syn-eruptive mingling between most- and least-evolved
magmas accounts for the mineralogical disequilibria and for the bimodality
of the glass compositions in the intermediate-composition rocks.
Pumice Sr-isotope ratios are positively correlated with degree of
differentiation. Feldspar crystals separated from pumices of different
compositions have a homogeneous Sr-isotope composition similar to
that of the least-evolved pumices. Interaction between fluids and
strongly fractionated Sr-poor less-dense magma can account for these
isotopic features.Geochemical, mineralogic, stratigraphic and volcanologic
data, together with the stratigraphic relations between most-, intermediate-
and least-evolved ignimbrite as detected at Mondragone and from bore-hole
drillings suggest that: (1) the CI magmatic system was composed of
two distinct magma layers - the upper layer was more differentiated
and homogeneous in composition, while the deeper was less evolved
and slightly zoned; and (2) the CI was mostly emplaced in three main
pulses of pyroclastic flows that tapped the chamber at variable levels
and with distinct withdrawal dynamics. The eruption began with emission
of the most differentiated magma, which gave rise to the fallout
deposit. It continued with generation of expanded, turbulent pyroclastic
flows that reached the Sorrento Peninsula in the southeast and Roccamonfma
volcano in the northwest. These flows, whose thickness was greater
than the overtopped relief, were able to travel over the water of
the bay of Naples. Subsequently an intermediate-composition magma
resulting from mingling of different portions of the magma chamber
generated similar flows that spread radially and traveled not less
than 65 km from the vent. During the last pulse the least-evolved
magma was tapped and generated flows that spread within the Campanian
Plain.Variation in eruptive dynamics and composition of magma during
the course of the eruption likely reflected variations of both geometry
of vent and plumbing system, and efficiency of water/magma interaction,
which in turns affected the dynamics of the magma chamber and the
withdrawal mechanism, and resulted from the dynamics of the caldera
collapse.
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