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of eruption (Fig.
4.24
). During this phase, there is violent upsurge of globular
masses of gas and vapour, which rise to several kilometres.
A detailed study of the pyroclastic deposits of the AD 79
Plinian
eruption of Vesuvius was studied by Cioni et al. (1995). The pyroclastic material is
characterized by K-tephritic to K-basanitic compositions. These were emplaced at
temperatures >1,150
“
Pompai
”
°
C. They had high volatile contents (2
5 wt%) comprising
-
H
2
O, Cl, F, and S.
Chemical analyses of lavas discharged during Plinian type of eruption of the
Somma-Vesuvius complex show that they form a distinct trend characterized by
higher values of alkali and lower values of silica with respect to the Somma lavas
(Cundari and Le Maitre 1970). The difference however, decreases towards the basic
end of trends, when there is increase in ma
c minerals with respect to alkali
alumino silicates. According to Cundari and Le Maitre, leucite is the dominant
alkaline phase in Vesuvius lavas associated with minor amounts of plagioclase and
sanidine, whereas leucite is a minor constituent in Somma lavas coexisting with a
large amount of plagioclase and sanidine. They found that products of the Plinian
type of eruptions are mainly glassy phonolitic derivatives, which are genetically
related to the lava
flows erupted earlier, and they formed their natural extension
towards higher silica values. The most salic types (Thornton and Tuttle differen-
tiation index equal to 88
93) plot near the low temperature minimum of the pho-
nolitic compositions (Qz = 27
-
34; Ne = 35
-
47; Ks = 25
-
27) of Hamilton and
Mackenzie (1965). The magma appears to have undergone crustal differentiation.
Peccerillo (2001) made some comparative geochemical studies among the lavas
of Vesuvius, Sromboli and Phlegrean Fields, and discussed about their geodynamic
and volcanological implications. Data reveal close compositional af
-
nities between
these two volcanoes. The abundant 13
15 Ka old Stromboli leucite-tephritic rocks
have radiogenic isotope signatures, and abundances and ratios of incompatible
elements with the exception of Rb and K, which are identical to those of Vesuvius.
The Phlegraean Fields also show close af
-
nities to these volcanoes.
Their data reveal close compositional af
nities between these two volcanoes.
The abundant 13
15 Ka old Stromboli leucite-tephritic rocks have radiogenic
isotope signatures, and abundances and ratios of incompatible elements with the
exception of Rb and K, which are identical to those of Vesuvius. The Phlegraean
Fields also show close af
-
nities to these volcanoes.
The most primitive rocks from Vesuvius, Phlegraean Fields and Stromboli
reveal intermediate compositions between arc and intraplate volcanics. It is sug-
gested that the mantle sources beneath these volcanoes consist of a mixture of
intraplate- and slab-derived components. Intraplate material was probably provided
by in
ow of asthenosheric mantle into the wedge above the subducting Ionian Sea
plate, either from the Apulian plate and/or from the Tyrrhenian Sea region. Fluids
or melts released from the sinking slab and associated sediments generated meta-
somatic modi
cation of the intraplate material, whose melting gave rise to the
Stromboli, Vesuvius and Phlegraean Fields magmas.
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