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other lava type of these major volcanic centres of the Roman Province. This may be
attributed to the fact that Mount Vulsini is located at the northern end of the Roman
Province, where this petrographic province overlaps a partially melted terrain,
which was extensively heated during the Tuscan magmatic event. They envisaged
both mixing of magmas and assimilation of Tuscan metasedimentary basement
rocks. It took place in conjunction with fractional crystallization of high K and low
K-series parent magmas, which resulted in making the isotopic effect at Mt. Vulsini
more complex.
According to Ferrera et al. (1986) during the last few million years, the upper
mantle beneath central Italy underwent a large scale mixing process that introduced
radiogenic Sr into the source regions of the leucite-bearing volcanic rocks. This
resulted in the production of voluminous high K-series magmas with
87
Sr/
86
Sr
varying between 0.7101 and 0.7103, and the
18
O ranging between + 5.5 and + 7.5.
δ
18
O values increased because of interaction of the magmas with the overlying
continental crust. According to them, towards southeast and away from the linear
belt there is another zone of major volcanoes (Ernici, Roccamon
The
δ
na, Vesuvius and
Phlegrean Fields), where the effect of this interactions was less insigni
cant. In
87
Sr/
86
Sr
these volcanoes
towards
southeast,
the
ratios
are much lower
18
O values do not change signi
δ
(0.7060
cantly. If this uniformity
in
87
Sr/
86
Sr ratio of this high K-series end member rocks (0.71025) is proved to be
true by future investigators then it would mean that a large volume of the upper
mantle beneath Italy was associated with continued magmatic and metasomatic
processes. These processes took place during the last few million years. The other
alternative explanation could be related to varying Rb/Sr ratios in the upper mantle
reservoir, which might have led to much greater heterogeneity in the
87
Sr/
86
Sr ratio.
The evolved phonolitic and trachytic tuffs were studied by Turbeville (1993),
and found that they have high chondrite-normalized Th, Zr, Rb and REE abun-
dances relative to Nb, Ta, Ba and Sr (Fig.
5.12
). According to him the large
-
0.7095), but the
10000
Trachyte
Phonolite
Tephriphonolite
Garnet Syenite
Quartz Syenite
1000
100
10
1
Ba Rb Th Nb Ta La Ce Sr Nd Sm Zr
Hf Eu Y
Yb Lu
Fig. 5.12 Spidergram plots of chondirte-normalized trace element concentrations in selected
samples of Latera pumice, syenites and lava (normalized values are from other sources) (after
Turbeville 1993)
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