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been hydrothermally altered. This breccia occurs on opposite sides of the 12 km
wide Phlegrean depression at Torrefumo and Soccavo. These rocks have been
regarded as a proximal facies and source rocks for Campanion ignimbrites, which
are represented by large pyroclastic
flow deposits that erupted 33,000 years ago.
These pyroclastic deposits cover the entire Campanian plain. This pyroclastic unit
probably erupted from a
fissure system, judging from the west
northwest
east
-
-
southeast mega-breccia alignment.
The Breccia Museo member (Melluso et al. 1995) was formed by explosive
eruption that occurred in the southwest sector of the Phlegrean Field about 20 ka ago.
The product of this eruption, range in composition from the trachytic to trachy-
phonolitic lavas with K
2
O content decreasing from 9.5 to 7 wt% and the Na
2
O
correspondingly increases from 2.6 to 7.2 wt% with differentiation (Nb from 23 to
122 ppm). The phenocrysts are mostly sanidine (Or
88
-
63
) with subordinate plagio-
clase (An
33
-
27
), clinopyroxene (Ca
47
Mg
44
Fe
09
-
Ca
46
Mg
35
Fe
19
), biotite, titanomag-
netite and apatite. The very recent post-caldera activity of the Phlegrean Field has
been subdivided into three main volcanic phases. The
first phase started with sub-
marine volcanic activity (38,500
-
10,500 years B.P.) erupting tuf
tes with interca-
lations of trachyte-latite lavas. This was followed by caldera-
lling with trachytic
tuffs, which erupted from several trachytic pyroclastic vents, both in marine and
subaereal environments. This is the Neopolitan yellow tuff phase. The second phase
(10,500
-
3,000 years B.P.) was mostly subaereal with eruption of pyroclastic tra-
chyte mainly from vents concentrated towards the west (Baia and Fondi di Baia) and
in the central eastern sectors. Some shoshonitic and latitic lavas also erupted from the
vents at Minopoli, Concola and Fondo Riccio. The last subaereal phase (4,500 years
B.P.
flows of
trachytic and trachyphonolitic composition. The eruption took place from a number
of vents, concentrated towards the caldera centre.
The spectrum of volcanic products in the Phlegrean Fields are petrologically
distinct and part of a co-magmatic sequence ranging from trachybasalts to
trachyphonolites, all appear to be related by a fractional crystallization process
(Armienti et al. 1983). The REE data are in agreement with this general scheme.
The Phlegrean Fields volcanism was characterized by strong block faulting of
the basement by a complex magma system at depths. According to Beccaluva et al.
(1990), this magma system was fed by parental magmas fractionating indepen-
dently in a partially open system. According to them, fractional crystallization
process gave rise to distinct liquid lines of descent developed in a complex fracture
system, where fractionating magmas are compositionally zoned upward and
sometimes mixed with each other with ma
1538 A.D.) resulted in the eruption of pyroclastic products and lava
-
c melts from depths.
4.8.4.7 Roccamon
na
Roccamon
na complex (Fig.
4.20
a) lies 50 km NNW of Somma-Vesuvius along
the western coast of Italy (Luhr and Giannetti 1987). The evolutionary history is
long and petrologically complex. Giannetti (1979) divided the history of the
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