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been active from about 1 Ma ago to a few thousands year B.P. (Amato et al. 1991).
The seismic velocity structure of the Vulsinian volcanic complex was determined
using seismic refraction method. According to Amato et al. the area is characterized
by complex structures in the shallow crust (0
7 km), which has strong lateral
-
heterogeneities. The upper layer (0.5
2.7 km thick) corresponds to a volcanic cover
-
and the upper part includes
flysch sequence. The region to the top of the middle
layer corresponds to the lower part of the
flysch unit and to the Mesozoic carbonate
sequence (2
5 km thick). The third layer extends beneath the whole Vulsinian
region and is characterized by high P-wave velocity (6.7
-
7.1 km/s) and at shallow
-
depth, it is 5
7 km/s. Such a high velocity is not related to the occurrence of high
grade metamorphic rocks such as granulites and metamorphosed tuffs. The velocity
pro
-
les and 3-D inversion showed that there is a velocity decrease by 5 % within
the deepest layer of the central part of the volcanic complex. They interpreted it to
be due to high temperature of the layer or presence of a liquid pool in the intrusive
body or deepening of the top of the third layer.
The silica-undersaturated lavas are characterized by leucitites, basanites, leucite
tephrites, phonolites, tephritic leucite phonolites and trachytes. Relatively more
silica-saturated K-rich rocks include shoshonites, shoshonitic andesites and high
K-trachytes, occurring at Latera Caldera (Schneider 1965). Neither mantle xenoliths
nor cumulate blocks are reported from these rocks. Blocks of well crystallized
ejecta containing clinopyroxene and leucite phenocrysts (present in almost equal
volume percentage) have been described. Most of the lavas are porphyritic with
1
15 vol% phenocrysts included in a matrix of clinopyroxene, leucite, plagioclase,
sanidine, dark phlogopite, minor amount of ore minerals with or without olivine.
The K-rich undersaturated rocks are intermediate to metaluminous. In contrast to
perpotassic rocks of East African Rift Valley, Leucite Hills and West Kimberly,
these rocks have high K
2
O/Na
2
O ratio (2
-
8), Al
2
O
3
(10.67
20 wt%), moderate
-
-
amount of total iron oxide (3.60
2.67 wt%) con-
tents. The degree of undersaturation is characterized by the presence of 20
8.64 wt%), and low TiO
2
(1.72
-
-
40 vol%
-
leucite and nepheline in the most basic lavas, and 0
10 % nepheline and leucite in
phonolites and trachytes. Diopside, leucite, anorthite and nepheline constitute more
than 80 vol% of the norm.
Barton et al. (1982) studied clinopyroxene zoning in the lavas of Vulsini Latium
in Italy. They studied two tephritic leucitites, two leucite phonolites and one tra-
chyte. They noted three compositionally distinct types of pyroxenes:
-
1. Salite (forming resorbed pyroxene phenocrysts),
2. Diopside (forming anhedral cores in pyroxene phenocrysts), and
3. Clinopyroxenes of intermediate composition between (a) and (b).
According to Barton et al. (1982) the compositional zoning of pyroxenes may be
related to mixing of two magmas both of tephritic leucitite in composition, one
relatively Fe-rich with salite phenocrysts and evolved, and the other more primitive
(Mg-rich) having diopside phenocrysts. Nappi et al. (1991) described evidence of
incremental growth in the Vulsinian calderas. On the basis of volcanological
field
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