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3, there is a systematic increase in plagioclase/alkali feldspar ratio and
increasing modal leucite towards tephritic types and total ma
From 1
-
c constituent of the
tephrites contain less than one third of the bulk rock.
The lavas erupted from the central vents are phonolitic leucite tephrites (e.g.
upper
flow of Mount Venere) and tephritic leucite phonolite (Mount Venere lower
flows). Cundari and Mattias (1974) also reported the occurrence of leucite trachyte
and tephritic leucite phonolite inside the caldera. All the above three lava types
were also erupted through the vents at the peripheries. Analyses of some leucite-
bearing rocks from the Vico complex are given in Table
4.16
.
More recently, Perini et al. (2003) discussed about the evolution of magmas
from the Vico region. According to them Vico volcano erupted potassic and ul-
trapotassic magmas, ranging from silica-saturated to silica-undersaturated types, in
three distinct volcanic periods over the past 0.5 Ma. During Period I magma
compositions changed from latite to trachyte and rhyolite, with minor phono-
tephrites. During periods II and III, there were eruption of magmas primarily
phono-tephrite to tephri-phonolite and phonolite; however, magmatic episodes
involving leucite-free eruptives with latitic, trachytic and olivine latitic composi-
tions also occurred. In Period II magmas [Sr
87
/Sr
86
0.71115]
were derived from a primitive tephrite parental magma. Modelling of phonolites
with different modal plagioclase and stroncium (Sr) contents indicates that low-Sr
phonolitic lavas were differentiated from tephri- phonolite by fractional crystalli-
zation of 7 % olivine + 27 % clinopyroxene + 54 % plagioclase + 10 % Fe-Ti
(initial) = 0.71037
-
Ti
oxides + 4 % apatite at low pressure, whereas high-Sr phonolitic lavas were gen-
erated by fractional crystallization at higher pressure. More differentiated phonolites
were generated from the parental magma of the high-Sr phonolitic tephra by
fractional crystallization of 10
-
29 % clinopyroxene + 12
15 % plagio-
-
-
clase + 44
67 % sanidine + 2
4 % phlogopite + 1
3 % apatite + 7
10 % Fe
Ti
-
-
-
-
-
[Sr
87
/Sr
86
oxides.
In contrast,
leucite-bearing rocks of Period III
(ini-
tial) = 0.70812
0.70948] were derived from a potassic trachybasalt by assimilation-
fractional crystallization with 20
-
0.5 (where
r is assimilation rate/crystallization rate) at different pressures. Silica-saturated
magmas of Period II [Sr
87
/Sr
86
40 % of solid removed having r = 0.4
-
-
0.71052] appear to have been
generated from an olivine latite similar to some of the youngest erupted products. A
primitive tephrite, a potassic trachybasalt and an olivine latite are inferred to be the
parental magmas at Vico. These magmas were generated by partial melting of a
veined lithospheric mantle
(initial) = 0.71044
-
sources with different vein-peridotite/wall-rock
proportions.
4.8.4.4 Monti Sabatini
The Sabatini volcanic complex (Fig.
4.20
b) includes a number of vents, which
cluster around the lake-
lled collapsed caldera of Bracciano located 30 km north-
west of Rome. Apart from Lago de Bracciano, the other important caldera in this
locality includes Lago de Martignano. The Bracciano caldera is located 30 km
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