Different Localities of Potassium-Rich Silica-Undersaturated Igneous Rocks and Their Silica-Rich Variants - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 168

northwest of Rome. The volcanic history of the area started with eruption of
tephritic ignimbrites and lavas, which preceded the eruption of Vico volcano. This
was followed by another stage of volcanic activities, which coincided with the
eruption of the Vico lavas. The third stage was associated with eruption of tephritic
and leucitic lavas. The major volcanic activity of Sabatini took place after the
completion of the eruption of the Vico complex, which is located adjacent to it.
Voluminous pumice-rich deposits were erupted from the Sabatini volcano,
which is far more in volume than the volume of lava

flows associated with the
eruption of pyroclasts, which took place repeatedly, forming well strati ed weakly

cemented tuffaceous materials. Bulk chemistry and mineralogy of the Sabatini lavas
show that they plot in the tephritic leucitite and tephritic phonolite field. According

to Cundari (1979), tephritic leucitites are uncommon in Vico, but common in
Sabatini. This may indicate that Sabatini lavas may constitute afterward extension
of the Vico volcanic

field.
The Sabatini leucitites do not contain feldspars, which are considered to be a rare
feature in lavas occurring north of Rome, but are common in Alban Hills (Fornaseri
et al. 1963). Another interesting feature of the Sabatini lavas is the occurrence of
olivine, nepheline and ha

ü

yne. Chemical analyses of some leucitites from Sabatini

are given in Table 4.16 .

4.8.4.5 Alban Hills

Fornaseri et al. (1963) described the detailed geology of the rocks of Alban Hills
(Fig. 4.20 b). The area is characterized by different sequence of lava ows.

(1) Vallerano ow,

(2) Osa ow,

(3) Capo di Bove

ow,

(4) Monte Falcone ow,

(5) Acqua Acetosa lava pile, and
(6) Santa Maria delle Mole lava ow.

There are also ejected lava blocks from Monte Cavo and Peperino and volcanic
tuffs haremmana Inferiore. Aurisicchio et al. (1988) classi ed the lavas as leucite

tephrites, phonotephrites, phonolites and foidite. According to Di Radicatti et al.
(1981) the Villa Senni Tuff, which is an important pyroclastic unit of Alban hills,
yielded an age of 338

8 ka. However, according to Ambrosetti et al. (1972) the
ages of the Alban Hills rocks vary from 43,000 to 700,000 years. The lavas from
Alban Hills often include xenoliths and ejecta of biotite leucitite, magnetite leuci-
tite, and biotite xenocrysts.
According to Aurisicchio et al. (1988) clinopyroxene crystals in the lavas of
Alban Hills, occur as phenocrysts and microphenocrysts in the groundmass. The
clinopyroxenes are the predominant phenocrystal phases in the post caldera sam-
ples. Aurisiccio et al. found coexistence of two core-rim evolutionary trends within
the same rock:

±