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One of the laccoliths is constituted of fergusite with phenocrysts of biotite, sanidine,
anorthoclase, pseudoleucite and small amount of clinopyroxene (diopside, aegrine
and augite). Calcite and apatite are also present in the
fine-grained matrix.
4.13 Potassium-Rich Rocks from Oceanic Islands
4.13.1 Volcanic Activity in the Aeolian Arc Region
The potassic volcanism of southern Italy (including the Roman Province) is well-
known and eruption of potassic lavas have been interpreted in terms of subduction-
related processes (Ellam 1986). The Roman Province lavas are characterized by
highly potassic volcanism, but the lavas of Aeolian islands located in the Tyrrhe-
nian sea is known for various types of rocks from calc-alkalic series to more
potassic shoshonitic lavas erupted within a period of 1 million years (Barberi et al.
1974; Dupuy et al. 1981). The archipelago is constituted of seven islands: Lipari,
Salina, Stromboli, Vulcano, Panarea, Filicudi, and Alicudi (Fig.
4.20
a). The
archipelago is located near the northern continental slope of Sicily. According to
Ellam et al. (1989) there is extensive variation in the K
2
O content of the lavas of
southern Italy. The Aeolian Arc lavas do not belong to the high K-series variety and
may be described as low K
2
O series rocks, and are also silica-saturated. They noted
however, that two volcanoes of Aeolian island namely Stromboli and Vulcano
erupted potassic-lavas. Rocks of Aeolian island form a continuous series from
strongly silica-undersaturated ultrapotassic lavas to hypersthene-normative calc-
alkaic rock series. It must be emphasized here that the majority of the Aeolian
magmatism is calc-alkalic to high K- calc-alkalic in af
nity.
Ellam et al. (1989) noted that the trace element pattern of K-rich ma
c lavas of
Roman and Aeolian lavas present negative anomalies for Nb and Ta. The rocks of
Stromboli however, present relatively smaller anomaly. All the lavas show
enrichment in LILE over REE. The trace element geochemistry of the Aeolian lavas
is consistent with the view that
they owe their origin to subduction-related
volcanism.
The
143
Nd/
144
Nd versus
87
Sr/
86
Sr plot of variety of rocks from Aeolian Arc, Etna
and Roman Province, shows continuous variation from high Nd isotopic ratios
(Etna and Salina to Roccamon
na and Vulsini lavas). In fact they display a linear
trend. Analyses of some of the rocks are given in Table
4.20
.
According to Barberi et al. the volcanism in this area is characterized by a
marked evolution in a restricted period of 1 million years, from typical cal-alkalic to
shoshonitic rocks. The
first volcanic stage was characterized by calc-alkalic series,
ranging from high alumina basalt to dacite. At the beginning of the second stage,
there was also eruption of potassic andesites at Lipari, whereas in Salina the last
stage of calc-alkalic volcanism took place. The most recent volcanism is charac-
terized by further increase of the potassium content, indicating their shoshonitic
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