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their geochemical characteristics are very different from those of continental rift
lavas and similar to potassic volcanics from island arcs (Foden and Varne 1980).
Af
nity of the potassic rocks of central and southern Italy and its possible asso-
ciation with subduction-related metasomatized mantle have been suggested by many
authors (Thompson 1977; Civetta et al. 1981; Rogers et al. 1985). The discovery of
two-pyroxene highly potassic calc-alkalic basalts, basaltic andesite and andesites
from the north-western margin of the Phlegraean Fields, buried under highly potassic
alkalic rocks (Di Girolamo 1977) threw new light on subduction-related volcanism in
this area. On the basis of this observation, Beccaluva et al. (1991; Fig.
14.3
) proposed
that the mantle wedge overlying the subducted slab was hybridized by melts pro-
duced by partial fusion of subducted material derived from the continental crustal
sediments. This process might have played a dominant role in the generation of the
mantle sources from which the high-K-calc-alkaline, shoshonitic, leucite basanitic
and leucititic magmas of the Roman Province, were derived.
14.1.3 Neogene Tectonics of Southern Spain
Lopez Ruiz and Badiola (1980) discussed about Neogene volcanic activity in Spain.
The volcanism in this region was associated with calc-alkalic, high K-calc-alkalic,
The volcanic episode started during Middle-Miocene and ended at the beginning of
Pliocene, 17 and 6 Ma ago. During this period, the volcanism was associated with
progressive enrichment of K
2
O and incompatible trace elements (Rb, Ba, Pb, Sr and
Zr) and ferromagnesian components towards north, and their age also progressively
decreases in the northward direction. Thus, the calc-alkalic rocks are oldest and they
erupted in the southern part at Cabo da Grata followed by slightly younger K-rich
calc-alkalic and shoshonitic rocks towards north and highly potassic silica-under-
saturated lamproites in the northernmost sector. According to them the last volcanic
episode associated with alkali volcanism took place 2
4 Ma ago.
They considered that the mineralogy and geochemistry of calc-alkalic, high
K-calc-alkalic and shoshonitic and K-rich rocks suggest that their genesis is asso-
ciated with subduction of the oceanic crust, which started in the Alboran Sea at the
end of Oligocene. The subduction of oceanic crust resulted in the genesis of
andesites at a depth of 100 km, and high K-calc-alkalic shoshonitic lavas at a depth
of 150 km. During the ascent of the
-
fluid, they might have picked up high quantity
of incompatible elements and also assimilated crustal material in varying degrees.
Apparently the ultrapotassic magmas were produced due to melting at greater
depth. According to Lopez Ruiz and Badiola (1980), the alkali basaltic magmatism
was apparently of subcrustal origin produced at a depth of 35
40 km. They con-
sidered this magmatism to be related to distensive phase, which started developing
during tortonian. Instead of assimilation with crust during ascent of the magmas,
this author prefers partial melting of metasomatised upper mantle after subduction
of crustle material.
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