Geology Reference
In-Depth Information
The question is whether the LVA zone is continuous over a large area. Some
degree of lateral discontinuity in the LVA zone is to be expected. It could result
from the localized up- and down-wellings near the lithosphere
asthenosphere
interface or rising blobs and pipes of hot volatile-rich mantle surrounded by cooler
asthenosphere entrained at shallower depth.
Hoernle et al. (1995) compiled the isotope data from Cenozoic volcanic rocks
overlying the LVA zone (Fig. 14.4 ). They subdivided the data into four groups
based on regional domain:
-
1. eastern Atlantic oceanic domain (western Canary and Madeira islands) erupted
on oceanic lithosphere,
2. eastern Atlantic continental margin domain (eastern Canary Islands) erupted on
lithosphere, which is transitional in character between oceanic and continental
lavas,
3. central European domain, erupted on continental lithosphere and apparently
affected to some degree by ancient subduction associated with the Hercynian
orogenic and
4. western Mediterranean domain, a region in
uenced by recent subduction.
The eastern Atlantic oceanic domain, includes western Canary and Madeira
islands. The eastern Atlantic continental margin domain, consists of eastern Canary
islands. The central European continental domain, includes K-rich mafic volcanic
rocks and carbonatites from the Massif Central, Rhenish massif, Rhine Graben,
Eger graben and Pannonian basin. The western Mediterranean domain, contains
Italy, Sicily, the Aeolian Islands and islands in the Straits of Sicily. According to
Hoernle et al. (1995) the data for each group is crudely consistent with binary
mixing. The arrays for each region has a restricted compositional range in the Low
Velocity Anomaly region (LVC), which has been interpreted by them as having
similar isotopic composition of the seismic low velocity anomaly; and is referred to
by them as low velocity composition (LVC).
The volcanic rocks from the western Mediterranean domain can be divided into
two groups based on isotopic composition and tectonic setting. The sodic lavas
(Na 2 O/K 2 O > 0.7 from Mt. Etna, Sicily, Straits of Sicily and Pietre Nere, south-
eastern Italy have Sr
Nd and Pb isotope ratios that overlap the LVC. Potassic and
calc-alkalic lavas from the regions are in
-
uenced by recent subduction of the
African plate (Western Italy and Aeolian Islands), as evidenced by very high
87 Sr/ 86 Sr and low 143 Nd/ 144 Nd and 206 Pb/ 204 Pb ratios. According to them, lavas
from western Italy have the most extreme isotopic composition with values similar
to deep sea sediments, whereas lavas from Aeolian Islands fall in the middle of the
array. Oxygen isotope and positive Eu anomalies in samples from western Italy
provide additional evidence related to contamination by sedimentary components
probably subducted to shallow depths in the upper mantle. They concluded that a
minor amount of a third, MORB-like component could also be seen in the lavas
from the Aeolian Islands, and in ancient tholeiites from Mt. Etna.
 
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