Age of Formation of the Aegean Active Volcanic Arc

Particularly the neotectonic studies have provided much information on within-plate motions, but these studies need to be greatly expanded, because the results obtained in a few specific sites can be hardly generalized.

The southern boundary of the Aegean plate is typical of a plate convergence environment. Seismic data suggest the occurrence here of a subduction zone dipping towards the Aegean Sea.

The existence in this area of a typical subduction process implies the occurrence of an outer sedimentary arc and of an inner volcanic arc and actually both are known and intensively studied.

Volcanic centres of the inner Aegean arc are limited in number and usually have limited dimensions. Their limit towards the outer sedimentary arc is well marked, whereas on the inner side small volcanoes occur at various distances from the main axis. These volcanoes can be considered as belonging either to the arc (Vilminot & Robert 1974) or to an older inner island arc (Fytikas et al. 1976). Orily precise petrological studies coupled with radiometric age dating can solve this problem.

Presently it is widely accepted that the volcanic arc forms an arcuate belt with a width ranging from 20 to 40 km and with 500 km length.

Recent petrological data are available for almost all recent volcanic centres, that is for those located on the outer arc (e.g. Crommyonia, Aegina, Methana and Milos), whereas information is scarce for Antimilos (Marinos 1960) and practically inexistent for Kimolos, Poliegos and Ananes cleefs.

Many recent data are available for the important Santorini and Nisyros centres and also for the Christiana islets. Several other recent papers discuss the relations between volcanism and tectonic setting and the significance of the potassium increase in calc-alkaline magmas of the innermost volcanic centres of the active arc.

The problem of defining the age of the beginning of volcanic activity on the active Aegean arc in its present position has been approached by several authors and with different lines of reasoning. Keller (1971), on the basis of chronostratigraphic data, proposed a starting age of Upper Pliocene. A Quaternary age for almost all the Aegean arc volcanism has been more or less explicitely accepted by several other authors who discussed volcanism and plate tectonics models for the Aegean area.

An Upper Pliocene age (2.7 m.y) was found only for Crommyonia zone, whereas all other ages were Quaternary. The historic Methana and Santorini eruptions and some radiocarbon datings indicate that several eruptions of the above mentioned centres have recent Holocene ages.

Although all these datings indicated very recent ages, Barberi et al. (1974) suggested a 3 m.y. age for the beginning of volcanism at the Aegean active arc, according to the geometry of the converging zone and the probable subduction rate. Radiometric dating data available up to now suggest the same age for the beginning of the volcanic activity. Statistically the radiometric dating data tend to indicate the last phases rather than the beginning of an eruptive cycle For this reason, it is advisable to make radiometric dating in buried older products.

It is sometimes possible to overpass this sampling difficulty by utilizing for radiometric dating xenoliths of buried rocks brought to the surface by violent explosive events. Some of these xenoliths came from acidic intrusive bodies formed by slow cooling of light residual liquids produced by fractionation of the calc-alkaline magmas. Intrusive bodies of this kind may easily form under tensional tectonic conditions determining the formation of "intra-arc" basins.

The graben formation with tensional faulting and block tilting can actually create tectonic conditions under which large masses of hot uprising magma rest within "tectonic and thermal traps". These traps are represented by wedge-shaped fissures of the tilted blocks forming the lowered steps of the graben structure.

Under these conditions magma slowly cools at depth, fractionating towards silicic liquids. Large intrusive silicic bodies may then form because it is difficult for the magma to rise up and erupt at the surface, owing to the fact that the wedge-shaped fissures are probably almost closed towards the top.

Such a tectonic process should be active mainly in the initial stages of the subduction process; in fact the tectonic regime became later dominantly compressive, and fracture systems, mainly transversal to the arc axis, allow the uprising of magmas to the surface where their products progressively build the volcanic arc.

Following such a model, silicic intrusive magmas should represent the early magmatic manifestations in extensional and tectonically unstable regions. To check this hypothesis new K-Ar dates have been measured on the following samples from the Aegean arc:

1. Isle of Kimolos. In one sample from a pyroclastic deposit containing many large xenoliths of a very fresh granitic rock.
2. Isle of Thera. In samples from xenoliths of intrusive or semi-intrusive rocks in explosive layers outcropping in the south-west of the island. These rocks range from quartz-diorites to granodiorites. Several attempts to obtain a radiometric age for these rocks have so far failed. As age data were lacking for the oldest volcanic products of the Santorini group, we decided to measure the K-Ar age of two samples from the south-western part of Thera, where the oldest volcanic products outcrop, e.g. one sample from a big dacitic lava flow forming the Cape Akrotiri and another sample from Cape Mavros from a submarine breccia with few pillow-lavas of andesitic composition.

Table 1: K/Ar ages on ingeous rocks from Aegean area 

Results (Table 1) indicate that 3 m.y. is probably the age of the beginning of magmatic activity at the outer limit of the active Aegean volcanic arc. It is to be noted that everywhere in the arc the oldest outcropping volcanic products are dominantly silicic (for instance the volcanics of Crommyonia, the ruffites of Milos and of Kimolos etc.).

Both the silicic xenolith age and the composition of the oldest outcropping volcanic rocks are therefore in agreement with the hypothesis that the initial tectonomagmatic stage of an island arc is characterized by the possible formation of thermal traps. Only the most evolved, light and volatile enriched residual liquids have the possibility of rising up to the surface from such traps.

The radiometric ages obtained on Thera samples confirm the stratigraphic conclusions of Nicholls (1971) and Pichler and Kussmaul (1972). Initial volcanic products are however probably buried in the basement of the islands, and it seems very reasonable to admit, also for the Santorini group, an Upper Pliocene date for the beginning of the magmatic activity.

Once again the existence of close interconnections between tectonics and magmatism is emphasized and the type of magmatic activity and the nature of magmatic products are revealed as important keys for the understanding of the earth dynamics.

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