Evolution of a section of the Africa-Europe plate boundary: Paleomagnetic and volcanological evidence from Sicily

New paleomagnetic data relative to Upper Cretaceous, Neogene and Quaternary volcanic rocks from eastern Sicily definitively indicate that Sicily is a part of the African plate, which collided with the European continental plate in Middle Miocene times. These data and the tectonic evolution of Sicily as inferred from the nature, age and distribution of volcanic products, are broadly consistent with the motions of Africa relative to Europe since the Upper Trias. During the Mesozoic, eastern Sicily was affected by extensional tectonics with associated alkali basaltic volcanism, and oceanic crust was produced in the meantime between the diverging African and European plates. Near the end of Mesozoic times the two plates started to converge with consequent consumption of oceanic crust. Different times of oceanic plate consumption along the Sicily-Calabria section of the plate boundary are suggested by the occurence of andesitic volcanism of different ages. The tectonic significance of late Tertiary to present basaltic activity in eastern Sicily is also discussed.     

Evolution and geodynamic significance of the tertiary orogenic volcanism in Northeastern Greece

The Tertiary volcanism of Eastern Macedonia and Western Thrace (Greece) developed in association with the sedimentary basin which formed, from Eocene to Oligocene, along the southern margin of the Rhodope Massif.The volcanic products, ranging in composition from basaltic andesites to rhyolites, show an overall calc-alkaline orogenic affinity, while chemical characteristics identify different groups of rocks, probably reflecting minor differences among parent magmas. The observed evolution within any group of rocks is compatible with fractional crystallization processes acting on relatively shallow magma bodies.The Sr isotopic composition of rhyolitic member shows an initial⁸⁷Sr/⁸⁶Sr ratio comparable with that of basaltic andesites, reinforcing the hypothesis of a crystal/liquid line of descent.Geochemical and petrographic evidence, on the whole, suggests that the investigated orogenic association developed on an active continental margin characterized by a relatively thick crust, acting as a density filter for the basic magmas and facilitating their storage and fractionation within the crust itself. Minor contamination by interaction with host materials may also have occurred.Stratigraphic and K/Ar geochronological data indicate that the volcanic activity started in Upper Eocene and reached its maximum development in Upper Oligocene. From Lower Miocene, the volcanism shifted southward in the Central Aegean area and in part of Western Anatolia, coming to an end by Middle Miocene.The southward migration of the volcanic front has been interpreted as a consequence of the increase in the dipping of the Benioff zone, due to the decrease of penetrative strength after the main phase of continental collision.     

On the geovolcanological and morphological evolution of an endogenous dome (Pantelleria,Mt. Gelkhamar)

The author describes the outcrops and chemico-petrographical characteristics of volcanites as referred to an eruptive centre in the NW of the Island of Pantelleria (Sicilian Channel).The survey carried out on this subject permitted the reconstruction of the particular eruptive mechanism which determined the formation of an endogenous dome.The volcanites in question, quartziferous sodatrachytes, alternate, in their outcrops, with hawaiitic lavas originating from eruptive centres situated in the immediate vicinity.The alternating of basic magmas and acid magmas within a restricted area supplies a further valid evidence for ascribing the origin of the quartziferous sodatrachytes to contact anatexis. On the other hand they have such a particular chemical composition, that cannot be found in other trachytes of orogenic and cratonic zones.

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Zusammenfassung Der Verfasser beschreibt das Vorkommen und die petrographisch-chemischen Eigenschaften der Vulkanite eines Ausbruchszentrums im NW-Teil der Insel Pantelleria. Aufgrund dieser Untersuchungen wird der Ausbruchsmechanismus rekonstruiert, der zur Bildung einer geborstenen Staukuppe führte.Die quarzführenden natrontrachytischen Laven dieser Kuppe wechsellagern mit hawaiitischen Vulkaniten, die aus benachbarten Ausbruchszentren stammen.Die abwechselnde Förderung von basischen und sauren Magmen in einem engbegrenzten Gebiet wird als Beweis für eine kontaktanatektische Entstehung der natrontraehytischen Magmen angeführt, deren eigenartige Zusammensetzung überdies von derjenigen der Trachyte aus Orogengebieten grundsätzlich abweicht.

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Résumé L'auteur décrit le gisement et les caractères pétrographiques et pétrochimiques des laves d'un centre éruptif dans la partie NO de l'Île de Pantelleria. Sur la base de ces observations il reconstitue le mécanisme éruptif qui a déterminé la formation d'un dôme endogène rompu.Les laves trachytiques sodiques quartzifères du dit dôme se trouvent intercalées dans les produits hawaiitiques provenants de centres éruptifs voisins.Cette alternance d'éruptions basiques et acides est un argument en faveur d'une origine anatectique de contact des magmas trachytiques sodiques qui, en outre, ont une composition minéralogique et chimique très différente de celle des roches trachytiques des régions orogéniques.

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Riassunto L'Autore descive la giacitura ed i caratteri chimico-petrografici delle vulcaniti riferibili ad un centro eruttivo posto nella parte Nord-occidentale dell'Isola di Pantelleria (Canale di Sicilia). Le osservazioni condotte su tale argomento hanno consentito la ricostruzione del particolare meccanismo eruttivo che ha determinato la formazione di un edificio vulcanico costituito da una cupola di ristagno.Le vulcaniti in questione, di natura sodatrachitica quarzifera, sono alternate, nella loro giacitura, a lave di tipo hawaiitico provenienti dai centri eruttivi posti nelle immediate vicinanze.L'altemarsi di eruzioni di magmi basici e di magmi acidi entro un'area ristretta fornisce un ulteriore valido elemento per ascrivere ad anatessi di contatto l'origine delle sodatrachiti quarzifere in oggetto, il cui chimismo è daltronde talmente particolare che non trova riscontro in analoghi termini di zone orogeniche e cratoniche.

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Dedicated to Professor Dr. A.Rittmann on the occasion of his 75. birthday     

Summer larval fish assemblages in the Southern Tyrrhenian Sea (Western Mediterranean Sea)

The purpose of this paper was to study the community structure, in terms of species composition, abundance and spatial distribution, of fish larvae in a wide coastal area of Sicily facing the Southeastern Tyrrhenian Sea, extending for 2300 km² from Cape Cefalù to the west, to Cape Rasocolmo in the east. This study analyses how species are assembled in relation to an inshore–offshore gradient and also how environmental conditions, determined by surface circulation patterns occurring in the Central Mediterranean at the local scale, determine the distribution patterns. Samples from 39 stations were collected using a 60‐cm Bongo net during an ichthyoplanktonic survey carried out in June 2006. In all, 62 taxa, representing 32 families, were identified. Cyclothone braueri (59.6%), Engraulis encrasicolus (9.2%) Lampanyctus crocodilus (4.3%) and Lampanyctus pusillus (4.1%) were the most abundant species. The results showed that the highest abundance value (14830.6 fish larvae per 10 m² sea surface) was observed in the western part of the study area. MDS, SIMPER and CCA analyses revealed well defined groups of stations and assemblages of larvae in accordance with an inshore–offshore gradient. The results of this study could have implications for the management of marine resources because the investigated area has already been identified as a nursery area for many pelagic and coastal fishes and a natural habitat for many species of high commercial interest.     

Evolution of Eolian arc volcanism (Southern Tyrrhenian Sea)

The Eolian arc is located at the boundary between the converging African and European plates. Its volcanism is characterized by a marked evolution in a restricted time (less than 1 my). A progressive transition is observed from typical calc-alkaline series toward shoshonitic rocks produced by past and present activity (Vulcano and Stromboli). By comparison with circum-Pacific island arcs, the evolution of Eolian volcanism can be related to a rapid deepening of the Benioff zone. The occurrence of shoshonitic rocks and the continental nature of the crust on both sides of the plate boundary suggest that the Eolian arc is in a senile stage of evolution. Petrology and Sr isotopic data indicate a probable mantle source for Eolian volcanism.     

The caldera of Pantelleria

The central part of the Island of Pantelleria is occupied by a caldera depression of ellipsoidal form, the major diameter of which is about 7 kilometers. The collapse causing the caldera to form seems to have taken place after a complex eruption which led to the extrusion of a great endogenous dome, the formation of which was followed by a localized explosive activity. The succession of the acid volcanites of Pantelleria shows periodical evolutions of the magmas, undergoing various cycles of differentiation. After a period of quiescence a new cycle of differentiation started again. This may be explained by admitting that the differentiation was accompanied by an increase in the gas content. The violent degassing re-established conditions analogous to those which had existed at the beginning of the previous cycle.     

Evolution of the volcanism in the area of interaction between the arabian, anatolian and iranian plates (Lake van, Eastern Turkey)

Geological investigations of the Lake Van area (Eastern Turkey) have shown the coexistence of a calc-alkaline volcanism, active at least since Lower Miocene, with an alkaline one, beginning around 6 m.y. ago. The calc-alkaline volcanic activity is related to subduction of the Arabian plate under the Anatolian—Iranian continental mass. The alkaline volcanism, on the other hand, is attributed to the fragmentation of such a mass and to the divergent motion of the Anatolian and Iranian plates; a process which began in Upper Miocene as a consequence of the continental collision with Arabia.     

Tertiary and quaternary volcanism of the Erzurumkars area (Eastern Turkey): Geochronological data and geodynamic evolution

This paper presents new geochemical and geochronological data on the Neogene and Quaternary volcanic products from the area of Erzurum and Kars (Eastern Turkey).The affinity of most of the volcanic rocks concerned is calc-alkaline, with the exception of minor Quaternary alkaline rocks that outcrop in the Kars sector. Ages range from 8 m.y. to 1.3 m.y.The time-space distribution of the calc-alkaline volcanism in Eastern Turkey - NW Iran is characterized by a sudden northward jump, that occurred around 6 m.y. ago, from the Van-Erzurum Southern Belt to the Kars Northern Belt. Available data indicate that this northward migration of volcanism was due to changes in the subduction geometry of the Arabian plate under Eurasia.     

Role of the alkali feldspars fractionation in the evolution of the pantelleria peralkaline silicic rocks

The bearing of fractional crystallization processes, involving mainly the separation of alkali feldspars, on the genesis and evolution of the Pantelleria peralkaline rocks suite has been studied. The separation of phenocrysts from representative vitrophyric rock samples allowed a comparison of the rock bulk compositions with those of their own glasses and alkali feldspars. The peralkaline character of the glassy matrix of a metaluminous trachyte showed the link between alkaline and peralkaline rock suites at Pantelleria, and pointed to a feldspars fractionation as the genetic main acting process (« plagioclase effect » of Bowen) of peralkaline trends. The results of the present study are consistent on the whole with the model proposed byNoble (1968) for the genesis and evolution of peralkaline liquids.