Magmatic constraints

The area extending from the circum-Aegean region to Central Anatolia is characterized by the presence of widespread Tertiary and Quaternary volcanism, showing a complex spatial and temporal distribution. Two extensive regions, separated by a narrow junction area, can be distinguished.

The Aegean-West Anatolia Area includes the Rhodope Massif-Thrace, the Central Aegean Sea and western Anatolia up to the South Aegean Active Volcanic Arc. In this area, a calc-alkaline and alkaline vulcano-plutonic associations occur, spanning in age from Upper Eocene to Present.

In the northernmost part of the area (Rhodopian Massif and western Thrace), apart from the Upper Cretaceous calc-alkaline rocks, the volcanism took place from Upper Eocene (~ 37 Ma, Priabonian) to Upper Oligocene (~24 Ma, Yanev, 1998; Innocenti et al., 1984). Erupted products have a petrogenetic affinity, variable from high-K calc-alkaline to shoshonitic with minor ultra-potassic products. Scattered K-basanites and horblend lamprophyres (camptonites) of 28-26 Ma with an OIB geochemical imprint are also found (Marchev et al., 1998). The volcanism shows a southward migration and a decreasing volume with time of the emitted products; a parallel average K2O decrease is also observed. The volcanic activity is associated with a plutonism encompassing a larger interval of time (41-15 Ma, Christofides et al., 1998).

In western Anatolia an Upper Eocene-Oligocene magmatism is also recognized; however, the volume of igneous (intrusive and volcanic) products is significantly smaller than that observed in the Rhodopian-Thracian zone (Yilmaz et al., 2001). Most of the magmatic activity developed during Lower-Middle Miocene (up to about 14 Ma), as well as in several islands of the Central Aegean Sea (Fytikas et al., 1984; Pe-Piper and Piper, 1989). The volcanic products are accompanied by cogenetic plutonic rocks (e.g. Mount Kozak, Altunkaynak and Yilmaz, 1998; Evciler Pluton, Genç, 1998; Izmir, Ayvalik and Bodrum, Savasçin, 1990). The igneous suite is ranging in composition from high-K andesite to rhyolite. Large dacitic-rhyolitic ignimbritic covers of lower Miocene age are widespread in western Anatolia and in eastern-northern Aegean Sea, as in the islands of Lesvos and Limnos. The upper part of this volcanic sequence is characterized by the presence of K-rich basic-intermediate rocks with shoshonitic affinity, sometimes showing ultrapotassic feature. Recently, lamproites as subvolcanic dykes and small lava flows have been found in the central part of western Anatolia (Gediz area), at the end of this orogenic volcanic cycle (14-15 Ma, Savasçin et al., 2000).

After 14-15 Ma the volcanic activity virtually ceased. It resumed in the Late Miocene after about 4 Ma. The erupted products are, in general, poorly evolved, being alkali basalts the most widespread terms; they frequently display a slight potassic affinity (e.g. Eskishehir, Francalanci et al., 2000). These alkali basaltic rocks are found in scattered occurrences in Western Thrace (Yilmaz and Polat, 1998), western Anatolia (Aldanmaz et al., 2000) and central and eastern Aegean Sea (Fytikas et al., 1979; Seyitoglu and Scott, 1992). The subduction-related geochemical signature of these products is generally slight or absent, and several lavas show an OIB character. In Quaternary times, Na-alkali basalts with a clear OIB signature were erupted in western Anatolia (Kula area) and in NW Aegean Sea, where they are located on the prolongation of the North Anatolia fault system (Psathura islet).

The outcropping area of the Eocene-Middle Miocene orogenic rocks is limited southward by the crystalline Actic-Cycladic and Menderes massifs, which are constituted by high P-T metamorphic rocks which reached the eclogite or blueschist-facies peak conditions around 40-50 Ma and successively overprinted by high T/ medium P metamorphism during Late Oligocene- Early Miocene (26-16 Ma, Bröcker et al., 1993). This latter metamorphic phase was accompanied by the intrusion of granites generally of Middle Miocene age (Altherr et al., 1982), locally associated with anatectic Late Miocene-Early Pliocene rhyolites. These volcanics are either exposed (e.g., Antiparos, Innocenti et al., 1982) or occurring as acid clasts in the tectono-sedimentary units overlain the blueschist unit (Sanchez-Gomez et al., 2002). Overall, geochronological data suggest that the Cycladic area has been invaded in the Early-Middle Miocene by large crustal granitic plutons of S- and I-type, that extended their presence also in the Menderes Massif (Delaloye and Bingöl, 2000).

The calc-alkaline volcanic activity started again in Early Pliocene south of the Attic-Cycladic Massif. In fact, a narrow active volcanic arc formed as expression of the active northeastward-directed subduction. The erupted products are typically calc-alkaline and constitute an association ranging in composition from orogenic basalts to rhyolites (Mitropoulos et al., 1987).

In Central Anatolia area, the volcanic activity developed essentially on the Central Anatolia Cristalline Massifs (Kirsehir-Akdag-Nigde, Whitney and Dilek, 1988) during the Middle Miocene-Quaternary, in two main areas located south-west of Konia and in Nevsehir-Kayseri area. The oldest products are represented by calc-alkaline Serravalian-Tortonian andesites (Besang et al., 1977); the climax of the volcanic activity took place in the Late Miocene-Pliocene when a huge ignimbritic sequence emplaced and formed the Ürgüp-Nevsehir plateau extending on about 12000 km2 (Aydar et al., 1995). The ignimbrite plateau includes lacustrine and fluvial deposits; it represents the basement of large composite volcanoes as Karacadag and Melendiz Dag (Mio-Pliocene) and Hasan Dag and Erciyes Dag (Quaternary). Many monogenetic Quaternary centers, either of basaltic or rhyolitic compositions, grew forming cinder and spatter cones and maars structures often with intra-crateric domes.

The erupted products constitute a high-K calc-alkaline association ranging from few basaltic andesites, to dominant andesites and dacites. The ignimbrite units are made up mainly by rhyolites (Temel et al., 1998) The lavas of the main composite Quaternary volcanoes (Hasan Dag and Ercyes Dag) are characterized by relatively lower K2O contents forming a typical calc-alkaline suite (Aydar and Gourgaud, 1998), whereas the basaltic rocks erupted by the monogenetic centers display a geochemical Na-alkaline OIB signature, as inferred by the composition of clinopyroxene (Aydar et al., 1995) and by the trace elements distribution (Kürkçüoglu et al., 1998).

The two areas are connected by a relatively narrow region, north-south-trending, from Afyon to Isparta angle (Kirka-Afyon-Isparta junction-area), in which an alkaline association developed from the Upper Miocene to the Pliocene. The volcanic rocks form a narrow belt, about 200 km long and 50 km wide, and they were erupted mainly along the Antalya fault zone (Yagmurlu et al, 1997). The oldest products (~21-15 Ma, Besang et al., 1977) are represented by the ignimbrite sheets of rhyolitic composition cropping out in the Kirka area and forming the base of the volcanic sequence in the northern part of the Afyon Massif. The igninbritic sequence is capped by K-alkaline lavas, breccias and pyroclastic flows with a Middle-Late Miocene age (from 14.8 Ma to 8.6 Ma, Besang et al., 1977; Savasçin et al., 1995). Two main potassic associations are documented in the Afyon area (Francalanci et al., 2000). The first one is almost-saturated potassic suite of composition ranging from trachybasalts to trachytes, through shoshonites and latites; these rocks display a typical orogenic geochemical signature with high LILE/HFSE ratios, and Nb, Ta and Ti negative anomalies in the primordial mantle normalized spidergrams. The second association is constituted by strongly alkaline, silica undersaturated to saturated rocks yet keeping a well-defined subduction-related geochemical imprint. The dominant products are phonolitic leucitites.

In Isparta region, two associations have been identified. The predominant rocks are K-rich undersaturated to saturated alkaline volcanics, varying from tephri-phonolites to latites and trachytes with a well-defined subduction-related trace elements signature. In this area a group of volcanic and subvolcanic ultrapotassic silica-undersaturated rocks is also present, significantly enriched in MgO (MgO>10%) and compatible elements, which display OIB- or intraplate-type geochemical imprinting with low LILE/HFSE and REE/HFSE and quite low Sr isotope ratios (~0.7038, Francalanci et al., 1991).

As a whole, in the Anatolia-Aegean area the space-time evolution of the volcanism indicate that the erupted products were fed by different mantle sources, whose activation was strongly related to the geodynamic evolution of the region. On the base of isotopic, geochemical and petrological characteristics of igneous rocks, three main mantle sources have been identified.

The dominant calc-alkaline to shoshonitic products show geochemical features indicative of an origin by a mantle source deeply modified by a subduction-related component. We consider that this source was located in the mantle wedge between the African subducting slab and the overriding Anatolia-Aegean plate. The geochemical variations observed in these products reflect not only the heterogeneity of the source, but also process of interaction of the calc-alkaline magmas with upper crust material, documented in several places in western Anatolia (e.g., Aldanmaz et al., 2000).

The Tertiary orogenic cycle locally ends with lamproitic rocks, interpreted as derived by a lithospheric refractory mantle, affected by a metasomatic event, which enriched the source in LILE and enhanced and lowered the 87Sr/86Sr and 143Nd/144Nd ratios, respectively. The geochemical imprinting of these rocks suggests that the metasomatizing component was yet linked with the subduction process, as stressed by the visible Nb, Ta and Ti negative anomalies in multi-element primordial-mantle normalized diagrams (Savasçin et al., 2000).

From Upper Miocene up to Quaternary, the scattered alkali basalts occurring in all the different sectors of the Aegean-Anatolia realm, exhibit geochemical signature typical of OIB- type source (e.g., Aldanmaz et al., 2000; Yilmaz et al., 2001; Wilson et al., 1997; Aydar et al., 1995). They are interpreted as an evidence of the activation of a relatively deep sub-slab mantle source, which was made possible by the absence of the subducting slab or by its inactive presence.

Figure 9. Time distribution

Time distribution

Time distribution of volcanism in Aegean–Awest Anatolia area with the indication of the main rock associations and the involved magma-sources. Mantle wedge source (MWS): CA, calc-alkaline and high-K calc-alkaline; SHO, shoshonites; UK-SHO, ultrapotassic shoshonites; Lithospheric source (LS): Lamp, Lamproites. OIB-type mantle source (OIB): AB, alkali basalts (sodic and potassic); UK-Und., undersaturated ultrapotassic phonolitic tephrites. Crustal source (CS): Granitoids and rhyolites; (GR) with dominant crustal signature.

Beside these mantle sources, in the Attic-Cycladic-Menderes Massifs several granites and rhyolites showing a dominant crustal signature emplaced during Middle-Late Miocene, in connection with a regional-scale extensional phase forming the Aegean Basin, (e.g., Altherr et al., 1982; Innocenti et al, 1982). The time-space evolution of the igneous recognized associations and the main involved magma-sources are schematically outlined in Figure 9. The southwestward migration of the subduction hinge was accompanied in the hangingwall by the southwestward migration of calc-alkaline, shoshonitic and lamproitic magmas. These volcanic products emplaced in an already extending area that eventually evolved in a more extreme setting of mantle uprise, source for the OIB basalts.