SUBDUCTION-RELATED AND INTRAPLATE-TYPE VOLCANISM IN CENTRAL ANATOLIA: AN OVERVIEW

In Central Anatolia Neogene volcanic rocks largely outcrop. In the region enclosed among Ankara, Konya, Adana and Sivas, large ignimbrite sheets (reported often as Kapadokyan Ignimbrites) and continental sedimentary rocks are found intercalated with lava flows, strato-volcanoes, volcanic domes and plateau lavas.

Some of these products have calc-alkaline affinity and the typical geochemical characters of subduction-related rocks, whereas some others are alkaline, strongly SiO₂-undersaturated and reveal an intraplate-type mantle source.

Calc-alkaline basalt, andesites and dacites are mainly found between Nevşehir and Niğde, around Kayseri and in the volcanic edifices of the two big stratovolcanoes of the region, the Hasan Dağ and the Ercyies Dağ, whereas basanites and alkali basalts occur as monogenetic cones in Karapınar, as eccentric monogenetic cones north-west of Hasan Dağ and as plateau lavas to the north-west of Nevşehir, south of Sivas, and west of Sivas, around the town of Şarkışla.

The occurrence of subduction-related and intraplate-type volcanic products in the same region is a common feature with surrounding areas in Aegean-Anatolian region. However, Central Anatolian lavas and pyroclastics have some peculiarities. Indeed, these two kind of lavas may be found in the same area, but have usually different time distribution: for instance in Western Anatolia, Central Aegean and Thrace calc-alkaline products are remarkably older than alkali basalts. Here, calc-alkaline basaltic to rhyolitic rocks and alkaline lavas (basanites, tephrites and alkali basalts) mostly have the same age (2-0 Ma) and sometimes are closely space-time related, especially in the northern portion of Hasan Dağ volcano, and around the city of Nevşehir. Moreover, alkali basaltic plateau lavas are sometimes remarkably older than calk-alkaline lavas and ignimbrites, as those around Şarkışla, dated by K-Ar method at 15.7-14.0 Ma ago, and those south of Sivas, which gave ages of 5.1-4.8 Ma ago.

In addition, mantle sources of subduction-related and intraplate-type lavas are easily distinguished by their Sr and Nd isotope ratios in Central-Eastern Mediterranean region: alkali basalts usually have ⁸⁷Sr/⁸⁶Sr around 0.7030-0.7035 and calc-alkaline and shoshonitic rocks show distinctly higher values, with ⁸⁷Sr/⁸⁶Sr varying from ≈0.7050 to ≈0.7080. Once again, in Central Anatolian volcanic products this sharp distinction is totally lacking, because both alkaline rocks (basalts and basanites), and calc-alkaline ones (basalts, andesites and dacites) vary in a quite small range of ⁸⁷Sr/⁸⁶Sr (0.7035-0.7055).

The different volcanism of Western and Central Anatolia may be linked with the different tectonic setting of the two regions: extension in Western Anatolia in a backarc position with respect to the Aegean subduction at Crete trench, and compression in Central Anatolia in the backarc of Cyprus arc subduction system.