Alkaline series related to Early-Middle Miocene intra-continental rifting in a collision zone: An example from Polatl?, Central Anatolia,Turkey

A large volcanic area (～7600 km²), the Galatean Volcanic Province (GVP), developed in northwest Central Anatolia during the Miocene along the Neo-Tethys Ocean suture zone possibly by post-collisional processes. The GVP mainly comprises 20–14 My old acid to intermediate volcanites with a geochemical signature indicating a mantle source modified by earlier (Late Cretaceous) subduction-related events. 100 km south of the GVP, near Polatl?, Ankara, basaltic rocks that cover large areas are intercalated with the Miocene deposits of the Beypazar? basin, an intra-continental subsidence zone at the southwest of the GVP. Field observations, geochemistry and K–Ar age dating of the Polatl? volcanites show that they are Early (19.9 Ma) to mid (14.1 Ma) Miocene in age, covering an area as large as 215 km². Variations in lava thickness and the thickness of the underlying silicified/baked zones suggest that the basaltic lavas erupted from a southern source, possibly from the Eski?ehir fault zone, and flowed northwards. Most Polatl? samples have chemical compositions that indicate derivation from a mantle source with crustal contamination during ascent. They do not display any characteristic to suggest a subductional component. Although the GVP and Polatl? lavas formed close in time and space, they were derived from different mantle sources. Considering the positions of these two magmatic regions with regard to the Tethyan suture zone, we propose that the mantle beneath the GVP and near the suture zone memorised the earlier subduction while the mantle beneath Polatl? that is located about 100 km further from the suture zone remained apparently unchanged. After a significant volume of magma was consumed in the GVP, a later (～10 My) and last activity (Güvem activity) has produced quantitatively much less basaltic rocks where this subductional signature seems to completely disappear. Considering that the western Anatolian crust is proposed to undergo extension since the Late Oligocene–Early Miocene times, the Early Miocene intra-plate Polatl? activity may have developed within this extensional tectonic regime. Combined with regional data, Polatl? data also provide broad estimations on how long a subductional event continues to modify the mantle after the subduction ceased (at least ～20 My), how long the subductional signature is preserved during significant magmatism (between 6 and 10 My) and how far the subductional effect disappears laterally on the mantle with respect to the collision zone (<100 km).