The transition from orogenic to intraplate Neogene magmatism in Western Anatolia and Aegean area.

During the Late Miocene to Pleistocene, Western Anatolia and the Aegean area were affected by scattered alkali basaltic activity that was temporally distinct from the older orogenic magmatism related to the subduction of Africa beneath the Anatolian and Aegean plates. On the basis of geochemical and isotopic data, two groups of alkali basalts have been distinguished. The fi rst group (Foça, Urla, Selendi, Samos, Chios, Patmos, and Psathoura) exhibits a wide variability in isotopic composition (87Sr/86Sr 0.7043-0.7079; 143Nd/144Nd 0.51278-0.51243) and trace-element distribution (Th/Ta 2.4-12.3; Ba/Nb 14-49) probably acquired from a subduction-related component. The second group (Kula, Biga, Kalogeri, and Thrace), on the other hand, retains typical intraplate features with no subduction-related imprinting (87Sr/86Sr 0.7031-0.7035; 143Nd/144Nd 0.5130-0.51275; Th/Ta 1.2-1.7; Ba/Nb 5-10). The fi rst group of basalts marks the transition between subduction-related and intraplate activity, characterized by the interaction of a mantle source with residual slab fl uids, whereas the second group is an expression of a mantle with any subduction signature. Within the second group, the geochemical and isotope variations highlight the involvement of both mid-ocean-ridge basalt (MORB)-like and ocean-island basalt (OIB)-like mantle domains. Overall, the intraplate character of this alkaline association indicates that the mantle wedge, previously metasomatized by slab-derived material, was replaced by the upwelling of subslab mantle. This process is considered to be the consequence of the extension of the hanging-wall Aegean-Anatolian lithosphere, coupled with the subducted African slab, which was stretched and torn. In this interpretation, the track of the alkali basalts would be a useful marker tool of ruptures in the slab.