Delamination Magmatism in Eastern Anatolia: A Geochemical Perspective

The Sr-Nd-Hf-Pb isotope geochemistry of the Late Miocene Tunceli Volcanics suggests that they are the products of mixed asthenospheric and lithospheric mantle melts. The combined elemental and mineral chemistry data additionally indicate that a pyroxenite component of lithospheric origin is involved in their genesis. Calculations favor melting depths of similar to 2 GPa for the Tunceli lavas, that is, deeper than the current lithosphere-asthenosphere boundary beneath Eastern Anatolia. Geochemical data suggest that during regional Neo-Tethyan subduction, dense (i.e., pyroxenite-bearing) domains formed by progressive melt intrusion into the lower lithosphere resulted in gravitational instabilities. This unstable density configuration eventually led to the foundering of the eastern Anatolian lithosphere in the Late Miocene, resulting in progressive melting of fusible pyroxenite-bearing domains at asthenospheric depths. We demonstrate that these pyroxenitic melts mixed with ambient asthenospheric melts and generated the Tunceli lavas. The Eastern Anatolian High Plateau (EAHP) that formed after the collision of Arabian and Eurasian continents hosts a huge volcanic system. The nature of the source region from which these volcanics originated and the geological dynamics that triggered this widespread volcanic activity are still under debate. We suggest that volcanism occurred when the lithospheric mantle beneath the EAHP separated physically from the overlying crust and sank into the deep asthenosphere. In this study, we explore the geochemical evidence for this model by focusing on the Late Miocene Tunceli Volcanics, one of the early stage members of post-collisional volcanics in the EAHP. Our data suggest that the Tunceli Volcanics are the products of mixed asthenospheric and lithospheric mantle melts. The lithosphere contains a dense pyroxenite component that forms when silica-rich melt invades the lower lithosphere during the subduction process. Calculations have shown that these dense materials melted at higher depths than the base of the lithosphere beneath the region. Thus, we propose that the dense domains in the lower lithosphere resulted in gravitational instabilities and eventually led to the foundering of the eastern Anatolian lithosphere in the Late Miocene. During Neo-Tethyan subduction, pyroxenite-bearing domains formed by melt intrusion into the lower lithosphere beneath Eastern Anatolia The pyroxenite-bearing domains resulted in gravitational instabilities and led to the foundering of the lithosphere in the Late Miocene The regional post-collisional volcanics represent the melts derived both from asthenospheric and delaminated lithospheric mantle domains