TIMING AND NATURE OF POSTCOLLISIONAL VOLCANISM IN WESTERN ANATOLIA AND GEODYNAMIC IMPLICATIONS

Abstract

Western Anatolia (Turkey) has experienced discrete pulses of widespread volcanism since the collision of the Sakarya and Tauride continental blocks in the early Eocene. Underplating of the leading edge of the Tauride platform at a north-dipping subduction zone beneath the Sakarya continent resulted in crustal thickening and detachment of the Neo-Tethyan oceanic lithosphere from continental lithosphere, causing slab break-off and the development of an asthenospheric window. Thermal perturbation caused by this asthenospheric upwelling led to melting of the metasomatized overriding mantle lithosphere that produced volcanism and granitic plutonism across the suture zone and the Sakarya continent. The products of this first episode of postcollisional volcanism in western Anatolia were subalkaline in character, with rocks ranging in composition from basalt, basaltic andesite, and andesite to dacite; they show enrichment in large-ion lithophile elements (LILE) and light rare earth elements (LREE) relative to the high field strength elements and display the highest 87Sr/86Sr (i) (0.7087 to 0.7071) and the lowest εNd (i) (-6.5 to -3.5) values in comparison to the rocks of the following volcanic episodes. Geochemical features and compositional variations of this subalkaline volcanic group indicate increasing amounts of crustal contamination and a decreasing subduction signature during their evolution from the Eocene through the Oligo-Miocene. Following the initial phases of orogenic collapse, collision-induced compression in western Anatolia was replaced by north-south extension in the early to middle Miocene that produced metamorphic core complexes and NNE-trending horst-graben structures in the region. The second major volcanic episode from 16 to 14 Ma produced mildly alkaline rocks ranging in composition from basalt, trachy-basalt, and trachyandesite to trachyte, showing enrichment in LILE and LREE (although less pronounced in comparison to the subalkaline lavas), with 87Sr/86Sr (i) (0.7075 to 0.7062) and εNd (i) (-3.6 to -1.6) values that are transitional between the earlier subalkaline and the later alkaline group lavas. Melting of a subduction-modified lithospheric mantle and asthenospheric melts appear to have contributed to the magma budget of the mildly alkaline group, which shows the effects of less crustal contamination or assimilation as a result of advanced crustal thinning associated with tectonic extensionin the region. The asthenospheric melt contribution likely resulted from lithospheric delamination or partial convective removal of the subcontinental lithospheric mantle. Alkaline volcanism, which started around 12 Ma and continued until the latest Quaternary, produced rocks in the tephrite, basanite, and foidite fields that show ocean island basalt-type trace-element patterns and 87Sr/ 86Sr (i) (0.7033 to 0.7030) and εNd (i) (+6.5 to +2.5) values. These features suggest enriched asthenospheric mantle-derived melts as their main magma source. Crustal contamination or assimilation was not an important process in the evolution of the alkaline group, suggesting that magma transport was facilitated by lithospheric-scale extensional fault systems that acted as natural conduits. Establishment of the Hellenic subduction zone and the associated slab roll-back from the middle Miocene onward produced arc volcanism on the south Aegean islands (Milos, Santorini, and Methana) and in southwestern Turkey (Bodrum). The postcollisional volcanism in western Anatolia thus displays compositionally distinct magmatic episodes controlled by slab break-off, lithospheric delamination, asthenospheric upwelling and decompressional melting, and oceanic lithospheric subduction as part of the geodynamic evolution of the eastern Mediterranean region throughout the Cenozoic. © 2006 Geological Society of America.