CONTINENTAL COLLISION, SLAB BREAKOFF AND POSTCOLLISIONAL CENOZOIC PLUTONISM IN WESTERN ANATOLIA, TURKEY

The southward emplacement of the Tethyan ophiolites onto the Anatolide-Tauride platform (ATP) in Turkey during the L. Cretaceous was followed by a collision between the Sakarya block (SB) & the ATP in the E. Eocene. The northern edge of the downgoing ATP was metamorphosed under HP/LT conditions at a subduction zone dipping N, beneath the SB. Blueschist rocks exposed along the Izmir-Ankara suture zone (IASZ) were metamorphosed during 108 – 88 Ma & exhumed by the Paleocene. The E-W trending Eocene-Miocene granitoids intruding the SB, ATP and the ophiolitic & blueschist rocks along the IASZ represent post-collisional plutons that mark an early phase of Cenozoic magmatism in W. Anatolia. The Eocene plutons – EP are composed of medium- to high-K rocks with SiO₂ contents between 59-68 wt.%. Their TiO₂, Al₂O₃, FeO_t, MnO, MgO & CaO values decrease continuously with increasing SiO₂, but at given SiO₂ these oxides show different values indicating that the rocks differentiated from at least two different calcalkaline parental magmas. The Oligo-Miocene plutons – OMP along & south of the IASZ comprise shoshonitic, high-K calcalkaline rocks with their SiO₂ contents c. 65-75 wt.%. These granitoids are more differentiated than the EP. The ε_Nd(t) values of the EP range from +1.97 to -4.45, closer to mantle values, whereas those of the OMP are all negative (-0.4 to -6.27) indicating crustal contamination of their melt source. These data suggest that moderately to strongly evolved magmas of the EP were derived from partial melting of a subduction-metasomatized SCLM source; magmas of the OMP were affected by crustal contamination and/or produced by melting of the lower crust. Partial melting of the western Anatolian SCLM was caused by asthenospheric thermal influx provided by slab breakoff, following the continental collision. Regional tectonic constraints do not support those models suggesting arc magmatism associated with an active subduction zone during this time. The slab breakoff-induced magmatism weakened the orogenic crust & led to its extensional collapse, resulting in widespread crustal exhumation & core complex development in the Aegean extensional province during the L. Cenozoic. This extensional regime facilitated a new phase of magmatism fed by the convecting mantle & produced alkaline/ultra-alkaline volcanism in the region.