Two contrasting magmatic associations of NW Anatolia and their tectonic significance

In northwestern Anatolia two magmatic episodes are distinguished. Initially an intermediate to felsic calc-alkaline association was formed during the Oligocene-Early Miocene. In this period, granitic plutons were intruded into shallow levels in the crust. They are associated with hypabyssal and volcanic rocks. This magmatic event is late/post collisional with respect to the Tethyan collision, which occurred during the Late Cretaceous-Eocene period. The magmatic activity occurred when the region was still suffering a N-S directed compression, which is the result of continuing convergence after the collision. Consequently the magmas passed through an excessively thickened continental crust and, therefore, were contaminated by the crustal materials. The magmatic rocks of this phase are commonly high-K calcalkaline and partly shoshonitic and hybrid. Their compositions reveal crystallization from mantlederived magmas contaminated by a high amount of crustal components. This magmatic event may thus be regarded as a Tibetan type. The geological signature of the magmas is also similar to the arc-derived magmas. The reason for this is that the metasomatic mantle where the magmas formed was permanently enriched when the subduction and total consumption of the NeoTethyan ocean floor occurred. The second magmatic phase occurred during the Late Miocene-Pliocene. Sporadically developed alkaline basalts were formed during this period. They show geochemical affinities similar to rift-type basalts. This genetical implication is supported by the structural data, which reveal that the E-W trending grabens of the western Anatolia developed in this period under N-S extensional regime.