Space and Time Relations of Three Plutonic Phases in the Eastern Pontides,Turkey

Granitoid plutons located between the towns of Dereli and Sebinkarahisar are differentiated into three groups, representing the products of three distinct phases; each of these phases is subdivided into several subunits. The first plutonic phase, intruding into the Santonian-Campanian volcanics, is composed of the Maastrichtian Tamdere quartz monzonite, Surmen granodiorite, and Aksu biotite monzogranite. It appears to have formed as differentiation products of a single hybrid magma source. The subunits of the second plutonic phase consist of the Paleocene Gökcebel syenite and Sebinkarahisar quartz syenite, which are intruded into the Aksu biotitemonzogranite to the north and the Santonian-Campanian volcanics to the south. The third plutonic phase consists solely of the Eocene-age Yücedere diorite/gabbro, which intrudes the subunits of the first plutonic phase. The first and second plutonic phases include microgranular mafic magmatic enclaves and display special microscopic textures, the former indicating magma mingling and the latter magma mixing. The first plutonic phase has cafemic (CAFEM)/calc-alkaline (CALK), I-type and arc-related magma characteristics. The second plutonic phase is derived from a magma exhibiting cafemic (CAFEM)/alkaline (silica oversaturated alkaline, ALKOS), I/A-type, and postcollisional characteristics. The third phase is a typical product of a cafemic (CAFEM)/low-K tholeiitic, M-type, postcollisional mafic magma source. Within a regional geologic context, the first plutonic phase is suggested as part of the well-known Eastern Pontide arc magmatism, induced by northward subduction of the northern branch of Neotethys beneath the Eurasian plate during Late Cretaceous time. The products of the second plutonic phase may have been formed from an alkaline magma generated in relation to the tensional regime after crustal thickening caused by the Anatolide-Pontide collision. The third plutonic phase appears to have orginated from a mantle-derived magma generated by the partial melting of upwelled mantle material during an advanced stage of extension.