Crystal fractionation of adakitic magmas in the crust-mantle transition zone: Petrology, geochemistry and U-Pb zircon chronology of the Seme adakites, eastern Pontides, NE Turkey

Plutonic and porphyritic intrusions of adakitic rocks are widely distributed in the southern part of the eastern Pontides orogenic belt in Turkey. Ranging in age from late Paleocene to early Eocene, these intrusions provide critical information to evaluate the early Cenozoic geodynamic evolution of the eastern Mediterranean region. Here we report the results from our field, petrologic, geochemical and zircon U-Pb geochronological studies on adakitic intrusions in the Seme area from the southern part of eastern Pontides belt. The Seme granitoid, Daldibi dacite and Aslanca dacite porphyry from this region carry abundant plagioclcase, amphibole and biotite phenocrysts and have geochemical affinities typical of adakites including high SiO2 (58.48-75.6wt.%), Al2O3 (15.78-18.28wt.%), Na2O (3.94-5.08wt.%), Sr (506-1413ppm), and La (12.6-52.9ppm), moderate K2O (1.77-2.76wt.%), and low Y (4.7-14.2ppm) and Yb (0.28-1.29ppm). The early Eocene intrusions in the Seme area have compositional similarities with high-slica adakites, and their geochemical features suggest the involvement of slab-derived melts. The zircons separated from these rocks show euhedral crystal shapes with oscillatory cores typical of crystallization from magmas, as also confirmed by their high Th/U ratios (0.78 to 1.15). U-Pb analyses of zircons from the quartz diorite yielded a weighted mean 206Pb/238U age of 53.16±0.45Ma. Zircon grains from the Aslanca dacite porphyry occurring as a late phase within the Seme body show U-Pb isotopic ages of 53 and 52Ma concordant with U-Pb ages of zircons from quartz diorite. Based on major, trace and rare earth element data, we present a model suggesting crystal fractionation and/or differentation processes of subduction-related adakitic magmas stored in the crust-mantle transition zone, and their transfer into shallow depths of the overriding continental lithosphere.