Akademik Veri Yönetim Sistemi
INTERNATIONAL GEOLOGY REVIEW, cilt.52, ss.142-186, 2010 (SCI-Expanded)
High-K volcanics were produced in two different Late Cretaceous cycles during the infilling of back-arc basins of the eastern Pontides. The first cycle, represented by shoshonitic trachyandesites and associated pyroclastics, containing high K2O (2.74-4.81 wt-%) and Na2O (3.60-5.51 wt-%), overlies a mid-Cretaceous ophiolitic-olistostromal melange formed during the rifting stage of a back-arc basin (Neotethys). Ar-40/Ar-39 dating of biotite from trachyandesite at the base of the first cyle indicates that shoshonitic volcanism in the far south of the eastern Pontides started in the early Campanian. Volcanic rocks of this cycle are overlain by upper Campanian-Maastrichtian rudist-bearing reefal limestones. The second cycle of high-K volcanism is represented by analcime-bearing volcanic rocks erupted in a Neotethyan lagoonal environment. These volcanic rocks, intercalated with continental detritus, are characterized by high Na2O (3.22-7.16 wt-%), now concentrated in secondary analcime crystals. Their K2O contents also range between 0.83 and 6.05 (wt-%). All of these units are disconformably overlain by Eocene turbidites with a basal conglomerate. Volcanic rocks belonging to both cycles show various enrichment degrees in LILE, HFSE, LREE, and HREE with respect to primitive mantle and chondrite [(La/Lu)(CN)=11.10-25.89]. Negative Nb, Ta, Zr, Hf, and Ti anomalies are typical of these subduction-related arc magmas. Geochemical similarities between the two volcanic cycles suggest that trachyandesites and analcime-bearing volcanics were derived from similar enriched mantle sources, and that they formed in the same geotectonic setting of a back-arc basin environment of the eastern Pontide magmatic arc during the Late Cretaceous. In addition, Nd-Sr and Pb isotope ratios of the investigated volcanic units indicate that their mantle melt sources were similar.