Geochemical characteristics of Upper Cretaceous volcanics in the north of Istanbul, Turkey: implication for the subduction zone magmatism

The volcanic rocks Upper Cretaceous in age are exposed along the Black Sea coasts east-west trending in the northwest Turkey and Srednegorie zone through Bulgaria. The geological and geochemical data, petrographic and mineralogical findings have been presented respectively. The basement rocks in the north of Istanbul are represented by sedimentary rock groups Paleozoic and Triassic in age. The volcanic rocks Upper Cretaceous in age overlay the basement rocks disconformably. The basement rocks and volcanic sequence are covered by Neogene sediments disconformably. Volcanic rocks are dominated by andesite, basaltic andesite and associated with lesser proportion of basalt, dacite, rhyodacite, less olivine basalt and their volcaniclastic equivalents. The volcanic rocks in the region have been differentiated volcanic breccias, poorly sorted volcaniclastic deposits, massive lavas and volcanic originated sandstone considerig their lithological facial features and field characteristics. In some locations hyaloclastite type of rocks are common. Some of them are reworked in the marine environment. This is the evidence that the volcanism was formed as submarine volcanic activities. The volcanic units are cut by some basaltic and/or basaltic andesite dykes. Lavas show microlitic porphyritic, hyaloplitic, spherulitic and hyaloporphyritic textures. When petrographically examined, the samples give evidence of hydrothermal alteration or devitrification. Corroded quartz, oscillatory zoning of plagioclase phenocrysts and reverse zoning are the evidences showing thermal and compositional disequilibrium produced by magma mixing. In this paper the geochemical characteristics of volcanic rocks have been discussed. Subduction of the Neotethys under the Istanbul zone (Pontides) created a new active continental margin arc. The volcanic rocks outcropped in the north of İstanbul are included in High-alumina basalt (>17 % Al₂O₃) group firstly with this study. The petrochemical characteristics of volcanism have been defined on the analyses of 24 representative lava samples covering the whole stratigraphic sequence, the major-oxide, trace and rare earth element data. In order to beter understand the genesis of subduction-related volcanism, we also performed an electron microprobe study on phenocryst phases of two hydrous lava samples an one unhydrous olivine basalt sample (e.g., pyroxenes, amphiboles and olivine). All lava suits are calc-alkaline with arc-like signatures and characterized by LILE and LREE enrichment and N-MORB-like patterns of HFSE and HREE. Such signatures are consistent with melts being derived from a mantle that was metasomatized by slab-derived aquous fluids and silicic melts. With these features İstanbul volcanics have been attributed to crystal fractination and interaction with the subduction zone and modified mantle wedge and to be enriched mantle origin. Geochemically volcanics show negative anomalies in Nb, Ta, P and Ti typical of arc magmas. Nb-Ta-Ti depletions are evidences of aqueous fluid metasomatism since fluids were depleted in these elements due to residuel rutile in the dehyrating slab. Mantle-derived rocks have The Zr/Nb, Nb/Y, Ba/Nb and Y/Nb ratios are akin to continental margin volcanics and also Zr/Y-Zr binary diagram show that there is similarity between continental margin volcanics and İstanbul volcanics. We assume that basaltic andesite and andesite dykes to be the source of volcanic products.