Petrographical, geochemical and petrological investigation of the İkizce (Ünye-Ordu) area volcanic rocks

SUMMARY Petrographical, Geochemical and Petrologjcal Investigation of the İkizce (Ünye-Ordu) Area Volcanic Rocks In this study, petrographical, geochemical and penological characteristics of Tertiary aged volcanic rocks in the ikizce (Ünye-Ordu) area were determined, and the evolution and origin of volcanism were investigated. In the studied area located at the south of Eastern Pontid, the basement is represented by Late Cretaceous-Paleocene aged Tekkiraz Member of Akveren Formation. This formation overlain unconformably by Paleocene-Eocene aged Koçevyam Basalt. Eocene aged Tekkeköy Formation, overlain unconformably this unit, is made up from bottom to top; Çaybaşı Member beginning localy with conglomerate, Kurttaşh Tepe Andesite, Kale Member, ikizce Andesite and Teknecik Andesite Porphyr. All these units are overlain unconformably by Quaternary terrace and alluvium. The investigated volcanic rocks contain commonly andesite, andesite porphyr and pyroclastics, rarely basalt and basaltic andesite, and show porphyric, microlitic porphyric, hyalo-microlitic porphyric, hyalopilitic and rarely intersertal, intergranular, fluidal and glomeroporphyric textures. The rocks include clinopyroxene (Wo44^gEn42^8Fs8.i2), plagioclase (A1132-50), olivine (Fog2-84), hornblende (Mg#= 0.48-0.81), biotite (Mg# = 0.48- 0.60), rarely magnetite, apatite and zircon. The rocks show disequilibrium textures such as oscillatory zoning, sieve texture and corrosion in plagioclase phenocrysts, breakdown and opaqued hornblendes and biotites, melting in clinopyroxene. Volcanics are generally calcalkaline and tholeiitic-alkaline transitional in character, and have medium-K contents. Major and trace element variations indicate significant role of clinopyroxene, olivine, hornblende, plagioclase, magnetite and apatite fractionation during the evolution of rocks. High LILE (Sr, K20, Rb, Ba) and LREE (La, Ce), low HFSE (Nb, Zr, Y, Ti02) contents show that the rocks evolved from a parental magma derived from an enriched source formed by mixing of slab and asthenospheric melts. Koçevyam Basalt formed olivine + augite controlled fractionation in high pressure (~2 kbar) whereas andesitic rocks developed augite + hornblende ± plagioclase controlled fractionation at shallow level low pressure (~1 bar), and assimilation ± magma mixing. Key Words: Eastern Pontid, Tertiary Volcanism, Arc Volcanism, Basalt, Andesite, Calcalkaline Volcanism, Mantle Metasomatism, Fractional Crystallisation. VII