Mineral Chemistry and Petrochemistry of Post-Collisional Tertiary Mafic to Felsic Cogenetic Volcanics in the Ulubey (Ordu) Area, Eastern Pontides, NE Turkey

Abstract

Post-collisional Tertiary volcanic rocks in the Ulubey (Ordu) area at the western edge of the eastern Pontides palaeo-arc are divided into four suites. The Yenisayaca basalt (TB) contains plagioclase (An_{61-83}), clinopyroxene (Wo_{42-44}En_{39-41}Fs_{15-18}) and olivine phenocrysts and titanomagnetite microphenocrysts, whereas the Çatal Tepe and Elekçioğlu Tepe suite (ÇES), Işık Tepe suite (ITS) and andesite/ trachyandesite suite (ATS) rocks include plagioclase (An_{23-78}), clinopyroxene (Wo_{27-48}En_{37-55}Fs_{11-26}), hornblende (Mg#= 0.63-0.76), biotite (Mg#= 0.63-0.82), sanidine phenocrysts and titanomagnetite and apatite microphenocrysts. Petrochemically, the volcanic rocks show tholeiitic-alkaline to the calc-alkaline affinities, and have medium to high-K contents. Most samples have low Mg#, Cr, and Ni, which indicates that they have undergone significant fractional crystallization from mantle-derived melts. The geochemical variations can be explained by fractionation of common mineral phases such as clinopyroxene ± plagioclase ± magnetite in the Yenisayaca basalt, and hornblende + biotite + plagioclase ± magnetite ± apatite ± sanidine in the Çatal Tepe and Elekçioğlu Tepe suite, Işık Tepe suite and andesite/trachyandesite suite rocks. N-Type MORB-normalized trace element patterns show that Ulubey volcanic rocks are enriched in LILE and to a lesser extent in Th and Ce, but depleted in Zr, Y and TiO_2. Besides, the rocks have depletion in Nb and Ta relative to LILE, moderate LREE/HREE ratios and high Th/Yb ratios, all of which indicate that parental magma(s) probably derived from an enriched source region (probably lithospheric mantle) which was previously modified by fluids. The C1-chondrite-normalized REE patterns are concave with low to medium enrichment, indicating similar source areas for the Yenisayaca Basalt, Çatal Tepe and Elekçioğlu Tepe suite, Işık Tepe suite and andesite/trachyandesite suite. The REE patterns also imply that negative Eu anomalies are probably associated with plagioclase fractionation in the evolution of the rocks.