Petrography,geochemistry and petrogenesis of The Borçka (Artvin, Northeast Turkey) area tertiary volcanics

Major, trace element, Ar-Ar age and Sr-Nd isotopic datas are presented for the Eocene Borçka volcanics in the eastern edge of the eastern Pontide orogenic belt (NE Turkey). Borçka (Artvin) volcanics are divided into three suites; Borçka Basalt, Civanköy member and basic dyke. These rocks contain plagioclase (An11-93), clinopyroxene (Wo38-49En38-54Fs8-25), hornblende (Mg#=0.57-0.74) phenocrysts and magnetit/titanomagnetite and apatite microphenocrysts. 40Ar-39Ar ages on hornblendes range from 46.9±0.1 to 39.9±0.5 Ma, within the Middle Eocene. The volcanic rocks show calc-alkaline affinities and have low to medium K contents. They are enriched in large ion lithophile (LILE) and light rare earth elements (LREE), with pronounced depleted of high field strength elements (HFSE). The chondrite-normalized REE patterns (Lacn/Lucn=1-19) show low to medium enrichment, indicating similar sources for the rock suite. Initial 87Sr/86Sr values vary between 0.70423 and 0.70495, while initial 143Nd/144Nd values change between 0.51263 and 0.51285. The main solidification processes involved in the evolution of the volcanics consist of fractional crystallization with minor amounts of crustal contamination±magma mixing. All evidence supports the conclusion that the parental magma(s) of the rocks probably derived from an enriched mantle, previously metasomatized by fluids derived from subducted slab in a post-collisional extension-related geodynamic setting.Key Words: Eastern Pontides, Sr-Nd isotopes, post-collisional magmatism, Borçka (Artvin) volcanics, Turkey