Petrogenesis of the post-collisional volcanic rocks from the Borçka (Artvin) area: Implications for the evolution of the Eocene magmatism in the Eastern Pontides (NE Turkey)

Whole-rock geochemistry, 40Ar-39Ar data, and Sr-Nd isotopes are presented for the Eocene Borçka volcanic rocks on the eastern corner of the eastern Pontide orogenic belt (NE Turkey). Borçka (Artvin) volcanic rocks are divided into three suites-basic dyke, Borçka basalt, and Civanköy suite. These rocks contain plagioclase (An49-88), clinopyroxene (Wo38-49En38-54Fs8-25), hornblende (Mg#=0.57-0.74) phenocrysts, and magnetite/titanomagnetite and apatite microphenocrysts. 40Ar-39Ar ages on hornblendes, ranging from 46.9±0.1 to 39.9±0.5Ma, within the middle Eocene. The volcanic rocks show tholeiitic-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 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 [(87Sr/86Sr)46Ma] vary between 0.70423 and 0.70495, while initial 143Nd/144Nd values [(143Nd/144Nd)46Ma] lie 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. All our evidences support the conclusion that the parental magma 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.