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)

Abstract

Whole-rock geochemistry, 40 Ar– 39 Ar 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 (An 49–88 ), clinopyroxene (Wo 38–49 En 38–54 Fs ), hornblende (Mg# = 0.57–0.74) phenocrysts, and magnetite/titanomagnetite and apatite microphenocrysts. 8–25 40 Ar– 39 Ar ages on hornblendes, ranging from 46.9 ± 0.1 to 39.9 ± 0.5 Ma, within the middle Eocene. The volcanic rocks show tholeiitic-calc-alkaline af?nities 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 ?eld strength elements (HFSE). The chondrite-normalized REE patterns (La 87 cn Sr/ /Lu 86 cn =1–19) show low-to-medium enrichment, indicating similar sources for the rock suite. Initial Sr values [( 87 Sr/ 86 Sr) 46 Ma ] vary between 0.70423 and 0.70495, while initial Nd values [( 143 Nd/ 144 Nd) ] lie between 0.51263 and 0.51285. The main solidi?cation 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 ?uids derived from subducted slab, in a post-collisional, extension-related geodynamic setting. 46 Ma 143 Nd/ 144