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

Abstract

Whole-rock geochemistry, Ar-40-Ar-39 data, and Sr-Nd isotopes are presented for the Eocene Borcka volcanic rocks on the eastern corner of the eastern Pontide orogenic belt (NE Turkey). Borcka (Artvin) volcanic rocks are divided into three suites basic dyke, Borcka basalt, and Civankoy suite. These rocks contain plagioclase (An(49-88)), clinopyroxene (Wo(38-49)En(38-54)Fs(8-25)), hornblende (Mg# = 0.57-0.74) phenocrysts, and magnetite/titanomagnetite and apatite microphenocrysts. Ar-40-Ar-39 ages on homblendes, ranging from 46.9 +/- 0.1 to 39.9 +/- 0.5 Ma, 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 (La-cn/Lu-cn = 1-19) show low-to-medium enrichment, indicating similar sources for the rock suite. Initial Sr-87/Sr-86 values [(Sr-87/Sr-86)(46 Ma)] vary between 0.70423 and 0.70495, while initial Nd-143/Nd-144 values [(Nd-143/Nd-144)(46) (Ma)] 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. (C) 2013 Elsevier B.V. All rights reserved.