Petrology of Eocene volcanic rocks from the Central Sakarya Zone (northwestern Anatolia, Turkey): new evidence from Ar-Ar and Sr-Nd isotope determinations

Abstract

In this study, petrological characteristics of the Early-Middle Eocene Bozanic volcanic rocks resulting from the closure of the Neotethys Ocean in the Central Sakarya region and geodynamic evolution of the Eocene magmatism in the region were investigated. Although previous researchers attained insightful findings about the geodynamic evolution of the region, the age of the post-collisional volcanism, petrological characteristics, and source of the magma remain uncertain. Therefore, volcanic rocks outcropping nearby Saricakaya-Mihalgazi (Eskisehir) are investigated in this study. For that purpose, geological map of the study area was drawn and mineralogical, petrographic, geochemical, and isotopic analyses were performed on rock samples collected from different locations. Bozanic volcanic rocks outcropping as lavas and pyroclastics (agglomerate, lapillistone, and tuff) seem to be derived from four separate volcanic chimneys located in the study area and their composition are typically andesite. According to petrographic data, hypocrystalline porphyritic and microlitic porphyritic are the dominant textures of the rocks. Plagioclase, amphibole, clinopyroxene, and biotite are the main minerals, while sanidine, opaque minerals, chalcedony, calcite, and chlorite can also be observed in small amounts. Geochemically, they show calc-alkaline characteristics and contain medium-high K. Fractional crystallization is the most important process in the development of the rocks. Bozanic volcanic rocks were derived from a lithospheric mantle source that developed in the active continental margin. According to Ar-40/Ar-39 age spectra of the samples, the ranges of 48.13 +/- 0.15-48.78 +/- 0.23 Ma plateau ages were obtained. (Sr-87/Sr-86)i ratios of the rocks range between 0.705404 and 0.705502, while (Nd-143/Nd-144) i ratios range between 0.512570 and 0.512581. Consequently, the age data obtained in this study show that the formation time of the first products of the post-collisional volcanism be as early as the Early Eocene and the presence of those is important in terms of the elucidation of geodynamic evolution of the region.