Eocene post-collisional volcanism in the Central Anatolian Crystalline Complex, Turkey: Petrology and geodynamic significance

In the Central Anatolian Crystalline Complex (CACC) the Late Cretaceous postcollisional granitic magmatism is followed by Eocene extension, resulting in formation of roughly E-W trending transtensional basins. The volcanic rocks, mainly submarine lava flows and subareal domes are concentrated along these Middle Eocene (Bartonian) basins. The volcanic rocks are basic to intermediate and are classified as basalt, basaltic andesite and rarely alkali basalt and trachy-andesite. Petrographically they are generally plagioclase + pyroxene ± olivine ± hornblende ± biotite pyhric, indicating a shallow crystallization level. They are characterized by several disequilibrium textures, which may suggest role of magma mixing/mingling process during their evolution. Eocene volcanic rocks are characterized by high phenocryst contents, low but variable MgO concentrations (0.54-9.30 wt %), low Mg numbers (19.57-55.57) and low compatible trace element concentrations (Ni 5-166 ppm; Co 7-32 ppm), which provide strong evidence for the mafic mineral fractionation. Their relatively high Zr and Y contents provide strong evidence for their transitional to mildly alkaline nature and also point out their within-plate characters. All studied samples are strongly and variably LREE enriched relative to chondrite with the (La/Sm) N ratio of 2.26 - to 6.17 and show small negative Eu anomalies (Eu/Eu*=0.65-1.00), suggesting plagioclase fractionation. The REE patterns of the studied rocks are consistent with the derivation from a shallow depth (e.g. spinel lherzolitic source). They have negative Nb-Ta and Ti anomalies in the primitive mantle normalized diagram and are characterized by low Nb/La (0.21 to 0.62), Ce/Pb (3.70-34.90) and Nb/U ratios (1.11-30), which may indicate an interaction with the Late Cretaceous granitic host rocks in the course of their ascent.
   The volcanic rocks display similar but variable ranges of Sr, Nd and Pb isotope ratios. εNd values range from 0.12 to 4.06, which is indicative of an isotopically depleted mantle source. They have relatively high and variable LILE/HFSE, LILE/LREE ratios (e.g. Ba/Nb 32-208 and Ba/La 16-46) and relatively radiogenic Sr, Pb isotope compositions (0.70404-0.70559 and 18.62-19.17 for ²⁰⁶Pb/²⁰⁴Pb 15.58 – 15.68 for ²⁰⁷Pb/²⁰⁴Pb and 38.65 – 39.00 for ²⁰⁸Pb/²⁰⁴Pb), indicating that they were derived from a heterogeneous lithospheric mantle that had been metasomatised by subduction related agents such as fluids and/or melts during a previous geodynamic event. On the other hand, high LILE and LRE contents of the rocks point out fluid dominated metasomatism rather than melt metasomatism.
   Eocene volcanic rocks are supposed to be formed as a result of post-collisional lithospheric extension that followed the Late Cretaceous thickening of the Central Anatolian Crystalline continental crust, related to the closure of the Neotethyan Izmir-Ankara branch of Neotethys. Geochemistry and geotectonic setting point out that lithospheric delamination was the most likely mechanism to generate these calcalkaline to mildly alkaline volcanic rocks in the CACC.