IS A SINGLE MODEL ADEQUATE TO EXPLAIN ORIGIN OF THE GABBROS FROM CENTRAL ANATOLIA, TURKEY?

Genesis of the Late Cretaceous gabbroic rocks playing a significant role in the geodynamic evolution of central Anatolia is debate. Contrasting models have been discussed so far by various researchers. Are they (a) member of ophiolitic units deriving from closure of Izmir-Ankara-Erzincan branch of Neotethys Ocean in Turkey, (b) mafic unit of intrusive bodies originating in a collisional setting, or (c) formed in a continental arc setting? To shed light on origin of these rocks, gabbroic rocks from two igneous associations, namely Agaçören and Ekecikdag, on the west of the Central Anatolian Crystalline Complex are investigated. Basement units in the study area are metamorphic rocks that are thrusted by ophiolitic units from north to south. Gabbros, having either layered or isotropic character, also tectonically overly the metamorphic rocks. Medium-coarse sized gabbros are cut by diabase dykes. Granitic rocks intrude both basement metamorphics and gabbros.

Gabbros are heteregeneous in composition and texture. Petrographical and mineral chemical data show that magnesio-hornblende (Mg/(Fe²⁺+Mg)=0.68-0.94) and high Ca plagioclase (An=69-98) are essential phases of the rocks while olivine (Fo=74-76), augitic diopside (Wo_46-47En_37-38Fs_14-15), and biotite are rarely present. Based on the presence of biotite, gabbroic rocks are named as biotite-gabbro or biotite-free gabbro.

The biotite-free gabbros together with the diabase dykes are tholeiitic in character whereas the biotite gabbros are calc-alkaline. The biotite-free gabbros and the diabase dykes display comparable geochemical features, i.e. similar ranges of major oxides. However, the biotite gabbros differ by their higher K₂O, Na₂O an TiO₂ and lower MgO and CaO contents. They have also higher large ion lithophile, high field strength and light rare earth elements compared to the biotite-free rocks. All the rocks show negative anomalies of Nb and Zr where the biotite gabbros have higher concentrations. The biotite-free gabbros and the diabase dykes are depleted in light rare earth elements (LREE) compared to heavy rare earth elements (HREE) while the biotite gabbros are enriched in LREE compared to HREE.

Low Ti/Y and Zr/Y, and high V/Ti ratios of the gabbroic rocks in the area infer plate margin. Lower K₂O and P₂O₅ contents compared to TiO₂ of the biotite-free rocks suggest oceanic setting while biotite gabbros with higher K₂O were likely to be derived in a continental setting. Mineral data from the biotite-free rocks are comparable with those of gabbros from island arc settings. Additionally, whole-rock geochemical characters of these rocks strongly indicate island arc – backarc setting in an intra-oceanic subduction zone. Although the biotite-free gabbros display ophiolitic origin, the biotite gabbros reveal similar geochemical characteristics to the Late Cretaceous syn- to post collisional granitoids in the area. Consequently, a single model is not sufficient to explain the origin of the gabbroic rocks in central Anatolia, since gabbros with different origins are present, as the ophiolitic gabbros and collision related intrusive gabbros.