RE-EVALUATION OF THE PETROLOGICAL FEATURES OF THE EKECIKDAG OCEANIC PLAGIOGRANITES IN CENTRAL ANATOLIA / TURKEY

Remnants of the Izmir-Ankara-Erzincan branch of the Alpine Neotethys are represented by dismembered ophiolitic units in the Ekecikdag area (central Anatolia/Turkey). In the Ekecikdag and nearby areas, the ophiolitic rocks thrust over the central Anatolian metamorphic basement and intruded by the Late Cretaceous granitoids. Among these ophiolitic rocks plagiogranites are significant for the petrological evaluation of the oceanic history. The available data and interpretations on the Ekecikdag plagiogranites (Göncüoğlu and Türeli, 1993) are re-assessed by using the new geochemical data. The Ekecikdag plagiogranites are composed of quartz, plagioclase, hornblende, biotite ± clinopyroxene, with accessory amount of zircon, titanite and apatite minerals. They are tonalite in composition on the normative Ab–An–Or plot, but display extremely low Or %. The plagiogranites in the Ekecikdag area show high SiO₂ (69.9–75.7 wt. %), but exceptionally low K₂O contents (less than 0.6 wt. %), which correspond to the oceanic plagiogranite description of Coleman and Peterman (1975). Furthermore they contain Na₂O contents ranging from 2.7 to 5.3 wt. %, Al₂O₃ contents ranging from 12.8 to 14.3 wt. %, and CaO contents ranging from 3.0 to 5.4 wt %. They are subalkaline, calcic, and have transitional characters from metaluminous to peraluminous, and from magnesian to ferroan. Furthermore, depletions in Th, Nb, La, Ce, P, Zr and Ti are noticeable on the primitive normalized spider diagrams. Chondrite-normalized REE diagram of plagiogranite samples shows LREE contents, increasing from La to Sm, and flat MREE and HREE profiles. Overall REE pattern reveals higher HREE with respect to LREE ([La/Yb]_N = 0.33–0.46] and negative Eu anomalies ([Eu/Eu*]_N = 0.67–0.97) suggesting plagioclase fractionation. Additionally, the plagiogranite samples plot into the VAG field within the Nb vs. Y diagram of Pearce et al. (1984), but still in the fields of plagiogranites from the Troodos and Oman ophiolites. The Ekecikdag plagiogranites likely to be derived from partial melting of the depleted mantle sources in a supra-subduction zone setting, and are comparable with other plagiogranites in the Tethyan realm.

References:

Coleman, R.G. and Peterman, Z.E., 1975. Oceanic plagiogranite. Journal of Geophysical Research 80:1099–1108.

Göncüoğlu, M.C. and Türeli, K., 1993, Petrology and geodynamic interpretation of plagiogranites from Central Anatolian Ophiolites (Aksaray-Turkey). Turkish Journal of Earth Sciences 2:195–203.

Pearce, J.A., Harris, N.B.W. and Tindle, A.G.W., 1984. Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. Journal of Petrology 25:956–983.