R E-EVALUATION OF THE PETROLOGICAL FEATURES OF THE

1. RE-EVALUATION OF THE PETROLOGICAL FEATURES OF THE EKECİKDAĞ OCEANIC PLAGIOGRANITES IN CENTRAL ANATOLIA / TURKEY Study area Serhat KÖKSAL Middle East Technical University, Central Laboratory, Radiogenic Isotope Laboratory Ankara, Turkey,skoksal@metu.edu.tr M. Cemal GÖNCÜOĞLU Middle East Technical University, Department of Geological Engineering, Ankara, Turkey, mcgoncu@metu.edu.tr Fatma TOKSOY-KÖKSAL Middle East Technical University, Department of Geological Engineering, Ankara, Turkey, ftkoksal@metu.edu.tr Introduction Remnants of the İzmir-Ankara-Erzincan branch of the Alpine Neotethys are represented by dismembered ophiolitic units in the Ekecikdağ area (central Anatolia/Turkey). In the Ekecikdağ 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 Ekecikdağ plagiogranites (Göncüoğlu and Türeli, 1993) are re-assessed by using the new whole-rock elemental and isotopic data. Geology Plagiogranites occupy large area on the south of the Ekecikdağ area.They intrude the ophiolitic gabbros and cut by several dolerite dykes in the Ekecikdağ area . Petrography The Ekecikdağ plagiogranites are equigranular rocks mainly composed of quartz and plagioclase. They are characterized by hypidomorphic to granophyric texture, with typical intergrowth ofquartz and plagioclase. Moreover, orthoclase and ferromagnesian minerals (biotite, hornblend, clinopyroxene) are present in minor amounts, with accessory amount of zircon, titanite, apatite and opaque minerals besides chlorite, epidote and sericite as alteration minerals. They are tonalite in composition on the normative Ab–An–Or plot, but display extremely low Or %. After Göncüoğlu and Türeli (1993) Geochemistry 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. DMM: Depleted MORB Mantle, MORB: Mid Ocean Ridge Basalt, EM: Enriched Mantle, BSE: Bulk Silicate Earth, TVG: Troodos volcanic glasses Sr & Nd Isotopes The Ekecikdağ plagiogranites are characterized by high initial Nd and low initial Sr isotopic ratios. On the ƐNd(T) vs. 87Sr/86Sr(T) diagram the distinct isotopic character of the Ekecikdağ plagiogranites compared to Late Cretaceous collisional to post-collisional granitic rocks in the CACC is significant. Initial Sr and Nd isotopic ratios typically reveal MORB character, and are similar to those of the Troodos volcanic glasses of Rautenschlein et al. (1985). Sr and Nd isotope analyses were performed at the Radiogenic Isotope Laboratory of the METU Central Laboratory As a consequence, we suggest that the Ekecikdağ plagiogranites, which are comparable with other plagiogranites in the Tethyan realm,were derived from partial melting of the depleted mantle sources in a supra-subduction zone setting. 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. Rautenschlein M., Jenner G.A., Hertogen J., Hofmann A.W., Kerrich R., Schmincke H.-U., White W.M., 1985. Isotopic and trace element composition of volcanic glasses from the Akaki Canyon, Cyprus: implications for the origin of the Troodos ophiolite. Earth and Planetary Science Letters 75: 369-383.