PETROGENETIC CHARACTERISTICS OF OYACA – KEDİKAYASI – BOYALIK ADAKITES IN SW ANKARA (CENTRAL ANATOLIA, TURKEY): EVIDENCE FOR SLAB MELT METASOMATISM

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The Early Miocene Oyaca, Kedikayası and Boyalık dacites, situated approximately 50-60 km southwest of Ankara have affinities similar to adakitic rocks. They have porphyritic texture with a variable amount of plagioclase feldspar, hornblende and lesser biotite phenocrysts and a groundmass of plagioclase and quartz microcrysts. They have high Sr/Y (55-79 ppm) and (La/Yb)n (21-32 ppm) ratios, and low Y (10-19 ppm) and heavy rare earth element contents. According to their SiO2 (62.3-69.70 % wt.) and MgO (0.62-2.23 % wt) contents, they are referred to as high silica adakites, indicating the effects of slab-derived melts in their genesis. The adakites in the study area are enriched in Large Ion Lithophile Elements (LILE) (e.g., Ba: 800-1395 ppm, Sr≥720 ppm) relative to High Field Strength Elements (HFSE) (e.g. Nb: 20-10 ppm, Ta: 0.8-1.2 ppm). Low Rb/Sr and high Ba/Sr ratios in these adakites indicate that they are resulted from an amphibole bearing mantle source, as amphiboles have low Rb concentrations. Thus, partial melting of an amphibole bearing mantle source would be responsible for low Rb concentrations). For that reason, nonmodal partial melting calculations from a 13 % amphibole bearing garnet peridotite were carried out in order to determine the source features of adakites. The variations between La/Yb vs La and (Tb/Yb)n vs (La/Yb)n in partial melting studies demonstrate that the adakites in the study area were most probably derived from an amphibole bearing garnet peridotite mantle source via 5-10 % degrees of partial melting.

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