The Behrekdag composite batholith, which crops out as a huge N-S-trending plutonic body in central Anatolia, Turkey, consists of five mappable granitoid units of Late Cretaceous age. They are (1) the S-type, peraluminous Danaciobasi biotite leucogranite, (2) the I-type, hybrid, metaluminous Konur K-feldspar megacrystic quartz monzonite, (3) the mafic A-type, alkaline Kizdede monzogabbro, (4) the felsic A-type, alkaline Hasandede quartz syenite/monzonite, and (5) the M-type, low-K tholeiitic Yenikoy tonalite. The S-type Danaciobasi biotite leucogranite constitutes the oldest intrusive unit in the mapped area. It has coarse- to medium-crystalline texture and consists Of quartz, orthoclase and plagioclase, with variable amounts of biotite and accessory minerals, including apatite, zircon and opaque phases. K-Ar age dating of biotite separates, yields cooling ages of 69.1 +/- 1.42 and 71.5 +/- 1.45 Ma for the Danaciobasi biotite leucogranite. Major-element, trace-element, and rare-earth element geochemical data suggest an exclusively peraluminous, S-type, high-K calc-alkaline, upper crustal genesis for the Danaciobasi biotite leucogranite. This petrogenetic interpretation is also Supported by oxygen-isotope data from quartz separates, with a mean value of 10.58 +/- 0.11 (0)/(00) of delta O-18(vsmow) value. The magma source of the Danaciobasi biotite leucogranite is proposed to have been a syn-collisional leucogranitic melt. derived by anatexis of high-grade metasediments of the Central Anatolian Crystalline Complex during peak conditions of regional metamorphism. This metamorphic event was induced by crustal thickening which was a result of Late Cretaceous collision between the Eurasia and Tauride-Anatolide Platform along the Izmir-Ankara-Erzincan Suture Zone in central Anatolia. Copyright (c) 2005 John Wiley & Sons, Ltd.