The composite Yozgat batholith consists of a S-I-A-type granitoid association intruding the supra-subduction zone-type (SSZ-type) central Anatolian ophiolite and medium-to high-grade metasedimentary rocks of the central Anatolian crystalline complex. These rocks are unconformably covered by Palaeocene to Early Eocene sedimentary rocks. The I-type granitoids are the most common rock association of this huge batholith. In an area between the towns of Sefaatli and Yerkoy, the southwestern part of the batholith can be subdivided into five mappable units: the Akcakoyunlu quartz monzodiorite ( mafic; hornblende K-Ar cooling ages of 77.6-79.3 Ma); the Cankili monzodiorite ( mafic; hornblende K-Ar cooling age of 71.1 Ma); the Adatepe quartz monzonite ( mafic; hornblende K-Ar cooling age of 68.0 Ma); the Yassiagil monzogranite ( felsic; hornblende + biotite K-Ar cooling ages of 69.9-79.8 Ma) and the Karakaya monzogranite ( felsic; hornblende + biotite K-Ar cooling ages of 71.3-77.0Ma). All the lithological units, except the Karakaya monzogranite, include large K-feldspar megacrysts and various types of mafic microgranular enclaves in field outcrops, indicating mingling and mixing. In addition, microscopic textures showing the hybridization between the coeval mafic and felsic magma sources are present. Whole-rock major element geochemistry shows a high-K calc-alkaline, metaluminous, I-type composition with an aluminium saturation index (ASI) less than 1.10 and with CIPW diopside content in all the lithological units. Large ion lithophile elements ( LILE), light rare earth elements (LREE), some high field strength elements (HFSE) ( except Nb) enrichments and significant crustal contribution revealed by the oxygen and sulphur stable isotope compositions in the mafic and felsic I-type granitoid units are consistent with mafic lower crustal and metasomatized mantle sources the latter of which were metasomatized by earlier supra subduction zone (SSZ)-derived fluids during the development of the SSZ-type central Anatolian ophiolite.