GEOLOGICAL JOURNAL Geol. J. 38: 1–14 (2003) Published online 10 January 2003 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/gj.921 Sarac ¸ko ¨ y Volcanic Suite: implications for the subductional phase of arc evolution in the Galatean Arc Complex, Ankara, Turkey ALI _ KOC ¸ YI _ GI _ T 1 , JOHN ALAN WINCHESTER 2 , ERDI _ N BOZKURT 1 * and GRENVILLE HOLLAND 3 1 Geological Engineering Department, Middle East Technical University, Ankara, Turkey 2 School of Earth Sciences and Geography, Keele University, Staffordshire, UK 3 Department of Geological Sciences, Science Laboratories, Durham, UK A sequence of upper Campanian volcanic rocks, hitherto considered as part of the Miocene calc-alkaline Galatean volcanic sequence, crops out near Sarac ¸ko ¨y, 40 km north of Ankara. These volcanic rocks, unconformably overlain by Eocene nummu- litic limestones, comprise thin flows and volcaniclastic deposits. They can be divided compositionally into three distinct sub- units: (i) basal massive trachyandesitic volcaniclastic rocks with a thin flow at the top, overlain by (ii) mixed volcaniclastic rocks and thin amygdaloidal flows of alkali basaltic composition, with (iii) trachyandesitic flows and subordinate volcanicla- stic rocks at the top. This alkaline volcanic activity, associated with late Campanian forearc peripheral sedimentation, is indi- cative of late Campanian extension within a collisional regime, pre-dating the compression accompanying Galatean arc development. Indeed, the composition of the Sarac ¸ko ¨y Volcanic Suite and field evidence are consistent with a slab roll-back process in an extensional supra-subduction setting which resulted from the northward-subducting, northern Neotethyan ocean floor during the late Campanian. They also imply that the present-day Galatean volcanic plateau is a magmatic arc development in three stages: (i) late Campanian–Lutetian subductional phase, (ii) post-Lutetian collisional phase, and (iii) Oligocene–Miocene post- collisional phase. Copyright # 2003 John Wiley & Sons, Ltd. Received 1 March 2001; revised version received 25 February 2002; accepted 27 March 2002 KEY WORDS Sarac ¸ko ¨y; Galatean arc magmatism; subduction; active margin; late Campanian; Turkey 1. INTRODUCTION The Galatean Arc Complex (GAC) occurs within the Pontide structural division of Turkey (Ketin 1966) and crops out on a high volcanic plateau directly north of Ankara, in Central Anatolia (Figure 1). The major outcrop of the GAC occurs in one large area elongated E–W (up to 12 800 km 2 ). Active segments of the North Anatolian Trans- form Fault System (NATFS) border the northern margin of the complex, while the Early Tertiary I _ zmir– Ankara–Erzincan Suture Zone (I _ AESZ) forms the southern boundary. Several additional small and isolated out- crops of the GAC also occur near Ankara to the south (Figure 1). The volcanic plateau comprises a sequence of 1500 m thick intermediate-acid lava flows (trachyandesites and dacites with minor rhyolites) and associated vol- caniclastics with minor alkali basaltic flows. Also present are several isolated basins of varying size, which contain thick fluvio-lacustrine sedimentary infills interfingering with volcanic rocks (see Toprak et al. (1996) and Wilson et al. (1997) for lengthy discussions). Although the Galatean Arc Complex has been studied by many workers (Stchepinsky and Lahn 1941; Erol 1951, 1954, 1955; Akyol 1969; Fourquin et al. 1970; O ¨ ngu ¨ r 1977; Ach 1982; Akyu ¨rek et al. 1984; Kazanci and Go ¨kten Copyright # 2003 John Wiley & Sons, Ltd. *Correspondence to: E. Bozkurt, Geological Engineering Department, Middle East Technical University, Tectonic Research Unit, Ankara TR- 06531, Turkey. E-mail: erdin@metu.edu.tr