Journal of the Geological Society, London, Vol. 170, 2013, pp. 159–173. doi: 10.1144/jgs2011-091. 159 Our discovery of previously unknown continental lithologies beneath a metamorphosed Mesozoic carbonate platform in the Tavşanlı Zone, NW Turkey (Fig. 1) has important implications for the Palaeozoic tectonic development of the Tethyan orogen. Meta- morphosed clastic sediments and a crosscutting metagranite, the Sarıkaya metagranite, represent the lowest known stratigraphic lev- els of the Tavşanlı Zone. The Triassic to Upper Cretaceous stratig- raphy of the Tavşanlı Zone is well documented (Okay 1984, 1986; Okay & Tüysüz 1999), but what is beneath has until now remained unknown. Palaeozoic lithologies have been mapped and dated and the results are used to shed light on the tectonic development of the Tethyan orogen. Evidence of the Palaeozoic lithologies is presented from the Dursunbey area, the most northwesterly outcrop of the Anatolides, based on new field, chemical and isotopic age data (Özbey 2010; Özbey et al. 2010; Fig. 2). We will first summarize the evidence for the high-pressure/low-temperature (HP/LT) metamorphism and associated deformation that affects the Anatolide block beneath its Cenozoic sedimentary cover. The metamorphism, of Late Cretaceous age, has to be taken into account when interpreting the Palaeozoic metasedimentary and meta-igneous rocks. We also draw on information concerning the Late Palaeozoic–Mesozoic geology of the Taurides and the Pontides in Turkey and the wider region to test alternative tectonic hypotheses of Tethyan assembly. Regional setting The Anatolides of Turkey are subdivided into the east–west-trending Tavşanlı Zone (50–60 km wide × 250 km long) in the north, the Afyon Zone to the south and the Menderes Massif to the SW (Fig. 1). Within the Tavşanlı Zone, a coherent Mesozoic carbonate platform succession has been metamorphosed to blueschist-facies conditions (Okay 1984, 2002; Davis & Whitney 2006; Çetinkaplan et al. 2008) during the Late Cretaceous. Glaucophane and phengite grains yielded ages of 108–88 Ma using the 40 Ar/ 39 Ar method (Harris et al. 1994). Phengites have also been dated more precisely using Ar/Ar laser probe at 88.5 ± 0.5 Ma (Okay & Kelley 1994). Late Cretaceous ages (78.5 ± 1.6 Ma and 82.8 ± 1.6 Ma) were later con- firmed by Rb/Sr dating of four samples of phengite from the Tavşanlı and Sivrihisar areas (Sherlock et al. 1999). We therefore emphasize that the HP/LT mineral assemblages of the various lithologies mak- ing up the Tavşanlı Zone are Late Cretaceous whereas the substra- tum of the Tavşanlı Zone formed during the Early Palaeozoic. The Tavşanlı Zone is widely interpreted as the northern edge of the Tauride–Anatolide microcontinent (Okay 1984; Sherlock et al. 1999; Robertson et al. 2009; Okay & Whitney 2011). The ocean to the north, known as the northern branch of Neotethys (Şengör & Yılmaz 1981) or the İzmir–Ankara–Erzincan ocean (Okay 1989) was consumed by northward subduction along a subduction trench during Late Cretaceous–Early Cenozoic time, creating suprasu- bduction-zone type ophiolites and related accretionary mélange (Robertson et al. 2009). During the Late Cretaceous, the subduction trench collided with the Tauride–Anatolide continental margin, which was deeply buried and metamorphosed to blueschist-facies conditions (Okay 2002). The HP/LT metamorphic rocks were exhumed by Palaeocene time (Okay & Kelley 1994; Sherlock et al. 1999). In contrast, the Afyon Zone to the south (Fig. 1), of less rel- evance here, underwent lower grade HP/LT metamorphism (Okay Tectonic significance of Late Ordovician granitic magmatism and clastic sedimentation on the northern margin of Gondwana (Tavşanlı Zone, NW Turkey) ZEYNEP ÖZBEY 1* , TIMUR USTAÖMER 1 , ALASTAIR H. F. ROBERTSON 2 & P. AYDA USTAÖMER 3 1 İstanbul Üniversitesi, Jeoloji Bölümü, Avcılar, 34850, İstanbul, Turkey 2 University of Edinburgh, School of GeoSciences, West Mains Road, Edinburgh EH9 3JW, UK 3 Yıldız Teknik Üniversitesi, Doğa Bilimleri Araştırma Merkezi, Davutpaşa-Esenler, 34210, İstanbul, Turkey *Corresponding author (e-mail: zeynep.uctas@gmail.com) Abstract: Metasedimentary schists and a small crosscutting metagranite underlie a Mesozoic metamorphosed carbonate platform within the regional Anatolide tectonic belt of northern Turkey. These lithologies are inferred to have formed part of the northern margin of Gondwana during the Palaeozoic. Immobile element geochemistry suggests that the country rock metasediments were derived from upper continental crust and arc-type magmatic rocks. Major and trace element analyses of the metagranite indicate affinities with upper continental crust or a continental margin magmatic arc. Tectonic discrimination of the metagranite is consist- ent with a rift or post-collisional setting. Niobium depletion relative to Ce on primitive mantle-normalized spidergrams suggests a subduction influence, possibly inherited from Late Precambrian (Cadomian) arc mag- matism. Ion microprobe U–Pb dating of zircons from the metagranite yielded a crystallization age of 446 ± 8 Ma (Late Ordovician). An inherited core gave a concordant age of c. 578 Ma, consistent with Cadomian or Pan-African basement. Regional comparisons indicate that the Palaeozoic lithostratigraphy is similar to that of the Taurides (Gondwana) rather than the Pontides (Laurasia). We infer Mid- to Late Ordovician crustal extension along the north Gondwana margin, followed by spreading of Palaeotethys. The entire stra- tigraphy experienced high-pressure/low-temperature metamorphism during Late Cretaceous closure of the İzmir–Ankara–Erzincan ocean. Supplementary materials: The mineral composition, fabric, coordinates (UTM) and modal composition of the Beyce schist and Sarıkaya metagranite samples are available at www.geolsoc.org.uk/SUP18556. research-articleResearch Article 170 X 10.1144/jgs2011-091Z. ÖzbeyCrustal Provenance of the Alexander Terrane 2013