Triassic blueschists and eclogites from northwest Turkey: vestiges of the Paleo-Tethyan subduction

Abstract

Triassic eclogite and blueschist facies rocks occur as a thrust sheet, 25-km long and over 2-km thick, in an Eocene fold-andthrust belt in northwest Turkey along the İzmir -Ankara suture. The thrust sheet consists mainly of metabasites with minor marble, phyllite and metachert, and rare lenses of serpentinite. The common blueschist facies mineral assemblage in the metabasites is sodic amphibole + epidote + albite + chlorite + phengite F garnet. Sodic amphibole commonly shows replacement by barroisite, and there is continuous petrographic transition from blueschist -metabasites to barroisite-bearing epidoteamphibolites. Eclogite with the mineral assemblage of garnet + sodic pyroxene + sodic -calcic amphibole + epidote is found only in one locality. P -T conditions of the epidote -blueschist facies metamorphism are estimated as 450 F 50 jC and 11 F 2 kbar. The blueschist formation was followed by a decrease in pressure and increase in temperature, leading to the development of barroisite-bearing epidote -amphibolites. Phengite, sodic amphibole and barroisite Ar/Ar ages from three metabasic rocks range between 215 and 205 Ma, and indicate Late Triassic high-pressure metamorphism. The Triassic blueschists in northwest Turkey constitute part of a much larger allochthonous tectonic unit of Triassic mafic volcanic rocks. They probably represent the upper layers of a Triassic oceanic plateau, which was accreted to the Laurasian margin during the latest Triassic. The close spatial association of the Triassic and Cretaceous blueschists along the İzmir -Ankara suture suggests that the suture represents a long-lived plate boundary of Late Palaeozoic to early Tertiary age. D

Devonian magmatism in the western Sakarya Zone, Karacabey region, NW Turkey

“…In Söğüt region, K-Ar biotite and Pb-Pb zircon evaporation ages are~290 Ma (Çoğulu & Krummenacher, 1967;Çoğulu, Delaloye & Chessex, 1965), and one Ar-Ar amphibole age is 272 ± 2 Ma (Okay, Monod & Monié, 2002). However, the granitoid ages in these regions are known to be Carboniferous, hence younger ages from the granitoids should represent later metamorphic (heating?)…”

The Ankara Mélange: an indicator of Tethyan evolution of Anatolia

“…The high pressure metamorphism of some Neotethyan ophiolites along the İzmir-Ankara-Erzincan suture zone supports this suggestion. The oldest radiometric ages from the high pressure metamorphic rocks associated with the Neotethyan ophiolite to the north of Eskişehir town range between 204 and 215 Ma, (Okay et al 2002). The average ages (204)(205) indicate that the subduction of the Neotethyan oceanic lithosphere began 30-40My after the rifting of the Neotethyan Ocean.…”

“…During Mesozoic times, the Tethyan Ocean formed a gulf towards the west between the two continents. Various hypotheses have been suggested on the shape and the evolution of the Tethyan embayment including single or multi-branched Tethys; opening and closure times of the Neotethyan Ocean; as well as the time and the polarity of the Tethyan subduction (Ricou et al 1975;Bijou-Duval et al 1977;Şengör & Yılmaz 1981;Knipper et al 1986;Okay & Tüysüz 1999;Stampfli 2000;Okay et al 2002;Robertson 2002;Çakır 2009). …”

Preface

“…Aral's work in the Biga Peninsula showed that the Karakaya Complex has a lower unit composed predominantly of metabasites, representing an accreted Permo-Triassic oceanic plateau or large number of oceanic islands, which is overlain by chaotically deformed volcanic and sedimentary rocks of the Upper Karakaya Complex. His discovery of latest Triassic eclogites and blueschists in the Lower Karakaya Complex in north-west Turkey (Okay & Monié, 1997;Okay, Monod, & Monié, 2002) showed convincingly a subduction rather than rift origin of the Karakaya Complex. The presence of Permo-Carboniferous limestone blocks were long known in the Upper Karakaya Complex.…”