Low‐pressure–high‐temperature metamorphism during extension in a Jurassic magmatic arc, Central Pontides, Turkey

Abstract

Magmatic arcs are zones of high heat flow; however, examples of metamorphic belts formed under magmatic arcs are rare. In the Pontides in northern Turkey, along the southern active margin of Eurasia, high temperature-low pressure metamorphic rocks and associated magmatic rocks are interpreted to have formed under a Jurassic continental magmatic arc, which extends for 2800 km through the Crimea and Caucasus to Iran. The metamorphism and magmatism occurred in an extensional tectonic environment as shown by the absence of a regional Jurassic contractional deformation, and the presence of Jurassic extensional volcaniclastic marine basin in the Pontides, over 2 km in thickness, where deposition was coeval with the high-T metamorphism at depth. The heat flow was focused during the metamorphism, and unmetamorphosed Triassic sequences crop out within a few kilometres of the Jurassic metamorphic rocks. The heat for the high-T metamorphism was brought up to crustal levels by mantle melts, relicts of which are found as ultramafic, gabbroic and dioritic enclaves in the Jurassic granitoids. The metamorphic rocks are predominantly gneiss and migmatite with the characteristic mineral assemblage quartz + K-feldspar + plagioclase + biotite + cordierite AE sillimanite AE garnet. Mineral equilibria give peak metamorphic conditions of 4 AE 1 kbar and 720 AE 40°C. Zircon U-Pb and biotite Ar-Ar ages show that the peak metamorphism took place during the Middle Jurassic at c. 172 Ma, and the rocks cooled to 300°C at c. 162 Ma, when they were intruded by shallow-level dacitic and andesitic porphyries and granitoids. The geochemistry of the Jurassic porphyries and volcanic rocks has a distinct arc signature with a crustal melt component. A crustal melt component is also suggested by cordierite and garnet in the magmatic assemblage and the abundance of inherited zircons in the porphyries.

Late Cretaceous infant intra-oceanic arc volcanism, the Central Pontides, Turkey: Petrogenetic and tectonic implications

“…The Sakarya Zone consists of Permo-Carboniferous crystalline basement and Upper Triassic distal turbidites with dismembered ophiolitic rocks (Nzegge et al, 2006;Okay et al, 2015;Ustaömer and Robertson, 1993. Middle Jurassic granitoids and related LP/HT metamorphic rocks form part of a magmatic arc within the Sakarya Zone (Okay et al, 2014;Yılmaz and Boztug, 1986). Upper Jurassic limestones and Lower Cretaceous turbidites unconformably cover rocks of both, _ Istanbul and Sakarya zones (Okay et al, 2013;Tüysüz, 1999).…”

The Intra-Pontide suture zone in the Tosya-Kastamonu area, Northern Turkey

“…the Sakarya continent of Şengör & Yılmaz, 1981) structurally underlies the IPS zone. It is made up of a Variscan basement (Okay et al, 2014) tectonically coupled with a deformed and metamorphosed Triassic subduction/ accretion complex known as the Karakaya Complex (e.g. Okay & Göncüoğlu, 2004).…”

Preface

“…With two TÜBİTAK projects, the geological picture of the Central Pontides changed completely. The metamorphic rocks including eclogites, blueschists and serpentinite were shown to have Early Cretaceous metamorphic ages , and the magmatic arc turned out to be Middle Jurassic in age (Okay et al, 2014). The northern part of the Central Pontides is occupied by a huge middle Cretaceous turbidite fan, which directly faced to a Tethyan ocean in the south.…”