Elsevier

Journal of Asian Earth Sciences

Volume 45, 2 February 2012, Pages 201-222
Journal of Asian Earth Sciences

Origin of the Early-Middle Devonian magmatism in the Sakarya Zone, NW Turkey: Geochronology, geochemistry and isotope systematics

https://doi.org/10.1016/j.jseaes.2011.10.011Get rights and content

Abstract

A unique example of isotopically-dated Devonian metagranitoid (the Çamlık metagranite; Okay et al., 1996) crops out in the Biga Peninsula, NW Turkey, although its contact relationships with the country rocks and geodynamic setting have remained to be enigmatic so far. Our field work, however, has shown that a number of metagranitoid bodies similar to the Çamlık metagranite intruded the country rocks and developed contact metamorphic zones, consisting of andalusite and calcsilicate hornfelses, garnet–epidote and diopsite–wollastonite skarns. The country rocks of these metagranitoids are made up of regionally metamorphosed metaclastic successions with subordinate metacarbonate–metachert–metabasites (the Kalabak formation), intercalated with tectonic slices of meta-serpentinites. The metagranitoids and the Kalabak formation are collectively termed here the Havran Unit which forms a NE–SW trending, 20 km wide and 80 km long belt in the Biga Peninsula. The Havran Unit is unconformably overlain by the Late Triassic shallow marine sediments and is in tectonic contact with the Permo-Triassic Karakaya Complex, interpreted as the Palaeotethyan subduction–accretion complex.

U–Pb SHRIMP-II and LA–ICP–MS dating of the zircons from four individual plutons yielded crystallization ages ranging from 389.1 ± 2.6 to 401.5 ± 4.8 Ma (i.e. Early to Middle Devonian). Based on their geochemical characteristics, the metagranitoids are divided into two groups named the Çamlık and Yolindi metagranitoids. Both granitoids display distinct subduction signature and plot in the volcanic arc granite and post-collisional granite fields on tectonic discrimination diagrams. 87Sr/86Sr(T) values of these intrusions vary between 0.707367 and 0.715588 while their 143Nd/144Nd(T) values range from 0.51166 to 0.51187. Very low values of εNd(T) (from −5.3 to −9.1) imply that the Lower to Mid-Devonian metagranitoids in the Havran Unit might have been generated by partial melting of lower continental crust, probably driven by a major magma underplating event. Delamination and/or slab-breakoff models are considered to be the most plausible mechanisms for the genesis of these metagranitoids.

The data obtained in this study has shown for the first time the widespread existence of Devonian and older basement in NW Turkey. Regional geological correlations imply that this basement may be an exotic terrane in the Sakarya Zone, emplaced during the Late Triassic, prior to its amalgamation with the Palaeotethyan Karakaya Complex.

Highlights

► Metagranitoid stocks intrusive into the Kalabak formation in the Sakarya Zone, NW Turkey are described. ► These metagranitoids have been divided into two units as the Çamlık and Yolindi metagranitoids. ► Geochemistry and isotope characteristics are compatible with the genesis at a magmatic arc or collisional setting. ► SHRIMP and LA–ICP–MS U–Pb zircon dating yielded Lower-Middle Devonian ages. ► The metagranitoids and the Kalabak formation (Havran Unit) may be a far-travelled exotic terrane.

Introduction

Understanding the tectonic development of orogenic belts requires correct identification of component litho-tectonic units, robust age and petrological data. Undated meta-igneous rocks in metamorphic terranes are of critical importance as they record the age of tectono-thermal events and tectonic setting of formation, thus allowing comparisons to be made with the tectonic events recorded in the adjacent litho-tectonic units or elsewhere along the orogeny. We use this approach in one of the pre-Jurassic basement units, the Havran Unit (new name), of the Sakarya Zone of NW Turkey. A number of metagranitoidic bodies are exposed within a regionally metamorphosed sedimentary-volcanic succession (the Kalabak formation) in the Havran Unit. Despite the fact that one of these metagranitic bodies were previously dated at 399 Ma by Pb-evaporation method (Okay et al., 1996, Okay et al., 2006), the contact relation of these bodies with the country rocks, their crystallization ages and tectonic-setting of formation have remained poorly understood.

In this paper, we present the first account of major- and trace-element geochemistry, 87Sr/86Sr and 143Nd/144Nd isotopic systematics and U–Pb SHRIMP-II and LA–ICP–MS zircon ages of the metagranitoids in the Havran Unit. We show that the granitoids, with arc-type geochemical signatures, were emplaced into the Unit during Early to Mid-Devonian and produced well-developed contact metamorphic aeoroles. The data presented in this paper demonstrate for the first time the widespread existence of Devonian and older basement in NW Turkey and shed light to the tectonic development of S Eurasian margin in Palaeozoic–Early Mesozoic, which was an active margin with important along-strike terrane dispersal.

Section snippets

Regional tectonic setting

Turkey is a tectonic mosaic of several continental blocks, amalgamated mainly during the Mesozoic and Cenozoic by the consumptional obliteration of intervening ocean basins, now marked by the anastomosing ophiolitic suture zones (Fig. 1). With an over 1500 km length, the Early Cenozoic Neotethyan İzmir–Ankara–Erzincan Suture (IAES) is the most significant suture zone in Turkey (Fig. 1). The IAES separates the Eurasian Pontides tectonic belt in the N from the Gondwanan Anatolide–Tauride Platform

Geological setting of the Havran Unit in the Biga Peninsula

Three structurally assembled pre-Cenozoic rock associations are exposed in the Biga Peninsula; forming SW–NE trending zones (Fig. 3). These are the Ezine Zone in the NW, the Sakarya Zone in the SE and northward narrowing wedge of high-grade metamorphics (i.e. the Kazdağ Metamorphics) in the middle.

The Ezine Zone comprises an ophiolitic mélange (the Cretaceous Çetmi mélange) and greenschist facies metasediments with tectonic lenses of blueschists and eclogites, Ar–Ar dating of which yielded Late

Analytical methods

The analytical methods (U/Pb SHRIMP-II, LA–ICP–MS, Sr–Nd isotope analyses and ICP–MS) used during this study is given in the Appendix A1.

Zircon U–Pb dating

Zircon crystals from the studied metagranitoids are generally characterized by granitic zircon morphology with partly-preserved oscillatory zoning along with some inherited cores. SHRIMP-II U–Pb zircon dating of the Yolindi stock yielded 389.1 ± 2.6 Ma (Fig. 9-a). LA–ICP–MS U–Pb zircon dating of three other samples taken from the Güveylerobası, Bayatlar and Karaaydın stocks yielded U–Pb Concordia ages of 401.5 ± 4.8, 401.4 ± 7.8 Ma and 401.4 ± 3.7 Ma, respectively (Fig. 9b–d and Table 1a, Table 1b, Table

Petrogenetic implications

Granitic magmas can be generated in various tectonic settings. As we presented earlier in Fig. 12, the Çamlık and Yolindi metagranitoids fall basically into either late/post-collisional or volcanic arc granite fields on tectonic discrimination diagrams. In collision/post-collision zones, magmas may be generated by melting of crustal rocks when they get closer to the hot asthenosphere after a major lithospheric delamination and/or slab breakoff event (e.g. Kay and Kay, 1993, Von Blanckenburg et

Conclusion

The data obtained in this study has shown for the first time the widespread existence of Devonian and older basement in NW Turkey. This basement, termed the Havran Unit, consists of a dominantly metasedimentary succession, intruded by a number of metagranitoidic stocks.

The first account of field, petrographical and geochemical characteristics of these metagranitoidic stocks and their contact zones with the country rocks are presented.

The metagranitoidic stocks have been divided into two units

Acknowledgments

This study builds on the first author’s PhD thesis and was supported by the Scientific Research Projects Coordination Unit of Istanbul University through the Projects No.: T026/23072002 and 436/13092005 (Geochemical analyses), 3859 (Sr–Nd isotope analyses), 435/13092005 (SHRIMP-II dating) and 4249 (LA–ICP–MS dating). Help given by Prof. Dr. Gernold Zulauf and Dr. Zeynep Özbey during the petrographical examinations are much appreciated. We thank Prof. Dr. Aral Okay for letting us use of his

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