Turkish Journal of Earth Sciences Turkish J Earth Sci (2020) 29: 208-219 http://journals.tubitak.gov.tr/earth/ © TÜBİTAK Research Article doi:10.3906/yer-1905-19 Middle Eocene high-K acidic volcanism in the Princes’ Islands (İstanbul) and its geodynamic implications Fatih ŞEN* Institute of Graduate Studies in Sciences, İstanbul University, Vezneciler, İstanbul, Turkey Received: 26.05.2019 Accepted/Published Online: 03.10.2019 Final Version: 02.01.2020 Abstract: The rock assemblages of the Princes’ Islands, which are located to the south of mainland İstanbul, are regarded as parts of the Lower Paleozoic quartz sandstones, although they were initially considered as volcanic rocks by Swan in 1868. They differ from quartz sandstones by their vesicular texture and are devoid of any stratigraphic layering. Their mineral constituents are plagioclase (30%–35%), feldspar (35%–40%), and quartz (20%–25%), corresponding to rhyolite. The crystallization age of the rhyolites is 45.66 ± 0.84 Ma on the basis of the U-Pb zircon data. They show high-K calc-alkaline affinity. On primitive-normalized spider diagrams, negative anomalies of Ba, Nb, Sr, P, and Ti and positive anomalies of Pb are noteworthy. Their chondrite-normalized REE patterns are characterized by strongly fractionated patterns with demonstrative negative Eu anomaly, whereby middle REE are not fractionated relative to the heavy REE. These geochemical features suggest a fractionating mineral assemblage of feldspar, apatite, and biotite without significant involvement of garnet. The Lutetian rhyolites of the Princes’ Islands are a part of the Middle Eocene magmatic associations of the West Pontides, related to collision of the Menderes-Taurus block with the Pontides. Key words: İstanbul, quartz sandstone, rhyolite, Middle Eocene, West Pontides 1. Introduction 2004; Karslı et al., 2010; Kaygusuz et al., 2011; Arslan et The Tethys Ocean, which began to subduct under al., 2013). Laurasia during the late Paleozoic-early Mesozoic (Topuz It is stated that Eocene magmatism is not present in et al., 2018), was completely consumed during the Eocene the western section of the İstanbul-Zonguldak Tectonic along the İzmir-Ankara-Erzincan suture (İAES) (Şengör Unit (İZTU), except for the Armutlu-Almacık zone (e.g., and Yılmaz, 1981). Destruction of the Tethys Ocean is Gülmez et al., 2013). In this study, I present new U-Pb associated with collision of the Pontides with the Kırşehir zircon age and geochemical data for volcanic rocks from block (KB) and the Menderes-Taurus block (MTB) along the Princes’ Islands, located in western section of the İZTU, the İAES (e.g., Okay and Tüysüz, 1999; Espurt et al., 2014). with the aim of shedding light on the Eocene geodynamic The models explaining the Eocene tectonic setting evolution. of the Pontides are a matter of debate. The suggested models include: (a) an arc-related environment (Yılmaz 2. Geological setting et al., 1981, 2001; Ercan et al., 1995; Robinson et al., 1995; The study area is located in the İZTU, forming the western Delaloye and Bingöl, 2000; Köprübaşı et al., 2000; Okay part of the Pontides, to the east of the Rhodope-Strandja and Satır, 2006; Ustaömer et al., 2009; Eyüboğlu et al., zone and to the north of the Sakarya zone (Figure 1). The 2010, 2011), (b) a postcollisional environment (Harris İZTU includes Ordovician to Carboniferous sedimentary et al., 1994; Genç and Yılmaz, 1997; Arslan et al., 2006; rocks, which unconformably overlie metamorphic rocks of Kaygusuz and Öztürk (2015), (c) a postcollisional setting Proterozoic age (e.g., Yiğitbaş et al., 1999). In the western comprising slab break-off (Altunkaynak, 2007; Keskin section of the İZTU, these sequences are locally intruded et al., 2008; Gülmez et al., 2013), (d) postcollisional by a Late Permian magmatic body (e.g., Yılmaz, 1977). All extension (Topuz et al., 2005, 2011; Kürkçüoğlu et al., these rocks are unconformably overlain by Permo-Triassic 2008; Kaygusuz et al., 2011; Temizel et al., 2012; Arslan siliciclastic and carbonate rocks (Türkecan and Yurtsever, et al., 2013; Aslan et al., 2014; Yücel et al., 2014), and (e) 2002; Özgül, 2012). The western part of the İZTU lithospheric delamination (Köprübaşı and Aldanmaz, was probably above sea level during Jurassic and early * Correspondence: [email protected] 208 This work is licensed under a Creative Commons Attribution 4.0 International License. ŞEN / Turkish J Earth Sci 24 27 30 + ++ 42 + Eocene-Olgocene + + + + Haskovo Black Sea + v v N +++ v v v v v v v +++ v v v v v v v ++ + v v v ++ + + +++ v v v v v v v v v v v +++ + v v v Edrne + + v v v v v v v + + v v v v v v + ++ v v v v v v v v ++ + v vv v v v v v v ++ + v v v v v v v Eocene + + + v v v v v v v v + + + + ++ v v v v v v + + ++ v v v v v v v v +++ v v v v v + v + v v v ++ v v v v v v v ++ + + v v İstanbul v ++ + ++ ++ v ++ v + + v v v Düzce v + v v Tekrdağ Fgure 2 vv v v v + v v v v v v v 41 v v v v v v Kocael v v v v v v v v v v v Prnces’ v v v Kavala v v v v v v v v v v v Bolu v v v v v v + v v v v Islands v Sakarya v v v v v v v v asos v v v v v v + + + v v v v Apula v + v v v Rhodope-Strandja + ++ + v essalonk v + Karabga + v v v v + v v v v Lapsek Zone v + + Bga Bursa v v +++ Karyes v v v v v + ++ Çanakkale v Gönen + Bleck + v v + + ++ + + + + v Çan v v v v v + + v v v vv v ++++ v v v v v + + + + v + v + + + v v v Olgocene volcanc rocks vvv v Pontdes + + + v v v + + v v v v 40 v v v vv + + v v + Bayramç v v v v v v + + + + BULGARIA BLACK SEA v v v + GEORGIA Ezne v v v + + + + + Edremt v + + Olgocene plutonc rocks Study Area v v Balıkesr + + + v + + + + T S RSZ İZTU ARMENIA GREECE N v I E v v v O D v v SZ v v İAES v v v v Eocene-Olgocene volcanc rocks ANKARA v v v v İAES IRAN P KB MTB Menderes-Taurus + + + + + ITS + + + Block + + + Eocene-Olgocene plutonc rocks BZS ++ + + + 39 MTB + + + + + IRAQ v v v v AEGEAN SEA v Eocene-Olgocene v v v v SYRIA v v v v v Eocene volcanc rocks MEDITERRANEAN sedmentary rocks v v v v 0 100 + + + + + + + + + + + + + Eocene plutonc rocks Eocene sedmentary rocks + + + km + + + + Figure 1. Distribution of the Middle-Late Eocene plutonic and volcanic rocks in the West Pontides. Red box shows the location of the study area. Inset shows the main continental blocks and suture in Turkey (modified from Moix et al., 2008; Gülmez et al., 2013; Elmas et al., 2016). RSZ: Rhodope-Strandja zone, İZTU: İstanbul-Zonguldak Tectonic Unit, SZ: Sakarya zone, KB: Kırşehir block, MTB: Menderes-Taurus block, AP: Arabian platform, İAES: İzmir-Ankara-Erzincan suture; ITS: Inner-Taurus suture, BZS: Bitlis-Zagros suture. Cretaceous times. Upper Cretaceous-Paleocene sequences the Ordovician sandstones are thrust over the Devonian sit on the older rock units with major unconformity carbonate rocks. This thrust is regarded as the southerly (e.g., Tüysüz et al., 2004). Additionally, there are local extension of the Maltepe-Beykoz nappe on the mainland granodiorite intrusions of Late Cretaceous age (Öztunalı of İstanbul (Seymen, 1995; Çılgın, 2006). To the north of and Satır, 1975). The Paleozoic sedimentary rocks are Heybeli Ada and northeast of Büyük Ada, Carboniferous thrust over the Upper Cretaceous volcanosedimentary clastic rocks, which comprise sandstone, mudstone, and rocks and Paleocene sedimentary rocks to the north of minor limestone, are juxtaposed with Ordovician quartz İstanbul from south to north (Türkecan and Yurtsever, sandstones by large normal faults (Çılgın, 2006; Özgül, 2002). 2012) (Figure 2). The Middle Eocene magmatic and volcanic rocks Felsic volcanic rocks, the topic of this study, extend in the Armutlu-Almacık zone are represented by basic roughly in the NW-SE direction and cover approximately to intermediate volcanic rocks, dykes-sills, and coeval 4 km2 (Çılgın, 2006) (Figure 2). They mostly crosscut granites. The volcanic rocks exhibit a continuous trend the Ordovician quartz sandstones (Swan, 1868). The from basalt to dacite. The Middle Eocene magmatic- felsic volcanic rocks, which show strong alteration on volcanic assemblages show subduction components and the Princes’ Islands, are distinguished from the Paleozoic display tholeiitic to low-K subalkaline affinities (e.g., quartz sandstones by their vesicular texture and massive Kürkçüoğlu et al., 2008; Gülmez et al., 2013). appearance (Figure 3). The discussion about these volcanics has a history of 3. Geology of the Princes’ Islands 150 years. The first person who dealt with these rocks was The geology of the Princes’ Islands is represented Swan (1868). His observations from Prinkipo to Antigoni, by Ordovician to Carboniferous sedimentary rocks the initial names for Büyük Ada and Burgaz Ada, (Ketin, 1953; Çılgın, 2006; Özgül, 2012) (Figure 2). The respectively, were as follows: (a) The units are similar to the Ordovician quartz sandstones are the dominant lithology quartz sandstones observed on all of the islands; however, on the islands (Figure 2). To the southwest of Büyük Ada, feldspar and quartz minerals in the rocks show euhedral there is a roughly N-S directed thrust zone along which crystal forms unlike those in sedimentary rocks. (b) The 209 ŞEN / Turkish J Earth Sci 29°5'30''E 29°3'0''E Thrust fault Alluvium 40°55'00''N 40°55'00''N Quaternary Debris flow N Strike-slip fault 32 Lutetian Rhyolite 12 Normal fault 14 22 Carboniferous Sandstone-mudstone and minor limestone Strike and dip 33 of bedding 22 20 23 Devonian Limestone Contact Fold axis KINALI ADA Ordovician Quartz sandstone sample BURGAZ ADA KAŞIK ADASI HEYBELİ ADA FS-Brg-1 50 24 41 42 28 32 30 23 FS-Brg-2 + BÜYÜK ADA FS-Brg-4 - 17 42 FS-Brg-3 46 26 60 28 Yalı + FS-Hyb-1 FS-Hyb-2 - FS-Hyb-4 FS-Hyb-3 40 33 FS-Ba-1 FS-Ba-2 35 20 28 42 28 23 33 14 30 40 Marmara Sea 43 27 24 24 30 37 50 24 48 15 22 30 SEDEF ADASI 55 20 50 40°50'30''N 44 44 50 40°50'30''N 33 22 24 57 0 1 29°3'0''E 29°6'30''E km Figure 2.