Lithos 192–195 (2014) 226–239

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Isotopic evidence for a transition from subduction to slab-tear relatedvolcanism in western Anatolia, Turkey

Özgür Karaoğlu a,⁎, Cahit Helvacı ba Yüzüncü Yıl Üniversitesi, Mühendislik-Mimarlık Fakültesi, Jeoloji Mühendisliği Bölümü, TR-65080 Van, Turkiyeb Dokuz Eylül Üniversitesi, Mühendislik Fakültesi, Jeoloji Mühendisliği Bölümü, TR-35160 İzmir, Turkiye

⁎ Corresponding author.E-mail address: ozgur.karaoglu@deu.edu.tr (Ö. Karaoğ

http://dx.doi.org/10.1016/j.lithos.2014.02.0060024-4937/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 6 March 2013Accepted 6 February 2014Available online 18 February 2014

Keywords:Western AnatoliaSlab rollbackTear faultMagmatismTransfer zones

Volcanic rocks in western Turkey show age progressive magmatismmigrating from northeast to southwest thatreflects a southward shift of the Aegean subduction zone during the Miocene. Slab segmentation during thisperiod of trench-roll back is thought to have imposed source region heterogeneity trending northwest to south-east. In this study, we present new Sr, Nd, Pb and O isotopic analyses from the Miocene volcanic rocks of theUşak–Güre basin and compare these to previously published data. The data demonstrate a change fromsubduction-related sources around the Menderes Core Complex to more asthenospheric sources in the Afyonregion.Isotopic compositions (Sr–Nd–Pb) of volcanic rocks from the Demirci and Selendi basins to the west and theAfyon volcanic area to the east indicate minimal upper crustal contamination. The most primitive lavas alsoreveal increasing K contents from west (the NE–SW-trending basins) to east (Afyon region).It is suggested that the composition of the western Anatolian volcanic rocks change from orogenic (withlithospheric mantle sources) associated to denudation of the Menderes Massif Core Complex (MMCC) toanorogenic (with asthenospheric mantle sources) in the vicinity of the Kırka–Afyon–Isparta (KAI) volcanicprovince with time, from Early Miocene to Quaternary. There is no asthenospheric contribution during the lateMiocene onwards in the eastern margin of the MMCC, while the asthenospheric upwelling occurred only in asmall area beneath the exhuming core complex. We interpret the Uşak–Güre basin to reflect a structuralboundary showing a transition from a subduction-influenced metasomatized mantle source to asthenosphericmantle source volcanism driven by slab-tearing between the Hellenic and Cyprus slab segments.The Uşak–Muğla Transfer Zone (UMTZ) most likely corresponds to slab-tear related westernmost faults thatwere induced by initiation of slab segmentation processes following the lateMiocene (circa 11Ma), and possiblysince the Early Miocene.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

The Aegean–West Anatolian region has undergone compressionaldeformation since the late Cretaceous. During this time the northernbranch of the Neo-Tethys seaway was consumed via northward sub-duction and is now represented by the İzmir–Ankara Suture zone. Thissuture zone thus separates two continental blocks; the Sakaryacontinent to the north and the Anatolia–Tauride block to the south —

that were juxtaposed after collision in the late Palaeocene–Eocene (cf.Şengör and Yılmaz, 1981). Continental collision was followed bydevelopment of an E–W-trending, early to late Eocene, subduction-related magmatic arc to the south of Marmara (Altunkaynak andGenç, 2008; Altunkaynak et al., 2012), which was subsequently partlycovered by extensive outcrops of Oligocene–Miocene volcanic andplutonic rocks that were emplaced throughout the northern Aegean

lu).

and western Anatolia (e.g., Aldanmaz et al., 2000; Dilek andAltunkaynak, 2009; Dilek et al., 2009; Erkül et al., 2005a,b; Ersoy andHelvacı, 2007; Ersoy et al., 2010a; Fytikas et al., 1984; Güleç, 1991;Innocenti et al., 2005; Karaoğlu et al., 2010; Pe-Piper and Piper, 2001,2007; Prelević et al., 2012; Yılmaz et al., 2001 and references therein).The Miocene magmatic products differ from those emplaced duringthe Eocene, in that the younger volcanics are characterized by morepotassic rocks; including high-K, shoshonitic and ultrapotassic serieswith more primitive high-field strength elements (HFSE) ratios (nearlyprimitive mantle-like), and they have much higher 87Sr/86Sr and lower143Nd/144Nd ratios than observed at equivalent Mg# values in theEocene rocks (Ersoy et al., 2012b).

Aegean–western Anatolian magmatism is closely associated withextensional tectonics that are partly related to southward retreat ofthe Hellenic trench (Le Pichon and Angelier, 1981; McKenzie, 1978).Although it is broadly accepted that trench retreat and extension-related volcanism started in theMiddle Eocene in the northern Aegean,as defined by development of core complexes in Rhodope (Brun and

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Sokoutis, 2010; Fytikas et al., 1984), the observation that Oligocene toMiocene magmatic activity also took place to the north of the earlyEocene magmatic rocks means that this relationship is not simple(Ersoy and Palmer, 2013). Recently, detailed seismic studies suggestthat the retreat of the African/Aegean trench was coupled with slabsegmentation likely induced by mechanical heterogeneity within theoceanic lithosphere (Biryol et al., 2011; de Boorder et al., 1998; Dilekand Altunkaynak, 2009; Govers and Wortel, 2005; van Hinsbergenet al., 2010; Wortel and Spakman, 2000). For example, tomographicstudies have been interpreted to indicate a discontinuity between thesubducted Aegean and Cyprus slab segments (e.g., Biryol et al., 2011)that results in a window lying largely beneath the Menderes Massifcore complex (MMCC) (e.g., Gessner et al., 2013). The evolution and up-welling of the asthenospheric mantle beneath the Anatolian plate (e.g.Kula volcanism; Afyon volcanism and the most of Eastern Anatolia)may thus be associated with a vertical tear in the Mediterranean litho-sphere (de Boorder et al., 1998; Dilek and Altunkaynak, 2009; Dilekand Sandvol, 2009; Tokçaer et al., 2005). We hypothesize, therefore,that slab segmentation or ‘slab tearing' exerts an important influenceon the geochemical characteristics of magmatism in the Aegean–westAnatolian region.

Recent studies on the tectonic evolution of western Anatolia showthat the exhumation of the Menderes Massif along the crustal-scalelow-angle detachment faults was accompanied by a series of NE–SW-trending strike-slip faults that extend across the İzmir–Ankara zone,and appear to delineate the western and eastern margins of theMMCC (Erkül, 2012; Erkül and Erkül, 2012; Ersoy et al., 2011, 2012a,b;Karaoğlu and Helvacı, 2012b; Oner and Dilek, 2011, 2013). A NE–SW-trending zone of deformation along the western margin of the MMCChas recently been recognized and is known as the İzmir–Balıkesir Trans-fer zone (İBTZ) (Erkül et al., 2005a,b; Ersoy et al., 2011; Uzel andSözbilir, 2008) proposed earlier by Ring et al. (1999). The zone is dom-inated by early-middle Miocene volcanic rocks with variable degrees ofpotassic and locally alkaline character that are thought to be derivedfrom mixing of mantle-derived and lower crustal melts. Karaoğlu (inpress) reported that although, this is an agreement with the exhuma-tion of theMMCC along low-angle detachment faultswas also accompa-nied by İBTZ, the İBTZ might be not active since Early Miocene (ca. 21Ma) to present for justmiddle part of this zone referred to Yuntdağı vol-canic region. More recently Karaoğlu and Helvacı (2012b) proposed theUşak–Muğla transfer zone (UMTZ) that juxtaposes theMenderesMassifand the İzmir–Ankara zone along the eastern margin of the massif, wasprobably controlled by successive extensional deformation since themiddle Miocene on the eastern edge of the MMCC (Fig. 1).

Miocene magmatism in western Anatolia was accompanied by de-velopment of metamorphic core complexes in which mid- to lowercrustal rocks were exhumed along low-angle detachment faults (e.g.,Işık et al., 2003; Okay and Satır, 2000; Oner and Dilek, 2011, 2013;Ring et al., 2003). For example, the MMCC is closely associated withsyn-extensionalmagmatism thatwas controlled by theNE-trending lin-eaments (Işık et al., 2003; Ring et al., 2003 and references therein). EarlyMiocene first-stage detachment faulting, together with the NE–SW-trending zones, was accompanied by extensive magmatic activityalong the northern flank of the MMCC (Gördes, Demirci, Selendi andUşak–Güre basins from west to east; Ersoy et al., 2010a, 2011;Karaoğlu et al., 2010; Karaoğlu and Helvacı, 2012a,b) (Fig. 2). Theseevents were responsible for deposition of the early Miocene HacıbekirGroup in which (1) widespread high-K calc-alkaline dacitic–rhyoliticand (2) local ultrapotassic volcanic intercalations occur. The second-stage detachment faulting in the central parts of the massif resulted inNE–SW-trending oblique faulting along the basin-bounding marginsthat again controlled the distribution of magmatic activity (Erkül et al.,2005a,b; Ersoy and Helvacı, 2007; Ersoy et al., 2010a,b, 2012a,b;Karaoğlu and Helvacı, 2012a,b; Karaoğlu et al., 2010; van Hinsbergenet al., 2010). This event was coeval with (1) high-K calc-alkaline andes-itic–dacitic and (2) shoshonitic–ultrapotassic, volcanism.

In addition, the coeval emplacement of lamproitic/ultrapotassicrocks ~16 Ma has been recognized in NE–SW-trending volcano-sedimentary basins in the Gördes to Afyon volcanic region along a nar-row WNW–ESE-trending zone (Fig. 1). Development of the high-K,calc-alkaline to potassic/ultrapotassic volcanism in western Anatolia islargely consistent with a progressive migration of their source towardsto south (Erkül and Erkül, 2012), and supports the Subduction Trans-form Edge Propagator (STEP) model for their origin (Biryol et al.,2011; Dilek and Altunkaynak, 2009; Govers and Wortel, 2005).Karaoğlu et al. (2010) recently reported new 40Ar/39Ar radiometricdata that demonstrate that Cenozoic volcanism commenced 17.29 Mawith the emplacement of the Beydağı volcanic unit in the northernpart of the Uşak–Güre basin, synchronously with deposition of thefluvio-lacustrine İnay group (Fig. 2). As the youngest radiometric ageobtained from the Beydağı volcanic unit (12.15 ± 0.15 Ma) is in theBeydağı caldera in the south of the area (Karaoğlu et al., 2010), theage progression of volcanism in the Uşak–Güre basin may be taken tofurther support this north to south migration of volcanism duringMiocene.

In this study, we present new Sr, Nd, Pb andO isotopic analyses fromtheMiocene volcanic rockswithin theUşak–Güre basin.Major and traceelement analyses for these rocks have been previously given byKaraoğlu et al. (2010). The data are comparedwith the geochemical fea-tures of the volcanic rocks from the Demirci and Selendi basins to thewest and the Afyon volcanic area to the east, with the aim of mappingthe spatial and temporal trends in the geochemistry of the volcanicsources. This template is then used to test geodynamic models of themechanical heterogeneity in the Aegean slab and the hypothesized‘slab tear’ mechanism as a driver for changes in volcanic source regionbeneath western Anatolia (Fig. 1).

2. Geological setting

The Uşak–Güre basin is one of the NE–SW-trending Neogene de-pressions developed on the northern flank of the Menderes core com-plex (Ercan et al., 1978; Ersoy et al., 2010a, 2011; Karaoğlu andHelvacı, 2012a; Karaoğlu et al., 2010; Seyitoğlu, 1997; Seyitoğlu et al.,2009). The basin contains volcano-sedimentary successions accumulat-ed during the Miocene and onwards, and includes the Hacıbekir Group,İnay Group and younger sedimentary units. The Hacıbekir Group is cor-relatedwith earlyMiocene units in the adjacent Selendi Basin (cf., Ersoyet al., 2011; Karaoğlu and Helvacı, 2012b; Karaoğlu et al., 2010;Seyitoğlu, 1997) that are interbedded with acid volcanic units yielding20.00–18.90 Ma Ar/Ar radiometric ages (Ersoy et al., 2008, 2010a;Seyitoğlu et al., 1997) (Fig. 2).

Continental deposits of the İnay Group within the Uşak–Güre basinare intercalated with the Beydağ and Payamtepe volcanic units(Fig. 2). The Beydağ volcanic unit comprises three volcanic centresforming, from north to south, the Elmadağ (17.29–16.28 Ma),İtecektepe (15.04–14.60) and Beydağ (13.10–12.15 Ma) calderas alonga NE–SW-trend with southward decreasing ages (Karaoğlu andHelvacı, 2012b; Karaoğlu et al., 2010; Seyitoğlu et al., 1997). These vol-canic occurrences are correlated in terms of emplacement timing withthe Asitepe and Yağcıdağ volcanics (high-K calc-alkaline felsic rocks)in the adjacent Demirci and Selendi basins. The Payamtepe volcanicunit is composed of small mafic extrusive rocks emplaced synchronous-ly with the deposition of the upper İnay Group. Ar/Ar ages for thePayamtepe unit yields middle Miocene ages; i.e., Karabacaklar volcanicrocks (16.01–15.04 Ma), Güre lamproite (14.30 Ma), Yeniköy dykes(16.01) and Kıran–Zahman basalts (15.67–15.32 Ma) (Innocenti et al.,2005; Karaoğlu et al., 2010; Prelević et al., 2012; Seyitoğlu et al.,1997). The Payamtepe volcanics can be correlated with the Kuzayırlamproite, Naşa basalt and Orhanlar basalt (shoshonitic to ultrapotassicmafic rocks) in the adjacent Demirci and Selendi basins. The İnay Groupin the Uşak–Güre basin is unconformably overlain by continental clasticsediments of the late Miocene Asartepe Formation (Ercan et al., 1978;

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Fig. 2. Stratigraphic sections of the NE–SW-trending basins (from Ersoy et al., 2011 and Karaoğlu et al., 2010). IAZ the rocks of the Izmir–Ankara zone, EG Eğrigöz Granite. Data sources areindicated by numbers in parentheses: (1) Ersoy et al., 2011; (2) Purvis and Robertson, 2005; (3) Ercan et al., 1996; (4) Ersoy et al., 2008 (5) Innocenti et al., 2005, (6) Karaoğlu et al., 2010.

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Karaoğlu et al., 2010; Seyitoğlu et al., 2009). These sediments are cut bythe Karaağaç dykes in the vicinity of Uşak (Karaoğlu et al., 2010). Theage-correlative sediments of the Asartepe Formation in the adjacentSelendi basin (the Kocakuz Formation; Ersoy et al., 2010a) are overlainby the Kabaklar basalt (8.50–8.37Ma; Ercan et al., 1996; Innocenti et al.,2005).

Structural data suggest that there were three deformation phases inthis area since the Early Cenozoic: the Early Miocene (D2); the MiddleMiocene (D3) and the Late Miocene (D4). This deformation sequencesuggests that the Uşak–Gure basin was affected by NE–SW-trendingprogressive extensional tectonics (Karaoğlu and Helvacı, 2012b)that most likely controlled emplacement of the Payamtepe volcanics(Karaoğlu and Helvacı, 2012a,b).

3. Field relations and volcanic rocks

Volcanic rocks can be grouped into: (1) the Beydağı unit —

composed of andesitic to rhyolitic lavas and pyroclastic deposits;(2) the Payamtepe unit — lava flows and dykes; and (3) the Karaağaçunit — latite/andesite dykes.

The Beydağı volcanic unit is represented by the NE–SW-trendingvolcanic centres (Beydağı, İtecektepe and Elmadağ) that trends fromhigh-K calc-alkaline to shoshonitic in character from southwest tonortheast, respectively (Fig. 2).

The Payamtepe volcanic unit is composed of: (i) shoshonitic Kıranrocks, (ii) Yeniköy latite dykes, (iii) ultrapotassic Güre lavas, and (iv)Karabacaklar lava flows. Lava flows and locally-developed dykes of the

Fig. 1(a) Tectonic map of Anatolia and distribution of the Cenozoic magmatic rocks in the regWestern Anatolian Volcanic Province; KAI: Kırka–Afyon–Isparta Volcanic Province; GVP: GalatKrV: Diyarbakır Karacadağ Volcanics; EAVP: Eastern Anatolian Volcanic Province; MCL: Mid-CyFBFZ: Fethiye–Burdur Fault Zone; SAVA: South Aegean Volcanic Arc; VİAS: Vardar–İzmir–AnkBlock; AP: Arabian Platform; EAFZ: Eastern Anatolian Fault Zone; NAFZ: North Anatolian Faulttectono-magmatic map of western Anatolia (Turkey) modified from 1:500,000 scale GeologiMa: Ar–Ar and K–Ar age data sources: (1) Işık et al., 2004; (2) Erkül, 2010; (3) Helvacı and Alonet al., 2011; (8) Ersoy et al., 2011; (9) Purvis and Robertson, 2005; (10) Innocenti et al., 2005; (11Oyman, 1998; (15) Besang et al., 1977; (16) Prelević et al., 2012; (17) Platevoet et al., 2008; (1

Payamtepe volcanic unit are most abundant in the northeastern partof the Güre region. Emplacement of these rocks appears to have beenstructurally controlled by NE–SW-trending oblique-slip faults.

The Karaağaç latite/andesite dykes show a NE–SW orientation andcut the late Miocene Asartepe formation. The Asartepe formation isalso observed in the Selendi basin where the Upper Miocene Kabaklarbasalt (8.37–8.50Ma, Ercan et al., 1985; Innocenti et al., 2005) conform-ably overlies the unit (Fig. 2). Ersoy and Helvacı (2007) suggested thatthe Kocakuz formation, which is correlated with the Asartepe forma-tion, is late Miocene in age according to stratigraphic relations in theSelendi basin.

4. Analytical methods

Representative samples of the volcanic rockswere analyzed for theirSr–Nd–Pb and O isotopic compositions. Major and trace element datafor these samples are presented in Karaoğlu et al. (2010). Sr and Nd iso-topic analyses were carried out at the Radiogenic Isotope Laboratory ofMETU Central Laboratory, using the analytical procedures described byKöksal and Göncüoğlu (2008). Powdered samples (~80 mg) wereweighed and transferred into Savillex PFA vials and leached with 4 mlof 52% HF for four days at 160 °C. The digested samples were thendried and dissolved overnight in 4 ml 6 N HCl at 160 °C. Sr was separat-ed in Teflon columns with 2.5 N HCl and 2 ml Bio Rad AG50 W-X8,100–200 mesh resin. The REE fraction was eluted using 6 N HCl afterwashing of Ba with 2.5 N HNO3. Sr was loaded on single Re-filamentswith Ta-activator and 0.005 N H3PO4 and its isotopic composition

ion (from Geological Map of Turkey (1:500,000), 2002 and Ersoy et al., 2012a,b). WAVP:ia Volcanic Province; KV: Konya Volcanics; CAVP: Central Anaolian Volcanic Province; D–cladic Lineament; İBTZ: İzmir–Balıkesir Transfer Zone; UMTZ: Uşak–Muğla Transfer Zone;ara–Erzincan Suture; BZS: Bitlis–Zagros Suture; PNT: Pontides; ATB: Anatolide–TaurideZone; DSFZ: Dead Sea Fault Zone; MMCC: Menderes Massif Core Complex. (b) Neogene

cal Map of Turkey (1:500,000), 2002). The numbers indicate the ages of the volcanics inso, 2000; (4) Seyitoğlu et al., 1997; (5) Semiz et al., 2012; (6) Ercan et al., 1996; (7) Ersoy) Karaoğlu et al., 2010; (12) Ercan et al., 1985; (13) Aydoğan et al., 2008; (14) Savaşçın and8) Gündoğan et al., 2012; (19) Paton, 1992.

230 Ö. Karaoğlu, C. Helvacı / Lithos 192–195 (2014) 226–239

determined by using a Triton Multi-Collector Thermal Ionization MassSpectrometer with static multi-collection. The 87Sr/86Sr data are nor-malized to 86Sr/88Sr = 0.1194. During the course of the measurementSr standard NIST SRM 987 was measured as 0.710240 ± 5 (n = 4).Nd was separated from the REE fraction on Teflon columns with 0.22N HCl using 2 ml biobeads (Bio Rad) coated with HDEHP (bis-ethyexylphosphate). Nd was loaded on double Re-filaments with 0.005 NH3PO4 and its isotopic composition was determined on a Triton TIMSusing static multi-collection. 143Nd/144Nd data are normalized to146Nd/144Nd = 0.7219. Measurement of the Nd La Jolla standard gavea value of 0.511842 ± 5 (n = 2). During the analyses BCR-1 USGSstandard was processed using the same conditions, and yielded 87Sr/86Sr = 0.705014 ± 5 and 143Nd/144Nd = 0.512638 ± 5. Analyticaluncertainties are given at the 2σ level. No corrections were applied tothe Nd and Sr isotopic compositions for instrumental bias.

Pb isotope analyses of 9 samples were carried out at ACTLABS(Canada). Pb was separated using an ion-exchange technique withBio-Rad 1 × 8 and the Pb isotope compositions analyzed on a FinniganMAT-261 multi-collector mass spectrometer. The measured Pb isotoperatios were corrected for mass fractionation calculated from replicatemeasurements of the Pb isotope composition of NBS SRM982 standards.External reproducibility of the lead isotope ratios is 206Pb/204Pb= 0.1%,207Pb/204Pb= 0.1%, 208Pb/204Pb=0.2% at the 2σ level, as demonstratedby multiple analyses of standard BCR-1.

Oxygen isotope analyses on 3 samples were carried out at ACTLABSLaboratory (Canada). The crushed samples were reacted with BrF5 atca.650 ºC in nickel bombs following the procedures described inClayton and Mayeda (1963). The isotopic analyses were performed ona Finnigan MAT Delta, dual inlet isotope ratio mass spectrometer. Thedata are reported in the standard delta notation as per mil deviationsfrom V-SMOW. External reproducibility is ±0.19‰ (1σ) based on re-peat analyses of an internal white crystal standard (WCS). The valuefor NBS 28 was 9.61 ± 0.10% (1σ). The results of isotopic analyses aregiven in Table 1, together with previously published isotopic analyses.

5. Major and trace element data

The major and trace element compositions of the Payamtepe andBeydağ volcanics have been discussed by Seyitoğlu et al. (1997),

Table 1Sr, Nd, Pb and O isotopic analyses from the Miocene volcanic rocks in the Uşak–Güre basin.

Sample (reference) USGS classification Rock group SiO2 (wt%) MgO (wt%)

Beydağ volcanicsU-7B (1) HK Latite Beydağ caldera 57.90 2.85U-9 (1) HK Dacite Beydağ caldera 61.79 2.32U-70 (1) UK Latite Elmadağ caldera 56.70 3.71U-164 (1) SHO Dacite Elmadağ caldera 62.95 2.36U-100 (1) HK Dacite Muratdağı 61.31 2.51

Payamtepe volcanicsU-109 (1) SHO Latite Karabacaklar volc. 57.83 2.81U-144 (1) SHO Trachyte Karabacaklar volc. 61.35 1.98IZ-64 (2) SHO Trachyte Karabacaklar volc 63.54 1.38IZ-65 (2) UK Latite Karabacaklar volc 57.15 3.40U-128 (1) UK Rhyolite Karabacaklar volc 85.91 0.24U-153 (1) UK Trachyte Yeniköy dykes 61.49 3.2905GUE02 (3) UK Shoshonite Güre lamproite 52.8 9.3705GUE03 (3) UK Latite Güre lamproite 50.0 6.8205GUE01 (3) UK Shoshonite Güre lamproite 52.8 7.02IZ-169 (2) UK Shoshonite Güre lamproite 51.78 10.69822 (4) UK Shoshonite Güre lamproite 50.27 9.82U-140 (1) UK Latite Kıran–Zahman basalt 52.42 8.34732 (4) UK Latite Kıran–Zahman basalt 52.82 4.76839 (4) UK Shoshonite Kıran–Zahman basalt 53.20 6.03

Karaağaç: dykesU-82 (1) UK Latite Karaağaç dykes 53.98 4.41U-85 (1) HK Andesite Karaağaç dykes 54.05 4.24

References: (1) this study, (2) Innocenti et al. (2005); (3) Prelević et al. (2012); (4) Ersoy et a

Innocenti et al. (2005), Karaoğlu et al. (2010), Ersoy et al. (2012a,b)and Prelević et al. (2012), and are summarized in Appendix 1. All thesamples from the Uşak-Güre basin, as well as from the Demirci andSelendi basins and Afyon region, are potassic in character. Sodic alkalinevolcanism in the region appears with the emplacement of the Kulabasalts in the centre of the Menderes Massif during the Quaternary(Aldanmaz et al., 2000; Alıcı et al., 2002; Chakrabarti et al., 2012).

The Beydağ volcanics are composed of high-K and shoshoniticseries rocks, including basaltic andesite, shoshonite, latite, trachyte,trachydacite, dacite and rhyolite with a small numbers of ultrapotassicrocks (Fig. 3; Karaoğlu et al., 2010). The broad compositional rangewith respect to other volcanic occurrences in the vicinity suggest thatthe Beydağ volcanics are transitional between the correlative unitsfrom the adjacent Demirci and Selendi basins (high-K calc-alkaline vol-canic rocks) and potassic rocks from the Afyon region. The Payamtepevolcanics contain examples of a more mafic alkaline series, includingshoshonitic (latite, trachydacite and trachyte) and ultrapotassic rocks(shoshonite and latite). One of the two samples of the late MioceneKaraağaç unit is classified as an ultrapotassic latite and the other is ahigh-K andesite. The Kıran–Zahman basalt and the Güre lamproite ofthe Payamtepe volcanics are also comparable with shoshonitic toultrapotassic rocks from both Demirci and Selendi basins and Afyonregion.

Primitive mantle-normalized trace element patterns of volcanicrocks from the Uşak–Güre basin reveal that all rock groups show rela-tive depletions in Nb, Ta and Ti with respect to large ion lithophile andlight rare earth elements (LILE and LREE) (Fig. 4). Although all rockunits show similar patterns, some differences exist between the early-middle Miocene Beydağ and Payamtepe volcanic units: the former ischaracterised by larger Nb–Ta trough (Karaoğlu et al., 2010). TheMuratdağı volcanics have similar patterns to the Beydağ volcanics.Among the Payamtepe volcanics, light to middle rare earth element(LREE to MREE) abundances increase from Yeniköy dykes to Kıran–Zahman basalt, although the heavy rare earth element (HREE) abun-dances are similar. HREE abundances of the Payamtepe volcanics, as awhole, are characterised by inclined patterns. The Güre lamproite ismost depleted in Ba (Fig. 4). One sample from the late MioceneKaraağaç dyke suite shows the smallest Nb–Ta trough (Karaoğlu et al.,2010).

87Sr/86Sr 143Nd/144Nd δ18O 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb

0.706811 ± 6 0.512498 ± 9 12.0 ± 0.19 19.055 15.693 39.0720.706194 ± 5 0.512444 ± 9 – 18.960 15.689 39.0920.707456 ± 7 0.512492 ± 7 9.7 ± 0.19 19.098 15.681 38.9800.707686 ± 6 0.512438 ± 4 – 19.093 15.732 39.1730.707342 ± 5 0.512488 ± 9 – 19.051 15.706 39.082

0.707132 ± 5 0.512467 ± 6 – 19.112 15.708 39.0940.707944 ± 5 0.512426 ± 4 11.8 ± 0.19 19.054 15.792 39.3880.707870 ± 4 0.512416 ± 4 – – – –

0.707911 ± 5 0.512371 ± 5 – – – –

0.709121 ± 5 0.512328 ± 9 – – – –

0.707394 ± 6 0.512459 ± 6 – 19.126 15.708 39.0990.710384 ± 6 0.512219 ± 4 – 18.980 15.750 39.3000.707997 ± 6 0.512225 ± 5 – 18.970 15.730 39.2500.708687 ± 7 0.512224 ± 5 – 18.940 15.750 39.250.710286 ± 7 0.512234 ± 7 – – – –

0.710280 ± 11 0.512249 ± 5 11.0 – – –

0.708793 ± 7 0.512282 ± 4 – 19.027 15.722 39.2700.708888 ± 5 0.512286 ± 4 11.6 – – –

0.707925 ± 5 0.512443 ± 9 8.9 – – –

0.707357 ± 5 0.512458 ± 4 – – – –

0.711172 ± 5 0.512220 ± 9 – – – –

l. (2010b).

Fig. 3. (a) Total alkali (K2O + Na2O) versus SiO2 (wt.%) (TAS), K2O versus SiO2 (wt.%) (b) and K2O/Na2O versus MgO (wt.%) classification diagrams for the volcanic rocks from the Uşak–Güre basin. IUGS fields in (a) and (b) are after LeMaitre (2002). Data are recalculated on anhydrous basis. Miocene volcanic rocks from the adjacent Demirci and Selendi basins and Afyonarea are also shown for comparison. Data sources: Seyitoğlu et al. (1997), Francalanci et al. (2000), Innocenti et al. (2005), Akal (2008), Ersoy et al. (2008), Ersoy et al. (2010a,b), Karaoğluet al. (2010), Chakrabarti et al. (2012), Prelević et al. (2012). See Appendix 1 for detailed analytical results.

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6. Sr, Nd, Pb and O isotope data

The Sr, Nd and Pb isotopic data from the Miocene volcanic rocks ofthe Uşak–Güre basin are shown on Fig. 5, together with previously pub-lished analyses from the area. Similar to otherMiocene potassic rocks inthe adjacent areas, all the samples plot in the enriched lower-right of aSr–Nd isotope diagram (Fig. 5a). In detail, the more mafic rocks of thePayamtepe volcanics generally have more radiogenic Sr and less radio-genic Nd than themore acidic Beydağ volcanics. Among the Payamtepevolcanics, the Güre lamproite has the most radiogenic Sr isotopic com-positions, except for two samples analyzed by Prelević et al. (2012).The two samples from the Karaağaç dykes show distinct Sr–Nd isotopiccompositions: sample U-85 (high-K andesite) has the highest Sr isoto-pic ratio, while sample U-82 (ultrapotassic latite) has similar Sr–Nd iso-topic ratios to the shoshonitic rocks of the Karabacaklar volcanics of thePayamtepe volcanics. This suggests that the lateMiocene volcanic activ-ity in the Uşak–Güre basin differs from that of the Demirci and Selendibasins in which the lateMiocene basaltic rocks are transitional betweenthe shoshonitic–ultrapotassic rocks and Quaternary Na-alkaline Kulavolcanics.

Lead isotope data show that 208Pb/204Pb vs 206Pb/204Pb ratios of theBeydağ volcanics are comparable with the shoshonitic–ultrapotassicsample, rather than the correlative units of the high-K calc-alkalinerocks from the adjacent Demirci and Selendi basins (Fig. 5b). For agiven 206Pb/204Pb ratio, the Payamtepe volcanics have generally higher208Pb/204Pb ratios than the shoshonitic–ultrapotassic rocks of theDemirci and Selendi basins. Coupled 87Sr/86Sr and 208Pb/204Pb ratiosalso indicate that the Miocene volcanic rocks of the Uşak–Güre basinare comparable with the correlative units from the adjacent areas(Fig. 5c). The oxygen isotopic ratios of the Payamtepe volcanics(δ18O= 11.6–8.9) are slightly higher than correlative shoshonitic–ultrapotassic rocks from the Demirci and Selendi basins (δ18O = 8.6–7.7; Ersoy et al., 2012a).

The isotopic ratios of the Beydağı volcanics generally reflect thetrend of progressively younger emplacement ages toward the south ofthe basin, with the southernmost Beydağı caldera (12.15 Ma; Karaoğluet al., 2010) having lower 87Sr/86Sr and rarely higher 143Nd/144Nd ratioswith respect to the older Elmadağ caldera (17.29 Ma; Karaoğlu et al.,2010).

7. Discussion

7.1. Fractional crystallization and assimilation

Major element variation diagrams (Fig. 6) can be used to assess therole of fractional crystallization (FC) processes in the petrogeneticevolution of the volcanic rocks from the Uşak–Güre. Variations in CaO,Al2O3, Fe2O3 and TiO2, with decreasingMgO contents can be interpretedto result from fractionation of olivine-dominated, followed byclinopyroxene- and then plagioclase-dominated assemblages. In thiscase, the mainly felsic rocks of the Beydağ volcanics are assumed to bederived fromFCof themoremafic andmainly shoshonitic–ultrapotassicrocks of the Payamtepe volcanics, and this relationship may also applyto the high-K calc-alkaline and ultrapotassic–shoshonitic rocks in theDemirci and Selendi basins (Fig. 6). As noted by Karaoğlu et al. (2010),this process should produce higher concentrations of highly incompat-ible trace elements in the most fractionated products, i.e., the Beydağvolcanics. However, the Cs, Rb, Nb, Ta, LREE and MREE contents of thePayamtepe volcanics are higher than those of the Beydağ volcanics(Fig. 4). This suggests that the Payamtepe volcanic rocks cannot beparental to the Beydağ volcanics.

Upper crustal contamination coupled with fractional crystallizationprocesses (AFC) of the shoshonitic to ultrapotassic mafic magmas(Payamtepe volcanics) represents another way of producing high-K toshoshonitic magmas (Beydağ volcanics). The upper crustal rocks inthe region comprise augen gneisses and schists of theMenderes Massif.

Fig. 4. Primitive mantle-normalized trace element patterns of the Miocene volcanic rocksin the Uşak–Güre basin. Normalizing values are from Palme and O'Neil (2004). Datasources: Seyitoğlu et al. (1997), Innocenti et al. (2005), Ersoy et al. (2010a,b), Karaoğluet al. (2010), Prelević et al. (2012). See Appendix 1 for detailed analytical results.

Fig. 5. Sr, Nd and Pb isotopic data of the Miocene volcanic rocks in the Uşak–Güre basin.Data source: this study, Innocenti et al. (2005), Ersoy et al. (2010a,b) and Prelević et al.(2012). Quaternary Kula volcanics are from Güleç (1991), Alıcı et al. (2002), Innocentiet al. (2005) and Chakrabarti et al. (2012). See Appendix 1 for detailed analytical results.

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The δ18O values of the Payamtepe and Beydağ volcanics are 8.9–11.0and 9.7–12.0, respectively, whereas the Menderes metamorphic rocksyield δ18O values of between ~9 and 14 (Satır and Friedrichsen, 1986),and may be considered as the contaminant for genesis of the evolvedfelsic rocks of the Beydağ volcanics. Similarly, Nd isotope data of theMenderes Massif (0.51218–0.51283; Ersoy et al., 2010b; Çoban et al.,2012) support this hypothesis. However, the 87Sr/86Sr ratios of theMenderes metamorphic rocks are too high (0.71655–0.77680) (Çobanet al., 2012; Ersoy et al., 2010b; Satır and Friedrichsen, 1986), suchthat upper crustal contamination of the mafic magmas would producefelsic magmas with 87Sr/86Sr ratios that are higher than those of thePayamtepe volcanic rocks (Fig. 6e). Whereas, the felsic rocks of theBeydağ volcanics have slightly lower 87Sr/86Sr ratios than the contem-poraneous mafic rocks. Hence, upper crustal contamination processesdo not appear to have played a significant role in the origin of theBeydağ volcanics.

Ersoy et al. (2012a,b) suggested that the early-middleMiocenehigh-K calc-alkaline rocks in the Demirci–Selendi basin are not geneticallyrelated to the contemporaneous mantle-derived shoshonitic–ultrapotassic rocks in the region; rather, they were derived mainlyfrom melting of lower crust, and then hybridized with the mantlederived magmas. By analogy, we hypothesize that the Beydağ volcanicrocks also originate from lower crustal melting processes. Given thatthe Payamtepe cannot be parental to the Beydağ, the Payamtepevolcanics were most likely derived from mantle sources (see below).Although FC and AFC have affected these two rock unit to varyingdegrees, it is clear that they were derived from distinct source regions(Fig. 6).

7.2. Mantle source characteristics

In order to explore the mantle source characteristics of the volcanicrocks, the most primitive samples are used with SiO2 b55.0 wt.%, MgON6.0 wt.% and Mg# N65, thus minimizing the effects of FC and AFC.The samples matching these criteria are ultrapotassic shoshonites(from the Güre lamproite) and ultrapotassic latites (from the Kıran–Zahman basalt) in the Uşak–Güre basin. Applying the same filter inadjacent areas, the most primitive samples include high-K toshoshonitic rocks in addition to ultrapotassic lamproites.

Fig. 6.Major element and Sr–Nd isotopic variations vs. MgO for the Miocene volcanic rocks in the Uşak–Güre basin. Data source: this study, Innocenti et al. (2005), Ersoy et al. (2010a,b)and Prelević et al. (2012). Quaternary Kula volcanics are from Güleç (1991), Alıcı et al. (2002), Innocenti et al. (2005) and Chakrabarti et al. (2012). See Appendix 1 for detailed analyticalresults.

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Samples of the Na-alkaline Quaternary Kula volcanics with MgOcontents N6 wt.%, which are assumed to be derived from mainly as-thenospheric upper mantle reservoirs, are characterised by lower SiO2

(~42.5–48.5 wt.%) but also lower Mg# (54.4–66.6) values (cf., Alıcıet al., 2002; Güleç, 1991) with respect to the most primitive lava sam-ples from the Uşak–Güre basin and environs (~65.5–77.9).

The 87Sr/86Sr ratios of these mafic potassic rocks are also higher(~0.7073–0.7100) than those of the Kula volcanic rocks (~0.7030–0.7035). In addition, the most primitive Miocene volcanic rocks fromthe Uşak-Güre, Demirci and Selendi basins and from the Afyon regionhave similar trace element patterns that show; (1) enriched LILE(~100–1000× PM) and LREE (50–120× PM) abundances, (2) negativeanomalies in Nb-Ta and Ti, and (3) LREE enriched patterns (Fig. 7a).All these data demonstrate that these potassic rocks originated from amantle sources that were anomalously enriched in incompatible ele-ments (other than HFSE), as well as having radiogenic Sr. As previouslysuggested by several researchers, the most likely mantle source for thevolcanic rocks under investigation is a lithospheric mantle enriched in

fluid mobile and incompatible trace elements via subduction-relatedmetasomatic events (Aldanmaz et al., 2000; Çoban et al., 2012; Dilekand Altunkaynak, 2007; Erkül et al., 2005a,b; Ersoy et al., 2010b,2012b; Fytikas et al., 1984; Güleç, 1991; Innocenti et al., 2005;Karaoğlu et al., 2010; Pe-Piper and Piper, 2001; Prelević et al., 2012;Yılmaz et al., 2001), but the timing and type of such a metasomatic en-richment remains controversial.

The LREE enriched patterns of the most primitive volcanic rocks(Fig. 7a) indicate that their mantle source contains a minor phase,such as garnet, that fractionated MREE/HREE ratios during melting.The Tb/Yb vs La/Yb ratios of the most primitive samples from theUşak–Güre basin are well-correlated with those of the most primitivevolcanic rocks from the Demirci and Selendi basin and with other Mio-cene potassic rocks in western Anatolia (Fig. 7b; cf. Aldanmaz et al.,2000), and also demonstrates that the mantle source region of theserockswas enrichedwith respect to theprimitivemantle. An intersectionpoint of hypothetical garnet-facies melting trend with a mantle enrich-ment trend (mantle array on Fig. 7b) gives an approximation of the

Fig. 7. (a) Primitivemantle-normalized trace element patterns and (b) Tb/Yb versus La/Yb(after Aldanmaz et al., 2000) for themost primitiveMiocene volcanic rocks (MgO N6 wt.%;Mg# N65; SiO2 b55 wt.%) in the Uşak–Güre basin and adjacent areas.

Fig. 8. K2O versus MgO (wt.%) variations for the most primitive Miocene volcanic rocks(MgO N6 wt.%; Mg# N65; SiO2 b55 wt.%) in the Uşak–Güre basin and adjacent areas.

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mantle source composition for the volcanic rocks (West AnatoliaMantle, WAM; cf. Aldanmaz et al., 2000). This approach suggests thatthe mantle source of the most-primitive shoshonitic to ultrapotassicvolcanic rocks in the region was a garnet-bearing peridotite. If this orig-inal mantle source had been intensely metasomatised by crustal fluids,it may have developed incompatible element-bearing metasomaticminerals (cf. Foley, 1992). There is debate, however, as to whether am-phibole is themost likely reservoir for K (e.g., Ersoy et al., 2012b;Helvacıet al., 2009), orwhether it is phlogopite (e.g., Çoban et al., 2012; Prelevićet al., 2012).

7.3. Implications for mantle enrichment processes

As noted above, the geodynamic scenario responsible for thesubduction-related enrichment processes of the Miocene magmaticrocks in the Aegean–west Anatolian region is debated. For example,the apparent southward migration of magmatic activity (from lateCretaceous in the north to the Recent in the south) in the Aegean andwestern Anatolia, Fytikas et al. (1984) could be interpreted to suggestthat orogenic potassic magmatism in the region results from southwardmigration of the Aegean arc. In this scenario, the enrichment processesare solely related to the Aegean subduction system (see also Erkül et al.,2005a,b; Innocenti et al., 2005; Ring et al., 2010; Tonarini et al., 2005).Although northward subduction of the northern branch of the Neo-Tethys beneath the Sakarya continent along the İzmir–Ankara suturehas been documented (Aldanmaz et al., 2000; Dilek and Altunkaynak,

2007; Güleç, 1991; Yılmaz et al., 2001), the Miocene volcanic rocks arelocated to the south of the İzmir–Ankara suture, so that subduction ofthe northern Neo-Tethys cannot have been responsible for enrichmentof the mantle beneath the Menderes Massif. On the basis of Pb isotopicratios of theMiocene volcanic rocks in theAegean andwestern Anatolia,Pe-Piper and Piper (2001) and Ersoy et al. (2010b) suggested that themantle source underwent multi-stage subduction enrichment events.Recently, Ersoy et al. (2012b) noted that Eocenemagmatismdiffers geo-chemically from the Miocene magmatism in the western Anatolia,implying that themantle sources of these magmatic events were differ-ent. Çoban et al. (2012) and Ersoy et al. (2012b) suggested that themantle sources of the Miocene magmatic rocks were contaminateddirectly by continental subduction. Such a geodynamic event, althoughpoorly defined in time and space, could explain the geochemical charac-teristics of themantle sources, such as high radiogenic Sr and incompat-ible element compositions.

Highly potassic volcanic rocks are also found in the Afyon volcanicarea to the east of the Uşak-Güre basinwhich, together with the Selendiand Uşak–Güre units extends the Miocene volcanic sequences along~150 km. A plot of K2O contents of the most primitive samples(Mg# N 65) against MgO contents (Fig. 8) shows a general eastwardincrease in K2O contents. This increase in incompatible trace elementcontents of the mafic lavas in western Anatolia increase from west toeast may be related to thickening in the lithospheric mantle. For exam-ple, Dilek and Altunkaynak (2009, 2010) suggested that variations inthe Sr–Nd–Pb isotopic signatures of the contemporaneous potassicand ultrapotassic rocks are indicative ofmelting of a heterogeneous lith-ospheric mantle source metasomatised by previous subduction events.

On the basis of the presence of late Miocene transitional basalts inSelendi Basin and mainly asthenosphere-derived Quaternary Kulabasalts near the centre of the Menderes Core Complex (the Kabaklarbasalt, Ersoy et al., 2008; Innocenti et al., 2005) it appears that, in gener-al, the composition of western Anatolian volcanic rocks has changedfrom largely orogenic (with lithospheric mantle sources) to largelyanorogenic (with asthenosphericmantle sources) from themiddleMio-cene to the Quaternary (cf. Innocenti et al., 2005). However, Ersoy et al.(2012b) proposed that the late Miocene basaltic rocks in the westernpart of the west Anatolia differ from the transitional Kabaklar basalt,in that they also have a lithospheric mantle source; thus, a clear as-thenospheric contribution to magma genesis is valid only for the centreof the Menderes Core Complex. Similarly, the Karaağaç dykes are alsothought to have a lithospheric mantle origin, such that asthenosphericupwelling only occurred in a small area beneath the exhuming corecomplex associated with emplacement of the Kula basalts during theQuaternary (Fig. 9).

Fig. 9. Cartoon illustrating the fundamental changes in the tectonic and tectono-magmatic regimes of western Anatolia at; (a) 20–17Ma interval and after (b− e) 15Ma. (a) Roll-back ofthe slabwhich triggered denudation ofMMCC via SDF duringOligocene–EarlyMiocene period. High-K calc-alkaline source of lower crust feeding the volcanism placed on northern part ofMMCC. Mantle wedge-flux melting began making enriched lower crust. Lavas and extrusive of the Early Arc system formed essentially by mantle wedge flux melting above thedehydrating Aegean slab about 20 Ma. (b) Possibly steepened Aegean slab forming a sub-horizontal slab tear developed beneath the vicinity of Uşak–Güre area. Volcanic centres showa southwestern migration. Partial melting of lowermost mafic arc crust lithospheric source generated a small volume of orogenic shoshonitic/ultrapotassic volcanism. (c) Slab tearsegmentation from west to east, roll-back and slab tear processes triggered the development of UMTZ (Karaoğlu and Helvacı, 2012b). Uşak–Güre region exhibits a transition domain interms of slab-tear forming around 11 Ma ago. (d) Ultrapotassic mafic rocks from an asthenospheric/anorogenic lamproitic source via the slab-tear beneath Afyon region. This slab-tearcaused a right-lateral fault zone known as FBFZ since 4 Ma. Mafic volcanism shows propagation in a southwest direction from 11 Ma to 4 Ma. (e) Intense strained steepened slabexperienced small-scale tearing, producing mafic Na-alkaline Kula volcanism fed by an astenospheric mantle source since 2 Ma. Model of slab tear propagation is based on Wortel andSpakman (2000).

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7.4. Geodynamic implications: the transition from subduction to slabtearing

During the lifecycle of subduction zones (such as the Aegeanslab), most subduction hinges migrate in a direction opposite tothe dip of subduction (Hale et al., 2010; Schellart, 2004). This processof subduction rollback is affected by interaction of the slab with theinduced or background asthenospheric mantle flow (Dvorkin et al.,1993; Hale et al., 2010; Schellart, 2004). In addition, Hale et al.(2010) noted that subduction rollback is a contributing factor tothe initiation of back-arc extensional basins, in particular when thevelocity of subduction rollback exceeds the velocity of plate conver-gence (Dewey, 1980).

The mechanical behavior of the Aegean arc in terms of its tear resis-tance at the edge of the slab is an important parameter controlling theevolution of the subduction zone (e.g., Hale et al., 2010). Comparedwith other subduction parameters, such as plate strength, plate viscos-ity, plate thickness and trench width, the dynamics of tearing have onlyrecently been addressed in the Aegean region (Biryol et al., 2011; deBoorder et al., 1998; Dilek and Altunkaynak, 2009; Govers and Wortel,2005; van Hinsbergen et al., 2010; Wortel and Spakman, 2000). Forexample, slab tears may result from mechanical heterogeneity, e.g.,

transform faults or other pre-existing weaknesses in the subductingslab. A slab tear beneath the Aegean domain and western Anatolia hasbeen proposed by de Boorder et al. (1998); Wortel and Spakman(2000); Govers and Wortel (2005); Dilek and Altunkaynak (2009);van Hinsbergen et al. (2010); Biryol et al. (2011) based on seismicdata and tomographic models that suggest the Aegean slab and theCyprus slab separated from each other, with the Aegean slab havingits eastern termination at the north part of the Pliny and Strabo trench-es/transforms (Biryol et al., 2011). Recent studies have re-emphasizedthat slab edges and associated tearing of subducting lithosphere alonghorizontal terminations of subduction trenches (transform faults) arecommon in the Mediterranean region. These kinks in plate boundarieshave been termed as Subduction Transform Edge Propagators (STEP)(Biryol et al., 2011; Govers and Wortel, 2005). Recent studies also sug-gest that a large part of the finite stretching was accommodated bysuch transform faults from east to west throughout the North AnatolianFault Zone (NAFZ), East Anatolian Fault (EAFZ), Dead Sea Fault Zone(DSFZ), İzmir–Balıkesir Transfer Zone (İBTZ), Cyprean and Kephaloniatransfer zones, and the more recently proposed Uşak–Muğla TransferZone (UMTZ). In this study, we believe that it is important to highlightthe causal link between major transform faults and major transferzones.

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Recent studies show that exhumation of the Menderes Massif alongcrustal-scale low-angle detachment faults was accompanied by NE–SW-trending high-angle strike-slip faults along its western and easternmargins (Erkül, 2012; Erkül and Erkül, 2012; Ersoy et al., 2011, 2012a,b;Karaoğlu andHelvacı, 2012b; Oner andDilek, 2011, 2013).More recent-ly, Karaoğlu and Helvacı (2012b) argue that a probable transfer zone(UMTZ), is directly controlled by NE–SW-trending strike-slip andoblique-slip normal faults, that led to successive extensional deforma-tion episodes since themiddleMiocene on the eastern edge of theMen-deres Massif Core Complex (MMCC; Fig. 9).

As noted above, Karaoğlu and Helvacı (2012b) pointed out that thefault zone (UMTZ) initiated in the Oligocene–Miocene, but that theNE–SW direction of extension was accommodated by a transfer zone.A deep crustal extensional phase linked to back-arc extension in theeastern part of the Aegean back-arc during the Oligocene–Miocene isthen thought to have acted as a transtensional transfer fault zone fromthe middle Miocene. They also infer that the probable transfer faultzone (UMTZ) was most active during the late Miocene with regard todeposition of the late Miocene Asartepe Formation, which may havebeen controlled by NE–SW-trending strike-slip and oblique-slip normalfaults (Fig. 9). In addition to these, each of the Neogene basins locatedin the south-eastern part of the Menderes Massif was stronglystructurally-controlled by UMTZ (Figs. 1 and 9; Karaoğlu and Helvacı,2012b). The authors emphasized that this transfer zone accommodatedthe differential stretching caused by crustal-scale extension accompa-nied by back-arc extension of the Aegean Arc, westward extrusion ofthe Anatolian plate and anticlockwise rotation of the Menderes Massifduring the Middle to Late Miocene

The southernmost, offshore part of the UMTZ is thought to bemarked by high angle, oblique faults. Supportive evidence for the exis-tence of the UMTZ is provided by Uluğ et al. (2005) who reportedright lateral strike-slip fault cutting the NE–SW-directed and NNW–

SSE-trending high angle faults in the Gökova Gulf (Fig. 1). We believethat this modification and reactivation of NE-directed deformationaltectonics of the regional stress field is associated with accommodationof the differential stretching caused by; (i) crustal-scale extensionaccompanied by back-arc extension of the Aegean Arc; (ii) westwardextrusion of the Anatolian plate and; (iii) anticlockwise rotation of theMenderes Massif during the Early Middle to Late Miocene. We suggestthat development of the UMTZ may reflect initiation of the STEP fault/tear between the Aegean and the Cyprus slabs. Sub-parallel faults ofthe FBZ and N–S-trending fault zone, which are splays of Isparta Angleand represent the eastern edge of this STEP fault/tear, may havetriggered development of the Kırka–Afyon–Isparta Volcanic provincesthat were produced by melting of the sub-slab asthenosphere (Dilekand Altunkaynak, 2009, 2010) (Fig. 9).

Recently, a significant amount of research on the radiometric agedeterminations has been reported on the volcanic area in NE–SW-trending supra detachment basins and adjacent Afyon volcanic rocks(Prelević et al., 2012 and references therein, Fig. 1b). Coevally, supra-detachment basins and related volcanism occurred on hanging-wall ofthe Simav Detachment Fault regard to exhumation of MMCC duringearly Miocene (Fig. 1b). The bulk of the volcanism in NE–SW-trendingbasins is characterized by typical calc-alkaline, subduction-relatedvolcanic rocks, emplaced between ~19–12 Ma. While, high-K calc-alkaline volcanic activity in the Selendi basin took place between 19and 15 Ma, correlative volcanism gives ages of 17–12 Ma in the Uşak–Güre basin. In addition, the oldest radiometric ages for the Beydağı vol-canic unit are from the Elmadağ volcanic centre in the north and rangesfrom 17.29 to 16.01Ma. The İtecektepe volcanic edifice from themiddlepart of the basin spans a period of 15.04–14.60Ma (Fig. 1). The youngestradiometric age for the Beydağı volcanic unit is obtained from theBeydağı caldera located (ca. 12 Ma) in the south (Karaoğlu et al.,2010). The radiometric age data indicate that the Beydağı volcaniccentres in the Uşak–Güre basin were active in the late middle Mioceneand migrated from north to south with time (Fig. 9). Shoshonitic/

ultrapotassic and lamproitic rocks exhibit a similar propagation fromnortheast to southwest as given by the successive north–south ages ofthe ultrapotassic lavas and intrusive rocks in the Uşak area: 16.01,15.93, 15.2, and 14.2 Ma Ar/Ar ages, respectively in Fig. 1.

The thermal perturbation associatedwith tearing has been observedin the northern Tyrrehenian Sea and Tuscany regions (Gasparon et al.,2009). The heat transfer resulting from slab tearing seems to havequickly propagated eastward, causing substantial continental uplift, asdemonstrated by the exposure of basement rocks in the vicinity of theregions defined as; (i) the Gediz Detachment Fault (see for detailErsoy et al., 2010a,b; Karaoğlu and Helvacı, 2012b) with subsequent ex-humation of the MMCC during the Middle Miocene, and (ii) the em-placement of Late Miocene syn-extensional granitoids in the centralMMCC (Dilek and Altunkaynak, 2009; Dilek et al., 2010). The episodicdenudation of the MMCC also resulted in extensive continental clasticdeposition within the Menderes Massif and its surroundings duringthe Late Miocene (Karaoğlu and Helvacı, 2012b; Oner and Dilek, 2011;Sümer et al., 2013).

The volcanism in western Turkey is locally contemporaneous withmore voluminous shoshonitic, ultrapotassic and high-K calc-alkaline,southward younging, Miocene–Pliocene volcanic episodes in theSimav, Selendi, Uşak–Güre, Köroğlu, Afyon, Denizli and Isparta–Gölcükareas (Fig. 1) of heterogeneous petrography, volcanological facies(abundant ignimbrites as well as effusive types), and geochemical com-position (rare basic and much more common intermediate types)(Prelević et al., 2012). These mafic rocks dominate the Afyon–Ispartadomain where lamproites are the one of the oldest magmatic products.The emplacement ages of these rocks are between 12.0 Ma and 8.0 Ma(Ar/Ar dating) at Afyon; the age span of the Isparta area is 4–0.024 Ma(Besang et al., 1977; Lefevre et al., 1983; Platevoet et al., 2008;Prelević et al., 2012). Those age data show propagation in a southwarddirection of the Afyon–Isparta mafic volcanism as controlled by the dif-ferent structural components of the Isparta Angle.

Prelević et al. (2012) suggest that an age-controlled lithosphere–asthenosphere interaction occurred in the mantle under the MenderesMassif in western Anatolia. Prelević et al. (2012) proposed thatultrapotassic volcanism is able to isolate the initial stages of this interac-tion, which proceeds to complete transformation of the lower parts ofthe lithosphere into asthenosphere (asthenospherization). This eventended with a transition to Na-alkaline lavas, which are observed locallyin many regions, such as the Selendi (Ersoy et al., 2008) and Uşak–Gürebasins (Karaoğlu et al., 2010). The magmatic source of the shoshonites,Na-alkaline e.g., Kıran lavas, and ultrapotassic volcanics in Güre impliesthat they may have been derived from a heterogeneous mantle sourcesince Middle Miocene. These rocks may be closely associated with theinitiation of thinning of the subducted slab under western Anatolia(e.g. Prelević et al., 2012).

We interpret the Uşak–Güre basin as reflecting a structural bound-ary showing a transition from a subduction-influenced metasomatisedmantle source to an asthenospheric mantle source driven by slab-tearing between the Hellenic and Cyprus slab segments (Fig. 8). Thisconclusion is based onwhole rock geochemistry and isotopic signaturesthat are intermediate between rocks from the northern part of theMen-deresMassif and theAfyon-Isparta domain. As noted above, theUMTZ ismost likely an upper crustal expression of the slab tear, i.e., it was in-duced by slab segmentation processes through the late Miocene(~11 Ma). Our new isotopic data and published radiometric age datashow two chronologically distinct magmatic episodes that highlightthe evolution from subduction-related magmatism to a second mag-matic episode that we interpret as initiation of the process of slab tear-ing around the MMMC and western Anatolia (Fig. 9).

In addition, it is widely accepted that the Isparta Angle is bounded bythe NW–SE striking Sultandağ Fault (SF) to the east (Boray et al., 1985)and the transtensional left-lateral NE–SW trending Fethiye–BurdurFault Zone (FBFZ) to the west (Dumont et al., 1979; Price and Scott,1994; Taymaz and Price, 1992). The southernmost, offshore part of the

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Isparta angle is marked by the Anaximander Mountains (Biryol et al.,2011). We propose that FBFZ the sub-parallel orientation of the FBFZto the UMTZ results from eastward migration of this tear fault, whichcommenced with the UMTZ in the late Miocene, and terminated atthe Isparta Angle domain whichwas fed by ultrapotassic mafic volcanicactivity via the step tear beneath the lithospheric mantle of westernAnatolia.

Post-Miocene hydrothermal, porphyritic mineralization (of whichthe best known is the TÜPRAG-Kışladağ Gold Mining district), andsome polymetallic veins were also developed along the extensional/transtensional faults of the UMTZ and FBFZ (see Figs. 1b and 9) andbasins. This suggests that enhanced heat flow linked with the processof slab-tear faulting may have driven mineralization between the areaof Uşak and Muğla. This is further supported by the observation oftravertine deposition in Denizli region and geothermal activity, espe-cially in the Afyon area (e.g. Gasparon et al., 2009;Mascaro et al., 2001).

8. Conclusions

The geochemical features of the Uşak–Güre volcanic units revealthat they were derived from distinct sources: (1) most probably lowercrust, and (2) lithospheric mantle. The lithospheric mantle source is ev-idenced by high LILE and LREE contents, relative Nb–Ta depletions andradiogenic 87Sr/86Sr contents. Sr–Nd and Pb isotopic compositions indi-cate theywere not affected significantly by upper crustal contaminationprocesses, although they have undergone variable effects of fractionalcrystallization. The geochemical features of the most primitive lavasalso reveal that there is an increase in K contents in the Miocene volca-nic rocks from west (the other NE–SW-trending basins) to east (Afyonregion), associated with thickening of the lithospheric mantle.

Anorogenic ultrapotassic mafic rocks occur in the Afyon–Isparta do-main, where lamproites display southward younging from 12.0 Ma to8.0 Ma at Afyon and 4–0.024 Ma at Isparta. These age data clearlyshow propagation of the anorogenic mafic rocks in a southward direc-tion within the Afyon–Isparta domain in which the mafic volcanismwas controlled by different structural components of the Isparta Angle.

We suggest that the composition of the western Anatolian volcanicrocks changes during Early Miocene to Quaternary, from orogenic(with lithospheric mantle sources) associated to denudation of theMMCC to anorogenic (with asthenosphericmantle sources) in the vicin-ity of the Kırka–Afyon–Isparta volcanic province. We infer that theultrapotassic rocksmay be closely associated with the initiation of thin-ning of the subducted slab under western Anatolia since the MiddleMiocene.

The UMTZmost likely corresponds to slab tear related westernmostfaults that were induced by initiation of slab segmentation processesfollowing the late Miocene (circa 11 Ma), and possibly since the EarlyMiocene.

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.lithos.2014.02.006.

Acknowledgement

E. Yalçın Ersoy and Fuat Erkül are thanked for their utmost contribu-tion anddiscussions to the earlier version of themanuscript.We are alsograteful for insightful comments and editing provided by MartinPalmer. Dan Barfod helped with the English of earlier version of themanuscript. We wish to express sincere thanks to Şafak Altunkaynakand an anonymous reviewer. We also thank Andrew Kerr for construc-tive comments and editorial handling.

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