Jurassic – Paleogene intraoceanic magmatic evolution of the Ankara Mélange , north-central Anatolia , Turkey

Oceanic rocks in the Ankara Mélange along the Izmir–Ankara–Erzincan suture zone (IAESZ) in northcentral Anatolia include locally coherent ophiolite complexes ( ∼ 179 Ma and ∼ 80 Ma), seamount or oceanic plateau volcanic units with pelagic and reefal limestones (96.6± 1.8 Ma), metamorphic rocks with ages of 256.9± 8.0 Ma, 187.4± 3.7 Ma, 158.4± 4.2 Ma, and 83.5± 1.2 Ma indicating northern Tethys during the late Paleozoic through Cretaceous, and subalkaline to alkaline volcanic and plutonic rocks of an island arc origin ( ∼ 67– 63 Ma). All but the arc rocks occur in a shale–graywacke and/or serpentinite matrix, and are deformed by southvergent thrust faults and folds that developed in the middle to late Eocene due to continental collisions in the region. Ophiolitic volcanic rocks have mid-ocean ridge (MORB) and island arc tholeiite (IAT) affinities showing moderate to significant large ion lithophile elements (LILE) enrichment and depletion in Nb, Hf, Ti, Y and Yb, which indicate the influence of subduction-derived fluids in their melt evolution. Seamount/oceanic plateau basalts show ocean island basalt (OIB) affinities. The arc-related volcanic rocks, lamprophyric dikes and syenodioritic plutons exhibit highK shoshonitic to mediumto high-K calc-alkaline compositions with strong enrichment in LILE, rare earth elements (REE) and Pb, and initial εNd values between+1.3 and +1.7. Subalkaline arc volcanic units occur in the northern part of the mélange, whereas the younger alkaline volcanic rocks and intrusions (lamprophyre dikes and syenodioritic plutons) in the southern part. The late Permian, Early to Late Jurassic, and Late Cretaceous amphibole-epidote schist, epidote-actinolite, epidote-chlorite and epidote-glaucophane schists represent the metamorphic units formed in a subduction channel in the northern Neotethys. The Middle to Upper Triassic neritic limestones spatially associated with the seamount volcanic rocks indicate that the northern Neotethys was an open ocean with its MORB-type oceanic lithosphere by the early Triassic (or earlier). The latest Cretaceous–early Paleocene island arc volcanic, dike and plutonic rocks with subalkaline to alkaline geochemical affinities represent intraoceanic magmatism that developed on and across the subduction–accretion complex above a N-dipping, southward-rolling subducted lithospheric slab within the northern Neotethys. The Ankara Mélange thus exhibits the record of ∼ 120–130 million years of oceanic magmatism in geological history of the northern Neotethys.

+1.7.Subalkaline arc volcanic units occur in the northern part of the mélange, whereas the younger alkaline volcanic rocks and intrusions (lamprophyre dikes and syenodioritic plutons) in the southern part.The late Permian, Early to Late Jurassic, and Late Cretaceous amphibole-epidote schist, epidote-actinolite, epidote-chlorite and epidote-glaucophane schists represent the metamorphic units formed in a sub-duction channel in the northern Neotethys.The Middle to Upper Triassic neritic limestones spatially associated with the seamount volcanic rocks indicate that the northern Neotethys was an open ocean with its MORB-type oceanic lithosphere by the early Triassic (or earlier).The latest Cretaceous-early Paleocene island arc volcanic, dike and plutonic rocks with subalkaline to alkaline geochemical affinities represent intraoceanic magmatism that developed on and across the subduction-accretion complex above a N-dipping, southward-rolling subducted lithospheric slab within the northern Neotethys.The Ankara Mélange thus exhibits the record of ∼ 120-130 million years of oceanic magmatism in geological history of the northern Neotethys.

Introduction
In the circum-Mediterranean mountain chains belonging to the Alpine-Himalayan system, subduction-related tectonic mélanges during pre-collisional stages are described, but they generally are overprinted by arc-continent and continent-continent collisions (Festa et al., 2010 and references therein).In northern Turkey, the 2600-km-long IAESZ extends from west to east, connecting the Vardar suture in the west and the Sevan-Akera suture zone in the east.The ophiolitic mélanges and ophiolite slabs are observed along this zone.At the central part of IAESZ, in the vicinity of Ankara, Kırıkkale, Çankırıand Çorum, the Ankara Mélange, first described by Bailey and McCallien (1950), is a wellknown subduction-accretion type mélange of the world.They defined metamorphic, limestone and ophiolitic rock blocks in era age from Paleozoic to Mesozoic in the mélange.
Published by Copernicus Publications on behalf of the European Geosciences Union.In previous works, three major tectonic units are identified in the Ankara Mélange from the northwest to the southeast.These are metamorphic block mélange, limestone block mélange and ophiolitic mélange (Norman, 1984;Akyürek et al., 1984;Koçyigit, 1991;Tüysüz et al., 1995;Tankut et al., 1998).The metamorphic block mélange contains a chaotic mixture of variably metamorphosed sedimentary, basic-ultrabasic, and pyroclastic rocks in age from Permian to Triassic while the limestone block mélange consists of neritic to pelagic limestone blocks in age ranging from Permian through Albian in a shale-graywacke matrix.The ophiolitic mélange includes several kilometer-size thrust sheets of mantle peridotite, oceanic basic crustal rock, and blocks of serpentinite, massive to pillow basaltic lava flows, radiolarite, chert and neritic-pelagic limestone in pelitic and/or serpentinite matrix (Norman, 1984;Akyürek et al., 1984;Koçyigit, 1991;Tüysüz et al., 1995).Tankut et al. (1998) emphasized that the ophiolitic mélange unit of the Ankara Mélange is represented by two major mappable coherent units -ophiolitic fragments and volcanic seamounts.They determined that the N-MORB character of the Neotethyan oceanic crust along with its seamounts was overprinted by a chemical signature related to subduction zone processes and associated magmatism prior to their incorporation into the subduction-accretion mélange.Göncüoglu et al. (2001) proved that the ocean floor generation in the Izmir-Ankara oceanic branch started in early late Carnian from the radiolarian fauna in the blocks of basalt-radiolarite association.Tekin and Göncüoglu (2007) have presented the radiolarian fauna giving late Ladinian to early middle Carnian from the ribbon cherts within the Bornova Flysch zone, western part of IAESZ.Çelik et al. (2011) reported the amphibolites in the ophiolitic mélange near Çankırı giving dates between 177.08 ± 0.96 Ma and 166.9 ± 1.1 Ma from amphibole ages.In the eastern part of the investigated area, the isotropic gabbros of Refahiye (Erzincan) ophiolite with MORB-like to island arc tholeiite (IAT) typical geochemical signatures of SSZ oceanic crust give uranium-lead (U-Pb) zircon age of 183 ± 1 Ma (Uysal et al., 2013).
In this study, we mapped the ophiolitic rocks, the megablocks and/or thrust sheets of seamount and metamorphic rocks in the Ankara Mélange, central part of IAESZ, and the products of island arc magmatism.Also, we document the internal structure of the Ankara Mélange along the IAESZ in north-central Anatolia, and present new geochemical and geochronological data from various magmatic rock assemblages that make up distinct tectonic units in this mélange.Our geochemical data and interpretations indicate that all units within the Ankara Mélange are intraoceanic in origin and appear to have formed during the seafloor spreading, seamount volcanism and island arc magmatism stages of the northern Neotethys.We also present new Pb, Sr, Nd isotopic compositional data and radiometric data belonging to both magmatic arc rocks and basic rocks from the Ankara Mélange.Thus, the Ankara Mélange displays a complete record of ∼ 120-130 m yr −1 of intraoceanic magmatism that took place prior to the continental collisional events in Anatolia in the Eocene.

Regional geology
The IAESZ forms the tectonic boundary between the Pontide tectonic belt that includes the Sakarya Continent, which represents the southern margin of Eurasia in the north, and the Anatolide-Tauride block that includes the Central Anatolian Crystalline Complex (CACC) in the south.The suture zone is marked by ophiolite units, ophiolitic mélanges, and seamount fragments.The Sakarya Continent in the Pontides represents the southern margin of Eurasia (Figs. 1 and 2).Carboniferous , high-grade metamorphic rocks (gneiss, migmatite, amphibolite and marble), currently exposed in the Kazdag, Sögüt, Devrekani, and Pulur massifs (from west to east), make up the continental basement of the Sakarya Continent (Duru et al., 2004;Okay et al., 2006;Nzegge et al., 2006;Topuz et al., 2007).These metamorphic basement rocks are intruded by the Carboniferous (295 Ma) granitoids (Çogulu et al., 1965;Delaloye and Bingöl, 2000).The Triassic Karakaya Complex, representing a subductionaccretion complex, tectonically overlies the crystalline basement units (Tekeli, 1981).It includes the Lower Karakaya, which comprises metabasite, marble and phyllite rocks, and the Upper Karakaya consisting mainly of unmetamorphosed clastic and basic volcanic rocks with blocks of Carboniferous and Permian neritic limestones (Bingöl et al., 1975;Okay et al., 2002;Okay and Göncüoglu, 2004).
Megablocks and imbricated thrust sheets of oceanic rocks occur as mappable units enveloped in a pelitic (clayey, sandysilty), serpentinite or volcanic matrix within the Ankara Mélange (Fig. 5).In some of these blocks or thrust sheets the mafic-ultramafic rock units and the associated sedimentary rocks make up coherent ophiolite complexes (e.g., the Eldivan ophiolite) (Figs. 6 and 7), representing the Neotethyan oceanic lithosphere.Plagiogranite dikes intruding the serpentinized peridotites near Eldivan (Çankırı) revealed U-Pb zircon ages of 179 Ma (Dilek and Thy, 2006), indicating that part of the Neotethyan oceanic crust preserved in the mélange is as old as the Early Jurassic.The radiolarian fauna in the chert blocks have yielded late Carnian-mid-Norian, and Middle Jurassic to Middle Cretaceous ages (Sarifakioglu et al., 2011).However, the whole-rock 40 Ar/ 39 Ar Fig. 3b.The generalized tectonostratigraphic columnar section showing the igneous pseudostratigraphy and internal structure of the Eldivan ophiolite, the Ankara Mélange and the island arc magmatic rocks, their tectonic basement, and sedimentary cover.
dating of basaltic pillow lava from an ophiolitic thrust sheet farther south in the Ankara Mélange has revealed an age of 80.3 ± 7.6 Ma, indicating that Late Cretaceous oceanic crustal rocks also exist within the mélange (Table 1).
Blocks (kilometer sized) of alkaline volcanic and pyroclastic rocks, debris flow deposits, and coarse-grained reefal limestones representing seamount and/or oceanic plateau fragments also occur in the Ankara Mélange (Fig. 8).We have obtained Middle-Upper Triassic and Cretaceous biostratigraphic ages from the reefal limestones overlying the seamount volcanic units, and 40 Ar/ 39 Ar whole-rock ages of 96.6 ± 1.8 Ma from the alkaline pillow lavas that are stratigraphically associated with the pink-colored pelagic limestones (Sarifakioglu et al., 2011).Rojay et al. (2004) obtained the late Barremian-early Aptian biostratigraphic ages from the reefal limestones resting on the pillow lavas with ocean island basalt (OIB) geochemical affinities.Blocks of these neritic carbonates and the underlying alkaline pillow lavas are also embedded in a turbiditic sequence consisting of chert and volcanic rock clasts in a fine-grained sandstone matrix.Volcanic debris flow deposits also occur within the turbiditic sequence.
In addition to the blocks of ophiolitic, seamount and oceanic plateau rocks, the Ankara Mélange also contains blocks of metamorphic rocks, mainly epidote-glaucophane, epidote-chlorite, and epidote-actinolite schists (Fig. 6).The geochemical fingerprinting of these rocks suggests that their protoliths were made of seamount volcanics and ophiolitic basic rocks and related sediments.Detailed descriptions and documentation of these metamorphic rocks will be presented elsewhere.We interpret these metamorphic rocks to have formed in an intraoceanic subduction zone.The 40 Ar/ 39 Ar dating of the epidote-glaucophane, epidote-chlorite and epidote-actinolite schists revealed the cooling ages of 83.5 ± 1.2 Ma, 158.4 ± 4.2 Ma, and 187.4 ± 3.7 Ma, respectively, whereas phyllite, actinolite schists and amphiboleepidote schists yielded 119.8 ± 3.3 Ma, 177.4 ± 5.8 Ma, 256.9 ± 8.0 Ma, respectively (Tables 2 and 3).
Lamprophyre dikes and a syenodiorite pluton of an island arc origin are intruded into the ophiolitic and seamount rocks and the mélange matrix along the Kızılırmak River near and east of Kalecik (Fig. 11).The brownish-grey  4b and c).
We have also obtained an 40 Ar/ 39 Ar biotite age of 75.9 ± 1.3 Ma from a syenodioritic pluton, approximately 1 km in diameter, indicating that the arc magmatism started as early as the Campanian and that it progressed with alkaline volcanism and dike emplacement throughout the Maastrichtian and early Paleocene (Table 4d).Andesitic lavas and volcaniclastic and pyroclastic rocks are intercalated with the Upper Cretaceous-lower Paleocene turbiditic rocks in the region.These turbiditic and flyschoidal rocks contain volcanic pebbles in the lower stratigraphic levels and grade upwards into sandstone and shale.The Paleocene rocks (Dizilitaslar Formation) are conformably overlain by the lower to middle Eocene sandstone, shale, clayey limestone and marl units that collectively make up the Mahmutlar Formation (Akyürek et al., 1984).All these Paleogene sedimentary rocks were deformed by south-vergent thrust faults and folds, indicating that they underwent N-S directed contractional deformation in the middle to late Eocene.

Petrography
In this section we describe the primary and secondary mineral assemblages and the textures of the main lithological types associated with the Neotethyan oceanic crust, seamount volcanic units, and island arc assemblages (e.g., volcanic rocks, lamprophyre dikes and syenodiorite plutons) that we investigated in the study area.

Basalt
The seamount-related alkaline basaltic rocks consist mainly of plagioclase (55-60 %) and clinopyroxene (approximately 40 %), displaying an intergranular texture (Fig. 12a).Some of the basalt samples contain olivine phenocrysts (about 15 %), ranging in size from 0.2 mm to 2 mm.Clinopyroxene grains (titanaugites) are partially altered into chlorite, olivine to serpentine and iddingsite, and plagioclase to sericite and chlorite.Apatite and opaque minerals (Fe-Ti oxides) occur as accessory minerals.Amygdals are filled with secondary carbonate and chlorite minerals.Tholeiitic basaltic rocks of the Neotethyan oceanic crust comprise microlitic plagioclase and clinopyroxene crystals in a fine-grained texture (Fig. 12b).They are partially or completely spilitized, with plagioclase replaced by albite, sericite, chlorite and epidote (saussuritization), whereas clinopyroxene replaced by actinolite (uralitization) and chlorite.The glassy material in the matrix is transformed into chlorite.Leucoxene and opaque minerals are present as accessories.Vesicles in the basaltic lavas are filled by secondary carbonate and chlorite.
The island arc basaltic rocks consist mainly of plagioclase (about 55 %) and clinopyroxene (45 %) crystals in the porphyritic textures with chloritized glassy and microcrystalline groundmass.Clinopyroxene (diopside) grains range in size from 0.2 mm to 2 mm in length (Fig. 12c and d), and locally display twinning.The plagioclases are partly altered to chlorite and carbonate minerals.Accessory minerals are made of fine crystalline Fe-Ti oxides.Basaltic andesites contain plagioclase, clinopyroxene, minor olivine and biotite within porphyritic and glomeroporphyritic textures (Fig. 12e).Ferromagnesian minerals are locally 1.5 cm-long.Fe-Ti oxide  12.4 Age spectrum: the sample yielded age spectrum with three-steps plateau, characterized by 66.7 % of 39 Ar, age value of 158.4 ± 4.2 Ma.On the inverse isochron plot, points form linear regression characterized by age value of 166.9 ± 5.9 and ( 40 Ar/ 36 Ar) 0 = 272 ± 8.

Basanite
The ultrabasic volcanic rocks consist of clinopyroxene, plagioclase and minor olivine occurring as euhedral and subhedral grains in a hyalomicrolitic, porphyritic texture.Plagioclase forms microlites or microphenocrysts, and is commonly altered to clay minerals.Clinopyroxene is mainly    augite, and displays zoning and twinning.Olivine is surrounded by a groundmass that is made entirely of serpentine minerals.Small analcime crystals occur as a replacement of leucite between plagioclase and clinopyroxene crystals within the groundmass.
Some tephrites display characteristic features of phonolitic tephrite with feldspar crystals (plagioclase > K-feldspar) and mafic minerals (phlogopite, hornblende) in a microcrystalline porphyritic texture.Plagioclase is partially altered to sericite and chlorite, whereas sanidine is partially altered to sericite and clay minerals.Leucite occurs as subhedral grains, is mostly altered to sanidine microlites, zeolite and clay minerals, and is surrounded by small phlogopite flakes.Euhedral apatite crystals and anhedral opaque minerals (Fe-Ti oxides) are present as accessories.

Lamprophyre
These alkaline dike rocks consist mainly of small prismatic clinopyroxene (diopside), phlogopite, minor hornblende, and leucite pseudomorphs embedded in a groundmass composed of feldspars (orthoclase > plagioclase), analcime crystals and glassy material (Fig. 12f and g).Both plagioclase and orthoclase are partly or completely altered to carbonate, clay and zeolite minerals; phlogopite is replaced by chlorite along its rims.Small, interstitial apatite laths are enclosed in the orthoclase crystals.In addition, euhedral prismatic apatite crystals up to 0.7 mm in length are also present in the groundmass.Opaque minerals occur as accessory crystals.
Lamprophyres are classified according to their mineralogical composition (Rock, 1987).The lamprophyre dikes in the investigated area were considered as they are minette by their defined mineralogical composition.

Syenodiorite
The main minerals in this intrusive rock include feldspar (plagioclase ≥ orthoclase), clinopyroxene, hornblende and biotite (Fig. 12h).Subhedral to anhedral plagioclase crystals form a granular texture; some large orthoclase crystals (∼ 2.5 cm) locally give the rock a porphyry texture.Plagioclase grains (An 28 -An 48 ) are locally surrounded by orthoclase.K-feldspar grains display a perthitic texture.Subhedral to anhedral clinopyroxene (diopside), hornblende and biotite crystals show partial chloritization.Subhedral hornblende crystals have opacite rims around them as a result of metasomatism during their reaction with melt (Plechov et al., 2008).The subhedral prismatic apatite and anhedral granular opaque minerals are present as accessories.

Analytical methods
We analyzed fifty-one (51) rock samples for major, trace, and rare-earth element chemistry at ACME Analytical Laboratories Ltd. (Canada).Inductively coupled plasma optical emission spectrometry has been used for major-element analysis, and inductively coupled plasma mass spectrometry has been used for the analysis of both trace elements and rare earth elements (REE).The results of these analyses are presented in Tables 5, 6, 7, 8 and 9.   Ar age dating was done at the Geochronology and Isotopic Geochemistry Laboratory of Activation Laboratories Ltd. (Actlabs), Ancaster, Ontario, Canada.We obtained 40 Ar/ 39 Ar ages of biotite separates from two samples of the arc rocks.In addition, whole rock fractions of six rock samples were analyzed.The samples wrapped in Al foil were loaded into an evacuated and sealed quartz vial with K and Ca salts and packets of LP-6 biotite interspersed with the samples to be used as a flux monitor.The samples were irradiated in the nuclear reactor for 48 h.The flux monitors were placed between every two samples, thereby allowing precise determination of the flux gradients within the tube.After the flux monitors were run, J values (n×10 −10 cc STP) were then calculated for each sample, using the measured flux gradient.LP-6 biotite has an assumed age of 128.1 Ma.The neutron gradient did not exceeded 0.5 % on sample size.Argon was extracted from the sample system as degassing at ∼ 100 • C during two days in a double vacuum furnace at 1700 • C. Argon concentration was determined using isotope dilution with 38 Ar spike, which was introduced into the sample system prior to each extraction.The obtained pure Ar was introduced into a customer-built magnetic sector mass spectrometer (Reinolds type) with a Varian CH5 magnet.Measurement of Ar isotope ratios was corrected for mass discrimination and atmospheric argon, assuming that 36 Ar was only from the air.After each analysis, the extraction temperature was elevated to 1800 • C for a few minutes.Then, Aliquot of each sample was weighted into a graphite crucible with lithium metaborate/tetraborate flux and fussed using an LECO induction furnace for K analysis.The fusion bead was dissolved with acid.Standards, blanks and sample were analyzed using a Thermo Jarrell Ash Enviro II ICP Spectrometer.
The Sr, Nd, and Pb isotopic compositions of six samples from the alkaline lamprophyric dikes were determined at the ACT Analytical Laboratories Ltd., Canada (Table 9).The Sr isotope analysis was performed with a Triton multicollector mass spectrometer in static mode.The weighted average of 15 SRM 987 Sr standard runs yielded 0.710258 ± 9 (2 s) for 87 Sr/ 86 Sr. Sm and Nd were separated by extraction chromatography on hexyl di-ethyl hydrogen phosphatecovered Teflon powder.The analysis was performed using a Triton multicollector mass spectrometer in static mode. 143Nd/ 144 Nd ratios were relative to the value of 0.511860 for the La Jolla standard.Pb was separated using the ion exchange technique with Bio-Rad 1 × 8. Pb isotope compositions were analyzed using a Finnigan MAT261 multicollector mass spectrometer.The measured Pb isotope ratios were corrected for mass fractionation calculated from replicate measurements of Pb isotope composition in the National Bureau of Standards-SRM 982.External reproducibility of lead isotope ratios -206 Pb/ 204 Pb = 0.1 %, 207 Pb/ 204 Pb = 0.1 %, 208 Pb/ 204 Pb = 0.2 % -on the 2σ level has been demonstrated through multiple analyses of US Geological Survey basalt standard BCR-1.

Geochemistry
We report below on the geochemistry of the representative samples of oceanic basaltic rocks in the Ankara Mélange, as well as the lamprophyric dikes, a syenodioritic pluton, and alkaline lavas that crosscut and/or cover the blocks of volcanic and volcaniclastic rocks, serpentinite, radiolarian chert, and shale in the Ankara Mélange.(Irvine and Baragar, 1971).(c) Ti-Zr-Y discrimination diagram (Pearce and Cann, 1973).(d) Ti-V diagram (Shervais, 1982).

Oceanic basaltic rocks
The Na 2 O+K 2 O values of basaltic blocks of the Neotethyan oceanic crust range from 1 wt % to 4.28 wt %, with the K 2 O values much lower than those of Na 2 O (Table 5).The Na enhancement of two samples (CE.07,CE.08) may be a result of spilitization caused by low-grade hydrothermal ocean floor metamorphism.Similarly, the total alkali values from the seamount volcanic blocks vary between 4.72 and 8.14 wt %, with the Na 2 O values (3.78-6.79wt %) much higher than that of oceanic crust (Table 6).
The N-MORB normalized multielement diagrams of the representative samples of basalts of oceanic crust and seamount volcanic rocks are shown in Fig. 13e.Basaltic samples of both MORB and SSZ (suprasubduction zone) affinities show enrichment in their LILE (the large ion lithophile elements: Rb, Ba, K, Sr, Cs, Th) contents.The HFSE (high field strength elements: Nb, Ta, Zr, Hf, Ti, Y) and REE (rare earth elements) contents of the MORBtype basaltic rocks display a slight increase, whereas the SSZ-related basaltic rocks (four samples) exhibit depletion in HFSE and REE.The LILE, HFSE, LREE (light-REE) contents of the seamount volcanic rocks are extremely enriched relative to the HREE (heavy-REE) values.Also, the Th/Yb (2.8-5.6) and Nb/Yb (27.6-54.8)values of the seamount volcanic rocks are high in comparison to those of the Neotethyan oceanic basalt samples (Th/Yb = 0.2-1.1;Nb/Yb = 0.7-2.7).However, the alkaline lava samples have the ratios of Nb/Y > 1.5 and Zr/Nb<6 that are typical for within-plate basalts (Edwards et al., 1991).The seamount volcanic rocks have Nb/Y ratios of 2.3-3.1 and Zr/Nb ratios of 3.1-4.1,indicating OIB-like geochemical characteristics, whereas the oceanic crust basalt samples have Nb/Y (0.1-0.4) and Zr/Nb (8.1-32) values.

Island arc rocks
A small syenodiorite pluton, a suite of volcanic rocks, and lamprophyric dikes in the Kalecik (Ankara) area collectively represent the products of island arc magmatism.These arc rocks mostly plot in the alkaline field on a TAS diagram (Fig. 14a and b).The alkaline rock samples with medium to high Al 2 O 3 contents (10-19 wt %) represent both silicasaturated and silica-undersaturated rock units (Tables , 7, 8  and 9).The lamprophyric dikes have picrobasalt, trachybasalt, trachyandesite, tephrite and phonotephrite compositions, whereas the volcanic rocks display basalt, basanite, tephrite, leucite tephrite and foidite compositions.The samples from small alkaline intrusions fall into the syenodiorite field in the TAS diagram (Fig. 14b; Cox et al., 1979).The Late Cretaceous-early Paleocene volcanic rocks (andesite, dacite, rhyolite), found nearly 60 km SW of Kalecik, show subalkaline (tholeiitic and calc-alkaline) compositions, except for a few trachybasalt and trachyandesite samples (Fig. 14a, c and d; Dönmez et al., 2009).
The alkaline volcanic rocks mostly display high-K shoshonitic compositions in the K 2 O vs. SiO 2 diagram (Fig. 14d; Peccerillo and Taylor, 1976).Some volcanic and dike rocks also plot in the fields of medium-to high-K, calc-alkaline series.Although some alkaline volcanic rocks show medium-K calc-alkaline characteristics as a result of hydrothermal alteration (LOI/loss on ignition > 2 wt %), they have high-K shoshonitic affinity since the leucite bearing, silica-undersaturated alkaline rocks experienced analcimization, resulting in low K 2 O values in favor of Na 2 O values.On the Hastie et al. (2007) and Pearce (1982) diagrams, which utilize the immobile elements and the ratios of immobile elements (Th vs. Co, and Ce/Yb vs. Ta/Yb), the arc-related plutonic, volcanic and dike rocks generally display high-K (K 2 O/Na 2 O = 1.5-3.4)and shoshonitic characteristics (Fig. 14e and f).However, seven samples from the volcanic rocks and lamprophyre dikes contain high K 2 O/Na 2 O ratios (18.16-24.52),showing ultrapotassic (K 2 O/Na 2 O > 3) characteristics.
When plotted on MgO vs. major element diagrams, the analyzed samples mainly exhibit negative correlations, except on the Fe 2 O 3 and TiO 2 plots, which show positive correlations (Fig. 15).Based on the MgO vs. trace element variation diagrams (Fig. 15), Co shows a positive trend while Ba, Rb, Sr, Th and Zr all exhibit negative trends.These majorand trace-element trends can be explained by fractionation of clinopyroxene, feldspar, black mica (biotite, phlogopite), Fe-Ti oxides and apatite.However, the scatter in Fig. 15 may also be caused by the alteration of the arc rocks and/or the involvement of subducted sediments in their melt regime.Pearce and Cann, 1973;Shervais, 1982;Muller et al., 1992;Pearce, 2008).
The rock samples from the small syenodiorite pluton with metaluminous characteristics plot in the VAG (volcanic arc granites) field (Fig. 16a, b and c).The Ti-Zr-Y and Ti-V diagrams (Pearce and Cann, 1973;Shervais, 1982) show that the alkaline basic samples and the subalkaline volcanic rocks (Dönmez et al., 2009) from the southwestern part of the study area all plot in the arc field (Figs.16d and e, respectively), whereas the TiO 2 -Al 2 O 3 and Y-Zr diagrams (Muller et al., 1992) show that these samples fall into the arc field (Fig. 16f and g, respectively).The analyzed alkaline rocks display shoshonitic characteristics in the Th/Yb vs. Nb/Yb diagram (Pearce, 2008), and their Hf/Th ratios are rather low, ranging from 0.11 to 0.57, consistent with their shoshonitic affinity.The island arc tholeiitic (IAT) basaltic rocks have Hf/Th > 3, whereas the calc-alkaline volcanic rocks have Hf/Th < 3 (Wood, 1980).Their Th enrichment and increased Th/Yb ratios along the mantle metasomatism trend indicate the influence of subduction-derived fluids in their magma source (Fig. 16h; Pearce, 2008).The samples derived from the blocks of N-MORB-, SSZ-and OIB-like oceanic basalts in the Ankara Mélange typically plot within the MORB-OIB mantle array (Fig. 16h).

Source characteristics
The subduction-accretion complex represented by the Ankara Mélange contains blocks of oceanic lithosphere, showing geochemical affinities ranging from MORB to IAT and calc-alkaline.The SSZ-type ophiolite assemblages in the melange display both IAT-like and boninitic geochemical signatures.The ophiolitic units with an IAT-like chemistry are the manifestation of partial melting of the upper mantle peridotites, which were modified by incompatible elementenriched hydrous fluids (or melt) released from the subducting Tethyan oceanic slab.The ophiolitic units with MORBlike signatures represent the products of a depleted mantle source.Some of the samples with MORB-like chemistry plot within or near the IAT field (Figs.13c and 16h), indicating that their magmas were influenced by subduction-derived fluids.These ophiolitic rocks are the oldest units, as constrained by the volcanic stratigraphy and crosscutting relationships.Some doleritic dikes and basaltic rocks in the ophiolites show boninitic affinities, consistent with their formation in a forearc setting (Dilek and Furnes, 2011 , 2011).Collectively, the ophiolitic units in the Ankara Mélange display a geochemical progression that is typical of the development of forearc oceanic crust in the early stages of subduction-induced magmatism, as also documented from other Tethyan ophiolites (Dilek andFurnes, 2009, 2011;Dilek and Thy, 2009;Pearce and Robinson, 2010;Saccani et al., 2011;Moghadam et al., 2013).Seamount volcanic rocks occurring in the Ankara Mélange have OIB-like geochemical features, showing tholeiitic to alkaline affinities (Fig. 13e) with enrichment in incompatible elements and LREEs.The tholeiitic OIB affinity of some of the seamount volcanic rocks may have resulted from the interaction of plume-derived melts with MORB-type melts near a seafloor spreading system.The depletion of the OIBtype volcanic rocks in immobile elements (especially Ti) suggests mixing of the plume-derived and MORB-type melts during seamount evolution.
Enrichments in Cs, Rb, Ba, Th, U, K, La, Ce and Pb of the alkaline rocks suggest that their melt source was modified by subducted slab material (mainly fluids, and pelagic and/or terrigenous sediments).Slab-derived fluids helped to form hydrous and K-rich minerals, such as amphibole, apatite and phlogopite with high Rb/Sr (0.04-0.71) and K/Ti (3.77-16.62)ratios relative to MORB-and OIB-like magmas, and resulted in a positive correlation between Ba/Nb and La/Nb ratios (Fig. 18a).Also, the high La (18.4-69.2ppm) contents and La/Yb ratios (9.5-34.6)reflect that the high-K magmas may have been produced by small degrees of partial melting of a subduction-metasomatized mantle source (Fig. 18b).
As illustrated in the 143 Nd/ 144 Nd vs. 87 Sr/ 86 Sr diagram (Fig. 19a), six lamprophyre samples plot on the mantle array, defining a subduction component during the evolution of their magmas.We also show in this diagram, for comparison, the Late Cretaceous-early Tertiary volcanic rocks from the southern part of central Anatolia and the eastern Pontides, and the Cenozoic volcanic units in western Anatolia (Alpaslan et al., 2004;2006;Eyüboglu, 2010;Altunkaynak and Dilek, 2006 and references therein).The relatively high Pb (up to 34 ppm in some samples) and 87 Sr/ 86 Sr contents, as well as the Rb/Sr ratios (0.02-0.71) of the lamprophyre rocks, all indicate the effects of subducted oceanic sediments added to the mantle melt source (Pearce and Peate, 1995)  206 Pb/ 204 Pb variation diagrams, the data points lie above the Northern Hemisphere reference line (NHRL), and the radiogenic isotope data fall close to the fields of MORB, enriched lithospheric mantle source (EMII) and oceanic sediments.These features collectively suggest that the magmas of the lamprophyre rocks were derived from a MORB-like mantle source that was enriched by subducted terrigenous and carbonate sediments (Fig. 19b-e).However, the postcollisional Late Cretaceous-early Tertiary volcanic rocks in the Ulukışla basin in the southern part of central Anatolia have higher 87 Sr/ 86 Sr and lower 143 Nd/ 144 Nd ratios than those of the lamprophyres in the Ankara Mélange, indicating an EMII with recycled, continent-derived material.The Late Cretaceous high-K volcanic rocks representing active continental margin arc units in the eastern Pontides with  (Alpaslan et al., 2004(Alpaslan et al., , 2006;;Altunkaynak andDilek, 2006, 2013;Dilek and Altunkaynak, 2007;Keskin et al., 2008;Gündogdu-Atakay, 2009;Sarifakioglu et al., 2013).
The depletion of HFSE with respect to LREE enrichment, and high LILE/HFSE and radiogenic isotope ratios suggest that the high-K shoshonitic rocks are likely to have formed by small degrees of partial melting of a lithospheric mantle modified by slab-derived hydrous fluids.

Tectonic model
The Ankara Mélange displays a heterogeneous structural architecture containing oceanic and crustal rocks with different internal structure, stratigraphy and geochemical compositions.The oldest ophiolitic rocks in the Ankara Mélange appear to have formed in a SSZ setting within the northern Tethys around 180 Ma (Dilek and Thy, 2006;Sarifakioglu et al., 2011).The ∼ 80 Ma (80.3 ± 7.6 Ma) ophiolitic rocks in the same mélange also indicate that oceanic crust formation in the northern Tethys was still in operation in the Late Cretaceous (Table 1).
We obtained Middle-Upper Triassic biostratigraphic age data from the neritic limestones that are spatially associated with the seamount volcanic rocks, indicating that an oceanic lithosphere of the Late Triassic and older ages must have existed in this ocean to make up the substratum of the seamounts.Thus, we know that the northern branch of Neotethys was already a wide-open ocean with its MORB-type oceanic lithosphere between the Pontide block to the north and the Anatolide-Tauride micro-continent to the south in the Early Triassic (or even before).The ophiolitic mélange units in the Kırıkkale-Ankara-Çankırı-Çorum area are unconformably overlain by basal volcanic conglomerates of an arc origin.The overlying volcano-sedimentary units contain clayey-and sandy-limestone, limey sandstone, and sandstone-claystone alternating with volcaniclastic rocks.These rock types and their internal stratigraphy suggest their deposition in a frontal arc-forearc basin.The clayeylimestones are intruded by dikes and sills and have late Santonian and Campanian-Maastrichtian ages based on their fossil contents (Sarifakioglu, unpublished data).The radiometric age data from an alkaline basaltic rock (YK.4) and a syenodiorite intrusion (YK.438) give ages of 67.8 ± 4.9 Ma and 75.9 ± 1.3 Ma, respectively (Table 4a and d), constraining the timing of intraoceanic arc magmatism at the latest Cretaceous.
In general, subalkaline (tholeiitic and calc-alkaline) volcanic arc rocks occur in the northern part of the study area, whereas the younger alkaline volcanic and plutonic rocks are in the south.We interpret this spatial and temporal relationship to have resulted from a southward progression of the arc magmatism from subalkaline to alkaline affinities through time due to arc rifting above the southward retreating Tethyan subduction system (Fig. 20).We therefore think that the arc-related late alkaline dikes and plutons were emplaced on and across the evolving subduction-accretion complex above the north-dipping, southward-rolling Tethyan slab.
The high-K and shoshonitic Eocene dikes and lavas in the Ankara Mélange formed from melts derived from partial melting of the metasomatized arc mantle that was triggered by the influx of slab breakoff-induced asthenospheric flow.This slab breakoff was a result of an arc-continent (Central Anatolian Crystalline Complex -CACC) collision, followed by the continent-continent collision (Sakarya and CACC) in the early to middle Eocene.Zindler and Hart (1986).The field for oceanic islands is from White (1985).NHRL = Northern Hemisphere reference line.

Conclusions
1. Blocks of Middle-Late Triassic seamount and upper Permian metamorphic rocks occurring in the Ankara Mélange represent an intraoceanic subduction-accretion complex that developed in the northern Neotethys during the late Paleozoic through Cretaceous.
2. Thrust sheets and/or megablocks containing SSZ ophiolite units with Liassic and Cretaceous ages were incorporated into this subduction-accretion complex during the early Late Cretaceous.
3. The Late Cretaceous tholeiitic to calc-alkaline volcanic rocks are the products of an intraoceanic island arc system.The tholeiitic and calc-alkaline arc rocks show enrichment in incompatible elements due to the influence of slab-derived fluids.The shoshonitic arc rocks representing the latest stage of island arc magmatism were produced by partial melting of a subduction-enriched mantle source.(D) Island arc construction and magmatism on and across the preexisting SSZ oceanic lithosphere and the subduction-accretion complex (i.e., Ankara Mélange units).With continued slab retreat, arc magmatism shifts southward, following the migrating trench, and becomes more alkaline in time, producing lamprophyric and syenodioritic intrusions.See text for further explanation.

Fig. 1 .
Fig. 1.Simplified ophiolite map of Turkey showing the distribution of the suture zones and some of the major tectonic entities in Turkey (from MTA, 2001).Pontide tectonic belt including the Sakarya Continent.The inset box refers to the map area in Fig. 2.

Fig. 3a .
Fig. 3a.Geological map of the Kalecik area, east of Ankara, showing the distribution of the ophiolitic, turbiditic and island arc rock units in the Ankara Mélange in north-central Turkey.

Fig. 15 .
Fig. 15.Major oxides and trace elements vs. MgO variation diagrams for various alkaline island arc units.

Fig. 18 .
Fig. 18.(a) Ba/Nb vs. La/Nb diagram for the high-K island arc rocks.The data for N-MORB, OIB and PM are from Sun and McDonough (1989).(b) La/Yb vs. La diagram for the island arc rock units, illustrating the effects of partial melting and fractionation in their melt evolution.

Fig. 20 .
Fig. 20.Sequential tectonic diagrams depicting the intraoceanic magmatic evolution of the Ankara Mélange in the northern Neotethys during the Jurassic through Paleocene.(A) Suprasubduction zone generation of the oldest Neotethyan oceanic crust (∼ 180 Ma) in the upper plate of a north-dipping intraoceanic subduction zone, and seamount construction (SM1 and SM2) in the down-going oceanic plate.High-grade metamorphic rock blocks and turbiditic sandstone-mudstone sequences in the Ankara Mélange formed in the subduction channel (blue) and then the accretionary prism formed.(B) Accretion of seamount-1 (SM1) into the accretionary complex and related deformation in the subduction-accretion system.(C) Slab rollback and associated extension and SSZ oceanic crust formation (∼ 85-80 Ma) in the upper plate, accretion of seamount-2 (SM2) into the accretionary complex, and the lateral growth and deformation in the subduction-accretion system.(D)Island arc construction and magmatism on and across the preexisting SSZ oceanic lithosphere and the subduction-accretion complex (i.e., Ankara Mélange units).With continued slab retreat, arc magmatism shifts southward, following the migrating trench, and becomes more alkaline in time, producing lamprophyric and syenodioritic intrusions.See text for further explanation.

Table 1 .
Whole-rock 40Ar/ 39 Ar age data for a basaltic rock sample (YK-11) from the youngest SSZ ophiolite in the Ankara Mélange, Turkey.

Table 2a .
Whole-rock 40Ar/ 39 Ar age data for an epidote-glaucophane schist rock from a metamorphic block in the Ankara Mélange, Turkey.Age spectrum: the sample yielded age spectrum with two three-steps plateaus, characterized accordingly by 40.1 % of 39 Ar, age value of 83.5 ± 1.7 Ma and 42.2 % of 39 Ar, age value of 72.2 ± 2.2 Ma.On the inverse isochron plot, points form two linear regressions characterized by age value of 87.8 ± 2.5 and ( 40 Ar/ 36 Ar) 0 = 285 ± 5, respectively.The presence of two age plateaus evidence to isotope heterogeneity of YK 6.

Table 2a .
Whole-rock 40Ar/ 39 Ar age data for an epidote-glaucophane schist rock from a metamorphic block in the Ankara Mélange, Turkey.

Table 2b .
Whole-rock 40Ar/ 39 Ar age data for an epidote-chlorite schist rock from a metamorphic block in the Ankara Mélange, Turkey.

:
The sample yielded age spectrum with 3 steps plateau, characterized by 66.7% of 39 Ar, Age value of 158.4 ± 4.2 Ma.On the Inverse Isochrone Plot points form linear regression characterized by age value of 166.9 ± 5.9 and ( 40 Ar/ 36 Ar)0 = 272 ± 8.

Table 2c .
Whole-rock 40Ar/ 39 Ar age data for an epidote-actinolite schist rock from a metamorphic block in the Ankara Mélange, Turkey.Sample YK-1: epidote-actinolite schist, J = 0.004428 ± 0.000051 T • C 40 Ar cc (STP)40Ar/ 39 Ar ±1σ Age spectrum: the sample yielded age spectrum with three-steps plateau, characterized by 50 % of 39 Ar, age value of 187.4 ± 3.7 Ma.On the inverse isochron plot, one can observe linear regression characterized by age value of 166.1 ± 12.3.

Table 2c .
Whole-rock 40Ar/ 39 Ar age data for an epidote-actinolite schist rock from a metamorphic block in the Ankara Mélange, Turkey.962

:
The sample yielded age spectrum with 3 steps plateau, characterized by 50% of 39 Ar, Age value of 187.4 ± 3.7 Ma.On the Inverse Isochrone Plot one can observe linear regression characterized by age value of 166.1 ± 12.3.

Table 3 .
Whole -rock K/Ar age data from metamorphic rock blocks in the Ankara Mélange, Turkey.Sample no.Rock %K 40 Ar/ 36 Ar 40 Ar rad , nl g −1 % 40 Ar air error Age, Ma

Table 4b .
40Ar/ 39 Ar biotite age data for a lamprophyre dike (Sample No. YK-19) from the island-arc unit in the Ankara Mélange, Turkey.

:
The sample yielded age spectrum with four steps Plateau characterized by 89.6% of 39 Ar, Age value of 63.6 ± 1.2 Ma.On the Inverse Isochrone Plot plateau points form linear trend, characterized by age value of 57.5 ± 4.1 Ma, MSWD = 1.5 970 971

Table 4c .
Whole-rock 40Ar/ 39 Ar age data for a lamprophyre dike (sample no.YK-20) from the island arc unit in the Ankara Mélange, Turkey.Age spectrum: the sample yielded complex age spectrum with noticeable hump after low temperature step containing three-steps intermediate plateau followed by high temperature two-steps intermediate plateau.Intermediate plateaus are characterized accordingly by 24.8 % of 39 Ar, age value of 67.2 ± 1.2 Ma and 44.7 % of 39 Ar, age value of 45.9 ± 0.9 Ma.

Table 4c .
Whole-rock 40Ar/ 39 Ar age data for a lamprophyre dike (Sample No. YK-20) from the island-arc unit in the Ankara Mélange, Turkey.972

Sample: YK-20 (whole rock): Lamprophyre, J=0.007258 ± 0.000137
The sample yielded complex age spectrum with noticeable hump after low temperature step containing three steps intermediate plateau followed by high temperature two steps intermediate Plateau.Intermediate plateaus are characterized accordingly by 24.8% of 39 Ar, Age value of 67.2 ± 1.2 Ma and 44.7% of 39 Ar, Age value of 45.9 ± 0.9 Ma.

Table 4d .
Age spectrum: the sample yielded age spectrum with three-steps plateau characterized by 48.6 % of 39 Ar, age value of 92.7 ± 1.2 Ma.On the inverse isochron plot, points do not form linear regression.

:
The sample yielded age spectrum with three steps plateau characterized by 48.6 % of 39 Ar, Age value of 92.7 ± 1.2 Ma.On the Inverse Isochrone Plot points don't form linear regression.

Table 5 .
Tankut et al., 1998) and REE data for a select group of volcanic and dike rocks from the Neotethyan ophiolitic units in the Ankara Mélange (first nine samples fromTankut et al., 1998).

Table 6 .
Major, trace element and REE data for a select group of seamount volcanic rocks from the Ankara Mélange.

Table 7 .
Major, trace element and REE data for a select group of island arc volcanic and syenodiorite rocks from the Ankara Mélange.

Table 8 .
Major, trace element and REE data for a select group of island arc volcanic rocks from the Ankara Mélange.

Table 9 .
Nd/ 144 Nd (0.512674-0.512690)and 87 Sr/ 86 Sr (0.704697-0.704892)isotopic compositions.The initial ε Nd values range from +1.3 to +1.7, whereas the modern ε Nd values vary between +0.7 and +1.0 indicating a relatively enriched mantle source.Their Pb isotope ratios range from 19.332 to 19.939 for 206 Pb/ 204 Pb, 15.655 to 15.691 for 207 Pb/ 204 Pb, and 39.192 to 39.612 for 208 Pb/ 204 Pb.The high 206 Pb/ 204 Pb, and relatively high 143 Nd/ 144 Nd and 87 Sr/ 86 Sr ratios seem to be compatible with a mantle source that is enriched by slab-derived fluids and/or subducted pelagic Major, trace element and REE data for a select group of lamprophyric dike rocks from the Ankara Mélange.