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International Geology Review, Vol. 49, 2007, p. XXX–XXX.Copyright © 2007 by V. H. Winston & Son, Inc. All rights reserved.

Cenozoic Crustal Evolution and Mantle Dynamics of Post-Collisional Magmatism in Western Anatolia

YILDIRIM DILEK1 Department of Geology, 116 Shideler Hall, Miami University, Oxford, Ohio 45056

AND SAFAK ALTUNKAYNAKFaculty of Mining, Department of Geology, Istanbul Technical University, Maslak, Istanbul, Turkey

Abstract

Post-collisional magmatism in western Anatolia followed a continental collision event in theEarly Eocene, and occurred in discrete pulses that appear to have propagated from north to southover time. The first episode occurred during the Eocene and Oligo-Miocene and was subalkaline innature, producing medium- to high-K calc-alkaline granitoids and mafic to felsic volcanic rocks.Partial melting and assimilation–fractional crystallization of enriched subcontinental lithosphericmantle–derived magma(s) were important processes in the genesis and evolution of the parentalmagmas, which experienced decreasing subduction influence and increasing crustal contaminationthrough the Early Eocene–Early Miocene. This magmatic episode coincided with continued regionalcompression and development of a thick orogenic crust, and was influenced by an influx of astheno-spheric heat and melts provided by lithospheric slab break-off. Extensional tectonics replaced theregional compression by the Middle Miocene, following the initial collapse of the western Anatolianorogenic welt, and resulted in the development of metamorphic core complexes and horst-grabenstructures. The second main episode of magmatism occurred during the Middle Miocene (16–14 Ma)and produced mildly alkaline rocks that show a decreasing amount of crustal contamination andsubduction influence through time. Although melting of a subduction-modified lithospheric mantlecontinued, an asthenospheric mantle–derived melt contribution played a major role in the genera-tion of these mildly alkaline magmas. The inferred asthenospheric melt contribution was a result ofdelamination of the lowermost part of the lithospheric mantle and/or partial convective removal ofthe sub-continental lithospheric mantle (SCLM). The third episode of post-collisional magmatismstarted around ~12 Ma and continued through the Late Quaternary. The main melt source for thisphase carried no subduction component and was generated by the decompressional melting ofasthenospheric mantle, which flowed in beneath the attenuated continental lithosphere in theAegean extensional province. Lithospheric-scale extensional fault systems acted as natural conduitsfor the transport of uncontaminated alkaline magmas to the surface. Post-collisional magmatism inwestern Anatolia thus displays compositionally distinct episodes controlled by slab break-off, litho-spheric delamination, and asthenospheric upwelling and decompressional melting, reflecting thegeodynamic evolution of the eastern Mediterranean region throughout the Cenozoic. These eventsand the associated processes in the mantle took place primarily in response to the plate tectonicevolution of the region and collectively constitute a time-progressive template for the mode andnature of the post-collisional magmatism common to most alpine-style orogenic belts.

Introduction

THE LATE MESOZOIC–early Cenozoic tectonic evolu-tion of the eastern Mediterranean region was con-trolled by the collapse and closure of variousTethyan seaways between the obliquely convergingEurasia and Afro-Arabia (Sengör and Yilmaz, 1981;Robertson and Dixon, 1984; Dilek and Moores,

1990; Dilek et al., 1999a; Stampfli et al., 2001;Dilek and Flower, 2003). Widespread ophioliteemplacement in the Balkan Peninsula, Anatolia,and along the periphery of the Arabian subcontinentin the Cretaceous represents the earliest phase ofthe collisional tectonics in the region (Fig. 1; Dileket al., 1999a). Following the final obliteration of theTethyan oceanic lithosphere in these basins, thebounding continental fragments collided in the1Corresponding author: e-mail: [email protected]

10020-6814/07/XXX/XXX-23 $25.00

2 DILEK AND ALTUNKAYNAK

Early Eocene to form a series of alpine-styleorogenic belts (Dewey et al., 1986; Harris et al.,1994; Okay et al., 1996; Dilek et al., 1999b;Sherlock et al., 1999). The collisions of the SakaryaContinent with the Anatolide-Tauride platform inwestern Anatolia, the Kirsehir Continent (or theCentral Anatolian Crystalline Complex) with theAnatolide-Tauride Platform in Central Anatolia, andthe Rhodope-Pontide fragment with the KirsehirContinent in eastern Anatolia mark the most impor-tant collisional events in the early Cenozoic. Collec-tively, these collisions resulted in the formation ofthe bulk of the orogenic crust in the Turkish segmentof the Alpine orogenic belt (Fig. 1; Dilek and Whit-ney, 2000). The collision of the Arabian promontorywith Eurasia in the Middle Miocene (~14 Ma) wasthe last major collisional event in the eastern Medi-terranean region. This collision resulted in develop-ment of the Turkish-Iranian high plateau, followedby extensive volcanism across the plateau and intothe Caucasus farther north (Dewey et al., 1986;Pearce et al., 1990; Yilmaz, 1990; Dilek andMoores, 1999).

A complete record of post-collisional magmatismin the Mediterranean region is best seen in westernAnatolia, where Eocene to Quaternary magmaticrocks with changing chemical compositions areexposed. This exposure provides us with an opportu-nity to examine the spatial and temporal relationsbetween different magmatic episodes and theirproducts. The spatial distributions of these plutonicand volcanic rocks in western Anatolia and thechanges in their compositions through time andspace were strongly controlled by post-collisionalmantle dynamics, thermal regimes, melt evolutionpatterns, and magma transport mechanisms beneatha young orogenic belt. The existing models forthe cause of early Cenozoic magmatism in westernAnatolia include orogenic collapse of overthickenedcontinental crust (Seyitoglu and Scott, 1992, 1996),subduction-induced magmatism (Fytikas et al.,1984; Pe-Piper and Piper, 1989; Gülen, 1990),changes in stress regimes and tectonic modes duringthe late-stage evolution of the western Anatolianorogenic belt (Yilmaz, 1989, 1990; Savasçin andGuleç, 1990; Güleç, 1991; Ercan et al., 1995;Yilmaz et al., 2001; Akay and Erdogan, 2004), andslab breakoff–related thermal changes and magma-tism (Aldanmaz et al., 2000; Köprübasi and Aldan-maz, 2004; Altunkaynak and Dilek, 2006). Each ofthese models has strong implications for the early to

late Cenozoic geodynamics of the region, althoughthey are not entirely mutually exclusive.

This paper presents an overview of the nature ofpost-collisional Cenozoic magmatism in westernAnatolia and its geochemical characteristics andpetrogenetic evolution through time based on exist-ing data and regional tectonic constraints. Our tec-tonomagmatic model provides a realistic templatefor the geodynamic evolution of post-collisionalmagmatism in alpine-style orogenic belts aroundthe world.

Cenozoic Magmatism and Its Productsin Western Anatolia

Post-collisional Cenozoic magmatism in westernAnatolia occurred in discrete pulses that producedplutonic and volcanic rock associations with varyingages and chemical compositions (Fig. 2). Plutonicrocks appear to be limited mostly to the northwest,whereas volcanic rocks are extensive throughout thewest, but are relatively scarce south of the Menderesmetamorphic massif and into the Tauride carbonateplatform. Timing of the formation of these rocks andtheir source magmas spans the regional compres-sional stress regime that persisted during and afterthe collision of the Sakarya and Anatolide-Tauridecontinental fragments as well as the subsequentextensional tectonic regime that was fully under wayby the Middle Miocene; this extensional tectonicscontinues to affect the entire Aegean extensionalprovince to the present day (McKenzie and Yilmaz,1991). Following the initial collapse of the westernAnatolian orogenic belt as early as the EarlyMiocene, and after the onset of the extensional tec-tonic regime in the region (Seyitoglu and Scott,1996; Dilek and Whitney, 2000; Okay and Satir,2000), the north-dipping subduction zone along theHellenic Trench was established around 12–11 Ma(Meulenkamp et al., 1988). The North and EastAnatolian transform fault zones developed shortlyafterward (~7 Ma; Sengör et al., 1985; Bozkurt,2002), facilitating the southwestward tectonicescape of the Anatolian plate away from the Arabiancollision zone in the east (Dewey et al., 1986). Plateboundary processes associated with these eventshave strongly affected the nature of crustal- andlithospheric-scale deformation (i.e., Ring and Layer,2003), melt generation and evolution, magma trans-port mechanisms, and the mode of magmatismthroughout the late Cenozoic.

POST-COLLISIONAL MAGMATISM IN WESTERN ANATOLIA 3

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FIG. 2. Simplified geological map of western Anatolia, showing the distribution of post-collisional volcanic and plu-tonic rocks ranging in age from the Eocene to Plio-Quaternary, major tectonic blocks (Sakarya continent, Anatolide belt,Tauride platform, and Kazdag and Menderes metamorphic massifs), and the Izmir-Ankara-Erzincan suture zone. Abbre-viations: KM = Kazdag Massif; MM = Menderes Massif.


Cenozoic plutonic rocks in western Anatolia aremostly Eocene and Oligo-Miocene in age and occuralong and north of the Izmir-Ankara-Erzincansuture zone (IAESZ; Fig. 2) as composite plutonsemplaced during the compressional buildup of thewest Anatolian orogenic crust (Ercan et al., 1984;Savasçin and Güleç, 1990; Harris et al., 1994;Altunkaynak and Yilmaz, 1998, 1999; Genç, 1998;Karacik and Yilmaz, 1998; Yilmaz et al., 2001;Köprübasi and Aldanmaz, 2004). In addition tothese mostly I-type post-collisional granitoids,sheet-like leucocratic granitic intrusions occur inthe Menderes metamorphic massif farther south.These leucocratic granitoids are intrusive into thegneissic and schistose rocks in the footwalls ofmajor detachment faults and shear zones in themassif, and are lineated and foliated parallel to thefabric elements in their mylonitic host rocks (Doraet al., 1987; Bozkurt, 2004; Isik et al., 2004; Ringand Collins, 2005). These structural relationshipssuggest that the granitoid rocks within the metamor-phic massif are syn-extensional intrusions whosemagmas were likely derived from decompressionalmelting of the exhumed continental lower crust, typ-ical of Cordilleran-type core complexes (Crittendonet al., 1980, and references therein; Coney andHarms, 1984). The emplacement and cooling ages ofthese syn-extensional granitoids have been dated at~21–20 Ma (Gessner et al., 2004; Isik et al., 2004;Ring and Collins, 2005), and are not discussed inthis paper.

The Eocene granitoid plutons occur in a nearlyE-W–trending zone between the Sea of Marmaraand the IAESZ (Fig. 2). They have subcircular toelliptical shapes, range in diameter from 10 to 25km, and become slightly younger in crystallizationages from the south near the IAESZ, ~54–48 Ma, tothe north near the Sea of Marmara, ~48–34 Ma,based on 40Ar/39Ar and/or K/Ar hornblende andbiotite ages, Rb/Sr biotite ages (Ataman, 1972;Bingöl et al., 1982, 1994; Harris et al., 1994; Birkleand Satir, 1995; Delaloye and Bingöl, 2000; Okayand Satir, 2000; Köprübasi and Aldanmaz, 2004).The suture-zone granitoids, which were emplacedinto the mafic-ultramafic ophiolitic and blueschistrocks at shallow crustal levels along the IAESZ inthe south (Harris et al., 1994), are composed mainlyof diorite, quartz diorite, granodiorite, and syenite(i.e., Topuk, Orhaneli, Gürgenyayla, and Göynük-belen plutons; Altunkaynak, 2004). Volcanic equiv-alents of the suture zone plutons have not beenrecognized in the area. The younger granitoids near

the Sea of Marmara (i.e., Fistikli, Armutlu, Lapseki,Kapidag plutons) are intrusive into the Paleozoic–early Mesozoic basement rocks of the Sakarya conti-nent, and consist mainly of monzogranite, grano-diorite, and granite. These Marmara granitoids areaccompanied by coeval pyroclastic rocks and basal-tic to andesitic lavas (i.e., Kizderbent andBalikliçesme volcanics) that likely constitute theirextrusive counterparts (Genç and Yilmaz, 1997).

The younger generation of plutons, which areOligo-Miocene in age (i.e., Kestanbol, Evciler,Eybek, Ilica, Kozak), mostly occur farther west inthe Biga Peninsula and to the south of the Gulf ofEdremit (Fig. 2). These plutons are commonly intru-sive into the crystalline basement rocks of theSakarya continent as individual bodies, and arecomposed of granite, granodiorite, quartz diorite,and quartz monzonite (Altunkaynak and Yilmaz,1998; Genç, 1998; Karacik and Yilmaz, 1998;Yücel-Öztürk et al., 2005). They are commonlyaccompanied by hypabyssal porphyry sheet intru-sions in the country rocks, and are locally intrusiveinto a volcanic carapace consisting of andesite,dacite, rhyodacite, rhyolite, and pyroclastic rocks,which collectively form a lower volcanic associationlargely coeval with the granitoids. These relationssuggest relatively shallow upper crustal levels ofemplacement for the Oligo-Miocene plutons.

Both the plutons and their volcanic counterpartsare stratigraphically overlain by rhyolitic ignimbriteflows, pumiceous air-fall and mudflow deposits, andlatitic, andesitic, and basaltic lava flows, which areintercalated with Lower–Middle Miocene lacustrinerocks and coal seams (Genç, 1998). Some of theplutons and their volcano-sedimentary cover consti-tute parts of collapsed caldera structures in the area(e.g., Kozak pluton; Altunkaynak and Yilmaz,1998). Radiometric dating of these plutonic andvolcanic rocks have revealed igneous ages rangingfrom ~31 Ma to 15 Ma (Borsi et al., 1972; Ataman,1974; Bingöl et al., 1982; Fytikas et al., 1984; Ercanet al., 1985, 1995; Birkle and Satir, 1995; Erkül etal., 2005), which are consistent with the fossil agesobtained from sedimentary intercalations within thevolcanic associations (Genç, 1998, and referencestherein). These Oligo-Miocene volcano-plutonicunits collectively cover a wide area in northwestAnatolia and constitute the most extensive post-collisional magmatic products in the region(Savasçin and Erler, 1994). The products of an EarlyMiocene (~21.5–16.5 Ma) volcanism, characterizedby extensive andesitic, dacitic, to rhyolitic and


pyroclastic rock formations, are also widespread onthe islands of Lesbos, Limnos, Agios Evstratios,and Samothraki in the northeastern Aegean Sea(Pe-Piper and Piper, 1992, 2002). These volcanicrocks are spatially and temporally associated withgranitic plutons and collectively form large strato-volcano remnants and volcano-plutonic associationsexposed in collapsed caldera structures.

As the collision-induced compression gave wayto widespread N-S extension during the MiddleMiocene, the nature and products of magmatismalso changed in western Anatolia. Volcanic rocks ofthis transitional phase are common along NNE- andNNW-trending oblique transtensional fault systems(Altunkaynak and Yilmaz, 1998; Yilmaz et al.,2000; Erkül et al., 2005) and are composed ofandesites and pyroclastics intercalated with and/orgrading into dark, mildly alkaline basaltic lavas(Altunkaynak and Dilek, 2006, and referencestherein). This transitional-phase volcanism has noplutonic equivalents exposed at the surface in west-ern Anatolia. The products of this bimodal transi-tional volcanism are also exposed on the adjacentAegean Islands of Chios, Evia, Samos, and Patmos,where andesitic to rhyolitic flows and tuffs are inter-calated with alkali basalt and basaltic trachyandes-ite flows and basinal sedimentary rocks (Pe-Piperand Piper, 2002). Radiometric dating of some ofthese volcanic rocks revealed ages around 15–14Ma (Fytikas et al., 1980; Pe-Piper and Piper, 2001,2002, 2006). Extrusion of basaltic flows on Samosappears to be spatially associated with N-S–orientedoblique-slip faults.

By the Late Miocene, the ongoing extensionaltectonics had affected the entire Aegean province(Jolivet et al., 1994; Yilmaz, 2002), significantlythinning the lithosphere and exhuming much of theKazdag and Menderes metamorphic massifs in west-ern Anatolia (Bozkurt and Satir, 2000; Okay andSatir, 2000; Isik et al., 2004; Ring et al., 2003; Ringand Collins, 2005). Continued high- to low-anglenormal faulting and associated extension within theMenderes core complex during the Late Miocenefurther facilitated its tectonic denudation andresulted in the formation of deep graben structures(Yilmaz et al., 2000; Ring et al., 2003; Bozkurt,2002). Widespread erosion accompanying thisextensional phase produced a regionwide peneplainsurface marking the Miocene–Pliocene boundary.Magmatism during this time produced alkalinemafic lavas with their potassic and sodic composi-tions progressively increasing in time (Yilmaz,

1989; Savasçin and Oyman, 1998; Aldanmaz et al.,2000). Upper Miocene and Pliocene alkaline basaltsare exposed in southeastern Thrace (Yilmaz andPolat, 1998), in Ezine (~9.7 Ma; Borsi et al., 1972;Innocenti et al., 2005), east of Çanakkale (LatePliocene) and Ayvalik (Early Pliocene; Yilmaz,1990), in Söke (~7 Ma; Yilmaz, 1990), and in Urla-Izmir (~11.3 Ma; Borsi et al., 1972; Innocenti et al.,2005). Strongly alkaline latest Miocene to latestQuaternary basaltic lavas consist of olivine-phyricand/or aphyric basalts, basanites, and phonoteph-rites with potassic to ultrapotassic compositionsand constitute the final magmatic phase in westernAnatolia (Richardson-Bunbury, 1996; Seyitoglu etal., 1997; Alici et al., 2002; Aldanmaz et al., 2000;Francalanci et al., 2000). These super-alkali volca-nic rocks are spatially associated with graben-bounding, E-W–oriented normal fault systems and/or with the intersections of the E-W– and NE-trend-ing grabens and faults (as in Kula volcanics), andthey represent the products of “rift volcanism” inwestern Anatolia. Similar alkaline rocks (sodicbasalts, hawaiites, trachybasalts) associated withextensional graben structures also occur on theIslands of Samos and Patmos in the Aegean Seafarther west (Pe-Piper and Piper, 2002).

Geochemistry and Petrogenesis

Geochemistry

The Eocene granitic and volcanic associationsnorth of the IAESZ are subalkaline in nature withthe exception of two volcanic samples, which fallwithin the alkaline field on the Na2O + K2O vs SiO2diagram (Fig. 3). This group is composed ofmedium- to high-K calc-alkaline rocks whose K2Ocontents increase with increasing SiO2 (Fig. 4).Their Mg numbers vary from 21 to 70 (granitoids:21–58; volcanic rocks: 33–70), and their A/CNK[Al2O3/(CaO + Na2O + K2O) molecular ratio] valuesrange between 0.72 and 1.16 (Table 1).

The trace-element abundances of these rocksshow large variations (e.g. Ba: 57–1150 ppm; Th: 2–13 ppm; La: 2.01–57.1 ppm), suggesting that theEocene granitic and volcanic rocks in particularwere moderately enriched in incompatibleelements, and indicating that their melts were mod-erately evolved (Frey et al., 1978; Pearce, 1982).They show enrichment in large-ion lithophileelements (LILE [K, Rb, Ba, Sr]) over light rare-earthelements (LREE) and MREE, and depletion in high-field-strength elements (HFSE [Zr, Nb, Ti, and P])


FIG. 3. Total alkali vs. SiO2 classification diagram showing alkaline-subalkaline subdivisions, according to Irvineand Baragar (1971; IB) and Kuno (1966; K).

FIG. 4. K2O vs. SiO2 diagram showing the distribution of post-collisional volcanic and plutonic rocks in westernAnatolia according to the classification scheme of Peccerillo and Taylor (1976).


TABLE 1. Representative Chemical Analyses of the Eoceneto Quaternary Magmatic Rocks from Western Anatolia

EoceneGranitoids

Eocenevolcanics

OligoMiocenegranitoids

OligoMiocenevolcanics

Middle Miocenevolcanics

Late Miocene–Quaternary volcanics

SiO2 61.10–75.05 48.11–63.75 55.91–74.87 48.60–61.37 45.91–58.92 41.54–50.26TiO2 0.10–0.69 0.65–1.70 0.12–0.91 0.61–1.19 0.76–2.90 1.80–3.22Al2O3 12.59–17.27 15.61–17.82 13.90–17.24 14.26–19.75 14.00–19.30 11.86–18.32Fe2O3 0.10–4.77 4.80–13.12 1.28–8.15 4.74–10.08 6.07–9.14 8.02–14.38MnO 0.02–0.16 0.11–0.19 0.02–0.15 0.04–0.17 0.06–0.16 0.12–0.26MgO 0.10–2.53 1.43–6.27 1.42–3.32 1.05–9.93 2.06–10.12 4.60–10.91CaO 0.73–5.86 4.59–11.92 3.22–7.19 4.90–11.03 3.66–12.11 7.87–14.61Na2O 2.94–5.94 2.80–4.95 3.25–4.42 2.30–4.04 1.47–4.39 2.61–6.66K2O 0.83–5.23 0.10–3.57 1.06–5.1 1.00–3.54 1.19–6.26 1.00–4.84P2O5 0.01–0.23 0.08–0.39 0.10–0.46 0.12–0.33 0.12–0.62 0.72–1.64

Trace elements in ppmLa 15.20–57.1 2.01–27.60 38.01–77.00 20.72–92.79 41.23–90.00 20.86–62.50Ce 27.70–83.21 15.00–52.00 66.10–161.70 37.71–178.90 42.00–134.80 42.86–136.04Pr 2.72–8.18 6.00 6.90–18.30 4.28–13.84 7.64–14.70 5.73–16.23Nd 9.81–33.80 22.40 23.70–68.80 20.60–52.75 22.00–55.80 34.50–55.60Sm 1.70–6.60 4.40–27.00 4.30–13.70 4.10–7.70 4.35–10.39 6.60–12.20Eu 0.30–1.81 1.20 1.10–2.78 1.08–1.94 1.20–2.65 1.94–3.70Gd 1.33–4.06 8.10 3.20–8.50 3.70–6.45 6.09–8.59 5.20–10.02Tb 0.26–0.67 0.59 n. d.1 0.62–0.89 0.58–8.88 0.85–1.47Dy 1.40–3.60 3.83 2.60–6.10 3.29–4.88 3.20–5.11 4.20–7.23Er 0.90–2.50 2.32 1.40–2.60 1.76–2.86 1.69–3.08 1.78–3.04Tm 0.14–0.33 0.34 0.32–0.37 0.30–0.50 0.30–0.40 0.26–0.38Yb 1.30–2.69 2.37 1.30–2.31 1.75–2.83 2.06–2.53 1.47–2.30Lu 0.14–0.44 0.37 0.21–0.31 0.27–0.43 0.27–0.37 0.21–0.30Rb 29–198.60 2–73 39–212 22–189 15–243 6–105Sr 45–1080 104–317 202–1240 466–1037 517–1002 823–1586Y 7.20–28.00 20.00–49.01 15.00–35.00 18.01–39.03 19.00–21.80 21.30–31.00Zr 29–175 78–292 61–372 72–297 153–1128 217–406Nb 2.00–16.50 1.00–11.00 1.00–15.00 2.00–23.00 15.00–55.20 67.30–116Cs 0.46 1.80–8.60 5.80–8.15 0.30–2.67Ba 81–1150 57–319 809–1229 200–1595 483–2000 649.40–1035Ni 1–9.30 1–43 13–30 20–222 10.0–343 34–183.3Hf 2.50–4.60 2.00 3.97 2.10–5.41 2.97–7.65 4.70–7.93Ta 0.50–2.30 0.70 0.68 0.50–1.29 0.97–1.52 2.16–6.00Pb 1.10–11.00 1.00–17.00 6.10–21.00 7.00–14.65 1.20–11.00Th 2.00–13.00 36.70–47.90 12.90–21.92 7.00–38.40 2.63–9.40U 1.39–8.30 1.62 2.30–5.35 2.00–6.90 0.56–2.65

ASI 0.80–1.16 0.72–1.01 0.79–1.13 0.56–1.04 0.50–0.95 0.38–0.67Mg# 21–58 33–70 47–72 55–77 59–81 70–84Eu/Eu* 0.38–0.98 0.61 0.79–0.91 0.75–0.91 0.79–0.98 1.00–1.1087Sr/86Sr 0.7064–0.7074 0.7059 0.7059–0.7090 0.7048–0.7090 0.7036–0.7076 0.7030–..7035143Nd/144Nd 0.51243–0.51244 0.5125 0.51234–0.51258 0.5120–0.5129 0.51250–0.51280 0.5127–0.5130

1n. d. = not determined.Sources: Gulec, 1991; Yilmaz, 1990; Ercan et al., 1984, 1985, 1990, 1995; Genc and Yilmaz, 1997; Altunkaynak and Yilmaz, 1998; Aldanmaz et al., 2000; Yilmaz et al, 2001; Alici et al., 2002; Altunkaynak and Dilek, 2006.


with respect to the adjacent LILE on MORB-normalized multi-element variation diagrams (Fig.5). They also display enrichment in LREE, flatHREE, and minor to moderate negative Eu anoma-lies (average Eu/Eu*: 0.38–0.98) on chondrite-normalized rare-earth diagrams, indicating stronglyfractionated REE patterns regardless of rock type(Fig. 6).

The Oligo-Miocene volcano-plutonic associa-tions comprise shoshonitic to high-K calcalkalinerocks (Fig. 4) with depleted TiO2 (


and depletion in HREE relative to the N-MORB(Fig. 5), suggesting an enriched mantle source (notsubduction related) for the origin of their magmas.

These alkaline rocks also exhibit LREE enrichmentrelative to chondrites and flat REE patterns from Erto Lu relative to the LREE (Fig. 6).

FIG. 6. Chondrite-normalized REE patterns for western Anatolian volcanic and plutonic rocks. Chondrite normaliz-ing values are from Boynton (1984).


Petrogenesis and Geodynamic Implications

The Eocene and Oligo-Miocene plutons in north-west Anatolia are predominantly metaluminous andslightly peraluminous I-type granitoids with the A/CNK values ranging between 0.79 and 1.16 (Table1). The major- and trace-element features of thesegranitoids and their volcanic counterparts suggestthat their melts were moderately and stronglyevolved, and that they were derived from a subduc-tion-influenced, enriched lithospheric mantlesource (Thorpe et al., 1982; Pearce et al., 1990;Saunders et al., 1991; Hawkesworth et al., 1993;Thirwall et al., 1994; Pearce and Peate, 1995).Because the subduction of the Tethyan oceaniclithosphere was effectively shut off by the partialsubduction of the northern edge of the Anatolide-Tauride platform beneath the Sakarya Continent bythe latest Cretaceous and by the subsequent conti-nent-continent collision in the Eocene (Okay et al.,1998; Okay, 2002; Altunkaynak and Dilek, 2006),the inferred subduction signature of the Eocene andOligo-Miocene volcano-plutonic units is consideredto have been inherited from the Late Cretaceoussubduction of the Neo-Tethyan oceanic lithospherebeneath the Sakarya Continent (Genç and Yilmaz,1997; Yilmaz et al., 2001; Aldanmaz et al., 2000;Köprübasi and Aldanmaz, 2004; Altunkaynak andDilek, 2006; Yücel-Öztürk et al., 2005). The sub-

continental lithospheric mantle is interpreted tohave been variously enriched by volatile supply anddiaprism associated with this subduction event andhence to have been highly heterogeneous.

The εNd(i) versus 87Sr/86Sr (i) diagram in Figure 7shows initial isotopic compositions of all volcano-plutonic rock groups examined in this study. TheOligo-Miocene plutonic and volcanic rocks have thehighest 87Sr/86Sr (i) [0.7087– 0.7071] and the lowestεNd(i) (–6.5 to –3.5) values. By contrast, the UpperMiocene–Quaternary alkaline lavas have the lowest87Sr / 86Sr (i) [0.7033–0.7030)] and the highest εNd(i)(+2.5 to +6.5) values. The Eocene to Oligo-Mioceneplutonic and volcanic rocks plot in a field betweenthe “asthenospheric melting” and “crust” subfields;this indicates a hybrid composition along the litho-spheric mantle-melting array (Fig. 7).

Positive correlations between K2O and SiO2 con-tents, REE patterns, and nearly constant values ofthe initial εNd and 87Sr/86Sr ratios (regardless of theSiO2 content of the rocks) indicate that assimilationand fractional crystallization of the primary maficmagmas played a major role during the evolution ofthe Eocene and Oligo-Miocene mafic to felsic rocks(Wilson, 1989; Güleç, 1991; Genç and Yilmaz 1997;Altunkaynak and Y€lmaz, 1998; Genç 1998; Aldan-maz et al., 2000; Yilmaz et al., 2001; Akay andErdogan, 2004; Yücel-Öztürk et al., 2005;

FIG. 7. εNd(t) vs. 87Sr/86Sr(t) diagram for western Anatolian volcanic and plutonic rocks. The fields for asthenosphericand lithospheric mantle melting are from Wilson (1989) and Davies and von Blackenburg (1995), and the referencestherein.


Altunkaynak and Dilek, 2006). Some of the Oligo-Miocene plutons and associated volcanic unitsdisplay less pronounced depletions in Ba, Sr, and P,and have higher contents of Pb, K, Ni, and SiO2 incomparison to the Eocene plutons and associatedvocanic rocks, suggesting a greater crustal involve-ment in their petrogenesis (Fig. 5). The degree ofcrustal contamination seems to have increasedthrough time, as evidenced by the comparison of the87Sr/86Sr ratios of the Eocene and Oligo-Mioceneplutonic and volcanic rocks (Figs. 7 and 8). Theseisotopic values and the La/Nb, Zr/Nb ratios alsoindicate decreasing subduction signatures in thevolcano-plutonic rocks from the Late Eocene to theEarly Miocene (Altunkaynak and Dilek, 2006).Thus, as post-collisional crustal build-up continuedthrough the Oligo-Miocene, widespread magmatismin western Anatolia produced extensive volcano-plutonic units with hybrid compositions that reflectincreasing amounts of crustal contamination, anddiminishing subduction-zone influence in the man-tle source region (Altunkaynak et al., 2004).

Melting of the previously subduction-enrichedmantle lithosphere was facilitated by astheno-spheric upwelling caused by slab breakoff in theaftermath of the Eocene regional continental colli-sion (Köprübasi and Aldanmaz, 2004; Altunkaynakand Dilek, 2006). The upwelling hot asthenosphereimpinged on the overlying mantle lithosphere and

resulted in the melting of the previously metasoma-tized and hydrated layers that in turn producedpotassic, calc-alkaline magmas. The geochemicalfeatures of the Eocene volcanic and plutonic rocksindicate that these calcalkaline magmas experi-enced various degrees of crustal contamination anddifferentiation during their ascent through the newlyformed orogenic crust and the Sakarya continentallithosphere. The continued convergence in the colli-sion zone and the cessation of subduction collec-tively led to the development of thick orogenic crust.The replacement of the cold lithosphere by theasthenosphere caused the formation of hybrid Oligo-Miocene magmas from both lithospheric mantle-derived and crustal melts (Altunkaynak and Dilek,2006).

The Middle Miocene volcanic rocks displaygeochemical characteristics that indicate their deri-vation from slightly to moderately evolved magmasthat underwent significant crystal fractionation(crystallization of olivine and clinopyroxene in par-ticular) (Aldanmaz et al., 2000; Akay and Erdogan,2004; Altunkaynak and Dilek, 2006). The lack ofsignificant negative Eu anomalies and the consis-tently high Sr contents of the rocks are inconsistent,however, with the conclusion that their magmasunderwent plagioclase fractionation. The negativeTa and Nb anomalies, enriched LREE, and low Rb/Sr ratios of the rocks in this group point to the

FIG. 8. 87Sr/86Sr vs. age diagram for western Anatolian volcanic and plutonic rocks, depicting temporal changes intheir degree of crustal contamination. See text for discussion.


involvement of a subduction-influenced, incompati-ble element–enriched mantle source in their magmaevolution. However, these Middle Miocene rockshave significantly lower La/Nb, Zr/Nb, and 87Sr/86Srratios, and higher 143Nd/144Nd ratios in comparisonto the Eocene and Oligo-Miocene rocks (Table 1).Although lithospheric mantle melts seem to havecontributed to the source of these transitional,mildly alkaline lavas, as suggested by the observa-tion that their eNd(t) vs.

87Sr/86Sr(t) values plot on thelithospheric mantle-melting trajectory (Fig. 7), othergeochemical features as stated above indicate theinfluence of incoming asthenospheric melts in theirmagmatic evolution. Thus, it is apparent that bothlithospheric and asthenospheric mantle melts wereinvolved during the generation of the MiddleMiocene lavas. Through 87Sr/86Sr values decreasingfrom 0.7090 to 0.7064, the gradually diminishingeffects of crustal contamination during the evolutionof these transitional rocks from the Early to MiddleMiocene is apparent (Fig. 8).

We think that the introduction of inferredasthenospheric melts to the source of the MiddleMiocene bimodal volcanic rocks (contemporaneouscalc-alkaline and alkaline rocks) was facilitated bypartial delamination of the lithospheric root beneaththe western Anatolian orogenic belt. The Eocenecontinental collision and continued shorteningthroughout the Oligo-Miocene likely resulted in theformation of overthickened crust and a deep litho-spheric keel with dense eclogitic material beneaththe orogenic belt (Seyitoglu and Scott, 1996; Dilekand Whitney, 2000). Foundering of the dense,unstable lithospheric root into the mantle throughdelamination caused the replacement of eclogite bybuoyant, warm asthenosphere that underwentdecompressional melting to produce basaltic liq-uids. Both the heat and the basaltic melts providedby the upwelling asthenosphere resulted in partialfusion of the previously metasomatized regions inthe mantle lithosphere, and led to formation of themildly alkaline lavas. Similar lithospheric delami-nation processes and their magmatic consequenceshave been suggested for the western and easternAnatolian orogenic belts and other mountains (i.e.,Pearce et al., 1990; Turner et al., 1996; Ducea andSaleeby, 1998; Aldanmaz et al., 2000; Keskin et al.,2003; Sengör et al., 2003; Jones et al., 2004;Williams et al., 2004; Altunkaynak and Dilek,2006). This period of inferred lithospheric delami-nation and bimodal volcanism coincides withthe onset of regional extension, crustal uplift, and

core complex exhumation in western Anatolia(Altunkaynak and Dilek, 2006).

The Upper Miocene and Quaternary alkalinerocks have OIB-like geochemical characteristics,consistent with an asthenospheric mantle source(Aldanmaz et al., 2000; Alici et al., 2002). The lackof negative Ta and Nb anomalies in their traceelement patterns also shows that the magmas ofthese alkaline rocks were not affected by subduc-tion-generated fluids (Fig. 5). This interpretation isalso supported by their Sr-Nd isotopic compositions(Table 1). The samples from this group plot in theasthenospheric melting array on the eNd(t) vs.

87Sr/86Sr(t) diagram (Fig. 7). However, the Rb/Nb and K/Nb ratios of these lavas are higher than those oftypical OIB, and combined with HFSE enrichmentlevels this observation indicates limited melt contri-bution from a lithospheric mantle source (Alici etal., 2002). The sharp negative jump in the 87Sr/86Srvalues from the Middle Miocene mildly alkalinelavas to the Upper Miocene alkaline lavas around11–10 Ma indicates a significant reduction in thedegree of crustal contamination of the magmas (Fig.8). The Kula volcanic field (8.4 Ma – 0.13 ± 0.005Ma) north of the Gediz graben (Fig. 2) represents thebest example of this latest Cenozoic alkaline volca-nism in western Anatolia, and contains olivine-phy-ric and/or aphyric basalts and basanites withpotassic-ultrapotassic compositions (Richardson-Bunbury, 1996; Alici et al., 2002; Aldanmaz et al.,2000). Assimilation–fractional crystallization mod-eling of these highly alkaline Kula volcanic rockssupports the idea that fractional crystallization was amajor process during the evolution of their magmas.This alkaline to ultra-alkaline volcanism in the lat-est Cenozoic was a direct result of decompressionalmelting of the asthenospheric mantle, which wasupwelling to isostatically compensate for thethinned lithosphere undergoing wholesale extensionin the Aegean Province. Major graben systems andthe intersections of lithospheric-scale extensionalfault systems played a significant role as naturalconduits for magma transport, thus minimizingcrustal contamination.

This synoptic evaluation of the post-collisionalmagmatism in western Anatolia in light of thegeodynamic evolution of the region indicates thatconditioning of the lithospheric mantle throughplate tectonic events (i.e., previous subductions,orogenic thickening) was an important factor affect-ing melt evolution through time. Collision-inducedslab breakoff, lithospheric delamination, and


asthenospheric upwelling all appear to havecontributed to the time-progressive evolution ofCenozoic magmatism in the region. These aspects ofthe western Anatolian orogenic belt are highly char-acteristic of the post-collisional evolution of alpine-style mountains.

Geodynamic Template for Post-collisional Magmatism in Orogenic Belts

A close examination of collision zones in differ-ent orogenic belts around the world shows that theevolutionary patterns of their post-collisionalmagmatism have many features in common (Fig. 9).Following a continental collision event, thereappears to be an episode of medium- to high-K,calc-alkaline magmatism producing linearly distrib-uted granitoids and extrusive sequences near thesuture zones (Fig. 9A). Development of calc-alka-line volcano-plutonic complexes in the early stagesof the evolution of mountain belts commonly coin-cides with continued regional shortening and crustalthickening (Fig. 9B). This calc-alkaline magmaticepisode is then followed by volcanism displayinggeochemical features transitional between calc-alkaline and alkaline compositions. This transi-tional phase is generally short-lived (~several mil-lion years) and gives way to an eruption of OIB-typealkaline basalts that is commonly synchronous withwidespread tectonic extension (Fig. 9C). In theabsence of the active subduction of oceanic litho-sphere in continental collision zones, the source ofcalc-alkaline magmas is likely to be from the melt-ing of a previously subduction-modified continentallithospheric mantle. The eruption of within-platealkaline basalts in the latest stages of post-colli-sional magmatism is most likely associated withmelting of upwelling asthenospheric mantle beneathhighly attenuated continental crust and thinnedlithosphere (Fig. 9D).

We observe many components of this template ofpost-collisional magmatism, albeit with some devia-tions and minor differences, in many discrete colli-sion zones within the broader Alpine orogenic belt.In the Betic-Rif mountain system, which evolvedfrom the collision of Iberia with Africa in the LateOligocene–Early Miocene, the first episode of mag-matism was marked by calc-alkaline volcanism insouthwestern Spain and northwest Africa (~15–8Ma) showing geochemical signatures of increasedcrustal contamination through time (Benito et al.,1999; Maury et al., 2000; Coulon et al., 2002).

Eruption of alkali basalts, basanites, and ultrapotas-sic lamproites ensued during this early episode ofcalc-alkaline magmatism and accompanied wide-spread crustal extension in the region around 10–6Ma (Maury et al., 2000). Moreover, seismic tomo-graphic data from this mountain belt provide strongevidence for post-collision slab detachment andasthenospheric upwelling (Zeck, 1996; Carminati etal., 1998; Zeck et al., 1998; Benito et al., 1999).

In the European Alps, which resulted from thecollision of Adria (an African promontory) withEurope starting around 55 Ma (Coward andDietrich, 1989), the products of post-collisionalmagmatism are represented by granitoid intrusions,mafic dike swarms, and associated mineralizationalong the peri-Adriatic lineament (De Boorder et al.,1998). Ranging in age from 33 Ma to 29 Ma (DelMoro et al., 1983; Barth et al., 1989; Hansmann andOberli, 1991; von Blanckenburg and Davies, 1995),these granitic plutons and dikes have geochemicalsignatures of basaltic magmas that were derivedfrom partial melting of a subcontinental lithosphericmantle (SCLM), and experienced variable degrees ofassimilation and fractional crystallization duringtheir ascent (Kagami et al., 1991; von Blanckenburget al., 1992). Cenozoic volcanic rocks are rare in theAlps; however, the Oligocene calc-alkaline, shosho-nitic, and ultra-potassic andesites in the northwest-ern Italian Alps are comparable in age andchemistry to the peri-Adriatic granitoids (Venturelliet al., 1984). Post-collisional volcanism was eitherhighly limited in space and time in the Alps, or itsproducts have been rapidly removed by erosionassociated with significant crustal and surface upliftthroughout the late Cenozoic (Gebrande et al., 2006,and references therein). Modeling of post-collisionalmagmatism, deformation, and exhumation of high-Procks in the Alps suggests that slab breakoff was thedriving mechanism for the late Cenozoic tectonicevolution of this collision zone (von Blanckenburgand Davies, 1995; Davies and von Blanckenburg,1995). The inferred slab breakoff is interpreted tohave occurred at depths around 100 km or more, toodeep to allow melting of asthenosphere (Davies andvon Blanckenburg, 1995). This lack of astheno-spheric melting is likely to have inhibited the devel-opment of late-stage alkaline volcanism in the Alps.

Farther east in the Dinaride-Hellenide segmentof the Alpine orogenic belt, post-collisionalmagmatic associations occur in the peri-AdriaticSava-Vardar zone and include Eocene-Oligocenegranitoids, Oligocene shoshonitic and calc-alkaline


FIG. 9. Conceptual geodynamic template for post-collisional tectonic and magmatic evolution of alpine-style oro-genic belts through time. A. Slab breakoff stage. B. Crustal thickening and anatectic melting stage. C. Lithosphericdelamination, tectonic extension, and crustal exhumation stage D. Asthenospheric decompressional melting and basal-tic volcanism, back-arc extension (if active subduction occurs), and block faulting stage. Spatial and temporal relationsof faulting and magmatism, geometry and kinematics of extensional deformation, nature of mantle unrooting, and dipangle of the subducting slab (if active subduction exists) may vary in different collision zones; not all stages depictedhere are fully developed in each orogenic belt. See text for discussion and for other factors that control the mode andnature of post-collisional tectonic and magmatic processes in orogenic belts.


volcanic rocks and lamprophyres, and LateOligocene (Chattian) calc-alkaline volcanic rocksand granitoids (Pamic et al., 2002; Prelevic et al.,2005). The oblique collision of Adria with Eurasia(Tsia-Moesia continental blocks) around 45 Ma wasfollowed by the partial subduction of the continentalmargin of Adria beneath Eurasia and the subse-quent breakoff of the leading edge of the subductingTethyan oceanic lithosphere, as inferred fromseismic tomographic studies in the region. TheOligocene granitoid plutons and shoshonitic to calc-alkaline volcanic rocks in the Dinarides show Sr,Nd, and O isotopic values that indicate that theirprimary basaltic magmas were derived from partialmelting of subduction-modified subcontinentallithospheric mantle and its asthenosphere, and thatthese magmas were contaminated to various degreesby continental crust (Pamic et al., 2002; Prelevic etal., 2005). This lithospheric/asthenospheric mantlemelting was also facilitated by slab breakoff, whichcoincided with significant orogen-parallel dextralstrike-slip deformation within the Dinarides as aresult of the oblique collision of Adria. Alkali basaltvolcanism appears to be absent in the post-colli-sional magmatic history of the Dinarides.

In the Carpathian-Pannonian region north of theDinarides, post-collisional magmatism followed thediachronous collision of the Alcapa and Tisia litho-spheric blocks with the southern edge of Eurasiaduring the Early to Middle Miocene (respectively)and produced calc-alkaline to alkaline rocks(Mason et al., 1998; Nemcok et al., 1998; Seghedi etal., 2004). The early stages of magmatism in theCarpathians produced medium- to high-K calc-alkaline and shoshonitic basalts and basaltic andes-ites showing isotopic evidence of crustal assimila-tion (Mason et al., 1998); slightly younger alkalinebasalts exhibit much less crustal contamination.Although the existing geodynamic models vary indetail, they all agree that slab breakoff was respon-sible for the generation of typical calc-alkalinemagmas and that the later stage alkaline basalticvolcanism with an OIB-like asthenosphere sourcewas associated with local extensional tectonics.

Slab breakoff appears to be the most common,major driving force for the early stages of post-colli-sinal magmatism, crustal uplift, and deformation incontinental collision zones (Fig. 9A; Wortel andSpakman, 2000; Kohn and Parkinson, 2002; Clooset al., 2005). The magmatism in this stage is repre-sented by calc-alkaline to transitional (in com-position) products that overlap in age with

compressional tectonics. In the case of post-colli-sional magmatism in western Anatolia, a completespectrum of magmatic products ranges from calc-alkaline volcano-plutonic units associated withcompressional tectonics to alkaline to super-alka-line basalts spatially and temporally associated withextensional faulting. This last stage alkaline volcan-ism that accompanied advanced degrees of litho-spheric-scale tectonic extension in western Anatoliaand in the broader Aegean region was either poorlydeveloped and/or is lacking in other segments of theAlpine orogenic belt. The alkaline volcanism inwestern Anatolia was likely a result of lithosphericdelamination and/or partial convective removal ofthe SCLM (Fig. 9C; Aldanmaz et al., 2000;Altunkaynak and Dilek, 2006). Collision-inducedlithospheric thickening is assumed to have resultedin destabilization of the SCLM and its subsequentremoval by thermal erosion or in delamination andsinking of part of the lithosphere (Houseman et al.,1981; Platt and England, 1994). In either explana-tion, decompressional melting of the upwellingasthenosphere is thought to have produced basalticmagmas, which ascended through the tectonicallyextended and thinned continental crust. This inter-pretation is similar to models developed for post-collisional high-K, medium-alkaline volcanism andshoshonitic volcanism and extension in the northernTibetan Plateau (Turner et al., 1996; Williams et al.,2004) and for calc-alkaline to alkaline basaltic,shoshonitic, and purely asthenospheric basalticmagmatism in the eastern Rhodopes (Marchev et al.,2004).

Some of the outstanding questions regarding thelate-stage, post-collisional magmatism in orogenicbelts are why lithospheric delamination and/or con-vective removal of the SCLM occurs in some orogensbut not in all of them, and what local or regionalgeodynamic processes may trigger these events. Inthe case of western Anatolia and the Rhodope belt,we know that the north-dipping subduction zone atthe Hellenic Trench and associated slab rollbackwere already in operation at the time of inferredlithospheric delamination (Meulenkamp et al.,1998), placing the Aegean Province in a back-arcextensional tectonic setting (Fig. 9C). Initiation ofthe north-dipping subduction zone at the HellenicTrench in the Middle Miocene (~13 Ma) and thepropagation of the North Anatolian transform faultinto the northeastern Aegean region around 7 to 5Ma are two significant, unique tectonic events, andthey may have been responsible for the observed


shifts in the character of the post-collisional magma-tism in western Anatolia and in the Aegean provincesince the Middle to Late Miocene (Pe-Piper andPiper, 2002).

It is apparent that although the nature of post-collisional magmatism in orogenic belts follows acommon pathway, deviations may exist in the flow-chart of this generalized tectonomagmatic template.The observed changes in the spatial and temporaldistribution and evolution of post-collisional mag-matic products in various collision zones within thebroader Alpine-Himalayan orogenic belt are likelyto have resulted from differences in: (1) types andsizes of colliding lithospheric blocks; (2) conver-gence directions and rates between colliding plates;(3) strength of the subducting plate and hence themode of its internal deformation; (4) depth of slabbreakoff; (5) the extent of slab breakoff–generatedasthenospheric window and the magnitude andintensity of related thermal perturbation; (6) natureof the subcontinental lithospheric mantle (hydrousvs. less hydrous, amphibole- vs. phlogopite-bear-ing); (7) thickness of orogenic crust; (8) nature ofmantle unrooting (delamination of mantle litho-sphere vs. convective thinning by thermal erosion ofthe thermal boundary layer); and (9) regional geody-namics and other plate boundary processes at workin the vicinity of the collision zones (Davies and vonBlanckenburg, 1995; Atherton and Ghani, 2002;Bonin, 2004).

Conclusions

The onset of post-collisional magmatism in west-ern Anatolia dates back to the Early to MiddleEocene, long before the Arabia-Eurasia collisionaround 14 Ma. The evolution of this magmatism hasoccurred in discrete pulses throughout the Ceno-zoic, and produced volcanic and plutonic associa-tions with varying ages and chemical compositions.Spatial, temporal, and geochemical relationships ofthese volcano-plutonic associations provide valu-able information on the crustal and mantle evolu-tion, thermal regimes, and magma transportmechanisms beneath a young orogenic belt in analpine-style collision zone.

The first products of the post-collisional magma-tism in western Anatolia are represented by nearlyE-W–trending Eocene granitoid plutons intrudedinto ophiolitic and blueschist rocks along the Izmir-Ankara-Erzircan suture zone and into the crystallinebasement of the Sakarya continent. Some of these

plutons are spatially associated with volcanic coun-terparts, and both plutonic and volcanic rocks showmoderately evolved compositions enriched inincompatible elements reminiscent of subductionzone– influenced subalkaline magmas.

The next pulse of magmatism is represented bywidespread Oligo-Miocene volcanic and plutonicrocks in northwest Anatolia and on the adjacentAegean islands that also display characteristicgeochemical signatures of subduction-zone mag-mas. However, the primary magmas of the Eoceneand Oligo-Miocene volcano-plutonic units werederived from a subduction-influenced, enrichedlithospheric mantle source beneath the suture zoneand the Sakarya continent, rather than from a man-tle wedge above an active subduction zone. Assimi-lation, fractional crystallization, and crustalcontamination processes played a major role in theevolution of these magmas. Slab breakoff–inducedasthenospheric upwelling was responsible forpartial melting of the previously subduction-enriched mantle lithosphere following continentalcollision in the Early Eocene. The Eocene andOligo-Miocene magmatism was mostly contempora-neous with collision-driven regional compression inwestern Anatolia.

Middle Miocene (16–14 Ma) magmatism in theregion is represented by mildly alkaline, bimodalvolcanic rocks that show diminishing effects ofcrustal contamination during their evolution, incontrast to the Eocene and Oligo-Miocene volcano-plutonic units. The geochemical features of theseMiddle Miocene rocks suggest that both lithosphericand asthenospheric mantle melts were involved dur-ing their evolution. These mildly alkaline volcanicrocks mark, therefore, a transitional period inthe post-collisional magmatic history of westernAnatolia that coincides with the region-wide exten-sional tectonic deformation. Introduction ofasthenospheric melts to the source of this transi-tional volcanism in the Middle Miocene is likely tohave resulted from asthenospheric upwelling causedby partial delamination of the lithospheric rootbeneath the western Anatolian orogenic belt.

Late Miocene to Quaternary magmatism in west-ern Anatolia is represented by alkaline to super-alkaline volcanic rocks showing OIB-like geochem-ical features. The lack of isotopic evidence forsubduction zone influence or significant crustalcontamination in the evolution of these alkalinerocks indicates that their magmas were derived fromdecompressional melting of the asthenospheric


mantle beneath significantly attenuated continentalcrust in the Aegean extensional province. Litho-spheric-scale extensional normal fault systemsprovided natural plumbing systems to deliverasthenospheric melts to the surface with limitedcontribution from the lithospheric mantle.

The time-progressive evolution of post-colli-sional magmatism in western Anatolia was affectedby slab breakoff, lithospheric delamination, andasthenospheric upwelling and melting, coincidingwith regional compressional and extensional tec-tonic deformation, in that order. This conceptualflowchart of the documented tectonic and magmaticevents, which developed in response to the post-collisional mantle dynamics in the Aegean region, ischaracteristic of many alpine-type collisionalorogenic belts and provides a realistic template fortheir post-collisional magmatic evolution.

Acknowledgments

Our work on Cenozoic magmatism in westernTurkey has been supported by research grants fromthe Scientific and Technical Research Council ofTurkey (TUBITAK-CAYDAG-101Y006 to S.A.) andthe Miami University Hampton Funds (to Y. D.). Ourideas on various aspects of the post-collisional tec-tonic and magmatic evolution of western Anatoliahave been influenced by discussions with E.Bozkurt, C. Genç, C. Helvaci, Z. Karacik, A. Okay,A. Polat, G. Seyitoglu, and Y. Yilmaz; we extend oursincere thanks to these colleagues for sharing theirdata and knowledge with us. Thorough reviews byKendall Hauer, Damian Nance, and Paul T. Robin-son helped us improve the paper.

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Cenozoic Crustal Evolution and Mantle Dynamics of Post-Collisional Magmatism in Western AnatoliaYildirim Dilek1and Safak AltunkaynakIntroductionCenozoic Magmatism and Its Products in Western AnatoliaGeochemistry and PetrogenesisGeochemistryFig. 3. Total alkali vs. SiO2 classification diagram showing alkaline-subalkaline subdivisions, according to Irvine and Baragar (1971; IB) and Kuno (1966; K).Table 1. Representative Chemical Analyses of the Eocene to Quaternary Magmatic Rocks from Western AnatoliaFig. 6. Chondrite-normalized REE patterns for western Anatolian volcanic and plutonic rocks. Chondrite normaliz ing values are from Boynton (1984).

Petrogenesis and Geodynamic ImplicationsFig. 7. eNd(t) vs. 87Sr/86Sr(t) diagram for western Anatolian volcanic and plutonic rocks. The fields for asthenospheric and lithospheric mantle melting are from Wilson (1989) and Davies and von Blackenburg (1995), and the references therein.

Geodynamic Template for Post-collisional Magmatism in Orogenic BeltsFig. 9. Conceptual geodynamic template for post-collisional tectonic and magmatic evolution of alpine-style oro genic belts through time. A. Slab breakoff stage. B. Crustal thickening and anatectic melting stage. C. Lithospheric delamination,...

ConclusionsAcknowledgmentsREFERENCESFig. 1. Tectonic map of the eastern Mediterranean, showing plate boundaries and major tectonic units in the region (modified from Altunkaynak and Dilek, 2006).Fig. 2. Simplified geological map of western Anatolia, showing the distribution of post-collisional volcanic and plu tonic rocks ranging in age from the Eocene to Plio-Quaternary, major tectonic blocks (Sakarya continent, Anatolide belt, Taur...Fig. 4. K2O vs. SiO2 diagram showing the distribution of post-collisional volcanic and plutonic rocks in western Anatolia according to the classification scheme of Peccerillo and Taylor (1976).Fig. 5. N-MORB-normalized multi-element patterns for western Anatolian volcanic and plutonic rocks. N-MORB nor malizing values are from Sun and McDonough (1989).Fig. 8. 87Sr/86Sr vs. age diagram for western Anatolian volcanic and plutonic rocks, depicting temporal changes in their degree of crustal contamination. See text for discussion.

cenozoic crustal evolution and mantle dynamics of post ...€¦ · 4 dilek and altunkaynak fig.2....

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