Lutetian arc-type magmatism along the southern Eurasian margin: New UPb LA-ICPMS and whole-rock...

ORIGINAL PAPER

Lutetian arc-type magmatism along the southern Eurasianmargin: New U-Pb LA-ICPMS and whole-rock geochemicaldata from Marmara Island, NW Turkey

P. Ayda Ustaömer & Timur Ustaömer & Alan S. Collins &

Jörg Reischpeitsch

Received: 25 March 2008 /Accepted: 30 March 2009 /Published online: 9 April 2009# Springer-Verlag 2009

Abstract The rocks of Turkey, Greece and Syria preserveevidence for the destruction of Tethys, the construction ofmuch of the continental crust of the region and theformation of the Tauride orogenic belt. These eventsoccurred between the Late Cretaceous and Miocene, butthe detailed evolution of the southern Eurasian marginduring this period of progressive continental accretion islargely unknown. Marmara Island is a basement high lyingat a key location in the Cenozoic Turkish tectonic collage,with a Palaeogene suture zone to the south and a deepEocene sedimentary basin to the north. North-dippingmetamorphic thrust sheets make up the island and areinterlayered with a major metagranitoid intrusion. We havedated the intrusion by Laser Ablation ICP-MS analysis of Uand Pb isotopes on zircon separates to 47.6±2 Ma. We also

performed major- and trace-elemental geochemical analysisof 16 samples of the intrusion that revealed that theintrusion is a calc-alkaline, metaluminous granitoid, markedby Nb depletion relative to LREE and LIL-elementenrichment when compared to ocean ridge granite (ORG).We interpret the metagranitoid sill as a member of a mid-Eocene magmatic arc, forming a 30 km wide and more than200 km long arcuate belt in NW Turkey that post-datessuturing along the İzmir-Ankara-Erzincan Suture zone. Thearc magmatism was emplaced at the early stages ofmountain building, related to collision of Eurasia with theMenderes-Taurus Platform in early Eocene times. Orogen-esis and magmatism loaded the crust to the north creatingcoeval upward-deepening marine basins partially filled byvolcanoclastic sediments.

Introduction

Understanding of the growth of mountain belts requiresrobust knowledge about the timing of tectono-thermalevents, kinematic analysis and tectonic setting of formation.

Marmara Island, the largest island in the Marmara Sea, isa metamorphic basement high at the edge of S MarmaraShelf, bordered by a deep marine basin (i.e. the Tekirdağtrough) in the north, which is dissected by the dextral NorthAnatolian Transform Fault (Okay et al. 1999; Seeber et al.2004). Marmara Island is located at a critical position innorthwest Turkey, where several tectono-stratigraphic ter-ranes are juxtaposed (Fig. 1). Several undated meta-sedimentary and meta-volcanic units are exposed in theisland, forming N-dipping thrust sheets. These units havebeen assigned to the Triassic Karakaya Complex (Okay2000) or have been interpreted as representing a passivemargin trench collision zone that was subsequently trans-

Miner Petrol (2009) 96:177–196DOI 10.1007/s00710-009-0051-8

Editorial handling: A. Möller

P. A. Ustaömer (*)Doğa Bilimleri Araştırma Merkezi, Yıldız Teknik Üniversitesi,34349 Beşiktaş-Istanbul, Turkeye-mail: [email protected]

T. UstaömerMühendislik Fakültesi, Jeoloji Mühendisliği Bölümü,İstanbul Üniversitesi,34850 Avcılar-Istanbul, Turkeye-mail: [email protected]

A. S. CollinsTectonics and Resources Exploration (TREX),School of Earth and Environmental Sciences,University of Adelaide,Adelaide, SA 5005, Australiae-mail: [email protected]

J. ReischpeitschDepartment of Geology, University of Hamburg,Hamburg, Germany

formed into an Andean-type arc setting before Permiantimes (Aksoy 1999).

A WSW-ENE trending, 4 km wide and 20 km long, sill-like intrusion forms a central spine of hills crossing thecentral part of the island (Fig. 2). This intrusion is highlysheared and contains both a N-dipping foliation and astretching lineation, trending parallel to the border, betweenthe intrusion and adjacent units. Owing to its deformationand metamorphic state, various ages were proposed for theigneous intrusion. The suggested ages range from Pre-Permian (Caledonian?; Aksoy 1996) to Late Palaeozoic(MTA 2004, 1/500.000 İstanbul sheet) and to Oligo-Miocene (Okay and Satır 2000). If correct, each of theseages has important tectonic implications concerning thetectonic evolution of NW Turkey during the Cadomian,Caledonian, Hercynian or Alpine deformation periods.

In this paper, we present new radiometric age data andgeochemical evidence for the timing and petrogenesis ofthe Marmara Island igneous intrusion. We show that themagmatism is of arc-type and formed in early mid-Eocene,shortly after the collision of the Sakarya Continent with theMenderes-Tauride Platform. A number of plutons were

emplaced into the crust in early to mid-Eocene and fedsurface eruptions, which generated a narrow and arcuateigneous belt along the suture zones in NW Turkey.

Regional geology

Marmara Island is surrounded by several tectono-stratigraphic terranes of continental and oceanic origin(Fig. 1). The most northerly of these terranes is the IstrancaMassif, which is a Palaeozoic-Mid Jurassic continental unitthat records earlier Late Palaeozoic (Hercynian) deforma-tion and amphibolite-facies metamorphism and subsequent(Triassic) crustal extension and basin formation (Ustaömerand Robertson 1993; Okay et al. 2001). This basin wasclosed by Late Jurassic times when the whole terraneunderwent a second metamorphism in greenschist facies(Okay et al. 2001). The Armutlu Peninsula, along strikefurther to the east, comprises two tectonically assembledcontinental metamorphic units (the Armutlu and İznikMetamorphics) and an ophiolitic suture zone, termed theIntra-Pontide Suture (Şengör and Yılmaz 1981; Göncüoğlu

Fig. 1 Tectonic map of the Ae-gean region showing the locationof the study area and the suturezones discussed in the paper. Datasources are: Şengör and Yılmaz1981; Brown and Robertson2000; Okay 1989; Okay andSatır 2006; Okay et al. 2001;Ustaömer and Robertson 1997;Marchev et al. 2005

178 P.A. Ustaömer et al.

and Erendil 1990; Yılmaz et al. 1995; Robertson andUstaömer 2004). The Armutlu Metamorphics comprise anamphibolite facies high-grade basement, transgressivelyoverlain by mixed clastic-carbonate and volcanogenic units,metamorphosed to greenschist facies. The volcanogenicrocks in the Armutlu Metamorphics are of MORB-typewith little subduction influence (Robertson and Ustaömer2004; Ustaömer and Robertson 2005; Yiğitbaş et al. 2004).The age of the Armutlu Metamorphics is unknown; withboth Precambrian and Palaeozoic ages previously proposed(Göncüoğlu and Erendil 1990; Göncüoğlu et al. 1992;Yılmaz et al. 1995; Robertson and Ustaömer 2004;Ustaömer and Robertson 2005; Yiğitbaş et al. 2004).Recent age data have shown that late Proterozoic (ca570 Ma) and Ordovician granitoids (ca 460 Ma) occurwithin the basement (Okay et al. 2008a). The İznikMetamorphics are in tectonic contact with the ArmutluMetamorphics and comprise a basement metavolcanicsuccession with a rift-like geochemistry (Robertson andUstaömer 2004). This is unconformably overlain by aTriassic-Late Cretaceous passive margin succession that isoverridden by an ophiolitic melange.

A third tectonic terrane occupies a large area to the S ofthe Marmara Sea and is termed the Sakarya Continent(Şengör and Yılmaz 1981) or Sakarya Zone (Okay 1989;Fig. 1). The pre-Jurassic basement of the Sakarya Zone ismade up of a chaotic rock assemblage, called the KarakayaComplex, interpreted as a subduction-accretion complex ofTriassic age related to northward subduction of Palae-otethys (Ustaömer and Robertson 1993; Pickett andRobertson 1996, 2004; Okay 2000). Several tectonic unitsare distinguished within the Karakaya Complex, the mostextensive of which is the Nilüfer Unit, consisting ofgreenschist facies metabasic and metasedimentary rocks.Geochemistry of the metabasic rocks indicates a seamountor LIP (Large Igneous Province) origin for the Nilüfer Unit(Pickett and Robertson 1996; Genç 1998; Okay 2000). TheKarakaya Complex is transgressed by Lower Jurassic toLate Cretaceous sedimentary rocks that are interpreted as anunmetamorphosed passive margin succession facing southto Neotethys (Yılmaz et al. 1997). The Sakarya Zone isbound by the İzmir-Ankara-Erzincan Suture Zone to thesouth and by the Intra-Pontide Suture to the north (Fig. 1).The İzmir-Ankara-Erzincan Suture was closed in

Fig. 2 Geological map of the Marmara Island (modified after Aksoy 1995)

Lutetian arc-type magmatism along the southern Eurasian margin 179

Palaeocene-Eocene times, leading to collision of theMenderes-Taurides continent (i.e. Menderes Massif) and theSakarya Zone (Şengör and Yılmaz 1981; Collins andRobertson 1998; Okay et al. 1996, 1998; Harris et al. 1994).The Intra-Pontide Suture Zone, on the other hand, had alreadyclosed in the Late Cretaceous, leading to the amalgamation ofthe Sakarya Continent with Eurasia (Şengör and Yılmaz 1981;Yılmaz et al. 1995; Göncüoğlu et al. 1992; Robertson andUstaömer 2004; Ustaömer and Robertson 2005).

Marmara Island is isolated in the Marmara Sea, which issurrounded by a complex tectonic mosaic of severaltectono-stratigraphic terranes and fault zones. Because ofthis, there is no direct evidence as to which terrane themetamorphic units of Marmara Island belong. A direct tiewith the İstanbul Fragment to the NE can be ruled out asPalaeozoic to younger units of the İstanbul Fragment do notshow any metamorphism (Abdüsselamoğlu 1977; Şengörand Yılmaz 1981; Şengör et al. 1984; Ustaömer andRobertson 1993). A connection with the other unitsdescribed above is possible when the metamorphic historyof the rock units is considered; however, there are importantstratigraphic differences between those terranes. Okay andSatır (2000) and Beccaletto and Jenny (2004) havesuggested that a suture between the Sakarya Zone and theRhodope Massif passes to the north of the Marmara Island,leaving Marmara Island at the northern edge of the SakaryaZone. It is yet not possible to constrain the provenance ofthe metamorphic units of the Marmara Island.

Tectono-stratigraphy

Three N-dipping thrust sheets are present in MarmaraIsland (Fig. 2). The lower thrust sheet is made up of ahigh-grade metamorphic unit (the Gündoğdu Complex ofAksoy 1995), composed of pelitic and psammitic schistsand marbles. The pelitic schists have a mineral assemblageof staurolite+garnet+kyanite+quartz+albite+biotite andsuggest metamorphism at amphibolite facies conditions(Tanyolu 1979; Aksoy 1995). This unit is over thrust by ametabasite unit (Erdek Complex; Aksoy 1995). Bothmeta-basic lavas and tuffaceous sedimentary rocks arepresent in this unit, together with marble lenses andtectonic blocks of serpentinite. Pelitic parts of this unithave a similar mineral assemblage to the meta-pelites ofthe underlying Gündoğdu Complex, which suggests nometamorphic break is present between the Erdek andGündoğdu Complexes. The Erdek Complex is overlain bya thick (ca. 1.5 km) marble unit (the Marmara Marble).The contact between the Erdek Complex and the overlyingMarmara Marble has been interpreted either as conform-able (Yüzer 1971; Tanyolu 1979) or as an unconformity(Aksoy 1995). The contact is cut by the metagranitoid

intrusion in the westerly areas, whereas in the eastern partof the island a transition occurs between the two units.

The Marmara Marble is overlain by the SaraylarComplex (Aksoy 1995). This is a chaotic unit, metamor-phosed to greenschist facies conditions. The matrix of theunit is represented by meta-pelitic-psammitic sedimentswith occasional interbedded layers of meta-carbonates andmetacherts. The blocks in this unit are dominantly marbles.Gabbro, metabasite and ultramafic blocks are also present(Aksoy 1995).

İlyasdağ metagranitoid

The İlyasdağ metagranitoid is a WSW-ENE trending sill-like intrusion that cuts the Marmara Marble and the ErdekComplex. It can be traced across the island (Fig. 2). At thepresent level of exposure its N-S width is nearly 4 km butthins down to 1.5 km in the central part of the island. Theintrusion is dark grey coloured on fresh surfaces. It is amedium to coarse grained intrusion and is cut by aplitic andpegmatitic veins. The grain size is reduced towards themargins. Feldspar, quartz, amphibole and biotite are themain minerals present in the intrusion. Screens of host rock(amphibolite) form narrow and semi-continuous zonesparallel to the general strike of the intrusion (Fig. 2).

A number of aplitic, pegmatitic and granitoid veins anddykes, related to intrusion of the İlyasdağ metagranitoid arefound in the nearby Marmara Marble and Erdek Complex.Evidence for contact metamorphism was determined locallyin the neighbouring metacarbonates (Tanyolu 1979; Aksoy1995). The contact metamorphic mineral assemblageindicates a metamorphism at albite-epidote hornfels(quartz+actinolite+epidote+sphene) and hornblende-hornfels (hornblende+epidote+plagioclase+quartz+garnet)facies (Tanyolu 1979).

The İlyasdağ metagranitoid was previously termedgneiss (Ketin 1946), orthogneiss (Yüzer 1971), granodio-ritic orthogneiss (Tanyolu 1979) and İlyasdağ Metagrano-diorite (Aksoy 1995). The metagranitoid exhibits a weaklydeveloped mylonitic foliation and a stretching lineation. Themylonitic foliation appears to have formed by the parallelalignment of micas and amphiboles on the foliation planes.The stretching lineation is marked by elongation of quartz andneedle-shaped amphibole crystals. The intrusion comprisesoccasional deformed mafic mineral enclaves (MME) andxenoliths of the country rocks, which were described andstudied by Aksoy (1995). The MMEs are parallel to foliationplanes and are elliptic in shape. The long axes of the MMEreach up to 1 m. The deformation amount was calculatedfrom the shape analysis of MME as 62% elongation (parallelto the mean strike; i.e. WSW-ENE) and 38% shortening(perpendicular to the foliation plane, i.e. N-S) (Aksoy 1995).


According to Aksoy (1995), the foliation and lineationof the metagranodiorite is at the same attitude as those inthe adjacent metamorphic units in the Marmara Island. Hetherefore thinks that all the units were deformed andmetamorphosed together at the same stage.

Petrography of the metagranitoid

The main mineral phases in the metagranitoid are quartz+plagioclase (andesine)+alkali feldspar (orthoclase and mi-crocline)+amphibole (hornblende)+biotite, accompanied bytitanite, zircon, apatite and some opaque minerals in theaccessory phases. A secondary mineral assemblage con-sisting of epidote ± chlorite ± actinolite ± muscovite isdeveloped as a result of hydrothermal alteration.

The igneous fabric of the medium to coarse grainedİlyasdağ metagranitoid has largely been replaced by a brittle-ductile deformation resulting in grain-size reduction, recrys-tallization and development of a pervasive mylonitic foliation.The most prominent feature of the cataclastic structure is thegrain size reduction, recrystallisation and development of amylonitic foliation which was observed in all thin sectionsexamined. In general, the mafic phases (biotite and amphib-oles) envelope the felsic mineral rich zones and creates ananastomosing foliation. A secondary shear band foliation isalso observed locally, forming an SC-structure.

Both plagioclase and alkali feldspars suffered brittledeformation and their grain sizes were reduced consider-ably. Such anhedral-subhedral feldspar porphyroclasts oraugens exhibit deformation lamellae and pericline twins.Feldspar shows core- and -mantle structures. Stronglyzoned plagioclase minerals are partly replaced by secondaryepidote group minerals and chlorites. Some feldsparcrystals have sericitized cores.

Quartz underwent dislocation creep that resulted newlyformed small recrystallized grains, the latter showingoccasional ribbon structure. Larger quartz grains in suchaggregates show undulose extinction and are composed ofelongate subgrains. Grain boundary migration is the princi-ple deformation mechanism of quartz in the metagranitoid.

Biotites are brown-greenish brown and exhibit plasticdeformation into elongate lenses. Such coarse grainedbiotites are seen to have been replaced with smallerneocrystallised grains of biotite, muscovite, titanite, chlo-rite, epidote and opaques. Kinking along, and tight foldingoutside the foliation planes are common microstructuresexhibited by biotite. In several sections, biotite shows afish-eye fabric, preserved along shear planes.

Amphiboles are also affected by grain size reduction as aresult of brittle deformation. Both euhedral to subhedralamphibole grains occur and show pale to dark greenpleochroism. Poikiloblastic amphiboles contain biotiteinclusions. Fine grained aggregates of biotite also mantles

the euhedral grains locally. The amphiboles break down toan assemblage of actinolite, chlorite and epidote.

Despite these deformation structures dominating the rockfabric, the relict igneous fabrics encountered at individualminerals or mineral groups record evidence for magmareplenishment or mixing processes. These include oscilla-tory zoning in plagioclase, poikilitic feldspars containingamphibole and plagioclase inclusions, and presence ofapatite needles in feldspars. Perthitic fabrics are encoun-tered occasionally in alkali feldspar porphyroclasts. Titaniteforms euhedral and subhedral grains. Euhedral zircons arecommonly present as small inclusions in plagioclase.

Analytical methods

Geochronology

Zircons were separated by conventional methods, then werehand-picked and mounted in epoxy resin. Zircon mounts wereimaged by cathodoluminescence (CL), using standard imagingtechniques on a Phillips XL20 SEM with attached Gatan CL.

U-Pb analysis of individual zircons was conducted usingthe laser inductively coupled plasma mass spectrometer(LA-ICPMS) at the University of Adelaide. Equipment andoperating conditions for zircon analysis are identical tothose reported by Payne et al. (2006). A spot size of 30 μmand repetition rate of 5 Hz was used for U-Pb dataacquisition, producing a laser power density of approxi-mately 8 J/cm−2. Zircon ages were calculated using theGEMOC GJ-1 zircon standard to correct for U-Pbfractionation (TIMS normalisation data 207Pb/206Pb=608.3 Ma, 206Pb/238U=600.7 Ma and 207Pb/235U=602.2 Ma—Jackson et al. (2004)), and the GLITTERsoftware for data reduction (Jackson et al. 2004). Over theduration of this study the reported average normalised agesfor GJ-1 are 607.9±3.6, 600.7±1.0 and 602.4±0.8 Ma forthe 207Pb/206Pb, 206Pb/238U and 207Pb/235U ratios, respec-tively (n=209). Accuracy was monitored by repeat analysesof the Sri Lankan in-house internal standard (BJWP-1,207Pb/206Pb=720.9±1.8 Ma, 206Pb/238U=720.4±0.5 Maand 207Pb/235U=720.5±0.6 Ma, unpublished MassachusettsInstitute of Technology TIMS data). Over the duration ofthis study the reported average ages for BJWP-1 are 744±22, 721.4±6.6 and 727.1±6.1 Ma for the 207Pb/206Pb,206Pb/238U and 207Pb/235U ratios, respectively (n=7).

U-Pb LA ICP-MS Zircon ages

The analysed zircon grains ranged in 206Pb/238U age from56.7±0.8 Ma to 46.1±0.7 Ma (Table 1, Fig. 3). Sixanalyses are older than 50 Ma and are considered to be


Table 1 Quadrupole laser ablation ICP-MS U/Pb analysis of zircons

Ages (Ma)

Analysis Pb206/U238 ±(1) Pb207/U235 ±(1) Pb207/Pb206 ±(1) Pb207/Pb206 ±(1) Pb206/U238 ±(1)

M1Q102 0.00761 0.00013 0.05554 0.00199 0.05297 0.00194 327.3 81.1 48.8 0.8

M1Q106 0.00748 0.00012 0.0504 0.00148 0.04888 0.00144 142.3 67.7 48.0 0.7

M1Q107 0.00741 0.00012 0.05327 0.00182 0.05215 0.0018 291.8 77.0 47.6 0.8

M1Q109 0.00718 0.00011 0.04649 0.00133 0.04693 0.00133 45.2 66.6 46.2 0.7

M1Q110 0.00747 0.00011 0.05264 0.00141 0.05115 0.00136 247.4 60.0 47.9 0.7

M1Q111 0.00741 0.00013 0.0507 0.00195 0.04962 0.00194 177.3 88.8 47.6 0.8

M1Q113 0.00743 0.00012 0.05196 0.00151 0.05073 0.00147 228.8 65.7 47.7 0.7

M1Q114 0.00855 0.00012 0.06126 0.00135 0.05197 0.0011 284.1 47.8 54.9 0.8

M1Q208 0.00737 0.00011 0.0529 0.0015 0.05204 0.00148 287.2 63.8 47.4 0.7

M1Q210 0.00745 0.00012 0.05086 0.00157 0.04953 0.00153 173.0 70.6 47.8 0.8

M1Q305 0.00728 0.00012 0.05421 0.00175 0.05401 0.00176 371.5 71.6 46.8 0.8

M1Q312 0.00752 0.00012 0.05527 0.00155 0.0533 0.00149 341.5 62.0 48.3 0.7

M1Q316b 0.00733 0.00012 0.06748 0.00203 0.06681 0.00204 831.7 62.4 47.1 0.8

M1Q318 0.00756 0.00014 0.05075 0.00231 0.04869 0.00227 132.7 106.3 48.6 0.9

M1Q403b 0.00742 0.00012 0.04895 0.00168 0.04787 0.00166 91.7 81.1 47.6 0.8

M1Q407 0.0074 0.00011 0.05247 0.00146 0.05142 0.00143 259.7 62.5 47.5 0.7

M1Q409 0.00732 0.00011 0.04884 0.00121 0.04838 0.00118 118.0 56.5 47.0 0.7

M1Q410 0.00744 0.00012 0.05284 0.00161 0.05147 0.00158 262.1 68.8 47.8 0.8

M1Q411 0.00764 0.00011 0.05465 0.00145 0.05188 0.00136 279.9 58.9 49.1 0.7

M1Q412 0.00751 0.00013 0.07896 0.00263 0.07627 0.00262 1,102.2 67.3 48.2 0.8

M1Q414a 0.00879 0.00013 0.06263 0.00154 0.05171 0.00125 272.7 54.3 56.4 0.8

M1Q415a 0.00735 0.00011 0.05776 0.00161 0.05697 0.00159 489.9 61.0 47.2 0.7

M1Q418 0.00747 0.00011 0.05057 0.00133 0.04909 0.00128 152.3 60.0 48.0 0.7

M1Q508 0.00755 0.00011 0.05085 0.00125 0.04885 0.00119 140.8 56.0 48.5 0.7

M1Q510 0.00768 0.00012 0.05842 0.00178 0.05514 0.0017 417.8 66.8 49.3 0.8

M1Q512 0.00736 0.00011 0.05108 0.00137 0.05038 0.00133 212.4 60.2 47.2 0.7

M1Q516a 0.00745 0.00011 0.05162 0.00138 0.05027 0.00133 207.6 60.4 47.8 0.7

M1Q518 0.00747 0.00011 0.04924 0.0014 0.04784 0.00135 90.2 66.7 48.0 0.7

M1Q606 0.00724 0.0001 0.04785 0.00121 0.04796 0.0012 96.2 59.2 46.5 0.7

M1Q607 0.00745 0.00012 0.05197 0.00164 0.05064 0.0016 224.6 71.4 47.8 0.8

M1Q608 0.00724 0.0001 0.04775 0.0011 0.04787 0.00107 91.6 53.0 46.5 0.7

M1Q609b 0.00736 0.00011 0.0502 0.00136 0.04949 0.00132 171.1 61.2 47.3 0.7

M1Q610 0.00733 0.00011 0.04844 0.0014 0.04795 0.00137 95.7 67.6 47.1 0.7

M1Q611 0.00779 0.00011 0.05407 0.00106 0.05034 0.00093 210.6 42.4 50.0 0.7

M1Q612 0.0074 0.00012 0.05363 0.00165 0.0526 0.00162 311.4 68.6 47.5 0.8

M1Q613a 0.00746 0.00013 0.05511 0.00201 0.05362 0.00199 354.9 81.5 47.9 0.8

M1Q613b 0.00756 0.00012 0.05395 0.0017 0.05177 0.00164 275.1 70.8 48.6 0.8

M1Q615a 0.00743 0.00011 0.05029 0.00112 0.04912 0.00105 153.6 49.4 47.7 0.7

M1Q615b 0.00735 0.00012 0.04998 0.0015 0.04933 0.00147 163.6 68.3 47.2 0.8

M1Q616 0.00738 0.0001 0.05114 0.0013 0.05021 0.00129 204.9 58.3 47.4 0.7

M1Q617 0.00883 0.00013 0.0636 0.00135 0.05227 0.00105 297.4 45.1 56.7 0.8

M1Q618 0.00718 0.00011 0.04857 0.00151 0.04905 0.00155 150.1 72.3 46.1 0.7

M1Q619a 0.00788 0.0001 0.05841 0.00095 0.05377 0.00079 361.1 32.8 50.6 0.7

M1Q619b 0.00731 0.00011 0.05008 0.00144 0.04971 0.00143 181.3 65.5 46.9 0.7

M1Q620 0.00797 0.00013 0.05201 0.00194 0.04736 0.0018 66.7 88.6 51.1 0.8

M1Q621 0.00795 0.00012 0.05438 0.00161 0.04962 0.00149 177.2 68.6 51.0 0.8

(1) Absolute errors, 1 sigma


mixed analyses that sampled different age domains withinthe zircon grain. These analyses are thought to consistpartially of xenocrystic zircon incorporated into the magmaduring its ascent through the crust. A tight age-group of 39analyses yield a precise 206Pb/238U age of 47.6±0.2 Ma(95% confidence, MSWD=0.99). This concordant popula-tion, that correlates with euhedral, oscillatory-zoned zircon,is interpreted as dating crystallisation of the İlyasdağmetagranitoid (Fig. 3).

As noted above, the age of the İlyasdağ metagranitoidwas previously thought to be Late Palaeozoic or olderbased on its deformation state. However, our new age datahas shown that the İlyasdağ metagranitoid intrusion is infact an early mid-Eocene pluton. The age of the brittle-ductile deformation that the pluton exhibits, therefore, alsooccurred in the syn- to post mid-Eocene period.

Magmatism of Eocene age is known in NWAnatolia andNE Greece (Serbo-Macedonian and Rhodope Massifs). Thedistribution of granitoids, volcanic rocks and sedimentaryunits of Eocene age in NW Turkey, together with theophiolitic thrust sheets of the Tethyan suture zones areshown on Fig. 4. The İzmir-Ankara-Erzincan Suture to thesouth was formed in Late Palaeocene-early Eocene bycomplete closure of N Neotethys (Şengör and Yılmaz 1981;Robertson and Dixon 1984; Collins and Robertson 1998;Okay 2000). The Intra-Pontide Suture to the north, on theother hand, is a Late Cretaceous feature (Yılmaz et al. 1995;Göncüoğlu et al. 1992; Robertson and Ustaömer 2004;Ustaömer and Robertson 2005). The only ocean basin thatknown to have survived Eocene times was South Neo-tethys, which was open until the Miocene and partiallyclosed along the Bitlis Suture Zone by collision of the

Arabian promontory with the E Taurides (Robertson et al.2006). The present Eastern Mediterranean basin is aremnant of South Neotethys in this area, which is activelyclosing by northward subduction along the Hellenic trench(Fig. 1).

Eocene granitoids are found in two separate, E-Wtrending, regions in NW Anatolia. The N-S widths of thenorthern and southern plutons are 25 and 30 km, respec-tively, and the N-S distance between the two groups ofplutons is ~100 km. The southern group (also termed as thesuture zone granitoids; Altunkaynak 2007; Karacık et al.2008) includes several plutons that intrude the ophiolites ofthe İzmir-Ankara-Erzincan Suture zone and the underlyingLate Cretaceous blueschist facies metamorphic rocks(Tavşanlı Zone; Okay 1984, 1989) of the Menderes-Taurus platform. These southerly plutons were dated by40Ar/39Ar (Harris et al. 1994), K/Ar (Delaloye and Bingöl2000) and U/Pb methods (Okay and Satır 2006) to early-middle Eocene (53 to 44 Ma), one being nearly identical inage to the granitoid of the Marmara Island at 47.8±0.4 Ma(the Topuk granodiorite; Harris et al. 1994). Eocenemagmatism further to the south of the blueschist belt isnot known. All the dated post-Triassic granitoids in theMenderes Massif yield Early Miocene emplacement ages(e.g. Ring and Collins 2005).

The northern group of plutons occurs in Armutlu andKapıdağ Peninsulas and in Karabiga, to the north of theIntra-Pontide Suture Zone (termed the Marmara gran-itoids; Altunkaynak 2007; Karacık et al. 2008). There areno known Eocene plutons in the Sakarya Continent,between the southern and northern plutonic belts (Fig. 4).This area is occupied by Oligocene to Miocene intrusions

Fig. 3 U-Pb concordia diagramof zircon analyses from theİlyasdağ tonalite. Insert is atypical cathodoluminescenceimage of an oscillatory zonedzircon from the rock and also aplot of the analyses used tocreate the weighted mean


and volcanics, constructed upon Cretaceous to oldersedimentary-metamorphic units (Okay et al. 2008b). Thenorthern plutons are also associated with Eocene volcanicsthat form a continuous belt from Çanakkale-Biga to theArmutlu Peninsula and further east (Fig. 4). In contrast tothe southerly plutons, the northern plutons have mostlybeen dated using the K/Ar method on mineral separatesand different minerals yielded different ages for individualplutons (Delaloye and Bingöl 2000). Uncertainty, there-fore, exists for the exact age of crystallisation of theseplutons. For instance, two biotite analyses yielded 48.2±1and 35.4±0.8 Ma and one muscovite analysis yielded 34.3±0.9 Ma cooling ages for the Fıstıklı pluton in ArmutluPeninsula (Fig. 4; Delaloye and Bingöl 2000). A similarlywide age range was gathered for the north and southKapıdağ plutons. Two biotite analyses yielded 39.9±0.8and 38.3±0.8 Ma, and one hornblende analysis yielded42.2±1 Ma for the N Kapıdağ pluton whereas 38.2±0.8and 36.1±0.8 Ma ages were determined from two biotite

analyses from the S Kapıdağ pluton (Delaloye and Bingöl2000). Only one biotite analysis was made from theKarabiga pluton that yielded an age of 45.3±0.9 Ma(Delaloye and Bingöl 2000). Recently, Beccaletto et al.(2007) dated xenotime in the Karabiga pluton andobtained a crystallisation age of 52.7±1.9 Ma. Thus, theU-Pb age dating of the İlyasdağ Pluton provided one ofthe most robust age data gathered from the northernplutons.

While the deeper part of the crust was invaded by theseplutons in mid- to late Eocene times, stratigraphic datasuggest that the surface was covered with a shallow to deepsea, as carbonates and clastics of this age are foundthroughout north west Turkey (Saner 1985; Okay and Satır2006; Siyako and Huvaz 2007). Before this, the earlyEocene was probably the time of uplift and erosion due toclosure of Neotethys and subsequent collision of theMenderes and Sakarya continents, since marine sedimentsof this age are scarce in NW Anatolia.

Fig. 4 Simplified geological map of NW Turkey showing distribution of Eocene magmatic rocks and sedimentary basins. Known suture zonesare also shown. Data sources are: Harris et al. 1994; Delaloye and Bingöl 2000; Okay and Satır 2006; Beccaletto et al. 2007


Below, we will present the geochemical properties ofthe İlyasdağ pluton (Table 2). We incorporate publishedgeochemical data from the northern and southern group ofplutons (Harris et al. 1994; Delaloye and Bingöl 2000;Köprübaşı and Aldanmaz 2004) with an aim to show thesimilarities and differences of the İlyasdağ pluton from therest of the Eocene plutons. As we will show, the İlyasdağpluton forms the most basic Eocene pluton in NWAnatolia.

Geochemistry

A total of 16 samples were selected for geochemicalanalysis. Fourteen of these are from the granitoid and twofrom the cross-cutting aplitic veins. The samples wereanalyzed at the University of Adelaide, South Australia.

Whole rock (XRF) chemistry of the granitoid

The analytical method has been described in Ustaömer etal. (2009) and the results are given in Table 2. The graphicsin this section were produced by using the softwareGCDKit (Janousek et al. 2006).

Major elements

SiO2 values of the İlyasdağ metagranitoid samples varybetween 57 to 69%. The cross-cutting aplitic veins,however, have higher silica values (> 71%). The Mgnumber of the granitoid samples varies between 39 and 46but that of the aplitic veins is less than 35.

The İlyasdağ metagranitoid samples plot in the tonalitefield, whereas the aplitic samples plot in the monzogranitefield on the Q’ versus ANOR nomenclature diagram (Fig.5A). Several other nomenclature diagrams (P versus Qdiagram of Debon and Le Fort 1983; R1 versus R2 diagramof De la Roche et al. 1980; TAS diagram, Middlemost1985) yielded a similar result (not shown). The K2Ocontent of the İlyasdağ metatonalite varies from 0.27 % to2 %. This is well illustrated on a SiO2 versus K2O diagram(Peccerillo and Taylor 1976) in which the metatonalitesamples plot in the low- to medium-K fields (Fig. 5B),indicating a tholeiitic to calc-alkaline nature of the melt.The aplitic samples, on the other hand, plot in the mediumto high-K fields on the same diagram. The metatonalite isclassified as a calcic pluton in a SiO2 versus MALI(Modified alkali-lime index: Na2O+K2O-CaO; Frost et al.2001) diagram (Fig. 5C). Two of the aplite samples plot incalc-alkaline and one in alkali-calcic field on the samediagram. The SiO2 versus FeOt/FeOt+MgO diagram ofFrost et al. (2001) is used to measure the iron enrichment ofthe intrusive rocks. The metatonalite and one aplite sample

plot in the magnesian field and two aplite samples plot inferroan field (Fig. 5D). The boundaries of Caledonian(post-collisional) and Cordilleran plutons (magmatic arc)are also shown on these diagrams. The İlyasdağ metatona-lite samples exhibit a similar composition to the Cordilleranplutons (Fig. 5C, D). The Shand index is used todistinguish aluminium saturation level of the samplesstudied (Fig. 6). The metatonalite samples plot in themetaluminous and the aplitic samples plot in the peralumi-nous field. Normative corundum in the aplitic set is,however, less than 0.8, suggesting that the aplites areweakly peraluminous.

There is a good correlation of the major and minoroxides with SiO2. The correlation coefficient for most ofthese oxides is above 0.97, except Na2O which has acorrelation coefficient of 0.68. Scatter of Na2O of the apliticand only a few intrusive samples is interpreted as anindication of mobility of this element during post-crystallisation alteration.

Published geochemical data on Eocene plutons of NWAnatolia are plotted along with the İlyasdağ data on Harkervariation diagrams (Fig. 7). Note that the ages of theseplutons are variable, recording at least 15 Ma of magmaticprocesses that operated in NW Anatolia between 50 to35 Ma. Field and petrographic features of the northernplutons suggest that magma mixing is one of the principlemechanisms for differentiation of these plutons, togetherwith assimilation and fractional crystallisation (Köprübaşıand Aldanmaz 2004). The trends observed on Harkervariation diagrams of Eocene plutons are the result of thesemagmatic processes. Overall, Al2O3, CaO, MgO, FeO*,TiO2 and P2O5 show strong negative correlation withincreasing SiO2, while K2O and Na2O show a positivecorrelation with increasing SiO2 (Fig. 7). The İlyasdağpluton is the most basic pluton in the data set with its lowSiO2 and high MgO content. Some of the Kapıdağ samplesplot off trend, otherwise the Eocene plutons lay on the samedifferentiation trend on major oxide Harker diagrams. Agreat scatter of Na2O of the Eocene plutons (except theİlyasdağ metatonalite) reflects the high mobility of thisoxide during weathering and alteration. Therefore, thenomenclature and variation diagrams involving Na2Ocontent of such plutons should be interpreted with caution.

Trace elements

The İlyasdağ metatonalite is characterised by low Nb, Rb,Th and high V, Sc and Pb contents. Unlike major oxides,the Eocene plutons display different variation trends basedon trace element compositions, indicating differences intheir source compositions and subsequent crystallisationhistory. For instance, the Kapıdağ and Orhaneli plutonsshow first enrichment and depletion whereas the İlyasdağ,


Tab

le2

Geochem

ical

analysisof

theİly

asdağtonalite

Sam

ple

MR1

MR4

MR5

MR6

MR7

MR8

MR10

MR12

MR14

aMR15

MR16

MR18

MR20

MR25

MR27

M1

SiO

262

,59

65,49

63,03

64,96

62,65

57,2

63,32

60,01

62,71

60,61

66,17

65,09

74,7

75,6

62,13

68,53

TiO

20,48

0,41

0,49

0,4

0,43

0,76

0,43

0,63

0,49

0,51

0,38

0,4

0,08

0,04

0,49

0,32

Al 2O3

17,65

17,18

17,62

16,94

17,75

19,53

17,49

18,32

17,54

17,96

16,7

17,39

13,46

13,9

17,99

16,27

Fe 2O3

4,63

4,04

4,56

4,11

4,56

6,97

4,58

5,99

4,66

5,05

3,67

3,99

0,83

0,39

4,91

3,21

MnO

0,15

0,13

0,14

0,13

0,13

0,16

0,12

0,16

0,13

0,14

0,12

0,13

0,04

0,02

0,13

0,12

MgO

1,73

1,53

1,68

1,58

1,99

2,25

1,93

2,26

1,85

2,02

1,32

1,46

0,21

0,06

2,06

1,13

CaO

6,16

5,35

5,93

5,32

6,17

7,82

5,73

7,15

6,4

6,11

4,88

5,23

1,4

0,97

6,44

4,25

Na 2O

4,03

4,06

4,01

3,95

3,82

3,78

3,68

3,87

3,91

3,79

3,96

4,03

3,71

5,28

3,88

4,15

K2O

1,2

1,11

1,2

1,41

1,24

0,27

1,47

0,85

0,85

1,08

1,2

1,4

3,81

2,86

0,94

2

P2O5

0,13

0,11

0,13

0,12

0,14

0,21

0,15

0,18

0,14

0,17

0,11

0,12

0,02

0,01

0,16

0,09

SO3

0,02

0,01

0,01

0,02

0,01

0,01

0,01

0,01

0,01

0,01

0,01

0,01

0,01

0,01

0,01

0,02

LOI

0,88

0,56

0,74

0,81

0,83

1,04

0,9

0,61

0,86

1,07

0,92

0,62

0,34

0,2

0,83

0,44

Total

99,65

99,97

99,55

99,74

99,71

100

99,81

100,04

99,54

98,53

99,44

99,88

98,6

99,32

99,98

100,52

Zr

9990

109

9179

9782

101

8293

97110

4468

9085

Nb

23

22

23

32

33

23

22

11

Y21

1823

1816

3516

2319

2015

199

1821

15

Sr

337

281

336

309

413

362

392

396

368

378

318

347

143

4238

629

2

Rb

3432

3536

375

4124

2736

3942

7660

2750

U2

11

23

12

01

21

12

32

1

Th

64

35

64

61

43

46

1013

46

Pb

1010

1212

157

1611

1111

1316

2828

1127

Ga

1817

1615

1620

1617

1516

1617

1014

1815

Cu

1814

1712

744

719

1315

927

716

159

Zn

6257

6257

5877

5870

5364

5655

153

5945

Ni

32

32

44

44

76

62

11

63

Ba

280

295

295

360

346

198

359

223

224

289

361

340

929

290

218

325

Sc

86

76

810

711

89

56

22

105

Co

6647

5354

3835

4548

5228

2936

4063

436

V79

6281

6492

154

88116

8386

6062

146

8852

Ce

4328

2730

2428

3018

1718

2026

2116

2225

Nd

1410

107

1113

94

97

611

44

95

La

168

76

74

80

34

48

103

38

Cr

11

30

30

25

178

162

25

87


Topuk, Lapseki and Armutlu plutons show depletion of Srwith increasing SiO2 (not shown). Different variations inthe Ba content of the plutons with SiO2 are also evident.The İlyasdağ pluton shows a moderate Ba enrichment,whereas the Kapıdağ pluton shows a strong enrichment inBa, and the Armutlu and Lapseki plutons exhibit strong Badepletion (not shown). Rb enrichment with increasing SiO2

is also strong in the Kapıdağ pluton, moderate in theİlyasdağ pluton, but strongly depleted in the Armutlupluton, although some variations in the data may relate tomobility of this element. Nb is highly depleted in theİlyasdağ pluton in comparison to rest of the Eocene plutons.Nb decreases weakly with increasing SiO2 in the İlyasdağ

pluton but it decreases strongly in all the Armutlu, Kapıdağand Lapseki plutons (not shown).

Differences in source characteristics of these plutons areclearer in compatible-incompatible element ratios diagram.In Th versus Th/Nb diagram (Fig. 8A), the İlyasdağ andKapıdağ-Orhaneli-Topuk-Lapseki plutons form two parallelpositive trends and the Armutlu pluton occupies the spacebetween these two trends. A similar feature is observed inthe Rb versus Rb/Nb, Y/Nb versus Rb/Nb and Rb/Zr versusNb diagrams (Fig. 8B, C, D), implying that the sourcecharacteristics of the İlyasdağ Pluton is different from therest of the plutons. Low Rb (4–49 ppm), Th (1–6.4 ppm),Nb (0.8–2.8 ppm), Rb/Sr (0.01–0.12) and high Y/Nb (6–

Fig. 5 a Q vs ANOR nomenclature diagram; b SiO2 versus K2O diagram (Peccerillo and Taylor 1976); c SiO2 vs MALI (Modified Alkali LimeIndex) diagram (Frost et al. 2001); D) SiO2 vs FeOt/FeOt+MgO diagram (Frost et al. 2001)


13) values of the İlyasdağ pluton distinguish it from theother Eocene plutons of NW Anatolia.

Further information can be gleaned from multi-elementspidergrams. On an ORG- (Ocean Ridge Granites) normal-ized spidergram, both aplitic and tonalitic samples givesimilar overall patterns, but the aplitic samples are richer inLIL- (Large Ion Lithophile) elements and are more depletedin HFS- (High Field Strength) elements compared totonalitic samples (not shown). There is a marked LIL-element enrichment and a Nb depletion relative to LREE(i.e. Ce) in all samples. Nb is depleted 5–10 times morerelative to ORG.

The samples are also plotted on primitive mantlenormalized spidergrams (Fig. 9). The distinction betweenthe aplitic and metatonalitic samples is much clearer in thisdiagram. The aplitic set is enriched in Th, U, Pb, and K anddepleted in Ti, Zr and P relative to the metatonalitic set.

Tectonic discrimination

In the Y versus Nb diagram (Pearce et al. 1984), all thesamples plot in the VAG (volcanic arc granites) and syn-COLG (syn-collisional granites) fields (not shown). The Y

Fig. 6 A/CNK (molar Al2O3/CaO+Na2O+K2O) versus A/NK (molarAl2O3/Na2O+K2O) diagram (Shand 1927). See text for explanation

Fig. 7 Major element Harkervariation diagrams of the Mar-mara Island intrusives. See textfor explanation (Lapseki-Armutlu-Kapıdağ plutons dataare compiled from Köprübaşıand Aldanmaz 2004; Orhaneli-Topuk plutons data are compiledfrom Harris et al. 1994)


Fig. 8 a- Th vs Th/Nb diagramof the NW Anatolian Eocenegranitoids (İlyasdağ andArmutlu, Kapıdağ-Orhaneli-Topuk-Lapseki plutons); b Rbversus Rb/Nb diagram; c Y/Nbversus Rb/Nb diagram; d Rb/Zrvs Nb diagram. See Fig. 7 forthe key and text for explanation

Fig. 9 Primitive Mantle normalised spidergrams of İlyasdağ tonalite and cross-cutting aplitic veins. See text for explanation. Normalising valuesare from Sun and McDonough (1989)


+Nb versus Rb diagram is used to separate between thesetectonic settings. All samples plot in the VAG field in thisdiagram (not shown). In R1 versus R2 diagram of Batchelorand Bowden (1985), the aplitic samples plot in the syn-collison to post-orogenic setting while the tonalitic samplesplot in pre-plate collision setting, near the mantle frac-tionates dividing line (Fig. 10a). The Nb versus Rb/Zrdiagram is used to compare the arc maturity of Brown et al.(1984). The Marmara Island’s plutonic rocks are grouped inprimitive island arcs or continental arcs fields rather thannormal or mature continental arcs (Fig. 10b).

Interpretation of geochemical data

Our study has shown that the İlyasdağ metatonalite is ametaluminous, calc-magnesian granitoid, emplaced at 47.6±2 Ma in the Lutetian. It is a unique pluton among the Eoceneplutons of NW Anatolia with its low SiO2 and K2O, highMgO, and the lowest Nb, Th, Rb/Sr and highest Y/Nb ratios.It is also characterised by having a high Pb content.

Harker variation diagrams indicate that compositionalvariation of the İlyasdağ tonalite was formed either fromfractional crystallisation of an andesitic melt (57 wt% SiO2)or by differentiation and fractional crystallisation of a basicmelt (more primitive than 57 wt% SiO2) at a magmachamber in the continental crust. Enrichment of Ba in theİlyasdağ pluton is controlled mainly by the crystallisationof clinopyroxene. Enrichment of Sr, up to SiO2 value of64%, and subsequent depletion in the more evolvedsamples is thought to have been controlled by clinopyrox-ene and plagioclase, respectively. Rapid depletion of Sc canalso be assigned to crystallisation of clinopyroxenes.Enrichment of Rb is much weaker in the İlyasdağ plutonthan in the Kapıdağ and Karabiga plutons. Crystallisationand removal of clinopyroxene and plagioclase should havecaused the slight enrichment of Rb in the remaining melt.The low K2O content of the İlyasdağ metatonalite rules outthe enrichment of Rb with the assimilation of continentalcrustal material. Minor phases such as titanite, apatite andzircon control the depletion of Zr, Nb, TiO2 and P2O5.Amphibole, on the other hand, had an effect on thedepletion of Y in the İlyasdağ pluton.

The cross-cutting aplitic set is different from themetatonalite in its geochemical properties. The aplitic setgenerally follows the same fractional crystallisation trendwith the metatonalite in major- and trace-element Harkervariation diagrams but they are more felsic (> 71 wt%SiO2) than the metatonalite and are peraluminous incomposition.

Aplitic veins are characterised by low concentrations ofAl2O3, CaO, MgO, Fe2O3 and TiO2 relative to themetatonalite. The enrichment in Th, K2O, Pb and U anddepletion in Zr and Y relative to the metatonalite suggestthat crustal contamination may have played an importantrole in the chemical evolution of the aplitic set in additionto differentiation and fractional crystallisation processes.Therefore we think that the aplites were originated from thesame tonalitic melt but differentiated from it by assimilationof crustal material and by fractionation of amphiboles,which is responsible for low Y concentrations.

Previous petrological studies on the Eocene magmatismof NW Anatolia indicated metasomatised mantle sources(Köprübaşı and Aldanmaz 2004; Altunkaynak 2007;Karacık et al. 2008). Major and trace element character-istics of the İlyasdağ pluton are also compatible with a

Fig. 10 a R1 versus R2 discrimination diagram (Batchelor andBowden 1985) for İlyasdağ Pluton; b Distribution of Eocene andOligocene plutons on the Nb versus Rb/Zr arc maturity diagram(Brown et al. 1984). Note that the Ilyasdağ Pluton is characterised bylow Nb values and plots in the primitive arc field. See Fig. 7 for thekey of the symbols


metasomatised mantle source. The metaluminous, ratherthan peraluminous nature, and high Mg # of the İlyasdağpluton rule out derivation from a lower crustal amphibolitic/metabasaltic source area.

Both metatonalites and aplitic veins are characterised byLIL-element enrichment, strong Nb depletion, LREEenrichment, negative Ti and P anomalies and a positivePb anomaly. These features are compatible with supra-subduction zone magmatism. In that regard, the İlyasdağpluton can be considered to have emplaced at a continentalmargin arc setting.

Discussion

Mantle involved magmatism is common in all tectonicsettings, including rifts, magmatic arcs and post-collisionalsettings. If the mantle concerned is metasomatised previ-ously with the fluids from a subducting slab, the resultinginitial melts will be of arc-type, regardless of the mantlemelting mechanism (e.g. Pearce et al. 1984; Seyitoğlu et al.1997; Reiners et al. 2000; Keskin 2003). This is the reasonfor a wide range of models proposed in the previous workfor the tectonic setting of the Eocene magmatism in NWAnatolia and NE Greece. All the previous studies emphas-ised that the Eocene magmatic rocks in this region are ofarc-type. Some tectonic models, however, inferred contem-poraneous subduction (Delaloye and Bingöl 2000; Okayand Satır 2006) whereas others suggested post-collisionalsetting with the view that Neothethys in this area wassutured by early Eocene (Köprübaşı and Aldanmaz 2004;Altunkaynak 2007; Karacık et al. 2008). Eocene activesubduction was also suggested for the north Greek plutons(Pe-Piper 2004).

Below we will discuss these alternative tectonic models,highlighting their positive and negative sides in the light ofregional geological data.

a) Mid-Eocene post-collision magmatism

A post-collision setting was first proposed for the southerlyplutons by Harris et al. (1994) and Okay et al. (1998).Harris et al. (1994) suggested that the Orhaneli and Topukgranodiorites are post-collisional granites emplaced duringthe 53–48 Ma interval (Ypresian, early Eocene). Theyshowed that the granodiorites are either derived from amantle wedge or by anatexis of lower continental crust; inthe latter case advective heating of the crustal source bymantle-derived melts is needed to generate crustal melting.They envisaged that cessation of subduction may haveheated the hydrated mantle and this resulted in melting.Okay et al. (1998), on the other hand, tied the mantlemelting mechanism to asthenospheric upwelling, which

occurred following a rapid isostatic rebound of subductedcontinental crust (the lower plate) by 60 km within a35 myr period between Late Cretaceous and early Eocene,due to slab breakoff. In that model, the upwellingasthenosphere underwent decompression melting and theascending melts triggered dehydration melting of lowercrust by providing convective heat. Okay et al. (1998)argued that the location of the granodiorites in theisostatically upwelling zone (i.e. in Tavşanlı Zone) supportstheir model.

Köprübaşı and Aldanmaz (2004) extended the post-collision model to the northern plutons and inferred thateither slab breakoff or delamination of a thermal boundarylayer of subcontinental mantle produced the Eocenemagmatism in this area. According to this model, magma-tism occurred by means of conductive heating of litho-spheric mantle by upwelling asthenosphere. Altunkaynak(2007) similarly suggested a slab breakoff model forgeneration of the Eocene magmatism in NW Anatolia. Inthis model, the southern plutons were assumed to be older(52 to 48 Ma) and the northern plutons younger (48 to35 Ma). The southern plutons were emplaced during theinitial slab breakoff (rift stage) and the northern plutonswere emplaced when the slab was detached and the slabwindow was formed completely. Melting of mantle wastriggered by asthenospheric upwelling through the slabwindow according to this model. The slab in these modelsis the northward descending Neotethyan oceanic litho-sphere attached to the northern passive margin of theMenderes-Taurus platform.

The main discrepancy of the post-collisional modelsinvolving the slab breakoff mechanism lies in inconsistencyof the timing of slab breakoff. The northern passive marginof the Menderes-Taurus Platform subducted to mantledepths (ca. 70 km) and underwent HP/LT metamorphismin Late Cretaceous times (80 Ma, Okay 2002). Thisblueschist belt, known as the Tavşanlı Zone, extends over500 km in NW-SE direction with a width of ca. 80 km inNE-SW direction. The blueschist belt was exhumed in thelatest Cretaceous since detritus derived from it is found inthe latest Cretaceous sediments of the Sakarya Continent(Okay and Satır 2006). The exhumation of the subductednorthern margin of the large Menderes-Taurus Platform inlatest Cretaceous has been proposed to be due to the slabbreaking off in late Campanian to early Maastrichtian(Okay 2002; Okay and Satır 2006).

Secondly, magmatism associated with slab breakoff (anddelamination) mechanism is shown to emplace intothickened crust with elevated topography at the surface(see plate tectonic diagrams in Atherton and Ghani 2002;Altunkaynak 2007). The stratigraphic record in NWAnatolia, however, shows that the crust was at its normalthickness when the mid-Eocene plutons were emplaced


(Okay and Satır 2006). For instance, an important mid-Eocene to Oligocene stratigraphic sequence is preserved tothe north of the Marmara Island, in the Thrace Basin to theNW of Marmara Sea. The sequence is unconformable onolder units and starts with neritic limestones and clastics ofmiddle Eocene age (the Soğucak Limestone) and deepensupward, marked by deposition of overlying turbiditicsandstones (Ceylan Formation). The basin was then filledwith volcanogenic conglomerates and sandstones. Aftervolcanism ended, a thick sequence of grey shales wasdeposited in the basin, together with thin interlayers ofturbiditic sandstones. The basin was then filled by thedeposition of channelised conglomerates and sandstones oflate Eocene to Oligocene age. Similar mid-Eocene shallowmarine limestones are exposed in the Biga and ArmutluPeninsulas, to the south of Marmara Island. The ophiolitesin the İzmir-Ankara-Erzincan Suture are also unconform-ably overlain by mid-Eocene shallow marine sediments.Thus, the stratigraphic record reveals that NWAnatolia wasbelow sea level during the emplacement of the İlyasdağpluton at ~48 Ma and remained as a subsiding area until theend of Eocene in this area. Apparently, the granitoidmagmatism aided this subsidence by thermally weakeningthe crust. The subsiding area became an erosional area inOligocene times, accompanied with folding and faulting.

Thirdly, the spatiotemporal distribution of the Eoceneplutons (53–48 Ma in the south and 48–35 Ma in the north)is not valid since the Tepeldağ pluton in the southernplutons and the Karabiga pluton in the northern plutonshave recently been dated as ~44 Ma and ~53 Ma,respectively by U/Pb method (Okay and Satır 2006;Beccaletto et al. 2007), and therefore the magmatismactually took place contemporaneously both in the northernand southern plutonic belts.

Fourthly, alkaline magmatism directly derived from theasthenospheric melts are absent in this area.

b) Mid-Eocene arc magmatism

Assuming that the Neotethyan slab was detached in LateCretaceous times, a new subducting slab is needed forgeneration of arc magmatism in NW Anatolia. Severalalternatives were proposed for such an oceanic lithosphere.

Northward subduction of the African Plate along theHellenic trench was proposed for the origin of Eocene toQuaternary magmatism in Anatolia and the Aegean Sea(Delaloye and Bingöl 2000). The present distance betweenMarmara Island and the Hellenic trench exceeds 800 km.However, the area between the two is known to be the oneof the fastest extending area in the world at present. Theextension of the upper plate commenced in Early Miocene(Seyitoğlu and Scott 1996), causing retreat of the trenchsouthward. Several lines of evidence suggest that the

extension factor is ~1.2 (Eyidoğan 1988). The distancebetween the two amounts to 600 km when this extension isrestored. Major discrepancies of this model are first, thegreat distance involved between arc magmatism and thetrench, which is possible only if the subduction angle wasunusually low, and second, the uncertainty in the age ofnorthward subduction initiation along the Hellenic trench.

Eocene subduction along the Vardar suture

Okay and Satır (2006) envisaged that the dual nature of themid-Eocene plutons in NW Anatolia is due to strike-slipdisplacement of a once contiguous, E-W trending plutonicbelt in post-Eocene times. This plutonic belt is interpretedto be formed by arc magmatism developed in response tonortheastward subduction of a remnant ocean basin, locatedin the north Aegean Sea region along the trace of the VardarSuture.

Blueschist and eclogite facies metamorphism of 52 to ca.40 Ma (Eocene) in the Cycladic Massif and equivalents inwest Anatolia have been reported (Oberhänsli et al. 1998;Tomaschek et al. 2003; Lagos et al. 2007). These ages areindicative of active subduction in this area. The subductiontrench is inferred to be NW-SE trending, oblique to thegeneral E-W trend of the Eocene plutons in NW Anatolia.Palaeomagnetic data collected from the Late Miocenevolcanics in NW Anatolia indicate dominantly anticlock-wise rotations at ca. 20 degrees (Kaymakçı et al. 2007).When such rotation is restored, the plutonic belt rotates toNW-SE direction and becomes parallel to the inferredactive margin. However, anticlockwise rotations weredetermined from the coastal areas only and it is not certainif the whole area in consideration underwent such rotation.

Discrepancies of this model are first, that no Eoceneblueschist has yet been found along the İzmir-Ankara-Erzincan Suture. The Eocene plutons are also found in theSivrihisar area some 200 km to the east and therefore a tiewith the subduction of the Vardar Ocean requires anticlock-wise rotation of whole Anatolian Peninsula, which isdifficult to visualise. Secondly, there are some differencesin the northern and southern plutons. The main differencebetween the two granitoid zones, apart from their locationin the upper (northern group) and lower plates (southerngroup) of the Sakarya-Menderes convergence system, is theabsence of late Eocene (?) plutons and associated volcanicsin the southern zone. Altunkaynak (2007) also found outthat both the northern and southern plutonic belts werederived from the metasomatised mantle however thenorthern plutons were modified by larger amount of crustalcontamination that resulted in their stronger depletion inEu, Ba, Sr and P and higher contents of Pb, K, Ni and SiO2.Due to the fact that the exact age of most of the northernplutons are not known, useful comparisons can only be


made with the İlyasdağ (~48 Ma) and Topuk (~48 Ma)plutons as they were emplaced at nearly the same time inthe upper and lower plates, respectively. The Topuk andİlyasdağ plutons display similar patterns on N-MORBnormalised spidergrams. However, the Topuk pluton isenriched in most of the elements in comparison to İlyasdağtonalite, except Ti and Y (not shown).

c) Mid-Eocene extensional magmatism associatedwith the collapse of the orogen

Late Palaeocene to early Eocene sedimentary rocks arelargely absent in NW Anatolia. This interval possiblycorresponds to uplift and erosion due to deformation ofolder rocks in response to collision of the Menderes-TaurusPlatform and the Sakarya Continent. The presence of a mid-Eocene shallow marine sedimentary cover on the olderunits in Tavşanlı Zone and NW Anatolia suggests that theuplifted area rapidly subsided below sea-level in mid-Eocene times. If the subsidence of uplifted orogen in themiddle Eocene is related to lithospheric extension, then thecoeval magmatism can easily be interpreted as an exten-sional magmatism. In that case, asthenospheric melts ofdecompressional origin could have provided the necessaryheat to melt the metasomatised subcontinental mantle.

In NW Anatolia there is no evidence for compressionaldeformation during the mid to late Eocene, as describedabove. However, in marked contrast, the mid-Eocene wasthe time when the Menderes Massif underwent its peakmetamorphism under the thrust load of the Lycian andBeyşehir-Hoyran-Hadim Nappes. Therefore, it appears thatwhile the Menderes Massif was telescoping by thin- tothick-skin thrusting, the Tavşanlı Zone and the area furthernorth subsided rapidly.

Delamination of the thermal boundary layer under theTavşanlı Zone and the areas further north is one of thepossible explanations for this regional subsidence. Howev-er, orogenic collapse, in most cases, requires renewedsubduction at a nearby ocean basin to accommodate theextension of the orogen. A remnant ocean basin located atthe present Aegean Sea to the north of Cycladic Massif mayhave started subduction northward at this stage.

The available evidence does not prove any one or otherof the models, although we favour a hybrid model, in whichextension in NW Anatolia occurred after the final collisionbetween the Menderes-Tauride Block and the Sakaryacontinent. We interpret that this extension was due to therapid retreat of the southern Eurasian subduction systemfrom the (present) central Aegean and the Izmir-AnkaraZone to the site of the active Hellenic trench. The reason forthis retreat, we suggest, was due to the Palaeocene-EarlyEocene collision of the Menderes-Tauride Block with theSakarya continent (the southern margin of Eurasia at the

time). This collision caused the >200 km translation of theLycian Thrust Sheets over the Menderes-Tauride Block tothe present Lycian front that lies along strike of thetermination of the Hellenic Trench (Collins & Robertson1997; 1998).

Conclusions

The İlyasdağ metatonalite is a N-dipping, sill-like intrusion,emplaced into a metamorphic rock association, made up ofseveral thrust sheets of metaclastics, metabasics andmetacarbonates. The intrusion exhibits strong brittle-ductile deformation fabrics, with the development of botha N-dipping foliation and an E-W plunging stretchinglineation.

Despite the previous estimates of Palaeozoic or olderintrusion ages, LA-ICP MS dating of U and Pb isotopes onzircon separates from the İlyasdağ metatonalite yielded anearly Lutetian (47.6±2 Ma) age for igneous crystallisation.

Eocene granitoids form two E-W trending belts in NWAnatolia; the one in the north is parallel to the LateCretaceous Intra-Pontide Suture and the other in the southis parallel to the Early Cenozoic İzmir-Ankara-ErzincanSuture. Robust radiometric ages were available for thesouthern belt whereas the published ages of the northerlyplutons have mostly been made using the K/Ar method onmineral separates that yielded diverse ages for individualplutons. Therefore, the age data presented in this paper arethe one of the most robust age data gathered from thenorthern plutonic belt.

Geochemical data shows that the İlyasdağ metatonalite isa calcic-magnesian, metaluminous intrusion with arc-typegeochemical characteristics. The İlyasdağ metatonalite wasderived from a depleted mantle source with some contam-ination from continental crustal rocks, and is the most basicand the northernmost member in the northern plutonic beltof NW Anatolia.

The İlyasdağ pluton was emplaced into a crust of normalthickness, following the collision of the Sakarya andMenderes platforms along the İzmir-Ankara-Erzincan Su-ture Zone in Late Palaeocene-early Eocene times. Thepluton underwent severe brittle-ductile deformation follow-ing its mid-Eocene emplacement.

All previous tectonic models have some sort of problemsin explaining the genesis of the Eocene magmatism in NWAnatolia. However, a model of extension in the hanging-wall of a rapidly retreating arc setting is favoured here forthe genesis of the İlyasdağ Pluton. The arc retreat beingrecorded in the rapid southerly stepping of subduction fromthe central Aegean (Cyclades) in the Eocene to the HellenicTrench at the present day. This retreat is recorded in thepunctuated history of thrusting recorded in the Lycian


Thrust Sheets (Collins and Robertson 1998). YoungerEocene and Oligocene granitoids in NWAnatolia may relateto post-collisional processes. Future radiometric dating ofthe plutons and the blueschists are needed to betterunderstand the tectonic setting of formation of magmatism.

Acknowledgement This study was partly supported by a TÜBİTAK(The Scientific and Technical Research Council of Turkey) grant to P.A.U. (TÜBİTAK post-doctoral overseas research grant). The authorswould like to thank John Stanley, David Bruce, Justin Payne and BenWade from the University of Adelaide for their help during thelaboratory studies. Mineral Research and Exploration Institute inAnkara is greatly appreciated for preparation of thin sections. We arealso grateful to Mehmet Keskin of Istanbul University for hissuggestions on geochemistry section and to Tolga Görüm of YıldızTechnical University for his assistance in drafting some of the figures.Constructive and helpful reviews, provided by Gernold Zulauf, AralOkay and A. Möller are greatly acknowledged.

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