Lithos 127 (2011) 354–372

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Adakite-like granitoid porphyries in the Eastern Pontides, NE Turkey: Potentialparental melts and geodynamic implications

Orhan Karsli a,⁎, Murat Ketenci b, İbrahim Uysal c, Abdurrahman Dokuz a, Faruk Aydin c, Bin Chen d,Raif Kandemir a, Jan Wijbrans e

a Department of Geological Engineering, Gümüşhane University TR-29000 Gümüşhane, Turkeyb Department of Geological Engineering, Istanbul Technical University TR-34000 Istanbul, Turkeyc Department of Geological Engineering, Karadeniz Technical University TR-61080 Trabzon, Turkeyd School of Earth and Space Sciences, Peking University, 100871 Beijing, Chinae Laboratory of Isotope Geology, Vrije Univ., De Boelelaan 1085, 1081 HV, Amsterdam, The Netherlands

⁎ Corresponding author. Tel.: +90 532 2087307; fax:E-mail address: okarsli@gmail.com (O. Karsli).

0024-4937/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.lithos.2011.08.014

a b s t r a c t

a r t i c l e i n f o

Article history:Received 5 April 2011Accepted 25 August 2011Available online 5 September 2011

Keywords:Adakite-like porphyryPartial meltingHeterogeneous lower continental crustEastern PontidesNE Turkey

The tectonic setting of the Eastern Pontides during the late Mesozoic to early Cenozoic remains a subject ofdebate. Petrogenesis of adakite-like granitoid porphyries plays a critical role in determining the nature ofthe lower continental crust and mantle dynamics during orogenic processes in the region. Here we describe,for the first time, the late Paleocene to early Eocene adakite-like granitoid porphyries from the northernpart of the Eastern Pontides although their counterparts in the southern part have recently been found.The adakitic porphyries, which emplaced into the subduction-related Turonian–Santonian volcanics fromnorthern part of the region, consist of I-type calc-alkaline quartz monzonite–tonalite (SiO2=62.89–65.07 wt.%) and high-K calc-alkaline granodiorite–granite associations (SiO2=69.06–70.43 wt.%). Theformer displays peraluminous to metaluminous signatures, whereas the latter shows peraluminousgeochemical character. The granite–granodiorite porphyries have high K2O (3.32–3.84 wt.%), and low Na2O(3.48–4.61 wt.%) and MgO (0.91–1.04 wt.%) relative to the quartz monzonite–tonalite association(K2O=1.50–1.92 wt.%; Na2O=4.08–6.45 wt.%; MgO=1.44–2.07 wt.%). Ar–Ar geochronology studies onthe amphibole separates reveal that the adakite-like porphyries have a crystallization ages of 51.34±0.27to 53.55±0.34 Ma. Here, we contend that these rocks were formed by partial fusion of a mafic lower conti-nental crust in a collisional phase but not in a subduction setting. All the samples exhibit the typical geochem-ical characteristics of adakite, that is, high Sr (250–1141 ppm), high Sr/Y ratios (16–147), low Y (6.8–14.8 ppm) and low HREE concentrations; they are similar to adakites formed by slab melting associatedwith the subduction zone. However, the rocks exhibit heterogeneity in isotopic composition, with ISr rangingfrom 0.70554 to 0.70986, εNd (51 Ma) from –8.5 to –0.9 and Nd model ages from 0.72 to 1.26 Ga. The samplesare characterized by relatively high Th, Th/U and no significant Eu anomalies, implying that garnet was stablein their source during partial melting. The compositional diversity between rock groups is probably related topartial melting of heterogeneous lower crustal source. All of the features are inconsistent with a slab-meltingorigin and slab-related petrogenetic model, but instead, they favor an origin by melting of heterogeneouslower continental crust due to a thermal anomaly that was induced by the upwelling of the asthenospherethrough a slab break-off in a collisional setting. These interpretations argue against the evidence for the pres-ence of an early Cenozoic arc setting in the Eastern Pontides. Instead, the early Cenozoic time in the regionmay be attributed to a geodynamic response to a post-collisional uplift phase that occurred along the conti-nent-continent collision between the Pontide and the Anatolide-Tauride blocks.

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1. Introduction

The term “adakite” is used to describe volcanic rocks and its intru-sive counterparts that are characterized by distinctive geochemical

signatures such as SiO2N56 wt.%, K2O/Na2O b0.6 and high Sr/Y ratioscoupled with strong depletion in Y, Yb and HFSE. Modern adakites aremostly found in island and continental arc settings (Defant andDrummond, 1990; Kay, 1978). Adakites that form along convergentplate boundaries have been described as melts of the subducted oce-anic slab (e.g., Beate et al., 2001; Defant and Drummond, 1990;Gutscher et al., 2000; Martin, 1999; Martin et al., 2005; Qu et al.,2004; Zhu et al., 2008). Hence, the presence of adakites is of special

Fig. 1. (a) Regional tectonic setting of Turkey with main blocks in relation to the Afro-Arabian and Eurasian plates [modified from Okay and Tüysüz (1999)]. (b) Simplified geolog-ical map of the Zigana area showing the early Eocene adakitic granitoid porphyries with sample locations.

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geodynamic significance, as they require particular generation in sub-duction zone (Defant and Drummond, 1990; Rapp et al., 1991; Şenand Dunn, 1994). However, many studies show that such a geochem-ical signature can be achieved via different processes within post-subduction settings, including fractional crystallization or AFC of a

parental basaltic melt (Bourdon et al., 2002; Castillo et al., 1999;Grove et al., 2003; Macpherson et al., 2006), mixing of mantle- andcrustal-derived melts (e.g., Guo et al., 2007), and partial fusion ofmafic continental lower crustal rocks (Atherton and Petford, 1993;Barnes et al., 1996; Guo et al., 2006; Hou et al., 2007; Huang et al.,

356 O. Karsli et al. / Lithos 127 (2011) 354–372

2008; Kay and Kay, 2002; Liu et al., 2008; Wang et al., 2004; Wang etal., 2006; Wang et al., 2007a,b; Xu et al., 2006; Zhao and Zhou, 2008).Additionally, Martin et al. (2005) classified modern adakites into twosubgroups, low-SiO2 and high-SiO2 adakites, and claimed that high-SiO2 adakites are products of reactions between the high-pressureslab melt and the overlying mantle wedge. The granitoid porphyriesfrom the northern part of the region have high-SiO2 contents andadakite-like geochemical signatures, but they are not similar to thehigh-SiO2 adakites described by Martin et al. (2005). Because theserocks are not strictly adakites as originally defined by Defant andDrummond (1990), we refer to them as “adakite-like or adakitic por-phyry”. Although some early Cenozoic volcanic and intrusive rocksfrom the Eastern Pontides clearly show adakite-like signatures ratherthan modern adakite, they were assumed as modern adakite formedin subduction zone. In such a case, our data provide a good opportu-nity to characterize adakitic melt petrogenesis, crustal growth andmantle dynamics in the region.

Here, we present new Ar–Ar geochronological, whole-rockgeochemical, and Sr–Nd isotopic compositional data from the earlyCenozoic, adakitic granitoid porphyries from the northern part ofthe Eastern Pontides. Our objectives are to clarify the petrogenesisof the adakitic rocks in the region and to characterize the geodynamicevolution of the Eastern Pontides during the late Paleocene to theearly Eocene.

2. Geological background

Turkey is essentially composed of four major tectonic blocks orterranes that are separated by the suture zones (e.g., Okay and Tüysüz,1999; Fig. 1a). The Eastern Pontides is a subset of the Sakarya zone,which is one of the major tectonic units of Turkey (Fig. 1a). Thebasement rocks of the Eastern Pontides are late Carboniferous granitoid.Early Carboniferous metamorphic rocks (e.g., Yilmaz, 1972; Topuz et al.,

Fig. 2. (a) Macroscopic view of the contact aurole between adakitic intrusives and Turoniangranidiorite, tonalite and quartz monzonite porphyries. The features are amphibole (A), pla

2007; Topuz et al., 2010; Dokuz, 2011) and late Carboniferous to earlyPermian shallow marine to terrigeneous sedimentary rocks areaccompanied with the granitoids (Çapkınoğlu, 2003; Okay and Leven,1996). Late Permian and Triassic rocks are not common in the EasternPontides.

The basement is overlain by early and middle Jurassic tuffs, pyro-clastic and interbedded clastic and carbonate sedimentary rocks. Thistime interval is attributed to break-up of a continental margin thatthe rift-related volcano-sedimentary sequences (Dokuz and Tanyolu,2006; Kandemir, 2004; Kandemir and Yilmaz, 2009) and basic volca-nic rocks (Şen, 2007) formed. Late Jurassic granitoids and their volca-nic equivalents emplaced into the volcano-sedimentary rocks ofŞenköy Formation during middle-late Jurassic (Dokuz et al., 2010).These granitoids are interpreted as the products of an arc-continentcollision event, in response to closure of Paleotethys during middleJurassic and the accretion of the Sakarya Zone to Laurasia in thenorth (Dokuz et al., 2010; Şengör et al., 1980; Şengör and Yilmaz,1981; Yilmaz et al., 1997). During the late Cretaceous, the EasternPontides is a magmatic arc due to the ongoing convergence be-tween Laurasia and Gondwana, resulting in a northward subductionof Neotethys along the southern border of Sakarya Zone (e.g., Akin,1979; Altherr et al., 2008; Çinku et al., 2010; Karsli et al., 2010a; Karsliet al., in review; Okay and Şahintürk, 1997; Okay and Tüysüz, 1999;Şengör et al., 2003; Şengör and Yilmaz, 1981; Topuz et al., 2007;Ustaömer and Robertson, 2010; Yilmaz et al., 1997). The magmaticarc is characterized by a more than 2 km-thick volcano-sedimentarysequence with local intrusion of hornblende–biotite granitoids inthe northern part of the Eastern Pontides (Karsli et al., in review;Boztuğ et al., 2006; Boztuğ and Harlavan, 2008; Karsli et al., 2004;Karsli et al., 2010a; Kaygusuz et al., 2008; Okay and Şahintürk,1997; Yilmaz and Boztuğ, 1996). The southern part represents afore-arc phase where flyschoid sedimentary rocks with limestoneolistoliths were accumulated. The early Paleocene plagioleucitites

–Santonian subduction-related volcanic rocks. (c–d) Macroscopic view of the granite,gioclase (Pl), quartz (Q) and orthoclase (Or).

Table 140Ar/39Ar dating values for the adakitic granitoid porphyry rocks from the Eastern Pontides.

Sample Rock type Mineral Weighted plateau MSWD Normal isochron 40Ar/36Ar(i)±2σ

MSWD 39Ar(K)% (n) K/Ca±2σ

Age±2σ (Ma) Age±2σ (Ma)

H3 Tonalite Hornblende 51.34±0.27 0.19 51.36±0.32 295.28±5.12 0.22 100.00 12 0.074±0.01Y4 Granodiorite Hornblende 53.55±0.34 2.06 52.49±1.58 303.04±10.98 1.88 72.99 16 0.171±0.01HK5 Tonalite Hornblende 65.80±5.35 4.00 27.65±28.29 305.21±7.06 2.52 79.10 15 0.172±0.02

Note: Plateaus ages were calculated over concordant steps (as defined by the MSWD value calculated for the plateau steps), the percentage of the gas release included in the plateaucalculation is given in the column 39Ar(K), the number of steps forming the plateau is n. 40Ar/36Ar(i) refers to the non-radiogenic intercept ratio of 40Ar/36Ar, which are in all casesindistinguishable from the value for modern air. Isochron ages were calculated over the steps that represent the plateau. Errors given are ±2σ. Ages were calculated on the basis ofan age for the laboratory standard sanidine DRA-1 of 25.26±0.2 Ma.

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was referred to last products of the northward subduction (Altherr etal., 2008). Paleocene time in the Eastern Pontides is attributed to con-tinent–continent collision between the Sakarya zone and the Tauride–Anatolide block due to the complete closure of Neotethys. Şengör

Fig. 3. 40Ar/39Ar ages spectra for hornblende se

and Yilmaz (1981), Okay and Şahintürk (1997) and Okay et al.(1997) propose a Paleocene to early Eocene (ca. 55 Ma) collision,causing crustal thickening. The Eastern Pontides have a quiescent pe-riod during the Paleocene. Early Cenozoic adakitic rocks, pointing to

parates from the adakitic porphyry rocks.

Table 2Major oxide and trace element analyses of the adakitic porphyry rocks from the Eastern Pontides.

Sample V2 V3 V5 V8 V12 ZT2 HK1 HK3 V6 V7 V9 V10

Rock type qmp qmp qmp qmp qmp qmp qmp qmp tnp tnp tnp tnp

SiO2 64.62 64.67 64.45 63.83 64.15 64.47 64.81 64.4 63.84 63.69 65.07 63.52TiO2 0.38 0.37 0.37 0.36 0.36 0.36 0.37 0.39 0.37 0.37 0.37 0.36Al2O3 17.19 17.26 17.29 17.14 17.09 17.3 17.32 17.12 17.13 16.68 17.06 16.86Fe2O3

tot 3.75 3.85 3.78 3.78 3.67 3.74 3.81 3.86 3.88 3.75 3.86 3.78MnO 0.07 0.07 0.07 0.07 0.07 0.08 0.07 0.08 0.06 0.07 0.06 0.08MgO 1.54 1.55 1.58 1.63 1.57 1.6 1.56 1.72 1.49 1.63 1.47 1.44CaO 2.21 3.12 3.09 3.22 3.02 2.72 2.14 2.16 3.87 3.81 4.02 5.01Na2O 6.27 5.25 5.45 5.24 5.88 6.37 6.45 6.24 4.78 4.82 4.61 4.08K2O 1.76 1.92 1.63 1.78 1.69 1.72 1.53 1.81 1.73 1.79 1.76 1.72P2O5 0.19 0.19 0.19 0.19 0.18 0.19 0.19 0.21 0.18 0.18 0.18 0.18LOI 1.80 1.50 1.80 2.50 2.10 1.20 1.50 1.70 2.40 3.0 1.30 2.70Total 99.75 99.73 99.74 99.74 99.74 99.77 99.73 99.73 99.75 99.76 99.74 99.77Mg# 0.45 0.44 0.45 0.46 0.46 0.46 0.45 0.47 0.43 0.46 0.43 0.43ASI 1.06 1.05 1.06 1.05 1.01 1.00 1.07 1.06 1.02 0.99 1.02 0.95Rb 40 41 35 39 35 37 30 39 35 42 40 38Sr 916 887 930 869 967 836 1123 1066 828 775 885 785Ba 836 981 869 880 854 777 740 798 873 851 918 853Cs 0.6 0.7 0.7 1.0 0.5 1.1 0.4 0.6 1.6 0.9 0.8 0.9Zr 106 103 110 108 101 99 110 109 118 104 106 105Hf 2.6 3.0 3.0 3.0 3.0 3.1 3.4 3.1 3.4 3.0 3.0 2.9Th 6.5 6.5 6.5 6.5 6.1 6.4 7.1 7.6 6.8 6.8 6.6 6.1Pb 13.8 9.3 9.7 10.1 9.7 10.7 12.9 11.7 6.2 6.4 7.2 3.6Zn 38 32 45 51 31 33 44 42 35 53 31 37Ta 0.4 0.5 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4Nb 7.3 7.2 7.1 7.0 7.0 6.8 7.0 7.1 7.0 7.0 7.3 7.0Y 7.1 7.6 7.9 6.8 7.7 7.6 8.1 9.8 7.1 7.5 7.4 7.5U 1.4 1.4 1.5 1.2 1.3 1.5 1.3 1.5 1.2 1.3 1.2 1.4Ni 7 8 9 9 8 6 7 8 9 9 8 10Co 7.3 7.4 7.5 6.8 7.5 7.4 7.6 8.1 7.4 7.2 7.3 6.9V 63 62 59 58 58 59 60 68 60 61 59 60Ga 18.5 18.7 18.9 18.8 18.5 19.0 19.0 19.1 18.9 18.6 18.8 19.1Sc 7 6 6 6 6 6 6 7 6 6 6 6La 29.9 31.2 31.4 31.0 29.7 30.6 31.6 34.4 30.4 30.5 30.8 30.6Ce 52.9 55.6 54.3 53.6 53.5 54.7 54.6 56.4 54.6 52.9 53.9 53.1Pr 5.61 5.86 5.91 5.78 5.73 5.82 6.04 6.53 6.02 5.86 5.82 5.70Nd 20.0 19.9 20.5 20.9 20.7 20.0 20.8 23.0 21.6 21.7 20.3 20.2Sm 2.93 2.99 3.05 2.93 3.02 2.95 3.16 3.25 3.15 3.16 3.08 3.04Eu 0.85 0.90 0.92 0.88 0.88 0.89 0.93 1.01 0.91 0.91 0.89 0.87Gd 2.12 2.24 2.30 2.11 2.18 2.20 2.26 2.45 2.19 2.21 2.24 2.18Tb 0.27 0.29 0.30 0.27 0.28 0.28 0.29 0.32 0.28 0.28 0.30 0.28Dy 1.41 1.47 1.52 1.35 1.42 1.48 1.46 1.64 1.37 1.42 1.45 1.47Ho 0.23 0.24 0.26 0.22 0.24 0.26 0.25 0.29 0.23 0.24 0.26 0.23Er 0.65 0.64 0.70 0.62 0.66 0.65 0.72 0.77 0.59 0.64 0.66 0.60Tm 0.10 0.10 0.11 0.09 0.10 0.,11 0.11 0.12 0.09 0.10 0.11 0.10Yb 0.58 0.63 0.66 0.57 0.64 0.65 0.67 0.66 0.58 0.63 0.62 0.6Lu 0.10 0.10 0.10 0.09 0.10 0.10 0.10 0.11 0.09 0.10 0.11 0.09(La/Yb)n 34.7 33.4 32.1 36.7 31.3 31.7 31.8 35.1 35.3 32.6 33.4 34.3(Yb)n 2.7 3.0 3.2 2.7 3.1 3.1 3.2 3.2 2.7 3.0 2.9 2.8Eu/Eu* 1.04 1.07 1.06 1.08 1.05 1.07 1.07 1.10 1.06 1.05 1.04 1.03

Sample V13 V14 V15 V16 V16A V17 ZT1 ZT3 ZT4 ZT5 H1 H2

Rock type tnp tnp tnp tnp tnp tnp tnp tnp tnp tnp tnp tnp

SiO2 64.13 63.72 64.14 63.28 63.18 64.02 64.58 62.68 63.46 63.82 63.45 63.45TiO2 0.38 0.37 0.35 0.37 0.37 0.39 0.37 0.38 0.39 0.35 0.37 0.38Al2O3 17.02 17.05 17.13 16.86 16.85 17.17 17.19 17.12 17.39 16.87 17.04 16.96Fe2O3

tot 3.72 3.85 3.57 3.79 3.82 3.85 3.71 3.87 4.04 3.60 3.77 3.82MnO 0.06 0.05 0.06 0.08 0.08 0.08 0.07 0.08 0.08 0.07 0.07 0.08MgO 1.47 1.48 1.54 1.56 1.54 1.63 1.49 1.70 1.70 1.51 1.56 1.57CaO 4.15 4.3 4.10 4.63 4,52 4.79 3.19 4.71 4.00 4.01 4.75 4.94Na2O 4.82 4.61 4.59 4.42 4.58 4.36 5.86 4.65 4.87 4.94 4.60 4.38K2O 1.68 1.78 1.70 1.66 1.69 1.70 1.69 1.67 1.71 1.50 1.62 1.71P2O5 0.17 0.18 0.18 0.19 0.19 0.20 0.20 0.21 0.21 0.18 0.19 0.20LOI 2.10 2.30 2.30 2.90 2.90 1.60 1.40 2.70 1.90 2.90 2.30 2.30Total 99.74 99.68 99.71 99.74 99.76 99.74 99.73 99.74 99.73 99.78 99.75 99.75Mg# 0.44 0.43 0.46 0.45 0.44 0.46 0.44 0.46 0.45 0.45 0.45 0.45ASI 0.98 0.98 1.02 0.96 0.96 0.97 0.99 0.95 1.02 0.99 0.95 0.94Rb 37 39 35 36 38 41 35 36 36 31 37 38Sr 862 1141 1077 888 859 914 1023 911 904 797 908 927Ba 947 1230 1107 866 866 902 893 825 851 751 809 848Cs 0.5 0.6 0.7 0.9 1.1 0.7 0.5 0.8 0.6 1.0 0.6 0.6Zr 109 110 94 108 102 111 122 114 116 99 109 106Hf 2.7 2.9 2.8 3.0 2.7 2.9 3.5 3.2 3.0 2.8 3.1 3.0Th 6.4 6.4 6.9 6.5 7.1 7.0 6.5 6.9 8.3 6.4 6.9 7.3

358 O. Karsli et al. / Lithos 127 (2011) 354–372

Table 2 (continued)

Sample V13 V14 V15 V16 V16A V17 ZT1 ZT3 ZT4 ZT5 H1 H2

Rock type tnp tnp tnp tnp tnp tnp tnp tnp tnp tnp tnp tnp

Pb 7.1 10.7 9.1 11.5 10.9 9.2 8.1 11.3 8.3 5.0 9.7 8.8Zn 33 27 32 51 57 37 38 61 45 35 40 38Ta 0.4 0.4 0.3 0.4 0.4 0.4 0.4 0.4 0.4 0.3 0.4 0.4Nb 7.0 6.8 6.7 6.8 6.7 6.9 7.1 7.0 7.7 6.8 6.7 6.7Y 7.4 7.8 7.6 7.5 7.5 7.9 7.7 9.2 13.1 7,2 7.6 7.8U 1.5 1.3 1.3 1.6 1.7 1.4 1.3 1.5 1.6 1.1 1.5 1.5Ni 7 9 8 10 10 8 6 10 9 9 6 7Co 6.5 7.0 6.7 7.6 7.1 7.7 7.4 7.7 8.3 6.4 7.6 7.5V 61 58 56 61 59 63 63 66 63 56 63 63Ga 18.2 18.2 18.2 18.4 18.3 18.7 19.2 18.4 19.5 18.1 17.9 18.6Sc 6 6 6 6 6 7 7 7 7 6 7 7La 28.3 30.6 30.7 29.9 29.6 31.1 30.7 32.1 35.6 29.7 30.0 30.4Ce 49.9 53.9 55.3 52.3 53.8 55.1 54.9 56.6 59.3 51.4 53.5 53.1Pr 5.44 5.79 5.95 5.65 5.75 5.97 5.89 6.23 7.05 5.66 5.72 5,88Nd 19.5 20.4 20.4 20.0 19.9 20.2 20.8 21.3 24.8 20.2 20.3 20.3Sm 2.93 2.96 3.07 2.98 2.93 3.17 3.10 3.05 3.84 2.84 2.91 3.03Eu 0.87 0.90 0.91 0.89 0.89 0.92 0.92 0.92 1.12 0.86 0.88 0.90Gd 2.02 2.16 2.22 2.06 2.16 2.24 2.18 2.28 3.05 2.17 2.22 2.21Tb 0.28 0.28 0.29 0.28 0.28 0.30 0.28 0.30 0.41 0.28 0.29 0.29Dy 1.34 1.47 1.50 1.42 1.35 1.44 1.39 1.53 2.18 1.34 1.37 1.43Ho 0.24 0.26 0.24 0.24 0.25 0.27 0.27 0.27 0.39 0.24 0.24 0.26Er 0.67 0.62 0.63 0.67 0.71 0.72 0.68 0.74 0.99 0.61 0.66 0.70Tm 0.11 0.09 0.11 0.10 0.09 0.10 0.10 0.11 0.14 0.10 0.10 0.11Yb 0.59 0.52 0.59 0.59 0.59 0.63 0.64 0.68 0.84 0.60 0.63 0.61Lu 0.10 0.10 0.10 0.09 0.09 0.10 0.10 0.11 0.14 0.10 0.10 0.10(La/Yb)n 32.3 39.6 35.0 34.1 33.8 33.2 32.3 31.8 28.5 33.3 32.1 33.5(Yb)n 2.8 2.4 2.8 2.8 2.8 3.0 3.0 3.2 4.0 2.8 3.0 2.9Eu/Eu* 1.09 1.09 1.07 1.10 1.08 1.06 1.08 1.07 1.01 1.06 1.06 1.04

Sample H3 H4 H5 H6 HK2 HK4 HK5 YY1 KH4 Y2 Y4 KH2 Y3

Rock type tnp tnp tnp tnp tnp tnp tnp grdp grdp grdp grdp grnp grnp

SiO2 63.84 63.37 63.95 62.89 63.65 63.87 62.93 69.79 69.73 69.06 69.66 69.8 70.43TiO2 0.39 0.39 0.38 0.4 0.38 0.37 0.38 0.27 0.26 0.28 0.27 0.26 0.26Al2O3 17.18 17.14 16.90 17.11 17.05 16.98 16.50 14.89 14.91 15.06 14.75 14.88 15.20Fe2O3

tot 3.85 3.89 3.83 3.95 3.95 3.82 3.73 3.00 3.00 2.95 3.06 3.03 2.38MnO 0.08 0.08 0.07 0.07 0.08 0.07 0.07 0.05 0.05 0.05 0.06 0.05 0.04MgO 1.61 1.65 1.61 1.80 1.75 1.66 2.07 0.97 0.92 1.04 0.97 0.91 0.95CaO 4.86 4.9 4.7 5.12 4.19 3.96 3.65 2.75 1.54 1.91 2.34 1.64 1.10Na2O 4.43 4.39 4.52 4.39 4.77 4.71 4.98 3.48 4.61 4.39 3.91 4.12 4.23K2O 1.63 1.67 1.63 1,55 1.6 1.54 1.14 3,32 3.43 3.33 3.32 3.84 4.64P2O5 0.21 0.21 0.2 0.22 0.19 0.19 0.17 0.07 0.06 0.07 0,07 0.07 0.08LOI 1.70 2.10 1.90 2.20 2.10 2.60 4.10 1.20 1.30 1.70 1.40 1.20 1.60Total 99.74 99.74 99.73 99.72 99.72 99.74 99.77 99.84 99.84 99.84 99.84 99.82 99.93Mg# 0.45 0.46 0.45 0.47 0.47 0.46 0.46 0.39 0.38 0.41 0.39 0.37 0.44ASI 0.96 0.96 0.95 0.94 0.99 1.02 1.03 1.04 1.06 1.05 1.03 1.07 1.04Rb 38 39 36 33 35 31 23 111 98 108 106 120 19Sr 942 946 990 1009 977 834 547 250 292 278 266 351 200Ba 886 862 889 902 943 904 843 829 804 820 763 925 174Cs 0.6 0.6 0.5 0.6 0.6 0.9 0.8 0.6 0.2 0.4 0.4 0.4 0.2Zr 100 107 106 106 115 97 101 118 114 113 111 116 106Hf 2.9 3.2 3.0 3.1 3.3 2.7 2.8 3.3 3.4 3.7 3.4 3.5 3.5Th 6.9 6.8 7.0 7.3 7.2 6.4 6.0 14.2 14.8 14.8 14.1 15.9 15.4Pb 8.1 9.3 9.7 6.7 5.4 4.7 5.1 37.4 38.3 42.1 39.5 38.3 21.2Zn 38 36 37 39 45 51 47 38 32 34 36 34 29Ta 0.4 0.4 0.4 0.4 0.4 0.5 0.3 0.6 0.6 0.6 0.6 0.6 0.6Nb 7.2 7.2 7.0 7.0 7.1 6.7 6.2 6.0 6.0 6.4 6.0 6.1 6.0Y 7.7 8.1 8.0 8.4 8.0 7.5 7.3 13.5 14.2 14.6 13.7 14.4 13.0U 1.7 1.5 1.5 1.5 1.8 1.4 1.5 3.3 3.4 3.4 3.7 3.7 2.8Ni 6 6 6 9 9 9 8 7 7 7 7 6 7Co 7.9 8.0 7.8 8.8 8.0 6.4 6.4 5.0 5.2 5.1 5.1 5.0 4.5V 67 64 65 71 64 53 52 42 40 42 39 43 46Ga 19.7 18.5 18.8 19.1 18.8 16.0 16.5 14.9 14.9 15.1 14.0 15.2 15.7Sc 7 7 7 7 7 7 7 6 7 7 7 6 6La 31.5 30.8 31.3 32.7 31.8 32.3 29.3 28.2 27.2 29.1 27.5 34.7 22.7Ce 55.5 55.6 56.8 58.4 55.4 52.5 50.7 51.7 50.2 54.2 50.6 62.7 44.1Pr 6.01 6.05 6.10 6.50 6.18 6.06 5.66 5.49 5.36 5.82 5.45 6.63 4.98Nd 20.7 21.6 20.6 22.3 21.8 22.2 21.3 18.7 18.5 19.0 19.3 22.2 17.2Sm 3.19 3.12 3.14 3.39 3.16 3.20 3.00 3.18 3.20 3.21 3.13 3.42 3.09Eu 0.92 0.93 0.91 1.01 0.92 0.91 0.90 0.72 0.67 0.72 0.65 0.71 0.64Gd 2.23 2.34 2.23 2.48 2.25 2.25 2.18 2.55 2.52 2.72 2.48 2.69 2.41Tb 0.3 0.3 0.3 0.33 0.3 0.25 0.25 0.38 0.4 0.43 0.41 0.43 0.37Dy 1.43 1.49 1.5 1.62 1.43 1.32 1.48 2.20 2.25 2.49 2.19 2.33 2.16Ho 0.27 0.26 0.26 0.29 0.26 0.25 0.27 0.42 0.45 0.48 0.46 0.46 0.42

(continued on next page)

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Table 2 (continued)

Sample H3 H4 H5 H6 HK2 HK4 HK5 YY1 KH4 Y2 Y4 KH2 Y3

Rock type tnp tnp tnp tnp tnp tnp tnp grdp grdp grdp grdp grnp grnp

Er 0.71 0.7 0.74 0.74 0.72 0.66 0.70 1.26 1.30 1.44 1.42 1.35 1.26Tm 0.11 0.11 0.10 0.11 0.11 0.11 0.10 0.21 0.22 0.22 0.23 0.22 0.21Yb 0.68 0.62 0.62 0.68 0.61 0.60 0.59 1.38 1.43 1.49 1.48 1.46 1.43Lu 0.1 0.11 0.1 0.11 0.1 0.11 0.10 0.22 0.23 0.24 0.23 0.24 0.22(La/Yb)n 31.2 33.4 34.0 32.4 35.1 36.2 33.4 13.7 12.8 13.1 12.5 16.0 10.7(Yb)n 3.2 2.9 2.9 3.2 2.9 2.8 2.8 6.6 6.8 7.1 7.0 6.9 6.8Eu/Eu* 1.06 1.05 1.05 1.07 1.06 1.04 1.08 0.77 0.72 0.75 0.71 0.72 0.72

Rock types: qmp quartz monzonite porphyry, tnp tonalite porphyry, grdp granodiorite porphyry and grnp granite porphyry. Mg# is 100×MgO/(MgO+0.9FeOtot) in molarproportions. ASI is alumina saturation index. Oxides are given in wt.%, trace elements in ppm.

Table 3Sr and Nd isotope data for the adakitic granitoid porphyry rocks from the Eastern Pontides.

Sample [Rb] [Sr] 87Rb/ 87Sr/ 2σ ISr [Sm] [Nd] 147Sm/ 143Nd/ 2σ εNd(0) εNd(T) f Sm/Nd TDM

ppm ppm 86Sr 86Sr (51 Ma) ppm ppm 144Nd 144Nd 51 Ma (Ga)

Quartzmonzonite–tonalite porphyriesZT2 36 836 0.1021 0.705650 6 0.70558 3.0 21 0.0864 0.512550 6 −1.7 −1.0 −0.56 0.72H5 36 990 0.1083 0.706330 3 0.70625 3.0 20 0.0907 0.512558 6 −1.6 −0.9 −0.54 0.73HK5 23 547 0.1219 0.705599 4 0.70551 3.0 21 0.0864 0.512551 6 −1.7 −1.0 −0.56 0.72V9 40 885 0.1311 0.705640 7 0.70555 3.0 20 0.0907 0.512549 8 −1.7 −1.1 −0.54 0.75ZT5 31 797 0.1128 0.705670 5 0.70559 2.8 20 0.0846 0.512543 6 −1.9 −1.1 −0.57 0.72V16 36 888 0.1174 0.705630 7 0.70554 2.9 20 0.0877 0.512553 6 −1.7 −1.0 −0.55 0.72

Granite–granodiorite porphyriesY2 108 278 1.1266 0.706570 6 0.70577 3.0 19 0.0955 0.512169 7 −9.1 −5.5 −0.51 1.10YY1 111 250 1.2876 0.710780 10 0.70986 3.0 18 0.0955 0.512171 10 −9.1 −8.5 −0.51 1.26

Note: εNd=((143Nd/144Nd)s/(143Nd/144Nd)CHUR−1)×10,000, fSm/Nd=(147Sm/144Sm)s/(147Sm/144Sm)CHUR−1, (143Nd/144Nd)CHUR=0.512638, and (147Sm/144Sm)CHUR=0.1967.The model ages were calculated using a linear isotopic ratio growth equation: TDM=1/λ×ln(1+((143Nd/144Nd)s−0.51315) /((147Sm/144Nd)s−0.2137)).

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syn- to post-collision phase have been determined in the region(Karsli et al., 2010b; Topuz et al., 2005; Topuz et al., 2011). MiddleCenozoic times are recorded by volcanic (e.g., Aliyazicioğlu, 1999;Çoban, 1997; Şen et al., 1998; Tokel, 1977) and granitoid rocks(Boztuğ et al., 2004; Karsli et al., 2007; Yilmaz and Boztuğ, 1996).These granitoid bodies formed in a post-collisional setting (e.g., Karsliet al., in review; Karsli et al., 2007; Yilmaz and Boztuğ, 1996). PostEocene terrigeneous units are observed in the area (e.g., Okay andŞahintürk, 1997). Neogene alkaline volcanics are ascribed to post-col-lision extensional tectonic setting (Aydin et al., 2008, 2009). The pres-sure release in the crust, due to the escape, triggered volcanism alongmajor fault planes in the Eastern Pontides during Plio-Pleistocene (ca.2 Ma; Yeğingil et al., 2002).

Fig. 4. K2O+Na2O vs. SiO2 classification diagram (Middlemost, 1994) for the adakitic g

The adakitic granitoid porphyries emplaced in the northern part ofthe Eastern Pontides and have a wide contact aureole in Turonian–Satonian andesitic rocks of the Çatak Formation (Fig. 1b). The intrusivesform outcrops with a length of less than 5 km and amaximumwidth of2 km. They are parts of the composite Kaçkar Batholith, which has beendated to between late Cretaceous and late Eocene [K–Ar and Ar–Ar onhornblende, Moore et al., 1980; Taner, 1977; K–Ar on biotite, Karsliet al., 2007; Ar–Ar on hornblende, Karsli et al., 2010a; SHRIMP zirconU–Pb, Kaygusuz et al., 2009; SHRIMP zircon U–Pb, Karsli et al., inreview; SHRIMP zircon U–Pb, Karsli et al., in review]. The adakiticintrusives studied are made of granite–granodiorite and quartzmonzo-nite–tonalite porphyry associations. All of the contact relations betweenrock types are transitional.

ranitoid porphyries. σ is a Rittmann index, defined as (K2O+Na2O)2/(SiO2−43).

Fig. 5. Chemical variation diagrams for the adakitic granitoid porphyries illustratingsome chemical features that distinguish between the granitoid rocks. (a) ASI versusSiO2 diagram. (b) Al2O3/Na2O+K2O (molar) versus ASI [after Maniar and Piccoli(1989)] diagram for the pluton. (c) K2O versus SiO2 diagram for the samples withlines separating tholoiitic, calc-alkaline, high-K calc-alkaline and shoshonitic series ofPeccerillo and Taylor (1976).

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3. Analytical methods

Thirty seven samples that are visibly free of alteration wereselected for major and trace element analyses from the granitoid

porphyry rocks in the Hamsikoy–Zigana areas from the northernpart of Eastern Pontides. To prepare whole-rock powders, 1–3 kg ofthe fresh samples were crushed in steel crusher and then, the sampleswere grinded in an agate mill to a grain size of b200 mesh. Major andtrace element contents were determined at the commercial ACMELaboratories Ltd in Vancouver, Canada. The compositions of the sam-ples were measured by ICP-AES for major oxides (0.2 g pulp sampleby LiBO2 fusion). Major element detection limits are about 0.001–0.04%. For the trace elements, 0.2 g of sample powder and 1.5 g ofLiBO2 flux were mixed in a graphite crucible and subsequently heatedto 1050°C for 15 min in a muffle furnace. The molten sample wasthen dissolved in 100 mL of 5% HNO3 (American Chemical Society–grade nitric acid in demineralized water). Sample solutions wereshaken for 2 h and then an aliquot was poured into a polypropylenetest tube and aspirated into Perkin-Elmer Elan 600 ICP mass spec-trometer. Calibration and verification standards together with re-agent blanks were added to the sample sequence. The elementalconcentrations of the samples were obtained using BCR-2 and BIR-2(concentrations from USGS) as known external standards. The detec-tion limits of trace element range from 0.01 to 0.5 ppm.

40Ar/39Ar incremental heating experiments were carried out inthe Geochronology Laboratory at the Vrije University, the Netherlands.For each sample ca. 200 mg of washed groundmass was packed in20 mmdiameter Al-foil packages and stackedwith packages containinga mineral standard into a 23 mmOD quartz tube. The mineral standardis DRA-1 sanidine (with a K/Ar age of 25.26 Ma). The quartz vial waspackaged in a standard Al-irradiation capsule and irradiated for 1 h ina Cd-lined rotating facility (RODEO) at the NRG-Petten HFR facility inTheNetherlands. Laser incremental heatingwas carried out using a Syn-rad 48–5 CO2 laser. A typicalmass spectrometer run consists of steppingthrough the argon mass spectrum. Details of the analytical methodwere described by Wijbrans et al. (1995).

Sr and Nd isotopic analyses were performed at the Institute of Ge-ology and Geophysics, Chinese Academy of Sciences (Beijing). Isotopeanalyses were performed using a multi-collector VG354 mass spec-trometer. Rb, Sr, Sm and Nd concentrations were measured using iso-topic dilution method. 87Sr/86Sr ratios were normalized against86Sr/88Sr=0.1194, and 143Nd/144Nd ratios were normalized against146Nd/144Nd=0.7219. 87Sr/86Sr ratios were adjusted to the NBS-987Sr standard=0.710250 and 143Nd/144Nd ratios to the La Jolla Ndstandard=0.511860. The uncertainty in concentration analyses byisotopic dilution is ±2% for Rb, ±0.4–1% for Sr, andb±0.5% for Smand Nd, depending on concentration levels. The overall uncertaintyfor Rb/Sr is ±2% and for Sm/Nd is ±0.2–0.5%. Procedural blanksare: Rb=120 pg, Sr=200 pg, Sm=50 pg and Nd=50–100 pg. Thedetailed analytical procedures for Sr and Nd isotopic measurementsare given in Qiao (1988).

4. Results

4.1. Petrography

The adakite-like granitoid porphyries crop out in the northern partof the Eastern Pontides (close to the Hamsiköy–Zigana region)(Fig. 1b). They intruded into the Turonian–Santonian subduction-related volcanic rocks (Fig. 2a). The adakite-like porphyries are occur-rences of stockswith a number of small (b2 km) apophyses. A remark-able feature of the rocks is that each stock is almost homogeneous inpetrographical features. Thus, there is no gradual change from onerock type to another. All of the stocks are more or less homogeneous,and no enclave was found. The adakite-like porphyries form associa-tions of quartz monzonite–tonalite and granite–granodiorite porphy-ries, and tonalite porphyry dominates the assemblage. The rocks havea microgranular porphyric texture. They contain 25–35% phenocrystsof plagioclase and hornblende and have a matrix that is composedprimarily of fine-grained plagioclase, hornblende, orthoclase and

Fig. 6. Variations of some major and trace element with SiO2 for the early Eocene adakitic granitoid porphyries from the Eastern Pontides.

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quartz (Fig. 2b–d). Accessory zircon, apatite and Fe–Ti oxides are pre-sent in all of the rock types. Plagioclase is generally euhedral and lessthan 5 mm in size although some reach 10 mm in diameter, and theyexhibit no zonation. Orthoclase and quartz are generally anhedral,which is indicative of relatively late crystallization. Hornblende isthe major mafic mineral in the rocks and is characterized by a lathshape; biotite and augite occur very rarely as microcryst in all of therock types.

4.2. Geochronology

The radiometric data were obtained by the 40Ar/39Ar incrementalheating method. The results of the dating analysis are given inTable 1. Fig. 3 shows the results in the form of age spectra.40Ar/39Ar dating of two hornblende separates from the adakitic

rocks yielded plateau ages between 51.34±0.27 and 53.55±0.34,while an hornblende separate has a plateu age of 65.80±5.35 Ma.The samples yielded good plateaus over 80–100% of the gas release.In this respect, they yielded the best results, with reasonable enrich-ment in radiogenic argon and good agreement between plateau andisochron ages, except for sample HK5 with a plateau age of 65.80±5.35 Ma. The sample contains a low amount of K2O and thereforethe enrichment in radiogenic argon is low. This can also be seen inthe isochron plots (not shown). The alignment of points in the iso-chron plots seems to indicate that there is extraneous, perhaps inher-ited or excess 40Ar in the system. As a consequence the isochronresults point to a younger (27.6 Ma) age and a non-radiogenic com-ponent that is higher than air: 305.3 versus 295.5. The effect of thisdifference is large because of the low K2O content/low enrichmentin radiogenic argon. In such cases, the plateau ages are interpreted

Fig. 7. Diagrams of (a) Sr/Y vs. Y and (b) chondrite-normalized La/Yb ratios vs. Yb.Fields of adakite and TTG and arc calc-alkaline lavas are fromMartin (1999). An EasternPontides gabbro G518 (Dokuz et al., 2006) was used as the source rock for the REEmodeling under amphibolite and eclogite conditions, with varying garnet contentsand respective partition coefficients proposed by Irving and Frey (1978), Fujimakiet al. (1984) and Sisson (1994).

Fig. 8. (a) Chondrite normalized (to values given in Boynton, 1984) rare earth elementabundance patterns for the selected samples from the adakitic samples. (b) N-MORB-normalized multi-element variation patterns (normalized to values given in Sun andMcDonough, 1989) for the adakitic granitoid porphyries.

Fig. 9. Plot of εNd (51 Ma) versus ISr (51 Ma) of the adakitic granitoid poprhyriesfrom the Eastern Pontides. Data source are as follows: Cenozoic subducted oceaniccrust-derived adakites are after Defant et al. (1992), Kay et al. (1993), Sajona et al.(2000) and Aguillón-Robles et al. (2001); 400–179 Ma MORB are from Mahoneyet al. (1998), Xu et al. (2003), Tribuzio et al. (2004) and Xu and Castillo (2004); CentralAnatolian and Eastern Pontide lower crustal-derived volcanic fields and Eastern Pon-tides lower crustal-derived Saraycik granodiorite were taken after Varol et al. (2007),Karsli et al. (2010b) and Topuz et al. (2005), respectively. The fields of lower crustal-derived I-type Harşit and A-type Pirnalli plutons are after Karsli et al. (2010a) and Karsliet al. (in review), respectively.

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as anomalously old due to the presence of extraneous 40Ar. In addi-tion, previous studies strongly suggest that adakitic melt generationin the Eastern Pontides occurred in time interval between 50 and55 Ma (Eyüboğlu et al., 2011; Karsli et al., 2010b; Topuz et al., 2005;Topuz et al., 2011). Therefore, approximate crystallization age of theadakitic porphyries seem likely between 51 and 53 Ma rather than65 Ma.

4.3. Major and trace elements

Table 3 lists major- and trace-element data for the representativeadakite-like porphyries studied. They plot within the fields of quartzmonzonite, tonalite, granodiorite and granite in the total alkali-silicadiagram of Middlemost (1994) (Fig. 4). Compositional gap isobserved for the adakite-like granitoid porphyries in the diagrams,dividing the suite into two parts: quartz monzonite-tonalite associa-tion (SiO2=62.89 to 65.07 wt.%, with Mg#=43–47) and granite-granodiorite association (SiO2=69.06–70.43 wt.%, with Mg#b44).The granite–granodiorite porphyries are richer in K2O contents

Fig. 10. Diagrams of (a) Th/U versus U and (b) Th/U versus Th for the adakitic samples.LCC, lower continental crust; MCC, middle continental crust. The composition of lowerand middle continental crusts were taken from Rudnick and Gao (2003). Those ofMORB are from Sun et al. (2008). The composition of Seme adakitic rocks were takenfrom Eyüboğlu et al. (2011).

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(3.32 to 3.84 wt.%) relative to the other group. K2O contents of all thesamples (1.50 to 3.84 wt.%) are normally higher than those of typicaladakites derived from oceanic-slab melting (Defant and Drummond,1990). The samples are of I-type character with ASI [=molar Al2O3/(-CaO+K2O+Na2O)] ranging from 0.93 to 1.06. The quartz monzo-nite–tonalite association shows metaluminous to peraluminousfeatures, whereas the granite–granodiorite association display pera-luminous signature (Fig. 5a, b). Based on the alkali versus silica plot,the adakitic samples demonstrate calc-alkaline and high-K calc-alka-line natures, respectively (Fig. 5c). There are no strong variations inthe selected major oxide binary diagrams that are depicted in Fig. 6.The samples have much lower Mg# at a given level of SiO2 than doadakites in modern arcs (e.g., Martin et al., 2005; Smithies, 2000; Tat-sumi, 2006), but they are similar to the experimental melts derivedfrom metabasites (e.g., Rapp et al., 1999; Rapp and Watson, 1995;Xiong et al., 2005). The relatively low Ni concentrations (5–10 ppm)of the rocks are clearly different from those of modern adakitesfound in subduction zones worldwide, but they are similar to thelow-Mg adakitic rocks from some regions of China (e.g., Huang etal., 2009; Wang et al., 2007a; Zhao and Zhou, 2008). The rocks have

high Sr (200 to 1141 ppm) and low Y (6 to 14 ppm) contents. Theyalso display elevated Sr/Y (20 to 150) and La/Yb (11 to 40) ratios(Table 3). It should be particularly noted that these samples fall intothe adakite fields when plotted in Sr/Y versus Y and (La/Yb)n versus(Yb)n diagrams (Fig. 7a, b). Compared with the oceanic slab-derivedadakites in the subduction zone, they have also higher Th concentra-tions (7 to 15 ppm) and relatively high Th/U ratios (4 to 6).

The adakitic samples exhibit an enrichment of light rare earth el-ements (LREE) relative to heavy rare earth elements (HREE) with(La/Yb)n ratios ranging from 10 to 39 (Fig. 8a). Compared with thequartz monzonite–tonalite group, the granite–granodiorite associa-tion show HREE-enriched patterns. All the samples demonstrate neg-ligible Eu anomalies (Eu/Eu*=0.72–1.09), and are mostly more than1 (Table 3) with no concave-upwards patterns (Fig. 8a). TheirN-MORB-normalized trace-element patterns are characterized byenrichment of large-ion lithophile elements (LILE, e.g., Rb, Ba andTh) and depletion of high-field-strength elements (HFSE, e.g., Nband Ta) and show negative Ti anomalies (Fig. 8b).

4.4. Whole- rock Sr–Nd isotope composition

The measured and age-corrected Sr–Nd isotopic compositions ofthe adakite-like granitoid porphyries are given in Table 3. The por-phyries are representative of the compositional range observedfrom the most primitive through the most evolved members. In gen-eral, all the rocks show isotopic heterogenity and have radiogenic Sr[ISr=0.70551 to 0.70986] and Nd [εNd(51 Ma)=−8.5 to −0.9] iso-tope compositions, with the Nd model ages (TDM) of the samples rel-ative to the depleted mantle ranging from 0.72 to 1.26 Ga. Comparedwith modern adakites in subduction zones (e.g., Kay et al., 1993;Stern and Kilian, 1996; Yogodzinski et al., 1995), all the rocks havemuch higher radiogenic Sr and lower Nd isotopic compositions andare contained within a distinct field in a conventional Sr–Nd isotopicdiagram. In this diagram, the porphyries plot within the lower-rightquadrant, where they occupy an area between the Central Anatolianlower crustal-derived adakitic rocks (Varol et al., 2007), the I-typeHarşit (Karsli et al., 2010a), and the A-type granitoid rocks from theEastern Pontides (Karsli et al., in review) (Fig. 9). The adakitic por-phyries almost overlap the field of the Eastern Pontide lower-crustal-derived adakitic volcanics (Karsli et al., 2010b) and are closeto the Eastern Pontide lower-crustal-derived adakitic granitoids(Topuz et al., 2005). They have clearly much lower εNd(t) than dothe 400–179 Ma MORB (Mahoney et al., 1998; Tribuzio et al., 2004;Xu et al., 2003; Xu and Castillo, 2004) or the Cenozoic adakitesformed by slab melting (Aguillón-Robles et al., 2001; Defant et al.,1992; Kay et al., 1993; Sajona et al., 2000) (Fig. 9).

5. Discussion

5.1. Petrogenesis of the adakite-like porphyries

Many intrusive and volcanic rocks have been described as “ada-kite” because they clearly fall within the adakite field on commonlyused discrimination diagrams such as Sr/Y versus Y and (La/Yb)n ver-sus (Yb)n (Fig. 7a, b). An important question here is whether theserocks should be classified as adakite or adakitic based on such a geo-chemical signature. Firstly, adakites have been described as exclusive-ly products that were derived from the partial melting of subductingmetabasaltic sources, and therefore, they are consistently K-poor. Arestrictive characterization of adakites as K-poor products of slabmelting could fail to accommodate natural variations in mainly slab-derived adakite compositions; such variations could arise from sedi-ments on slabs or from interactions with the continental crust.Many studies have proposed possible models for the formation of K-rich adakitic signatures, including (i) crustal assimilation and frac-tional crystallization from parental basaltic magmas (e.g., Castillo

Fig. 11. Plots of (a) εNd (51 Ma) versus SiO2%, (b) ISr (51 Ma) versus SiO2%, (c) ISr (51 Ma) versus 1/Sr, (d) LogSr versus LogEu/Eu* and (e) La/Yb versus La (ppm) for the adakiticgranitoid porphyries from the Eastern Pontides.

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et al., 1999; Macpherson et al., 2006; Zhu et al., 2008) and (ii) partialmelting of delaminated (Kay and Kay, 1993; Wang et al., 2006; Xu etal., 2002; Xu et al., 2006) or thickened mafic lower continental crust(Atherton and Petford, 1993; Huang et al., 2009; Xiong et al., 2003).Hence, there is still considerable debate as to the origin of adakiticmelts. These models are briefly discussed below for the adakiticrocks studied.

5.2. Partial melting of the subducted oceanic crust

The possibility that melting of the subducting Neotethyan oceaniccrust generated the Eastern Pontide adakitic porphyries can be ruledout based on their geochemical and isotopical signatures and on thetectonic context of the region during the early Cenozoic time. Theadakitic rocks are different from slab-derived adakites in that: (i) oce-anic slab-derived adakites are generally metaluminous, whereas theadakitic rocks studied are metaluminous to peraluminous (Fig. 5a,b); (ii) the samples have much higher K2O (0.64 to 3.25%) and Th(7 to 15 ppm) contents, and Th/U (4 to 16) ratios (Table 2; Fig. 10a,b); (iii) they have much lower εNd (−8.5 to −0.9) and higher ISr(0.70551 to 0.70986) isotopic values than those of the subducted oce-anic slab-derived adakites and MORB (Fig. 9); and (iv) the sampleshave low Mg# (37–47) values and Ni (5–10 ppm) contents. Indeed,these characteristics differ markedly from those of oceanic slab-derived

adakite melts (Mg#N47; Martin, 1999; Smithies, 2000). However,based on only whole-rock major- and trace-element data, a recentwork (Eyüboğlu et al., 2011) has claimed that the Seme adakitic rocks,which have the same age and are ~150 km south of the studied porphy-ries, originated from partial melting of a subducted oceanic crust, al-though the rocks have similar geochemical characteristics to those ofthe studied adakite-like porphyries. Moreover, Topuz et al. (2011) hasmore recently showed that the Seme sampleswere derived frompartialmelting of thickened lower crust rather than melting of oceanic slab.The Seme samples have low εNd (51 Ma) values (−1.1 to 1.0) andboth their low Mg# (22 to 43) and higher Mg# (51 to 62) values,completely peraluminous geochemical nature, positive Sr anomaliesand no Eu anomalies (Topuz et al., 2011) arguing against such an originof oceanic slab-derived melt. Furthermore, the Th/U values of both theadakitic porphyries and the Seme samples plot in proximity to thefield of the lower continental crustal melts rather than in the area ofthe N-MORB-derived melts (Fig. 10a, b). Due to chemical disequilibri-um, slab-derived, siliceous melts can rise into and metasomatize theoverlying mantle wedge during their ascent (Beard et al., 1993; Şenand Dunn, 1994). Interactions between the slab melts and the mantlewedge cause enrichment of Mg and Fe in the melt (Killian and Stern,2002; Şen andDunn, 1994; Xiong et al., 2006). Although the terrigenoussediments in the subduction zonesmight contribute to the negative εNd(t) values and the arc-like signature, the presence of sediments does not

Fig. 12. (a) MgO vs. SiO2 diagram for the studied samples. The adakites produced bymelting of oceanic slabs, adakites fromed by delamination of lower crust, and metaba-saltic and eclogite experimental melts (1–4.0 GPa) are fromWang et al. (2006) and ref-erences therein. Adakites formed by partial melting of the thickened mafic lower crustare the reference field (Atherton and Petford, 1993; Chung et al., 2003; Johnson et al.,1997; Muir et al., 1995; Petford and Atherton, 1996; Wang et al., 2005). (b) Ternarydiagramof Fe2O3–K2O–MgO for the adakitic rocks from the Eastern Pontides. Data for ada-kitic rocks originated by partial melting of the subducted oceanic crust (Aguillón-Robleset al., 2001; Defant and Drummond, 1993; Kay et al., 1993; Stern and Kilian, 1996;Yogodzinski et al., 1995) and adakitic rocks originated by partial melting of thickenedmafic lower crust are given for reference field using the data source in Fig. 12a. The grayfield represents experimental melts of metabasalts at 16–22 Gbar (Rapp and Watson,1995).

Fig. 13. (a) Nb/Ta versus Zr/Sm diagram (after Condie, 2005) for the samples. (b) Com-positions of the adakitic rocks in comparison to compositional fields of experimentallyderived partial melts of metapelites, metagreywackes and amphibolites.Data were taken from Patiño Douce (1999).

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explain the lowMg# values and Ni contents that are similar to those ofthe porphyries and the Seme samples; this suggests that they did notexperience interaction with the mantle rocks and then the samplesfrom the region clearly do not show a MORB-like signature. Therefore,Pontide adakitic rocks have not to be addressed as partialmelts of a sub-ducted oceanic slab. They favor a lower continental crustal melt, as isclearly stated in Topuz et al. (2011). More importantly, in the late-Mesozoic-to-early-Cenozoic continental configuration of the region,there is broad agreement that the Neotethyan Ocean to the south ofthe Pontides was completely exhumed by northward subduction andconsequently the Pontide block collided with the Anatolide block (e.g.,Boztuğ et al., 2006; Çinku et al., 2010; Karsli et al., 2010a; Karsli et al.,in review; Meijers et al., 2010; Okay and Şahintürk, 1997; Okay andTüysüz, 1999; Rice et al., 2006; 2009; Robinson et al., 1995; Ustaömerand Robertson, 2010). Lack of coeval-arc magmatism and associatedaccretionary prism assemblage is in accordance with the adakiticmelt generation in a subduction setting. All of these findings clearly

rule out the adakitic-melt generation from oceanic-slab meltingand thus the possibility of subduction beneath the region duringthe early Cenozoic time.

5.3. Generation by assimilation and fractional crystallization from aparental basaltic magma

Melt generation for adakitic products is formed by high-pressurefractionation (involving garnet) of hydrous basaltic magma (e.g.,Macpherson et al., 2006; Prouteau and Scaillet, 2003) and crustal assim-ilation and low-pressure fractional crystallization (involving olivine+clinopyroxene+plagioclase+hornblende+titanomagnetite) from aparental basaltic magma (e.g., Castillo et al., 1999). The adakitic rocksformed by high-pressure fractional crystallization (involving garnet) ofhydrous basaltic melts show unique geochemical trends (Macphersonet al., 2006; Prouteau and Scaillet, 2003). The positive correlationbetweenSr/Y ratios and SiO2 content which is expected for fractionation is not ob-served for the adakitic porphyries (Fig. 6g). This suggests that the highSr/Y ratio and low HREE content were not related to fractionation

Fig. 14. R1–R2 diagram of Batchelor and Bowden (1985) for the adakitic granitoid por-phyry samples. The adakitic granitoid porphyry rocks fall into the post-collision upliftfield. R1=4Si–11(Na+K)–2(Fe+Ti); R2=6Ca+2 Mg+Al.

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processes. Similarly, the absence of increasing Al2O3 and the decrease ofLa contents with increasing SiO2 content (Fig. 6a, h) weaken the possibil-ity of high-pressure fractional crystallization of hydrous basaltic melts ingeneration of the rocks. Castillo et al. (1999) claim that adakitic rockscould be produced by assimilation and fractionation of a parental basalticmagma. However, the adakitic porphyries lack clear compositional trendsthat would be indicative of fractional crystallization (Fig. 6a–h). The ab-sence of these major element variations suggests that the fractionationprocess does not play amajor role in their generation. The samples exhibitpositive Sr and have no significant Eu anomalies (Fig. 8a, b), which is ev-idence against a significant role for plagioclase fractional crystallization intheir genesis. The samples display relatively flat patterns between LREEand HREE (Fig. 8a). If fractional crystallization was the dominant processcontrolling their evolution, removal of plagioclase and amphibole wouldproduce concave-upwards patterns. Additionally, there are no co-existingmafic rocks associated with these adakitic rocks in the region. All of theevidence suggests that the adakitic rocks could not have been producedby a high- or low-pressure fractional crystallization process involving aparental basaltic magma. The lack of variations on the binary plots oflogSr versus logEu/Eu* (Fig. 11d) are accordance with the unfractionatedcharacter of the adakitic samples; instead, the trend seen in the plot ofLa/Yb ratios versus La contents is more consistent with the suggestionthat the partial melting process controlled the compositional variation.The εNd (t) values and the ISr (t) ratios of the adakitic samples shown inFigs. 11a and b exhibit no obvious variations with increasing SiO2 con-tents, indicating that crustal assimilation did not play a significant rolein their generation. The lack of visible variation in the plot of ISr (t) ratiosversus 1/Sr values (Fig. 11c) rules out the probability of magma mixingduring the magma evolution of the adakitic porphyries.

5.4. Partial melting of the delaminated lower crust

Lower crustal delamination may represent an important processin the differentiation of the continental lithosphere (e.g., Ducea andSaleeby, 1998; Kay and Kay, 1993; Zandt et al., 2004). Partial meltingof the delaminated lower crust is a probable petrogenetic model forgeneration of adakitic melts (e.g., Kay and Kay, 1993; Liu et al.,2008; Wang et al., 2006; Xu et al., 2002; Xu et al., 2006; Zhai et al.,2007). The delamination process is believed to be the thinning of lith-osphere as a consequence of the sinking of crustal material from thebase of crust into the underlying mantle. In this case, the delaminatedsection of the lower crust is heated by the surrounding relatively hotmantle and undergoes partial melting (e.g., Kay and Kay, 1993). Theresultant melt significantly interacts with the mantle peridotite, ele-vating the MgO and Mg# values of the melt, while the melt risesthrough a zone of mantle peridotite en route to its emplacement inthe upper crust (e.g., Smithies, 2000; Wang et al., 2006). However,the low MgO contents and Mg# values (mostly b47) of the EasternPontide adakitic porphyries suggest that they did not experience in-teraction with mantle material. The samples have lower MgO andMg# relative to those adakitic rocks formed by partial melting ofthe delaminated lower crust and the subducted oceanic slab(Fig. 12a). This petrologic evidence combined with regional geologi-cal data leads to the suggestion that the adakitic porphyries werenot formed by partial melting of the delaminated lower crust.

5.5. Partial melting of the thickened lower crust

The most probable petrogenetic model for generating the adakiticmelts is partial melting of a mafic lower crust in the region during theearly Cenozoic. In the plot of La/Yb ratio versus La content, the sam-ples display compositional trends that are consistent with partialmelting (Fig. 11e), suggesting that this process was responsible fortheir generation. In contrast to the delamination model, the adakiticmelts that originated from the thickened lower continental crust donot need to pass through the mantle wedge; therefore, they lack

indications of interaction with mantle rocks that would elevate theirMgO and Ni contents. If the adakitic magmas are derived from amafic lower crust, they should have a relatively low MgO content,and they should be compositionally similar to the experimentalmelts of metabasalts at 1.6 GPa of Rapp and Watson (1995) and ofmetabasalt and eclogites at 1–4.0 GPa of Rapp and Watson (1995)and Rapp et al. (1999). Indeed, the adakitic samples show trendsthat are very similar to those of the experimental melts producedby the melting of metabasalts (Fig. 12a, b). In addition, the adakiticporphyries with low Mg# (b47) have same coherent trends as dothe adakitic samples that were formed by partial melting of a thick-ened lower crust (Fig. 12a, b), suggesting that they are more likelyto have been produced by partial melting of a thickened maficlower crust. The data for adakites worldwide show that typical slabmelts have low Rb/Sr ratios (0.01–0.05); this is in contrast to thewide range of Rb/Sr ratios (0.01–0.4) for the adakitic rocks that orig-inated from thickened continental lower crust (Huang et al., 2009).Actually, the relatively higher Rb/Sr ratios (0.04–0.5) of the adakiticporphyries are consistent with their derivation from the thickenedlower continental crust rather than a slab melting. Experimentalstudies have shown that garnet and rutile, but little or no plagioclase,are present in the residual phase, adakitic melts can be produced bymelting of mafic rocks at pressures equivalent to crustal thickness ofN40 ~50 km (1.2–1.5 GPa) (e.g., Rapp et al., 1999; 2002; Xiao andClemens, 2007; Xiong et al., 2005). On a plot of Nb/Ta versus Zr/Sm,granite–granodiorite porphyries plot in the field of amphibole andhornblende eclogite melting, while quartz monzonite–tonalite por-phyries commonly lie in the field just above or close to the boundarybetween amphibole and hornblende eclogite melting and rutile eclo-gite melting (Fig. 13a). This suggests that the crustal thickness of theEastern Pontides was of N50 km and that their protolith underwentpartial melting at depths of lower crust is heterogeneous. Similarly,the negative Nb, Ta and Ti anomalies of the adakitic porphyries sug-gest that their source region may have melted under higher pressureconditions in which rutile was a residual phase at a pressure of morethan 1.5 GPa (Barth et al., 2002; Xiong et al., 2006). Such an evidenceimplies that the crust beneath the region was thickened during theearly Cenozoic. We also conclude that the protolith of the adakiticporphyries is a thickened mafic lower continental crust that under-went partial melting by underplated hot basic magma, possiblyinduced by asthenospheric upwelling due to slab break-off of oceanicslab during the early Cenozoic in the Eastern Pontide region.

Fig. 15. Tectonic framework illustrating the crustal evolution and mechanism for crustalmelting beneath the Eastern Pontide region. (a) Subduction of the İzmirAnkara-Erzincanoceanic crust (e.g. Akin, 1979; Okay and Şahintürk, 1997; Şengör and Yilmaz, 1981)and emplacement of subduction-related granitoids (Karsli et al., 2010a; Karsli et al., inreview). (b–c) during the early Paleocene to early Eocene period, beginning of conti-nent–continent collision and initiation of slabreakoff (e.g., Okay et al., 1997; Okay andŞahintürk, 1997). (d) Partial melting of the thickened mafic lower crust produced the pri-mary adakitic melts which emplaced into the upper crust.

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5.6. Source characteristics of the adakitic porphyries

Adakitic affinities, such as the relative enrichment of Sr and thenegligible or lack of Eu anomalies, indicate that the source residuewas plagioclase-free after melt extraction. Depletion of HREEs and Yin adakitic rocks requires melting of mafic source rocks within a sta-bility field of garnet under eclogite-facies conditions (e.g., Defantand Drummond, 1990; Wilson, 1989). Adakitic melt to form adakiticrocks from such a garnet-bearing source can be achieved via partialmelting of the subducted oceanic slab (Defant and Drummond,1990) and thickened lower continental crust (Petford and Atherton,1996). In terms of their Sr–Nd isotope compositions, the adakitic

porphyries also show a resemblance to the lower crustal-derived ada-kitic rocks (Karsli et al., 2010b; Topuz et al., 2005; Topuz et al., 2011;Varol et al., 2007) from eastern Turkey. As demonstrated in the previ-ous sections, a thickened mafic lower continental crust seems likelyfor the potential magma source. Accordingly, Rapp et al. (1991) andChung et al. (2003) suggest that the adakites may form by the meltingof the mafic lower continental crust which could be either eclogite orgarnet amphibolite. In addition, experimental studies show that gar-net will be sufficiently stable within the residual assemblage (e.g.,garnet–amphibolite, amphibole-bearing eclogite and eclogite) to pro-duce adakitic melt at sufficient depths ofN40 km (Rapp et al., 2003).The HREE of the adakitic porphyry samples show flattened patterns.This suggests that amphibole played a more important role than gar-net during partial melting, and that garnet-amphibolite rather thaneclogite is likely to be the source lithology. If amphibole is residualin the source, it will induce concave-upward patterns between mid-dle and heavy REE (Rollinson, 1993). This was further supported bythe modeling using element pairs La/Yb and Yb (Fig. 7b). The La/Ybversus Yb modeling shows that the adakitic porphyries can be gener-ated by ~5–20% melting of the presumed garnet-amphibolite with itsgarnet constituents varying from ~3–15%; eclogite melting does notmatch the data (Fig. 7b). In comparison with experimentally derivedmelts, the adakitic samples have both low Al2O3/(FeO+MgO+TiO2)and high Al2O3+FeO+MgO+TiO2 values. They mainly plot in thefield of amphibolite-derived melts (Fig. 13b), which also suggeststhat they are derived from an amphibolitic source. In summary, an or-igin via partial melting of a thickened, garnet-bearing, amphiboliticlower continental crust to produce the adakitic melts in the EasternPontide region is consistent with specific chemical and isotopic data.

5.7. Geodynamic mechanisms

The occurrence of many adakitic volcanic and intrusive rocksthroughout the Eastern Pontides during the late Paleocene to theearly Eocene shows that melt generation was not a local petrologicevent; their formation and interpretations are of regional significance.The thermal sources and melting models have long been subjects ofdebate. There is a general consensus that throughout the late Creta-ceous there was a northward subduction beneath the Eastern Pon-tides in which many subduction-related intrusives were emplaced(e.g., Altherr et al., 2008; Boztuğ et al., 2004; Boztuğ et al., 2006;Çinku et al., 2010; Karsli et al., 2010a; Karsli et al., in review; Meijerset al., 2010; Okay and Şahintürk, 1997; Okay and Tüysüz, 1999;Rice et al., 2006; Rice et al., 2009; Robinson et al., 1995; Şengör andYilmaz, 1981; Topuz et al., 2011; Ustaömer and Robertson, 2010). AMaastrichtian–early Paleocene plagioleucitite body in the Bayburtarea of the region is regarded to be one of the final products of thenorthward subduction of the northern branch of the Neotethys(Altherr et al., 2008).

Major crustal shortening that probably coincides with quiescentperiod in the Eastern Pontides occurred during the Paleocene. Thecollisional stage that produced a tectonically thickened crust in theregion is attributed to this period (e.g., Boztuğ et al., 2004; Karsliet al., 2010b; Okay et al., 1997; Okay and Şahintürk, 1997; Okay andTüysüz, 1999). According to Karsli et al. (2010b), the first stageof the post-collision extensional events that followed the crustalthickening began at 50 Ma. In contrast, Eyüboğlu et al. (2011) haveclaimed that the late Cretaceous subduction event continuedthroughout the early Cenozoic time. This claim was based onwhole-rock major- and trace-element data of Seme adakitic rocks(ca. ~50 Ma) without N-MORB-like isotopic and coherent geody-namic data. On the other hand, Topuz et al. (2011) has more recentlyargued against that the Seme samples were derived from partialmelting of oceanic slab in a subduction setting, but favor an originof partial melting of thickened mafic lower crust in a post-collisionuplift phase. Additionally, paleomagnetic data of Hisarli (2011) from

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the Eastern Pontides clearly show evidence of oroclinal bending thatresulted from the convergence between Arabian block and Eurasiain the Paleocene. The shape of oroclinal bending is interpreted to beresult of the continent–continent collision in the region. All these ar-guments are inconsistent with the subduction events continuing inthe region during the early Eocene.

The adakitic granitoid porphyries were emplaced into the Turo-nian–Santonian subduction-related volcanic rocks. Hornblende sepa-rates from the adakitic granitoid porphyries yielded an age of 51.34±0.27 to 53.55±0.34 Ma. The time interval is considered to be relatedto the post-collisional uplift phase pointing to compression in theEastern Pontides (e.g., Boztuğ et al., 2004; Boztuğ et al., 2006; Karsliet al., 2010b; Okay et al., 1997; Okay and Tüysüz, 1999; Topuz et al.,2005; Topuz et al., 2011; Ustaömer and Robertson, 2010). Additional-ly, both the adakitic porphyries of this study and the Seme samplesplot within the field of post-collision uplift on the R1–R2 diagram(Fig. 14) that was designed by Batchelor and Bowden (1985). Thisclearly implies that subduction events had ended before these rockswere formed. Furthermore, the peraluminous to metaluminous na-ture, the lack of significant Eu anomalies, the negative εNd(51 Ma)values and the high ISr values (mainly N0.705) of the Eastern Pontideadakitic rocks rule out their generation in a subduction zone by par-tial melting of oceanic slab. There is no an explanation in the recentworks as to what caused the magmatic gap and the ungularunconformity that is evident throughout the Paleocene if the subduc-tion events had continued during this time interval. Partial melts havegeochemical and isotopic signatures as stated in earlier sections thatcan easily be produced by the melting of a thickened mafic lowercontinental crust.

Proposed models for the thermal-anomaly sources and melt pro-duction in a post-collision setting are (a) convective removal of thelithosphere or delamination (Houseman et al., 1981; Turner et al.,1996), (b) intracontinental subduction (Arnaud et al., 1992; Dinget al., 2003; Tapponnier et al., 2001), and (c) slab break-off of oceanicslab (Kohn and Parkinson, 2002; Miller et al., 1999). All of thesemodels for the adakitic melts emphasize potential heating from as-thenospheric upwelling beneath the region. Xu et al. (2002) proposedthat the lower part of the thickened crust sank into the relativelyhot mantle and then themelting of the lower crustalmaterial generatedadakitic melts that have highMgO andMg# values due to melt–mantleinteraction during its ascent. This is not the case for the adakite-likegranitoid porphyries, indicating that the melts were not derived frompartial melting of a delaminated lower crust and a subducted continen-tal crust. We therefore favor the slab break-off model, in which the as-thenospheric upwelling is also the major heat source for derivation ofadakitic magmas by the melting of a thickened mafic lower continentalcrust. In the region, once the rupture is initiated, detachment of oceaniclithosphere occurs along the subduction plane, triggering the upwellingof asthenosphere under the collision zone. Then, partial melting of theenriched upper-lithospheric mantle modified by earlier subductionfluids in response to a hot asthenospheric upwelling possibly resultingfrom the slab break-off produced underplated mafic magma in thelower part of a thickened mafic lower continental crust. The under-plated basic magma led to partial melting of a thickened heterogeneouslower continental crust to form the adakitic melts in the region duringthe early Cenozoic (Fig. 15a–d). Therefore, our results are inconsistentwith the possibility that subduction existed and played an importantrole in the petrogenesis of the adakitic activity in the Eastern Pontidesduring early Cenozoic time.

6. Conclusions

The metaluminous-to-peraluminous, I-type-calc-alkaline andhigh-K-calc-alkaline adakitic granitoid porphyries from the EasternPontides are characterized by low Mg#, extremely high Sr and Sr/Y,low Y and HREE concentrations, lack of significant Eu anomalies,

and crustal-like ISr (0.70574 to 0.70940) and εNd(51 Ma) (−8.5 to−1.1) isotopic compositions. 40Ar/39Ar technique of incrementalheating suggests that the adakitic rocks were emplaced in the Paleo-cene to early Eocene (51 to 53 Ma). The absence of coeval arc magma-tism and associated accretionary prism assemblage, together with aquiescent period of magmatism soon after the late Cretaceous sub-duction or continent–continent collision tectonic regime during thelate Paleocene to early Eocene, strongly imply that the adakiticmelts of the region postdated the late Cretaceous subduction-related magmas. Instead, the adakitic melts are considered to havebeen produced by partial fusion of a thickened garnet-bearing,amphibolitic heterogeneous lower continental crust in response tohot upwelling of asthenosphere, possibly triggering by the slabbreak-off of oceanic slab in a post-collision uplift phase representingcompression. Therefore, the generation of the early Cenozoic EasternPontide adakitic rocks does not necessitate contemporaneous sub-duction of a young and hot oceanic slab as proposed by a previous hy-pothesis for the region.

Acknowledgments

We aremost grateful to the staff of the Isotopic Laboratory in the In-stitute of Geology, Chinese Academy of Geological Sciences in Beijingfor the Sr–Nd isotopic analyses. Financial support for this workwas pro-vided by the Scientific and Technological Research Council of Turkey(TÜBITAK) with grant # 107Y177. Detailed comments of Nelson Ebyand two anonymous reviewers significantly improved the manuscript.We wish to acknowledge discussions for geodynamic processes withMurat Erduran.

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