The petrogenesis of Sarıçimen (Çaldıran-Van) quartz monzodiorite: Implication for initiation of magmatism (Late Medial Miocene) in the east Anatolian collision zone, Turkey

Lithos 119 (2010) 607–620

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The petrogenesis of Sarıçimen (Çaldıran-Van) quartz monzodiorite: Implication for initiation of magmatism (Late Medial Miocene) in the east Anatolian collision zone, Turkey Ali Riza Çolakoğlu a,⁎, Greg B. Arehart b a b

Department of Geological Engineering, Yüzüncü Yıl University, TR-65080 Van, Turkey Department of Geological Sciences, University of Nevada-Reno, Reno, NV 89557, USA

a r t i c l e

i n f o

Article history: Received 24 February 2010 Accepted 7 August 2010 Available online 13 August 2010 Keywords: Geochemistry Geodynamics Quartz monzodiorite Late medial miocene Eastern Anatolian Accretionary Complex

a b s t r a c t The Sarıçimen porphyry is exposed as a sub-volcanic pluton within the Upper Cretaceous ophiolitic rocks in East Anatolian Accretionary Complex. The pluton is quartz monzodioritic in composition consisting of feldspar, hornblende, and biotite phenocrysts set in a fine-grained matrix. Major element geochemistry indicates the pluton is of high-K, calc-alkaline, metaluminous character, with a low (0.81–0.90) Aluminum Saturation Index (ASI). Trace element and sulfur isotope geochemistry suggests that the Sarıçimen porphyry was mantle-derived and contaminated by crustal materials during ascent. Tectonically, this and related volcanic and plutonic rocks in eastern Turkey and Iran are subduction-related and comprise the earliest documented neotectonic igneous activity associated with the final closure of the neo-Tethys between the Arabian and Eurasia plates at ~ 14–13 Ma. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Eastern Anatolia is a very special area from the point of view of global tectonics. In fact it faces the northward-moving Arabian plate, and separates two diverging microplates, Anatolia to the west and Iran to the east (McKenzie, 1972). It has a high plateau caused by strong N–S compression since the Miocene due to the continuing convergence between the Arabian and Anatolian (Eurasian) plates; this deformation continues today (Şengör and Kidd, 1979; Dewey et al., 1986; Yılmaz et al., 1987; Yılmaz, 1990; Yılmaz et al., 1998; Şengör et al., 2003, 2008; Dhont and Chorowicz, 2006). Because of the young volcanic activity, neotectonism, and uplift in the region, north of Lake Van peak elevations of volcanic cones such as Nemrut, Suphan, Tendurek and Mount Ağrı increase from southwest to northeast (Fig. 1). Mount Ağrı, at 5156 m, is the highest mountain of Turkey. The whole area is covered by thick volcanic products from eruptions that began in the Miocene (e.g., Innocenti et al., 1976; Şengör and Kidd, 1979; Şengör et al., 2008) and continued almost without interruption into historical times (Yılmaz, 1990; Şengör et al., 2008). In Eastern Anatolia, volcanic-related studies are abundant because of the regional interest in global tectonics. These studies can be summarized as follows: geology-based studies (e.g., Özpeker, 1973; Lambert et al., 1974; Şengör et al., 2008), age and geochemistry based

studies (Innocenti et al., 1976; Ercan et al., 1990; Pearce et al., 1990; Keskin, 2003; Keskin et al., 1998; Şengör et al., 2008), geochemistry and genesis of the volcanism-based studies (Innocenti et al., 1980; Pearce et al., 1990; Yılmaz et al., 1987; Yılmaz, 1990; Keskin, 2003; Keskin et al., 1998; Buket and Temel, 1998; Yılmaz et al., 1998) and proposed model studies for magma generation and crustal thicknesses (e.g., Keskin, 2003, 2007; Şengör et al., 2003; Keskin et al., 2006; Barazangi et al., 2006; Şengör et al., 2008). In these studies, some general models for volcanism have been proposed. Keskin (2003) and Şengör et al. (2008) indicate that characteristics of the volcanic rocks are calc-alkalic in the north, transitional in the central, and alkalic in the south parts of the plateau. Innocenti et al. (1976, 1980, 1982) indicates a temporal trend in magmatism, beginning with calc-alkaline in Burdigalian, transitional high-K calc-alkaline (Serravallian to Pliocene), to alkaline in the past ~6 Ma. In this study, we present major and trace elemental compositions, K–Ar age and sulfur isotope data from a set of first-described quartz monzodioritic rocks in this region. The purpose of this paper is to contribute to understanding of the magmatism and its relationship to tectonism in Eastern Anatolia region based on the new geological data. 2. Geological setting 2.1. Regional geology

⁎ Corresponding author. Tel.: + 90 432 2251024x2964; fax: + 90 4322251732. E-mail address: [email protected] (A.R. Çolakoğlu). 0024-4937/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2010.08.014

In Eastern Anatolia, basically four different type of rock units have been described which are from oldest to youngest: (i) Palaeozoic to

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Fig. 1. General geological map of Eastern Turkey showing the location of the study area. Modified from Keskin, 2003.

Lower Mesozoic metamorphic rocks that form the basement (Perinçek, 1980; Yılmaz et al., 1993; Göncüoğlu and Turhan, 1984); (ii) Upper Cretaceous ophiolitic melange (Ketin, 1977; Şengör and Yılmaz, 1981; Yılmaz, 1993; Yılmaz et al., 1993) typically described as Late Cretaceous to Oligocene East Anatolian Accretionary Complex (EAAC) (Şengör et al., 2003; Şengör et al., 2008). The EAAC can be regarded as a remnant of a large accretionary prism located between the Pontides and the Bitlis-Pötürge Massif, having been formed on a northward-subducting oceanic lithosphere (Fig. 1). The EAAC is composed of ophiolitic melange and flysch, produced by progressive consumption of Tethyan oceanic lithosphere (Şengör and Yılmaz, 1981; Yılmaz, 1990). (iii) Eocene to Early Miocene oceanic sedimentary rocks comprise the remnants of the Tethyan ocean as it was closed by compressional events (Şaroğlu and Yılmaz, 1986; Şengör and Yılmaz, 1981; Şengör et al., 2008) and (iv) Mid-Miocene and younger calc-alkaline to alkaline volcanic rocks (Innocenti et al., 1980; Yılmaz et al., 1987; Keskin, 2003; Şengör et al., 2008). Volcanic rocks of Mid-Miocene age and younger are the result of subductionrelated volcanism, typically post-collisional in nature (Yılmaz et al., 1987; Ercan et al., 1993; Keskin, 2003; Şengör et al., 2008). 2.2. Local geology The study area is situated within a typical ophiolitic mélange comprising serpentinite, peridotite, marine and continental sedimentary units, and mafic to intermediate igneous units (Fig. 2). Pelagic sediments composed of claystone, marl and clayey limestones are the main units in the study area. Serpentinite covers the

western and northern sides of the map area. The serpentinite contains some chrysotile (1–3 cm), calcite (2–5 cm), dolomite (5– 20 cm) and opal-chalcedony (10–150 cm) veins and veinlets which are elongated in the late stage NW and NE directions (Fig. 2). Fe–Ni laterites are present in the center of the map area, and were deposited as slightly ferruginous conglomerates and clast-supported conglomerates. The clasts within these conglomerates confirm that minerals and components have not formed in a primary lateritic crust but by erosion of the original lateritic components and their transport in a shallow marine environment near the ophiolities (Çolakoğlu, 2009). These laterites are cut by the Sarıçimen quartz monzodiorite porphyry (originally described petrographically as diorite porphyry by Çolakoğlu, 2009), which crops out as several sub-volcanic plutons within the Upper Cretaceous ophiolitic rocks of the EAAC. Additional outcrops of micro-diorites are present to the southeast of the map area and extend farther southeast outsite of the map area. Pliocene basalts (probably of broadly similar age to Tendürek alkali volcano, 10–15 km north) stratigraphically overlie all other rock units and are the youngest units of the study area (Fig. 2). Structurally, the area is complex (Fig. 2). Thrust faults are the oldest mapped structures in the area. Some of these thrusts can be related to obduction of the ophiolites beginning in Cretaceous time, and may have been active as late as the early–mid Miocene as they clearly cut Miocene laterites related to the paleotectonic era. However, they do not cut the pluton, thus movement likely ceased, at least on these particular faults, prior to mid-Miocene. Strike-slip faults clearly cut the pluton; although they may have earlier origins, the latest movements are clearly post mid-Miocene in age.

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Fig. 2. Geological map of the study area. Modified from Çolakoğlu, 2009.

2.2.1. Quartz monzodiorite porphyry Quartz monzodiorite porphyry crops out in five different locations in the map area (Fig. 2). The maximum dimensions of the largest stock are 300 × 400 m. One of these intrusions cuts the lateritic zone (Fig. 2). Macroscopically, these rocks exhibit typical onion-skin weathering texture due to cooling and weathering (Fig. 3a). In the fresh hand

specimen, samples are white to beige in color and fine- to mediumgrained in size (Fig. 3b). In thin section, the Sarıçimen quartz monzodiorite porphyry is holocrystalline with a porphyritic texture. It consists of euhedral prismatic plagioclase phenocrysts up to ~3 mm in size with minor amphibole, biotite and K-feldspar. The groundmass is completely microcrystalline. Green-colored amphibole

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Fig. 3. a–b, macro-photos and c–d, photomicrographs of the quartz monzodiorite porphyry. a. Onion-skin weathering textured of pluton. b. Hand specimen view of the rock. c. Zoned plagioclase, biotite and hornblende in quartz monzodiorite porphyry (cross-polarized transmitted light). d. Transmitted plane of the same view. Pl. plagioclase; Hb. hornblend; Bi. biotite (hammer is 35 cm long).

(hornblende), reddish-brown biotite, K-feldspar and minor clinopyroxene and quartz are set in this groundmass (Fig. 3c–d). Titanite, apatite and disseminated opaque minerals are the accessories. Opaque minerals are scattered and mainly occur within the biotite and the groundmass. 3. Analytical methods Nine fresh samples were collected from the study area and analyzed for major, trace and REE composition. All samples were analyzed at ALS Chemex Laboratory in Canada. Major oxides were analyzed using ICP-AES and trace and REEs were analyzed using inductively coupled plasma mass spectrometry (ICP-MS). Analytical data are reported in Table 1. Two fresh samples were taken from the study area and analyzed for K–Ar age at CSIRO in Australia. These samples were washed with de-ionized water and dried at 60 °C overnight. Several grams from each sample were crushed with a shatter box (Siebtechnik ring grinder with chrome steel barrel) to b750 micron. Potassium content was determined in duplicate by atomic absorption (Varian Spectra AA 50) using Cs at 1000 ppm concentration for ionisation suppression. The pooled error of duplicate K determination of all samples and standards is better than 2%. The K blank was measured at 0.35 ppm = 0.000035% K. Sample splits for Ar analysis were preheated under vacuum at 80 °C for several hours to reduce the amount of atmospheric Ar adsorbed onto the mineral surfaces during sample handling. Argon was extracted from the separated mineral fractions by fusing samples within a vacuum line serviced by an on-line 38Ar spike pipette. The isotopic composition of the spiked Ar was measured with a high-sensitivity on-line VG3600 mass spectrome-

ter. The 38Ar spike was calibrated against international standard biotite GA1550 (McDougall and Roksandic, 1974). The error for Argon analyses is below 1.00% and the 40Ar/36Ar value of the air standard that was used is 295.36 ± 0.28. The K–Ar ages were calculated using 40K abundance and decay constants recommended by Steiger and Jager (1977). The age uncertainties take into account the errors during sample weighing, 38Ar/36Ar and 40Ar/38Ar measurements and K analysis. K–Ar age errors are reported as 2 sigma uncertainty. Analytical results are reported in Table 2. Total sulfur was extracted from 5 whole-rock powder samples using the Kiba method (Sasaki et al., 1979) and sulfur isotope analyses of Ag2S performed on a continuous-flow Micromass Isoprime mass spectrometer using SO2 gas as the analyte. Precision on sulfur isotope analyses is ±0.25‰. Analytical results are reported in Table 3. 4. Geochemistry Major, trace and rare earth element contents of the quartz monzodiorite plutons from the Sarıçimen area are presented in Table 1. All analyzed rock samples plot in the quartz monzodiorite field using the Q–P classification of Debon and Le Fort (1983) (Fig. 4). In addition, all samples plot in field IV of the B–A classification of Debon and Le Fort (1983) (Fig. 5). These geochemical data are very consistent with the mineralogical composition of the Sarıçimen quartz monzodiorites, which typically have biotite + hornblende ± pyroxene assemblages. Whole-rock K concentrations are higher than 2.4 wt. % (Table 1). Major element data indicate that the Sarıçimen quartz monzodiorite porphyries are sub-alkaline (Fig. 6a), calc-alkaline (Fig. 6b), high-K calcalkaline (Fig. 6c) and metaluminous (Fig. 6d) in composition. The ASI

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Table 1 Whole-rock major (wt.%) trace and REE composition (ppm) of the Sarıçimen samples. Sample #

SiO2

Al2O3

tFe2O3

CaO

MgO

Na2O

K2O

Cr2O3

TiO2

MnO

P2O5

SrO

BaO

LOI

Tot.

S1 S2 S3 S4 S5 S6 S7 S8 S9

58.2 58 57 57.5 57.6 57.5 56.2 57.9 58.6

16.35 16.5 16.25 16.55 16.35 16.3 16.2 16.2 16.55

7.47 7.43 7.16 7.2 7.04 7.06 6.97 6.83 7.19

5.62 4.88 5.21 4.92 4.99 4.75 6.02 6.29 5.52

2.69 2.78 2.89 2.79 2.74 2.74 2.41 2.39 2.73

3.6 4.01 4.01 4.08 3.88 3.88 3.71 3.68 3.7

2.49 2.58 2.5 2.5 2.87 2.86 2.43 2.39 2.55

b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01

0.85 0.87 0.86 0.87 0.85 0.85 0.85 0.84 0.87

0.14 0.17 0.12 0.12 0.13 0.12 0.14 0.13 0.12

0.22 0.26 0.2 0.21 0.21 0.18 0.18 0.17 0.23

0.06 0.06 0.05 0.06 0.05 0.05 0.06 0.06 0.06

0.12 0.13 0.12 0.12 0.12 0.12 0.12 0.12 0.12

1.8 2 1.8 1.6 2.3 1.9 2.9 2.8 1.5

99.6 99.7 98.1 98.5 99.1 98.4 98.2 99.8 99.7

Ba

Cs

Cu

Ga

Hf

Nb

Pb

Rb

Sr

Ta

Th

Tl

U

V

Y

S1 S2 S3 S4 S5 S6 S7 S8 S9

1085 1205 1135 1140 1115 1110 1105 1080 1170

2.86 3.76 3.23 3.6 1.49 1.4 2.97 3.13 3.22

230 61 81 34 41 33 47 21 49

21.3 21.9 21.5 22.2 21.5 21.3 21.2 20.8 21.6

6.2 6.3 6.4 6.4 6.2 6.2 6.4 6.1 6.7

21.6 21.9 21.6 21.6 21.3 21.6 21.6 21 22.4

134 113 75 64 76 88 69 55 70

74.3 73.8 73.2 73.6 71.8 71.6 70.5 68.9 76.7

498 475 474 480 438 441 502 501 509

1.3 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.3

14.9 14.4 14.3 14.1 14.4 14.6 14.5 14.1 15.2

b0.5 b0.5 b0.5 b0.5 b0.5 b0.5 b0.5 b0.5 b0.5

4.81 4.03 3.94 4.06 4.37 4.58 4.62 4.86 4.71

132 141 138 141 138 136 136 130 138

27.2 28.1 28 28.4 27.7 26.5 28 28 28.3

Zn

Zr

Ce

Dy

Er

Eu

Gd

Ho

La

Lu

Nd

Pr

Sm

Tb

Tm

S1 S2 S3 S4 S5 S6 S7 S8 S9

225 170 155 115 142 123 158 114 145

246 261 252 255 253 249 255 249 261

82.6 84.9 84.6 84.1 85.6 83.5 83.2 82.1 85.9

5.21 5.29 5.22 5.38 5.22 5.08 5.2 5.19 5.33

2.98 3.08 3.12 3.03 2.98 3.01 3.09 3.02 3.09

1.7 1.74 1.85 1.91 1.8 1.79 1.85 1.8 1.8

6.4 6.42 6.73 6.88 6.54 6.35 6.44 6.41 6.58

1.03 1.05 1.11 1.08 1.03 1.02 1.05 1.04 1.06

45.6 46.8 46.6 45.9 46.6 45.4 45.5 45.1 47

0.44 0.42 0.47 0.45 0.45 0.43 0.44 0.46 0.45

36 37.3 37.3 37 36.9 35.5 36.1 35.2 37.4

36 37.3 37.3 37 36.9 35.5 36.1 35.2 37.4

6.73 6.77 7.17 7.06 6.94 6.7 6.57 6.58 6.74

1 0.9 1 1 1 0.9 1 1 0.9

0.43 0.43 0.46 0.44 0.44 0.41 0.44 0.43 0.44

Yb

ΣREE

Eu/Eu*

Ce/Pb

Nd/Pb

La/Nb

Th/Ta

ASI

(La/Yb)CN

S1 S2 S3 S4 S5 S6 S7 S8 S9

2.82 2.8 2.88 2.92 2.66 2.86 2.74 2.77 2.81

228.9 235.24 235.83 234.12 235.04 228.47 229.71 226.26 236.94

0.79 0.81 0.82 0.84 0.82 0.84 0.87 0.85 0.83

0.62 0.75 1.13 1.31 1.13 0.95 1.21 1.49 1.23

0.269 0.33 0.497 0.578 0.486 0.403 0.523 0.64 0.534

2.11 2.14 2.16 2.13 2.19 2.10 2.11 2.15 2.10

11.5 12 11.9 11.7 12 12.2 12.1 11.7 11.7

0.86 0.9 0.86 0.9 0.88 0.89 0.82 0.81 0.88

11.6 12 11.6 11.3 12.6 11.4 11.9 11.7 12

Explanation: tFe2O3, total iron oxide as ferric iron; LOI, loss of ignition; ASI, Aluminum Saturation Index [White and Chappell, 1988; molar (Al2O3)/molar (CaO + Na2O + K2O)].

(Aluminum Saturation Index = molar Al2O3/molar (CaO + Na2O + K2O); White and Chappell, 1988) value of the Sarıçimen quartz monzodiorite porphyry samples ranges from 0.81 to 0.90 (Table 1), which seems to be associated mainly with I-type granites (Chappell and White, 1974). Low ASI is consistent with the Sarıçimen mafic mineral assemblage of amphibole + biotite+ pyroxene (Chappell and White, 1974; White and Chappell, 1988) and also consistent with B–A diagram of Debon and Le Fort (1983) in Fig. 5. The limited silica range of the samples precludes drawing any significant conclusions from major oxide vs. silica plots. In a chondrite-normalized rare earth element (REE) spider diagram, the Sarıçimen quartz monzodiorite samples show enrichment of light rare earth elements (LREE) compared to heavy rare earth elements (HREE) and HREEs exhibit a flat pattern (Fig. 7a). The Sarıçimen samples have a weak negative Eu anomaly between 0.87–0.79 (n =9 average 0.83) where Eu/Eu* [=EuCN/(SmCN × GdCN)0.5]. The depletion of Eu is

indicative of feldspar involvement during fractionation and/or melting (Rollinson, 1993). A primitive mantle-normalized trace element spider diagram of the Sarıçimen quartz monzodiorite samples has negative anomalies of Nb, P, Ti and positive anomalies in large-ion lithophile elements (LILE) such as Rb, Ba, Th, U and K (Fig. 7b). In particular, the Pb contents of the Sarıçimen samples are high, between 55–134 ppm, (n =6, average 83 ppm). Typical Pb values in various types of oceanic basalts range from b1 to ~3.2 ppm, (Sun and McDonough, 1989) whereas continental crust values are ~3.3–18 ppm, (Taylor and McLennan, 1985; Rudnick and Taylor, 1987; Shaw et al., 1994; Rudnick and Fountain, 1995; Gao et al., 1998; Rudnick and Gao, 2004). Ce content of the Sarıçimen samples are between 82–87 ppm (n =9, average 84 ppm). For comparison, Ce values are 20–21 ppm in lower crust; ~53 ppm in middle crust and ~63 ppm in upper crust (Rudnick and Gao, 2004). Nd/Pb of the

Table 3 Whole-rock sulphide data δ34S for Sarıçimen samples. Table 2 Whole-rock K–Ar dating of Sarıçimen quartz monzodiorite samples. Sample no.

K%

Rad. 40Ar (mol/g)

S2 S8

2.19 2.05

4.83E−11 58.59 4.26E−11 57.13

Rad. (%)

40

Ar

Age Uncertainty (Ma) (Ma)

Age (Gradstein et al., 2004)

12.6 11.9

Serravallian Serravallian

0.3 0.35

Sample no.

Sample type

Method

δ34SVCDT (‰)

S1 S2 S3 S8 S9

Whole-rock Whole-rock Whole-rock Whole-rock Whole-rock

Kiba Kiba Kiba Kiba Kiba

5.3 6.5 5.6 6.8 6.0

612

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Fig. 4. QP plot (Debon and Le Fort, 1983) for samples from the Sarıçimen quartz monzodiorite pluton.

Sarıçimen samples are between 0.26–0.64, (n= 6, average 0.47). These values in MORB are ~11; and in upper crust are ~1.6–3.6. All of these data are indicative of an upper crustal component in the Sarıçimen plutons. All samples from the Sarıçimen monzodiorite porphyries plot very close to the arc volcanic subfield in Ce/Pb vs. Ce and Nb/Th vs. Nb diagrams (Fig. 8a,b). Such an arc-like geochemical composition is most likely the result of subduction zone enrichment or crustal contamination as shown by the Rb/Y vs. Nb/Y plot (Pearce et al., 1990) (Fig. 8c, c′). Calc-alkalic Sarıçimen samples display a consistent displacement from the mantle metasomatism array towards higher Th/Yb ratio on a Ta/Yb vs. Th/Yb diagram (Taylor and McLennan, 1985) (Fig. 8d). The samples form a sub-parallel trend (Fig. 8d′) with respect to the main mantle metasomatism array, supporting the idea that the mantle source had a distinct subduction component similar to the Erzurum–Kars region plateau mantle source (Şengör et al., 2008). This trend may indicate the main mantle metasomatism was affected by fractional crystallization and assimilation. The Ba/Nb vs. La/Nb plot (Jahn et al., 1999) indicates the possibility of a multi-sourced material involvement in the genesis of the Sarıçimen quartz monzodioritic, possibly including, arc volcanic rocks and/or clastic continental sediments of the EAAC (Fig. 8e). The Yb/Ta vs. Y/Nb variation diagram (Best and Christiansen, 2001) shows that the rocks from the Sarıçimen monzodioritic porphyries plot as expected, with an arcrelated signature (Fig. 8f). On the tectonic discrimination diagrams of Batchelor and Bowden (1985) the Sarıçimen quartz monzodiorite plutons fall near the boundary between pre-plate collision and post-collision uplift (Fig. 9a). The data also reveal a subduction-related signature when plotted on the Rb vs. Y + Nb, Rb vs. Ta + Yb and Ta vs. Yb diagrams of Pearce et al. (1984) (Fig. 9b–d). On the tectonic discrimination diagrams of Schandl and Gorton (2002) the Sarıçimen samples plot in the active continental margin fields (Fig. 9e–f). 5. Geochronology Two fresh whole-rock samples from separate igneous bodies (Fig. 2) were analyzed for their K–Ar age. The data are given in Table 2. Sample S2 yielded a K–Ar age of 12.6 ± 0.30 Ma and sample S8 yielded a K–Ar age of 11.9 ± 0.35 Ma. Although not identical within error, the obtained K–Ar ages of samples S2 and S8 are very close in age. 6. Sulfur isotope data Whole-rock sulfur isotope analyses were made on five samples and these data are presented in Table 3. The δ34S values from the Sarıçimen pluton range between 5.3–6.8‰ with a mean value of 6‰.

Fig. 5. Classification of the Sarıçimen quartz monzodiorite pluton on B–A diagram of Debon and Le Fort (1983). I, II and III fields represent the peraluminous, and IV, V and VI fields display the metaluminous domains. mu: muscovite, bi: biotite, hb: hornblende, cpx: clinopyroxene, px: pyroxene.

Stable isotope data for S reported in this study are supportive of small to large degrees of magma-crust interaction for the pluton. The only reasonable contaminant that would yield these signatures is crustal sulfate (δ34S = 10–25‰; Ohmoto and Rye, 1979); reduced crustal sulfur nearly always has δ34S values at or below zero. In addition, there are no known fractionation processes that might reasonably yield positive δ34S values from initial mantle values of 0‰, particularly at magmatic temperatures where fractionation factors are very small. Therefore, the most likely source of crustal contribution is from crustal sulfates of the sedimentary section or from the lithospheric mantle metasomatized by earlier subduction-derived fluids and accreted as tectonic slices into the subcontinental lithosphere (Bonin, 2004) during the collision in Eastern Anatolia (analogous to, but younger than, the collision described by Boztuğ and Arehart, 2007). 7. Discussion It is clear that the bulk geochemical properties of the Sarıçimen plutons are compatible with an origin as subduction-related granitoids, probably associated with the closure of the neo Tethyan ocean. The magmas have been derived from partial melting of a metasomatized lithospheric mantle source and also have been contaminated by upper crustal material during ascent through the crust. Major element geochemistry data indicate that the plutons have a high-K, calcalkaline, metaluminous character, with a low (0.81–0.90) Aluminum Saturation Index (ASI). In addition, the La/Nb ratios are high (2.1: n = 9; Fig. 8e) indicative of a lithospheric signature (DePaolo and Daley, 2000). The Sarıçimen plutons plot in the field of volcanic-arc granitoids in tectonic discrimination diagrams (Figs. 8,9). In addition, trace element data and spider diagrams are supportive of this interpretation. Sarıçimen monzodiorite samples show negative anomalies of Nb, P, Ti and positive anomalies in large-ion lithophile elements (LILE) such as Rb, Ba, U, K and Pb (Fig. 7b) indicating a subduction zone-related magmatic signature for their origin. The high values of Pb and U (Table 1) as well as stable isotopes of sulfur clearly indicate magmacrust interaction during ascent of the magma. The Th/Ta ratio is diagnostic of magmas (52 b SiO2 wt.%) having subduction vs. intraplate signatures (Şengör et al., 2008) and the very high Th/Ta ratio

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Fig. 6. Classification of the Sarıçimen quartz monzodiorite rocks (a) on the Na2O + K2O vs.SiO2 diagram (Irvine and Baragar, 1971); (b) on the AFM diagram; (c) on a K2O vs. SiO2 diagram (Rickwood, 1989); and (d) on the A/CNK vs. A/NK diagram (Shand, 1943).

(~12, Table 1) of the Sarıçimen pluton indicate that it was derived from partial melting of a metasomatized lithospheric mantle source containing a distinct subduction signature. The Ta/Yb vs. Th/Yb diagram (Fig. 8) implies a similar source. In this paper, we have reported two ages for monzonite plutons, located 12 km NE of Alikelle Mountain, at 11.9 and 12.6 Ma. These ages are more consistent with the age for dacitic volcanism near the Sarıçimen plutons reported by Innocenti et al. (1976) and clearly indicate that magmatism was beginning in this part of eastern Anatolia by ~ 13 Ma. In addition, several monzodiorite intrusions from northwestern Iran have similar chemistries and yield similar ages (Khalatbari-Jafari et al., 2004). K-feldspar phenocrysts from the Yakmaleh intrusion close to Turkey–Iran border yield an age of 14.0 ± 0.3 Ma (Fig. 10; pluton 1). Feldspar and biotite phenocrysts from a pluton south of Dizaj Aland village yielded ages of 13.8 ± 0.4 Ma, and 11.5 ± 0.3 Ma, respectively (Fig. 10; pluton 3). Feldspar phenocrysts from the Avrine village intrusion yielded 12.2 ± 0.3 Ma, and amphibole from the same sample gave 10.5 ± 0.7 Ma (Fig. 10; pluton 4). Moderate excess argon in the feldspar phenocrysts in these three intrusions may be responsible for the slight discrepancies between mineral ages, with somewhat older ages given by the feldspars (Khalatbari-Jafari et al., 2004). Nonetheless, these plutons are coeval with the Sarıçimen plutons. These plutons are aligned geographically

roughly parallel to the Zagros fold and thrust zone, which is approximately parallel to the paleo-subduction zone. 7.1. Implications for the closure of the neo-Tethys There are many hypotheses about the evolution and geodynamics of volcanism related to the Eurasian–Arabian collision in Eastern Anatolia (e.g., McKenzie, 1972; Innocenti et al., 1976, 1980, 1982; Yılmaz et al., 1987; Yılmaz, 1990; Ercan et al., 1990; Pearce et al., 1990; Westaway, 1994; Keskin, 1994; Keskin et al., 1998; Buket and Temel, 1998; Yılmaz et al., 1998; Keskin, 2003, 2007; Şengör et al., 2003; Gök et al., 2000, 2003; Al Lazki et al., 2003; Keskin et al., 2006; Barazangi et al., 2006; Şengör et al., 2008). Various authors have proposed different models for the Miocene–Quaternary alkaline and calc-alkaline volcanism, and its temporal relationship to the closure of the neo-Tethys. According to the stratigraphic record the thrust zone was above sea level by the end of the early Miocene (16 Ma.) (Okay et al., 2010). The youngest marine fossils from near the Lake Van area in eastern Turkey are reported as Serravallian age (11.5–13.6 Ma) by Gelati (1975). As noted by Şengör and Kidd (1979) and Şengör et al. (2008), the initial contact between Arabia and Anatolia is at least 12 Ma based on the lack of marine sedimentary rocks and the presence of

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Fig. 7. (a) Chondrite-normalized REE; and (b) primitive mantle-normalized spider diagrams for samples from the Sarıçimen quartz monzodiorite plutons. Normalization values after Sun and McDonough (1989).

flysch/molasse in the stratigraphic column. It has been accepted that at least by ~ 13 Ma, the Anatolian plateau had been uplifted to ~ 2 km accompanying the start of Arabia and Eurasia in this area (Şengör and Kidd, 1979; Dewey et al., 1986). New apatite fission-track data (Okay et al., 2010) suggest that uplift occurred between 18–13 Ma and this uplift showed a rapid increase at ~ 12 Ma. In the western part of the region, the oldest volcanic age determination thus far reported is 11.1 ± 0.5 Ma from north of Horasan (Keskin et al., 1998), 11.4 ± 0.9 Ma from the Erzurum–Eleşkirt–Kösedağ and 11.2 ± 1.5 Ma. Erzurum–Horasan by Ercan et al. (1990). Our new data, along with the data of Innocenti et al. (1976: Alikelle Mountain) and Khalatbari-Jafari et al. (2004): multiple ages of plutons), clearly extend the earliest magmatic activity to ~ 14–13 Ma. Thus, if one accepts that volcanism marks the closure of the neo-Tethys, it is clear that the neo-Tethys had been eliminated by ca. 14–13 Ma.

7.2. Tectonic implications of the new ages The age we report for these plutons is not consistent with the age sweep proposed by Şengör et al. (2008) for most magmatism in eastern Turkey. In their model, magmatic activity began at 11.4 Ma in the northern part of the EAAC as calc-alkaline volcanism and progressed southward in time, culminating in dominantly alkaline volcanism since ~ 6 Ma. Our data suggest that there is a much larger zone of early magmatism (~ 14–10 Ma), stretching from the Erzurum area southeastward to the Sarıçimen area and continuing into northwestern Iran. This belt of igneous rocks represents the original collision between the Arabian and Eurasian continents immediately following the closure of the neo-Tethys ocean. This belt parallels the boundary between the major tectonic blocks of the EAAC and the Northwest Iranian Fragment (e.g., Şengör et al.,

Fig. 8. (a) Ce/Pb vs. Ce, (b) Nb/Th vs. Nb, (c) Rb/Y vs. Nb/Y, (Pearce et al., 1990), (d) Th/Yb vs. Ta/Yb (after Pearce, 1983), (e) Ba/Nb vs. La/Nb (Jahn et al., 1999) and (f) Yb/Ta vs. Y/Nb (Best and Christiansen, 2001) plots of the Sarıçimen samples. PM (primitive mantle) from Hofmann (1988); CC (continental crust), MORB (mid-ocean ridge basalts), OIB (oceanisland basalts), and arc volcanic rock compositions from Schmidberger and Hegner (1999) in a, b and e. Compositions of the LC (lower continent), BC (bulk continent), MM (mantle metasomatism array), SZE (subduction zone enrichment), FC (fractional crystallization), AFC (assimilation combined with fractional crystallization curve) and UC (upper crust) from Taylor and McLennan (1985); the vectors for the subduction zone enrichment or crustal contamination and WPE (within-plate enrichment) are based on the data of Pearce et al. (1990) in c. Th/Yb vs. Ta/Yb, diagrams for basic and intermediate lavas (SiO2 b 60 wt.%) of Sarıçimen compositions in d. IAB and OIB refer the island arc and oceanic island basalts, respectively, in f. c′ shows sample trending in c. d′ shows sample trending in d. Diamond shapes are data from Sarıçimen area. Fields from Boztuğ (2007).

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Fig. 9. (a) R1 vs. R2; (b) Rb vs. Y + Nb; (c) Rb vs. Ta + Yb; and (d) Ta vs. Yb; (e) Th/HF vs. Ta/Hf; (f) Th/Ta vs. Yb geotectonic discrimination diagrams for the quartz monzodiorite Sarıçimen plutons. Syn-COLG = syn-collisional granitoids; WPG = within-plate granitoids; VAG = volcanic-arc granitoids; ORG = ocean-ridge granitoids; MORB = mid-ocean rift basalt. a–d fields from Batchelor and Bowden (1985) and Pearce et al. (1984), d and e from Schandl and Gorton (2002). All elements are given as ppm.

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Fig. 10. (a) Location map of the study area (b) showing the monzodiorite plutons and Quaternary large volcanic centers on Google earth image.

2003, Fig. 1; Şengör et al., 2008, Fig. 14; Çakır, 2008, Fig. 9). The chemistry of these igneous rocks indicates that continental crust was involved in their genesis that can belong to either EAAC or old crust. Our new data, coupled with the ages reported from Iran, suggest that there is a fundamental difference between younger and older magmatism in eastern Turkey, thus suggesting some fundamental difference in the tectonics. The ages of volcanic rocks in eastern Turkey shows a significant gap between the older, calc-alkaline rocks (~ 14–10 Ma) and younger alkaline rocks (generally b6 Ma) suggesting different origins for the two suites. The calc-alkaline rocks are undoubtedly the result of subduction-related processes from the collision following the closure of the neo-Tethys, whereas the alkaline rocks are most likely related to younger processes associated with slab breakoff and incursion of asthenospheric mantle through the slab gap (e.g. Şengör et al., 2003; Keskin, 2005; Yılmaz et al., 1998; Şengör et al., 2008).

Thus, the southeastward sweep of volcanism proposed by Şengör et al. (2008) may not truly represent a progressive change in volcanism with time but rather, a fundamental difference in tectonic regime. Early magmatism is related to compressional tectonism associated with final closure of the neo-Tethys, whereas later magmatism occurred after continental welding and is associated with the breaking off of the subducting slab. The geometry of the continental collision in eastern Turkey is sufficiently complex that it is difficult to assess the exact nature of the subducted slab, but we suggest that the distinct alignment of younger volcanic edifices (from Nemrut to Ağrı: Fig. 10) may reflect a large crustal-scale discontinuity in the subducted slab (perhaps a subducted transform fault that opened along its length to facilitate magmatism?). Şengör et al. (2003) pointed out that the region has no mantle lithosphere, based on the results obtained from two independent seismic studies (Gök et al., 2000, 2003; Al Lazki et al., 2003). Delamination of mantle lithosphere (Pearce et al., 1990; Keskin et al.,

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Fig. 11. Simplified and schematised tectonic evolution of the eastern Anatolian high plateau during Miocene time. (a) Early Miocene, showing the latest sea in the area. (b) First initiation of magmatism. (c) Falling slab and extensive volcanism in the area. EAAC = eastern Anatolian accretionary complex; R-PA = Rhodope-Pontide arc; R-PLM = RhodopePontide lithospheric mantle; AP = Arabian plate; BPM = Bitis-Pötürge massif; LM = lithospheric mantle. Modified from Şengör et al. (2008).

1998) and detachment and northward movement of a subduction slab models proposed by Innocenti et al. (1980, 1982) are the best plausible model for the area but are not consistent with the new data to explain the eastern Anatolia region. Therefore, we propose (slab steepening and breakoff) a modification of the model of (Şengör et al., 2008) especially in the Middle Miocene (~ 14–13 Ma) which is a very critical era for initiation of magmatism in the region. (Fig. 11a–c) explains our interpretation of Miocene volcanism in eastern Anatolia. Early–Middle Miocene events include shortening and uplift of the EAAC and elimination of the neo-Tethys (Fig. 11a). Our model contrasts with the model of Şengör et al. (2008) which proposed a progression of magmatism from NW to SE with time. Our data argue for early high-K, calc-alkaline magmatism in an arcuate pattern roughly parallel to the northern boundary of the EAAC in the period 14–10 Ma. (Fig. 11b). This was followed by a hiatus in igneous activity until at least period 9 Ma and most dates are 6 Ma

or younger. The younger magmatism was alkaline in nature and was related to slab breakoff (Fig. 11c). 8. Conclusions The Sarıçimen quartz monzodiorite plutons are clearly subduction-related, on the basis of their geochemical properties. These plutons, as well as nearby plutons in Iran and volcanic rocks of Alikelle Mountain, and the volcanic rocks described by Şengör et al. (2008) are the earliest manifestation of subduction-related neotectonic volcanic activity having a high-K, calc-alkaline nature. These rocks parallel the suture zone between the EAAC and the continental crust of the Eurasian continent. This magmatic activity was associated with the final closure of the neo-Tethys ocean and the inception of continent–continent collision beginning at about 14–13 Ma (Late Medial Miocene).

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