Geochemical features of collision-related volcanic rocks in central and eastern Anatolia, Turkey

Journalofvokanology andgeothertnalresearch Journal of Volcanology

and Geothermal

Research 64 (1995)

171-192

Geochemical features of collision-related volcanic rocks in central and eastern Anatolia, Turkey K. Notsuav*, T. Fujitanib, T. Ui’,**, J. Matsudad, T. Ercane “Laboratoryfor Earthquake Chemistry, University of Tokyo. Bunkyo-ku, Tokyo 113, Japan bMarine Technical College, Ashiya, Hyogo 659, Japan ‘Department of Earth and Planetary Sciences, Kobe University, Nada-ku, Kobe 657, Japan dDepartment of Earth and Space Science, Osaka University, Toyonaka, Osaka 560, Japan ‘Geological Research Department, MTA, Ankara, Turkey Received 2 May 1993;

revised version accepted23 June 1994

Abstract In central and eastern Anatolia, volcanism related to continental collision occurred in Neogene to Quatemary times. Majorand trace-element compositions, R7Sr/86Srand K-Ar ages were determined for volcanic rocks from selected volcanoes in this region. 87Sr/86Srfor basal& from Erciyes volcano in the eastern part of central Anatolia, as well as Ararat, Nemrut and Karacadag volcanoes in eastern Anatolia are in the range 0.7035-0.7040. These ratios are higher than those of N-type MORB but overlap arc volcanic rock values. This suggests that the mantle source, which was possibly contaminated by fluids and melts with higher 87Sr/86Sr during the pre-collisional events, has similar Sr isotopic compositions in wide areas of the eastern part of central Anatolia and eastern Anatolia, even across the collision plate boundary. In addition, chemical compositions of the mantle source are not homogeneous with respect to HFS element contents, indicating that the slab-derived components resulting from past subductions are not homogeneously distributed. The mantle source beneath the western part of central Anatolia, which corresponds to the transitional region from collision tectonics in the east to subduction tectonics in the west, has different chemical and Sr isotopic signatures from other parts of Anatolia. In the western part of central Anatolia, the 87Sr/86Srof basaltic rocks from three regions characterized by monogenetic volcanoes are in the range 0.70427 to 0.70581 and differ from region to region, in spite of indistinguishable chemical compositions. This indicates either source heterogeneity on a small scale, or different contributions of local crustal contaminants. Calc-alkaline lavas from Ararat and Kars in eastern Anatolia have chemical compositions with subduction signatures inherited from pre-collision subduction events. In contrast, the chemical and Sr isotopic features of volcanic rocks from Erciyes volcano in central Anatolia indicate that the combined process of fractional crystallization of basaltic magma and crustal assimilation, without injection of slab-derived components, is the dominant process generating talc-alkaline andesite, under the collision tectonics regime operating in this region.

1. Introduction Two types of volcanism

typically

occur at conver-

* Corresponding author. **Present address: Division of Earth Planetary Sciences, Hokkaido University, Kita-ku, Sapporo 060, Japan. 0377-027?i95f$O9.50 0 1995 Elsevier Science B.V. Al1 rights reserved SSDfO377-0273(94)00077-S

gent plate boundaries: subduction-related and collision-related. Many geochemical studies of subductionrelated volcanic rocks have been carried out and several petrogenetic models have been proposed (see Gill, 1981, for a comprehensive review). Tatsumi (1989) and Sakuyakma and Nesbitt ( 1986) proposed sophis-

Fig.

1.Quaternary

volcanic zones in and around Turkey (modified from lnnocenti et al., 1982a. K7‘ (Kizirtepe region), EC (Egrikuyu region)

and KP (Karapinar region) are volcanic zones with abundant monogenetic volcanoes. EC/= Eurasian plate; AR= Arabian plate; AF= plate; AC=

Aegean microplate; T=Turkish

ticated models, stressing the injection of slab-derived fluid into the mantle wedge. In contrast, few studies have been carried out on collision magmatism, mainly on the Alpine-Himalayan belt (Jackson and McKenzie, 1984). Harris et al. ( 1986) summarized geochemical characteristics of collision-zone magmatism, proposing an evolutionary model for magma genesis, and recently Pearce et al. ( 1990) proposed a model of collision magma genesis in eastern Anatolia. However, it is important to present new geochemical data on collision magmatism in order to further our understanding of the process. In this study, we present major- and trace-elemental compositions, and *‘Srl*“Sr and K-Ar ages of volcanic rocks from central Anatolia, where the Eurasian and Afro-Arabian plates collide (Pasquare et al., 1988), and compare these with data from eastern and western Anatolia.

2. Plate tectonic and volcanological

African

microplate

settings

The plate boundary between the Eurasian, African and Arabian plates, which corresponds to the western half of the Alpine-Himalayan belt (Jackson and

McKenzie, 1984), is irregular in the Mediterranean region. In southern Italy and the Aegean region, the African plate subducts beneath the Eurasian plate, forming the Aeolian and Aegean volcanic arcs, respectively (Keller, 1982). In contrast, collision tectonics control the geology and geomorphology of other sections of the plate boundary in the Mediterranean. Turkey is located on the collision boundary to the east of the Aegean arc. According to McKenzie ( 1970, 1972), seismicity and fault plane solutions in the Mediterranean region show that the current deformation is the result of small continental plates (Aegean, Turkish and other microplates) moving between the Eurasian and African or Arabian plates. The plate-tectonic model that best explains the gross geologic structure and evolution of the Alpine system during the past 200 Ma was proposed by Dewey et al. (1973). Dewey and Sengiir (1979) further discussed the complex multiplate and continuum tectonics in the Aegean and surrounding regions, and SengGr and Yilmaz (1981) discussed the plate tectonic evolution of Turkey since the Permian. The inferred distribution of the microplates in and around Turkey differs among the various models proposed by McKenzie (1970, 1972), Dewey et al.

K. Notsu et al. /Journal of Volcanology and Geothermal Research 64 (1995) 171-l 91

(1973) and Sengor and Yilmaz (1981). Nevertheless, it is evident that the tectonic setting of Turkey changes from east to west, resulting in different types of volcanism in three sectors of Anatolia: western, central and eastern (Innocenti et al., 1982a; Yilmaz, 1990). In Fig. 1 the Quaternary volcanic zones in and around Turkey after Innocenti et al. (1982a) are shown, using plate tectonic boundaries modified from Sengor and Yilmaz (1981). Eastern Anatolia is governed by collision tectonics between the Arabian and Eurasian plates (Sengor and Kidd, 1979). The collision was initiated during Early Miocene times and induced magmatic activity and thickening of the crust (Dewey et al., 1986). On the northern (Eurasian plate) side of the plate boundary, volcanism has occurred in the thickened crust from Late Miocene to historical times. (Yilmaz et al., 1987; Yilmaz, 1990). An initial minor phase of alkaline volcanism was followed by widespread eruptions of talc-alkaline rocks during the Pliocene. In Quaternary times, large stratovolcanoes composed of alkaline and/ or talc-alkaline rocks were formed along NE-SW trends. On the opposite (Arabian plate) side of the plate boundary, basaltic shield volcanism erupted alkaline rocks from the Late Miocene until the Quaternary, forming a large shield volcano (Ercan et al., 1991). Collision volcanism between the Afro-Arabian and Eurasian plates occurred mainly along a NE-SW direction in Neogene and Quaternary times in central Anatolia (Pasquare et al., 1988). Volcanism started with effusive activity in Middle-Late Miocene times and was followed by the emplacement of a thick ignimbrite sequence with an aerial distribution covering about 11,000 km2. During Late Pliocene to present times, large andesitic-basaltic stratovolcanoes, such as Erciyes and Hasan, and a number of monogenetic volcanic centers (KT, EG and KP regions in Fig. 1) were formed. The monogenetic volcanism in the southern part (KP region) is characterized by abundant maar craters (Keller, 1974). Western Anatolia is completely different from the other two regions, since the eastern end of the Aegean volcanic arc, resulting from the subduction of the African plate, is present (Dewey and Sengor, 1979). Calcalkaline volcanic activity began in Late OligoceneEarly Miocene times in a N-S-directed compressional regime, and was followed by alkaline basalt volcanism in the Late Miocene, corresponding to the abrupt

173

change from the N-S-directed compressional to the NS-directed tensional regime in the Middle Miocene (Yilmaz, 1989, 1990). Alkaline volcanism has continued until the most recent activity of Kula volcano about 10,000 years ago. Geochemical and petrographical features of volcanic rocks from the three sectors of Anatolia were reviewed by Innocenti et al. ( 1982a). Recently, detailed geochemical studies on volcanic rocks from eastern and western Anatolia have been carried out (Pearce et al., 1990; Yilmaz, 1989, 1990). However, geochemical data on volcanic rocks from central Anatolia are sparse and there has been limited discussion on the origin of collision volcanism there.

3. Samples Volcanic rock samples for this study were collected from selected Quatemary volcanoes in Turkey (Fig. 1) . Rock types, occurrences and locations of analyzed samples are listed in the Appendix. Quatemary volcanism in central Anatolia is represented by two large stratovolcanoes (Erciyes and Hasan) and a number of monogenetic volcanoes (Pasquare et al., 1988). Thirteen rock samples from Erciyes volcano and seventeen samples from three regions (Kizirtepe, Egrikuyu and Karapinar) of monogenetic volcanoes were selected for analysis. In addition, one sample from Hasan volcano was analyzed. In the case of Erciyes volcano, samples were selected on the basis of rock type and eruptive stage. These samples constitute the first systematic geochemical data set. Only a few samples were used in previous studies of Erciyes volcano (Innocenti et al., 1975; Ayranci and Weibel, 1973). No previous geochemical studies have been carried out on the monogenetic volcanoes, with the exception of a major-element study of volcanic rocks near Karapinar by Keller ( 1974). In eastern Anatolia, rock samples were collected from four Quaternary stratovolcanoes; Nemrut, Suphan, Tendurek and Ararat (Agri), which are aligned in a NE-SW direction to the north of the plate boundary. Nine rock samples from Nermut volcano and seven samples from three other volcanoes were analyzed. In addition, eleven volcanic rock samples were analysed from Karacadag, a great shield volcano on the Arabian plate side. Previous work on the volcanoes of

“SrPSr

45 324 327

Rb tppmi Sr Ba SC Y La Ce Nd Sm EU Tb Yb LU Th U Zr Hf Nb Ta Cr co

0.70493

22 23.8 41.4 20.4 3.89 1.09 0.55 2.15 0.33 6.6 2.3 184 4.2 12 0.8 48 16.3

II.5

98.72

Total

1.63

0.20

W P*O,

MgO CaO Na,O

AI& FeO” MnO

62.02 0.69 16.73 4.45 0.08 2.93 6.34 3.65

ERll And

Sample No: Rock type:

SiO, (wt.%) TiOi

Erciyes

Volcano:

0.70487

84 286 476 11.5 27 30.2 59.9 24.3 4.53 1.05 0.71 2.67 0.38 13.7 4.9 278 6.4 14 1.2 20 14.4

98.59

63.76 0.73 16.02 4.42 0.07 2.24 4.57 4.17 2.45 0.16

ER14 And

0.70541

106 217 487 5.5 21 27.4 s2.0 20.9 3.65 0.84 0.56 2.14 0.32 14.7 5.4 173 4.3 13 1.I 11.2 6.5

98.47

68.97 0.42 15.11 2.63 0.06 1.1.5 3.26 3.96 2.19 0.12

ER15 Dac

0.70548

117 207 552 4.4 21 30.0 58.2 20.6 3.46 0.80 0.63 2.18 0.32 17.8 6.4 171 4,s 12 I.5 11.8 4.9

97.1 I

69.14 0.34 14.80 2.24 0.05 0.78 2.93 3.89 2.8.5 0.09

ER16 Dac

of volcanic rocks from central Anatolia, Turkey

0.70472

106 229 5.53 8.6 27 30.1 58.2 25.2 4.67 1.09 0.76 2.70 0.38 16.8 5.4 343 7.5 I4 I .2 13.4 10.1

98.86

66.44 0.66 15.49 3.75 0.05 I .40 3.20 4.54 2.99 0.14

ER13 Dac

Table 1 Elemental and Sr isotopic compositions

0.705 14

79 281 504 10.0 25 33.2 60.6 23.7 4.7 1 1.24 0.68 2.37 0.37 12.4 4.0 212 5.1 14 I .3 44 12.9

81 290 500 11.5 27 36.1 51.1 30.1 5.33 1.38 0.86 2.51 0.38 14.1 5.3 216 5.1 11 1.1 31 15.7 0.70477

99.4:1

65.69 0.59 16.13 3.86 0.07 2.19 4.67 3.75 2.32 0.16

ER20 Dac

98.19

64.24 0.72 16.52 4.29 0.06 2.08 4.00 3.77 2.36 0.15

ER17 And

0.70487

3.3 174 4.5 12 0.9 38 16.4

10.5

64 318 370 11.6 23 24.9 49.3 20.4 3.93 1.06 0.61 2.23 0.33

98.17

63.18 0.68 16.39 4.14 0.07 2.62 5.28 3.84 I.81 0.16

ER2 1 And

0.70515

12

185

22

9s 259 458

99.46

66.20 0.56 15.02 3.48 0.06 2.20 4.82 4.4 1 2.55 0.16

ER22 Dac

0.70530

70 315 405 10.4 22 23.5 47.8 17.5 3.49 0.97 0.53 I .76 0.27 11.0 2.8 172 3.9 12 0.9 60 15.0

98.50

63.48 0.60 15.68 3.96 0.07 2.9 I 5.91 3.49 2.23 0.17

ER23 And

0.70387

6 464 191 27 41 20.7 47.3 25.8 6.10 2.03 I .02 3.75 0.52 I .5 0.7 266 5.3 12 1.o 61 41

96.82

47.56 2.08 16.37 10.5s 0.17 5.80 9.13 3.91 0.80 0.45

ER24 B&-Y

5

5 g Q z rI: -? G ?-

; z z 3 % P B

2 x 3‘ P 9 -Y % a i & B g

2 %

5 2

3

,I,

87Sr/86Sr 0.70454

26 394 342 15.9 27 27.8 58.4 24.4 4.97 1.51 0.74 2.71 0.38 4.7 1.6 224 4.9 14 1.2 67 23

48 303 355 9.7 21 23.2 46.0 19.0 3.58 0.94 0.57 1.95 0.29 8.5 2.6 175 4.0 11 0.6 44 12.5

Rb (ppm) Sr Ba SC Y La Ce Nd Sm Eu Tb Yb LU Th U zr Hf Nb Ta Cr co

0.70512

99.14

98.80

Total

KzO PZOS

MgO CaO NazO

Al& FeoB h4llO

57.90 1.05 17.45 6.16 0.10 3.90 6.77 3.96 1.50 0.35

ER26 And

64.33 0.56 16.38 3.82 0.07 2.43 5.40 3.87 1.78 0.16

ER25 And

Sample No: Rock type:

SiO* (wt.%) TiOl

Erciyes

Volcano:

Table 1 (continued)

8,

8,

0.70498

19 612 388 25 27 28.4 58.8 26.3 5.10 1.57 0.81 2.47 0.38 5.0 1.3 153 3.0 10 1.1 20 35

100.24

49.61 1.39 18.10 8.68 0.14 6.01 10.72 4.21 1.02 0.36

HA01 BaS

HaMIl

,,

0.70464

30 619 391 19.2 28 29.0 59.6 26.0 5.06 1.63 0.67 2.25 0.36 4.8 0.9 197 4.1 20 1.3 161 33

99.39

51.84 1.54 17.12 7.91 0.13 6.58 8.24 4.00 1.57 0.46

KTll Ba.S

,,

0.70516

17 624 399 27 27 29.3 61.2 25.2 5.02 1.58 0.72 2.41 0.35 5.1 1.4 153 3.4 16 0.9 227 37

98.53

48.97 1.26 16.36 8.15 0.13 7.38 11.33 3.44 1.09 0.42

KT12 BriS

Kizirtepe region

,>,

0.705 12

19 605 408 30 27 29.5 60.0 26.6 5.19 1.61 0.73 2.29 0.34 5.0 1.0 143 3.3 16 0.9 207 36

98.31

48.50 1.30 16.65 8.50 0.14 7.16 11.34 3.34 1.00 0.38

KT13 BaS

0.705 14

18 614 419 29 27 29.6 62.2 26.8 5.19 1.61 0.75 2.38 0.35 5.4 1.5 151 3.5 16 0.9 272 40

98.83

16.52 8.44 0.14 7.77 10.86 3.55 1.05 0.39

1.28

48.83

KT14 BriS

0.70503

21 606 415 24 27 30.2 63.3 26.9 5.12 1.63 0.74 2.44 0.35 5.4 1.2 173 3.7 18 1.0 274 39

99.50

50.57 1.26 16.44 8.14 0.14 8.14 9.72 3.46 1.20 0.43

KT16 BaS

0.70478

16 697 389 27 22 20.3 42.4 19.3 3.85 1.32 0.56 2.05 0.31 3.2 0.8 97 2.4 10 0.6 414 44

99.76

48.52 1.23 16.08 8.11 0.13 10.20 11.07 3.08 1.03 0.31

EG12 BaS

0.70440

13 783 326 27 17 24.9 48.0 20.8 3.81 1.21 0.50 1.67 0.23 5.3 1.1 64 2.1 9 0.5 563 44

97.47

49.11 0.80 14.91 7.48 0.13 10.59 10.38 2.96 0.86 0.25

EG14 BaS

Egrikuyu region

0.70434

13 535 279 25 26 20.2 43.7 21.7 4.63 1.51 0.65 2.46 0.37 3.0 1.2 155 3.2 15 0.9 287 42

99.60

50.38 1.50 16.98 8.59 0.14 8.10 8.88 3.57 1.10 0.36

EG15 BaS

2:

s7Sr/“6Sr

0.70427

13 503 264 28 27 17.8 37.9 17.9 3.84 1.34 0.62 2.24 0.33 3.5 0.7 133 2.9 12 0.9 305 38

96.24

Total

Rb (ppm) Sr Ba SC Y La Ce Nd Sm Eu Tb Yb LU Th U zr Hf Nb Ta Cr co

41.13 1.47 16.03 8.30 0.14 7.97 9.80 3.49 0.99 0.32

EG16 Bas

Sample No: Rock type:

SiOz (wt.%) TiO, AhO, FeO” MnO MgG CaO Na,O K,O PZOF

Egrikuyu

I (conrinued)

volcano:

Table region

0.70465

0.70446

0.70438

0.67 2.28 0.32 3.8 0.9 183 3.7 17 1.1 244 37

1.55

22 s72 336 23 27 24.1 51.2 23.4 4.82

26 743 387 21 26 31.8 66.7 31.3 6.28 1.80 0.68 2.12 0.31 5.9 1.4 190 3.9 14 0.8 269 33

13 659 587 24 21 25.6 52.5 24.5 4.80 1.54 0.64 2.18 0.34 4.0 1.1 152 3.3 14 0.8 233 37

98.56

97.68

98.15

50.21 1.55 16.62 8.30 0.14 7.5 1 8.70 3.77 1.33 0.43

EG19 Bas

49.46 1.38 16.19 7.85 0.13 8.10 9.04 3.56 1.52 0.45

EGl8 BaS

49.61 1.44 16.67 8.32 0.14 7.52 9.47 3.41 1.12 0.39

EG17 Bas

0.70434

26 715 407 24 21 31.5 64.5 31.0 6.48 1.83 0.72 2.21 0.31 5.5 1.3 184 3.7 14 1.o 267 37

97.45

16.18 8.09 0.13 7.90 9.31 3.60 1.48 0.43

1.45

48.82

EG2 1 Bas

0.70476

14

134

2.5

13 574 324

99.19

48.46 1.32 15.29 8.59 0.14 10.21 10.46 3.45 0.87 0.40

EG22 Bas

0.70536

0.70539

135 2.9 11 0.7 456 45

1.5

20 566 356 27 24 29.5 55.9 22.1 4.56 1.37 0.62 2.24 0.35 7.2

99.12

96.90 17 ~569 317 28 24 27.8 56.0 22.9 4.42 1.36 0.61 2.30 0.34 6.7 1.4 127 3.1 I1 0.6 474 47

49.81 1.09 15.40 8.05 0.14 10.47 9.73 3.18 0.93 0.32

KP16 Bas

region

47.62 1.08 15.07 8.17 0.13 10.72 9.96 2.99 0.84 0.32

KP15 Ba.S

Karapinar

0.70563

25 606 478 24 24 34.6 67.2 27.1 4.54 1.41 0.67 2.17 0.36 10.3 2.0 147 3.3 12 0.7 267 36

98.27

52.29 1.00 15.95 7.07 0.12 7.86 9.1 I 3.36 1.19 0.32

KP17 Bas

0.7058 1

58 669 766 18 21 40.8 76.3 27.3 4.59 I .26 0.63 2.14 0.33 15.2 3.4 137 3.3 15 0.9 234 26

98.94

56.38 0.78 15.80 s.99 0.11 6.40 7.62 3.5 I 2.09 0.26

KPl8 And

a’SrPSr

Sr Ba SC Y La Ce Nd Sm Eu Tb Yb Lu Th U zr Hf Nb Ta Cr co

0.70435

0.70402

0.70398

37 446 433 14.6 33 24.4 53.1 23.9 5.00 I .47 0.90 2.91 0.43 5.4 1.9 243 5.1 I3 0.8 15.6 22

56 381 554 11.8 26 27.5 56.5 25.7 4.78 I .27 0.87 2.48 0.36 7.6 2.5 259 5.4 8 I.1 10.4 18.8

41 389 524 7.0 23 26.0 50.1 23.4 4.27 1.32 0.61 2.06 0.31 6.2 1.8 218 4.6 IO I.1 2.5 11.9

Rb @Pm)

98.85

99.60

100.25

Total

Na& GO P*Os

Mgo CaO

Al,03 Fee” MnO

ARII And 58.73 I.31 16.08 6.80 0.11 3.36 5.91 4.41 I.81 0.33

AR02 And 62.64 0.93 16.46 5.00 0.08 2.33 4.86 4.92 2.15 0.23

63.83 0.82 17.11 4.95 0.09 1.64 4.63 5.03 I .85 0.30

AROI And

Sample No: Rock type:

SiOl (wt.%) TiOz

Ararat

Volcano:

0.70528

7.1

99 291 585 5.6 55 54.5 117.7 51.1 8.83 2.02 1.39 5.86 0.87 13.2 4.1 591 11.0 34 3.0

99.18

58.53 I .25 18.06 5.48 0.15 I .45 3.23 6.90 3.71 0.42

TBOI T-and

Tendurek

Table 2 Elementaland Sr isotopic compositions of volcanic rocks from eastern Anatolia, Turkey

0.70563

23 629 462 14.3 38 35.7 76.4 36.2 7.21 2.11 I .07 3.26 0.51 4.2 0.9 299 5.8 21 1.7 9.3 30

101.14

51.76 2.02 17.93 10.30 0.16 4.41 7.47 4.93 1.44 0.72

TBo2 T-has

0.70627

0.70467

64 243 344 15.8 42 22.6 52.0 29.3 6.74 1.88 I .oo 4.07 0.60 8.5 2.8 288 6.4 7 I.1 5.9 20

I05 194 480 10.8 46 36.3 75.9 35.7 7.62 I .74 I.18 4.61 0.69 13.3 4.3 297 7.1 I2 2.1 9.5

99.03

59.06 1.56 16.08 7.39 0.13 2.38 5.09 4.95 2.13 0.26

suo2 And

98.75

65.25 0.86 15.42 4.69 0.09 I .32 3.45 4.45 2.99 0.23

SUOI Dac

Suphan

0.70665

85 44 975 5.6 72 52.4 124 54.0 11.3 2.05 1.94 7.83 I.11 16.4 2.0 784 15.1 40 2.9 7.0 0.47

99.10

66.93 0.38 15.10 4.36 0.12 0.07 1.31 5.76 5.01 0.06

NE13 Trac

Nemrut

0.70733

0.3 129 97.8 220 95.0 19.4 0.55 3.42 14.5 I .98 34 II.2 1310 26.7 66 5.8 12.8 0.20

85 <20 <20

98.73

72.44 0.23 12.22 3.32 0.08 0.01 0.28 5.54 4.61 0.00

Rhy

NE15

?W~Sr

0.70522

90 <20 <20 0.7 118 65.5 148 63.1 13.9 0.30 2.67 12.0 1.65 26 9.7 741 17.5 60 5.5 11.1 0.12

97.27

Total

Rb (ppm) Sr Ba SC Y La Ce Nd Sm Eu Tb Yb LU Th U zr Hf Nb Ta Cr co

72.92 0.15 12.34 2.01 0.05 0.01 0.2 1 5.01 4.50 0.01

59

54

0.70457

737

636

0.70516

114

107

0.70527

50 595 458 12.2 50 38.0 86.3 44.6 10.2 3.38 1.48 4.45 0.65 7.4 2.3 349 6.7 26 2.0 6.5 24

98.46

98.95 91 120 <20

so.57 2.48 16.37 10.03 0.19 3.42 7.19 4.93 2.33 0.95

NE22 T-has

73.77 0.16 12.52 2.15 0.06 0.03 0.27 5.46 4.52 0.01

Rhy

NE20

92 <20 <20

98.81

73.84 0.15 12.78 1.99 0.05 0.02 0.25 5.13 4.59 0.0 1

Rhy

NE17

Rhy

Sample No: Rock type:

SiO, (wt.%) TiO, Al,Ox FeO” MnO MgO cao Na,O K,O P?O,

NE19

Nemrot

volcano:

Table 2 (continued)

0.70823

59

1170

79

92 <20 <2O

98.83

71.14 0.27 12.84 3.73 0.1 1 0.03 0.4 I 5.51 4.75 0.05

Rhy

NE24

0.70416

8 518 104 21 31 14.2 34.5 19.7 4.98 1.80 0.94 2.83 0.4 I 1.9 0.7 201 4.2 12 0.8 39 50

15 392 s2 28 40 16.7 40.2 22.4 5.70 1.84 I .oo 3.45 0.50 2.4 1.1 245 4.8 14 0.9 49 46 0.70352

99.01

46.79 2.16 17.02 11.19 0.17 7.07 10.05 3.55 0.74 0.27

NE27 BaS

99.03

47.50 2.8 1 16.00 12.39 0.18 6.12 9.16 3.67 0.82 0.38

NE26 Bas

0.70389

0.23 3.0 0.8 197 4.6 31 2.0 294 54

1.60

13 597 249 21 24 26.2 56.4 29.0 6.38 2.18 0.84

98.97

48.42 2.61 13.97 11.61 0.16 8.52 8.63 3.21 1.36 0.48

KRll Ba.S

Karacadag

0.70336

9 663 241 21 23 27.6 58.1 30.2 6.55 2.12 0.82 I so 0.20 2.1 0.9 205 4.7 28 2.1 362 58

98.97

46.53 2.93 14.10 12.41 0.16 9.14 8.72 3.17 I .30 0.5 I

KR12 Bas

0.70359

0.70442

12 403 162 24 24 16.9 37.7 20.2 4.72 1.63 0.85 1.74 0.27 2.3 0.6 141 3.5 16 1.2 342 62

98.50

48.79 1.99 14.18 12.33 0.16 8.51 8.36 3.04 0.87 0.27

KR18 BaS

0.70348

9 661 195 23 25 20.3 46.9 26.3 5.84 2.05 0.87 1.51 0.22 1.9 0.6 181 4.2 27 1.9 137 51

98.92

46.24 3.09 15.18 12.64 0.16 6.45 10.30 3.33 1.09 0.44

KR19 BaS

0.70340

7 552 113 23 20 18.5 41 .o 22.7 5.18 1.87 0.74 1.54 0.18 1.6 0.6 141 3.8 21 1.6 286 68

97.67

45.01 2.66 13.98 13.31 0.17 8.89 9.27 3.16 0.85 0.37

KRZO BaS

0.70382

10 514 177 23 23 20.4 46.1 24.6 5.65 1.93 0.96 1.71 0.23 2.2 0.8 170 4.2 26 1.7 287 59

98.32

47.29 2.65 14.26 12.40 0.16 8.18 8.83 3.11 1.06 0.38

KR21 BaS

Rock type: Bas = basalt; And = andesite; Dac = dacite; Rhy = rhyolite; Trac = trachyte; T-has = trachybasalt; T-and = trachyandesite. Total iron as FeO.

%rPSr

Ba SC Y La Ce Nd Sm EU Tb Yb Lu Th U zr Hf Nb Ta Cr co 0.70380

15 575 227 22 23 22.7 49.6 25.9 5.54 1.93 0.74 1.44 0.21 2.8 0.9 172 4.1 28 1.8 310 60

12 525 184 23 23 21.8 47.0 25.0 5.84 1.95 0.89 1.72 0.24 2.5 0.6 177 4.3 26 1.6 370 53

15 812 214 20 26 35.9 73.3 35.8 7.84 2.61 1.01 1.44 0.22 3.1 1.1 224 5.1 36 2.3 262 57

Rb (mm) Sr

0.70334

96.94

97.81

98.63

Total

KzO p24

MgO CaO Na*O

Al,4 Few MIIO

46.24 2.34 13.49 11.84 0.16 8.86 9.19 3.16 1.23 0.43

KR17 BaS

47.36 2.52 13.49 11.49 0.15 8.33 9.70 3.24 1.14 0.39

KR14 BaS

45.47 2.84 13.56 12.55 0.16 8.74 9.24 3.76 1.62 0.69

KR13 BaS

Sample No: Rock type:

SiO, (wt.%) TiO*

Karacadag

Volcano:

Table 2 (conrinued)

0.70354

10 706 303 20 22 25.0 53.1 26.3 5.68 1.96 0.71 1.58 0.23 2.3 0.8 219 4.7 33 2.1 231 53

97.49

44.96 3.11 15.08 11.88 0.15 7.57 9.63 3.38 1.25 0.48

KR25 BaS

0.70350

20 764 187 18 23 35.7 77.1 39.0 8.10 2.56 0.92 1so 0.22 3.6 1.3 225 5.5 30 2.5 262 62

96.81

46.47 2.52 13.23 12.51 0.15 8.06 8.02 3.84 1.41 0.60

KR28 BaS

I80

K. Notsu et al. /Journal

Table 3 Elemental and Sr isotopic compositions ofvolcamc ern Anatolia, Turkey Volcano:

Klda

Sample No: Rock type:

KU1 I T-has

SiOz (wt.%) TiOz

KU12 T-bas 4.5.20 2.08 16.73

P?O5

47.58 2.17 16.55 8.14 0.13 7.05 10.19 6.26 1.16 0.68

Total

99.91

96.96

AI?03 Fe0 MnO M@ CaO Na,O KzO

Rb (wm) Sr Ba SC Y La Ce Nd Sm EU Tb Yb LU Th u Zr Hf Nb Ta Cr co X7Sr/X6Sr

s.1 852 675 2s 24 37.5 73.8 33.1 6.28 I .98 0.72 I .96 0.29 5.8 I .6 I85 4.0 59 3.8 179 35 0.70346

8.3.5 0.1.5 6.69 9.4x 559 1.5.5 1.14

3.5 1150 I220 18.8 29 63.7 122.7 51.3 8.81 2.69 0.89 2.35 0.36 8.4 2.3 238 5.4 86 6.2

102 34 0.70329

ctf’ Volcanology

rocks from west-

KU13 T-baS 48.36 I.85 18.66 6.99 0.13 4.54 7.59 6.28 3.73 0.78 98.9

I

61 91 I 97s 14.9 25 47.x 87.2 34. I 6.22 2.04 0.78 2.28 0.33

a.0 2.0 ?I9 4.6 78 S.S 48 26 0.70307

Rock type: T-has = trachybasalt “Total iron as FeO.

Anatolia has been carried out by Lambert et al. ( 1974), Ota and Dincel ( 1975)) Innocenti et al. ( 1976, 1980, 1982a) and Pearce et al. ( 1990). In western Anatolia, three volcanic rock samples from Kula volcano were also analyzed.

eastern

mui Geothermal Research 64 (I99.5) 171-191

4. Experimental

methods and results

Major-element compositions were determined using a Rigaku Model 3080 X-ray fluorescence spectrometer at the Earthquake Research Institute, University of Tokyo, after powdered samples were fused with nine times their weight of LiBO, flux into glass tablets. Six trace elements (Rb, Sr, Ba, Y, Zr and Nb) were determined using the same X-ray fluorescence spectrometer, after powdered samples were pressed into disks without binding agent. In addition, fifteen other trace elements (SC, La, Ce, Nd, Sm, Eu, Tb, Yb, Lu, Th, U, Hf, Ta, Cr, Co) were determined using instrumental neutron activation analysis, after the method of Fujitani ( 1988). Neutron activation was carried out using the KLJR reactor at the Research Reactor Institute of Kyoto University and the JRR4 reactor at Japan Atomic Energy Research Institute. Experimental procedures for Sr isotopic analysis have been described by Notsu ( 1983). The Sr isotopic compositions were determined on a VG-Micromass Sector-J multi-collector mass spectrometer at the Tokyo National Research Institute of Cultural Properties. The mean and 1u error of four separate *‘Sr/s?Sr analyses of the NBS 987 standard during this study was 0.7 IO244 5 0.000007 (No&u and Hirao, 1990). Total rock K-Ar age determinations were carried out on selected volcanic rock samples. The argon isotopic composition was determined for 60-l 00 mesh fractions of crushed samples or small pieces of whole rocks using mass spectrometers at Okayama University (Nagao et al., 1991) and Kobe University. The potassium concentrations of the 60-100 mesh fractions and whole rocks were determined using an atomic absorption spectrometer or an X-ray fluorescence spectrometer in Kobe University. Tables 1, 2 and 3 give analyses of volcanic rocks from central, eastern and western Anatolia, respectively. The standard analytical uncertainties (ppm) associated with trace-element determinations by X-ray fluorescence analysis are: Rb -5, Sr - 20, Ba - 20, Y 3, Zr - 10, Nb - 2. For the neutron activation analysis, uncertainties due to the gamma-ray counting statistics were less than 3% for all elements except Tb (10%) and Ta (10%). The standard uncertainty associated with “Sr/?r is estimated to be +0.00003, from duplicate analyses of the same rock sample.

K. Notsu et al. /Journal of Volcanology and Geothermal Research 64 (1995) 171-191

181

Table 4 Total rock K-Ar ages of volcanic rocks from Turkey Volcano

Central Anatolia Erciyes

Hasan

Monogenetic Kizirtepe region

Sample number

K (wt.%)

Radiogenic 40Ar ( X 10-8cmS STP/g)

ERll ER13 ER14 ER20 ER23 ER24 ER25 ER26 HA01 HA02

1.39 2.53 2.08 1.97 1.90 0.69 1.50 1.29 0.87 I .36

1.15 24.95 20.88 5.33 4.39 4.63 0.48 0.86 0.05

KTll KT12 KTl3 KT14 KT16 EG13 EG15 EG16 EG17 EG19 EG21 EG22 KPOl KP15 KP17 KP18

AR01 AR02 AR11 TEOI TEo2 suo2 NE18 NE19 NE20 NE22 NE27 KRll KR12 KR13 KR16 KR17 KR18 KR19 KR20 KR28

Air 40Ar (%)

Age (Ma)

1.43

99.2 19.0 34.6 95.8 95.2 83.2 95.6 95.7 99.1 93.0

0.21 f 0.04 2.54f0.31 2.59kO.10 0.70 f0.05 0.60 & 0.04 1.73*0.07 0.08 fO.O1 0.17*0.01 0.02*0.01 0.27 k 0.02

1.34 0.93 0.86 0.90 1.05 0.56 0.95 0.85 0.96 1.15 1.28 0.73 1.47 0.72 1.02 1.79

0.34 0.35 0.40 0.23 0.38 2.52 1.54 1.07 1.77 2.34 2.09 2.02 I .44 0.94 1.44 1.12

96.2 99.0 98.9 98.0 92.1 73.7 73.7 86.8 87.3 86.0 70.1 81.6 98.9 92.1 96.8 96.5

0.06 f 0.02 0.10*0.01 0.12*0.02 0.07*0.01 0.09 *0.01 1.15&0.08 0.42 f 0.03 0.33 j, 0.08 0.47 f 0.06 0.53 f 0.08 0.42f0.16 0.71 zho.11 0.25 f 0.05 0.34 *0.02 0.36 f 0.02 0.16+0.01

1.57 1.65 1.56 3.11 1.14 1.80 3.79 3.86 3.80 2.00 0.62 1.18 1.12 1.39 1.06 1.06 0.75 0.94 0.73 1.21

0.11

< 0.62 <0.13 0.16 1.76 0.69 < 0.35 <0.14 0.35 0.77 0.19 0.46 8.38 1.26 I .63 0.92 0.79 4.49 4.03 5.44

99.9 100 100 97.6 87.0 98.1 100 100 92.9 81.9 93.1 98.1 49.6 96.3 93.5 88.8 96.5 57.3 73.6 84.3

0.02 f 0.03 CO.10 < 0.02 0.013 f 0.002” 0.40 f 0.02 0.10*0.02 < 0.02 < 0.01 0.02*0.01 0.10*0.05 0.08 f 0.02 0.10*0.01 1.93*0.14 0.23 f 0.01 0.40 f 0.02 0.22 * 0.02 0.27*0.15 1.24*0.15 1.42 f 0.07 1.16*0.14

volcanoes

Egrikuyu region

Karapinar region

Eastern Anatolia Ararat

Tendurek Suphan Nemrut

Karacadag

“Average value and

I CT standard deviation of five separate determinations

(Nagao et al., 1991).

182

K. Norm et al. /Journal

sio2

c?f’Volcanology

(%I

and Geothermal

Research 64 (1995) 171-191

sion (%I

Fig. 2. Selected trace elements (Sr, SC, Cr. Rb. Nb and La) plotted against SiO, for volcanic rocks from Erciyes volcano.

Table 4 shows total rock K-Ar ages determined in this work. For some samples in which radiogenic 40Ar from in-situ 40K decay is less than the analytical uncertainty, the upper limit ages are calculated from the upper limit amount of the radiogenic 4”Ar in the Ar isotope analysis.

5. Geochemical characteristics

of volcanic rocks

5.1. Erciyes volcano in central Anatoliu Erciyes is a large stratovolcano (39 18 m high) composed mostly of talc-alkaline andesite lavas with a small quantity of basalt lava (Ayranci and Weibel, 1973; Ota and Dincel, 1975; Innocenti et al., 1982a). Sampled rocks include andesites and dacites and one basalt (ER24). Based on the K-Ar age data (Table 3). volcanic activity has continued from at least 2.6 Ma until present with the basalt eruption occurring at 1.7 Ma. There is no simple correlation between the age and chemistry of the lavas analyzed. Selected trace-element variation diagrams of some Erciyes volcanic rocks are shown in Fig. 2. Rb increases significantly and La to a lesser extent with increasing SiO, content. In contrast, Cr and SC decrease sharply and Sr decreases more gradually. Incompatible elements such as Rb and La are enriched in the more fractionated magmas, whereas compatible elements such as Cr and SC are accommodated in early crystallizing minerals. The behaviour of these trace elements

may be generally explained in terms of fractional crystallization of basaltic magma (Pearce, 1982). Chondrite-normalized REE and MORB-normalized multi-element patterns for volcanic rocks from Erciyes volcano are displayed in Fig. 3a and b. In both diagrams, ER24 basalt has a different slope and pattern from other andesites and dacites. The ER24 basalt shows a REE pattern which decreases gradually and continuously with atomic number. In contrast, the andesites and dacites have similar REE patterns which show steeply decreasing light REE, followed by flatter heavy REE and a small Eu anomaly for the most siliceous rock. These features imply amphibole-bearing crystallization for andesites and dacites during fractionation from basaltic magma, as suggested for talc-alkaline rocks in eastern Anatolia by Pearce et al. ( 1990). The multi-element pattern of the ER24 basalt resembles those of tholeiitic to transitional basalts occurring within-plate (Pearce, 1982). The depletion of HFSE (high field strength elements) like Ta and Nb, which is a signature of subduction-related volcanic rocks (Pearce, 1982), is observed in lavas from Kars and Ararat in eastern Anatolia (Pearce et al., 1990) but is not obvious in the ER24 basalt. Erciyes andesites and dacites are highly enriched in LILE (large ion lithophile elements), such as K, Rb, Ba, Th, and are depleted in P and Ti, in comparison with the ER24 basalt. This behaviour is compatible with the fractional crystallization process.

K. Notsu et al. / Joumal of Volcanology and Geothermal Research 64 (1995) 171-l 91

(b)

(a)

-

ER24(47.6%,)

-

ER24 (47.6%)

:

----A---

ER26 (57.9%)

----&----

ER26 (57.9%)

ERll(62.0%) ER20 (65.7%) ERl6(69.1%)

-

ER11 (62.0%)

.

_._*_--

-

ER20 (65.7%)

----*----

ER16(69.1%)

1

183

““n”“u”’

LaCe

Nd

Sm Eu

Yb Lu

Tb

Sr K RbBaThTaNbCc

P ZrHfSmTi

Y YbScCr

Fig. 3. (a) Chondrite-normalized REE patterns and (b) MORB-normalized multi-element patterns for volcanic rocks from Erciyes volcano. SiO, (%) contents are shown in parentheses.REE contents of Leedy chondrite (Masuda et al., 1973) are used for chondritic REE values and MORB elemental contents are from Pearce ( 1982).

(a)

(b)

0.‘06I

.

: .-

0.705 l

F I.

. . l

.

l

2

6

0.704-

. l

0.703 40

50

60 SiOl

70

ER24

0.703- . a 0.0 0.1

8 0.2

8 0.3

' 0.4

' . 0.5 0.6

(9,,)

Rbl Sr Fig. 4. Plot of (a) a’Sr/‘?Q vs. SiOz and (b) a7Sr/“Sr vs. Rb/Sr for volcanic rocks from Erciyes volcano. Modelling of the ACF process using the equations of DePaolo ( 1981) is shown in (b) Two reference trajectories are shown for different r (the ratio of the rates of mass assimilation to mass crystallization) values; r=0.2 (upper) and r= 0.1 (lower), assuming that initial magma has ER24 composition, wall rock has Rb = 50 ppm, Sr = 100 ppm and *‘Sr/a?Sr = 0.720, and that bulk solid/liquid partition coefficients are DRb= 0 and Ds’ = 1.

The 87Sr/86Sr of volcanic rocks from Erciyes volcano varies widely from 0.70387 in ER24 basalt to 0.70548 in ER16 dacite. When 87Sr/86Sr is plotted against Si02 content (Fig. 4a), a broad positive trend is observed, suggesting significant assimilation of local crustal materials during fractional crystallization of basaltic magma (AFC model) in the thickened crust formed by the continental collision. In a *‘Sr/%r vs.

Rb/Sr diagram (Fig. 4b), Erciyes rocks are distributed along a trend expected for the AFC model (DePaolo, 1981). In a Th/Yb vs. Ta/Yb diagram (Fig. 5), a mixing line between metasomatized mantle and a crustal component, favoring the AFC model, is formed similar to the case of Nemrut-Mus-Tendurek rocks (Pearce et al., 1990). Therefore, crustal assimilation coupled with fractional crystallization and with no

184

,011

“.‘.‘a

““.” 1

.Ol

1

“.,.,,’

10

Ta i Yb Fig. 5. Th/Yb-Ta/Yb

diagram for volcanic rocks Irom Erciyes vo-

cane. Vectors of mantle metasomatism from N-type MORB, (subduction zone). I;C (fractional crysrallization) et al. (1990).

S%

arc after Pearce

Erciycs data are plotted along AFC (assimillation

fractional crystallization)

vector.

direct contribution of the slab-derived components is the dominant process generating talc-alkaline rocks in Erciyes volcano.

A number of monogenetic volcanoes arc located inside a WSW-ENE-oriented depression in central Anatolia (Pasquarc et al., 1988). The petrogenesis of these monogenetic volcanoes and the stratovolcanoea

like Erciyes is thought to be related to continental collision between the Afro-Arabian and Eurasian plates. As shown in Table 4, the monogenetic volcanism in the three regions occurred over a period of about I .2 Ma and each region represents a different time span [ the Kizirtepe region (KT), 0.2 - 0 Ma; the Egrikuyu region (EC), 1.2 and 0.7 - 0.3 Ma; the Karapinar region (KP), 0.4 - 0.1 Ma]. This suggests that the activity of the monogenetic volcanoes has migrated from one region to another. Monogenetic volcanoes in the three regions have produced olivine basalts with similar chemical compositions. REE and multi-element patterns (Fig. 6a and b) of representative volcanic rocks from the three regions are similar to one another and resemble those of HA01 basalt from Hasan volcano. The tensional stress field relating to the tectonic depression may be responsible for the formation of the monogenetic volcanoes (Nakamura, 1977). The chemical compositions ofbasalts from the monogenetic volcanoes and Hasan volcano are clearly diffcrent from those of ER24 basalt from Erciyes volcano. Light REE, Ba, Th and Cr enrichments are observed for the former. The lowest Sr isotopic ratio (0.70427) for basalts from the monogenetic volcanoes is clearly higher than that of the Erciyes basalt. These features indicate that source magmas are genetically independent between the monogenetic volcanoes and Erciyes

(b)

(a)

‘O”Y,,,

-

EK24

.I

-

ER24

---*--.

HA01 KT,

-

EG2l KP16

---t--.

La Cc Fig. 6. (a)

Nd

Chondrite-normalized

Sm Eu

Tb

REE patterns and (b)

Yb Lu

MORB-normalized

Sr

K

RbBaThTaNbCe

1

P

ZrHfSmTi

Y

YbScO

multi-element patterns for representative volcanic rocks from

monogenetic volcanoes and Hasan volcano in central Anatolia. HA01 is from Hasan volcano. KTI I, EC21 and KP16 are from monogenetic volcanoes in Kizirtepe, Egrikuyu and Karapinar regions. respectively. ER24 basalt data are also plotted for comparison.

185

K. Notsu et al. /Journal of Volcanology and Geothermal Research 64 (1995) 171-l 91

(a)

(b)

100 lWr-----l

II

’

La Ce

”

”

Nd

’

Sm Eu

”

”

Tb

’

.11.,,,,,,,,,,,,,.1

“1 Yb Lu

Sr K RbBaThTaNbCk

P ZrHfSmTi

Y YbScCr

Fig. 7. (a) Chondrite-normalized REE patterns and (b) MORB-normalized multi-element patterns for volcanic rocks from Karacadag volcano in eastern Anatolia.

volcano, suggesting a heterogeneous source mantle in central Anatolia. This may be related to the fact that the western part of central Anatolia is governed by a transitional tectonic setting somewhat between collision and subduction. It is interesting that a7Sr/a6Sr of volcanic rocks from the monogenetic volcanoes show regional differences, though their chemical compositions are hardly distinguishable. These are 0.70464-0.70516 for Kizirtepe (KT), 0.70427-0.70478 for Egrikuyu (EC) and 0.70536-0.70581 for Karapinar (KP) regions, respectively. R7Sr/RhSr for HA01 Hasan basalt (0.70498) is in the range of the KT group. As the Kizirtepe (KT) region is close to the northern foot of Hasan volcano, the magma supplying the monogenetic volcanoes of the KT group may have been derived from the same reservoir as that of Hasan volcano. The regional variation in a’Sr/‘%r among three regions of monogenetic volcanoes suggests source heterogeneity. Since 87Sr/ 8”Sr of volcanic rocks from Karapinar region is observed to correlate with Si02 content, crustal assimilation may also play a role. At present, it is difficult to elucidate the mechanism of 87Sr/86Sr ratio variation in the monogenetic volcanoes. 5.3. Karacadag volcano in eastern Anatolia Karacadag volcano is a large shield volcano located on the southern side of the collision boundary, where thickening of the crust has not occurred. Pearce et al.

( 1990) first reported the geochemistry of Karacadag lavas and discussed a model of foreland volcanism, indicating that the lavas are derived from the mantle lithosphere enriched by small volumes of asthenospheric melts. In this work, olivine basalt and augiteolivine basalt samples from Karacadag were collected, representing the different evolutionary stages of the volcano. K-Ar ages of eight samples (Table 4) are in the range 1.9-0.1 Ma. This range includes previous data1.45 + 0.13 and 1.04 f 0.10 Ma by Sanver ( 1968) and 0.9 + 0.3 and 0.8 + 0.9 Ma by Pearce et al. ( 1990). Our age data show that the volcanic activity has changed with time within at least three eruptive periods: ca. 1.9, 1.5-l .O and 0.3-0.1 Ma. REE and multi-element patterns of Karacadag samples are shown in Fig. 7a and b, respectively. The REE patterns of the eleven samples are similar: monotonously decreasing with atomic number, although the enrichment factors of light REE against heavy REE are somewhat different (La/Yb ratio varies from 9.7 to 25). As the Yb contents are relatively uniform, the difference in La/Yb ratio may be due to differences in the degree of partial melting of the mantle source. This ratio does not correlate with age. The multi-element patterns of the eleven samples are also similar, and resemble those of within-plate alkaline to transitional basalts (Pearce, 1982). In Karacadag volcano, the magma source composition has not changed apparently during ca. 2 Ma of volcanic activity; magma chemistry

186

0.706

-

65 tz

.

2 co

. 0.705

k

0 n

-

II V

[I

?

0.704 -

0

0 b II

Western Fip. 8. East-west

variations

-

Central

in X7Sr/X”Sr of basaltic rocks from Anatoha.

Eastern Open circles and bars indicate basal& and solid trachybasalts.

Ararat

basalt data are from Pearce et al. ( 1990).

has been generally constant with only small modifications due to the slight change in the degree of the partial melting, as suggested from the variation in light REE enrichment. The “Sr/a%r ratios of Karacadag eleven samples range from 0.70334 to 0.70442 and when the highest sample (KR 18) is omitted, the ratios are concentrated between 0.70334 and 0.70389 (Table 2). This range is lower than that reported by Pearce et al. ( 1990), who determined “Sr/?Sr for three Karacadag samples to be 0.70398-0.7045 1. Because Karacadag volcano is located on the thin crust within a tensional stress field, it may be supposed that crustal assimilation was not significant. Therefore, the lower “Sr/?Sr of the samples could represent mantle source values.

6. Discussion 6.1. Chemical and isotopic nature of the mantle under the collision zone According to the evolutionary model of Harris et al. ( 1986), continental collision is preceded by the subduction of oceanic lithosphere and the subducted slab remains under the thickened crust during the syn-collision stage. In the Anatolian case, prior to Late Eocene

times, two oceans existed to the north of the Arabian Platform and to the north of the Anatolides, and the oceanic crusts were subducting northward from both oceans (Sengiir and Kidd, 1979). This suggests that the slab-derived fluids transferred during past subductions may survive under the thickened crust even to the present day. Pearce et al. ( 1990) postulated lithosphere enriched by fluids and melts during the earlier subduction event, in order to explain the volcanic arc chemical signature of rocks from the Kars-Ararat region, in the absence of active subduction. The spatial variation in a’Sr/?Sr of basaltic rocks in Turkey is shown in Fig. 8. Excluding trachybasalts, the pattern is a simple one. The 87Sr/86Sr range for ten olivine basalts from Karacadag volcano in eastern Anatolia is 0.7033-0.7039, which overlaps those from Nemrut volcano (0.70352) and Ararat volcano (0.70389, Pearce et al., 1990) in eastern Anatolia and Erciyes volcano (0.70387) in the eastern part of central Anatolia. This suggests that a mantle source with “Sri ?r of 0.70354I.7040 is widely distributed in eastern Anatolia and the eastern part of central Anatolia, even across the collision boundary. In contrast, in the western part of central Anatolia, basalts from Hasan volcano and many monogenetic volcanoes show much higher Sr isotopic ratios of up to 0.7056. The trace-element compositions of volcanic rocks from Erciyes volcano

K. Notsu et al. /Journal of Volcanology and Geothermal Research 64 (1995) 171-191

-

TEOZ

-

---9--.

NE22

---a--. ---a--.

K”03

Sr K RbBaThTaNbCc

P

Zr HfSmTi

Y YbScCr

187

ER24 EGZI NE26 KR,J

Sr K RbBaThTaNbCe

P ZrHfSmTi

Y YbScCr

Fig. 9. MORB-normalized multielement patterns for basaltic rocks from different regions of Anatolia. (a) Trachybasalts: TE02, NE22 and KU03 are from Tendurek. Nemrut and Kula volcanoes, respectively. (b) Basalts: ER24, NE26 and KR13 are from Erciyes, Nemmt and Karacadag volcanoes, respectively, and EG21 is from a monogenetic volcano in Egrikuyu region.

are clearly different from those of Hasan and the monogenetic volcanoes; LIL elements and Cr are enriched in basaltic rocks from the latter volcanoes, compared with basalt in Erciyes volcano. This may reflect the heterogeneous nature of the mantle source in central Anatolia and may be related to the transitional collision-subduction tectonic setting in the western part of central Anatolia. The heterogeneous nature of *‘Sr/*% among the three regions of monogenetic volcanoes may also be related to such transitional tectonics. The 87Sr/86.Srof the mantle source in the eastern part of central and eastern Anatolia is clearly higher than those of olivine basalt-phonolite (high-Ti) series in western Anatolia (0.702-0.703, Yilmaz, 1990) or Ntype MORB (0.70246-0.70297, Le Roex, 1987)) and is rather similar to those of typical island arc basalts and andesites (0.703-0.704, Gill, 1981). The 3He/4He of the mantle source in eastern Anatolia is interpreted to be 1.06 X lo-‘, based on measurements on bubbling gas released from a hot spring inside the summit crater of Nemrut volcano (Nagao et al., 1989), and is similar to that of gases in island arc magmas (Sano and Wakita, 1985). Therefore, the mantle source of the collision magma in the eastern part of central and eastern Anatolia may be contaminated by some components with higher *‘Sr/*%r, analogous to the arc magma. It may be that, in eastern and central Anatolia, the contamination of the asthenosphere by slab-derived materials

took place in a complex manner during pre-collision events (Harris et al., 1986). Pearce et al. ( 1990) identified the island arc affinity for basalts from the KarsArarat region, based on HFS element depletion. Multi-element patterns of basaltic rocks from different regions in Turkey are shown in Fig. 9. The patterns of Karacadag, Nemrut and Erciyes basalts are similar to one another except for Cr enrichment in Karacadag basalts, depending on olivine fractionation. It must be pointed out that the HFS elementdepletion is negligible for ER24, NE26, KR13 and EG21 basalts, in contrast to Ararat basalt (Pearce et al., 1990). This suggests that, in eastern to central Anatolia, there is chemical heterogeneity in the source mantle, presumably corresponding to the spatial distribution of past subduction. As shown in Fig. 9, multi-element patterns of trachybasalts from Nemrut and Tendurek volcanoes in the collision zone of eastern Anatolia are similar to those from Kula volcano, where collision tectonics are absent. However, the *‘Sr/*‘Sr of the trachybasalts differ between eastern and western Anatolia. The trachybasalts at Nemrut and Tendurek volcanoes have *‘Sr/ *?Sr of 0.70527 and 0.70563, respectively, which are higher than that of Kula volcano (0.7030-0.7035). It may be postulated that the Sr isotopic composition of the mantle source generating trachybasalt magma is heterogeneous between eastern and western Anatolia. Moreover, the *‘Sr/*%r of trachybasalts are distinctly

188

K. Norm er ~11./ Journul of Volcunology und Geothermal

higher than those of olivine basalts in the vicinity. In eastern Anatolia, the ratios of trachybasalts from Nemrut and Tendurek volcanoes are higher than that of Nemrut basalt. In western Anatolia, the ratios of trachybasalts from Kula volcano are higher than those of nearby olivine basal& (0.702-0.703, Yilmaz, 1990). 6.2. Origin of calc-alkaline collision tectonics

andesites related to

It is well established that talc-alkaline andesites are typical products of arc volcanism and their geochemical features have been summarized by many authors (Gill, I98 1; Pearce, 1982; Hawkesworth, 1982). Gill ( I98 1) concluded that the main petrogenetic process giving rise to talc-alkaline andesites is the differentiation of basalt by crystal fractionation, although other mechanisms, such as partial melting of crust, mixing of basaltic and felsic magmas or assimilation of crustal materials along with fractional crystallization also take place. Saunders et al. (1980), Sakuyama and Nesbitt ( 1986) and Tatsumi ( 1989) proposed models for the genesis of island-arc basalts, emphasizing the role of slab-derived fluids in the mantle wedge. In these models, talc-alkaline andesites are related closely to the subduction of the oceanic crust. However, talc-alkaline andesites magmatism also occurs even where the subducted slab does not extend beneath the volcanic belt in the initial stage of subduction, as shown in the Southwest Japan arc (Notsu et al., 1990). Furthermore, calcalkaline andesites occur in tectonic settings having no subducting slab. The best example is along the collision boundary in central and eastern Anatolia (Innocenti et al., 1992a). Innocenti et al. (1983b) proposed that talc-alkaline andesites in eastern Anatofia were generated by subduction. However, this model was rejected by Pearce et al. ( 1990) and Yilmaz ( 1990)) because there is no seismic evidence for a downgoing slab beneath eastern Anatolia. Pearce et al. ( 1990) postulated the lithosphere was enriched by fluids and melts during the precollision subduction event, in order to explain the origin of talc-alkaline volcanic rocks with volcanic arc chemistry in eastern Anatolia. As shown in the previous section, talc-alkaline rocks of Erciyes volcano in central Anatolia are likely to be the fractionation products of basalts, accompanied by assimilation. They do not show distinct HFS element

Research 64 (1995) 171-191

depletion in the multi-element diagram, suggesting negligible amount of fluids and melts supplied during the past subduction events. This may be evidence that the subducting slab or slab-derived components are not necessary for the genesis of talc-alkaline andesites. During collision tectonics, the combined process of assimilation and fractional crystallization of basalts is the dominant process likely to generate talc-alkaline andesites.

7. Conclusions ( 1) In wide areas of the eastern part of central Anatolia and eastern Anatolia, the mantle source generating olivine basalts has uniform s’Sr/‘?Sr of 0.703% 0.7040, even across the collision plate boundary. As the ratio is higher than that for western Anatolia (0.702-0.703, Yilmaz, 1990) which has N-type MORB values, the asthenosphere must be contaminated by materials with high 87Sr/86Sr during pre-collisional events. In addition, chemical compositions are not homogeneous with respect to HFS element depletion, suggesting heterogeneous distribution of the slabderived components due to past-subduction. The mantle source beneath the western part of central Anatolia, which is governed by transitional tectonics from collision in the eastern region to subduction in the west, has different chemical and Sr isotopic signatures. (2) The 87Sr/86Sr of volcanic rocks from three regions characterized by monogenetic volcanoes in the western part of central Anatolia are in the range 0.70427-0.70581. The regional variation in “St+/*% and K-Ar age suggests either source heterogeneity on a small scale, or different contributions of local crustal materials. (3) Chemical and Sr isotopic features of Erciyes volcanic rocks in central Anatolia show that calc-alkaline andesite magma is derived from basaltic magma by fractional crystallization accompanied by crustal assimilation. In view of the absence of HFS element depletion, it is concluded that the subducting slab or slab-derived components are not necessary for the genesis of talc-alkaline andesites. In the collision tectonic regime of central Anatolia, the combined process of assimilation and fractional crystallization of basal&, without contribution of slab-derived materials, is the

K. Notsu et al. /Journal of Volcanology and Geothermal Research 64 (1995) 171-191

dominant process for the petrogenesis of talc-alkaline andesites and dacites.

Acknowledgements

This work was carried out under a joint Japan-Turkey program entitled “Geochemical Study of Collision Volcanism at the Plate Boundary in Turkey”, under the auspices of International Scientific Research Program, Ministry of Education, Science and Culture, Japan (Grant Nos. 62042015 and 63041085). We wish to thank other members of our joint program, Mrs. I.T. Cakmak and E. Gunay of MTA (General Directorate of Mineral Research and Exploration), Turkey, Dr. K. Nagao of Okayama University, and Dr. I. Kita of Akita University, Japan for their excellent cooperation during the field work in Turkey. We thank Dr. S. Aramaki of the University of Tokyo for providing the XRF facilities and Dr. H. Mabuchi of Tokyo National Institute of Cultural Properties for making the mass spectrometer available to us. We thank Dr. S. Nakai of the University of Tokyo for his helpful discussions.

189

Kizirtepe region

KTll KT12 KT13 KT14 KT16

Olivine basalt, lava flow, Kizirtepe Olivine basalt, lava flow, Mahmutlutepe Olivine basalt, lava flow, Anatepe Olivine basalt, lava flow, Anatepe Olivine basalt, lava flow, Yapraktepe

Egrikuyu region

EG12 EG14 EG15 EG16 EG17 EG18 EG19 EG21 EG22

Augite-olivine basalt, maar ejecta, Kuforen Olivine basalt, bomb, Obmk Olivine basalt, lava flow, south of Obruk Olivine basalt, lava flow, south of Obmk Olivine basalt, lava flow, east of Obmk Olivine basalt, bomb, southeast of Obmk Olivine basalt, lava Bow, southeast of Obmk Olivine basalt, lava flow, south of Obmk Olivine basalt, lava flow, Igredag

Karapinar region KP15

KF’16 KP17 KP18

Augite-olivine basalt, welded air fall?, east of Karapinar Augite-olivine basalt, lava block, east of Karapinar Augite-olinive basalt, lava flow, east of Karapinar Olivine-hypersthene andesite, lava flow, east of Karapinar

Eastern Anatolia Ararat volcano

AR01 AR02

Appendix: Rock type, occurrence and location of samples

AR11

Tendurek volcano

Central Anatolia

TEOl

Erciyes volcano

TEo2

ERll ER13 ER14 ER15 ER16 ER17 ER20 ER21 ER22

ER23 ER24 ER25 ER26

Augite-hypersthene andesite, lave flow, south of Incesu Augite-hypersthene dacite, lava flow, Dag Evi Augite-hypersthene andesite, lava flow, 1.4 km north of ER13 Hypersthene-hornblende dacite, lava flow, Goktepe Hypersthene-hornblende dacite, pumice, Goktepe Hypemthene-augite andesite, lava flow, 3.9 km south of ER13 Hypersthene-augite andesite, lava flow, 3.5 km south of ER13 Augite-hypersthene andesite, lava Row, west of Develi Augite-hypersthene dacite, lava flow, west of Develi Augite-hypersthene andesite, lava flow, west of Develi Aphyric basalt, lava flow, west of Devell Augite-hypersthene andesite, lava flow, Karigtepe Hypersthene andesite, lava flow, south of Kayseri

Hasan volcano

HA01

Olivine-augite basalt, lava flow, western foot of Hasan

Aphyric andesite, lava flow, southern foot of little Ararat Hypersthene-augite andesite, lava flow, southern foot of greater Ararat Augite-hypersthene andesite, lavaflow, southeast of Igdir

Olivine-augite trachyandesite, lava flow, southeastern foot of Tendurek Olivine-augite trachybasalt, lava flow, northeastern foot of Tendurek

Suphan volcano

SUOl suo2

Augite-hornblende dacite, lava flow, eastern slope of Suphan Augite-hypersthene andesite, lava flow, 4 km south of SUOl

Nemrut volcano NE13

NE15 NE17 NH19 NE20 NH22 NE24 NE26 NE27

Trachyte, lava flow, Kirkor mountain Rhyolite, obsidian flow, eastern slope of Nemrut Rhyolite, air fall pumice, southeast caldera wall Rhyolite, obsidian flow, central site of caldera floor Rhyolite, obsidian flow, northern site of caldera floor Aphyric trachybasalt, lava flow, northwest caldera wall Titanaughite rhyolite, dome lava, west of Sihmiran Augite basalt, lava Bow (1441), northern slope of Nemrut Olivine basalt, lava flow, east of Tatvan

KRll KR12 KR13 KR14 KR17 KR18 KR19 KR20 KR2 I KR25 KR28

Augite-olivine Augite-olivine Olivine basalt, Olivine basalt, Olivine basalt, Olivine basalt, Olivine basalt, Olivine basalt, Olivine basalt, mit Olivine basalt, Augite-olivine

basalt, lava tlow, south ofOvagab basalt, lava flow, south of Ovabag lava flow, Ovabag lava flow, east of Beykortepe lava flow, west of Kizitepe lava flow, north of Cinar lava Row, on Karacadag summit lava flow, close to Karacadag summit lava flow, northwest from Karacadagsumlava flow, I I km north of Siverek basalt, Juvenile fragment in cinder cone.

30 km northwest of Diyarbakir Western

KU11 KU12 KU13

Anatolia

Olivine-titanaugite trachybasalt, lava flow, BurgaL Olivine-titanaugite trachybasalt, lava flow, east of Kula Titanaugite-homblemde trachybasalt, lava flow. north of Kula

site: Orogenic Andesite and Related Rocks. Wiley, New York, NY, pp. 549-57 I. Innocenti, F., Mazzuoli, R.. Pasquare, G., Radicati di Brozolo, F. and Villari, L., 197.5. The Neogene talc-alkaline volcanism of Central Anatolia: geochronological data on Kayseri-Nigde area. Geol. Msg., 112: 349-360. Innocenti. F., Mazzuoli, R., Pasquare, G., Radicati de Brozolo, F. and Villari, L., 1976. Evolution of the volcanism in the area of interaction between the Arabian, Anatolian and Iranian plates (Lake Van, Eastern Turkey). 103-l 12.

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