Geochemistry of the Middle Miocene Collision-related ... · In this paper, we present geochemical characteristics and K-Ar data on lavas from the Yamadağı volcanism in the Eastern

Geochemistry of the Middle Miocene Collision-relatedYamadağı (Eastern Anatolia) Calc-alkaline

Volcanics, Turkey

TANER EKİCİ1, MUSA ALPASLAN2, OSMAN PARLAK3 & ALİ UÇURUM1

1 Cumhuriyet University, Department of Geological Engineering, TR−58140 Sivas, Turkey(E-mail: [email protected])

2 Mersin University, Department of Geological Engineering, TR−33343 Mersin, Turkey3 Çukurova University, Department of Geological Engineering, TR−01330 Adana, Turkey

Received 07 December 2008; revised typescript received 08 January 2009; accepted 04 March 2009

Abstract: Major, trace element and K-Ar age determinations are reported for a suite from the Yamadağı volcanics inthe Eastern Anatolia. The exposed rocks mainly consist of medium-potassium calc-alkaline basaltic andesites, andesitesand dacites. Petrographical data exhibit disequilibrium mineral textures, such as resorption of the ferromagnesianphases, clinopyroxene-mantled orthopyroxene, and sieve-textured plagioclases. The Yamadağı volcanics have a calk-alkaline character, and trace element characteristics exhibit that the volcanics resemble subduction zone volcanicsand/or volcanics assimilated by continental crust. K/Ar age determinations show that the Yamadağı volcanics wereformed during the 12 ± 0.5 − 15 ± 0.5 Ma time interval. Geochemical characteristics of these volcanics can be attributedto complex petrogenetic processes, including magma mixing and crustal assimilation along with fractionalcrystallization.

Key Words: calc alkaline, volcanics, collision, Eastern Anatolia, Turkey

Çarpışmayla İlişkili Orta Miyosen Yaşlı Yamadağ (Doğu Anadolu)Kalkalkalin Volkanizmasının Jeokimyası

Özet: Doğu Anadolu’daki Yamadağı volkaniklerinden ana, eser element ve K-Ar yaş determinasyonları yapılmıştır.Yamadağı volkaniklerindeki kayaçlar ortaç potasyumlu kalkalkalin bazaltik andezitler, andezitler ve dasitlerdenoluşmaktadır. Petrografik olarak elek dokulu plajiyoklaz, ortopiroksenler tarafından mantolanmış klinopiroksenler,resorbe olmuş ferromagnezyan fazlar ve birbirleriyle dengede olmayan mineral toplulukları içermektedir. Kalkalkalinkarakterdeki Yamadağı volkanitlerinin iz element karakteristikleri bu volkanitlerin dalma-batma zonu ve/veya kıtasalkabuk tarafından kirletilmiş volkanitlere benzeştiğini göstermektedir. K/Ar yaş tayinleri Yamadağı volkanitlerinin 12 ±0.5 − 15 ± 0.5 My yaş aralığında oluştuğunu göstermektedir. Yamadağı volkanitlerinin jeokimyasal karakteristikleri buvolkanitlerin evriminde fraksiyonel kristallenmenin yanı sıra magma karışımı ve kabuksal bulaşma süreçlerinin etkinolduğunu göstermektedir.

Anahtar Sözcükler: kalkalkalin, volkanikler, çarpışma, Doğu Anadolu, Türkiye

IntroductionAnatolia (Turkey) is tectonically complex because ofthe involvement of three major tectonic plates:Arabia and Africa in the south and Eurasia in thenorth. The Neotectonic evolution of Turkey reflectsthe interaction between these plates and the minorAnatolian plate (Şengör & Yılmaz 1981; Şengör et al.

1985; Dewey et al. 1986; Figure 1). The northwardmotion of the African and Arabian plates triggeredsubduction in Eocene−Miocene times, followed bydiachronous collision along the Bitlis suture zone(e.g., Şengör & Yılmaz 1981). Therefore, geodynamicmodels suggest that the Anatolian plate wasdeformed as a result of the collision of the Eurasian

511

Turkish Journal of Earth Sciences (Turkish J. Earth Sci.), Vol. 18, 2009, pp. 511–528. Copyright ©TÜBİTAKdoi:10.3906/yer-0712-1 First published online 17 August 2009

and Arabian plates along the Bitlis suture zone(McKenzie 1972; Şengör 1980). There is noconsensus on precisely when collision between theEurasian and Arabian plates began. The ageestimates of collision range from 12 Ma, based onstratigraphic discontinuities in Eastern Anatolia(Şengör & Yılmaz 1981) and the beginning ofcollision-related volcanism (Pearce et al. 1990), to 20Ma, based on the convergence rate of the two plates(Dewey et al. 1986). This collision, which occurredduring the Neogene period, resulted in shortening ofEastern Anatolia (McKenzie 1972; Şengör & Kidd1979; Şengör et al. 1985) as well as youngerextensional tectonics (Şengör & Kidd 1979; Şengör1980; Yılmaz 1990).

Extensive volcanic activity took place in EasternAnatolia during the neotectonic period (MiddleMiocene to present), as a result of which volcanicrocks were erupted over large areas. Calc-alkalinevolcanic rocks were produced when thecompressional regime led to crustal thickening.Calc-alkaline volcanics, which have chemicalcompositions with subduction signatures inheritedfrom pre-collision subduction events (Notsu et al.1995), were succeeded by alkaline volcanics duringthe final stage of the compressional regime (Yılmaz1990).

Many researchers have discussed the origin, ageand tectonic settings of these volcanic rocks(Lambert et al. 1974; Innocenti et al. 1976; Şaroğlu &

YAMADAĞI CALC-ALKALINE VOLCANICS, E ANATOLIA

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Yılmaz 1984; Gülen 1984; Tokel 1984; Alpaslan &Terzioğlu 1996; Keskin et al. 1998; Yılmaz et al. 1998;Buket & Temel 1998).

In this paper, we present geochemicalcharacteristics and K-Ar data on lavas from theYamadağı volcanism in the Eastern Anatolia. Theorigin, evolution and tectonic significance of thesevolcanics are then discussed.

Geological SettingThe study area is located in central-eastern Anatolia(north of Malatya, Figure 1) and is a part of theregion that is under approximately north−south andNNE−SSW shortening, related to the collisionbetween the Anatolian and Arabian plates along theBitlis suture zone (Bozkurt 2001). The eastern part ofAnatolia has experienced intra-continentalconvergence (McKenzie 1969) that resulted incrustal thickening and uplift (Şengör & Kidd 1979),as a direct result of the collision between Arabianand Anatolian plates and extrusion of collisionrelated volcanics (Pearce et al. 1990; Yılmaz et al.1998; Ekici 2003). This compressional tectonicregime was replaced by a new compressional-extensional tectonic regime by early Pliocene timefollowing continental collision (Koçyiğit et al. 2001).This has resulted in the generation ofintracontinental strike-slip faults, namely the NorthAnatolian and East Anatolian faults (Figure 1).Structural elements of the study area are dominatedby a left-lateral strike-slip fault zone, the Malatya-Ovacık fault zone, which has been suggested as theboundary between the Anatolian and Arabian plates(Figures 1 & 2; Westaway & Arger 2001). Based onexisting geological studies, three lithostratigraphicunits are distinguished within the LowerMiocene−Quaternary (Alpaslan & Terzioğlu 1996;Ekici 2003; Figures 2 & 3). The Yamadağı volcanicsconsist of intermediate to acidic or silicic lava flows,and their pyroclastic derivatives cover large areas andrest on the Lower Miocene limestones (Figure 2;Alpaslan 1987; Ekici 2003). The age of thesevolcanics varies from 11.99 ± 0.49 to 14.82 ± 0.57 Ma(Table 1), representing middle to late Miocene agesbased on K/Ar geochronology. Pliocene units arerepresented by lacustrine sediments (Figure 2).

PetrographySilica and total alkalis (Na2O+K2O) were used toclassify the rocks on the TAS diagram of Le Maitre etal. (1989) (Figure 3). The composition of thevolcanic rocks ranges from basaltic andesite to daciteon this diagram (Figure 3).

Basaltic andesites are grey and greyish brown andhave a hypocrystalline-porphyritic-pylotaxitictexture. They contain olivine, clinopyroxene,plagioclase and scarce hornblende phenocrysts.Their groundmass consists of plagioclase, olivine,pyroxene, hornblende and opaque mineral microlitesand devitrified glass. Reacted hornblendephenocrysts have occasionally been observed in thebasaltic andesites.

Andesites are dark grey to black and have anaphanitic texture. They display a hypocrystallineporphyritic-pylotaxitic texture. Phenocryst andmicrophenocryst phases in the andesites includeplagioclase, clinopyroxene, orthopyroxene,hornblende, apatite and opaque minerals. Thegroundmass also contains palagonitizated volcanicglass.

Dacites are dark-grey and have a hypo-hyalineporphyritic texture. They consist of plagioclase,clino- and ortho-pyroxene and either green orreddish brown hornblende phenocrysts. Theirgroundmass comprises plagioclase microlites,pyroxene and apatite microphenocrysts, opaqueminerals and volcanic glass. Occasional, millimetre-sized crystal-rich enclaves occur in the dacites(Figure 4A). The microlitic nature of these enclavespossibly indicates that they are chilled blobs of basicmagma.

Olivine occurs as phenocrysts andmicrophenocrysts in the basaltic andesites andandesites, sometimes these are penetrated bymicrocrystalline groundmass (Figure 4B). Someolivine phenocrysts are embayed and olivine alsooccurs as resorbed phenocrysts in the basalticandesites. Olivine also was observed as xenocrystsmantled by orthopyroxene in the dacites (Figure 4C).

Clino- and ortho-pyroxenes are found in all rocktypes as phenocrysts and microphenocrysts.Clinopyroxene is most common in the basalticandesite whereas orthopyroxene is more prevalent in

T. EKİCİ ET AL.

513

the andesites. Occasionally, clinopyroxene cores aremantled by overgrowths of orthopyroxene in basalticandesites (Figure 4D). Some clinopyroxenephenocrysts in the andesites have abundantinclusions of groundmass material in the core(Figure 4E) and the others have embayed marginssuggesting resorption (Figure 4F). Orthopyroxenealso occurs in glomeroporphyritic aggregates with

plagioclase and Fe-oxides and in reaction rimsaround olivine phenocrysts (Figure 4C).

Plagioclase phenocrysts in the Yamadağıvolcanics show clear evidence of multiple origins andperiods of dissolution and growth. Based on texturalcriteria, plagioclase phenocrysts can be identified asone of three types: (a) unsieved, with no dissolutiontexture, (b) sieve-cored, where the cores are riddled


514

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with glass and overgrown with clear rims, and (c)sieve-ringed, where a clear core is mantled by aresorption zone followed by a clear rim (Figure 4G).

Amphibole occurs as scarce dark reddish brownphenocrysts, which are observed as reactedphenocrysts. They typically have thin rims of fine-grained plagioclase, pyroxene and Fe-Ti oxide(Figure 4H). This feature probably reflects volatile

loss during ascent of magma in conduits (Rutherford& Hill 1993). Reacted amphibole phenocrysts arepartially replaced by acicular pyroxene and fine-grained oxide minerals [magnetite?]. Amphibolephenocrysts and microphenocrysts with a yellowish-green to green pleochroism are common in thedacites. These amphibole phenocrysts haveabundant inclusions of groundmass material in thecores (Figure 4I).

T. EKİCİ ET AL.

515

Pic

robasalt

Basalt

Ba

sa

ltic

an

de

site

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de

site

Da

cite

Rhyolite

Trachyte

Trachydacite

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andesite

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trachy-

andesite

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basalt

Tephrite

Basanite

Phono-

tephrite

Tephri-

phonolite

Foidite

40 50 60 70 80

05

10

SiO2

Na

O/K

O2

2

Figure 3. Total alkali-silica nomenclature diagram (Le Bas et al. 1986) for the Yamadağı volcanics.Dividing line between alkaline and subalkaline fields after Irvine & Barager (1971).

Table 1. K/Ar age (Ma).

Sample Rock name Grain size 40Ar radius 40Ar radius K (%) K-Ar Age (Ma)(ccSTP/g) (%)

AR-38 Dacite 90-250 μ 1.171 × 10-6 55.5 2.503 11.99 ± 0.49AR-40 Dacite 250-400 μ 7.560 × 10-7 66.3 1.424 13.61 ± 0.53AR-78 Andesite <90μ 5.827 × 10-7 48.1 1.073 13.91 ± 0.59AR-100 Dacite 90-250 μ 8.198 × 10-7 53.1 1.461 14.37 ± 0.59AR-68 Dacite 250-400 μ 8.731 × 10-7 72.2 1.509 14.82 ± 0.57


516

Figure 4. (A) Crystal-rich enclave in dacite, (B) embayed olivine crystal in basaltic andesite,(C) olivine mantled by orthopyroxene, (D) clinopyroxene mantled byorthopyroxene, (E) groundmass inclusions in clinopyroxene, (F) clinopyroxeneindicating resorption, (G) sieve-textured plagioclase, (H) amphibole with thin rimsof fine-grained plagioclase, pyroxene and Fe-Ti oxide, (I) amphibole with abundantgroundmass inclusions in the core, (J) dusty apatite microphenocryst in dacite.

(A) (B)

(C) (D)

(E) (F)

(G) (H)

(I) (J)

Apatite is an accessory phase in the Yamadağıvolcanics. It occurs mostly as microphenocrysts inthe dacites (Figure 4J). These microphenocrysts aredusted with fine, brown specks and are interpreted tobe apatite grains xenocrysts.

Analytical TechniquesRock powders were prepared by removing alteredsurfaces, crushing and then grinding. Major elementabundances were measured on fused discs. Fuseddiscs were prepared by using five parts of lithiumtetraborate and one part of rock powder. Themixture was fused in crucibles of 95%Pt and 5%Au at1150 °C to form a homogenous melt. The melt thenwas poured into a preheated mold to chill a thickglass disk. Major element analyses were performed atLausanne University using an X-ray spectrometerusing USGS and GEOSTANDARD rock standards.Trace element concentrations were analyzed atACME laboratories (Vancouver, CANADA) by ICP-MS with better than ± 3% accuracy using dissolvedfusion beads.

For the K-Ar dates, the samples were degassed ina conventional extraction system using inductionheating and were measured by mass in spectrometricisotope dilution with a 38Ar spike. Potassiumdeterminations were made using standard flamephotometric techniques. K and Ar determinationswere checked regularly by interlaboratory standards;HD-B1,LP-6,GL-0 and Asia1/65. Atomic constantssuggested by Steiger & Jaéger (1977) were used forcalculating the radiometric ages. All analytical errorsrepresent one standard deviation (68% confidencelevel). Details of the instruments, the methodsapplied and results of calibration have beendescribed by Balogh (1985).

Major- and Trace-element Geochemistry Major- and trace-element analyses were carried outon twenty-eight Yamadağı samples (Table 2a, b). Thevolcanic rocks have a wide range of chemicalcomposition with SiO2 contents ranging between54% and 70% without a compositional gap, and havebeen classified on the basis of their alkali and silicacontents using the total alkali − SiO2 diagram (TAS)of Le Bas et al. (1986) and K2O−SiO2. On the TASdiagram (Figure 3) volcanic rocks with intermediate-

acidic and sodic compositions are represented bybasaltic andesites, andesites and dacites. ThePeccerillo & Taylor (1976) diagram shows that allsamples are similar to calc-alkaline rocks, fallingwithin the medium-K series (Figure 5). The volcanicrocks are dominantly characterized by subalkalinetrends on the total alkali − silica diagram (Figure 3),and generally show a typical calc-alkalinedifferentiation trend on an AFM diagram (Figure 6).In the Harker diagrams, as SiO2 increases, Fe2O3,MgO, CaO, TiO2 and P2O5 decrease and K2Oincreases (Figure 7). Such negative and positivecorrelations can be explained by removal of theferromagnesian phases such as olivine and pyroxene,and apatite. Compatible trace elements such as Co, Vand Y show strong negative correlation withincreasing SiO2, whereas incompatible traceelements correlate positively (Figure 7). These majorand trace element trends are broadly consistent withplagioclase+pyroxene+Fe-Ti oxides+hornblende, allof which are present as phenocrysts in the Yamadağıvolcanics.

Primitive mantle-normalized trace elementpatterns of the Yamadağı volcanics (Figure 8) arecharacterized by a Nb-Ta trough and are enriched inincompatible trace elements. Negative and positivePb anomalies occur in all the rock types of theYamadağı volcanics (Figure 8).

When compared with the multi-elementdiagrams of the Yamadağı volcanics, the basalticandesites (Figure 8) are characterized by a lessmarked enrichment in Rb, Ba, Th, K, and a negativeNb anomaly. The andesites and dacites display multi-element patterns consistent with their possiblederivation from the associated basaltic andesitesthrough crystal fractionation: enrichment in Rb, Th,K and negative anomalies in Ba, Nb, and Ti. The REEpatterns of the basaltic andesites (Figure 8a) exhibitenriched light REE but the (La/Yb)N ratios (7.81) arelower than in the andesites (9.01) and dacites (10.42).

DiscussionFractional CrystallizationThe new data reported in this study indicate that theYamadağı volcanic rocks have similar petrographicaland geochemical features and define typical calc-

T. EKİCİ ET AL.

517


518

Tabl

e 2a.

Who

le-r

ock

maj

or el

emen

t com

posit

ions

of t

he Y

amda

ğı v

olca

nics

. Maj

or o

xide

s are

giv

en as

wei

ght p

er ce

nt (F

e 2O3

as to

tal i

ron,

LO

I as l

oss o

n ig

nitio

n).

Sam

ple

SiO

2Ti

O2

Al 2O

3Fe

2O3

MnO

MgO

CaO

Na 2O

K 2OP 2O

5LO

I

Ar-

1757

.55

1.26

18.3

86.

910.

082.

626.

404.

361.

390.

260.

88A

r-19

68.1

40.

5016

.35

3.23

0.03

1.05

3.85

4.61

1.68

0.14

0.66

Ar-

2555

.75

0.87

19.3

67.

810.

163.

027.

843.

680.

930.

170.

38A

r-34

63.7

40.

7217

.01

4.69

0.08

1.32

5.14

4.32

1.76

0.47

0.76

Ar-

3754

.60

1.25

17.8

57.

600.

124.

478.

213.

611.

120.

240.

71A

r-38

63.2

50.

8616

.53

4.81

0.08

2.13

4.67

3.66

2.94

0.22

1.04

Ar-

4066

.02

0.61

16.8

73.

590.

051.

244.

564.

081.

670.

161.

12A

r-41

60.4

80.

9917

.47

5.83

0.08

1.80

5.37

4.74

2.02

0.33

0.69

Ar-

4264

.13

0.79

16.7

84.

160.

071.

874.

334.

082.

550.

220.

62A

r-45

59.8

71.

0317

.27

6.06

0.09

2.56

5.41

4.65

2.01

0.34

0.41

Ar-

4963

.98

0.58

17.6

04.

560.

080.

833.

584.

852.

370.

250.

97A

r-51

54.9

51.

1817

.18

8.28

0.12

5.34

6.91

4.14

1.12

0.08

0.21

Ar-

5362

.23

0.81

16.3

15.

390.

093.

165.

133.

812.

090.

180.

64A

r-54

54.1

71.

3117

.37

8.62

0.14

4.95

7.25

4.20

1.31

0.33

0.20

Ar-

5559

.79

0.85

16.0

56.

240.

095.

025.

963.

851.

380.

200.

45A

r-58

55.6

51.

1517

.62

8.50

0.13

5.47

5.37

4.15

1.16

0.21

0.50

Ar-

6454

.66

1.39

18.2

28.

280.

123.

817.

484.

391.

120.

290.

33A

r-67

62.3

40.

7616

.66

4.99

0.08

2.45

5.82

4.13

1.66

0.28

0.81

Ar-

6864

.09

0.75

16.8

84.

550.

061.

564.

734.

251.

770.

201.

23A

r-77

61.0

40.

8117

.12

5.23

0.08

3.39

5.99

3.65

1.86

0.18

0.58

Ar-

7859

.76

0.99

17.0

76.

140.

083.

426.

144.

161.

430.

200.

62A

r-80

59.3

10.

9917

.08

5.69

0.09

3.94

6.39

3.88

1.52

0.19

0.42

Ar-

8359

.84

0.99

17.0

26.

120.

093.

616.

174.

221.

430.

200.

30A

r-85

67.2

00.

4816

.09

3.04

0.05

1.48

3.79

3.68

2.52

0.13

1.29

Ar-

8667

.64

0.46

15.9

82.

960.

051.

463.

713.

542.

510.

121.

62A

r-87

68.2

50.

4115

.89

2.87

0.05

1.41

3.57

3.64

2.58

0.12

1.34

Ar-

8861

.36

1.05

18.2

85.

490.

091.

485.

494.

511.

160.

210.

94A

r-89

60.6

41.

0418

.39

5.57

0.09

2.20

5.76

4.53

1.13

0.23

0.44

Ar-

100

67.9

90.

4916

.30

3.07

0.05

1.51

4.02

4.46

1.74

0.13

0.49

T. EKİCİ ET AL.

519

Tabl

e 2b.

Trac

e ele

men

t com

posit

ions

of t

he Y

amad

ağ v

olca

nics

. Tra

ce el

emen

ts ar

e giv

en as

ppm

.

Sam

ple

RbBa

ThN

bLa

Ce

SrSm

ZrY

PrN

dEu

Gd

TbD

yH

oEr

TmYb

LuP

Co

VTi

K

Ar-

1736

.318

65.

99.

818

.636

.842

23.

915

921

.34.

418

.71.

223.

870.

633.

710.

762.

160.

281.

820.

256.

7628

138

7552

1153

9

Ar-

1960

.822

06

4.8

1832

312

2.9

146

17.2

3.85

16.9

0.84

2.92

0.44

2.79

0.58

1.56

0.21

1.71

0.24

4.3

2352

2997

1394

6

Ar-

2530

195

4.8

5.1

14.1

29.3

329

410

925

.53.

6316

.31.

123.

690.

674.

350.

912.

610.

372.

790.

415.

2239

135

5215

7720

Ar-

3444

.145

711

.229

.644

.671

.354

74.

321

818

.47.

4227

.61.

213.

320.

582.

980.

581.

730.

221.

540.

2514

.44

2164

4316

1461

0

Ar-

3732

140

37.

414

.129

.342

33.

514

623

3.59

16.8

1.26

3.9

0.63

3.76

0.76

2.19

0.34

2.17

0.29

7.37

4915

874

9292

97

Ar-

3873

.827

67

11.4

19.4

37.8

374

3.2

168

18.8

4.11

170.

863.

130.

472.

880.

611.

640.

251.

350.

256.

7630

8051

5524

406

Ar-

4055

.627

97

7.3

20.8

39.8

305

3.3

151

16.4

4.34

16.7

0.85

2.86

0.44

2.75

0.55

1.48

0.23

1.47

0.2

4.91

2465

3656

1386

3

Ar-

4151

.628

28.

424

.230

.956

.353

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alkaline trends from subalkaline basaltic andesites todacites. Major- and trace-element abundances varyalong continuous trends of decreasing MgO, TiO2,Fe2O3*, CaO, V and Co, and increasing K2O, Rb, Zr,and Y with increasing SiO2 (Figure 7). Incompatible

(Rb) versus incompatible (K and Y) trace elementvariations are linear (Figure 9a, b), with trends fromlow abundances in basaltic andesites towards higherabundances in dacites (Figure 9). Normalized REEpatterns of the Yamadağı volcanics form paralleltrends, and total REE contents increase from basalticandesite to dacite (Figure 8). La/Sm data points(Figure 9c) plot along a line, a feature restricted tothe process of fractional crystallization (Allegre &Minster 1978).

The above-mentioned characteristics show thatthe Yamadağı volcanics evolved predominantlythrough fractional crystallization of thepetrographically observed phenocryst assemblage,which is olivine+plagioclase+augite+Fe-Ti oxides inmafic volcanic rocks and plagioclase+twopyroxene+hornblende+Fe-Ti oxides in the acidicrocks.

Crustal ContaminationThe chemical data of the Yamadağı Volcanic rocksprovide few constraints on whether or not there wassignificant crustal contamination, particularlybecause there is no data on the composition of the


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Figure 5. K2O−SiO2 diagram (Peccerillo & Taylor 1976) for the Yamadağı volcanics.

Figure 6. AFM diagram (Irvine & Barager 1971) for theYamadağı volcanics.

country rocks that may represent the potentialcontaminants. However, the LILE (e.g., Rb and K)and Zr are incompatible with respect to the majorcrystallizing phenocryst assemblage (plagioclase,pyroxene, Fe-Ti oxides) and ratios like K/Rb andRb/Zr do not significantly change by simplefractional crystallization of this assemblage.Variations in these ratios are preferably related tocrustal contamination by assimilation fractionalcrystallization processes (Davidson et al. 1987).

Examination of the Yamadağı volcanic rocks showsthat, in most of the intermediate volcanic rocksamples, it is very significant for both Rb/Zr andK/Rb (Figure 10). Therefore, the role of significantcrustal assimilation in the genesis of the intermediateYamadağı volcanics is unlikely, but cannot becompletely ruled out.

In theory, fractional crystallization of magnesianminerals plus plagioclase leads to production thatfalls within narrow coronal bands characterized by

T. EKİCİ ET AL.

521

Figure 7. Selected major and trace element variations against SiO2 content: (a) major element; (b) trace element.

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T. EKİCİ ET AL.

523

Figure 8. Primitive mantle normalized basaltic andesite, andesite and dacitepatterns for the Yamadağı volcanics (normalized values from Sun& McDonough 1989).

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large increases in K2O compared with smallerincreases in K2O/MgO and decreases in MgO. Incontrast, contamination of magmas with crustalmelts produces trajectories with low slopes on suchplots. Figure 11 illustrates a plot of K2O-K2O/MgOfor the Yamadağı volcanic, and shows these calc-alkaline rocks defining a low angle trajectory, whichimplies that the compositions of all the volcanicswere affected by crustal interaction and so none canbe assumed to be direct uncontaminateddifferentiates of primary mantle-derived magmas.

Magma MixingPetrographic data provide evidence for magmamixing in the Yamadağı volcanics. All rocks contain

disequilibrium mineral textures such as sieve-textured plagioclases, resorption of theferromagnesian phases such as olivine, pyroxene andhornblende. Fine-grained resorption zones inplagioclase are probably caused by superheating, asdescribed by Tsuchiyama (1985). The clearovergrowth rims on the sieved cores demonstratethat the reaction took place before crystallization ofthe inclusion groundmass began. Phenocrysts whichare reacted and resorbed in the Yamadağı volcanicsformed when their host magma interacted with amore basic one.

The Yamadağı volcanics contain bothclinopyroxene and orthopyroxene. In some andesitesamples, clinopyroxene is surrounded by a thinorthopyroxene rim (Figure 4D), possibly showingthat both pyroxene types originated from differentend members.

Hornblende phenocrysts within the Yamadağıvolcanics have quite a different origin. Sincehornblende in the dacitic member of the Yamadağıvolcanics has a light green to green pleochroism, and


524

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Figure 10. (a) Rb/Zr and (b) K/Rb variations against Rb for theYamadağı volcanics.

that in the basaltic andesite and andesite hasyellowish-brown to reddish-brown pleochroism,hornblende phenocrysts within the Yamadağıvolcanics probably had at least two different origins.

One is a basaltic andesitic origin, as indicated by thepresence of hornblende in the basaltic andesites andandesites whereas the other is a dacitic origindemonstrated by the presence of hornblende in thedacitic rocks. Yellowish-brown to reddish-brownrounded hornblendes mantled by resorption zonespossibly indicate a xenocrystic origin within theYamadağı volcanics.

Figure 12 tests the validity of mixing origin of theandesitic and dacitic rocks of the Yamadağı volcanicsusing a ratio/ratio diagram based on trace elementdata. In Figure 12, andesitic and dacitic rocks, exceptfor three andesitic samples, fall on or near thehyperbolic mixing curve between basic magmaderived from the mantle (basaltic andesite) andacidic magma derived from continental crust (datafrom Yılmaz et al. 1998).

Source CharacteristicsDepletions of HFSE and enrichments of LILErelative to neighbouring elements in diagrams suchas Figure 8 are widely considered diagnostic of

T. EKİCİ ET AL.

525

Figure 11. CaO/Al2O3 and CaO/P2O5 diagrams for Yamadağıvolcanics.

Figure 13. (a) Ba/Nb-SiO2 and (b) Th/Y-Nb/Y diagrams for theYamadağı volcanics.

Figure 12. Trace element ratio/ratio diagram testing the validityof the mixing origin of the andesitic magmas of theYamadağı volcanics.

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magmas generated from subduction processes (e.g.,Thirwall et al. 1994 and references therein).However, magmas contaminated by continentalcrust also have depletions of HFSE and enrichmentsof LILE as continental crust is depleted in HFSErelative to LILE (Weaver & Tarney 1984; Taylor &McLennan 1985; Wilson 1989; Winter 2001). Toaddress the problem whether the elevatedLILE/HFSE ratios in the Yamadağı calc-alkalinevolcanics reflect that of the source or crustalcontamination, or both, Figure 13a plots Ba/Nbratios of the Yamadağı volcanics against SiO2. For thesuite as a whole, Ba/Nb ratios increase as a functionof differentiation. We consider these relationships toindicate that the Yamadağı volcanics haveassimilated crustal material.

A Th/Y − Nb/Y plot (Figure 13b) provides someuseful constraints concerning the different sourcecomponents which may be involved in thepetrogenesis of the magmas (Wilson et al. 1997).Samples from the Yamadağı volcanics define acoherent trend, with a Th/Nb ratio close to 0.1,which may be attributed to the combined effects ofcrustal assimilation and fractional crystallisationi.e.,.AFC. The displacement of this data array tohigher Th/Y ratios than those of the oceanic basaltarray (MORB and OIB) is strongly indicative of themetasomatism of the mantle source by subductionzone fluids carrying the trace element signature of acrustal component.

The subduction-related geochemicalcharacteristics are therefore probably inherited frommantle lithosphere modified by slab fluids releasedduring northward subduction of the Afro-Arabianplate beneath the Eurasian plate during Eocene toMiocene times (e.g., Pearce et al. 1990).

Conclusions1. The Yamadagı volcanic rocks range from

basaltic andesite to dacite and show a typicalcalc-alkaline differentiation trend.

2. Major- and trace-element variations indicatefractional crystallization.

3. Tectonic discrimination diagrams indicatethat the mafic samples of the serie fall in to thecalc-alkaline basalt field and intermediate-acidic members have a syn-collisionalcharacter.

4. HFSE depletions and LILE enrichments onthe primitive mantle normalized traceelement patterns imply that the magmas werederived from a mantle domain enriched byearlier subduction processes or assimilation ofcontinental crust.

5. Disequilibrium mineral textures within theYamadağı volcanics and incompatible elementratio plots imply that magma mixing is animportant process on their evolution.

6. The variable characteristics of this collision-zone magmatism seem to have developed as aresult of the superimposition of geotectonicsettings, such as continent-continent collision,with a four-stage process (Harris et al. 1986).Therefore, the genesis of the Yamadağıvolcanics can be attributed to complexpetrogenetic processes, including partialmelting of a metasomatized mantle, crystalfractionation, magma mixing, andassimilation of crustal materials along withfractional crystallization.


526

ALLEGRE, C.J. & MINSTER, J.F. 1978. Quantitative models of traceelement behaviour in magnetic processes. Earth and PlanetaryScience Letters 38, 1−25.

ALPASLAN, M. 1987. Mineralogical and Petrographical Features of theNeogene Volcanics around Arguvan (NW Malatya). MScThesis, Cumhuriyet University [unpublished].

ALPASLAN, M. & TERZİOĞLU, N. 1996. Comparative geochemicalfeatures of the Upper Miocene and Pliocene volcanics aroundArguvan (NW Malatya). Geological Bulletin of Turkey 39,75−86 [in Turkish with English abstract].

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