Composite volcanoes in the south-eastern part of İzmir–Balıkesir Transfer Zone, Western Anatolia ...

14 Pages·2015·6.57 MB·English

by  Ioan Seghedi

JournalofVolcanologyandGeothermalResearch291(2015)72–85
ContentslistsavailableatScienceDirect
Journal of Volcanology and Geothermal Research
journal homepage: www.elsevier.com/locate/jvolgeores
Composite volcanoes in the south-eastern part of İzmir–Balıkesir
Transfer Zone, Western Anatolia, Turkey
IoanSeghedia,CahitHelvacıb,ZoltanPécskayc
aInstituteofGeodynamics,RomanianAcademy,Jean-LouisCalderon19-21,Bucharest020032,Romania
bDokuzEylülÜniversitesi,MühendislikFakültesi,JeolojiMühendisliğiBölümü,TR-35160İzmir,Turkey
cInstituteofNuclearResearch,HungarianAcademyofSciences,P.O.Box51,Bemter18/c,H-4001,Debrecen,Hungary
a r t i c l e i n f o a b s t r a c t
Articlehistory: DuringtheEarly–MiddleMiocene(WesternAnatolia)severalvolcanicfieldsoccuralongaNE–SW-trending
Received9September2014 shearzone,knownasİzmir–BalıkesirTransferZone.Thisisadeformedcrustal-scalesinistralstrike-slip
Accepted28December2014 faultzonecrossingtheBornovaflyschandextendingalongtheNW-boundaryoftheMenderesMassifby
Availableonline9January2015
accommodatingthedifferentialdeformationbetweentheCycladicandMenderescorecomplexeswithinthe
Aegeanextensionalsystem.HerewediscussthevolcanicactivityinYamanlarandYuntdağıfieldsthatisclosely
Keywords:
relatedtotheextensionaltectonicsoftheİzmir–BalıkesirTransferZoneandinthesametimewiththeepisodic
WesternAnatolia
İzmir–BalıkesirTransferZone corecomplexdenudationoftheMenderesMassif.
Volcanology Thisstudydocumentstwocompositevolcanoes(YamanlarandYuntdağı),whosepresentventareaisstrongly
K/Arage erodedandcutbyavarietyofstrike-slipandnormalfaultsystems,thetranscurrentNW–SEbeingthedominant
Geochemistry one.Theerosionalremnantsoftheventareas,resemblingashallowcraterintrusivecomplex,illustratethe
Miocenevolcanism presenceofnumerousdykesorvariablysizedneck-likeintrusionsandlavaflows,typicallyassociatedwith
hydrothermalalterationprocesses(propyliticandargillic).Suchventareaswereobservedinboththeexamined
volcanicfields,having~6kmindiameterandbeingmuchmoreerodedtowardthesouth,alongtheNW–SEfault
system.Lavaflowsandlavadomesaresometimesassociatedwithproximalblockandashflowdeposits.Inthe
cone-buildingassociationpart,besideslavaflowsandremnantsoflavadomes,rareblockandashandpumice-
richpyroclasticflowdeposits,aswellasaseriesofdebris-flowdeposits,havebeenobserved.
The rocks display a porphyritic texture and contain various proportions of plagioclase, clinopyroxene,
orthopyroxene,amphibole,rarebiotiteandcorrodedquartz.Theexaminedrocksfallatthelimitbetween
calc-alkalinetoalkalinefield,andplotpredominantlyinhigh-Kandesiteanddacitefieldsandoneisrhyolite.
Thetraceelementdistributionsuggestsfractionalcrystallizationprocessesandmixinginuppercrustalmagma
chambersandsuggestsametasomatizedlithosphericmantle/lowercrustsource.Thispreliminaryvolcanological–
petrologicalandgeochronologicalbasestudyalloweddocumentingtheYamanlarandYuntdağıascomposite
volcanoesgeneratedduringpost-collisionalEarly–MiddleMiocenetranstensionaltectonicmovements.
©2015ElsevierB.V.Allrightsreserved.
1.Introduction Western Anatolian Volcanic Province from the Eocene to Miocene
(e.g.,Helvacıetal.,2009;ErsoyandPalmer,2013;Göktaş,2014).The
Western Anatolia is characterized by two main continental exhumationoftheMenderesMassifalongthecrustal-scalelow-angle
assemblagesknownastheSakaryamicro-platetothenorthandthe detachmentfaultswasaccompaniedbyNE–SW-trendinghigh-angle
Anatolide–Taurideblocktothesouth.Theseterraneswereamalgamat- strike-slipfaultsalongitswesternandeasternmargins(Erkületal.,
edduringLateCretaceous–Paleocenecontinentalcollisionalongthe 2005a;Ersoyetal.,2011,2012a;Erkül,2012;ErkülandErkül,2012;
northernbranchoftheNeo-Tethys,whichismarkedbytheVardar– KaraoğluandHelvacı,2012b).TheNE–SW-trendingzoneofdeforma-
İzmir–AnkaraSutureZone(seeinsetFig.1).TherocksoftheVardar– tionalongthewesternmarginoftheMenderesCoreComplexistermed
İzmir–AnkaraSutureZoneinwesternAnatoliaarerepresentedbythe theİzmir–BalıkesirTransferzone(İBTZ)(Erkületal.,2005a;Uzeland
ophiolitic mélange units of the Bornova Flysch Zone and Tavşanlı Sözbilir,2008;Ersoyetal.,2011,2012b;Karaoğlu,2014).TheİBTZ
Zone. Widespread orogenic magmatic activity developed in the representstheNEextensionintowestTurkeyofaNE–SW-trending
crustal-scaleshearzone(themid-Cycladiclineament),alongwhich
MiocenegranitoidswereemplacedintheCyclades(Pe-Piperetal.,
E-mailaddress:[email protected](I.Seghedi). 2002).TheNE–SW-trendinghigh-anglestrike-slipfaultsarerecorded
http://dx.doi.org/10.1016/j.jvolgeores.2014.12.019
0377-0273/©2015ElsevierB.V.Allrightsreserved.
I.Seghedietal./JournalofVolcanologyandGeothermalResearch291(2015)72–85 73
Fig.1.Geologicalmapofthestudyareaandtwosimplifiedprofiles,eachcrossingthecraterareasofYamanlarandYuntdağıvolcanoes(modifiedfromMTAGeologicalmaps,
1/100000Scale).Theinsetisshowingthestudyareasituatedatthesouth-westernpartofİzmir–BalıkesirtransferzoneandtheedgewithMenderesCoreComplex.
inseveralNeogenebasins,suchasalongtheeasternmarginsofthe wediscussforthefirsttimetwocompositevolcanoesgeneratedin
NE–SW-trending Kocaçay (Sözbilir et al., 2011), Kocaçay and a post-collisional setting, associated with İBTZ area. Crater-shaped
Cumaovasıbasins(Kayseri-Özeretal.,2014)andGördesbasins(Ersoy “calc-alkalinecenters”havebeenintuitivelyrecognizedinprevious
etal.,2011).(Fig.2) publications,andhavebeendrawnongeologicalmaps(e.g.Agostini
Compositevolcanoesareessentiallyconfinedtoasingleventora etal.,2010;Karaoğlu,2014),althoughtheymissasuitablevolcanolog-
group of closely spaced central vents showing a simple conical or icaldescription.TheİBTZzonewasactiveduringEarly–LateMiocene
shield-likeoverallsymmetryandhavingmorethanoneevolutionary whenitgeneratedaseriesofNE–SWelongatedfluvio-lacustrinebasins
stage in their existence (e.g. Francis, 1993). Such volcanoes have (Seyitoğlu et al., 1997; Aydar, 1998; Yılmaz et al., 2000; Uzel and
beendominantlyfoundassociatedwitharcvolcanismandsubduction Sözbilir,2008;Ersoyetal.,2012a,b;KaraoğluandHelvacı,2012a,b;
processes,orpost-collisionalprocesses(e.g.Lexaetal.,2010).Here, Uzeletal.,2013)associatedwithcontemporaneousvolcanicactivity.
74 I.Seghedietal./JournalofVolcanologyandGeothermalResearch291(2015)72–85
Fig.2.GeologicalmapofthewesternAnatoliashowingthedistributionoftheNeogenebasins,radiometricagesofthevolcanicintercalationsintheNeogenesedimentsandmajor
structuresandourstudyarea(modifiedfrom1/500000scaledgeologicalmapofTurkey(MTA);Abbreviationsforgranitoids:EyG:Eybek,KzG:Kozak,AG:Alaçamdağ,Kog,Koyunoba,
EG:Eğrigöz,BG:Baklan,TG:TurgutluandSG:Salihligranitoids.Abbreviationsforbasins:KB:KocaçayBasin,BB:BigadiçBasin;CuB:CumaovasıBasin;SoB:SomaBasin,GB:Gördes
Basin,DB:DemirciBasin,SeB:SelendiBasin,UGB:Uşak–GüreBasin,EB:EmetBasin.Abbreviationsfordetachments:GDF:Gediz(Alaşehir)DetachmentFault,SDF:SimavDetachment
Fault,BMDF:BüyükMenderesDetachmentFault(afterErsoyetal.,2014withmodifications).
The volcanic products consist of high-K calc-alkaline and minor formation,consistingofconglomeratesatthebasegradingupwards
K-alkalineandultrapotassicrocks,suggestedtoderivefromthelitho- into sandstone-shale alternations and the Sabuncubeli formation,
sphericmantleand/ormixingwithlowercrustalmelts(Borsietal., consistingofmudstonesandthenlimestones.Thesedimentaryrocks
1972;AkayandErdoğan,2004;Erkületal.,2005a,b;Innocentietal., passintovolcanicdeposits.LateMiocenesedimentsarealsoknown
2005;Agostinietal.,2010;Ersoyetal.,2010,2012a,b;Chakrabarti along İBTZ area, sometimes associated with basaltic volcanic rocks
etal.,2012). (e.g.Ersoyetal.,2014).
Thispaperisbasedonvolcanologicalfieldobservationsperformed
onseveralprofilescrossingtwovolcanicareas,onK–Aragedetermina- 3.Methods
tionsandgeochemicalanalysesofrepresentativesamples.Herewe
discusstheeruptivehistoryoftheYamanlarandYuntdağıcomposite 3.1.Geochemistry
volcanoes, attempting to evaluate their generation in the post-
collisionalextensionalgeodynamiccontext. Rockpowdersoftheselectedfreshrocksampleswerepreparedby
removingthealteredsurfacesandpowderedinashatterboxatDokuz
2.Geologicsetting EylülUniversity.Thegeochemicaldatafor17sampleswereperformed
byACMEAnalyticalLaboratoriesLtd.inVancouver(Table1).Element
The pre-Miocene basement is characterized by the Bornova abundancesweredeterminedbyICP–AES(majorelements)andICP–
FlyschZone(BFZ),(Erdoğan,1990;Okayetal.,2012),knownasan MS(traceelements),followingalithiummetaborate–tetraboratefusion
olistostrome–mélangebelt,lyingbetweentheMenderesMetamor- anddilutenitricaciddigestionofa0.1gsample.WeightLossonignition
phicCoreComplexandSakaryazone,aspartoftheİzmir–Ankara (LOI)wasdeterminedbyweightdifferenceafterignitionat1000°C.The
Tethyansuture(Figs.1,2).Itiscomposedoftectonizedgravitymass precisionformajorelementswaslessthan1%.Theprecisionfortrace
flows, locally metamorphosed, of Late Cretaceous–Paleocene age, elementswasintheorderof10%relative.
with blocks of Mesozoic limestones, serpentinites and submarine
ophiolitic mafic volcanic rocks. The formation of the BFZ coincides 3.2.K–Ardatingmethodology
with the Cretaceous subduction and HP/LT metamorphism of the
northernpassivecontinentalmarginoftheAnatolide–TaurideBlock TheK–ArmeasurementsobtainedattheK–Arlaboratoryofthe
(Okayetal.,2012). InstituteforNuclearResearch,DebrecenHungaryusedthefollowing
The area was active during the Early Miocene extension, when methodology:Afterthefifteenrocksampleswereopticallyexamined,
various deposits unconformably cover the BFZ. The Early–Middle about1kgofeachsamplewasbrokenintosmallpiecesfreeofweather-
Miocenedepositsofthestudiedareaconsistoftwosedimentaryunits ing, xenoliths and joints. These whole-rock pieces were retained,
overlain by the Yamanlar volcanics (acc. to Uzel et al., 2013), or crushedandsievedto150–300μmsizefractionusingcoppersieves.
Yuntdağıvolcanicunit(acc.toÖzkaymaketal.,2013),i.e.theKızıldere Thefinepowderwaselutriatedwithdistilledwaterandovendriedat
I.Seghedietal./JournalofVolcanologyandGeothermalResearch291(2015)72–85 75
Table1
Whole-rockmajorandtraceelementanalysesofrepresentativesamplesfromYamanlarandYuntdağıvolcanoes.
Sample 52 52A 53 54 55 55A 56 57 59 60 61 62 65 67 68 69 70
Volcanic Lava Dyke Dyke Dyke Lava Lava Lava Lava Lava Lava Dyke Intrusion Lava Lava Lava Lava Lava
form
SiO2 55.46 59.75 59.84 61.40 57.46 59.18 59.04 59.69 56.76 57.67 59.17 57.52 59.78 61.64 60.04 68.66 54.77
Al2O3 15.50 16.44 15.23 15.56 14.96 16.15 16.88 14.90 17.40 16.73 15.54 16.17 16.82 16.42 16.64 15.04 17.32
Fe2O3 6.11 5.51 5.32 4.79 5.46 5.32 6.15 6.01 7.40 6.39 5.62 5.91 5.33 5.03 5.50 3.14 5.86
MgO 2.81 2.74 2.77 2.10 2.98 2.19 2.94 3.17 2.73 2.95 1.99 2.81 2.90 2.46 3.13 0.26 2.52
CaO 6.51 4.52 4.38 3.76 6.97 4.75 6.04 5.38 6.82 6.06 4.05 6.11 5.13 4.99 5.43 1.60 7.52
Na2O 2.78 3.54 3.03 2.75 2.50 3.18 3.13 2.89 3.12 2.93 2.56 2.54 3.33 3.31 3.42 3.47 2.99
K2O 3.70 2.44 3.92 3.44 3.21 2.41 2.91 4.00 2.57 3.44 3.29 3.40 3.07 3.20 3.16 5.31 3.49
TiO2 0.77 0.60 0.64 0.53 0.64 0.58 0.75 0.74 0.81 0.84 0.55 0.79 0.68 0.62 0.69 0.38 0.99
P2O5 0.32 0.19 0.32 0.22 0.35 0.18 0.26 0.40 0.30 0.38 0.15 0.32 0.26 0.21 0.25 0.09 0.49
MnO 0.11 0.08 0.11 0.09 0.13 0.08 0.09 0.08 0.11 0.11 0.15 0.08 0.10 0.09 0.10 0.03 0.13
LOI 5.60 3.90 4.10 5.10 4.70 5.70 1.40 2.30 1.60 2.10 6.80 4.00 2.30 1.70 1.30 1.70 3.50
Total 99.64 99.70 99.64 99.71 99.36 99.72 99.65 99.60 99.66 99.62 99.81 99.61 99.69 99.73 99.68 99.68 99.54
Cs 6.0 1.3 2.6 5.1 4.0 1.9 3.3 5.0 3.1 6.3 19.4 2.1 4.2 6.5 5.9 15.9 5.9
Rb 163.1 62.4 141.7 127.1 110.7 69.5 104.8 155.3 92.3 149 139.1 118.7 117.2 124.5 120.8 250.7 134.4
Ba 1266 1044 1325 1073 1364 959 1063 1307 1174 1301 629 1,336 930 921 960 1234 1695
Sr 664.5 610.4 526.5 427.9 750.5 688.5 658.8 689.3 677.8 717.3 244.7 777.6 611.1 522.4 601.6 420 918
Pb 2.3 14.5 13.7 10 2.9 12.1 3.3 2.8 4.5 3.6 7.4 4.9 4.2 2.4 3.4 6.8 3.7
Th 29.80 15.20 17.30 16.60 17.70 15.10 15.30 23.80 13.90 23.40 13.60 31.20 23.50 21.40 21.20 49.20 19.70
U 6.8 4.2 4.8 4.6 5.3 3.7 4.1 6.5 3.5 5.7 4.1 7.6 6.6 5.8 5.9 10.4 4.9
Zr 222.7 138.4 177.8 172.3 179.9 146.3 189.0 261.6 165.9 211.2 153.7 241.0 163.8 171.3 175.5 451.5 215.3
Hf 5.8 3.6 6.1 4.8 5.6 4.5 5.0 7.3 5.1 5.8 4.9 7.4 4.4 5.2 4.3 12.7 6.2
Ta 0.8 0.7 0.9 0.7 0.8 0.8 0.7 1.2 0.6 0.7 0.6 1.0 0.7 0.7 1.0 1.5 0.7
Y 20.5 23.9 24.3 22.6 21.5 22.9 26.9 27.4 24.7 23.2 21.6 22.0 19.4 19.2 24.0 21.5 22.1
Nb 14.4 10.2 13.7 11.1 11.6 9.9 10.6 15.6 9.0 12.5 9.7 14.9 11.7 11.6 13.4 25.0 13.5
Sc 15 13 14 11 17 12 18 17 19 18 12 15 13 12 13 6 18
Ni 13.8 6.2 67.9 21.3 49.2 4.3 8.9 168.8 7.3 9.7 4.9 13.7 10.9 12.0 28.0 1.4 19.2
Co 19.1 18.8 52.5 33.1 48.1 21.4 44.9 52.9 30.4 30.5 11.7 27.4 35.3 23.9 30.3 28.2 43.5
V 139 90 95 85 131 79 157 125 176 160 94 136 107 95 118 10 181
W 23.7 50.9 304.6 195.3 248.7 73.7 218.7 235.6 78.3 88.2 42.6 99.5 127.1 87.3 107.9 217.8 191.2
Ga 17.6 17.6 16.7 16.9 17.1 17.4 18.6 16.9 18.9 18.8 17.8 17.9 17.7 16.8 17.9 16.3 19.8
Zn 46 60 55 53 45 50 40 44 51 44 49 59 34 46 41 46 56
Cu 31.5 14.9 18.0 12.8 29.5 10.6 22.2 30.2 24.3 38.4 10.3 28.5 20.7 19.9 31.3 5.8 31.1
La 48.1 36.8 37.9 36.9 40.2 39.1 40.8 51.7 37.5 43.4 32.4 51.0 52.2 40.7 46.7 83.6 47.8
Ce 91.5 70.8 70.8 67.0 78.0 73.2 75.5 92.2 71.0 84.2 61.9 93.6 92.9 75.5 82.1 127.8 92.8
Pr 9.53 7.51 7.99 7.31 8.55 7.79 8.69 11.62 7.95 9.32 6.51 9.90 9.58 7.69 8.63 14.17 10.08
Nd 37.1 28.8 28.4 27.5 29.4 30.0 35.5 44.9 29.6 35.6 23.3 34.8 33.5 25.3 29.1 47.9 36.7
Sm 6.22 5.25 5.76 5.38 5.72 5.65 6.16 7.83 6.07 6.19 4.48 5.90 5.46 4.77 5.21 7.46 6.48
Eu 1.36 1.14 1.23 1.17 1.23 1.19 1.42 1.54 1.43 1.42 0.97 1.33 1.34 1.10 1.28 1.47 1.53
Gd 4.86 4.43 4.89 4.29 4.59 4.52 5.13 5.95 5.18 5.15 4.10 4.73 4.44 4.07 4.71 5.28 5.32
Tb 0.71 0.74 0.76 0.69 0.71 0.70 0.83 0.93 0.80 0.79 0.64 0.73 0.66 0.61 0.69 0.77 0.81
Dy 3.82 3.74 3.57 3.8 3.79 3.57 4.4 4.43 4.46 4.45 3.67 4.22 3.29 3.11 3.66 3.95 3.78
Ho 0.77 0.85 0.89 0.79 0.83 0.85 0.95 1.00 0.90 0.80 0.72 0.74 0.73 0.63 0.80 0.80 0.80
Er 2.04 2.35 2.33 2.28 2.04 2.27 2.58 2.89 2.67 2.26 2.13 2.16 2.24 1.95 2.33 2.46 2.37
Tm 0.36 0.37 0.42 0.37 0.37 0.40 0.43 0.45 0.40 0.35 0.35 0.35 0.34 0.30 0.33 0.41 0.39
Yb 2.13 2.61 2.58 2.38 2.27 2.41 2.64 2.65 2.48 2.33 2.39 2.26 2.02 2.21 2.15 2.59 2.11
Lu 0.36 0.35 0.41 0.35 0.37 0.38 0.40 0.40 0.40 0.38 0.34 0.33 0.32 0.30 0.37 0.40 0.30
110°Cfor24h.Oneportionoftheready-madewhole-rocksamplewas DetailsoftheanalyticalmethodshavebeenreportedbyBalogh
groundedinagatemortarforpotassiumanalysescarriedoutwithflame (1985),Pécskayetal.(2006)andOdinetal.(1982).K–Arageswere
photometry. calculated using decay constants suggested by Steiger and Jäger
Table2
K/ArdataofrepresentativesamplesfromYamanlarandYuntdağıvolcanoes.
Nr.crt. No.ofK/Ar Samplename Rocktype K 40Arrad 40Arrad K/Arage Volcanicarea
(%) (ccSTP/g)×10−6 (%) (Ma)
1 8207 052 Andesitelava 2.404 1.5561 19.9 16.57±1.16 Yamanlar
2 8208 052A Andesitedyke 1.772 1.1738 54.8 16.96±0.58 Yamanlar
3 8209 053 Andesitedyke 3.184 1.8573 42.9 14.94±0.58 Yamanlar
4 8210 054 Dacitedyke 2.463 1.6816 32.9 17.48±0.80 Yamanlar
5 8211 056 Andesitedomelava 2.039 1.2509 11.2 15.71±1.94 Yamanlar
6 8212 057 Andesitedomelava 2.325 1.5972 83.0 17.58±0.54 Yamanlar
7 8213 059 Andesitelava 2.057 1.2886 74.7 16.04±0.50 Yamanlar
8 8214 060 Andesitelava 2.959 1.9958 46.9 17.27±0.64 Yuntdağı
9 8215 061 Daciteintrusion 3.015 1.8687 35.5 15.87±0.69 Yuntdağı
10 8216 062 Andesiteintrusion 2.545 1.4921 11.6 15.02±1.75 Yuntdağı
11 8217 065 Andesitelava 2.52 1.5533 46.0 15.79±0.59 Yuntdağı
12 8218 067 Andesitelava 2.67 1.6579 42.6 15.90±0.62 Yuntdağı
13 8219 068 Andesitelava 2.24 1.5065 43.0 17.22±0.67 Yuntdağı
14 8220 069 Rhyolitelava 4.13 2.8270 44.2 17.52±0.67 Yuntdağı
15 8221 070 Andesiteclastinpyroclasticflow 2.78 1.8078 46.6 16.65±0.62 Yuntdağı
76 I.Seghedietal./JournalofVolcanologyandGeothermalResearch291(2015)72–85
(1977).Theinter-laboratorystandardsAsia1/65,LP-6,HD-B1andGL-O, associationisdominatedbylavaflowsorlavadomeswheremostof
aswellasatmosphericArwereusedtocontrolthemeasurements.All them are eroded. Toward the periphery, insignificant pyroclastic
analyticalprecisionisreportedatthe1σlevel. depositshavebeenfoundinYuntdağı,duetoerosionandburialin
younger Miocene sedimentary deposits (Fig. 1). The most suitable
4.Volcanologicalobservations termforsuchkindofcomplexstructuresdominatedbylavaflowsare
compositevolcanoes(e.g.DavidsonanddeSilva,2000).Toillustrate
4.1.Structureofvolcanicedifices thedistributionofdepositsrelativetoventfordiscussedcomposite
volcanoweusedtheterminologyofDavidsonanddeSilva(2000).
Thehighlyerodedpresentedificeheightisupto~800m.Inboth
cases(YamanlarandYuntdağı)itcanberemarkedanerodedquasi- 4.2.Mainventarea
circularcrater-likerimandamuchdeeplyerodedcentralventarea
thathas6–7kmindiameter(Fig.1).Thecentralventareasareoccupied Thecentralventareas,inbothcaseshaveasimilarcircularshape
by a shallow subvolcanic intrusion complex. The cone-building ~3 km in diameter, suggesting former craters characterizedby the
Fig.3.a:14.9MaerodeddykeinYuntdağıintrusivefeedingcomplex;b.15.02Mairregularintrusivebody(neck?)showingcomplexjointinginYamanlarshallowsubvolcanicintrusions;
c.flowbandinginYamanlarlavaswithinclusionsofcognateinclusions;d.flowbandinginlavas—Yuntdağıvolcano;e.dominantlyfall-outorigindepositsintercalatedinthelavadome
flowsinYuntdağıvolcano(seedescriptioninthetext);f.pyroclasticblockandashflowdeposits(seedescriptioninthetext)inYuntdağıvolcano;g.Pyroclasticflowdeposit(ignimbrite)in
Yuntdağıvolcano(seedescriptioninthetext);h.sequenceoffluvialconglomerates(showingcross-beddingatthebase)intopoflaharic(debrisflow)deposits,characterizedby
heterolithic,varioussize,dominantlyroundedandsubroundedlithoclast,withishpumices,inaprevailingmuddy–ashymatrix.Thesequencerepresentspartofring-plainassociation
aroundYuntdağıvolcano.
I.Seghedietal./JournalofVolcanologyandGeothermalResearch291(2015)72–85 77
presenceofnumerousdykes(Figs.1,3a,b)orvarious-sizedandirregu- 4.4.2.Fall-outdeposits
larlyshaped(neck-like)intrusionsandlavas.Thepervasivehydrother- Avolcaniclasticbeddedsequenceofwhitishcolorand~1.5–2m
malalterationofthesubvolcanicintrusioncomplexischaracteristic; thickness is intercalated between dome lava flows at point 066
propyliticalterationdominatesoverargillicaspreviouslydescribedin (Fig.3e).Atitsbaseitshowsa0.7msequenceofcm-sizebedsconsisting
Yamanlar,withquartzveinsassociatedwithgold(Dora,1964;Sayılı of various size, well sorted, ash to lapilli and angular-subangular
andGonca,1999). massiveandesitelithoclasts.Intheupperpartthereisadiscordant
ashy-dominated layer, 0.8–1.4 m thick, showing a mixture of ashy
materialandrarelapilli-sizelithoclasts.
4.3.Proximalconeassociation
Initslowerpartthissequenceischaracterizedbyseveralpulsesofa
Lavaflowsarethemostcommonattheedgesupto1–2kmdistance fall-outoriginthatallowedthesubaerialsortingprocesses.Itresulted
duringthedome-formingprocesses.Theupperdiscordantashymassive
towardtheexteriorforbothvolcanoesandarecharacterizedbyatypi-
cal flow banding underlined sometimes by the structural variation layercouldbeareworkedmaterialofthesimilarfall-outoriginasthe
lowersequence.
givenbydifferenceinvesicularity.Theyareeasilyassignedtothevent
areaduetothedivergentdiptrendofflowbanding.Lavadomesare
alsoobservedatthepresent-dayedificeedges,thoughitisdifficultto 4.4.3.Ignimbrites
Atpoint064outcropsa~3mthickdepositshowinglargevolumesof
imaginetheextentoftheirdistributionbecauseofthestrongerosional
processes(Fig.4).Dome-shapemorphologycanbeviewedonaprofile ashymatrix,reddishincolor,witharatherhomogeneousdistributionof
crossingthemarginoftheYuntdağıcrater(Fig.1).Theyaretheresult irregularpumiceclaststhathavevarioussize(~5–20cm),suggesting
of a more viscous flow and are characterized by a more massive absenceofgrading.Atthetop,thelastseveralcmshowsmallersize
pumices(~1–2cm)(Fig.3g).Itiscovereddiscordantlybyablocky
appearance,sometimesshowinglargesub-verticalpolygonaljointing.
lavaflowandoverlaysadifferentlavasequence.
Polygonalscatteredblocksareoccasionallyexposedatthetopofthe
edifices,suggestingformerdomestructure,presentlyeroded. Thedepositislikelytorepresentapyroclasticflowunit(ignimbrite)
mostprobablyemplacedinbetweenlava-anddomegeneratingevents.
Theupperthinzonewithsmallersizepumicessuggeststobereworked,
4.4.Cone-buildingassociation
asoftenthesituationinnon-weldedignimbrites.
Itisplaced2–10kmfromtheventanditisdominatedbyfluidallava
4.5.Ring-plainassociation
flowsandrarelavadomes,whichdivergeallaroundthecone,some-
timesassociatedwithvolcaniclasticdepositsthathavebeenobserved
Thisareaisnotwelldevelopedsinceitispresentlyburiedunderthe
onlyinYuntdağıvolcano.Severalvolcaniclasticlithologieshavebeen
surroundingmuchyoungeralluvialfans(Fig.1).Itlocallysurroundsthe
detected: volcanoesandmaynotbeconsideredpartoftheconstructionaledifice
itself.SuchdepositswereobservedatthesouthernmarginofYuntdağı
4.4.1.Blockandashflowdeposits (point063onFig.1).Herethereisasequenceofinterbeddedfluvial
Asequenceofmonolithicbrecciawithash-sizematrixcontaining conglomeratesandlahar(debrisflow)deposits(Fig.3h),whichform
whitishlapilli-sizepumices(1–2cmindiameter),scoriawithuneven thedistaledgesofwedge-likefansofdebrisatthelowerconeslopes.
roundedmargins(2–3cmindiameter)andvariable,highcontentsof
angular lithoclasts, consisting of monolithic andesite from 1 cm to 5.Petrographyandgeochemistry
1.5msize(Figs.1,3f).Thesequenceconsistsofseveralunitsofmassive
faintlybeddeddepositsofvariablethickness(0.5–2m),coveringthe 5.1.Petrography
eastern slopes of the Yuntdağı volcano and is intercalated in lava
flows.Thevariousbedsaredominatedbyangularmonolithiclithoclasts 5.1.1.Yamanlarvolcano
and are unsorted (Fig. 3f), main differences between the units Theanalyzedsamplesareporphyritic,showingvariablephenocryst
consistinginthesizeandvolumeoftheangularlithoclasts. load. However, there are some petrographic differences between
Thefeaturesaretypicalforasequenceofpyroclasticblockandash volcanoessetbytheirmainphenocrystphases.Plagioclase(andesine–
flowdepositsthatsucceededatshorttimeintervals. bytownite,opticallyestimated),amphiboleandbiotite(13.5–18%,as
Fig.4.a.PanoramaofYuntdağıerodedcraterviewingfromtoptowardtheSWmargins;notetheerosionaldome-shapeaspectofanintrusivebodyinsidethecrater(photocourtesyby
ErginUmutSayıl);b.panoramaofYamanlarerodedcraterviewedfromtop-middleedgetowardtheSW.Notetheremnantblocksofformerdomestructureintheforegroundandrounded
shapeofpresenterodedmarginofthecrater.
78 I.Seghedietal./JournalofVolcanologyandGeothermalResearch291(2015)72–85
Fig.5.Microscopeviewofselectedsamples(detaileddescriptioninAppendix1).Location:Yamanlarvolcano:a.sample053,andesitedyke,notetheroundedmarginsofplagioclaseand
irregularshapeofcognateinclusions;b.sample054,dacitedyke,notethepresenceofroundedmarginsofplagioclaseandopacitearoundbiotite;c.sample059,andesitelava,notethe
presenceofroundedmarginsofplagioclase,opacitearoundbiotiteandirregularshapeofcognateinclusions;Yuntdağıvolcano:d.sample060,andesitelava,notetheslightrounded
marginsofplagioclase,opacitearoundbiotiteandamphiboleandirregularshapeofcognateinclusion;e.sample065,andesitelava,notethesievedandmarginallycorrodedplagioclase
andresorbedandmarginallyopaciteamphibole;f.sample069,rhyolitelava,notethelargecrystalofK-feldspar,resorbedamphiboleandfreshbiotite.Abbreviations:Pl—
plagioclase;K-f—potassiumfeldspar;Amph—amphibole;Bi—biotite;Cpx—clinopyroxene;Opx—orthopyroxene;Op—opaqueminerals;Ci—cognateinclusions.
modal)arethemainphenocrystofYamanlar(Appendix1).Plagioclase roundedmarginswithhostandesites,whileothersarecharacterized
is rarely sieved, but mostly presents irregular margin. Pyroxenes bysomeembayments.OtherspecificfeaturesofYuntdağıareplagio-
(clinopyroxeneandorthopyroxene)andcorrodedquartz,besidesof clasesievetexturesand/orirregularmarginsandbiotiteandamphibole
theubiquitousapatite,opaquemineralsandsphenearethemainacces- marginalortotaloxidation,andrareamphiboleovergrowthsonbiotite
soryminerals.Cognateinclusions(hypidiomorphicquasi-equigranular (sample067,Appendix1).
aggregatesofplagioclase,pyroxene,amphiboleandopaqueminerals)
showingsharpcontactsandroundedmarginswithhostandesiteand 5.2.Geochemistry
resorptionofplagioclaseandopaciterimsaroundbiotitearefrequent
physiognomies (Fig. 5, Appendix 1).Thesizeof cognate inclusions TheLOIofthemajorityoftheeffusiverocksisabout1.3–2.3wt.%.All
rangesindiameterfromlessthan0.5cmtoatleast20cm,withmedian theshallowsubvolcanicintrusiverocksandafewlavasarevariablyaf-
diameterslessthan10cm.Theyshowrounded,sphericaltolobate fectedbyhydrothermalalteration.Itwasnotpossibletoentirelyavoid
shapesandsuggesttobegeneticallyrelatedtothehostrocks. themandthesesamplesdisplayhigherLOI(3.9–6.8wt.%).Thisisthe
reasontheresultshavebeenevaluatedonananhydrousbasisinthe
5.1.2.Yuntdağıvolcano classificationandotherdiagrams.
Plagioclase(andesine–bytownite opticallyestimated),pyroxenes Onthetotalalkalisversussilicadiagram(Fig.6a)mostsamplesplot
,
(clinopyroxeneandorthopyroxene),amphiboleandbiotite(upto24%, withinthesub-alkalinefieldatthelimitwiththelatitefield.According
asmodal)arethemainphenocrystphasesinYuntdağı(Appendix1). toPeccerilloandTaylor(1976)diagram(Fig.6b)alargeK Ovariation
2
RareK-feldsparphenocrystsarepresentalongplagioclase,amphibole (2–4%)inarelativelylargerangeofSiO —56–65%canbeobserved,
2
andbiotiteinasinglerhyolitesample(069),wherethegroundmasspre- withnoevidentcorrelationbetweenKandsilica.Mostoftherocksfall
sentsparallelunevenbandsshowingdifferenttypesofglassdevitrifica- randomlyinthehigh-Kandesitefield,exceptingthreesamples:two
tion,possiblysuggestingflowbanding.Cognateinclusions(aggregateof dacites (61, 54) and a rhyolite (69). Primitive mantle-normalized
holocrystallineplagioclase,pyroxeneandamphiboleandopaquemin- incompatibleelementpatternsshownegativeanomaliesforNb–Ta,as
erals)reachdecametricsize.Someofthemshowsharpcontactsand well for P and Ti (Fig. 7a, b). There is a positive Pb anomaly that
I.Seghedietal./JournalofVolcanologyandGeothermalResearch291(2015)72–85 79
randomanalyticalerrorshouldbeinconnectionwiththesecondary
alterationprocesses,however,thehydrothermalactivitydidnotaffect
significantlytheapparentages.Thustheseageshavealsogeological
meaningandtherandomanalyticalerrorcanbeoverestimated.The
ageintervalof volcanic activity in both edifices spansfor ~2.5 Ma,
between 17.52–14.94Ma with an average of 0.82 Ma 1σ precision
rangeifweconsiderallthemeasurements,or0.62ifwetakeoutthe
analyticalerrorsgreaterthan1,suggestingsimilartimelengthforboth
ofthem(Fig.8).Thereisonlyoneknownagedeterminationinour
studyarea(S-906,yielding17.0±0.3Ma,Rb–Srage)ofErcanetal.
(1996);howeveritsexactlocationisonlyguessed(Fig.2).Ourdata
fallsintheintervalsreportedbypreviouspublisheddatainthesur-
roundingarea(Borsietal.,1972;Savaşçın,1978;Ercanetal.,1996;
Agostinietal.,2010;Ersoyetal.,2014)correspondingtoEarly–Middle
Miocene,andtheyindicatetherelativetimebetweentheinception
andvanningofthevolcanicactivityinYamanlarandYuntdağıvolcanoes.
Itistonotethatthemostevolvedrock,rhyolite(69),isalsooneof
the oldest (17.52 ± 0.67 Ma). The youngest dated rocks (14.94 ±
0.58Ma;15.02±1.75Ma)arerepresented,inbothcases,byintrusive
rocksinsidetheshallowsubvolcanicareainsidethecraters.
7.Discussion
7.1.Volcanicevolution
TheK–Aragedeterminationsalongvolcanologicalobservationsare
essentialinunderstandingthecomparativevolcanicevolutionofthe
volcanoes. The initial stages of volcanic activity were dominantly
effusiveandsubaerial.Bothvolcanoesshowawell-exposedshallow
intrusivecomplexthatinthecaseofYamanlarexposesalargerange
ofagesforthevariousintrusivebodies(17.48±0.80Ma;16.96±
0.58Ma;14.94±0.58Ma),suggestiveofacontinuousactivityconnect-
Fig.6.a.Totalalkali–silica(TAS)andb.K2O–SiO2(PeccerilloandTaylor,1976)diagrams edtotheventarea.Theintervalofvolcanicactivityissimilarforboth
forthestudiedrocksamples.ThesamplelabelsreportedintheFig.6amisstheinitialzero. volcanoes(Fig.8).Therhyolitelavaexposedat~4kmfromtheeastern
border of Yuntdağı volcano represents one of the initial eruptions
(17.52±0.67Ma).Itistonotethecontemporaneousactivityinboth
characterizesmostlytheintrusiverocks.TheChondriteCI-normalized edificesandthefactthattheyoungestdatedrocks(14.94±0.58Ma;
REEpatterns(Fig.5c,d)shownoessentialdifferencesbetweensamples, 15.02±1.75Ma)arerepresented,inbothcases,byintrusiverocks
allhavingaminorEunegativeanomaly(Eu/Eu*=0.67–0.81)and insidethecraterarea.Theyrepresenttheupperfeedingsystem,and
increasedLREE,highestfortheevolvedrhyolite(069). theirpresenceatthesurfacesuggestsintensiveerosionmostlyfacilitat-
edbythepervasivehydrothermalalterationofsomeofit.
6.Timingofthevolcanism Theerosionismainlyvisibleonthelavasanddomesnearthecrater
border.Differentialerosionofthecraters,openedandstronglyeroded
Resultsofdatingonindividualsamplesmaynotalwaysbeindicative towardtheSWresultedviablockfaultingalongaNW–SEsystemand
oftheageoftherockthatmightbeaffectedbyyoungerprocesses,such enhancederosionduetoaninitiallowerandlargerfan-likealluvial
asalteration,lossorincorporationofexcessradiogenicargon,etc.Sever- drainagesysteminsidethecraters(e.g.Schummetal.,2000;Fielitz
alwayshavebeenusedtoeliminatepossibleerrorsintheageassign- andSeghedi,2005;Tibaldietal.,2006).Ifwetakeintoaccountamissing
mentofrocks.Besideweselectedfreshsamplesourdatasetwasnot ageintervalofca.0.7±0.62Mafromtheyoungestintrusiveinthecra-
dependonsinglesampleagedetermination.Aconsistentsetofresults, terareauptothefirstdatedlavaflowsoftheproximalconeassociation
inthiscasefifteen,diminishesthepossibilityoferrorintheageassign- (15.71±1.94MainYamanlarand15.79±0.59MainYuntdağı)thatis
ment.Sincewedidnotgetsignificantcontroversialresultsitwasnot largerthanassumedmainrepose-timeof~40kabetweensupposed
necessarytodatevariousgravityandmagneticfractionsofthesamples eruptions(Fig.8),thismaybesusceptibletorepresentalreadyeroded
andconstructisochronstoeliminatetheinfluenceofradiogenicArgon effusivepartatthetopedifices.SincetheK–Aragesarguetoarather
lossorexcess.FifteenK–Aragedata(7forYamanlarand8forYuntdağı) continuousvolcanicactivity(Fig.8),itmaybesuggestedthatmagma
arepresentedhereinordertorevealthepictureofvolcanicevolution outputvolumeresultedfromaconstantnumberofactiveperiodsin
duringtheformationofcompositevolcanoes(Table2).Sampleswere eachvolcanoeshistory,separatedbyrepose-timeintervalsthatcould
collectedfromthecentralpartofthevolcanoesrepresentedbyintrusive beintheorderof~405years.
rocks,fromthelavafloworerodeddomestructureslocatedatthe
topographic edges of the former crater areas (Figs. 1, 4) and from 7.2.Petrogeneticconsiderations
cone-building lava flows. In one case, a pyroclastic flow deposits
(blockandashflow)fromseveralkmoutsidethevolcanowasalso Recent studies indicate that Early–Middle Miocene medium- to
sampled(seethesamplesitedistributioninFig.1). high-Kseriesandesitestorhyolitesvolcanicrocksalongthestrike-slip
There are three ages with 1σ precision higher than 1 Ma (two dominatedtheİzmir–BalıkesirTransferZonethatincludeourstudy
belongstoYamanlarandonetoYuntdağı)howevertheseagesfallin area, were most probably derived from mixing between mantle-
the interval of volcanic activity (Table 2), suggesting that they are derivedmagmasandlowercrustalmeltsandformedthemainsource
meaningful and worth to be taken into consideration. The greater ofthemostprimitivemedium-tohigh-Kandesiticrocks.Subsequent
80 I.Seghedietal./JournalofVolcanologyandGeothermalResearch291(2015)72–85
Fig.7.aandb:PrimitiveMantle(PM)-normalizedandcandd:CI-chondritenormalizedREEdiagramsforYamanlarandYuntdağıvolcanoes.NormalizingfactorsarefromSunand
McDonough(1989).SymbolsasinFig.6.
fractionalcrystallization(FC)and/orassimilation-fractionalcrystalliza- contentsmaybeindicativeofextensivefractionationofclinopyroxene,
tion(AFC)processesthatoccurredareresponsibleforthewiderange butincreasingtheMgOuptoSiO b62%maybeusedtorefusethis
2
of chemical andpetrographic types (Ersoy et al., 2012a;Ersoy and interpretation.Inthiscase,adecreaseinFe O andTiO ,VandScmay
2 3 2
Palmer,2013). belinkedtoFe–Tioxidefractionation.Extensiveplagioclasefraction-
Ournewgeochemicaldataareinagreementwiththishypothesis ationisclearlyevidencedbydecreasingCaO,Sr,Eu,andEu/Eu*.CaO
andwewillattempttodemonstratethis.Fromageochemicalpointof depletion(anddecreaseinCaO/Al O )canalsobelinktoplagioclase+
2 3
view,alltherocksareevolved,showinginthespiderdiagram/primitive apatitefractionation,insteadofremovalofclinopyroxene.Smalldegree
mantleanegativeanomalyforPandTithatmaycharacterizeforpartial apatite fractionation is also supported by the decreasing in REEs,
melting(PM)ofmetasomaticmantlesourcesorfractionalcrystalliza- especiallyinLREE,suchasCe.Theseobservationsledustoconclude
tionprocesses(FC)(Fig.7a,b),withorwithoutcrustalcontamination. thatthefractionatingassemblageresponsibleforthedifferentiation
Isotopedatafromthestudyareasconfirmtheimplicationofcrustal oftheYamanlarandYuntdağıvolcanicscanbetakenasplagioclase+
assimilation(e.g.Agostinietal.,2010;Ersoyetal.,2012a).FCand/or biotite + Fe–Ti-oxides + apatite. Additionally, small amounts of
PMisalsosuggestedbyhighLREE/HREE(Fig.7c,d)andasmallnegative clinopyroxenemayalsobeincluded.
Euanomalywhichlikelyinfersplagioclasefractionation,typicalforan Byusing anassemblageofplagioclase(70%)+biotite(13%)+
uppercrustmagma storage system. ThesimilarEu anomaly of the clinopyroxene(5%)+magnetite(5%)+apatite(5%),Sr/Vvs.Vsystem-
rhyolite(069)suggestsimilarsourceasforandesitesandimportant aticsofthevolcanicrockshavebeenmodeledtotestthevalidityofthe
fractionationprocessesandrelatesthemto“wettypecold-oxidized fractional crystallization processes. In Fig. 10, the PETROMODELER
rhyolites”thatmayderivefromasubduction-derivedmetasomatized program(Ersoy,2013)isusedwiththeincrementsof10%andthe
mantlesource(e.g.,BachmannandBergantz,2008). partitioncoefficientscompiledfromGERMpartitionCoefficientdata
TodemonstratetheroleofFCprocessesinYamanlarandYuntdağı base(www.http://earthref.org/).Theresultsshowthattheandesitic
volcanoeswemadefurthergroupofHarkerdiagrams(Fig.9).Increase rocksmayhavebeenproducedby0–30%fractionalcrystallizationof
inMgO,fortherockswithSiO b62,andabsenceofNidepletionwith themostprimitivesample(sample70withlowestsilicacontent).The
2
respecttoincreasingSiO ,suggestthatolivinewasnotinvolvedinthe rhyoliticsample(69),ontheotherhand,requireupto80%fraction-
2
fractionatingphases.BroaddecreaseinBacoupledwithdecreasing ation.Itisalsotonotethatcrustalcontaminationandmixingprocesses
MgOfortherockswithSiO N62%maybeexplainedbyfractionation mayhaveoperatedduringthisfractionation,butitisnotpossibleto
2
ofMgandK-bearingphasesuchasbiotiteoramphibole.Inthiscase, monitor this effect, since there are no available isotopic data from
lateralvariation(orevenslightlydecreasing)inNbisnotcompatible thesesamples.
withamphibolefractionation.Furthermore,thehighlyfractionated Ratios of high field strength elements (HFSE) such as Nb and
(high-SiO )rhyolitesamplehasthehighestNbcontent,refusingthe Zrcanprovideinsightintovariationsinmagmasourcecomposition
2
amphibolefractionation.Therefore,decreasingMgOcontentsofthe (e.g.Davidson,1996;Singeretal.,1996).NbandZraredepletedin
rockswithSiO N62%canbeexplainedbybiotitefractionation.Ade- subduction-related magmas and are assumed to be dominantly
2
creaseinCaO/Al O ratiocoupledwithdecreasingV,Sc,CaOandTiO mantle-derived, being relatively immobile under hydrothermal
2 3 2
I.Seghedietal./JournalofVolcanologyandGeothermalResearch291(2015)72–85 81
Fig.8.DiagramofK/ArageswithanalyticalerrorsandSiO2vsK/AragesinYamanlarandYuntdağıvolcanoes.SymbolsasinFig.6.
conditionsandstronglyfractionatedonlyduringmeltingormagma- etal.,2010;Ersoyetal.,2012a),somostprobablythesourceareais
mixingprocesses(Thirlwalletal.,1994;Davidson,1996).Different situatedatthemantle–crustboundary.
Nb/Zrratiosaregenerallyinterpretedintermsofvariationsinsource Nevertheless,therearesomeadditionalpetrogeneticconsiderations
compositionorchangesindegreeofpartialmeltingofthemantle, that derive from a combined petrographic and geochemical study
sincelineartrendsofNbenrichmentsuggestfractionalcrystallization inbothvolcanoes.Sometexturalcharacteristicsofplagioclase,amphi-
(e.g.Davidson,1996).TheNb/Zr–Nbdiagramformostprimitiverock boleandbiotitecangiveconstraintsonadditionalmagmachamber
(~56wt.%SiO2)(Fig.11)showsZr–Nbvaluesstartingfrom~0.6– processes.
0.7,notfarfromatypicalMORBvalue,butclosertotheLowerand Thecommonpresenceofirregularandroundedmarginswhichare
Middle Crust average values (Rudnick and Fountain, 1995). The observedforalargenumberofplagioclasecrystals,besidecombination
plots follow both linear trends, starting from the most primitive withsievetextures(Fig.5,samples053,054and060)pointstomargin-
rocksfoundinYamanlarandYuntdağı,inasmallrangealongthe almeltingofthesecrystalsandconsequentlyaprocessofdisequilibrium
mantlesourcecompositionorchangesindegreeofpartialmelting (inthiscasethermal(e.g.NelsonandMontana,1992).Thismayindicate
trendandmostlydevelopedalongthehorizontaltrendtowardmore an influx of hotter magma inflow into a shallow, partially evolved
differentiatedrockssuggestingfractionalcrystallization,havingat magma storage system that implies mixing. The plagioclases with
theendoftherangethesolitaryrhyolitesample(69),withhigher irregularandroundedmarginsindicatethatre-equilibriumwasnot
Nbcontents. reached, suggesting that eruption occurred soon after mixing. The
Theseobservationsareinagreementwithfractionalcrystallization massbreakdownoxidationofamphibolesisalsoasignofthermody-
modeling(Fig.10).Anotherimportantobservationisthattherhyolite namicdisequilibrium,whichmayberelatedtotheincreaseinoxygen
sample (Yuntdağı) by showing similar Nb/Zr trend as the most fugacitywithincreasingtemperaturewhichfavorsdehydroxylation
primitiverockinthestudiedareaisolder(17.52±0.67Ma)thanthe (Grahametal.,1984).Suchfeaturesofincreasingtemperaturecanbe
otherrocksinYuntdağı,buthassimilaragewithandesitesofYamanlar mosteasilyexplainedintermsofmagmamixing(Fig.5,sample065).
volcano.Thismaysuggestthattherhyolitecouldderiveviafractional Thereactionrimsofamphibolesandbiotites,ontheotherhand,suggest
crystallizationprocessesfromthesimilarsourceasotherandesites, thatH Oisleavingthesystemduetoloweringpressure.
2
notfromadifferentone.Withoutisotopedataisdifficulttosaythat In the Peccerillo and Taylor (1976) diagram, the scatter plot
thesource,assuggestedalsobyFig.11,isaslightlyenrichedmantle distributionshowinglargeK Ovariation(2–4%)inarelativelylarge
2
sourceorlowercrust,orabouttheinfluenceofassimilationprocesses, rangeofSiO —56–65%mayalsoconfirmthelocaluppercrustalmixing
2
however,asalreadymentioned,thepublisheddatawithisotopesin processes(Fig.6a).ThedecreaseinSiO at~16.5Ma(Fig.8)forboth
2
the surrounding areas are suggesting that both mantle and crustal volcanoes could be interpreted in terms of mixing of an andesitic
melts are implicated in generation of these magmas (e.g. Agostini magmawithaleastevolvedbasalticandesite.Overall,theidentification