Mt. Nemrut volcano (Eastern Turkey): Temporal petrological evolution

Journal of Volcanology and Geothermal Research 209-210 (2012) 33–60

Contents lists available at SciVerse ScienceDirect

Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores

Mt. Nemrut volcano (Eastern Turkey): Temporal petrological evolution H.E. Çubukçu a,⁎, İ. Ulusoy a, E. Aydar a, O. Ersoy a, E. Şen a, A. Gourgaud b, H. Guillou c a b c

Hacettepe University, Department of Geological Engineering, 06800, Beytepe-Ankara, Turkey University Blaise Pascal, OPGC, Lab. Magmas et Volcans, UMR-6524 CNRS, 5 rue Kessler, 63038 Clermont Ferrand Cedex, France Laboratoire des Sciences du Climat et de l'Environnement, Domaine du CNRS, Bat. 12, Avenue de la Terrasse, 91198 Gif-sur-Yvette, France

a r t i c l e

i n f o

Article history: Received 17 November 2010 Accepted 23 August 2011 Available online 3 September 2011

a b s t r a c t Quaternary active Nemrut volcano is situated 12 km north of the Bitlis–Zagros suture zone, southern margin of continental collision between Arabian and Anatolian plates. The latest activity of the volcano dates back to historic times. Volcanic evolution of the volcano is investigated under two main stages: Pre-caldera and post-caldera separated by paroxysmal caldera forming eruptions not older than 90 ka. The majority of the products are silica oversaturated peralkaline {([Na2O + K2O]/Al2O3) N 1} felsic rocks with rare transitional-to-mildly alkaline basalts and mugearites. A compositional gap (Daly Gap) between 53% and 59% SiO2 is partly filled with benmoreitic enclaves in peralkaline rhyolites. Benmoreitic enclaves display evidence of interminglement between mafic and felsic magmas. Observed mineral assemblages represent typical peralkaline mineralogy with aenigmatite, arfvedsonite-riebeckite, aegirine, fayalite and chevkinite. Geochemical evolution trends and modelling depict that protracted crystal fractionation dominated by feldspar, clinopyroxene, olivine and Fe-Ti oxides and crustal contamination would produce peralkaline rhyolites from the actual mafic compositions taken as parents. Mineralogical and petrographical observations indicate that the magma chamber is zoned compositionally having a crystal rich density layer between mafic and felsic melts. The genesis of Nemrut peralkaline magmatism has been ascribed to the ascension of slightly subduction modified asthenospheric melts into upper crustal high level reservoirs in localized extension in Muş ramp basin. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Mount Nemrut is an active Quaternary volcano, located at the western shore of soda Lake Van, culminating at 2948 m. The volcano, with approximately 22 km of basal diameter, has an elliptic summit caldera with 8.5× 7 km in diameter. The eastern half of the caldera is filled by post-caldera pyroclastic deposits related to maar-like explosion craters, lava domes and flows (Aydar et al., 2003). The western half of the caldera is filled by a fresh water lake, covering a surface area of 12.36 km2 with a maximum depth of 176 m (Ulusoy et al., 2008). Besides, there is another lake with hot springs having a maximum depth of 11 m. Fumarolic activity emitting almost pure mantle He (Güleç et al., 2002) is present on a dome at the northern part of the caldera. The latest volcanic activity, although previously known to occur in 1441 (e.g. Oswalt, 1912), dates back to 1692 (Karakhanian et al., 2002). The products of Nemrut volcanism exhibit the petrological characteristics of a peralkaline association: Silicic rocks have molecular excess of alkalis over alumina (Na + K N Al; Peralkalinity Index N 1) and bear phases associated with peralkaline magmas (aenigmatite, aegirine, arfvedsonite–riebeckite, chevkinite). Volcanic rocks present a bimodal

⁎ Corresponding author. E-mail address: [email protected] (H.E. Çubukçu). 0377-0273/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2011.08.005

distribution having a Daly Gap between 53 and 59% SiO2 which is partly filled by benmoreitic enclaves observed in peralkaline rhyolites. Peralkaline magmas are widely distributed in the regions of continental upwelling and/or rifting (Fitton and Upton, 1987) as well as within various geodynamical settings including oceanic islands (Caroff et al., 1993; Mungall and Martin, 1995) and island arcs (Mahood, 1981; Civetta et al., 1984; Macdonald, 1987), local extensional regimes in convergent margins, and active or recently abandoned mid-ocean ridge segments (Bohrson and Reid, 1997). Located at East Anatolia, exceptionally active continental collisional site of the Alpine–Himalayan orogeny since the middle Miocene, existence of peralkaline silicic volcanism, ~12 km north of the Bitlis–Zagros suture, represent a remarkable and unique site for studying the genesis and the development of peralkaline volcanism. Petrogenetic models proposed to explain the generation of peralkaline felsic magmas include the evolution from alkali basaltic or mugearitic magmas by fractional crystallization (Nelson and Hegre, 1990; Mungall and Martin, 1995; Bohrson and Reid, 1997; Civetta et al., 1998) maybe with moderate assimilation of crust (Peccerillo et al., 2003), partial melting of lower crustal gabbros (Lowenstern and Mahood, 1991) and partial melting of heterogeneous source regions, including metasomatized subcontinental mantle (Bailey and Macdonald, 1987) and upper crustal lithologies (Davies and Macdonald, 1987; Macdonald et al., 1987; Black et al. 1997). However, no experimental study of crustal anatexis has yet produced peralkaline melts (Scaillet and Macdonald, 2003).

34

H.E. Çubukçu et al. / Journal of Volcanology and Geothermal Research 209-210 (2012) 33–60

Peralkaline felsic rocks are generally associated with alkali basalts with a lack of intermediate compositions, as a widely observed feature of most peralkaline associations independent of geodynamic settings (Avanzinelli et al., 2004). Although fractional crystallization remains one of the most satisfactory models for the magmatic evolution from basaltic to peralkaline magmas, the absence or scarcity of intermediate rocks (Daly Gap) and the predominance of silicic (daughter) with respect to mafic (parental) magmas are serious problems (Peccerillo et al., 2003). Despite the number of various processes thought to occur within, a significant consensus is established on ascribing magmatic evolution from mafic to peralkaline magmas and their bimodality to compositionally zoned magma chambers (e.g. Baker, 1987; Peccerillo et al., 2003; Macdonald and Scaillet, 2006). In this study, we present mineralogical and petrological data of all eruptive units concerning the volcanostratigraphy of Nemrut stratovolcano and discuss the magmatic evolution, the existence of peralkaline magmatism and the origin of the observed Daly Gap.

al., 2003) with varying eruption styles and geochemical characteristics from over 20 central eruption sites (Fig. 1b). Additionally, according to their Curie Depth Point and Pn velocity data, Elitok and Dolmaz (2007) propose that mantle lithosphere also gets thinner in the Arabian Foreland towards Bitlis–Zagros suture probably due to its mechanical removal/erosion at the southernmost Anatolian side of the suture. Further studies on mantle shear wave velocity structure, which provide insight into the rheology and dynamic processes of the upper mantle, signifies a warm (above solidus, molten), lowdensity upper mantle beneath the Turkish–Iranian plateau (Maggi and Priestley, 2005). Volcanism is still active in the region, represented by major Quaternary volcanic centers (Fig. 1b) (Mt. Nemrut, Mt. Tendürek, Mt. Ağrı, etc.) documented in historical records (e.g. Karakhanian et al., 2002; Aydar et al., 2003; Ulusoy et al., 2008). Active Nemrut volcano is being monitored by Hacettepe University (Ankara) against a probable eruption risk (Ulusoy et al., 2008).

2. Tectonic history, neotectonics and volcanism of Eastern Anatolia

3. Volcanic history of Nemrut volcano

Anatolian peninsula, an important component of the Alpine–Himalayan orogeny, was situated at the collisional boundary between Gondwana in the south and Laurasia in the north and comprised of sutured continental fragments (Bozkurt et al., 2000) which were formed by the closure of Neo-Tethyan Ocean during the late Cretaceous–Eocene (Şengör and Yılmaz, 1981) and have been accreted to Eurasia throughout the Mesozoic and Cenozoic. The final amalgamation of these fragments into a single continental mass occurred at Late Tertiary, when the Arabian plate collided with the Anatolian plate (Okay and Tüysüz, 1999) along Bitlis–Zagros thrust zone which was exhumed in the Early to Mid-Miocene between 18 and 13 Ma (Okay et al., 2010). In Eastern Anatolia, the intracontinental convergence and N–S directed compressional–contractional tectonic regime remained till the end of Late Miocene and was replaced by compressional–extensional tectonic regime in the early-late Pliocene (Fig. 1a), (Koçyiğit et al., 2001). Along the eastern portion of NAF, the present day state of deformation is dominantly right-lateral strike slip (Faccenna et al., 2006). However, an early Quaternary change from regional transpressional to transtensional regime has been reported (Over et al., 1997). Eastern Turkey Seismic Experiment (ETSE), (Sandvol et al., 2003), showed that there were no subcrustal earthquakes beneath the Arabian–Eurasian collision zone or beneath the Anatolian plateau suggesting no or very little underthrusting/subduction of the Arabian plate beneath Eurasia (Al-Lazki et al., 2003; Türkelli et al., 2003). Source properties of seismic events suggest that in eastern Turkey, most of the collision is taken up by strike slip faults, indicating the northward convergence of Arabia is being accommodated by escape tectonics, where compressive features (e.g. thrust faulting) are still active but are of lesser importance (Örgülü et al., 2003). Furthermore, seismic evidence indicates an average crustal thickness of 45 km for East Anatolian plateau (Zor et al., 2003), which was hitherto presumed to be ~ 55 km (e.g. Şengör, 1980). Moreover, seismic models show that most of the East Anatolian plateau is devoid of mantle lithosphere, which is ascribed to the break-off of northward-subducted slab beneath East Anatolian Accretionary Complex (EAAC), and, melting due to the direct contact with asthenosphere has resulted in widespread volcanism in the region (Sandvol et al., 2003; Şengör et

Volcanic evolution and interrelations of volcanic units of Nemrut volcano have been a subject of debate. Atasoy et al. (unpublished) divided the volcanic evolution into four stages, namely pre-cone, cone building, caldera-forming and post-caldera stages, whereas Yılmaz et al. (1998) proposed pre-cone, cone building, climactic, post-caldera and late phases. On the other hand, Aydar et al. (2003) suggested precaldera and post-caldera stages separated by a paroxysmal eruption leading to the caldera collapse. Conversely, Karaoğlu et al. (2005) and Özdemir et al. (2006) proposed pre-caldera, post-caldera and late stages. In this study, the volcanological evolution of Nemrut volcanism has been investigated under pre-caldera and post-caldera stages. Pre-caldera stage is comprised of building of the volcano and peripheral eruption centers followed by the paroxysmal caldera forming ignimbrite eruptions. Post-caldera stage includes the intracaldera activity and the bimodal rift volcanism on the northern flank of the volcano. We present seven new geochronological analyses for Nemrut volcanism (Table 1). Although, Keskin (2003) report a maximum age of 3.9 Ma for Mt. Nemrut, both our new radiometric data and available ages (Fig. 2) depict that the oldest volcanic rocks of Mt. Nemrut are 1.01 ± 0.04 Ma (Atasoy et al., unpublished).

3.1. Pre-caldera stage The oldest known volcanic units of Nemrut volcanism are metaluminous trachytes exposed on the southern and southwestern flanks of the volcano overlain by peralkaline rhyolites (comendite) (Atasoy et al., unpublished; Pearce et al., 1990). Central cone is believed to be formed after ca 500 ka producing dominantly comenditic trachytes, comendites and scarce pantellerites as lava flows and peripheral dome flows. Kirkor Dome Complex and Mazik Dome are two of the largest silicic domes located at southwest and west of the volcano respectively. Pre-caldera activity produced scarce basaltic trachyandesitic (mugearite) lava flows circa 100–80 ka (Notsu et al., 1995) which are observed on the southern and southwestern escarpments. Abundant enclaves with benmoreitic composition are observed in 89–99 ka precaldera comendites which are exposed along the flanks. Numerous pre-caldera trachytic intrusions are observed on the caldera walls.

Fig. 1. a) Tectonic plate movements of Anatolia and Eastern Anatolian Volcanic centers. b) Layout of volcanic rocks and main volcanic centers in the eastern Anatolia. AV: Ağrı volcano, AçV: Akça volcano, AkV: Akdoğan caldera, BV: Bilican volcano, BiV: Bingöl caldera, BoV: Bozdağ caldera, ÇV: Çıplak (Topdağı) volcano, GV: Gel volcano, GiV: Girekol volcano, HV: Hayal volcano, KV: Kandil volcano, KaV: Karacadağ volcano, KPV: Kargıpazarı volcanoes, KPT: Kars Plateau tuffs, MV: Meydan caldera, NV: Nemrut caldera, SV: Süphan volcano, TV: Tendürek volcano, YV: Yıllık volcano, ZV: Zor volcano. Data from: MTA, 1964; Aydar, 1992; Pawlewicz et al., 1997; Ulusoy, 2008. c) Location of Nemrut volcano and the surrounding region.

H.E. Çubukçu et al. / Journal of Volcanology and Geothermal Research 209-210 (2012) 33–60

35

36

H.E. Çubukçu et al. / Journal of Volcanology and Geothermal Research 209-210 (2012) 33–60

Table 1 K/Ar ages of samples from Nemrut volcano. Age calculations are based on the decay and abundance constants from Steiger and Jager (1977).

Post-caldera

Sample

Weight molten (g)

K (wt.%)

40Ar* (%)

40Ar* (10–13 mol/g)

N-305

0.94719 1.08099 0.96572 2.60063 1.17125 2.35809 1.08036 1.15475 1.01908 1.49449 1.53223 1.08822 1.26988 1.57533

3.686 ± 0.037 3.686 ± 0.037 3.873 ± 0.039 3.873 ± 0.039 4.284 ± 0.043 4.284 ± 0.043 4.059 ± 0.041 4.059 ± 0.041 3.852 ± 0.039 3.852 ± 0.039 3.868 ± 0.039 3.868 ± 0.039 4.203 ± 0.042 4.203 ± 0.042

0.145 0.072 1.28 2.513 11.159 7.915 4.734 2.927 7.72 7.555 18.025 10.14 14.477 12.734

0.611 0.404 0.79 1.005 6.719 6.561 6.317 6.878 7.081 6.341 10.586 10.719 19.007 19.379

N-051 N-80 N-181

Pre-caldera

N-98 N-149 N-238

3.2. Caldera forming pyroclastic activity During its construction, the stratovolcano has been calculated to culminate at 4500 m (Aydar et al., 2003). Following the construction of the volcano, explosive eruptions, which occurred in at least two rapid consecutive stages, produced plinian fallout and ignimbrite deposits. Prior to the caldera forming sequence, multiple episodes of plinian activity separated by paleosol levels have been observed

Age ± 2σ (ka)

Rock type Comendite

8±3 Comendite 15 ± 1 89 ± 2 93 ± 3

Comenditic trachyte Comenditic trachyte Pantellerite

99 ± 3 Comendite 158 ± 4 264 ± 6

Comenditic trachyte

(Ulusoy, 2008). Products of caldera forming eruptions begin with felsic pre-ignimbrite plinian fallout, which is followed by relatively thinner dark colored fallout tephra. These two plinian phases are sometimes overlain by a level of ground surge especially in eastern flanks. The surge would represent the lowermost section of the main ignimbrite flow deposit (Nemrut Ignimbrite). Its thickness reaches up to several tens of meters on the flatlands, while getting thicker in the valleys into which it canalized. Occasionally, the basal

Fig. 2. Geological map of Nemrut Stratovolcano. Available radiometric ages with respective sample numbers have been depicted. Samples with “N-” prefixes are from this study. (1) Atasoy et al., unpublished (2) Pearce et al., 1990 (3) Notsu et al., 1995) (4) Matsuda, 1988 (5) Ercan et al., 1990 (6) Aydar et al., 2003.

H.E. Çubukçu et al. / Journal of Volcanology and Geothermal Research 209-210 (2012) 33–60

section of Nemrut ignimbrite is strongly welded with significant fiammes exhibiting eutaxitic texture. Nemrut Ignimbrite is overlain by multiple levels of plinian fallback units followed by a second unit of ignimbritic flow (Kantaşı ignimbrite) which is observed dominantly on the north of the caldera (Fig. 2). It is reddish to pale brown in color and moderately-to-strongly welded. Atasoy et al. (unpublished) presented one K/Ar age (270 ka) for a “brown tuff” sample from eastern flatlands, and Mouralis et al. (2010) suggested one Ar/Ar age (117± 5 ka) for plinian fallout overlying their “Obuz ignimbrite” which they correlate to the Nemrut ignimbrite. According to our new radiometric data, the youngest effusive products prior to caldera forming pyroclastic events were circa 100–90 ka old (Table 1). Therefore, we suggest a younger age for ignimbritic flows, at most 89 ka. Although Mouralis et al. (2010) suggested an age of ~116 ka for “Nemrut ignimbrite”, they could not explain the contradiction regarding the younger ages for the domes covered by the ignimbrite. Yet, we propose that the timing of caldera forming eruptions, which has evacuated the magma chamber leading to the caldera collapse, was younger than ~90 ka. Although the exact time of caldera collapse is unknown, we do assume that this major event has possibly taken place between ~90 and 30 ka ago (Fig. 2). Nemrut caldera was collapsed in a piecemeal manner and constituted of four main blocks. (Ulusoy et al., 2008).

3.3. Post-caldera stage Post-caldera activity at Nemrut volcano is confined to intra-caldera region and to the northern flank where a “rift zone” has formed in historic times (1597 AD). Effusive activity is dominantly confined to the eastern half of the intracaldera region. Only one notable post-caldera lava flow is observed on the southwestern caldera rim along the structural caldera boundary (Fig. 2). Phreatomagmatic deposits, originating from several explosion craters, overlay comendite lavas and domes (Fig. 2).One pumice sample (N-305) from these phreatomagmatic units has yielded 8 ± 3 ka (Table 1). Intra-caldera lavas and domes are peralkaline rhyolites (Comendite2) and dates back to 30 ka (Fig. 2; Matsuda, 1988). Furthermore, one comendite sample (N-051) collected from the northern intra-caldera region yielded a K-Ar age of 15± 1 ka. These intracaldera comendites bear benmoreitic enclaves abundantly. Furthermore, abundant lithic fragments have been observed in phreatomagmatic base surges. Two types of phreatomagmatic lithic ejecta have been discriminated. One type, being porphyritic, bears highly destabilized (xeno)crysts noticed even by naked eye, and another type exhibits holocrystalline (semi) equigranular texture. The fissure eruption mentioned in historical records, produced a spectacular outcrop of bimodal rhyolite and basalt forming a rift

37

zone of 5 km length with an overall width of 50 m. Bimodal rift zone activity commences with scarce basaltic lavas that flowed towards east, west and northern directions.. Although, Özdemir et al. (2007) proposed that basalts have injected into rhyolites, we observed that comenditic lava flow overlies earlier basaltic flow (Çubukçu et al., 2007). 4. Methods 4.1. Electron microscopy and EPMA Mineral abundances have been obtained by point counting on an average of 3000 points per sample. Scanning electron microscopy imaging has been realized in Electron Microscopy and Microanalysis Laboratories of Hacettepe University (Ankara) using Cameca SU-30 SemProbe and Zeiss EVO-50 EP SEMs equipped with W filaments. Analytical conditions for Backscattered Electron Imaging (BSE) and EDS (Bruker-Axs XFlash 3001) mapping were 12–15 kV accelerating voltage, 10–15 nA beam current, 3–5 μ beam diameter and 10 mm working distance. Quantitative mineral microanalyses have been carried out on 54 samples on a Cameca SX-100 EPMA in Laboratoire Magmas et Volcans in Clermont-Ferrand (France). The analytical conditions for mineral analyses were 15 kV accelerating voltage, 15 nA beam current with 5 μm diameter. Counting times were 10 s for peaks (20 s for Cr and Ni) and 10 s for backgrounds. For glass analyses, beam current decreased to 6 nA with the minimum diameter possible (~ 1 μm). In order to minimize the migration effect of alkalis, Na and K were analyzed first. Upon the acquisition of raw data, ZAF correction procedure has been applied. 4.2. Geochemical analyses Samples have been crushed into rock chips by hammering. Gravels are further cleaned form dirt by compressed air before they are crushed and milled in agate mortar. Enclaves are separated from the host rock by chipping, trying to prevent any interference of the host rock. 4.2.1. Major and trace elements Major element (Si, Al, Fe, Mn, Mg, Ca, Na, K, P, and Ti) analyses have been performed on 52 samples at Blaise Pascal University (ClermontFerrand, France) using Jobin Yvon 70 ICP-AES. 200 mg of powdered samples have been molten with LiBO2, and dissolved by HNO3. Prepared solutions have been analyzed by the automatic procedure installed in the system. Trace element analyses have been conducted in Service d'Analyse des Roches et des Minéraux, CRPG–CNRS–Nancy, France using Perkin Elmer 500 Mass Spectrometer on 38 samples.

Fig. 3. Nomenclature of Nemrut volcanic rocks. a) Total alkali versus Silica diagram (Le Bas et al., 1986) b) Al2O3 versus FeO* diagram (Macdonald, 1974).

38 Table 2 Feldspar analyses of Nemrut volcanic rocks selected from 490 analyses. Calculated for 8 oxygens. Arrow indicates the alkali feldspar analysis mantling destabilized plagioclase. Rock names: T: Trachyte (Metaluminous), CT: Comenditic Trachyte, P: Pantellerite, C: Comendite, BM: Benmoreite, B: Basalt, M: Mugearite. Location:c – center, r – rim. Pre-caldera rocks

Nemrut ignimbrite

Sample

N-141

N-141

N-281

N-188

N-239

N-136

N-083

N-016

N-016

N-209

N-080

N-080

N-098

N-098

N-194

N-022

N-047

N-247

N-023

N-233

N-036

Rock

T

T

CT

P

CT

T

CT

M

M

M

CT

CT

P

P

C

T

T

T

T

T

T

c

c

c

c

c

c

c

c

r

c

CT

r

r

c

c

c

c

c

c

c

c

Size (μ) SiO2 Al2O3 FeO MgO CaO Na2O K2O Total Si Al Fe Mg Ca Na K Tot An Ab Or

500 65.57 20.79 0.42 0.04 1.97 7.84 2.49 99.11 11.64 4.38 0.06 0.01 0.38 2.71 0.57 19.78 10.30 74.19 15.52

1650 60.93 23.94 0.52 0.00 5.85 6.77 1.12 99.12 10.94 5.09 0.08 0.00 1.07 2.37 0.26 19.82 29.00 64.05 6.95

6000 67.32 19.41 0.33 0.02 0.44 6.92 6.95 101.38 11.90 4.04 0.05 0.00 0.08 2.37 1.57 20.03 2.07 58.95 38.98

2000 67.85 18.58 0.48 0.00 0.05 7.59 6.14 100.68 12.04 3.89 0.07 0.00 0.01 2.61 1.39 20.01 0.25 65.10 34.65

2000 64.70 21.23 0.17 0.00 2.00 7.49 4.76 100.35 11.52 4.46 0.03 0.00 0.38 2.59 1.08 20.07 9.44 63.85 26.71

2000 66.69 19.37 0.25 0.00 0.47 7.28 6.48 100.55 11.88 4.07 0.04 0.00 0.09 2.52 1.47 20.07 2.20 61.68 36.12

500 65.72 20.62 0.15 0.02 1.36 7.41 5.57 100.84 11.66 4.31 0.02 0.01 0.26 2.55 1.26 20.08 6.35 62.67 30.98

3000 54.60 28.09 0.52 0.08 10.92 5.35 0.41 99.96 9.89 5.99 0.08 0.02 2.12 1.88 0.09 20.08 51.80 45.91 2.29

3000 52.91 28.97 0.59 0.12 11.87 4.80 0.29 99.55 9.65 6.23 0.09 0.03 2.32 1.70 0.07 20.10 56.78 41.57 1.65

35 57.84 25.09 0.83 0.06 8.24 6.40 0.79 99.25 10.48 5.36 0.13 0.02 1.60 2.25 0.18 20.03 39.69 55.77 4.55

1500 66.87 19.17 0.27 0.00 0.20 7.07 6.88 100.46 11.93 4.03 0.04 0.00 0.04 2.44 1.56 20.05 0.93 60.40 38.67

1500 67.07 18.57 0.75 0.00 0.11 7.74 6.19 100.43 11.98 3.91 0.11 0.00 0.02 2.68 1.41 20.11 0.52 65.18 34.29

500 67.22 18.44 0.79 0.03 0.04 7.81 5.68 100.00 12.02 3.88 0.12 0.01 0.01 2.71 1.29 20.03 0.18 67.52 32.30

30 68.68 16.43 2.85 0.00 0.05 8.29 4.34 100.64 12.22 3.45 0.42 0.00 0.01 2.86 0.98 19.96 0.26 74.18 25.55

2000 66.93 19.01 0.31 0.00 0.10 7.47 6.19 100.02 11.96 4.00 0.05 0.00 0.02 2.59 1.41 20.04 0.50 64.39 35.11

1000 67.53 18.92 0.45 0.01 0.05 7.92 5.95 100.83 11.98 3.95 0.07 0.00 0.01 2.72 1.35 20.08 0.24 66.77 32.98

1500 65.31 20.77 0.20 0.00 1.86 7.34 5.00 100.48 11.61 4.35 0.03 0.00 0.35 2.53 1.13 20.03 8.83 62.95 28.23

500 57.99 26.04 0.36 0.03 8.45 6.64 0.56 100.06 10.41 5.51 0.05 0.01 1.62 2.31 0.13 20.05 39.97 56.88 3.16

150 57.00 25.50 0.57 0.06 8.73 6.72 0.85 99.43 10.35 5.46 0.09 0.02 1.70 2.36 0.20 20.18 39.86 55.50 4.64

1500 64.08 21.85 0.28 0.00 2.94 7.92 3.02 100.09 11.40 4.58 0.04 0.00 0.56 2.73 0.69 20.01 14.10 68.67 17.23

1200 57.57 27.06 0.35 0.06 9.22 6.30 0.59 101.14 10.24 5.67 0.05 0.02 1.76 2.17 0.13 20.06 43.26 53.43 3.32

Post-caldera rocks Sample

N-146

N-011

N-011

N-050

N-050

N-256

N-256

N-256

N-258

N-258

N-254

N-254

Rock

C

BM

BM

BM

BM

BM

BM

BM

B

B

C

C

Location

c

c

r

c

r

c

c

r

c

r

c

r

Size (μ)

1500

1200

1200

2500

2500

4000

4200

4200

1000

1000

2000

2000

SiO2 Al2O3 FeO MgO CaO Na2O K2O Total Si Al Fe Mg Ca Na K Tot An Ab Or

67.57 19.20 0.17 0.00 0.19 7.21 6.47 100.81 11.98 4.01 0.02 0.00 0.04 2.48 1.46 19.99 0.89 62.33 36.78

52.15 30.71 0.55 0.13 13.29 3.94 0.13 100.90 9.40 6.52 0.08 0.03 2.57 1.38 0.03 20.03 64.56 34.68 0.76

51.74 31.29 0.72 0.15 13.67 3.79 0.11 101.47 9.29 6.62 0.11 0.04 2.63 1.32 0.03 20.05 66.13 33.21 0.66

58.20 25.82 0.37 0.02 7.93 6.99 0.44 99.76 10.47 5.47 0.06 0.01 1.53 2.44 0.10 20.07 37.57 59.98 2.45

58.65 25.49 0.28 0.00 7.46 7.22 0.51 99.61 10.54 5.40 0.04 0.00 1.44 2.52 0.12 20.07 35.29 61.82 2.89

54.56 28.14 0.42 0.08 10.86 5.22 0.19 99.46 9.91 6.02 0.06 0.02 2.11 1.84 0.04 20.01 52.91 45.99 1.10

54.02 28.22 0.54 0.08 11.45 4.95 0.18 99.44 9.83 6.05 0.08 0.02 2.23 1.75 0.04 20.00 55.52 43.43 1.06

64.14 21.73 0.20 0.03 3.28 8.90 1.49 99.76 11.40 4.55 0.03 0.01 0.62 3.07 0.34 20.01 15.49 76.15 8.36

52.04 30.28 0.56 0.14 13.54 3.98 0.09 100.61 9.41 6.45 0.08 0.04 2.62 1.39 0.02 20.05 64.97 34.53 0.50

55.54 27.93 0.90 0.12 10.93 5.54 0.33 101.30 9.92 5.88 0.13 0.03 2.09 1.92 0.07 20.10 51.22 46.96 1.82

66.94 18.46 0.17 0.01 0.12 7.86 5.99 99.55 12.01 3.90 0.03 0.00 0.02 2.74 1.37 20.08 0.57 66.23 33.21

67.77 18.79 0.23 0.00 0.11 7.67 6.37 100.94 12.00 3.92 0.03 0.00 0.02 2.63 1.44 20.05 0.51 64.36 35.13

H.E. Çubukçu et al. / Journal of Volcanology and Geothermal Research 209-210 (2012) 33–60

Location

H.E. Çubukçu et al. / Journal of Volcanology and Geothermal Research 209-210 (2012) 33–60

4.2.2. Sr/Nd isotopic analyses Sr and Nd isotopic ratios have been determined at Laboratoire Magmas et Volcans in Clermont-Ferrand, France using VG Sector 54E multicollector and Triton mass spectrometers. For 143Nd/144Nd isotopic ratios of 4 samples have been analyzed by VG 54E. For these runs, Nd isotope ratios were normalized to AMES 143Nd/144Nd= 0.511953. 10 samples have been analyzed by Triton. For these runs, Nd isotope ratios were normalized to AMES 143Nd/144Nd= 0.511959. Precision (2σ) for these runs is better than± 0.000009. The mean of these two normalizing values (143Nd/144Nd= 0.511956) has been used as a reference for all 143Nd/144Nd isotopic ratios, including the data obtained from literature (i.e. Gülen, 1984). Sr isotope ratios of 5 samples have been analyzed by VG 54E and were normalized to NBS 987 87Sr/86Sr= 0.710266 and precision (2σ) is better than± 0.000017. 4.3. K/Ar dating Radiometric analyses have been realized on seven samples representing the volcanological history of the volcano, using K-Ar methodology. Analyses have been performed at "Laboratoire des Sciences du Climat et de l'Environnement" – Unité Mixte de Recherche CEA – CNRS, in Gif-sur-Yvette, France. Selected samples have been crushed, ground and sieved to 0.25–0.125 mm size fraction and washed by HC2H3O2 in ultrasonic cleaner. Groundmass of the rocks has been separated according to Guillou et al. (1998). Phenocrysts and xenocrysts were removed from the groundmass using density (with heavy liquids) and magnetic separation methods. K was analyzed by atomic emission spectrometry in Blaise Pascal University. For Ar extraction and analysis methods see Guillou et al. (1998). The isotopic

39

composition of Ar and the total Ar content were determined using an unspiked K-Ar technique (Cassignol and Gillot, 1982; Guillou et al., 1998). 5. Classification and nomenclature Nomenclature of the rocks, which have been re-calculated on a water free basis, has been realized according to the TAS (Total Alkali versus Silica) scheme of Le Bas et al. (1986). TAS classification diagram (Fig. 3.a) depicts that volcanic products consist dominantly of trachytes and rhyolites with minor mugearites (Na2O − 2.0 ≥ K2O) and transitional-to-mildly alkaline olivine basalts. A compositional gap (Daly Gap) between 53% and 59% SiO2 is partly filled with benmoreitic (Na2O–2.0 ≥ K2O; Le Bas et al., 1986) enclaves in rhyolites. Excluding the enclaves, rocks of Nemrut volcanism show bimodal distribution. The great majority of the felsic rocks are peralkaline ({Peralkalinity Index, P.I., (molecular [Na2O + K2O]/Al2O3) N 1}) except the trachytic pyroclastic products of caldera forming eruptions, metaluminous trachyte lavas of pre-caldera period and ejecta in post-caldera phreatomagmatic products (N-272). Additionally, peralkaline trachytes and rhyolites are dominantly comenditic trachytes and comendites respectively according to FeO* versus Al2O3 discrimination diagram (Macdonald, 1974; Fig. 3.b). However, pantellerites are very rare and confined to the pre-caldera stage. 6. Petrography and mineralogy Representative microprobe analyses have been given in Electronic Supplementary Data (Table 2: Feldspar, Table 3: Pyroxene, Table 4:

Table 3 Pyroxene analyses of Nemrut volcanic rocks selected from 370 analyses. Calculated for 6 oxygens. Rock names: T: Trachyte (Metaluminous), CT: Comenditic Trachyte, P: Pantellerite, C: Comendite, BM: Benmoreite, B: Basalt, M: Mugearite. Location:c – center, r – rim. Pre-caldera rocks

Nemrut ignimbrite

Post-caldera rocks

Sample

N-141

N184

N-188

N151

N136

N-239

N-016

N209

N080

N098

N228

N022

N047

N-023

N036

N225

N256

N220

N272

N254

N255

Rock

T

P

P

T

T

CT

M

M

T

P

C

T

T

T

T

BM

BM

CT

T

C

C

Location c

c

c

c

c

c

c

c

r

r

c

r

r

c

r

c

c

c

c

c

c

Size (μ) SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O Total

40 51.89 0.87 0.36 30.12 0.36 0.02 0.40 13.21 97.23

250 48.58 0.63 0.22 30.26 1.46 0.42 17.90 1.15 100.61

50 52.04 4.89 0.25 26.24 0.41 0.04 2.98 11.46 98.32

750 51.14 0.63 1.42 12.25 0.66 11.78 20.94 0.51 99.34

400 51.05 0.72 0.77 17.68 0.79 8.17 20.99 0.42 100.59

500 51.28 0.67 1.63 13.09 0.43 12.66 20.55 0.35 100.67

400 51.44 0.95 1.73 10.77 0.29 14.33 20.12 0.34 99.97

200 47.51 0.69 0.47 28.52 1.32 0.96 18.93 0.71 99.12

500 48.77 0.35 0.16 28.92 1.24 0.34 17.90 1.62 99.29

1700 47.60 0.29 0.14 30.25 1.17 0.11 18.52 1.21 99.28

500 47.62 0.32 0.20 30.72 1.27 0.12 18.53 1.20 99.99

1200 50.00 0.42 0.99 18.41 0.93 7.92 20.34 0.44 99.44

250 50.83 0.58 1.21 15.98 0.83 10.45 20.45 0.42 100.73

250 50.81 0.42 1.08 16.06 0.58 10.09 20.49 0.36 99.89

300 52.41 0.89 2.43 10.73 0.00 14.39 18.07 0.21 99.13

120 48.33 2.14 4.45 11.90 0.40 13.01 19.03 0.52 99.78

700 49.70 0.57 0.14 27.76 1.11 1.04 14.51 4.36 99.18

300 49.96 1.50 3.09 11.42 0.39 11.65 21.51 0.45 99.96

500 48.18 0.22 0.20 29.60 1.23 0.80 18.60 0.78 99.60

10 47.94 0.38 0.38 29.99 1.29 0.57 16.77 0.95 98.27

4 cations 6 oxygens 1.99 1.99 2.00 0.01 0.01 0.00 0.00 0.00 0.01 0.03 0.02 0.14 0.94 0.07 0.55 0.02 0.96 0.29 0.01 0.05 0.01 0.00 0.03 0.00 0.02 0.78 0.12 0.98 0.09 0.85 4.00 4.00 3.99 97.92 7.20 65.33 1.97 92.80 33.37 1.61 41.39 12.54 0.10 1.32 0.20 98.29 57.29 87.26 0.10 2.36 0.24

1.95 0.05 0.01 0.02 0.05 0.35 0.02 0.67 0.85 0.04 4.00 4.68 94.28 44.18 34.56 21.26 63.14

1.97 0.03 0.01 0.02 0.01 0.57 0.03 0.47 0.87 0.03 4.00 0.63 98.43 44.87 24.29 30.84 45.16

1.92 0.07 0.00 0.02 0.07 0.34 0.01 0.71 0.83 0.03 4.00 6.72 93.28 42.16 36.14 21.69 63.27

1.92 0.08 0.00 0.03 0.05 0.28 0.01 0.80 0.81 0.02 4.00 5.31 94.69 41.32 40.97 17.71 70.37

1.96 0.02 0.00 0.02 0.07 0.92 0.05 0.06 0.84 0.06 4.00 5.69 94.31 43.47 3.06 53.47 5.65

2.00 0.00 0.01 0.01 0.09 0.90 0.04 0.02 0.79 0.13 3.99 9.74 89.42 42.72 1.09 56.20 1.98

1.97 0.01 0.00 0.01 0.13 0.92 0.04 0.01 0.82 0.10 4.00 11.08 88.92 42.85 0.37 56.78 0.66

1.96 0.01 0.00 0.01 0.15 0.91 0.04 0.01 0.82 0.10 4.00 12.68 87.32 42.42 0.42 57.17 0.75

1.96 0.04 0.00 0.01 0.05 0.55 0.03 0.46 0.85 0.03 4.00 5.34 94.35 43.79 23.72 32.49 43.42

1.94 0.05 0.00 0.02 0.07 0.44 0.03 0.59 0.83 0.03 4.00 6.38 93.62 42.49 30.21 27.31 53.81

1.95 0.05 0.00 0.01 0.06 0.46 0.02 0.58 0.84 0.03 4.00 5.79 94.21 43.11 29.54 27.34 52.84

1.98 0.02 0.08 0.03 0.00 0.34 0.00 0.81 0.73 0.02 4.00 0.00 91.38 38.91 43.07 18.02 70.51

1.82 0.18 0.02 0.06 0.08 0.29 0.01 0.73 0.77 0.04 4.00 8.95 89.32 40.69 38.73 20.58 66.06

1.99 0.01 0.00 0.02 0.32 0.61 0.04 0.06 0.62 0.34 4.00 33.37 66.63 37.67 3.76 58.57 6.26

1.89 0.11 0.03 0.04 0.03 0.33 0.01 0.66 0.87 0.03 4.00 3.49 93.76 45.81 34.54 19.64 64.50

1.99 0.01 0.00 0.01 0.07 0.95 0.04 0.05 0.82 0.06 4.00 6.53 93.47 42.47 2.53 54.99 4.58

2.00 0.00 0.02 0.01 0.03 1.02 0.05 0.04 0.75 0.08 3.99 2.86 95.12 39.98 1.87 58.16 3.24

250 49.12 0.72 0.89 21.16 0.68 7.61 19.76 0.14 100.09

Formula based on Si 1.93 Aliv 0.04 vi Al 0.00 Ti 0.02 Fe+ 3 0.08 Fe+ 2 0.61 Mn 0.02 Mg 0.45 Ca 0.83 Na 0.01 Total 4.00 Ae 5.02 Aug 94.98 Wo 41.67 En 22.34 Fs 35.99 Mg# 39.07

N-256

BM

c

250 36.32 0.07 0.04 32.49 0.80 29.88 0.23 99.84

1.00 0.75 0.02 1.22 0.01 61.52 37.54 0.94

N-011

c

200 38.44 0.06 0.00 22.41 0.37 39.11 0.29 100.67

0.99 0.48 0.01 1.51 0.01 75.37 24.23 0.40 0.99 0.71 0.01 1.27 0.02 63.71 35.70 0.59 0.99 1.91 0.09 0.00 0.01 0.15 95.56 4.30 1.00 1.88 0.10 0.01 0.01 0.52 94.58 4.90 1.00 0.91 0.02 1.07 0.01 53.75 45.49 0.76 1.01 1.74 0.10 0.12 0.01 6.20 88.60 5.20 0.99 1.51 0.07 0.40 0.02 20.16 76.39 3.45 1.00 1.57 0.07 0.32 0.02 16.24 79.95 3.81 Formula calculated for 4 oxygens Si 0.96 1.01 1.93 1.83 Fe+ 2 Mn 0.11 0.12 Mg 0.02 0.01 Ca 0.02 0.02 Fo 0.77 0.70 Fa 93.78 93.42 Tp 5.45 5.89

Size (μ) SiO2 TiO2 Al2O3 FeO MnO MgO CaO Total

1.00 0.83 0.01 1.14 0.01 57.61 41.66 0.73

0.99 1.82 0.12 0.05 0.03 2.76 91.37 5.88

0.99 1.92 0.09 0.00 0.01 0.05 95.51 4.44

1.00 1.19 0.04 0.75 0.01 37.59 60.22 2.19

0.99 1.54 0.08 0.38 0.01 18.90 77.16 3.95

1.00 1.54 0.08 0.37 0.02 18.49 77.35 4.16

1.00 1.28 0.03 0.68 0.01 34.35 64.11 1.54

1.00 1.44 0.07 0.46 0.01 23.38 73.04 3.57

0.99 0.48 0.01 1.52 0.01 75.51 24.05 0.44 0.99 1.83 0.11 0.06 0.00 3.21 91.08 5.71 0.99 1.91 0.08 0.02 0.01 0.96 94.90 4.14

50 36.15 0.18 0.04 31.19 0.51 31.23 0.51 99.80

c

300 29.72 0.03 0.01 65.50 4.05 1.30 0.12 100.73 1500 32.02 0.12 0.03 55.05 2.66 9.89 0.42 100.19

750 37.86 0.00 0.07 22.17 0.40 39.05 0.25 99.80

c c

250 28.97 0.06 0.00 67.18 2.89 0.38 0.16 99.64

c

500 32.79 0.04 0.02 50.15 1.19 15.08 0.31 99.57 200 31.24 0.08 0.00 57.49 3.05 7.71 0.48 100.06

c c

800 30.89 0.03 0.08 57.30 2.89 7.87 0.42 99.48 750 33.30 0.03 0.01 47.41 1.70 16.61 0.30 99.36

c

250 29.08 0.04 0.04 67.32 2.99 0.06 0.37 99.90 200 29.40 0.03 0.00 65.93 3.37 0.20 0.31 99.25

c c c

1500 36.18 0.08 0.03 35.75 0.62 27.74 0.30 100.70

250 29.46 0.01 0.00 64.76 4.11 1.10 0.79 100.23

r

1500 35.51 0.05 0.05 38.64 0.64 25.61 0.34 100.85 200 30.39 0.09 0.01 62.38 3.61 2.45 0.28 99.22

c c

200 31.22 0.21 0.05 56.77 2.53 8.41 0.62 99.81 200 30.99 0.05 0.03 58.15 2.74 6.63 0.70 99.27

r Location

r c

300 28.03 0.03 0.00 67.19 3.86 0.31 0.43 99.85

Rock

300 29.92 0.06 0.00 65.15 4.05 0.27 0.45 99.89

c

c

300 28.83 0.03 0.00 66.97 3.08 0.02 0.31 99.23

c

c

B

N-258

B

N-220

T C

N-065 N-259

T T

N-036 N-233

T T

N-023 N-247

T

N-022

T

N-246

C

N-098

P CT

N-181 N-209

M M

N-209 N-239

CT T

N-136 N-136 N-188

P

Sample

N-188

T

Nemrut ignimbrite Pre-caldera rocks

Olivine, Table 5: Fe-Ti Oxides, Table 6: Amphibole, Table 7: Aengimatite). Nomenclature of pyroxenes and amphiboles has been realized according to Morimoto et al. (1988) and Leake et al. (1997) respectively. 6.1. Basalts

P

Kantaşı Ignimbrite

Post-caldera rocks

N-257

BM

H.E. Çubukçu et al. / Journal of Volcanology and Geothermal Research 209-210 (2012) 33–60 Table 4 Olivine analyses of Nemrut volcanic rocks selected from 210 analyses. Calculated for 4 oxygens. Rock names: T: Trachyte (Metaluminous), CT: Comenditic Trachyte, P: Pantellerite, C: Comendite, BM: Benmoreite, B: Basalt, M: Mugearite. Location:c – center, r – rim.

40

Basalts, confined to post-caldera historic eruption on the rift zone, are highly porphyritic (phenocryst abundance: 47–51 vol.%) with coarse, subhedral sometimes variolitic plagioclase (An66–36) and forsteritic olivine (Fo76–50) (Fig. 4a,c) phenocrysts which occasionally exhibit glomeroporphyritic texture due to synneusis. Groundmass is microcrystalline with pilotaxitic microlites of plagioclase, olivine, Fe-Ti oxides and rare clinopyroxene (Fig. 4b). 6.2. Mugearites Mugearites are dominantly porphyritic (6–29 vol.% phenocrysts.), with rare aphyric samples. Phenocrysts exhibit seriate texture and consist of plagioclase (An58–29), augitic clinopyroxene (Wo40En32Fs29–Wo33En41Fs16), olivine (Fo68–36) and Fe-Ti oxides (Fig. 4). Titanomagnetite is frequently oxy-exsolved to ilmenite and magnetite, exhibiting fine to coarse trellis-type exsolution (terminology after Buddington and Lindsley, 1964). Apart from the oxy-exsolved crystals, ilmenite (Ilm83–96) and magnetite (Mgn100–Usp30–69) coexist. Some plagioclase and to a lesser degree clinopyroxene and olivine phenocrysts display evident disequilibrium with occasional reverse zoning (Fig. 5a). Instable plagioclases are spongy or boxy cellular and frequently anhedral and embayed. Besides, subordinate axiolitic amphibole (ferrorichterite–edenite) occurrences have been observed to be restricted to the instable inner zones of decomposed plagioclase phenocrysts, originating from the plagioclase itself (Fig. 5b). On the other hand, euhedral plagioclase without disequilibrium textures is also observed. Groundmass has pilotaxitic to trachytic texture and consists of feldspar, clinopyroxene, olivine and Fe-Ti oxides. From their petrographical/mineralogical characteristics, it is clear that mugearites display evidence of heterogeneity due to probable magma mixing. Apatite is a common accessory phase. 6.3. Metaluminous trachytes Pre-caldera metaluminous trachyte lavas are porphyritic (20 vol.% phenocrysts) with seriate euhedral/subhedral plagioclase (An10–29) and anorthoclase (Or18–41). Besides, calcic plagioclase (An57–68) (Fig. 4) having disequilibrium textures is also present. Clinopyroxene is dominantly augite and magnesian phenocrysts (Mg# N 40) frequently display resorption and disequilibrium textures with irregular Mg-rich intra-crystal patches and reverse zoning (Fig. 5c). Olivine varies from Fo31 to Fo5 while xenocrysts with Fa75–80 is also present. Both magnetite (Mag99) and titanomagnetite (Usp71–57) are observed. Caldera forming pyroclastic activity is represented by metaluminous trachytic Nemrut and Kantaşı ignimbrites, which exhibit petrographical and mineralogical variation from bottom to the top of the sequence. Fallout underlying the Nemrut ignimbrite is pale yellow/white in color with white pumice at the bottom whereas it is reddish to black in color and comprised of dark colored pumice at the top. Stratigraphically lowermost pale colored plinian air fall bears euhedral anorthoclase (Ab65–67An0Or33–35), clinopyroxene Wo43En0Fs57, olivine (Fa95Tp5) and ilmenite. Dark colored fallout bears spongy anhedral anorthoclase (Ab63–69An9–22Or10–28), clinopyroxene (Wo44En23Fs23), olivine (Fo19Fa77Tp4) and titanomagnetite (Usp73). Nemrut Ignimbrite flow unit exhibits eutaxitic texture with flattened pumice/glass shards and occasionally contains xenoliths of older rocks. Nemrut ignimbrite bears occasionally spongy cellular oligoclase and anorthoclase (Ab46An1–42Or3–50), slightly-to-moderately resorbed

H.E. Çubukçu et al. / Journal of Volcanology and Geothermal Research 209-210 (2012) 33–60

41

Table 5 Magnetite and ilmenite analyses of Nemrut volcanic rocks selected from 190 analyses. Rock names: T: Trachyte (metaluminous), CT: Comenditic Trachyte, P: Pantellerite, C: Comendite, BM: Benmoreite, B: Basalt, M: Mugearite. Location:c – center, r – rim. Magnetite Pre-caldera rocks

Nemrut ign.

Kantaşı ign.

Post-caldera rocks

Sample

N-141

N-184

N-136

N-209

N-016

N-047

N-023

N-036

N-259

N-011

N-256

N-258

Rock

T

P

T

T

M

T

T

T

T

BM

BM

B

Location

c

c

c

c

c

c

c

c

c

c

c

c

Size TiO2 Al2O3 FeO Fe2O3 MnO MgO Total

250 25.25 0.47 53.76 17.59 1.12 0.02 98.21

250 3.33 0.09 32.01 62.69 2.28 0.04 100.44

10 21.18 0.41 48.87 26.18 1.17 0.22 98.02

1300 24.79 2.10 50.30 19.21 0.56 2.35 99.30

250 3.40 5.42 32.27 56.13 0.17 1.66 99.05

1000 24.29 1.32 51.26 20.04 1.44 0.64 99.00

150 25.78 1.40 52.30 17.27 1.29 0.94 98.98

30 23.36 1.47 48.27 21.54 0.85 1.21 96.69

200 2.21 2.66 29.05 61.95 2.35 1.36 99.58

100 22.45 2.16 47.61 22.18 0.54 2.35 97.29

100 1.39 0.53 31.50 58.72 0.16 1.27 93.58

50 24.84 2.55 50.87 16.02 0.72 1.79 96.80

and 4 oxygens 0.10 0.61 0.00 0.02 1.02 1.56 1.80 0.75 0.07 0.04 0.00 0.01 3.00 2.99 9.77 61.42

0.68 0.09 1.54 0.53 0.02 0.13 2.99 68.73

0.09 0.24 1.00 1.56 0.01 0.09 2.99 9.74

0.68 0.06 1.61 0.57 0.05 0.04 3.00 68.78

0.72 0.06 1.63 0.49 0.04 0.05 3.00 72.62

0.66 0.06 1.51 0.61 0.03 0.07 2.94 66.27

0.06 0.12 0.91 1.75 0.07 0.08 2.99 6.59

0.63 0.09 1.48 0.62 0.02 0.13 2.97 63.13

0.04 0.02 1.03 1.73 0.01 0.07 2.91 11.87

0.69 0.11 1.58 0.45 0.02 0.10 2.95 71.78

Formula based on 3 cations Ti 0.72 Al 0.02 +2 Fe 1.70 Fe+ 3 0.50 Mn 0.04 Mg 0.00 Total 2.97 Usp% 73.64 Ilmenite Pre-caldera rocks

Nemrut ignimbrite

Post-caldera rocks

Sample

N-188

N-016

N-080

N-227

N-098

N-022

N-047

N-268

N-036

N-220

N-146

N-255

Rock

P

M

CT

C

P

T

T

T

T

T

C

C

Location

r

c

c

c

c

r

c

c

c

c

c

c

Size TiO2 FeO Fe2O3 MnO MgO Total

150 51.80 44.64 1.78 1.99 0.01 100.22

300 45.42 32.85 16.38 1.19 3.88 99.72

150 51.75 44.48 2.60 1.91 0.08 100.82

50 50.58 43.49 3.28 1.98 0.00 99.32

150 52.15 45.16 0.92 1.73 0.00 99.96

50 51.61 44.85 1.78 1.58 0.00 99.82

1000 52.46 43.28 1.91 1.83 1.18 100.66

200 52.18 45.22 0.78 0.75 1.75 100.68

50 53.20 43.21 0.81 0.72 2.09 100.03

200 52.93 43.82 0.17 3.72 0.00 100.65

50 50.34 43.69 2.86 1.35 0.12 98.37

50 50.71 43.94 2.99 1.54 0.06 99.25

0.97 0.93 0.06 0.04 0.00 2.00 96.832

0.99 0.95 0.02 0.04 0.00 2.00 99.048

0.98 0.95 0.03 0.03 0.00 2.00 98.271

0.98 0.90 0.04 0.04 0.04 1.99 98.002

0.98 0.94 0.04 0.02 0.06 2.04 101.866

1.00 0.90 0.00 0.02 0.08 1.99 99.680

1.00 0.92 0.00 0.08 0.00 2.00 99.837

0.96 0.93 0.05 0.03 0.00 1.97 96.273

0.97 0.93 0.06 0.03 0.00 2.00 97.049

Formula based on 2 cations Ti 0.98 Fe+ 2 0.94 +3 Fe 0.03 Mn 0.04 Mg 0.00 Total 2.00 Ilm% 98.206

and 3 oxygens 0.84 0.97 0.68 0.93 0.30 0.05 0.02 0.04 0.14 0.00 1.99 2.00 84.364 97.489

clinopyroxene (Wo42En1–36Fs21-55, Mg#:2–64), olivine (Fo12–40Fa58–83 Tp2–5) and titanomagnetite (Usp71-94) with trellis type exsolution into ilmenite and magnetite. Dark colored plinian fallout overlying Nemrut Ignimbrite and underlying Kantaşı Ignimbrite comprised of subhedral andesine (An29–43), augite (Wo43En30Fs27, Mg#:53), olivine (Fo35Fa63Tp2) and titanomagnetite (Usp67–72). Kantaşı Ignimbrite is unwelded-to-welded with occasional eutaxitic texture with anorthoclase (Ab62–70An2–33Or4–34), augite (Wo43–44En26–34Fs23–30 Mg#:46– 60), olivine (Fo24Fa73Tp3) and titanomagnetite (Usp26–100) with trellistype exsolution into ilmenite and magnetite. Apatite is observed as an accessory phase in the pyroclastic sequence. 6.4. Peralkaline trachytes and rhyolites Pre-caldera comenditic and pantelleritic trachyte lavas are hypocrystalline (11–36 vol.% phenocrysts), with frequent vesicular flow

bands. Their typical mineral assemblage consists of euhedral/subhedral alkali feldspar (Ab53–72An0–9Or21–44), augite (Wo31–46En1–34 Fs20–67 Mg#:2–64), olivine (Fo1–31Fa66–94Tp3–7), titanomagnetite (Usp62–84) and ilmenite (Fig. 4). Microphenocrysts/microlites of aegirine–augite (Ae80–20) and microlites of aegirine (Ae N 80 mol%) are omnipresent in comenditic trachyte and pantellerite lavas. Precaldera comenditic trachytes occasionally contain sodic (arfvedsonite) and sodic–calcic amphibole (ferrorichterite/ferrowinchite) microphenocrysts. The occurrence of sodic amphiboles indicates the equilibrium at low temperature (b750 °C) and low oxygen fugacity conditions in a H2O rich magma (Scaillet and Macdonald, 2001). Aenigmatite, a common mineral of peralkaline rocks, is present in pre-caldera peralkaline trachytes and rhyolites. Apatite and quartz are accessory phases. Peralkaline rhyolite lavas are poorly porphyritic, hypo/holohyaline and sometimes observed as completely aphyric obsidian. Flow

42

H.E. Çubukçu et al. / Journal of Volcanology and Geothermal Research 209-210 (2012) 33–60

Table 6 Selected amphibole analyses of Nemrut volcanic rocks. Nomenclature after Leake et al. (1997). Rock names: T:Trachyte (Metaluminous), CT: Comenditic Trachyte, P: Pantellerite, C: Comendite, BM: Benmoreite, M: Mugearite. Location: c – center, r – rim. Mineral Names: Arf: Arfvedsonite, Fwc: Ferrowinchite, Frch: Ferrorichterite, Rbc: Riebeckite, Kaer: Kaersutite, Tsch: Tschermakite, Hst: Hastingsite, Fhb: Ferrohornblende, Fac: Ferroactinolite. Pre-caldera rocks

Post-caldera rocks

Sample

N-149

N-149

N-239

N-083

N-209

N-227

N-227

N-227

N-220

N-256

N-256

N-050

N-050

N-050

N-050

Rock

C

C

CT

CT

M

C

C

C

CT

BM

BM

BM

BM

BM

BM

Location

c

c

c

c

c

c

c

c

r

c

c

c

c

c

c

Size (μ) SiO2 TiO2 Al2O3 FeOt MnO MgO CaO Na2O K2O Total

750 49.64 0.28 0.21 34.55 1.52 0.01 2.06 7.22 1.21 96.69

300 49.26 0.58 0.15 34.04 1.64 0.08 2.79 6.87 1.42 96.85

75 50.26 3.49 0.50 30.13 0.91 1.72 3.74 4.08 1.45 96.28

90 52.92 1.74 14.46 13.49 0.63 0.77 0.72 4.43 4.33 93.47

20 55.89 1.81 11.58 15.49 0.24 2.48 4.30 4.18 3.51 99.47

200 50.07 0.39 0.21 33.78 1.23 0.00 1.47 7.90 1.40 96.45

200 50.35 0.36 0.19 34.19 1.10 0.00 1.64 5.18 1.44 94.49

400 52.07 0.67 0.21 31.57 1.20 0.02 1.00 6.40 1.57 94.75

400 49.68 0.78 0.90 29.31 1.14 4.65 3.78 6.17 1.18 97.59

25 40.81 5.11 8.30 18.79 0.26 9.53 10.84 2.23 0.73 96.61

30 40.74 3.88 8.93 18.05 0.10 10.48 9.63 2.23 0.74 94.79

50 39.19 3.70 10.40 21.42 0.52 7.54 9.58 2.63 0.80 95.78

30 43.48 1.46 7.68 19.53 0.77 8.15 14.88 0.95 0.26 97.17

50 50.67 0.77 6.68 16.22 0.93 6.53 12.37 0.80 1.15 96.11

50 42.44 1.95 9.42 19.98 0.63 9.47 10.21 2.34 0.67 97.10

7.97 0.03 0.00 8.00

8.17 0.00 0.00 8.17

8.28 0.00 0.00 8.28

8.15 0.00 0.00 8.15

8.14 0.00 0.00 8.14

8.43 0.00 0.00 8.43

7.71 0.16 0.09 7.97

6.27 1.50 0.22 8.00

6.23 1.61 0.16 8.00

6.08 1.90 0.02 8.00

6.83 1.17 0.00 8.00

7.92 0.08 0.00 8.00

6.39 1.61 0.00 8.00

0.06 0.42 0.32 0.41 3.67 0.12 5.00

2.63 0.20 0.00 0.18 1.74 0.08 4.83

2.02 0.20 0.00 0.55 1.92 0.03 4.72

0.04 0.05 0.28 0.00 4.32 0.17 4.85

0.04 0.04 1.08 0.00 3.55 0.15 4.86

0.04 0.08 0.25 0.01 4.02 0.16 4.57

0.00 0.00 0.86 1.08 2.94 0.12 5.00

0.00 0.37 0.39 2.18 2.02 0.03 5.00

0.00 0.29 1.05 2.39 1.26 0.01 5.00

0.00 0.41 0.93 1.74 1.85 0.07 5.00

0.25 0.17 0.00 1.91 2.57 0.10 5.00

1.15 0.09 0.00 1.52 2.12 0.12 5.00

0.06 0.22 0.98 2.12 1.53 0.08 5.00

0.00 0.00 0.00 0.64 1.25 1.89

0.00 0.00 0.00 0.12 1.32 1.44

0.00 0.00 0.00 0.68 1.20 1.88

0.00 0.00 0.00 0.26 1.74 2.00

0.00 0.00 0.00 0.28 1.62 1.91

0.00 0.00 0.00 0.17 1.83 2.00

0.00 0.00 0.03 0.63 1.34 2.00

0.00 0.00 0.00 1.78 0.22 2.00

0.00 0.00 0.00 1.58 0.42 2.00

0.00 0.00 0.00 1.59 0.41 2.00

0.00 0.00 0.00 2.50 0.00 2.50

0.00 0.00 0.00 2.07 0.00 2.07

0.00 0.00 0.00 1.65 0.35 2.00

0.00 0.29 0.29 15.18 Fwc

0.00 0.85 0.85 15.30 Frch

0.00 0.66 0.66 15.54 Frch

0.75 0.29 1.04 16.04 Arf

0.00 0.30 0.30 15.20 Rbc

0.18 0.32 0.51 15.51 Arf

0.52 0.23 0.75 15.72 Frch

0.45 0.14 0.59 15.59 Kaer

0.24 0.14 0.38 15.38 Tsch

0.38 0.16 0.54 15.54 Hst

0.29 0.05 0.34 15.85 Fhb

0.24 0.23 0.47 15.54 Fac

0.33 0.13 0.46 15.46 Tsch

Formula calculated for 23 oxygens T Si 8.04 8.02 iv Al 0.00 0.00 Ti 0.00 0.00 SumT 8.04 8.02 C vi Al 0.04 0.03 Ti 0.03 0.07 Fe+ 3 0.58 0.35 Mg 0.00 0.02 Fe+ 2 4.09 4.28 Mn 0.21 0.23 SumC 4.96 4.98 B Mg 0.00 0.00 Fe+ 2 0.00 0.00 Mn 0.00 0.00 Ca 0.36 0.49 Na 1.64 1.51 SumB 2.00 2.00 A Na 0.62 0.65 K 0.25 0.30 SumA 0.87 0.95 Sum 15.87 15.95 Name Arf Arf

banding and micro-foliations are common. Post-caldera comendites and some pre-caldera counterparts bear benmoreitic enclaves which occur as millimeter to decimeter sized blobs or diffused droplets with irregular margins. Petrographical features indicate strong evidence for mixing of rhyolitic magma with the mafic melt. Post-caldera phreatomagmatic eruptions from intra-caldera maars or explosion craters produced base surge deposits in which bread-crust bombs and juvenile pumice are common and reach up to several decimeters in size. Besides, centimeter-to-decimeter sized holocrystalline xenolithic fragments, whose dimensions reach up to block size in intra-caldera Göl Tepe maar, are observed as ejecta in base surge deposits. Peralkaline rhyolites contain sub/euhedral anorthoclase (Ab58–74), augite (Wo38–45En0–13Fs43–61, Mg#: 0–24), olivine (Fo0–2Fa93–96Tp4–5) and ilmenite (Ilm96–100). Ca-Na pyroxenes are observed only in pre-caldera comendites as aegirine–augite and pure aegirine. Precaldera comendites occasionally bear sodic amphibole (arfvedsonite and riebeckite) microphenocrysts and coronas, usually formed along the rims of decomposed fayalitic olivine. Post-caldera comendites contain REE-Ti silicate chevkinite-group [(Ce, La, Ca)4- (Fe2+, Mg)2 (Ti, Fe3+)3 Si4O22] mineral as inclusions in fayalite and as free crystals along the

rims of fayalite and ilmenite. Zircon and apatite are occasionally found together with chevkinite (Fig. 5d).

6.5. Benmoreitic enclaves Comendites of post-caldera and latest pre-caldera effusive stages bear significant enclaves with benmoreitic compositions. They occur as millimeter to decimeter-sized blobs or diffused droplets with irregular rounded margins. Dispersed phenocrysts and/or phenocrystic cumulates surrounded by mafic matrix are occasionally observed in rhyolitic glass (Fig. 5e,f). These textural properties indicate the interminglement of mafic and felsic magmas. Between enclave and comendite glass, significant interface zone is formed probably because of thermal difference between the inclusion and the host (Fig. 5e). The interface between host and the enclave is sometimes represented by two distinct sub-zones: 1) fine grained microcrystalline margin, composed of mafic minerals of those in the enclaves and, 2) highly vesicular pumiceous aphyric glass in contact with the felsic host.

H.E. Çubukçu et al. / Journal of Volcanology and Geothermal Research 209-210 (2012) 33–60 Table 7 Representative analyses of aenigmatite in Nemrut volcanic units. Aenigmatite Sample N-149

N-151

N-098

N-080

N-188

Rock

Comendite Comendite Pantellerite Comenditic trachyte Pantellerite

SiO2 TiO2 Al2O3 FeO Fe2O3 MnO MgO CaO Na2O K2O Total Si Ti Al Fe2 Fe3 Mn Mg Ca Na K

41.01 8.49 0.40 38.83 1.01 1.48 0.05 0.33 6.91 0.07 98.59 5.96 0.93 0.07 4.72 0.11 0.18 0.01 0.05 1.95 0.01

40.78 6.86 0.24 37.03 6.25 1.04 0.10 0.20 7.05 0.09 99.63 5.89 0.75 0.04 4.47 0.68 0.13 0.02 0.03 1.97 0.02

40.77 7.84 1.18 36.64 4.03 0.94 0.06 0.62 7.16 0.25 99.47 5.85 0.85 0.20 4.40 0.44 0.11 0.01 0.10 1.99 0.05

41.35 6.16 0.13 38.01 5.30 1.05 0.05 0.22 6.90 0.02 99.19 6.00 0.67 0.02 4.61 0.58 0.13 0.01 0.03 1.94 0.00

40.51 6.71 0.27 35.94 6.53 1.04 0.00 0.31 7.14 0.13 98.57 5.90 0.74 0.05 4.38 0.72 0.13 0.00 0.05 2.02 0.02

43

Alkali feldspar is observed to mantle plagioclase (Table 2). Subordinate axiolitic calcic amphibole is observed within the embayed inner portions of decomposed Ca-rich plagioclase (Fig. 5b). Clinopyroxene is euhedral to subhedral, whereas olivine is often embayed and display opaque rims. These mafic-intermediate enclaves can be interpreted in several ways: The fragments of minor intermediate melts which form the interface between mafic and silicic magmas in a zoned magma chamber (Peccerillo et al., 2003), the results of the intrusion of a fresh batch of basaltic magma into the more felsic part of a stratified magma chamber or they reflect fragmentation of wall-rocks, which are the earlier crystallization products of the magma chamber system, during a caldera collapse (Ferla and Meli, 2006).The textural relationships observed in Nemrut rocks strongly testify to the concurrent presence of mafic and silicic melts and their interaction, a common feature of well-known bimodal peralkaline associations. 7. Geochemistry Results of geochemical analyses including major, trace elements and calculated CIPW norms have been given in Table 8 (Supplementary data). 7.1. Major elements

Disequilibrium textures and xenocrysts are often observed. These enclaves are moderately porphyritic with phenocrysts (36–40 vol.%) of ternary feldspar, clinopyroxene and olivine. Feldspars occasionally exhibit extreme cellular skeletal texture with compositions An37–57.

Basalts are nepheline normative, and transitional to mildly alkaline. Mugearites are silica saturated with normative hypersthene up to 14%. Excluding two lava samples (ne-normative N-136 and marginally peralkaline N-238) all the trachytes and rhyolites of Nemrut volcanism are silica oversaturated with quartz in the norms. The degree of peralkalinity increases with increasing SiO2 content. However, we note a slight decrease in P.I. as the SiO2 content exceeds 73% in

Fig. 4. Summary of microprobe data of Nemrut volcanic rocks. a) Ab-An-Or diagram for feldspars, b) Wo-En-Fs diagram for clinopyroxenes (Morimoto et al., 1988), c) Tp-Fo-Fa diagram for olivines d) TiO-FeO-Fe2O3 diagram for Fe-Ti oxides.

44

H.E. Çubukçu et al. / Journal of Volcanology and Geothermal Research 209-210 (2012) 33–60

Fig. 5. a) Reversely zoned, instable clinopyroxene in mugearite. EDS profile of Mg, Fe and Al concentrations between points A and B is depicted. b) Subordinate axiolitic amphibole (amp) crystallization in intracrystal ponds of instable plagioclase (plg). Axiolitic amphibole crystallizes from plagioclase c) Reversely zoned clinopyroxene in metaluminous trachytes. EDS profile of Mg and Fe concentrations between points A and B is shown. d) EDS generated compositional map of REE-Ti silicate chevkinite (chvk) with clinopyroxene (cpx), apatite (ap) and zircon (zrn) in post-caldera vitric comendite. e) Benmoreitic enclave and blebs in pumicified vitric comendite. Microcrystalline interface between host and the enclave is evident. Plane polarized light. E) Backscattered Electron Image (BSE) of mingling of rhyolitic melt with benmoreitic melt.

post-caldera comendites (Comendite-2) compared to those of precaldera counterparts. According to the Harker variation diagrams of major elements (Fig. 6), a continuous decrease in MgO, TiO2, CaO, Fe2O3 is observed. Al2O3 displays slight decrease from basalts towards mugearites and increases towards caldera related trachytes, sharply decreasing afterwards. Na2O positively correlates with silica but decreases significantly towards rhyolites, exhibiting significant scatter in benmoreitic and trachytic compositions. K2O increases with increasing silica but remains constant in rhyolites. MnO exhibit scatter with respect to silica and trachytic lavas with 61–67% SiO2 bear the highest content (N0.2%), whereas metaluminous pyroclastic products fits to the general linearly decreasing trend. However, MnO sharply decreases after SiO2 exceeds 70%.

7.2. Trace elements 7.2.1. Compatible elements Trace element analyses of Nemrut volcanism are given in Table 8. Concentrations of compatible trace elements versus SiO2 are plotted in Fig. 7. Sr, Ni, and V exhibit significant depletion from basalt to rhyolite. However, Ni and Cr contents of basalts depict that they are not primitive (Ni: ~ 50 ppm, Cr: ~60 ppm) and that they have experienced some degree of olivine fractionation en route to the surface. Ba increases from basalts to mugearites, to metaluminous and marginally peralkaline trachytes. Trachytic pumice and post-caldera trachytic ejecta of phreatomagmatic eruptions show the highest concentrations of Ba followed by metaluminous trachytic lavas and some peripheral trachytic domes. Ba is almost depleted in peralkaline

H.E. Çubukçu et al. / Journal of Volcanology and Geothermal Research 209-210 (2012) 33–60

45

Fig. 6. Major element concentrations versus SiO2 for Nemrut volcanic rocks.

rhyolites and trachytes. Such enrichment of Ba may be due to feldspar accumulation (White et al., 2009) and/or significant crustal contamination in syn-caldera trachytic pumice, followed by abrupt depletion due to feldspar fractionation. Pre-caldera rocks span from mugearite to peralkaline rhyolite with evident scatter in trace element contents, whereas strongly bimodal post-caldera rocks do not exhibit such dispersion of compatible elements. 7.2.2. Incompatible elements Correlations of various incompatible elements versus highly incompatible Th define general linear trends especially for pre-caldera stage (Fig. 8). The non-zero intercepts on bivariate plots of incompatible elements indicate AFC processes (Powell, 1984). Slight-tomoderate scattering of pre-caldera samples may be the result of varying degrees of crustal contamination. Besides, post-caldera basalt and comendite (including post-caldera benmoreitic enclaves) define another lineation than that of the pre-caldera stage. Basalts are significantly deviate from general linear trend, exhibiting enrichment in incompatible elements. This might be due to the evolved (and contaminated) nature of post-caldera basalts, compared to probable mafic parents of pre-caldera stage. The most differentiated comendites of pre-caldera stage have higher concentrations of incompatible HFSE compared with the post-caldera counterparts (Comendite-2). 7.2.3. Multi-element patterns In the N-type MORB normalized (Hart et al., 1999; Ta from Hofmann, 1988) multi element spider diagrams (Fig. 9), the most primitive samples of Nemrut volcanism, the post-caldera basalts exhibit

enrichment in LIL elements (Sr, K, Rb, Ba, Th) relative to N-MORB abundances. Rb and Th present a humped pattern with Ba and Nb troughs. The absence of Ta and Ti troughs accompanying Nb indicates a lesser contribution from a subduction component in the mantle source or crustal assimilation. Mugearite defines marked negative anomalies of Nb, Sr and Ti. Trachytes emphasize the systematic enrichment of incompatible elements with significant negative anomalies in Ba, Sr, P and Ti from metaluminous to peralkaline trachytes. Pre-caldera comendites display the depletion of these elements. Fractional crystallization of feldspars, Ti-oxides and apatite would yield such trend. 7.3. Rare earth elements Chondrite normalized (Nakamura, 1974) rare earth element (REE) patterns for post-caldera basalt are slightly LREE (light–REE) enriched (Fig. 10). LREE/HREE fractionation is weak with [La/Yb]N ratio is nearly 3. HREE (heavy–REE) abundances are ~20 times chondritic indicating the absence of residual garnet in the source. Mugearites are relatively LREE enriched with [La/Yb]N ratio is approximately 5. Trachytes exhibit significant depletion in Eu from metaluminous to peralkaline samples. [La/Yb]N in trachytes varies between 4.5 and 8 comparable to the trachytes of Eburru volcanic complex, Kenya (Ren et al., 2006). Pre-caldera comendites define a significant Eu depletion and [La/Yb]N is 3.5–5.5, lower than the comendites of Gedemsa Volcano, Central Ethiopian Rift (Peccerillo et al., 2003) Trachytic caldera forming pyroclastic units exhibit mixed behavior for Eu. However, plinian fall underlying the Nemrut

46

H.E. Çubukçu et al. / Journal of Volcanology and Geothermal Research 209-210 (2012) 33–60

Fig. 7. Compatible trace element concentrations versus SiO2 for Nemrut volcanic rocks.

ignimbrite (sample N-022) bears Eu trough indicating its fractionated nature. Plinian fall overlying the Nemrut ignimbrite exhibits similar patterns without a significant Eu anomaly. Juvenile pumices of Nemrut ignimbrite exhibit Eu enrichment. For the caldera forming pyroclastics [La/Yb]N is ~ 5, lower than Gedemsa ignimbrites (Peccerillo et al., 2003). In post-caldera comendites (Comendite-2) Eu depletion is significant with [La/Yb]N being approximately 4. In holocrystalline granular ejecta negative Eu anomaly has not been observed, however porphyritic ejecta define a positive Eu spike. Ejecta have weakly enriched LREE with [La/Yb]N is 5. Post-caldera comendites have only ~ 4 times chondritic Eu, whereas pre-caldera comendites reach up to 21 times chondritic Eu abundances. Rare earth element patterns of felsic rocks of Nemrut volcanism lie sub-parallel to the patterns of basic rocks and have large Eu anomalies, typical of amphibole-free POAM (plagioclase olivine clinopyroxene magnetite) fractionation. A plot of Ce/Yb versus Ce (after Xu et al., 2010) depict that fractional crystallization processes govern the magmatic evolution both in pre- and post-caldera stages. Mafic parental magmas of each stage might have undergone different degrees of partial melting in pre- and post-caldera stages (Fig. 11).

7.4. Sr and Nd isotopes Sr isotopic ratios have been measured for basaltic rocks only and range from 0.70350 to 0.70591 (Table 9). The vast majority of trachytes and rhyolites have very low concentrations of Sr (b15 ppm) as they would have been highly susceptible to the effects of contamination. Hence, they are not analyzed for their 87Sr/ 86Sr ratios. However, Sr isotopic ratios from literature (Gülen, 1984; Notsu et al., 1995) have been normalized to the standards and their obtained average values used in this study. 87 Sr/ 86Sr isotopic ratios of basalts (SiO2 b 48%) are between 0.70350 and 0.70356, whereas their 143Nd/ 144Nd ratios plot between 0.512881 and 0.512897. Mugearites (48 b SiO2 b 55%) have 87Sr/ 86Sr ratios of 0.70591 and 0.70583. However, one mugearite sample has a much lower 143Nd/ 144Nd ratio than the basalts: 0.512637. Two distinct 87Sr/ 86Sr variation trends are identified from mugearite (and hawaiites of Gülen, 1984) to rhyolite. The three pre-caldera samples (trachyte and rhyolite) of Notsu et al. (1995) exhibit an increase in 87 Sr/ 86Sr isotopic ratios, which plot between 0.70665 and 0.70823. On the contrary, the three post-caldera rhyolites of Notsu et al.

H.E. Çubukçu et al. / Journal of Volcanology and Geothermal Research 209-210 (2012) 33–60

47

Fig. 8. Incompatible trace element concentrations versus Th for Nemrut volcanic rocks.

(1995) are within a relatively narrow interval of 0.70457–0.70522. For 143Nd/ 144Nd, pre-caldera trachytes define curvilinearly decreasing trend from post-caldera basalts. Moreover, pre-caldera rhyolites and mugearite (N-016) have similar 143Nd/144Nd ratios. Post-caldera comendites represent higher 143Nd/ 144Nd ratios compared to the pre-

caldera counterparts. This testifies that post-caldera comendites have been subjected to relatively lesser degrees of crustal contamination. Variations of 87Sr/ 86Sr and 143Nd/ 144Nd isotopic ratios with respect to SiO2 (Fig. 12a, b) rule out an evolution from post-caldera basalt to pre-caldera mugearite via only crystal fractionation. Besides,

48

H.E. Çubukçu et al. / Journal of Volcanology and Geothermal Research 209-210 (2012) 33–60

Fig. 9. Multielement spider diagrams normalized to N-MORB (Hart et al., 1999; Ta from Hofmann, 1988).

striking discrepancy in Sr and Nd isotopic ratios is evident between post-caldera and pre-caldera rhyolites indicating the different paths responsible for their evolution. Moreover, during the construction of the volcano and its magmatic chamber in pre-caldera stage, the degree of crustal contamination might have been higher than that of in post-caldera stage. Starting from mugearite; pre-caldera silicic isotopic ratios could be obtained via fractional crystallization with limited crustal assimilation. However, intermediate compositions (trachytic ejecta and least evolved trachyte, N-239) could indicate probable interaction between basaltic and felsic melts, as evidenced earlier by petrographical–mineralogical observations. 8. Modeling and source characteristics 8.1. Geochemical modeling of fractional crystallization 8.1.1. Whole rock major element modeling Major element data have been tested to reproduce similar trends of fractional crystallization by using both least squares mass balance

estimations (Bryan et al., 1969). The aim of the least squares analysis is to determine whether an assumed residual liquid might be derived from assumed parent magma by removal or addition of components which have the compositions of mineral phases or contaminants (Bryan et al., 1969). Since a well-constrained assumption of two end-members is the key for a reasonable calculated trend, representative samples have been chosen according to their mineralogical and geochemical properties. Removed phases are compiled from the microprobe data of each sample (Table 10). Post-caldera basalts (Sample N-258) have been tested as starting composition for both pre- and post-caldera magmatic evolution. For pre-caldera series, fractional crystallization processes from N-016 mugearite to marginally peralkaline (P.I.: 1.02) N-281 trachyte (Step-1) and then to highly evolved (P.I.: 1.26) pre-caldera glassy N-092 comendite (Step-2) have been modeled. Pre-caldera pantellerites are not incorporated into mass balance calculations as they exhibit subsolidus crystallization under low temperature, relatively oxidized conditions. For the post-caldera series, fractional crystallization processes from N-258 basalt to N-013 benmoreite (Step-A) and then to N-051 comendite (Step-B) have been tested. Although

H.E. Çubukçu et al. / Journal of Volcanology and Geothermal Research 209-210 (2012) 33–60

49

Fig. 10. Rare earth element concentrations normalized to chondrite (Nakamura, 1974).

benmoreites are observed only as enclaves in Nemrut volcanism, they are incorporated into models as they represent the intermediate compositions for the evolution from basic to felsic melts (e.g. Macdonald et al., 2008). Mass balance models (Table 10) indicate that 50% (Melt Fraction, F = 0.5) crystallization of plagioclase, olivine and magnetite from N258 basalt does not yield pre-caldera mugearitic compositions satisfactorily (ΣR 2 = 1.61) verifying that the parental basaltic magmas for 80–100 ka mugearites differ from historic (1441–1597 BC) basalts. For the pre-caldera series, 72% (F = 0.28) removal of plagioclase

(60%), clinopyroxene (20%), olivine (5%) and accessory phases (magnetite, ilmenite and apatite) from N-016 mugearite leads to a composition similar to a marginally peralkaline trachytic N-281 composition (Step-1). Further 60% crystallization (F = 0.40) of an assemblage of plagioclase (84%) with clinopyroxene (9%), olivine (5%) and titanomagnetite (2%) reproduces comenditic compositions comparable to sample N-092 (Step-2). For post-caldera series, from a basaltic parent (N-258), % 56 crystallization (F = 0.44) of an assemblage comprised of plagioclase (54%), olivine (23%), titanomagnetite (16%) and apatite (7%) yields

50

H.E. Çubukçu et al. / Journal of Volcanology and Geothermal Research 209-210 (2012) 33–60

Fig. 11. Ce/Yb versus Ce plot (after Xu et al., 2010) for Nemrut volcanic rocks indicating the trends for partial melting and fractional crystallization.

a composition similar to N-013 benmoreite (Step-A). Moreover, % 71 fractionation (F = 0.29) of plagioclase (31%), alkali feldspar (28%), clinopyroxene (21%), olivine (11%), titanomagnetite (7%) and ilmenite (1%) leads to a post-caldera comendite composition represented by sample N-051 (Step-B). ΣR 2 for all steps are bb1, hence acceptable, excluding the post-caldera basalt–pre-caldera mugearite calculation (ΣR 2 = 1.61). 8.1.2. Trace element modeling In order to test the trace element variation during magmatic evolution steps (Steps 1 and 2 for pre-caldera, Steps A and B for post-caldera stages), the same parent–daughter pairs and the proportions of fractionating assemblages obtained from the major element mass balance models (Table 10) have been used. Rayleigh fractionation equation (Arth, 1976) has been applied for fractional crystallization (FC). The effects of crustal assimilation (AFC) have been simulated using the equations of De Paolo (1981) and Powell (1984), where upper crustal contaminant is from Taylor and McLennan (1985). Bulk partition coefficients are calculated according to the calculated assemblages for each step with the Nernst coefficients (compiled from Villemant et al., 1981; Fujimaki, 1986; McKenzie and O'Nions, 1991; Caroff et al., 1993; D'Orazio et al., 1998; White et al., 2003). Trace element (HFSE and REE) modeling results of AFC processes for pre- and post-caldera stages have been depicted in Fig. 13. The degrees of crustal contamination (r) are 0.4 and 0.2 for pre- and postcaldera stages respectively. For pre-caldera stage, ~60% fractionation of the suggested assemblage in Table-10 from N-016 mugearite (Step-1) produces trace element compositions similar to marginally peralkaline trachytes (N-281) (Fig. 13a, b). Further ~65% crystal fractionation of alkali feldspar with minor clinopyroxene, olivine and titanomagnetite (Step-2, Table-10) yields highly evolved pre-caldera comedite (N-092, Fig. 13a, c). For post-caldera stage, ~ 50% fractionation of an assemblage proposed in Table-10 (Step –A) from post-caldera basalt (N-258) reproduces trace element concentrations similar to that of benmoreitic enclaves (N-013, Fig. 13a, d). Consequently, ~ 60% removal of a feldspar rich assemblage (Step-B) from a benmoreitic parent (N-013) produces trace element concentrations of postcaldera comendites represented by sample N-051 (Fig. 13a, e). Calculated REE trends are consistent with observation.

8.2. Source characteristics Covariation diagram for 87Sr/ 86Sr and 143Nd/ 144Nd isotopic ratios for Nemrut basalts (Fig. 14) has been plotted, on which East Anatolian volcanics (Gülen, 1984; Pearce et al., 1990), Afar, Main Ethiopian Rift (Hart et al., 1989; Deniel et al., 1994; Trua et al., 1998) and Kenya Rift (Naivasha, Davies and Macdonald, 1987) has also been shown for comparison. Nemrut basalts, as well as other east Anatolian volcanics clearly plot within OIB field as East African volcanics (Afar plume, Main Ethiopian Rift and Naivasha of Kenya Rift). The composition of average granitic upper crust (Wedepohl, 1995) is also shown. However, regarding the fairly evolved character of Nemrut basalts (e.g. Ni = 45–51 ppm, Cr = 64–55 ppm), we might expect considerable shift in Nd and Sr isotopic ratios compared to their sources. 9. Discussion 9.1. Genesis of peralkaline magmas in a continental collision setting Alkaline rocks such as peralkaline quartz trachyte and rhyolite are generally considered to be characteristic of intraplate magmatism (Bowden, 1974). However, occurrences of peralkaline magmatism have also been reported for local extensional regimes in convergent margins (e.g. D'Entrecasteaux Island, Papua New Guinea; Mayor Island, New Zealand; Bohrson and Reid, 1997; White et al., 2006 and references therein). Geodynamics of East Anatolia presents similarities with both Trans-Pecos, Texas (White et al., 2006) and Iforas, Mali (Liégeois and Black, 1987) magmatic provinces. Post-collisional magmatism in Trans-Pecos magmatic province is generally summarized in four basic evolutionary steps (Bonin, 2004; White et al., 2006): 1) Collision (flat subduction of Laramide plate resulting in lithospheric stacking and orogenic growth). 2) Post-collisional relaxation, following the roll-back or break-off of subducting slab resulted in asthenospheric upwelling, partial melting of lithospheric mantle and crust. 3) Post-orogenic, early stage of continental rifting and asthenospheric magmatism, represented by bimodal and peralkaline volcanism because of delamination and the sinking of the detached lithosphere. 4) Relaxation with a clear extensional intra-plate regime, and magmatism with a greater influence from

0.512736 0.512657 0.512759 0.512766 0.703519 0.512884 0.703503 0.512878 0.703564 0.512894 0.512657 0.512647 0.512652 0.705908 0.705827 0.512634 0.512658 0.512690 0.512637 Sr/86Sr Nd/144Nd 143

87

Trachyte

N-272 N-220

Trachyte Comendite

N-051 N-305

Comendite Basalt

N-264 N-010

Basalt Basalt

N-258 N-098

Pantellerite Trachyte

N-080 N-181

Trachyte Mugearite

N-209 N-016

Mugearite Comendite

N-149 N-239

Trachyte Trachyte

N-151

Lava flow Lava flow Comenditic

Lava flow Dyke

Comenditic

Type

Rock

Sample

Ejecta Ejecta

Comenditic

Lava flow Pumice Lava flow Lava flow Lava flow Comenditic Comenditic

Lava flow

Lava flow

Lava flow

Lava flow

Post-caldera Pre-caldera

Table 9 87 Sr/86Sr and 143Nd/144Nd isotopic compositions of selected samples from Nemrut eruptive units. NBS 987 standard gave an average value of 143 Nd/144Nd = 0.511957 ± 0.000005.

87

Sr/86Sr = 0.710260 ± 0.0000015 and the AMES standard gave an average value of

H.E. Çubukçu et al. / Journal of Volcanology and Geothermal Research 209-210 (2012) 33–60

51

the asthenosphere. Iforas province (Mali) displays similar geodynamic evolutionary steps (Liégeois and Black, 1987): Starting with subduction of passive continental margin of West African craton and related calc-alkaline island arc magmatism, initiation of alkaline magmatism after a major uplift and the break off of the subducted plate, asthenospheric mantle induced alkaline–peralkaline magmatism along lithospheric mega-shear zones and related wrench faults producing distension. Following the initial continental collision between Eurasian and Arabian plates and the subduction of northern margin of the Arabian plate, the final continental collision and suturing of Arabia with Eurasia plates is thought to have happened in the Middle Miocene (Okay et al., 2010). Subducted oceanic lithosphere, caused upwelling in the surrounding asthenosphere (Krienitz et al., 2006). The slab has been detached from Arabian continent due to its downward force probably 11 Ma ago (Şengör et al., 2003). Upon the break off of the subducting slab, tectonic regime in the region changes from compressional–contractional to compressional–extensional (Koçyiğit et al., 2001). Meanwhile, hot subduction-unmodified asthenosphere residing beneath the Arabian Foreland, might have migrated towards the slab window which was opened during the detachment and invaded the mantle wedge beneath East Anatolian Collision zone (Keskin, 2003; Şengör et al., 2003; Elitok and Dolmaz, 2008). The westward extrusion and counterclockwise rotation of Anatolian plate via NAF and EAF have been accompanied by the intracrustal deformations; the present crust between east Anatolian plate and Arabian foreland gets thinner from north (44 km) to south (~ 36 km) (Elitok and Dolmaz, 2008). Besides, rapid westward extrusion of Anatolia and its counterclockwise rotation following the slab break off eased by the amalgamated crustal structure of east Anatolian region, where stretching and crustal thinning dominated by wrench tectonics (Elitok and Dolmaz, 2008). Nemrut volcano is situated on the east of Muş basin (Fig. 1c), the deformed and dissected remnant of the WNW trending Oligocene– Miocene Muş-Van ramp basin, which has been deformed and dissected during its formation, located at the northern foot of the Bitlis–Zagros suture zone (Koçyiğit et al., 2001). A ramp basin is an intermountain basin where outward directed active thrusts bound its opposite sides (Cunningham, 2005) representing syn-tectonic depression. The reverse fault on the northern margin of Muş ramp basin has a considerable dextral strike-slip component since Pliocene (Koçyiğit et al., 2001) which would result in localized extension. The existence of prevailing extension in the Nemrut region is evident with the rift associated with post-caldera activity. Whilst the source of alkaline magmatism is to be sought in the deep mantle, its location and nature are largely controlled by the structure, composition and dynamics of the overlying continental lithosphere (Black et al., 1985) or of the crust in Nemrut case, where mantle lithosphere is extremely thin or even absent. Following the slab break off, pre-existing shear zones and neotectonic extensional fractures would have eased the emplacement and ascension of partial melts in the middle crust in the region. Further magmatic differentiation in upper level magmatic reservoirs would be the probable reason for its peralkaline character. Furthermore, Nemrut volcano represents analogous structural settings with the intracontinental rifting sites. Silicic peralkaline volcanoes are typically situated in tectonic settings characterized by moderate rates of extension (Mahood, 1984). Comendites occur in subvolcanic ring complexes associated with non-peralkaline volcanics during prerifting, whereas in rift valleys they are more closely linked with peralkaline trachytes and pantellerites during initial rifting. However, abundance of comendites diminishes and they become virtually absent, and replaced by pantellerites, in well-developed, continued rifts where crustal attenuation is more extensive (e.g. Mohr, 1970; Bowden, 1974). Thus, it should be emphasized that pantelleritic volcanism is directly related to the extent of rifting. However, the more the rift evolves,

52

H.E. Çubukçu et al. / Journal of Volcanology and Geothermal Research 209-210 (2012) 33–60

Fig. 12. Variation of (a) 87Sr/86Sr and (b) 143Nd/144Nd isotopic compositions versus silica content of selected Nemrut samples. Data from literature and discrimination between postand pre-caldera comendites are also plotted.

the less the abundance of peralkaline rhyolites, which are replaced by intermediate basalts between alkaline and tholeiitic nature, and minor trachytes. We suggest that, due to the predominance of comenditic products and the regional tectonic settings, Nemrut volcano bears imprints of initial stages of intracontinental rifting. 9.2. Crystal fractionation in a stratified shallow magma chamber 9.2.1. Shallow magma chamber Experimental studies suggest that peralkaline magmas indicate the differentiation under relatively low oxygen fugacity conditions in an extensional environment, characterized by a high degree of crustal fracturing that allows rapid upward migration of mafic parental magmas and formation of shallow magma reservoirs (Caricchi et al., 2006). According to seismic data, gathered by Hacettepe University Mt. Nemrut volcano-seismic monitoring network, depth of the volcanic events occur around 6 km under the Nemrut caldera (Ulusoy, 2008). Besides, observed mineral assemblages (ferrorichterite, aenigmatite) reflect crystallization under low pressures. Furthermore, a shallow chamber is implied by experiments which demonstrate that differentiation of mildly alkaline basalt at 1 kbar produces silica-saturated (pantelleritic) magmas whereas at moderate pressures, silicaundersaturated (phonolitic) magmas are produced (Mahood and Baker, 1986). Hence, we suggest that one of the primary parameters for developing peralkaline magmatism in Nemrut volcano is the existence of a shallow magma chamber. 9.2.2. Fractional crystallization in a zoned magma chamber and Daly Gap Bimodal volcanism and the absence of intermediate compositions (Daly Gap) are typical in most peralkaline suites. The origin of a Daly Gap has been explained by crustal melting or re-melting of mafic intrusions (e.g. Chayes, 1963), although several alternatives consistent with fractional crystallization have been suggested (e.g. Peccerillo et al., 2003). For the Nemrut volcanism, fractional crystallization has been suggested in precedent sections. However, the origin of Daly Gap during fractionation processes should be explained. In order to simulate fractional crystallization processes in Daly Gap (SiO2 53–59%), MELTS modelling has been performed (Ghiorso and Sack, 1995) under various pressures (1.2 and 3 kbar) and QFM

redox conditions. Because MELTS does not predict the crystallization of hydrous phases (i.e. amphibole) which have been observed in trachytes occassionally, felsic rocks have been excluded and simulations have been limited to be between 1000 and 1200 °C. The models performed under 2 and 3 kbar conditions are more realistic as the volcano-seismic events exhibit that the top of the magma chamber exist around 6 km (Ulusoy, 2008) which roughly corresponds to ~2 kbar pressure. When a basaltic parent (sample N-258) is selected, density of the melt, crystallizing phases and their proportions to the total mass changes abruptly within a narrow temperature interval (1100– 1140 °C) in MELTS simulations (Fig. 15). The mass of crystallized solids increases sharply between 1100 and 1140 °C accompanied with an abrupt decrease in the mass of total liquid (Fig. 15a). Furthermore, between 1100 and 1140 °C, calculated SiO2 of the melt changes from 46 to 53% for 1 kbar, whereas it changes from 46 to 55% for 2 and 3 kbar conditions. These data, as previously suggested by Peccerillo et al. (2003), point out that only little quantity of intermediate melt will be produced while fractionating basaltic magma pass rapidly through the intermediate stages. Besides, White et al. (2009) suggest that rapid crystallization in the compositional interval between hawaiite and trachyte may be at least partially for the presence of the “Daly gap” at Pantelleria. Moreover, between 1100 and 1140 °C, calculated mass of crystallized phases depicts an acute increase (Fig. 15b). Crystallized assemblage comprised of plagioclase, clinopyroxene and Fe-Ti oxides with minor olivine. Especially for 3 kbar conditions, Fe-Ti oxides represent the highest amount of crystallized mass. Shellnut et al. (2009) suggest that convective fractionation coupled with en masse oxide mineral crystallization is responsible for Daly Gap in A-type granitoids. The density of the liquid tends to increase between 1200 and 1150 °C, but when the temperature falls below 1140 °C, a sharp decrease is observed (Fig. 15c). This indicates that there is a significant difference in density between calculated compositions of 45% and 55% SiO2. Hence, the different densities of mafic, intermediate and silicic melts induce the stratification of the magma chamber, where silicic melts occupy the top and mafic melts pond at the bottom of the reservoir, while crystal-rich intermediate melts form the interface between the two zones (Turner and Campbell, 1986; Xu et al., 2010). The zoning of the magma chamber provides a “density filter” that

H.E. Çubukçu et al. / Journal of Volcanology and Geothermal Research 209-210 (2012) 33–60

53

Table 10 Least-squares mass balance calculations for major elements for magmatic evolution steps from basaltic to rhyolitic compositions for pre-caldera (Steps 1 and 2) and post-caldera (Steps A and B) stages. Chemical compositions of fractionating phases are given. ΣR2 is the sum of the least squares. (obs: observed, calc:calculated).

Pre-caldera stage Basalt Obs. Sample

N-258

Calc.

Post-caldera stage

Mugearite

Com. Trachyte

Comendite

Basalt

Obs.

Obs.

Obs.

Obs.

N-092

N-258

Calc.

N-016

Calc.

N-281

Comendite

Benmoreite Calc.

Obs.

Calc.

N-013

Obs. N-051

SiO2

46.75

46.77

51.85

51.85

66.42

66.40

72.93

46.75

46.82

56.44

56.47

TiO2

2.85

2.61

2.49

2.49

0.49

0.47

0.25

2.85

3.18

1.83

1.83

0.14

Al2O3

16.36

17.08

15.64

15.69

13.96

13.85

10.89

16.36

16.40

14.61

14.58

12.68

Fe2O3

14.06

14.11

12.32

12.31

6.20

6.21

4.59

14.06

13.94

9.51

9.51

2.36

MnO

0.21

0.20

0.19

0.18

0.21

0.25

0.1

0.21

0.20

0.15

0.21

0.05

MgO

6.49

6.45

3.27

3.27

0.14

0.12

0

6.49

6.52

3.97

3.93

0.03

CaO

9.13

7.55

6.89

6.89

1.24

1.26

0.25

9.13

8.98

5.80

5.76

0.31

3.7 7

3.36

4.30

5.77

5.45

3.77

3.04

4.52

4.49

5.36

1.02

1.94

4.09 2.06

5.32

0.81

5.09

5.11

4.39

0.81

0.81

2.17

1.85

4.50

Step-1

40.22

Na2O K2O

Step-2

Melt fraction (F)

50.43

28.24

Crystal fraction

49.57

71.76

59.78

44.08 55.92

Step-A

28.78 71.22

Step-B

Feldspar (%)

59.57

59.97

84.37

53.71

30.99

SiO2

52.04

57.62

67.24

52.03

55.54

65.08

Al2O3

30.28

26.04

18.83

30.28

27.39

20.66

28.17

FeO

0.56

0.78

0.55

0.56

0.54

0.52

CaO

13.54

8.50

0.25

13.54

10.28

2.97

Na2O

3.98

5.95

7.06

3.98

5.64

8.12

K2O

0.09

1.47

6.66

0.09

0.29

2.37

Clinopyroxene (%)

20.31

8.78

21.13

SiO2

50.44

47.95

48.47

TiO2

1.11

0.62

1.70

Al2O3

2.36

0.40

3.94

FeO

11.76

28.14

13.08

MgO

13.69

1.33

12.24

CaO

19.82

19.37

18.65

Na2O

0.36

0.63

0.49

Olivine (%)

28.32

4.89

4.87

23.17

11.27

SiO2

36.77

35.90

29.76

36.77

36.86

FeO

28.29

34.81

63.40

28.29

35.80

MgO

32.76

28.79

1.47

32.76

24.15

Fe-Ti oxides (%)

12.11

10.47

4.31

1.97

15.98

7.04

1.41

TiO2

21.61

7.97

49.62

26.19

21.61

19.64

50.05

Al2O3

3.93

3.86

0.26

0.36

3.93

3.23

0.39

FeO

62.70

80.24

43.43

66.33

62.70

68.13

43.30

MnO

0.62

0.29

0.80

1.50

0.62

0.67

1.02

MgO

2.73

1.32

3.90

0.12

2.73

2.03

1.07

0.67

0.64

Apatite (%)

0.06

Peralkalinity Index

0.59

2

ΣR

1.613

73.85

7.13 0.57 0.003

1.02

1.08

1.26

0.090

0.050

1.08

0.140

Bold data indicate the amount (%) of crystallized phases, italicized data indicate the overall fractions of melt and crystals.

prevents dense, viscous and crystal-rich intermediate magmas from erupting (Turner and Campbell, 1986). Furthermore, between 1100 and 1140 °C, crystallized andesine–labradorite (An53–46) would suspend in the liquid due to similar densities of the crystals and the liquid (2.6 g/cm3) (Fig. 15c). It has been suggested that feldspar crystals near the transitional boundary between peralkaline trachyte and more evolved liquid batches can be exchanged by various fluid dynamic processes (Troll and Schmincke, 2002). Such a crystal transfer model is consistent with the disequilibrium textures in benmoreitic enclaves, which exhibits evidence for several generations of crystals with significant disequilibrium textures and inconsistent crystal chemistry with the whole

rock (i.e. anorthoclase and trachytic ponds within plagioclase in benmoreites. This manifests that magma mixing/mingling may be an operational process in Daly Gap, occurring between mafic and the more evolved portions of the magma body as reported by Ferla and Meli (2006). 9.3. Temporal/petrological evolution of Nemrut volcano Upon the emplacement of asthenosphere induced partial melts into upper crust beneath Nemrut volcano, their ascension in high level storage reservoirs would have been eased by right lateral strike

54

H.E. Çubukçu et al. / Journal of Volcanology and Geothermal Research 209-210 (2012) 33–60

Fig. 13. Results of trace element modeling for Nemrut volcanism. a) Y versus Zr diagram showing the pre-caldera (Steps 1 and 2) and post-caldera (Steps A and B) AFC trends. Calculated and observed REE patterns and generalized fields of respective rock compositions for b) Step-1 c) Step-2 for pre-caldera stage d) Step-A and e) Step-B for post-caldera stage.

slip extension of pre-existing shears. The oldest rocks (~ 1.0 Ma) were metaluminous trachytes/rhyolites (Fig. 16. Meanwhile magma chamber should have evolved into stratified settings. In the construction period between 570 and 160 ka (Fig. 17), Nemrut volcano produced preferentially peralkaline trachytes and rhyolites of which the degree of peralkalinity in some trachytes extends into pantelleritic affinity

(Fig. 16). This testifies that magma chamber has been zoned and its upper levels, consisting of rhyolites and trachytes, were being continuously tapped. Peripheral doming (Fig. 17) has been intensified suggesting that magma chamber has expanded laterally, perhaps via extensional crack network, producing occasional distant. During 100 ka timespan, mugearite have reached surface (Figs. 16, 17).

H.E. Çubukçu et al. / Journal of Volcanology and Geothermal Research 209-210 (2012) 33–60

55

Fig. 14. 143Nd/144Nd versus 87Sr/86Sr plot for Nemrut basalts. Data for East Anatolian volcanics from Gülen, 1984 and Pearce et al., 1990. For Afar, Main Ethiopian Rift: Hart et al., 1989; Deniel et al., 1994; Trua et al., 1998 and for Kenya Rift (Naivasha), Davies and Macdonald, 1987.

However, the reason for the mugearitic flows can be controversial; either due to evacuation of the upper zones of the chamber by extensive felsic eruptions, or due to the distension of chamber boundaries helping low level mugearites to reach towards the surface via cracks. Subsequently, Nemrut continues to produce peralkaline rhyolites and trachytes with comenditic affinity (Fig. 16, 100–90 ka). During this time interval, a relatively fresh batch of replenishment causes an acute change in chamber dynamics as an eruption trigger. This resulted in paroxysmal pyroclastic eruptions, which produced extensive ignimbritic flows. According to our radiometric dating of preceding units, we suggest this pyroclastic activity should have likely to occur between 89 and 30 ka ago (Fig. 17). Intense pyroclastic flows resulted in the evacuation of the magma chamber and the collapse of its roof formed Nemrut caldera. Volcanic activity begins in intracaldera region with comenditic domes and flows. Phreatomagmatic deposits suggest the existence of a water body during their eruption. Meanwhile N–S extending rift on the northern flank of the volcano has been formed, from which historic bimodal basalt -comendite eruption has occurred. 10. Conclusions 1. Nemrut is an active Quaternary volcano situated at 12 km north of the Bitlis Suture Zone, within a continental collision domain between the Arabian and the Anatolian plates. 2. Volcanic history of the volcano has been investigated under preand post-caldera stages, separated by a caldera collapse which has occurred between 90 and 30 ka ago according to the new radiometric ages presented in this study. 3. Volcanic products bear typical low pressure peralkaline mineral assemblages including aenigmatite, arfvedsonite, chevkinite, aegirine, fayalite, and form typical peralkaline series: mildly alkali transitional olivine basalt–mugearite–(peralkaline) trachyte–(peralkaline) rhyolite. Felsic rocks predominate, basaltic rocks are scarce and intermediate compositions are almost absent or represented by benmoreitic enclaves in

evolved melts. A Daly Gap is present between 53 and 59% SiO2. 4. Geochemical simulations depict that protracted crystal fractionation dominated by feldspar, clinopyroxene, olivine and Fe-Ti oxides with crustal assimilation would produce pre-caldera peralkaline rhyolites starting from pre-caldera mugearites, and post-caldera rhyolites starting from post-caldera basalts. 5. MELTS simulations in the Daly Gap depict that the shallow magma chamber is compositionally stratified, where crystal-rich intermediate melts form the interface between mafic and felsic melts. The absence of intermediate compositions is ascribed to a) the rapid evolution of basalts through intermediate to felsic melts, accompanied with acute crystallization of plagioclase+clinopyroxene+Fe-Ti oxides dominant assemblage within a narrow temperature interval (1140–1100 °C) b) The existence of a density filter which prevents dense, viscous and crystal rich intermediate magmas from erupting within a zoned magma chamber. The existence of disequilibrium textures in intermediate rocks indicates the mixing/mingling processes in the Daly Gap. 6. The genesis of Nemrut peralkaline magmatism in this collisional context has been ascribed to the penetration and ascension of asthenospheric partial melts into upper crustal high level reservoirs under Muş ramp basin, whose pre-existing shears have gained strike slip component thus leading localized extensions where Nemrut volcano has been located. Supplementary materials related to this article can be found online at doi:10.1016/j.ejphar.2011.09.177. Acknowledgements The authors would like to acknowledge the through and stimulating reviews and comments of reviewers John C. White and Bruno Scaillet. Joan Marti is thanked for the editorial handling. This work benefited from a research grant of Hacettepe University Scientific Research Unit (Project No.: 01 01 602 021). It was also financially supported by UMR-CNRS 6524 and benefited from PhD grants from the French Ministry of Foreign Affairs.

56

H.E. Çubukçu et al. / Journal of Volcanology and Geothermal Research 209-210 (2012) 33–60

Fig. 15. MELTS modeling of fractional crystallization (a) proportions of fractionating phases, (b) relative masses of removed crystals and remaining melt and (c) variation of density.

H.E. Çubukçu et al. / Journal of Volcanology and Geothermal Research 209-210 (2012) 33–60

57

Fig. 16. Synthetic table depicting rock types, age intervals, representative samples and summarized nomenclature of Nemrut volcanism. Nomenclature is based on TAS (Total Alkali– Silica) diagram (Le Bas et al., 1986). Peralkalinity of Nemrut samples are expressed by Peralkalinity Indices (P.I.).

58

H.E. Çubukçu et al. / Journal of Volcanology and Geothermal Research 209-210 (2012) 33–60

Fig. 17. Stratigraphical timeline of Nemrut volcanism. Sources for radiometric data are: (1) Atasoy et al., unpublished (2) Pearce et al., 1990 (3) Notsu et al., 1995 (4) Matsuda, 1988 (5) Ercan et al., 1990. Age data marked with white stars have been obtained during this study. The respective sample numbers and rock names are also depicted.

References Al-Lazki, A., Seber, D., Sandvol, E., Turkelli, N., Mohamad, R., Barazangi, M., 2003. Tomographic Pn velocity and anisotropy structure beneath the Anatolian plateau (eastern Turkey) and the surrounding regions. Geophysical Research Letters 30 (24). Arth, J.G., 1976. Behaviour of trace elements during magmatic processes — a summary of theoretical models and their applications. Journal of Research of the U. S. Geological Survey 4, 41–47. Atasoy, E., Terzioğlu, N., Mumcuoğlu, Ç., (unpublished). Nemrut Volkanı Jeolojisi ve Jeotermal Olanakları, TPAO Arama Grubu Başkanlığı, Rapor No:393. Avanzinelli, R., Bindi, L., Menghetti, S., Conticelli, S., 2004. Crystallisation and genesis of peralkaline magmas from Pantelleria Volcano, Italy: an integrated petrological and crystal-chemical study. Lithos 73, 41–69. Aydar, E., 1992. Etude volcano-structurale et magmatologique du strato-volcan Hasan Dagi (Anatolie centrale-Turquie). These de Doctorat. Univ. Blaise Pascal-France, 200p. Aydar, E., Gourgaud, A., Ulusoy, I., Digonnet, F., Labazuy, P., Sen, E., Bayhan, H., Kurttas¸, T., Tolluoğlu, A.Ü., 2003. Morphological analysis of active Mount Nemrut stratovolcano, eastern Turkey: evidences and possible impact areas of future eruption. Journal of Volcanology and Geothermal Research 123, 301–312. Bailey, D.K., Macdonald, R., 1987. Dry peralkaline felsic liquids and carbon dioxide flux through the Kenya rift zone. In: Mysen, B.O. (Ed.), Magmatic processes: physiochemical principles. Geochem Soc Spec Publ, 1, pp. 91–105. Baker, B.H., 1987. Outline of the petrology of the Kenya rift alkaline province. In: Fitton, J.G., Upton, B.G.J. (Eds.), Alkaline Igneous Rocks. Geological Society London Special Publications, 30, pp. 293–311. Black, R., Lameyre, J., Bonin, B., 1985. The structural setting of alkaline complexes. Journal of African Earth Sciences 3, 5–16. Black, S., Macdonald, R., Kelly, M.R., 1997. Crustal origin of peralkaline rhyolites from Kenya: evidence from U-series disequilibria and Th-isotopes. Journal of Petrology 38, 277–297. Bohrson, W.A., Reid, M.R., 1997. Genesis of silicic peralkaline volcanic rocks in a ocean island setting by crustal melting and open system processes: Socorro Island, Mexico. Journal of Petrology 38, 1137–1166. Bonin, B., 2004. Do coeval mafic and felsic magmas in postcollisional to within-plate regimes necessarily imply two contrasting, mantle and crustal, sources? A review. Lithos 78, 1–24. Bowden, P., 1974. Oversaturated alkaline rocks: granites, pantellerites and comendites. In: Sørensen, H. (Ed.), The Alkaline Rocks. A Wiley-Interscience Publication, John Wiley and Sons, London- New York, pp. 109–123. Bozkurt, E., Winchester, J.A., Piper, J.D.A., 2000. Tectonics and Magmatism in Turkey and the Surrounding Area, vol 173. Geological Society of London, London, United Kingdom, pp. 489–512. Bryan, W.B., Finger, L.W., Chayes, F., 1969. Estimating proportions in petrographic mixing equations by least-squares approximation. Science 163, 926–927. Buddington, A.F., Lindsley, D.H., 1964. Iron-titanium oxide minerals and synthetic equivalents. Journal of Petrology 5, 310–357. Caricchi, L., Ulmer, P., Peccerillo, A., 2006. A high-pressure experimental study on the evolution of the silicic magmatism of the Main Ethiopian Rift. Lithos 91, 46–58.

Caroff, M., Maury, R.C., Leterrier, J., Joron, J.L., Cotten, J., 1993. Trace element behavior in the alkalic basalt comenditic trachyte series from Mururoa atoll, French Polynesia. Lithos 30, 1–22. Cassignol, C., Gillot, P.Y., 1982. Range and effectiveness of unspiked potassium-argon dating: experimental groundwork and applications. In: Odin, G.S. (Ed.), Numerical Dating in Stratigraphy. Wiley, Chichester, pp. 159–179. Chayes, F., 1963. Relative abundance of intermediate members of the oceanic basalt– trachyte association. Journal of Geophysical Research 68, 1519–1534. Civetta, L., Cornette, Y., Crisci, G., Gillot, P.-Y., Orsi, G., Requejos, C.S., 1984. Geology, geochronology and chemical evolution of the island of Pantelleria. Geological Magazine 121, 541–668. Civetta, L., D'Antonio, M., Orsi, G., Tilton, G.R., 1998. The geochemistry of volcanic rocks from Pantelleria Island, Sicily channel: petrogenesis and characteristics of the mantle source region. Journal of Petrology 39, 1453–1491. Çubukçu, H.E., Aydar, E., Gourgaud, A., 2007. Comment on “Volcanostratigraphy and petrogenesis of the Nemrut stratovolcano (East Anatolian High Plateau): The most recent post-collisional volcanism in Turkey” by 189–211. Chemical Geology 245, 120–129. Cunningham, D., 2005. Active intracontinental transpressional mountain building in the Mongolian Altai: defining a new class of orogen. Earth and Planetary Science Letters 240, 436–444. D'Orazio, M., Armienti, P., Cerretini, S., 1998. Phenocryst/matrix trace-element partition coefficients for hawaiite-trachyte lavas from the Ellittico volcanic sequence (Mt. Etna, Sicily, Italy). Mineralogy and Petrology 64, 65–88. Davies, G.R., Macdonald, R., 1987. Crustal influences in the petrogenesis of the Naivasha basalt–comendite complex: combined trace element and Sr–Nd–Pb isotope constraints. Journal of Petrology 28, 1009–1031. De Paolo, D.J., 1981. Trace element and isotopic effects of combined wall rock assimilation and fractional crystallization. Earth and Planetary Science Letters 53, 189–202. Deniel, C., Vidal, P., Coulon, C., Vellutini, P.J., Piguet, P., 1994. Temporal evolution of mantle source during continental rifting: the volcanism of Djibouti (Afar). Journal of Geophysical Research 99, 2853–2869. Elitok, Ö., Dolmaz, M.N., 2008. Mantle flow-induced crustal thinning in the area between the easternmost part of the Anatolian plate and the Arabian Foreland (E Turkey) deduced from the geological and geophysical data. Gondwana Research 13 (3), 302–318. Ercan, T., Fujitani, T., Matsuda, J., Notsu, K., Tokel, S., Tadahide, U.I., 1990. Doğu ve Güneydoğu Anadolu Neojen-Kuvaterner volkanitlerine ilişkin yeni jeokimyasal, radyometrik ve izotopik verilerin yorumu. MTA Dergisi, 110, pp. 143–164. Faccenna, C., Bellier, O., Martinod, J., Piromallo, C., Regard, V., 2006. Slab detachment beneath eastern Anatolia: a possible cause for the formation of the North Anatolian fault. Earth and Planetary Science Letters 242, 85–97. Ferla, P., Meli, C., 2006. Evidence of magma mixing in the ‘Daly Gap’ of alkaline suites: a case study from the enclaves of Pantelleria (Italy). Journal of Petrology 47, 1467–1507. Fitton, J.G., Upton, B.G.J. (Eds.), 1987. Alkaline Igneous Rocks. Special Publication, vol. 30. Geological Society, London. 568 pp. Fujimaki, H., 1986. Partition coefficients of Hf, Zr and REE between zircon, apatite, and liquid. Contributions to Mineralogy and Petrology 94, 42–45. Ghiorso, M.S., Sack, R.O., 1995. Chemical mass transfer in magmatic processes. IV. A revised and internally consistent thermodynamic model for the interpretation and extrapolation of liquid-solid equilibria in magmatic systems at elevated temperatures and pressures. Contributions to Mineralogy and Petrology 119, 197–212.

H.E. Çubukçu et al. / Journal of Volcanology and Geothermal Research 209-210 (2012) 33–60 Guillou, H., Carracedo, J.C., Day, S.J., 1998. Dating of the Upper Pleistocene–Holocene volcanic activity of La Palma using the unspiked K–Ar technique. Journal of Volcanology and Geothermal Research 86, 137–149. Güleç, N., Hilton, D.R., Mutlu, H., 2002. Helium isotope variations in Turkey: relationship to tectonics, volcanism and recent seismic activities. Chemical Geology 187, 129–142. Gülen, L., 1984. Sr, Nd, Pb isotope and trace element geochemistry of calc-alkaline and alkaline volcanics, Eastern Turkey. PhD thesis, Massachutsetts Institute of Technology. 232 p. Hart, W.K., WoldeGabriel, G., Walter, R.C., Mertzman, S.A., 1989. Basaltic volcanism in Ethiopia: constraints on continental rifting and mantle interaction. Journal of Geophysical Research 94, 7731–7748. Hart, S.R., Blusztajn, J., Dick, H.J.B., Meyer, P.S., Muehlenbachs, K., 1999. The fingerprint of seawater circulation in a 500-meter section of ocean crust gabbros. Geochimica et Cosmochimica Acta 63, 4059–4080. Hofmann, A.W., 1988. Chemical differentiation of the Earth: the relationship between mantle, continental crust, and oceanic crust. Earth and Planetary Science Letters 90, 297–314. doi:10.1016/0012-821X(88)90132-X. Karakhanian, A., Djrbashian, R., Trifonov, V., Philip, H., Arakelian, S., Avagian, A., 2002. Holocene-historical volcanism and active faults as natural risk factors for Armenia and adjacent countries. Journal of Volcanology and Geothermal Research 113, 319–344. Karaoğlu, Ö., Özdemir, Y., Tolluoğlu, A.Ü., Karabıyıkoğlu, M., Köse, O., Froger, J.-L., 2005. Stratigraphy of the volcanic products around Nemrut Caldera: implications for reconstruction of the caldera formation. Turkish Journal of Earth Sciences 14, 123–143. Keskin, M., 2003. Magma generation by slab steepening and breakoff beneath a subduction-accretion complex: an alternative model for collision related volcanism in Eastern Anatolia, Turkey. Geophysical Research Letters 30 (24), 8046. doi:10.1029/2003GL018019. Koçyiğit, A., Yılmaz, A., Adamia, S., Kuloshvili, S., 2001. Neotectonics of East Anatolian Plateau (Turkey) and Lesser Caucasus: implication from transition from thrusting to strike–slip faulting. Geodinamica Acta 14, 177–195. Krienitz, M.S., Haase, K.M., Mezger, K., Eckardt, V., Shaik-Mashail, M.A., 2006. Magma genesis and crustal contamination of continental intraplate lavas in northwestern Syria. Contributions to Mineralogy and Petrology 151, 698–716. Le Bas, M.J., LeMaitre, R.W., Streckeisen, A., Zanettin, B., 1986. A chemical classification of volcanic rocks based on the total alkali-silica diagram. Journal of Petrology 27, 745–750. Leake, B.E., Woolley, A.R., Arps, C.E.S., Birch, W.D., Gilbert, M.C., Grice, J.D., et al., 1997. Nomenclature of amphiboles: report of the subcommittee on amphiboles of the International Commission on New Minerals and Mineral Names. Mineralogical Magazine 61, 295–321. Liégeois, J.P., Black, R., 1987. Alkaline magmatism subsequent to collision in the Pan-African belt of the Adrar des Iforas (Mali). In: Fitton, J.G., Upton, B.G.J. (Eds.), Alkaline Igneous Rocks. Geological Society, London, Special Publications, 30, pp. 381–401. Lowenstern, J.B., Mahood, G.A., 1991. Petrogenesis of high-silica rhyolite on the Alaska Peninsula. Geophysical Research Letters 18, 1565–1568. Macdonald, R., 1974. Nomenclature and petrochemistry of the peralkaline oversaturated extrusive rocks. In: Bailey, D.K., Barberi, F., Macdonald, R. (Eds.), Oversaturated Extrusive Peralkaline Volcanic Rocks. Bulletin Volcanologique (Special Issue), 38, pp. 498–516. Macdonald, R., 1987. Quaternary peralkaline silicic rocks and caldera volcanoes of Kenya. In: Fitton, J.G., Upton, B.G.J. (Eds.), Alkaline Igneous Rocks. Geological Society, London, Special Publications, 30, pp. 313–333. Macdonald, R., Scaillet, B., 2006. The central Kenya peralkaline province: insights into the evolution of peralkaline salic magmas. Lithos 91, 59–73. Macdonald, R., Davies, G.R., Bliss, C.M., Leat, P.T., Bailey, D.K., Smith, R.L., 1987. Geochemistry of high-silica rhyolites, Naivasha, Kenya rift valley. Journal of Petrology 28, 979–1088. Macdonald, R., Belkin, H.E., Fitton, J.G., Rogers, N.W., Nejbert, K., Tindle, A.G., Marshall, A.S., 2008. The roles of fractional crystallization, magma mixing, crystal mush remobilization and volatile^melt interactions in the genesis of a young basalt peralkaline rhyolite suite, the Greater OlkariaVolcanic Complex, Kenya rift valley. Journal of Petrology 49, 1515–1547. Maggi, A., Priestley, K., 2005. Surface waveform tomography of the Turkish–Iranian plateau. Geophysical Journal International 160, 1068–1080. Mahood, G.A., 1981. The chemical evolution of a late Pleistocene rhyolitic centre : The Sierra La Prima vera, Jalisco, Mexico. Contributions to Mineralogy and Petrology 77, 129–149. Mahood, G.A., 1984. Pyroclastic rocks and calderas associated with strongly peralkaline magmatism. Journal of Geophysical Research 89, 8540–8552. Mahood, G.A., Baker, D.R., 1986. Experimental constraints on depths of fractionation of mildly alkalic basalts and associated felsic rocks: Pantelleria, Strait of Sicily. Contributions to Mineralogy and Petrology 93, 251–264. Matsuda, J.I., 1988. Geochemical study of collision volcanism at the plate boundary in Turkey (comparison with subduction volcanism in Japan). Initial report of Turkey–Japan Volcanological Project Part I, pp. 31–36. McKenzie, D., O'Nions, R.K., 1991. Partial melt distributions from inversion of rare earth element concentrations. Journal of Petrology 32, 1021–1091. Mohr, P.A., 1970. Volcanic composition in relation to tectonics in the Ethiopian rift system: a preliminary investigation. Bulletin Volcanologique 34, 141–157. Morimoto, M., Fabries, J., Ferguson, A.K., Ginzburg, I.V., Ross, M., Seifert, F.A., Zussman, J., Aoki, K., Gottardi, G., 1988. Nomenclature of pyroxenes. Mineralogical Magazine 52, 535–550. Mouralis, D., Kuzucuoglu, C., Akköprü, E., Dogu, A.F., Scaillet, S., Christol, A., Zorer, H., Brunstein, D., Fort, M., Guillou, H., 2010. Les pyroclastites du sud-ouest du Lac de Van: implications sur la paléo-hydrographie regional. Quaternaire 21, 417–433.

59

MTA, 1964. Geological Map of Turkey, (1:500,000), MTA, (Institute of Mineral Research and Exploration). Ankara, Turkey. Mungall, J., Martin, R.F., 1995. Petrogenesis of basalt–comendite and basalt–pantellerite suites, Terceira, Azores, and some implications for the origin of ocean–island rhyolites. Contributions to Mineralogy and Petrology 119, 43–45. Nakamura, N., 1974. Determination of REE, Ba, Fe, Mg, Na and K in carbonaceous and ordinary chondrites. Geochimica et Cosmochimica Acta 38, 757–775. Nelson, S.A., Hegre, J.A., 1990. Volcan Las Navajas, a Pliocene–Pleistocene trachyte/peralkaline rhyolite volcano in the northwestern Mexican volcanic belt. Bulletin of Volcanology 52, 186–204. Notsu, K., Fujitoni, T., Ui, T., Matsuda, J., Ercan, T., 1995. Geochemical features of collision related volcanic rocks in central and Eastern Anatolia, Turkey. Journal of Volcanology and Geothermal Research 64, 171–192. Okay, A.I., Tüysüz, O., 1999. Tehtyan sutures of northern Turkey. In: Durand, B., Jolivet, L., Horvath, F., Seranne, M. (Eds.), The Meditarranean basins: Tertiary extension within the Alpine orogen, Geological Society of London, Special Publication, 156, pp. 475–515. Okay, A., Zattin, M., Cavazza, W., 2010. Apatite fission-track data for the Miocene Arabia-Eurasia collision. Geophysical Research Abstracts, Vol.12. EGU2010-3942. Örgülü, G., Aktar, M., Türkelli, N., Sandvol, E., Barazangi, M., 2003. Contribution to the Seismotectonics of the Eastern Anatolian Plateau from Moderate and Small Size Events. Geophysical Research Letters 30 (24). doi:10.1029/2003GL018258. Oswalt, F., 1912. Armenian. Handbuch der regionalen Geologie 10. Heidelberg. Over, S., Bellier, O., Poisson, A., Andrieux, J., 1997. Late Cenozoic stress state changes along the central North Anatolian fault zone (Turkey). Annales Tectonicae XI (1– 2), 75–101. Özdemir, Y., Karaoglu, Ö., Tolluoglu, A.U., Gulec, N., 2006. Volcanostratigraphy and petrogenesis of the Nemrut stratovolcano, East Anatolian High Plateau): the most recent post-collisional volcanism in Turkey. Chemical Geology 226, 189–211. Özdemir, Y., Karaoglu, Ö., Tolluoglu, A.U., Gulec, N., 2007. Reply to discussion by Evren Çubukçu, Erkan Aydar and Alain Gourgaud of “Volcanostratigraphy and petrogenesis of the Nemrut stratovolcano (East Anatolian High Plateau): The most recent post-collisional volcanism in Turkey” by Y. Özdemir, Ö. Karaoğlu, A.Ü. Tolluoğlu and N. Güleç [Chemical Geology 226 (2006) 189–211]. Chemical Geology 245, 130–134. Pawlewicz, M.J., Steinshouer, D.W., Gautier, D.L., 1997. Map showing geology, oil and gas fields, and geologic provinces of Europe including Turkey. USGS Open file Report, 97-470-I. Pearce, J.A., Bender, J.F., De Long, S.E., Kidd, W.S.F., Low, P.J., Güner, Y., Şaroğlu, F., Yılmaz, Y., Moorbath, S., Mitchell, J.J., 1990. Genesis of collision volcanism in eastern Anatolia Turkey. Journal of Volcanology and Geothermal Research 44, 189–229. Peccerillo, A., Barberio, M.R., Yirgu, G., Ayalew, D., Barbieri, M., Wu, T.W., 2003. Relationships between mafic and peralkaline silicic magmatism in continental rift setting: a petrological, geochemical and isotopic study of the Gedemsa Volcano, Central Ethiopian Rift. Journal of Petrology 44, 2003–2032. Powell, R., 1984. Inversion of the assimilation and fractional crystallization (AFC. equations; characterization of contaminants from isotope and trace element relationships in volcanic suites. Journal of the Geological Society of London 141, 447–452. Ren, M., Omenda, P.A., Anthony, E.Y., White, J.C., Macdonald, R., Bailey, D.K., 2006. Application of the QUILF thermobarometer to the peralkaline trachytes and pantellerites of the Eburru volcanic complex, East African Rift, Kenya. Lithos 91, 109–124. doi:10.1016/j.lithos.2006.03.011. Sandvol, E., Türkelli, N., Zor, E., Gök, R., Bekler, T., Gürbüz, C., Seber, D., Barazangi, M., 2003. Shear wave splitting in a young continent–continent collision: an example from Eastern Turkey. Geophysical Research Letters 30. doi:10.1029/2003GL017390. Scaillet, B., Macdonald, R., 2001. Phase relations of peralkaline silicic magmas and petrogenetic implications. Journal of Petrology 42, 825–845. Scaillet, B., Macdonald, R., 2003. Experimental constraints on the relationships between peralkaline rhyolites of the Kenya Rift Valley. Journal of Petrology 44, 1867–1894. Şengör, A.M.C., 1980. Türkiye'nin neotektoniğinin esasları. Türkiye Jeoloji Kurumu Yayını, p. 42. Şengör, A.M.C., Yılmaz, Y., 1981. Tethyan evolution of Turkey: a plate tectonic approach. Tectonophysics 75, 181–241. Şengör, A.M.C., Ozeren, S., Genc, T., Zor, E., 2003. East Anatolian high plateau as a mantle-supported, north–south shortened domal structure. Geophysical Research Letters 30 (24), 8045. Shellnut, J.G., Zhou, M.F., Zellmer, G.F., 2009. The role of Fe–Ti oxide crystallization in the formation of A-type granitoids with implications for the Daly gap: an example from the Permian Baima igneous complex, SW China. Chemical Geology 259, 204–217. Steiger, R.H., Jager, E., 1977. Convention on the use of decay constants in geo- and cosmochronology. Earth and Planetary Science Letters 36, 359–362. Taylor, S.R., McLennan, S.M., 1985. The Continental Crust: its Composition and Evolution. Blackwell, Oxford. Troll, V.R., Schmincke, H.-U., 2002. Magma mixing and crustal recycling recorded in ternary feldspar from compositionally zoned peralkaline ignimbrite “A”, Gran Canaria, Canary Islands. Journal of Petrology 43, 243–270. Trua, T., Deniel, C., Mazzuoli, R., 1998. Crustal control in the genesis of Plio-quaternary bimodal magmatism of the Main Ethiopian Rift (MER): geochemical and isotopic, (Sr Nd, Pb) evidence. Chemical Geology 155, 201–231. Türkelli, N., Sandvol, E., Zor, E., Gok, R., Bekler, T., Al-Lazki, A., Karabulut, H., Kuleli, S., Eken, T., Gurbuz, C., Bayraktutan, S., Seber, D., Barazangi, M., 2003. Seismogenic zones in Eastern Turkey. Geophysical Research Letters 30 (24). doi:10.1029/ 2003GL018023. Turner, J.S., Campbell, I.H., 1986. Convection and mixing in magma chambers. Earth Science Review 23, 255–352.

60

H.E. Çubukçu et al. / Journal of Volcanology and Geothermal Research 209-210 (2012) 33–60

Ulusoy, İ., 2008. Etude volcano-structurale du volcan Nemrut (Anatolie de l'Est Turquie) et risques naturels associés. PhD. Thesis, Univ. Clermont-Ferrand II – France, Hacettepe Univ. – Turkey, 233 p (in English, with Turkish and French Abstr.). Ulusoy, İ., Labazuy, P., Aydar, A., Ersoy, O., Çubukçu, E., 2008. Structure of the Nemrut caldera (Eastern Anatolia, Turkey) and associated hydrothermal fluid circulation. Journal of Volcanology and Geothermal Research 174 (4), 269–283. Villemant, B., Jaffrezic, H., Joron, J.L., Treuil, M., 1981. Distribution coefficients of major and trace-elements — fractional crystallization in the alkali basalt series of Chaine-DesPuys (Massif Central, France). Geochimica et Cosmochimica Acta 45, 1997–2016. Wedepohl, K.H., 1995. The composition of the continental crust. Geochimica et Cosmochimica Acta 59, 1217–1232. White, J.C., Holt, G.S., Parker, D.F., Ren, M., 2003. Trace-element partitioning between alkali feldspar and peralkalic quartz trachyte to rhyolite magma. Part I: Systematics of trace-element partitioning. American Mineralogist 88, 316–329.

White, J.C., Benker, S.C., Ren, M., Urbanczyk, K.M., Corrick, 2006. Petrogenesis and tectonic setting of the peralkaline Pine Canyon caldera, Trans-Pecos Texas, USA. Lithos 91, 74–94. White, J.C., Parker, D.F., Ren, M., 2009. The origin of trachyte and pantellerite from Pantelleria, Italy: insights from major element, trace element, and thermodynamic modeling. Journal of Volcanology and Geothermal Research 179, 33–55. Xu, Y.G., Chung, S.L., Shao, H., Bin, H., 2010. Silicic magmas from the Emeishan large igneous province. Southwest China: Petrogenesis and their link with the end Guadalupian biological crisis. Lithos 119, 47–60. Yılmaz, Y., Guner, Y., Şaroğlu, F., 1998. Geology of the Quaternary volcanic centres of the east Anatolia. Journal of Volcanology and Geothermal Research 85, 173–210. Zor, E., Gurbuz, C., Turkelli, N., Sandvol, E., Seber, D., Barazangi, M., 2003. The crustal structure of the East Anatolian Plateau from receiver functions. Geophysical Research Letters 30, 8044.