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Mediterranean Geoscience Reviews (2019) 1:167–178 https://doi.org/10.1007/s42990-019-00013-5

ORIGINAL PAPER

Ages and glass compositions for paired large‑volume eruptions from the Acigöl volcanic complex, Cappadocia (Turkey)

Gökhan Atici1 · Axel K. Schmitt2  · Bjarne Friedrichs2 · Stephen Sparks3 · Martin Danišík4 · Esra Yurteri1 · Evren Atakay Gündoğdu1 · Julie Schindlbeck‑Belo2 · Mehmet Çobankaya1 · Kuo‑Lung Wang5,6 · Hao‑Yang Lee5

Received: 13 August 2019 / Revised: 22 November 2019 / Accepted: 26 November 2019 / Published online: 4 December 2019 © Springer Nature Switzerland AG 2019

AbstractAcigöl volcanic complex is a bimodal volcanic field located at the western margin of the Neogene–Quaternary Cappadocian volcanic province in Central Anatolia (Turkey). Its most voluminous eruptions are preserved as a pair of widespread tuffs known as Lower Acigöl Tuff (LAT) and Upper Acigöl Tuff (UAT). Both are nearly aphyric high-silica rhyolites that are compositionally identical; they can be distinguished, however, by the presence of obsidian lithics in UAT and the absence thereof in LAT. Here, we report the discovery of medial deposits of these tuffs at ~ 60–65 km distance from their source, and re-estimate the volume for LAT in the range of 21.4–26.7 km3 (similar constraints are lacking for UAT, but a subequal volume is assumed). Development of paleosol between LAT and UAT provides evidence for a hiatus between both events which is further supported by radiometric dating using combined U–Th and (U–Th)/He zircon geochronology. The revised age for LAT based on sequence modeling of U–Th zircon crystallization and (U–Th)/He zircon eruption ages is 190 ± 11 ka, whereas new and published (U–Th)/He zircon ages indicate eruption of UAT at 164 ± 4 ka (uncertainties generally stated at 1σ). The compositional similarity between both eruptions previously known from whole-rock analyses also extends to glass major, minor, and trace element compositions. Moreover, U–Th zircon crystallization ages are also indistinguishable between LAT and UAT in proximal and medial locations. U–Pb zircon ages for crystals in secular equilibrium indicate a provenance from Cretaceous granitic basement and Miocene volcanic overburden typical for Cappadocia, and such xenocrysts appear to be slightly more common in LAT compared to UAT. The compositional similarity between LAT and UAT along with the apparent recycling of zircon from the non-erupted LAT magma during the UAT event indicates sequential tapping of rhyolite melt from a magma system that maintained thermal and compositional uniformity over a protracted time interval. This can be explained by rhyolite melt extraction from a thermally buffered crystal mush that remained viable to produce voluminous high-silica eruptions for few 10s of ka. The subsequent migration of eruptive activity in the Acigöl volcanic complex from east to west also led to a change in magma chemistry and triggered a new pulse of zircon crystallization, indicating abandonment of the previous magmatic focus, and a shift of magma input to a new location. The identification of sizable medial deposits of LAT and UAT implies that both tephra units are potentially useful marker horizons for marine isotope stage (MIS) 6 sedimentary sequences in and around Anatolia. The recent discovery of UAT in a Black Sea sediment core supports this view.

Keywords Tephrochronology · Rhyolite · Zircon · (U–Th)/He · Anatolia · Marine isotope stage 6

Introduction

Sedimentary sequences deposited in terrestrial and marine environments are important geological archives that can provide unique records of paleoenvironmental condi-tions, and reveal important insights into biologic evolu-tion including human origins. Reconstruction of these archives on a local scale can be correlated with global records when reliable time markers are present. Volcanic

Electronic supplementary material The online version of this article (https ://doi.org/10.1007/s4299 0-019-00013 -5) contains supplementary material, which is available to authorized users.

* Axel K. Schmitt axel.schmitt@geow.uni-heidelberg.de

Extended author information available on the last page of the article

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tephras from fall-out deposits are widely used as such time markers (e.g., Lowe 2011). The Eastern Mediterranean and the Black Sea in southeastern Europe border several vol-canically active regions (Italy, Greece, Turkey, Armenia, Azerbaijan, Georgia, Russia, and Iran) that are the source for large-magnitude explosive eruptions producing wide-spread tephra fall-out deposits on land and in the marine environment. These widespread tephra deposits allow unique correlations between paleoenvironmental archives throughout the Quaternary (e.g., Albert et al. 2017; Lowe et al. 2015; Satow et al. 2015; Tomlinson et al. 2015; Wulf et al. 2002), and in some cases constrain the timing of anthropological and archaeological relicts in a crossroads region for human evolution (e.g., Mouralis et al. 2002; Slimak et al. 2008; Tryon et al. 2009). Among the poten-tial volcanic sources for tephra in this region, Cappadocia in central Anatolia stands out as a particularly long-lived silicic volcanic province. Cappadocian volcanism had its peak activity during the Miocene–Pleistocene (Aydar et al. 2012). Large-volume silicic volcanism in the region has waned since then, but two major stratovolcanoes, Hasan Dağı and Erciyes, in the periphery of Cappadocia remain active (e.g., Sarıkaya et al. 2019; Schmitt et al. 2014). These stratovolcanoes are potential or established sources for Holocene tephra in the Levant (e.g., Develle

et al. 2009; Hamann et al. 2010; Neugebauer et al. 2017). Another type of late Pleistocene volcanism in Cappado-cia is represented by bimodal volcanic fields (Gölludağ, Acigöl; Fig. 1). The Acigöl volcanic complex is the young-est of these fields, and based on existing chronology has been identified as a potential source for tephra during the late Pleistocene (Bigazzi et al. 1993; Druitt et al. 1995; Schmitt et al. 2011).

In this study, we focus on the two largest eruptions of Acigöl, the Lower and Upper Acigöl Tuffs (LAT and UAT, respectively; Druitt et al. 1995; Schmitt et al. 2011), and investigate the potential of the Acigöl volcanic complex as a source for tephra dispersal in eastern Anatolia and adja-cent regions. This research was motivated by the discovery of medial (10s of cm thick, compared to meter-thick in proximal locations) fall-out deposits ~ 60–65 km east of the inferred source for LAT and UAT during mapping of the Erciyes stratocone. These deposits were unequivocally correlated with proximal deposits of LAT and UAT by glass geochemistry (including major and trace elements) and through zircon geochronology. This recognition of medial LAT and UAT fall-out deposits leads to revised estimates of eruptive volumes. New geochronologic con-straints provide a benchmark for identifying Acigöl tephra deposits in sedimentary archives of marine isotope stage

Fig. 1 Overview map showing location of the Acigöl volcanic com-plex with its inferred caldera boundaries along with locations where medial LAT and UAT was identified on the western flanks of Erciyes volcano and in the Kaletepe Dere 3 archaeological excavation to the south of Acigöl (Slimak et  al. 2008). Published isopachs for the P1

unit of LAT with deposit thickness in cm from Druitt et  al. (1995) are shown along with the new, extrapolated 70-cm-thickness isopach from this study (dashed red line). Samples were collected at location 1

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MIS 6, which can be distinguished based on new glass chemistry data presented here from other coeval tephras sourced from volcanoes in the Aegean (Kos) or eastern Anatolia (Nemrut).

Geologic background

The Acigöl volcanic complex is part of the extensive (> 20,000 km2 in area) Central Anatolian Volcanic Prov-ince, which has produced voluminous silicic ignimbrites in two major pulses at ca. 9–8 and 7–5 Ma (Aydar et al. 2012). These deposits are preserved as erosional relicts which mark the picturesque landscape of Cappadocia (Çiner and Aydar 2019). During the Quaternary, volcan-ism in this province became bimodal, as represented by the adjacent volcanic complexes of Gölludağ and Acigöl. Both complexes have erupted high-silica rhyolite tuffs and domes, and are surrounded by basaltic scoria cones and associated lava flows (Aydin et al. 2014; Druitt et al. 1995; Schmitt et al. 2011; Siebel et al. 2011). Gölludağ is mid-dle Pleistocene in age (Aydin et al. 2014; Mouralis et al. 2019), whereas Acigöl—located ~ 40 km N of Gölludağ—is dominantly late Pleistocene. Eruptive activity at Acigöl started with the emplacement of LAT, a > 13 km3 (uncom-pacted tephra volume) crystal-poor rhyolite with several fall-out deposits and near-source ignimbrite (Druitt et al. 1995). Radiometric dating of LAT using (U–Th)/He zir-con geochronology yielded an age of 206 ± 17 ka (errors stated at 1σ throughout this study unless otherwise indi-cated; Schmitt et al. 2011), which is broadly consistent with obsidian fission track ages of ca. 180 ka (Bigazzi et al. 1993) and more precise (U–Th)/He zircon ages of ca. 190 ka for overlying lavas of the eastern dome complex (Schmitt et al. 2011). These lavas are in turn overlain by the UAT, for which (U–Th)/He zircon dating yielded an eruption age of 163 ± 7 ka (Schmitt et al. 2011). A tempo-ral gap between LAT and UAT is indicated from a paleosol between the two tuffs (Druitt et al. 1995). The younger, western rhyolite dome complex of Acigöl formed after another temporal hiatus between ca. 26 and 20 ka (Schmitt et al. 2011).

Compositionally, Acigöl volcanic rocks are bimodal between basalt to andesite on one hand, and nearly aphyric high-silica rhyolite on the other (e.g., Siebel et al. 2011). Although subtle chemical differences exist between eastern (older) and western (younger) rhyolite lavas, whole-rock compositions of LAT and UAT pumice are indistinguishable (Druitt et al. 1995; Siebel et al. 2011). One field criterion that distinguishes LAT and UAT is the lack of obsidian lith-ics in LAT, whereas obsidian lithics are abundant in UAT (Druitt et al. 1995).

Methods

Previously undescribed outcrops of aphyric pumiceous lapilli fall-out deposits were investigated during mapping of the volcanic stratigraphy of the Erciyes stratocone, located ~ 60–65 km E of the Acigöl volcanic complex. These deposits are the focus of this study, because of the necessity to distinguish their provenance from proximal (Erciyes) or medial–distal (e.g., Acigöl) sources. In addi-tion to mapping these deposits in outcrop, two tephra samples were collected (17-BF-27 and  -28 with ~ 1 kg of material per sample). Samples were gently crushed in an agate mill and sieved to < 125 µm. From this fraction, zir-con crystals were enriched using hydraulic and magnetic separation. Inclusion-free euhedral zircon crystals were then hand-picked and pressed into In metal for U-series analysis using secondary ionization mass spectrometry (SIMS) on the Heidelberg University CAMECA ims1280-HR following analytical methods described in Schmitt et al. (2017). Zircon crystallization ages were calculated using an isochron approach, but individual model ages were also determined as zircon-melt two-point isochrons in the (230Th)/(232Th) vs. (238U)/(232Th) diagram (Supple-mentary Table 1). Melt composition was approximated by using average Th/U of upper continental crust (3.8; Rudnick and Fountain 1995) and assuming secular equi-librium of the melt, a reasonable assumption given near-equilibrium values found in highly differentiated rhyolites (e.g., Reid et al. 1997).

Subsequently, selected zircon crystals from 17-BF-27 were extracted from the In mount for (U–Th)/He analysis at Curtin University, Perth, following methods described in Danišík et al. (2017). Disequilibrium-corrected (U–Th)/He ages (Table 1) were then calculated employing the average U–Th zircon crystallization age for LAT, UAT, and eastern rhyolite domes of 189 ± 9 ka (Schmitt et al. 2011), which is consistent with but more precise than the average U–Th zircon rim ages determined for LAT and UAT from the medial locations at Erciyes volcano. Initial disequilibrium for the 238U and 235U decay series used measured Th/U in zircon and melt, the latter approximated by Th/U of upper continental crust (3.8; Rudnick and Fountain 1995), and an initial (231Pa)/(235U) = 3 (modified from Schmitt 2007). In addition, a selection of zircon rims were analyzed by U–Pb using methods described in Schmitt et al. (2017); only pre-Quaternary ages were retained, whereas for younger zircon crystals only U–Th ages were sufficiently precise to yield meaningful age constraints (Supplementary Table 2).

Accuracy of both, U–Th and (U–Th)/He zircon geo-chronology was assessed by analyzing reference zircon during the same analytical sessions as the unknowns. Anal-ysis of secular equilibrium zircon AS3 (1099 Ma; Paces

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and Miller 1993) yielded (230Th)/(238U) = 1.002 ± 0.011 (MSWD = 0.41; n = 9) and (230Th)/(238U) = 1.003 ± 0.007 (MSWD = 1.12; n = 25) in two session for samples 17-BF-27 (UAT) and 17-BF-28 (LAT), respectively. (U–Th)/He ages were determined from Fish Canyon Tuff reference zircon, and an average age of 29.2 ± 0.5 Ma was obtained. This age is consistent with the reported (U–Th)/He age of 28.3 ± 1.3  Ma (Reiners 2005) and a U–Pb age of 28.5 ± 0.1 Ma (Schmitz and Bowring 2001).

Glass shards from the size fraction 63–125 µm were embedded in epoxy, sectioned and polished to expose flat areas for analysis. Major elements were determined by elec-tron microprobe (EMP) analysis using a JEOL JXA 8200 with wavelength-dispersive spectrometers at GEOMAR, Kiel. Care was taken to mitigate alkali mobility by using a defocused (to 5 µm) beam, and procedures were followed as described in Kutterolf et al. (2011). Lipari obsidian (Hunt and Hill 2001) was analyzed under the same conditions as a secondary reference (Supplementary Table 3).

Trace element compositions were determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) using a 193-nm excimer laser system (Teledyne CETAC Analyte G2) coupled with a quadrupole ICP-MS (Agilent 7900) at Academia Sinica, Taipei. Laser repetition rate was 4 Hz using a spot diameter of 35 µm and a fluence of 6.7 J/cm2. Analyzing times were ~ 1.5 min including 30 s of the background noise and ~ 70 s of the sample ablation. NIST SRM 612 was used for calibration with abundance values listed in Norman et al. (1996). Intermittent analyses of international reference glass (BCR-2G) were carried out repeatedly during the analytical session (Supplementary Table 4). Data reduction was performed using Version 4.4 of “real-time on-line” GLITTER© software immediately fol-lowing each ablation analysis. Average silica and calcium concentrations, measured by EMP, were used as internal standards to calibrate the trace element analyses. The limit of detection (LOD) for most trace elements is generally no greater than 100 ppb. For REEs, the LOD is generally around 10 ppb. The analytical precision is better than 10% for most trace elements.

Field observations and sample descriptions

Primary and reworked pumice fall-out deposits consist-ing of nearly aphyric pumiceous lapilli were mapped on the western flanks of Erciyes volcano (Fig. 2). Location 1 (UTM Zone 36 N: 702391E 4280025 N; 1313 m) is a ~ 30–40 m wide and ~ 7 m tall quarry, where a ~ 70-cm-thick white fall-out deposit mantles dark-brown paleosol (Fig. 2). This well-sorted deposit contains aphyric coarse ash to lapilli pumice with a maximum pumice diameter (MP) of ~ 3 mm. Lithic components comprise (meta-)Ta

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171Ages and glass compositions for paired large-volume eruptions from the Acigöl volcanic complex,…

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diorite and mafic volcanic rocks with a maximum lithic (ML) diameter of ~ 1 mm; obsidian lithics are absent. The fall-out deposit is overlain by several m of thick stratified volcaniclastic sediment with wavy cm-scale bedding dis-rupted by numerous syn-depositional soft sediment faults characteristic of down slope slumping (Fig.  2). These reworked deposits grade into tan volcaniclastic-rich coarse sand and a second light-brown paleosol (Fig. 2). This paleosol is overlain by another well-sorted (MP = 10 mm; ML = 5 mm) white pumice fall-out deposit ~ 50 cm thick

mantling the underlying strata. This deposit is also com-posed of coarse ash and small lapilli. It is overlain by a ~ 30-cm-thick ash layer followed by reworked volcano-genic sandy deposits that grade into a third paleosol > 3 m thick at the top of the deposit (Fig. 2). Pumice lapilli are aphyric, and lithic clasts are dominated by dark, glassy obsidian with devitrification features such as spherulites. Bulk tephra samples were collected from location 1 from the lower (17-BF-28; immediately overlying the bottom paleosol) and upper (17-BF-27) fall-out deposits.

A

B C

D

Fig. 2 Outcrop pictures from the western flanks of Erciyes Dag showing medial LAT and UAT as fall-out deposits in location 1 (a), 2 (b), and 3 (c). Interpretative stratigraphic section for location 1 (d). Sampling locations are indicated by red stars

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Location 2 (701367E 4281033N; 1218 m) is a ~ 6-m-high roadcut exposing subhorizontal layering with basal dark-brown sand and clay rich in volcaniclastic components. This layer is overlain by ~ 70 cm white pumice fall-out (MP = 10 mm) which is slightly reversely graded in its lower, and normally graded in its upper part. Laminated fine ash is present at the top of the fall-out deposit and is overlain by 1.5 m light-brown paleosol. Gray surge deposits (between ~ 30 and 100 cm thick) cover the paleosol; these surges are assigned to the Cora Maar ~ 1.2 km NW of loca-tion 2.

Location 3 (701373E 4271849N; 1646 m) is near Göğdağ Dome. It exposes a ~ 40-cm-thick fall-out deposit with lapilli-sized pumice, which grades into thin layers of fine ash. Debris from Göğdağ Dome mantles this fall-out deposit (Fig. 2).

Analytical results

Glass chemistry

Pumice from samples 17-BF-27 (medial UAT) and -28 (medial LAT) are high-silica (SiO2 = 75.7–76.8 wt%), sub-alkaline (Na2O + K2O = 8.0–8.9 wt%) rhyolites with closely overlapping compositions (Fig. 3). They are indistinguish-able from glass compositions of proximal LAT and UAT for

A B

C D

Fig. 3 Selected major elements against SiO2 for glass from pum-ice collected at proximal and medial locations of Lower Acigöl Tuff and Upper Acigöl Tuff in comparison to Holocene tephra from Erci-yes volcano (Tomlinson et al. 2015). Major elements for glass from coeval marine tephra attributed to Kos Plateau Tuff (KPT; Satow et al. 2015) and other potential source volcanoes in eastern Anatolia

(Nemrut; Sumita and Schmincke 2013a) are plotted for comparison; glass compositions from tephra recovered at the Kaletepe Deresi 3 archaeological excavation site are also outlined underscoring their provenance from Acigöl caldera (Tryon et al. 2009; Fig. 1). Fields are defined by manually drawing encompassing lines for the bulk of the analyses available for each source

A B

C D

E F

Fig. 4 Selected trace elements against Zr for glass from pumice col-lected at proximal and medial locations of Lower Acigöl Tuff and Upper Acigöl Tuff. Field for Kos Plateau Tuff (KPT) is shown (data from Satow et al. 2015). See caption for Fig. 3 for additional explana-tions

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major oxides (e.g., Al2O3, CaO, FeO; Fig. 3). Trace element abundances (Fig. 4) are plotted against Zr, which is expected to decrease in zircon-saturated evolved melts with decreas-ing temperature (e.g., Boehnke et al. 2013); in the case of LAT and UAT glasses, the corresponding zircon saturation temperatures calculated from averaged glass data in Supple-mentary Tables 3 and 4 are 703 °C and 695 °C, respectively. Compatible trace elements (Sr, Ba) show the expected posi-tive trend, but positive correlations also exist between Zr and incompatible trace elements (Nb, Yb; Fig. 4). This could imply that Nb and Yb behave compatibly, which would be the case if accessory minerals were present controlling their abundances (e.g., rutile and xenotime, respectively). However, these phases are not petrographically observed, and their presence would be inconsistent with the limited variability in incompatible element ratios such as La/Yb and Th/U. The covariation of Zr with both, compatible and incompatible trace elements, thus more likely reflects over-lap between glass and trace element poor phenocrysts (e.g., quartz, oxides) during laser ablation. Regardless of this vari-ability, trace element abundances in samples 17-BF-27 and -28, along with proximal LAT and UAT, closely overlap.

Major oxides and trace element abundances—where available—for late Pleistocene tephras with ages in the same range as for LAT and UAT were compiled from the literature, namely for Kos Plateau Tuff (KPT; 40Ar/39Ar age = 161 ± 2  ka; Bachmann et  al. 2010; Smith et  al. 1996) and Nemrut (AP-1 and AP-2 with 40Ar/39Ar ages of 189.9 ± 5 ka and 160.2 ± 5 ka, respectively; Sumita and Schmincke 2013a, b). KPT tephra is present in a core from the Aegean Sea (Satow et al. 2015). It has higher SiO2 and lower Al2O3, FeO, and CaO compared to proximal and medial LAT and UAT (Fig. 3). Trace element abundances of KPT glasses are also distinct: Zr, Sr, and Y are lower, whereas Ba and Nb are slightly more enriched than in 17-BF-27, -28, LAT, and UAT glasses, and resolvable differences also exist in La/Yb (higher in KPT) and Th/U (lower in KPT; Fig. 4). Nemrut volcano is an important tephra source in eastern Anatolia, which has had frequent explosive eruptions in the late Pleistocene and into the Holo-cene (Schmincke et al. 2014; Sumita and Schmincke 2013a, b). Major element glass compositions of Nemrut, however, are distinctly more alkaline and have much higher FeO/CaO compared LAT and UAT. Comparison of LAT and UAT glass data with tephra data from the Paleolithic record of the Kaletepe Deresi 3 site (Mouralis et al. 2002; Slimak et al. 2008; Tryon et al. 2009) indicates a correlation between these sequences (Fig. 3).

Geochronology

Pre-Quaternary U–Pb zircon ages for sample 17-BF-27 (medial UAT; n = 6) and 17-BF-28 (medial LAT; n = 13)

fall in two populations: late Mesozoic between ca. 68 and 112 Ma, and Mio- to Pliocene between ca. 2.4 and 11 Ma (Fig. 5). The presence of xenocrysts was also confirmed by U–Th analyses for samples 17-BF-27 and -28, where some

Fig. 5 U–Pb concordia diagram of pre-Quaternary zircon from medial locations of Lower Acigöl Tuff (LAT; sample 17-BF-28) and Upper Acigöl Tuff (UAT; sample 17-BF-27). Concordia ages in Ma

A

B

Fig. 6 U–Th isochron diagram for zircon from a proximal and b medial locations of Lower Acigöl Tuff (LAT; sample 17-BF-28) and Upper Acigöl Tuff (UAT; sample 17-BF-27). Data in panel a are from Schmitt et al. (2011). Reference isochrons for the preferred eruption ages for LAT (190 ka) and UAT (165 ka) are plotted for comparison

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zircon rims yielded secular equilibrium values, but most data display significant depletion in intermediate daughter isotope 230Th relative to its parent 238U (Fig. 6). Because both populations overlap within uncertainty, we used the unmixing model (“TuffZirc”) of Ludwig and Mundil (2002) as implemented in Isoplot 3.75 (Ludwig 2012) to distinguish between autocrystic and xenocrystic zircon. For 17-BF-27 (medial UAT), only one crystal was identified in secular equilibrium (slope = 1.020 ± 0.029). For 17-BF-28 (medial LAT), a larger number of secular equilibrium crystals was identified, with the corresponding slope for the older population of 1.005 ± 0.017 (MSWD = 0.74; n = 8), indi-cating secular equilibrium. The remaining disequilibrium populations yield overlapping isochron ages of 192 ± 14 ka (MSWD = 0.77; n = 24) for 17-BF-27 (medial UAT) and 179 ± 21 ka for 17-BF-28 (medial LAT). The combined isochron age for both samples is 188 ± 12 ka (MSWD = 0.82; n = 46), which is indistinguishable from the zircon crystal-lization age of 189 ± 9 ka (MSWD = 1.1; n = 80) for the eastern Acigöl complex (including proximal LAT and UAT) reported in Schmitt et al. (2011). Combining data for proxi-mal and medial LAT and UAT yields average zircon crys-tallization ages of 181 ± 14 ka (MSWD = 0.98; n = 33) and 191 ± 11 ka (MSWD = 0.98; n = 43), respectively.

(U–Th)/He zircon ages of seven crystals yielded a disequilibrium-corrected age of 165 ± 5 ka (goodness of fit = 0.188; n = 7; Fig. 7). One crystal with a (disequilib-rium-uncorrected) (U–Th)/He age of 9.6 ± 0.5 Ma was

omitted as a xenocryst which was not completely degassed during the eruption. The ca. 165 ka age overlaps closely with the (U–Th)/He zircon age for UAT of 163 ± 7 ka pre-viously reported in Schmitt et al. (2011).

To further refine eruption ages for both tephras, we aver-aged published (Schmitt et al. 2011) and new (U–Th)/He ages and obtained an eruption age for UAT of 164 ± 4 ka. The published eruption age of LAT (206 ± 17 ka) has com-paratively high uncertainties, due to the limited number of usable zircon crystals analyzed in Schmitt et al. (2011) where only 5 out of 8 analyzed crystals were completely degassed at the time of eruption. This old age is consistent with a large number of xenocrysts present in medial LAT (sample 17-BF-28). A sequence model (OxCal 4.3; Bronk Ramsey 2009) was set up to refine the eruption age of LAT based on the constraint that zircon crystallization dated by U–Th predates eruption dated by (U–Th)/He. This con-strains the eruption age for LAT to 190 ± 11 ka (Fig. 8). This age is consistent with the eruption ages of ca. 190 ka for rhyolite lavas that overlie LAT (Schmitt et al. 2011).

A

B

Fig. 7 (U–Th)/He zircon ages for Upper Acigöl Tuff (UAT) from a proximal and b medial locations with probability density functions. Data in panel a are from Schmitt et al. (2011)

A

B

Fig. 8 Probability density curves for a (U–Th)/He zircon ages and b U–Th crystallization ages for Lower Acigöl Tuff (LAT) combining data available for proximal and medial samples (proximal data from Schmitt et al. 2011). Model curve in a is the result of a sequence anal-ysis using OxCal 4.3 (Bronk Ramsey 2009) applying the a priori con-straint that zircon crystallization must precede eruption. The resulting model eruption age for LAT is 190 ± 11 ka (1σ), or 168–213 ka (95% confidence range)

175Ages and glass compositions for paired large-volume eruptions from the Acigöl volcanic complex,…

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Discussion

Reassessing the timing and distribution of LAT and UAT as a paired rhyolite eruption

U–Th zircon crystallization ages for 17-BF-27 and -28 overlap closely with those of proximal LAT and UAT samples. Moreover, (U–Th)/He ages for 17-BF-27 and UAT are indistinguishable within analytical uncertainties. Furthermore, all samples are identical in their major and trace element glass compositions. Collectively, these data demonstrate that tephra fall-out on the western flanks of Erciyes volcano can be correlated with proximal samples of LAT and UAT. This correlation is further supported by the characteristic composition of lithic clasts where obsid-ian is absent in LAT and conspicuously present in UAT (Druitt et al. 1995). Moreover, U–Pb zircon ages indicate that xenocrysts in the pumice fall-out deposits are from source rocks that are typical for Cappadocia, including Cretaceous granites and Miocene ignimbrites (Aydar et al. 2012). Archean zircon crystals postulated to exist in con-cealed basement rocks in the Cappadocia region (Paquette and Le Pennec, 2012) were neither detected here nor in the study of Aydar et al. (2012); the reasons for this discrep-ancy presently remain unresolved.

Given the limited overlap between the modeled LAT eruption age between ca. 168 and 213 ka at 95% confi-dence, and the corresponding 95% confidence range for UAT between ca. 156 and 172 ka, two temporally dis-tinct eruption events are likely. A temporal gap between LAT and UAT is also indicated by the development of a paleosol (Fig. 2), and—in a proximal location—the forma-tion of a scoria cone (Druitt et al. 1995). It is intriguing that despite this hiatus, zircon rim crystallization ages are indistinguishable in both, LAT and UAT. This could be explained by near-isothermal magma storage at ~ 700 °C, where new zircon crystallization was limited, as suggested by closely overlapping zircon saturation temperatures for LAT and UAT. Alternatively, the magma reservoir could have been in subsolidus storage after the LAT eruption, where no zircon crystallization would occur, and then hav-ing experienced rejuvenation by magma recharge imme-diately prior to the UAT eruption. In this case, however, it would be coincidental if magma remobilization during the UAT stage would produce identical magma compositions to the preceding LAT stage. Only when magmatism in the Acigöl volcanic complex migrated westward (with erup-tions between ca. 26 and 20 ka; Schmitt et al. 2011) did rhyolite compositions change significantly (Siebel et al. 2011). Regardless of the causes for invariant melt com-positions between two major eruptions over a protracted hiatus, the compositional homogeneity of glass between

LAT and UAT hampers their unique identification. The only distinguishing features besides age appear to be the presence of obsidian lithics in UAT and their absence in LAT, and the higher abundance of xenocrystic zircon in LAT compared to UAT.

The similarity between Acigöl tephras and those in sandy sediments from the Kaletepe Deresi 3 archaeological excavation has been previously recognized (Mouralis et al. 2002; Slimak et al. 2008; Tryon et al. 2009). This location is ~ 30 km south of Acigöl, highlighting that LAT and UAT are important regional marker horizons. At Kaletepe Deresi 3, five rhyolitic tephras with compositions similar to LAT and UAT were identified (Slimak et al. 2008; Tryon et al. 2009), whereas medial locations investigated by Druitt et al. (1995) and in this study indicate only two primary fall-out events. Further study is warranted to resolve this discrepancy.

Estimating volume and magnitude of LAT and UAT

Lithologic and zircon evidence strongly indicates that pyro-clastic veneer on the western flanks of Erciyes volcano rep-resents medial (~ 60–65 km from their source) equivalents of UAT and LAT. These outcrops also provide new thickness measurements, whereas those in Druitt et al. (1995) only extend out to about 35 km for unit P1 of the LAT. We have made a new estimate of the volume including the new data, assuming that the distal pumice correlates with P1 for which Druitt et al. (1995) presented thicknesses at 11 locations.

Significant informed subjectivity is necessary to con-struct isopach maps and using them to estimate volumes (e.g., Engwell et al. 2015). Thus it is necessary to state our assumptions here. The vent is assumed to be within the cal-dera, but the exact location is not known, which results in an uncertainty for the dispersal axis of at least 5 km. Isopach contours are assumed to enclose the vent in the caldera, and to extend 5 km upwind based on observations of compa-rable Plinian deposits (e.g., Bond and Sparks 1976). We approximated the 600 cm and 325 cm contours from Druitt et al. (1995) with an elliptical shape. Most of the thickness data are to the north of the east–west dispersal axis, so we assumed that the contours are symmetrical around the dis-persal axis. For the 70-cm contour the downwind distance was constrained to be 60 ± 2.5 km, but the crosswind width is not well constrained. We observed that there are several data points along a north–south profile at about 20 km from the caldera center allowing a very approximate estimate of the width of the 20 cm isopach at this distance (~ 30 km). We then drew isopach contours to maximize and minimize areas consistent with the data and the above assumptions. We plotted the square roots of the three isopach areas against log thickness. In all cases the plots were approximately log-linear consistent with the exponential decrease of deposit

176 G. Atici et al.

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thickness with distance from the source (Pyle 1989). Minor adjustments were made so that the three points were on regression lines with r > 0.99.

The results give a range of bulk volumes between 17.7 and 23 km3 with source thickness (T0) of 13.5 and 18 m, and thickness half distances (bt) of 11.2 and 9.9 km, respectively (variables according to Pyle 1989). These values are about double the bulk volume of 9.5 km3 for P1 reported by Druitt et al. (1995). Thus adding in the other units, the total esti-mated volume of the LAT is in the range 21.4 and 26.7 km3. The uncertainties are likely much larger and may be as high as 50% (Engwell et al. 2015). They are also expected to be minimum volumes because at great distances the thinning rate of tephra fall deposits decreases markedly (Bonadonna et al. 1998) so volumes are always underestimated from rela-tively proximal isopach maps typically by factors of 2 or more. Assuming a pumice deposit density of 500 kg/m3 and a magma density of 2300 kg/m3, the dense rock equivalent volume is calculated as ~ 4.7 km3 of magma so the erup-tion had a magnitude of 6 on the volcanic explosivity index (VEI). In the absence of similar constraints for UAT, we can presently only infer a similar magnitude based on subequal thicknesses for LAT and UAT fallout at the medial loca-tions documented here. Recent reports of distal deposits of Acigöl tephra in marine sediments of the Black Sea (Weg-werth et al. 2019) support that LAT and UAT tephra might be more widespread than is currently recognized.

Conclusions

The Acigöl volcanic complex has produced voluminous tephra in a pair of late Pleistocene eruptions (LAT and UAT). Although indistinguishable in major and trace ele-ment glass chemistry, LAT and UAT are separated by a tem-poral hiatus of few 10s of ka, with the UAT eruption being reliably dated in proximal and medial locations to 164 ± 4 ka by (U–Th)/He zircon geochronology. Zircon crystallization ages in LAT and UAT are also indistinguishable. Taking into account the combined U–Th zircon crystallization age for proximal and medial LAT, we refine the published (U–Th)/He zircon eruption age using sequence modeling to 190 ± 11 ka. Discrimination of LAT and UAT from coeval tephras is possible because their subalkaline rhyolitic glass composition is distinct from the more alkali- and Fe-rich compositions of Nemrut in eastern Anatolia. LAT and UAT glasses are also distinctively less evolved compared to Kos in the Aegean. Existing tephra dispersal patterns indicate comparatively thick (several 10s of cm) primary deposits up to 60–65 km from its source, with a dominant W–E distribution. Based on the re-estimation of LAT and UAT tephra volumes (VEI 6), this pair of tephras is considered a

potentially widespread age marker for paleoenvironmental and archaeological archives within MIS 6.

Acknowledgements We thank M. Thöner for support with EMP analy-ses, A. Frew for assistance with zircon dissolution, C. May and C. Scadding (TSW Analytical) for support with solution ICP-MS analy-ses, and I. Dunkl for sharing PepiFLEX software for geochronological ICP-MS data reduction. Reviews by F. Di Traglia and an anonymous reviewer are acknowledged. This work was supported by DFG (Ger-man Research Foundation) grant SCHM2521/3-1, a DAAD (Ger-man Academic Exchange Service) doctoral scholarship, TUBITAK 116Y480 project, and by AuScope and the Australian Government via the National Collaborative Research Infrastructure Strategy (NCRIS). M.D. was supported by Australian Research Council (ARC) Discovery funding scheme (DP160102427) and Curtin Research Fellowship.

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Affiliations

Gökhan Atici1 · Axel K. Schmitt2  · Bjarne Friedrichs2 · Stephen Sparks3 · Martin Danišík4 · Esra Yurteri1 · Evren Atakay Gündoğdu1 · Julie Schindlbeck‑Belo2 · Mehmet Çobankaya1 · Kuo‑Lung Wang5,6 · Hao‑Yang Lee5

1 Department of Geology, General Directorate of MTA, Dumlupınar Bulvarı No: 139, 06800 Ankara, Turkey

2 Institute of Earth Sciences, Heidelberg University, Im Neuenheimer Feld 234-236, 69120 Heidelberg, Germany

3 School of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Clifton BS8 1RJ, UK

4 John de Laeter Centre, Curtin University, GPO Box U1987, Perth, WA 6845, Australia

5 Institute of Earth Sciences, Academia Sinica, 128, Sec. 2, Academia Road, Nangang, Taipei 11529, Taiwan

6 Department of Geosciences, National Taiwan University, Taipei, Taiwan