Abstract

The Eğrikuyu monogenetic field is located in the southwestern part of the Central Anatolian volcanic province and includes numerous scoria cones and related lava flows, as well as a few maars. Eğrikuyu monogenetic field basalts are mainly olivine-nepheline (Ol-Nph) normative, with higher MgO (7.9–11.5 wt%) and Ni (up to 226 ppm) contents compared to other Central Anatolian volcanic province basalts. Enrichment in large ion lithophile elements compared to high field strength elements and depletions in Nb, Ta, P, and Ti in the multi-element diagrams are typical trace-element characteristics of all Central Anatolian volcanic province basalts. Mineral, trace element, and isotope compositions of Eğrikuyu monogenetic field basalts revealed the necessity of at least two distinct and variously enriched components responsible for their formation. Decompressional melting of metasomatized subcontinental lithospheric mantle (enriched mid-ocean-ridge basalt–like), with or without the contribution of upwelling deep asthenospheric melt (oceanic-island basalt–like), may explain the geochemical characteristics of all Central Anatolian volcanic province basalts. The lower 207Pb/204Pb ratios and some distinct clinopyroxene compositions (titaniferous augite and Fe-rich diopside) in Eğrikuyu monogenetic field basalts are indicators for the presence of upwelling asthenosphere, which is also evident in the low-seismic-velocity anomalies and Cyprian slab tear that underlie the field. The Eğrikuyu monogenetic field is a good example of mantle source heterogeneity in a monogenetic basaltic field and therefore constitutes a suitable region within the Central Anatolian volcanic province where different mantle source components can be traced.

INTRODUCTION

Monogenetic basaltic fields are spectacular volcanic landscapes that consist of numerous scoria (or cinder) cones, maars, and related lava flows (e.g., Wood, 1980; Connor and Conway, 2000; Valentine and Gregg, 2008; Kereszturi and Németh, 2012; Németh and Kereszturi, 2015; Smith and Németh, 2017), and they occur in different tectonic settings, including subduction zones (e.g., Trans-Mexican volcanic belt; Luhr et al., 1989) and extensional environments (e.g., Auckland volcanic field; McGee et al., 2013). Source heterogeneity, either in an individual monogenetic volcano (sensu lato) or the entire monogenetic basaltic field with respect to the geographical distribution and size of the eruptive centers, and the temporal evolution of monogenetic fields have been recently reported from different parts of the world (e.g., Shaw et al., 2003; Strong and Wolff, 2003; Brenna et al., 2010; McGee et al., 2013; Rasoazanamparany et al., 2015; Báez et al., 2017). Possible mechanisms proposed to explain this variation are various degrees of crystallization or melting, crystal-melt interaction, multiple components of the mantle source, and ascent dynamics (e.g., Kereszturi and Németh, 2012; McGee et al., 2013; Smith and Németh, 2017). Therefore, investigation of primitive basalts in monogenetic fields will provide new insight into the evolution of basaltic melt from the mantle, and monogenetic fields should be analyzed systematically in terms of their morphological, petrological, and volcanological processes (McGee et al., 2013).

Alkalinity is an important parameter of basaltic volcanic fields that helps in understanding possible mantle sources. Melts formed at high pressures have a deficiency in SiO2 compared to those formed at low pressures (e.g., Hirose and Kushiro, 1993). Therefore, SiO2 undersaturation (e.g., nepheline [Nph] normative) in many alkali basaltic rocks is believed to represent magmas formed by partial melting at greater depths (i.e., asthenosphere) than subalkali magmas (Gill, 2010, and references therein). The transition from calc-alkaline (i.e., subduction-related) to alkaline (i.e., within-plate–like) magmatism is a common process in postcollisional settings (e.g., circum-Mediterranean region; Lustrino and Wilson, 2007). Na-alkaline affinity with oceanic-island basalt (OIB)–like trace-element compositions is one of the main indicators of alkaline magmatism in such regions. Various hypotheses have been proposed for the origin of Na-alkalinity in basaltic magmas (e.g., recycled lithologies in the asthenosphere), but the melting of metasomatized lithosphere with a contribution from low-degree asthenospheric melts is the most suitable model, especially for explaining the similar major- and trace-element compositions of both K- and Na-alkaline magmas in continental settings (Pilet, 2015, and references therein).

The Central Anatolian volcanic province is a SW-NE–trending volcanic belt limited by strike-slip fault systems (i.e., Tuz Gölü and Ecemiş; Toprak, 1998), and it is thought to have evolved in an extensional or transtensional environment since Late Cretaceous time (Genç and Yürür, 2010) in between extensional western Anatolia and compressional eastern Anatolia (Fig. 1A). Plateau-like topography (∼1 km elevation), low seismic (Pn and shear) velocities in shallow depths (15–25 km; e.g., Gans et al., 2009; Mutlu and Karabulut, 2011; Abgarmi et al., 2017; Delph et al., 2017; Reid et al., 2017), and various Curie point depths (2–22 km; Ateş et al., 2005) and crustal thicknesses (Uluocak et al., 2016; Abgarmi et al., 2017) are observed throughout central Anatolia. The different episodes of uplift associated with asthenospheric upwelling (Cosentino et al., 2012) or isostatic adjustments (Schildgen et al., 2012; Abgarmi et al., 2017) after slab break-off or drip-tectonism (Göğüş et al., 2017) are some of the most important geodynamic processes in the Neogene–Quaternary evolution of central Anatolia. The ∼25,000 km2 Central Anatolian volcanic province is defined by widespread ignimbrite eruptions (ca. 9.1–2.5 Ma), Miocene–Pliocene polygenetic volcanoes (e.g., Tekkedağ, Develidağ), Quaternary stratovolcanoes (Hasandağ and Erciyes), and more than 800 monogenetic volcanoes (mainly scoria cones with subordinate maars and lava domes; Toprak, 1998). There is still no consensus on the Neogene–Quaternary petrological evolution of the Central Anatolian volcanic province basalts, which have strong arc-like trace-element patterns (depletion in high field strength elements [HFSEs]) in multi-element diagrams (e.g., Deniel et al., 1998; Kürkçüoğlu et al., 1998; Alıcı-Şen et al., 2004; Gençalioğlu-Kuşcu and Geneli, 2010; Gençalioğlu-Kuşcu, 2011; Reid et al., 2017), unlike their mildly Na-alkaline affinity (e.g., Aydin et al., 2014; Uslular, 2016). Despite some questionable attempts at claiming “OIB-like” trace-element compositions, especially for Quaternary Central Anatolian volcanics (Aydin, 2008; Güçtekin and Köprübaşı, 2009), it is almost impossible to reveal a clear transition from orogenic (i.e., subduction-related) to anorogenic (within-plate–like) volcanism in the Central Anatolian volcanic province using whole-rock geochemistry alone. However, various source components have been recently proposed for the petrological evolution of volcanics in the Central Anatolian volcanic province, where the presence of OIB-like affinity is fairly limited and only revealed in some trace-element ratios (e.g., Reid et al., 2017). This subject is certainly open to discussion and has been tackled in detail throughout this paper.

Some studies on the Central Anatolian volcanic province are mainly focused on the monogenetic fields (Keller, 1974; Gençalioğlu-Kuşcu, 2011), while most studies (Ercan et al., 1992; Notsu et al., 1995; Alıcı-Şen et al., 2004; Gençalioğlu-Kuşcu and Geneli, 2010; Aydin et al., 2014; Reid et al., 2017) concern the source and possible mechanism that triggered the extensive volcanism within the Central Anatolian volcanic province in general. The Eğrikuyu monogenetic field is one of the basaltic monogenetic fields in the Central Anatolian volcanic province with numerous scoria cones and related basaltic lava flows, and at least six maars (Kutören, Obruk, Develiini, Çayankışlası, Karafatma, and Leşkeri; Fig. 2; Uslular et al., 2015, and references therein). Petrological studies have revealed that despite their Na-alkaline affinity, the mildly alkaline Eğrikuyu monogenetic field basalts have typical arc-like trace-element compositions, and they could be derived from enriched subcontinental lithosphere with or without a contribution from deep asthenosphere that may have risen through a slab tear (Ercan et al., 1992; Notsu et al., 1995; Alıcı-Şen et al., 2004; Uslular, 2016; Reid et al., 2017).

This paper reports a detailed petrological investigation of the Eğrikuyu monogenetic field basalts using a monogenetic volcanism perspective that included mineral, major and trace element, and isotope geochemistry. We emphasize the geochemical diversity observed in this small monogenetic basaltic field that was possibly generated by different degrees of melting and/or various mantle source components with minor contributions of mafic mineral fractionation and crustal contamination.

CENTRAL ANATOLIAN VOLCANIC PROVINCE: A MONOGENETIC OVERVIEW

Some segments of the northern Neo-Tethys ocean were closed during the middle-to-late Paleogene collision between the Anatolide-Tauride and Kırşehir blocks along the İzmir-Ankara-Erzincan suture zone to the north, and the Inner Tauride suture zone to the south (e.g., Görür et al., 1984). Subsequently, the collision of the Afro-Arabian and Eurasian plates along the Bitlis suture zone (Fig. 1A) closed the southern branch of Neo-Tethys ocean in the early Miocene (Okay et al., 2010). The interplay of collisional events and subsequent processes (i.e., extensional tectonism after at least the late Miocene, with localized strike-slip faults; e.g., Koçyiğit and Beyhan, 1998; Genç and Yürür, 2010; Özsayın et al., 2013) likely controlled widespread volcanism throughout the Central Anatolian volcanic province. In addition, uplift and geodynamic processes, such as delamination and slab break-off (Bartol and Govers, 2014), associated with ca. 15 Ma volcanism in Eastern Anatolia point to coeval processes and related origins for both the Eastern and Central Anatolian volcanic provinces. However, recent geophysical (Abgarmi et al., 2017; Delph et al., 2017), geodynamic modeling (Göğüş et al., 2017), and petrological (Reid et al., 2017) studies revealed difficulties in calling for single-slab delamination along Central and Eastern Anatolia (Bartol and Govers, 2014). The causes of different episodes of uplift that have varied geographically throughout Central Anatolia (i.e., different in northern, central, and southern parts; Schildgen et al., 2014) have been recently discussed in the literature (Abgarmi et al., 2017; Delph et al., 2017; Göğüş et al., 2017). Uplift in the central and southern parts of Central Anatolia is especially important in terms of volcanism, but whether uplift was caused by asthenospheric upwelling (Genç and Yürür, 2010; Cosentino et al., 2012), topographic response after slab break-off (Schildgen et al., 2012, 2014; Abgarmi et al., 2017), or alternatively after lithosphere dripping (Göğüş et al., 2017) is still debatable. Therefore, we believe that further investigation, especially regarding the relationship between uplift and widespread volcanism in the Central Anatolian volcanic province, will provide new insight into the geodynamic evolution of Central Anatolia.

The recently proposed geodynamic models used to explain widespread Central Anatolian volcanism can be summarized as follows (Bartol and Govers, 2014; Schildgen et al., 2014; Reid et al., 2017; Abgarmi et al., 2017; Delph et al., 2017; Schleiffarth et al., 2018): (1) In the mid-late Miocene, rollback of the delaminating Anatolide-Tauride and Kırşehir mantle lithosphere gave rise to asthenospheric upwelling and interaction with the crust, initiating widespread explosive ignimbritic volcanism. (2) Continued slab rollback caused SW migration of volcanism, and subsequent slab break-off in the latest Miocene led to the initiation of uplift that accelerated in the late Pliocene–early Pleistocene owing to crustal rebound. This model is mainly based on geophysical interpretations, but petrological characteristics of volcanics, especially in the southernmost part of the Central Anatolian volcanic province, also support the contribution of upwelling asthenosphere (Reid et al., 2017). Delamination of the Anatolide-Tauride and Kırşehir mantle lithosphere around mid-Miocene time was also suggested by Kadıoğlu and Dilek (2010). However, the late Miocene to recent processes beneath Central Anatolia are especially crucial for a better understanding of Central Anatolian volcanic province volcanism.

Monogenetic volcanoes display spectacular landscapes throughout the Central Anatolian volcanic province, either as scattered cones on the flanks of stratovolcanoes (e.g., Hasandağ, Erciyes, Keçiboyduran) or as monogenetic fields (e.g., Eğrikuyu, Karapınar; Fig. 1B). Volcanic fields in the Central Anatolian volcanic province that include monogenetic volcanoes are briefly explained as follows:

Hasandağ-Karacadağ Volcanic Field

Hasandağ-Karacadağ volcanic field consists of numerous scoria and parasitic cones, several lava domes, and a few maars related to different evolutionary stages (paleovolcano, mesovolcano, and neovolcano) of the Hasandağ stratovolcano (Aydar et al., 1995; Aydar and Gourgaud, 1998; Deniel et al., 1998). Different types of monogenetic volcanoes (i.e., scoria cones, lava domes) formed randomly during the evolution of Hasandağ, and therefore there is no clear temporal evolution within these edifices based on available radiometric ages (e.g., Aydar and Gourgaud, 1998; Reid et al., 2017). These cones are mainly clustered in the southern (NE of Eğrikuyu monogenetic field) and northwestern (around Karataş village) parts of the Hasandağ (Fig. 1B). The cones around Karataş are elongated in a NW-SE direction (parallel to the Tuz Gölü fault zone), whereas those in the southern part have generally N-S or NE-SW (parallel to the Ecemiş fault zone) trends. While scoria cones are mainly basaltic, lava domes are more felsic in composition (Aydar et al., 1995; Notsu et al., 1995; Deniel et al., 1998). Reid et al. (2017) recently published Ar-Ar ages (1.75–0.37 Ma) for some of the basaltic scoria cones and related lava flows in the southern part of Hasandağ and Karataş field (Table 1). Karacadağ volcano is an andesitic polygenetic volcano of Miocene–Pliocene age (5.98–4.68 Ma; Platzman et al., 1998) in the area. Karapınar monogenetic field and Eğrikuyu monogenetic field are individual monogenetic fields of Quaternary age (Table 1) to the southwest of Karacadağ and Hasandağ, respectively. Therefore, they are considered as distinct clusters in this paper.

Karapınar Monogenetic Field

Karapınar monogenetic field is located at the southwestern tip of the Central Anatolian volcanic province, close to the Karacadağ stratovolcano (Fig. 1B), and it consists of basaltic scoria cones and maars of Pleistocene age (Table 1; Keller, 1974; Ercan et al., 1992; Notsu et al., 1995; Alıcı-Şen et al., 2004; Reid et al., 2017). The main alignment of the monogenetic volcanoes in the Karapınar monogenetic field is NE-SW, consistent with the elongation of the Karacadağ stratovolcano (Toprak, 1998). Meke maar is a spectacular monogenetic edifice in the Karapınar monogenetic field that includes a scoria cone in the center (Fig. 1B; Keller, 1974). Considering the available age data, there is no clear transition between magmatic and phreatomagmatic eruptions in the region. In Pleistocene time, the southwestern part of the Central Anatolian volcanic province close to the Karapınar monogenetic field was occupied by pluvial lakes with high water levels (e.g., Bayer-Altın et al., 2015, and references therein). In addition, during that time, Central Anatolia is thought to have been uplifted due to Cyprian slab break-off (e.g., Schildgen et al., 2014). All these resulted in variable hydrogeological conditions in both the Karapınar monogenetic field and the Central Anatolian volcanic province, where water probably was supplied by a combination of porous media and fracture-controlled aquifers, similar to the Tihany Maar volcanic complex in the Pannonian Basin (Németh et al., 2001). However, the link between hydrogeological conditions and the formation of various types of monogenetic volcanoes in the Central Anatolian volcanic province still needs to be resolved.

Niğde Volcanic Field

Niğde volcanic field includes three stratovolcanoes, namely, Melendiz, Keçiboyduran, and Tepeköy, along with numerous monogenetic volcanoes (mainly scoria cones and a few domes). There is a wide spectrum in the composition of the volcanics, from basalt to rhyolite (Batum, 1978; Aydin, 2008; Gençalioğlu-Kuşcu and Geneli, 2010; Aydin et al., 2014). The monogenetic volcanoes are clustered in three main groups, similar to those to the northwest of Keçiboyduran volcano, south of Melendiz volcano, and northwest of Tepeköy volcano (Toprak, 1998). The monogenetic volcanoes in the first two clusters (e.g., 0.65–0.22 Ma for the scoria cones of Keçiboyduran; Aydin et al., 2014) are partly buried by the recent ash deposits of the Hasandağ volcano in the north and recent alluvial deposits in the south. Göllüdağ rhyolitic dome complex (Batum, 1978) is located north of the Tepeköy stratovolcano, with a surface area of almost 90 km2 (Toprak, 1998). It is composed of rhyolitic and rhyodacitic lava flows and related obsidian and perlitic rocks (Batum, 1978; Aydin et al., 2014). Recent age data (U-Pb on zircon) reveal that rhyolitic lavas and pumice clasts from the Göllüdağ dome are 1.08 Ma and 0.89 Ma, respectively (Aydin et al., 2014; see also Table 1 herein).

Acıgöl Volcanic Field

Acıgöl volcanic field is represented by widespread Pleistocene pyroclastic fallout and flow deposits related to the Acıgöl caldera (∼6 × 5 km), and it is divided into two groups, the Lower Acıgöl Tuff and the Upper Acıgöl Tuff (Druitt et al., 1995). In addition, there are two clusters of rhyolitic domes, older domes to the east (e.g., Kocadağ, Boğazköy), and N-S–oriented younger domes with well-preserved tuff rings to the west (e.g., Karnıyarık, Kuzey, Güneydağ, Korudağ; Schmitt et al., 2011). The first group of domes is exposed along a N-S–oriented scarp resulting from downfaulting during Upper Acıgöl Tuff eruption (Druitt et al., 1995). However, the caldera margins are absent in the west and the north, and they predate the eruption of Upper Acıgöl Tuff in the south (Druitt et al., 1995), probably resulting from a fissure eruption or trapdoor caldera system with a N-S–oriented hinge zone (Türkecan et al., 2004). Acıgöl volcanic field also displays a great variety of monogenetic volcanoes (i.e., scoria cones, maars, lava domes, and tuff cones), especially in terms of different eruption styles and compositions (Toprak, 1998). Basaltic scoria cones and rhyolitic lava domes are the dominant monogenetic volcanoes in the region. Maars (e.g., Acıgöl) represent the youngest volcanic activity (20.3 ka; 14C and U-Th/He methods; Kuzucuoğlu et al., 1998; Schmitt et al., 2011; see also Table 1 herein) in the area and spatially coincide with the younger domes.

Erciyes Volcanic Field

Erciyes volcanic field is a unique area marked by the occurrence of hundreds of parasitic cones located on the flanks of the Erciyes stratovolcano along the radial fissures of its central vent (Toprak, 1998). The new Erciyes stage (Kürkçüoğlu et al., 1998; Şen et al., 2003) also includes rhyodacitic domes (e.g., Dikkartın Dağ; Şen et al., 2002), a basaltic-andesitic maar volcano (Cora maar; Gençalioğlu-Kuşcu et al., 2007; Gençalioğlu-Kuşcu, 2011), and numerous basaltic parasitic scoria cones on the flank of the volcano (Toprak, 1998; Şen et al., 2003). Based on published radiogenic ages, monogenetic volcanism in the Erciyes volcanic field lasted between 2.56 and 0.88 Ma (Notsu et al., 1995; Şen et al., 2003; Gençalioğlu-Kuşcu, 2011; Doğan-Külahçı et al., 2018; see also Table 1 herein).

Eğrikuyu Monogenetic Field

Eğrikuyu monogenetic field is a Quaternary basaltic intraplate monogenetic field represented by more than 100 scoria cones and a few maar volcanoes (e.g., Obruk, Kutören) within an area of 900 km2 located in the southwestern sector of the Central Anatolian volcanic province (Figs. 1B and 2). Toprak (1998) analyzed the relationship between tectonic lineaments and the spatial distribution of scoria cones and suggested that the clustering of volcanic centers in the area follows two dominant trends. Arcasoy (2001) also studied the clustering and alignments of point-like features (i.e., volcanic centers) and revealed the same dominant trends within the Eğrikuyu monogenetic field. These trends are NE-SW in the west (i.e., NE of Karacadağ stratovolcano) and N-S to NW-SE toward the east (i.e., between Karacadağ and the south of Hasandağ; Fig. 2; Toprak, 1998). As in the case of the other monogenetic fields in the Central Anatolian volcanic province, clustering directly indicates that the occurrence of monogenetic volcanoes in the Eğrikuyu monogenetic field was directly controlled by the main fault systems in the region (Figs. 1B and 2). For instance, the NE-SW trend of scoria cones to the NE of Karacadağ is a direct indication of a fault buried by the accumulation of younger sediments, which also gave rise to the formation of Karacadağ stratovolcano parallel to the Central Anatolian volcanic province long axis (Fig. 2; Toprak, 1998). However, the scoria cones clustered further to the northeast (between Karacadağ and Hasandağ, and south of Hasandağ) are related to the southern extension of the Tuz Gölü fault zone, which changes direction from NW-SE to N-S (Toprak, 1998). Additionally, some of the cones (e.g., Leşkeri Hill) are cut by faults within the Tuz Gölü fault zone, indicating contemporaneous activity of faulting and volcanism (Fig. 2; Toprak, 1998).

Most of the scoria cones within the Eğrikuyu monogenetic field are breached-type features (Uslular et al., 2015), due to possible flow emittance, flank collapse, and basal inclination (e.g., Németh et al., 2011). Detailed information on the morphological features of the cones in the Eğrikuyu monogenetic field can be found in Uslular et al. (2015). The interplay of eruptive styles from “dry” magmatic to “wet” phreatomagmatic processes is clearly observed in the Eğrikuyu monogenetic field (Fig. 2). Hydrological conditions in the Eğrikuyu monogenetic field during the Quaternary were variable, similar to other regions in the Central Anatolian volcanic province (especially Karapınar monogenetic field), owing to regional uplift and climatic changes (e.g., Schildgen et al., 2014; Bayer-Altın et al., 2015, and references therein). Considering the fact that maar volcanoes within the Eğrikuyu monogenetic field have not been studied in detail, the relative age relationships between maars and scoria cones are not clear.

Lava flows are vesicular, dense, and olivine-phyric, displaying typical ‘a‘ā type lava morphology with blocky outcrops (Figs. 3A and 3B). In addition, carbonate minerals filled the vesicles of lava flows, especially those sampled in the western part of the Eğrikuyu monogenetic field. Lapilli- and bomb-size basaltic scoria are the main pyroclastic deposits of the scoria cones in the region (Figs. 3C and 3D). Some of the scoria cones have ash- to lapilli-sized tephra layers at their lower parts. These slightly stratified tephra layers are related to either former maar formation or initial phreatomagmatic activity during the formation of scoria cones (e.g., Kereszturi and Németh, 2012). Maars in the Eğrikuyu monogenetic field display typical base surge deposits with lapilli-sized scoria-fall deposits (Figs. 3E and 3F). Basaltic scoria cones and associated lava flows have been studied in terms of their petrological evolution (Ercan et al., 1992; Notsu et al., 1995; Reid et al., 2017). In general, volcanics in the Eğrikuyu monogenetic field are high-Mg basalts with a typical subduction signature in multi-element diagrams. Reid et al. (2017) recently studied the petrological evolution of volcanic rocks in the Eğrikuyu monogenetic field and surrounding the Hasandağ stratovolcano, and they published Ar-Ar ages (2.57–0.21 Ma) that are mainly clustered around 0.4 Ma, similar to those proposed by Notsu et al. (1995).

SAMPLING AND METHODOLOGY

Fresh fallout deposits and lava flows of scoria cones and maars for thin section and whole-rock analysis were collected in and around the Eğrikuyu monogenetic field. Most of the samples are lava flows associated with scoria cones, because eruptive products of scoria cones exposed on their surfaces are generally altered, and quarried scoria cones are not always available for collecting fresh samples. In total, 25 thin sections were prepared at Vommak Mechanical Engineering Ind. Trade., Co., Ltd. (Ankara, Turkey). Of those, 20 relatively fresh samples were selected for geochemical analyses that were performed in the ACME Analytical Laboratories Ltd. (Canada) using an inductively coupled plasma–atomic emission spectrometer (ICP-OES) for major elements and inductively coupled plasma–mass spectrometer (ICP-MS) for trace and rare earth elements (REEs). The standards (STD SO-18) used for the geochemical analysis of all Eğrikuyu monogenetic field basalts are reported in the Supplemental File1.

After performing the CIPW norm calculations, Eğrikuyu monogenetic field basalts were divided into two main groups based on the classification of Thompson (1984). The majority of the samples are olivine-nepheline (Ol-Nph) normative alkali basalts (AB), and only two samples are olivine-hypersthene (Ol-Hyp) normative transitional basalts (TB) (mineral abbreviations throughout the paper are from Whitney and Evans, 2010). Therefore, samples for further analysis, such as mineral and isotope chemistry, were selected depending on this classification. In addition, this nomenclature is also used throughout the paper for geochemical interpretations. The compositions of olivine (Ol), clinopyroxene (Cpx), plagioclase (Pl), and some Fe-Ti oxides were analyzed in 10 polished sections using an electron microprobe (JEOL JXA 8900 R) at the University of Minnesota (USA) and University of Göttingen (Germany). Detailed descriptions for the analysis are given in the Supplemental File (footnote 1). The Sr, Nd, and Pb isotope analyses of 10 samples were performed at the Radiogenic Isotope Laboratory of Central Laboratory, Middle East Technical University, Ankara, Turkey. Detailed descriptions of analytical procedures for Sr, Nd, and Pb isotopes, explained by Köksal et al. (2017), are given in the Supplemental File (footnote 1).

RESULTS AND DISCUSSION

Petrography

In total, 25 thin sections from fallout and flow deposits of scoria cones and maars within the Eğrikuyu monogenetic field were examined under the microscope in terms of mineral contents and textural characteristics. The main mineral assemblages of the lava flows are Pl, Ol, Cpx, and oxides found as phenocrysts and microphenocrysts, and microlites. The only difference in these rocks is the relative proportions of the mineral phases (i.e., lava flows have a higher amount of Pl compared to fallout deposits; cf. Figs. 4A and 4D). The samples display porphyritic or seriate texture with the microphenocrysts or microlites and interstitial glass (Fig. 4B). The abundance of Ol and Cpx microphenocrysts and microlites in AB samples increases with an alkaline affinity (sample GU-16 has the highest normative Nph content; Fig. 4C). Lava flows mainly display trachytic texture (Fig. 4D), with some exceptions showing pilotaxitic texture (Fig. 4E), whereas all the fallout samples are pilotaxitic (Figs. 4A and 4E). Some vesicles in scoria samples are filled by secondary minerals (generally calcite). Cumulates of Cpx and Ol microphenocrysts are especially observed in fallout deposits (Fig. 4F). Subidiomorphic plagioclase microphenocrysts in some rocks display zoning (Fig. 4B). There are many indications of disequilibrium between minerals and melt (or rapid cooling of magma) in the Eğrikuyu monogenetic field basalts, such as skeletal Ol and Cpx minerals, zoning, especially in Ol, and sector (or hourglass) zoning in Cpx (Figs. 4A, 4D, and 4E).

Rock Classification

To quantify the degree of silica saturation in the Eğrikuyu monogenetic field basalts in comparison to other Anatolian basalts, major-element analyses of the rocks were used to calculate the CIPW normative mineralogy (Johannsen, 1931). However, it may be unreliable to calculate the normative mineralogy of rocks that are derived from the continental lithosphere where the interaction with silica-rich wall rocks is inevitable, because this decreases the normative nepheline (Nph) or increases the normative hypersthene (Hyp) contents (e.g., DePaolo and Daley, 2000). Therefore, normative classification of relatively evolved samples might result in misleading interpretations and should be used considering the effects of crustal assimilation processes (e.g., DePaolo and Daley, 2000). On the other hand, the loss on ignition (LOI) value is an important indicator for understanding the alteration effects or analytical problems. In order to minimize such effects, samples with less than ∼3 wt% LOI were considered for the interpretation of both Eğrikuyu monogenetic field (0.3–2.4 wt%) and Anatolian basalts (0–3 wt%).

Ternary diagrams based on normative mineralogy (Thompson, 1984) reveal that most of the Eğrikuyu monogenetic field basalts are silica undersaturated (0.3%–5.8% Nph) and plot in the olivine-alkali basalt field, except two more evolved (silica-saturated) Ol-Hyp–normative (1.1%–2.8% Hyp) samples (Fig. 5A). The Quaternary Central Anatolian volcanic province basalts, including the Eğrikuyu monogenetic field basalts (e.g., Ercan et al., 1992; Aydar et al., 1995; Notsu et al., 1995; Deniel et al., 1998; Kürkçüoğlu et al., 1998; Alıcı-Şen et al., 2004; Güçtekin and Köprübaşı, 2009; Kürkçüoğlu, 2010; Aydin et al., 2014; Reid et al., 2017), also plot in both silica-saturated and silica-undersaturated fields (Fig. 5A). In contrast to Pliocene–Quaternary Anatolian basalts from the Eastern Anatolian volcanic province (Pearce et al., 1990; Notsu et al., 1995; Alıcı-Şen et al., 2004; Lustrino et al., 2010, 2012; Özdemir et al., 2011; Ekici et al., 2014; Lebedev et al., 2016; Oyan et al., 2016), which also have both alkaline and subalkaline affinity (Ol-Nph and Ol-Hyp normative), Kula basalts are entirely Ol-Nph normative with strong alkaline affinity (Fig. 5A; Ercan, 1993; Notsu et al., 1995; Aldanmaz, 2002; Alıcı et al., 2002; Innocenti et al., 2005; Tokçaer et al., 2005; Chakrabarti et al., 2012; Grützner et al., 2013). In addition, all Pliocene–Quaternary Anatolian basalts, including the Eğrikuyu monogenetic field, exhibit Na-alkaline affinity (ΔQ = [quartz – {leucite + nepheline + olivine}] < 0) according to the diagram of Foley et al. (1987) (Aydin et al., 2014; Uslular, 2016).

The total alkali versus silica (TAS) diagram of Le Maitre et al. (2002) also revealed the same interpretation obtained using CIPW normative mineralogy for both Eğrikuyu monogenetic field and Anatolian basalts (Fig. 5B). Color-coding is given to all basalts according to their normative composition, either Ol-Hyp normative transitional basalts (TB) or Ol-Nph normative olivine-alkali basalts (AB) (Fig. 5B). AB samples have 47–49 wt% SiO2 with mildly alkaline to alkaline affinity, whereas TB samples are more evolved (50–51 wt% SiO2) with subalkaline affinity, based on the division line of Irvine and Baragar (1971) (Fig. 5B). As the exact correlation between TAS and silica saturation has never been achieved due to various planar surfaces in the basalts, a diagram originally proposed by Bellieni et al. (1983) for the separation of subalkali and alkali basalts can be used (Fig. 5B, basalt field with bold line; Le Maitre et al., 2002). AB samples mainly plot on the overlap field in this diagram, suggesting that they are three times as likely to be alkali basalts than subalkali basalts (Fig. 5B). AB with a high normative Nph, on the other hand, plot throughout the alkali field, supporting their strong alkaline affinity. Only one TB sample plots in the subalkali field, implying its subalkali affinity with 89% (Fig. 5B). Except for the Kula basalts, which are exclusively alkaline (i.e., basanite, tephrite, and phonotephrite), most of the Central Anatolian volcanic province (including Eğrikuyu monogenetic field) and Eastern Anatolian volcanic province basalts have transitional to mildly alkaline compositions and plot in the overlap field (Fig. 5B).

The following geochemical interpretation of major-and trace-element and isotope geochemistry data will be envisaged based on the normative classification of both Eğrikuyu monogenetic field and other Anatolian basalts.

Mineral Chemistry and Geothermobarometry

As discussed above, many microphenocrysts found in the Eğrikuyu monogenetic field basalts show disequilibrium textures (e.g., skeletal crystals, reaction rims) mainly related to rapid decompression rather than magma mixing. Therefore, we have avoided selecting minerals that display disequilibrium textures to limit the physical conditions under which magmas were crystallized. Representative Ol, Cpx, Pl, and some Fe-Ti oxide analyses are listed in Table 2.

Estimated pressure and temperature values for Eğrikuyu monogenetic field basalts were calculated using mineral-liquid equilibria based on the calibrations of Putirka et al. (2007) and Putirka (2008). The whole-rock composition was considered as liquid (or melt) composition in the geothermobarometric calculations. In addition, pressure and temperature values were also estimated by using whole-rock composition (Table 3; Lee et al., 2009), and pressure values were used as input for geothermobarometric calculations on mineral chemistry. In order to provide constraints for this simplification and eliminate misleading results (e.g., owing to the high proportion of xenocrysts or cumulates, even when not evident in petrographical observations), we also considered the mineral-liquid equilibrium according to partition coefficient (KD) tests of Roeder and Emslie (1970). Representative chemical compositions and structural formulae of the minerals were calculated using Microsoft Office Excel macros published at https://serc.carleton.edu/research_education/equilibria/index.html.

The following interpretations and discussions based on the mineral chemistry of the Eğrikuyu monogenetic field basalts are based on their two distinct geochemical affinities (i.e., alkali and transitional basalts).

Olivine

Olivine core composition in the alkali Eğrikuyu monogenetic field basalts ranges between Fo64 and Fo89 (Fo = 100 × Mg/[Mg + FeT], where T is total iron), whereas the range is Fo70–Fo90 in transitional Eğrikuyu monogenetic field basalts (Table 2; Supplemental Data [footnote 1]). Fo contents of Eğrikuyu monogenetic field basalts are also similar with those of the Central Anatolian volcanic province basalts (Aydar et al., 1995; Aydin, 2008; Köprübaşı et al., 2014). The olivine phenocrysts are essentially unzoned, or normally zoned with decreasing Fo content toward the rim (Fig. 6A). Most of the olivine crystals have Mg-rich cores, but they display a relatively sharp decrease in Fo content toward the rim (Fig. 6A), with a wide range of composition (Fo67 to Fo88), especially in the transitional basalts (Supplemental Data [footnote 1]). The lower Mg rim compositions might be related to rapid cooling or fast growth of the crystals, also evident in petrographic observations (e.g., skeletal olivine; Fig. 4D), or they might be related to the proportion of heterogeneity in the source (i.e., transitional vs. alkali affinity; see whole-rock geochemistry section for further discussion).

Most of the olivine crystals in the Eğrikuyu monogenetic field basalts are not in equilibrium with the melt (Fig. 7A; i.e., outside of KD = 0.30 ± 0.03), which has also been reported for some Central Anatolian volcanic province basalts (e.g., Köprübaşı et al., 2014), suggesting that olivine crystals either formed as accumulated xenocrysts or crystallized in evolved magmas. However, some of the olivine crystals in Eğrikuyu monogenetic field basalts only plot below the equilibrium boundary (Fig. 7), and therefore they seem to have formed either in a more evolved magma or due to rapid cooling. Mg# versus major oxides (e.g., variable CaO content) plots of olivine crystals (Supplemental Data [footnote 1]) also imply a variety of magmas from different mantle sources, as in the case of Kula basalts (Grützner et al., 2013) and some other basaltic monogenetic fields (e.g., Tenerife—Albert et al., 2015; Kissomlyò monogenetic volcano—Jankovics et al., 2015). Thermometry calculations based on olivine crystals in the Eğrikuyu monogenetic field basalts that equilibrated with the melt yielded temperatures of 1272–1280 °C for TB and 1273–1356 °C for AB.

Plagioclase

Anorthite (An) contents range from 37% to 96% in the TB samples and from 16% to 71% in the AB samples (Fig. 6B; Table 2). Most of the plagioclases in the Eğrikuyu monogenetic field basalts is labradorite and bytownite in composition, similar to other Central Anatolian volcanic province basalts (Fig. 7B; Aydar et al., 1995; Aydin, 2008; Köprübaşı et al., 2014; Doğan-Külahçı et al., 2018). Plagioclase crystals of the Eğrikuyu monogenetic field basalts generally display normal zoning (Figs. 4B and 6B), possibly due to the rapid cooling of the magma. The very few plagioclase crystals are in equilibrium with the melt (i.e., out of KD = 0.1 ± 0.11; Putirka, 2008), as many Pl crystals in the Eğrikuyu monogenetic field basalts are microphenocrysts or microlites. Thermobarometry calculations based on Pl phenocrysts equilibrated with the melt yielded temperatures of 1231–1242 °C and pressures of 5.6–8.4 kbar.

Clinopyroxene

Core-to-rim compositional variation, detected as reverse, oscillatory, and sector (or hourglass) zoning, is a common feature of the pyroxenes in the Eğrikuyu monogenetic field basalts (Figs. 4A, 6C, and 6D). Some of the Cpx crystals in the Eğrikuyu monogenetic field basalts contain high amounts of Al2O3 and TiO2, ranging 3–11 wt% and 0.3–2.8 wt% in AB and 2–7 wt% and 0.5–2 wt% in TB, respectively (Table 2; Supplemental Data [footnote 1]). According to Morimoto (1988), Cpx is mainly diopsidic and rarely augitic in composition (Wo38–45En40–48Fs9–21 in TB, and Wo42–50En36–49Fs6–19 in AB), similar to other Quaternary Central Anatolian volcanic province basalts (Fig. 8A; Aydar et al., 1995; Aydin, 2008; Köprübaşı et al., 2014; Doğan-Külahçı et al., 2018) and Pliocene Sivas basalts (Kocaarslan and Ersoy, 2018). Cpx is generally ferroan diopside (Fe2+ >0.1; Morimoto, 1988) with subordinate ferrian alumina diopside or fassaite (Fe3+ >0.1; Morimoto, 1988). Some of the Cpx from AB samples plot with those from alkali basalts (Leterrier et al., 1982), similar to the Kula alkali basalts (Wo45–51En35–45Fs10–17; Fig. 8B; Grützner et al., 2013). In addition, some of the AB samples have similar Cpx compositions (i.e., titanian augite and ferrian diopsides) to the Eifel alkali basalts (Duda and Schmincke, 1985). On the other hand, most of the Cpx compositions (ferroan diopside) in the Eğrikuyu monogenetic field and Central Anatolian volcanic province basalts are identical to basaltic rocks in Kissomlyò monogenetic volcano (western Pannonian Basin; Jankovics et al., 2015), where the petrological evolution of postextensional basaltic volcanism (e.g., Harangi et al., 2015) is almost identical to the Central Anatolian volcanic province. The Mg# (100 × Mg/[Mg + FeT], where T is total iron) of the Eğrikuyu monogenetic field basalts ranges from 66 to 89 in the AB samples and 65–84 in the TB samples (Table 2; Supplemental Data [footnote 1]). If only the Fe2+ is used for calculation of Mg# (100 × Mg/[Mg + Fe2+], where Fe2+ is calculated from the structural formula), values increase to 70–93. Mg# values are negatively correlated with CaO, Al2O3, TiO2, and Na2O, whereas Cr2O3 contents display almost positive correlation (Supplemental Data [footnote 1]). Abrupt changes in enstatite (En) composition of some Cpx (especially in TB) through the core-to-rim traverses may imply the arrival of more primitive melt (e.g., Albert et al., 2015). The occurrence of Mg-poor Cpx cores (Fig. 6D), especially in the TB samples, also indicates that they may be derived from a different melt source (i.e., in lithospheric mantle) that was subsequently integrated into more primitive magma (e.g., derived from asthenospheric mantle, causing the alkaline affinity in the Eğrikuyu monogenetic field basalts), as in the case of the Kula (Grützner et al., 2013) and Kissomlyó alkali basalts (Jankovics et al., 2015). Although the calculated Fe3+ values are high in some Cpx, possibly due to growth kinetics (Mollo et al., 2010), most of the Cpx in the Eğrikuyu monogenetic field basalts has lower Fe3+ compared to Fe2+, reflecting the effect of oxidation state in its evolution. Therefore, very few Cpx crystals in the Eğrikuyu monogenetic field basalts (only those from the AB samples) are in equilibrium with the melt (i.e., outside of KD = 0.27 ± 0.03), and these were used for the geothermometry and geobarometry calculations. Thermobarometry calculations on the Cpx of Eğrikuyu monogenetic field basalts (those in equilibrium with the melt), using calibrations of Putirka (2008), yielded temperatures of 1235–1272 °C and pressures of 4.7–8.3 kbar.

Fe-Ti Oxides

Most of the Fe-Ti oxide microlites and inclusions in major phenocryst phases (e.g., olivine; Fig. 6A) are close to the binary solid solution series of hematite and ilmenite, whereas some plot along the magnetite-ulvospinel line (Supplemental Data [footnote 1]).

Whole-Rock Geochemistry

The geochemistry data of the Anatolian basalts (45–52 wt% SiO2) of Pliocene–Quaternary age, including the Kula, Central and Eastern Anatolian volcanic province basalts, were compiled to discuss possible similarities with the Eğrikuyu monogenetic field basalts in terms of petrological evolution. In addition, geochemical features of Miocene–Pliocene Anatolian (Central and Eastern Anatolian volcanic province) basalts, circum-Mediterranean anorogenic Cenozoic igneous (CiMACI) basalts, and Aegean arc basalts (Santorini and Nisyros) were also added for comparison in order to better interpret the temporal and petrological evolution of the Eğrikuyu monogenetic field basalts.

Major- and Trace-Element Geochemistry

Eğrikuyu monogenetic field basalts are quite primitive, with comparably high Mg numbers (63–72; Mg# = 100 × Mg/[Mg + FeT], where T is to total iron, and Fe3+/FeT = 0.15) and MgO contents (7.9–11.5 wt%; Fig. 9; Table 2). Sample GU-16 is probably the most primitive Central Anatolian volcanic province basalt, with a high Mg# (∼72) and high MgO (11.2 wt%), and low SiO2 (46.9 wt%) and 87Sr/86Sr contents (0.7046; Tables 3 and 4). The Central Anatolian volcanic province and Eğrikuyu monogenetic field basalts have higher Mg# values compared to other Anatolian basalts (Fig. 9). TB samples are more evolved (50.1–50.7 wt% SiO2), with almost identical major-element composition to AB samples (46.7–49.8 wt% SiO2; Fig. 9A). There is a negative trend in Mg# versus some major oxides (Figs. 9A, 9B, and 9C) in the evolved Eğrikuyu monogenetic field basalts (Mg# < 68), suggesting that fractionated phases (Ol + Cpx ± Pl) played a role in magma differentiation. However, Eğrikuyu monogenetic field basalts with high Mg# display scattered patterns in all major-element diagrams, indicating that fractionation had a negligible or only minor effect on basalt genesis (even for the most evolved sample, GU-19, with 50.7 wt% SiO2; Figs. 9A–9D). The role of differentiation in the evolution of the Eğrikuyu monogenetic field basalts can also be discussed by using the differentiation index of Thornton and Tuttle (1960) (the ratio of normative Qtz, Ab, Or, Nph, Lct, Kls, Na2CO3, and Na2SO4 to the weight total of the norm). The differentiation index of more evolved Eğrikuyu monogenetic field basalts decreases with increasing Mg# (Fig. 9E), whereas those of relatively primitive basalts have scattered patterns, as in the case of major-element binary diagrams. Comparison of the major-element composition of the Eğrikuyu monogenetic field basalts with other Anatolian basalts also revealed the low TiO2 contents of the Eğrikuyu monogenetic field (0.8–1.5 wt%) and other Central Anatolian volcanic province basalts in general (0.8–2.1 wt%; Fig. 9B). This might be related to a distinct source composition of the Central Anatolian volcanic province basalts (e.g., rutile-bearing mantle source). The negligible effect of fractionation and scattered patterns of differentiation index, especially in the genesis of comparably primitive Eğrikuyu monogenetic field basalts, suggest that other processes may have been at play (i.e., variable melting degrees and/or heterogeneity in the mantle source, crustal assimilation, and/or sediment input), beyond the possible minor fractionation of Ti-bearing minerals, to explain the low-TiO2 characteristics of the Central Anatolian volcanic province and Eğrikuyu monogenetic field basalts.

Eğrikuyu monogenetic field basalts have relatively higher Cr (∼253–520 ppm; Fig. 9F) and Ni (∼100–226 ppm; Table 3) contents compared to other Anatolian basalts, reinforcing their almost primitive composition. The role of Ol fractionation in the differentiation of more evolved samples is also apparent (positive correlation between Cr and Mg#; Fig. 9F). The Sc contents (22–29 ppm) are generally scattered or display a slight positive correlation with increasing Mg#, possibly indicating minor fractionation of Cpx in the evolution of Eğrikuyu monogenetic field basalts (Fig. 9G). The most primitive Eğrikuyu monogenetic field basalt (GU-16) has a high Sc content (28.3 ppm), consistent with its elevated CaO/Al2O3 value. The incompatible elements of the Eğrikuyu monogenetic field basalts poorly correlate with Mg#, except for Y (15.4–24.2 ppm), which shows a mildly negative trend as a possible indicator of apatite fractionation (Fig. 9H); this might also be related to the enrichment of Y in the residual liquid (e.g., Pearce and Norry, 1979). The Nb content of Eğrikuyu monogenetic field (5.8–13.3 ppm) and Central Anatolian volcanic province (6.2–20.4 ppm) basalts is also comparatively low (Fig. 9I), consistent with the low TiO2 content (Fig. 9B). The low P2O5 (0.2–0.4 wt%) and Ta (0.2–0.7 ppm) contents with enrichment in large ion lithophile elements (LILEs; Rb, Ba, Th, U) also suggest a subduction-related geochemical affinity for the Eğrikuyu monogenetic field basalts, which is also evident in multi-element diagrams normalized to both primitive mantle (PM) and mid-ocean-ridge basalt (MORB), which show depletion in Nb, Ta, P, and Ti (Figs. 10A and 10C). AB samples also show a slight depletion in Y and Yb anomalies in the multi-element diagrams, possibly indicating the occurrence of garnet in the source (Figs. 10A and 10C). The PM-normalized REE patterns of the Eğrikuyu monogenetic field basalts display light (L) REE enrichment (La/Sm ∼6) and relatively flat middle (M) REE to heavy (H) REE patterns (Dy/Yb ∼2), which are also evident in other Anatolian basalts (Fig. 10E). The almost linear trend in Eu indicates that plagioclase is not the primary fractionating mineral phase, as documented as well in the variation diagrams (Figs. 9C, 9D, and 10E).

The Kula and Eastern Anatolian volcanic province alkali basalts (Ol-Nph normative) display an OIB-like pattern in the PM-normalized multi-element diagram (Fig. 10C). Decompression of a mainly asthenospheric mantle source, with or without the limited contribution of a lithospheric mantle component, has been suggested as the possible mechanism that explains the OIB-like geochemical affinity of these basalts (e.g., Alıcı et al., 2002; Keskin, 2007; Aldanmaz et al., 2015). In addition, CiMACI basalts (Lustrino and Wilson, 2007) and Miocene–Pliocene–age Central (including Koçdağ and Develidağ samples; Kürkçüoğlu et al., 1998; Kürkçüoğlu, 2010) and Eastern Anatolian volcanic province (Karacadağ basalts; Lustrino et al., 2010, 2012; Ekici et al., 2012, 2014) basalts are also compared to Eğrikuyu monogenetic field basalts in both multi-element and REE diagrams (Figs. 10B, 10D, and 10F). While CiMACI basalts display typical signatures of anorogenic magmatism with OIB-like patterns (hump in Nb and Ta, and depletion in K) in multi-element diagrams, almost all Miocene–Pliocene Anatolian basalts, including those with both transitional (Ol-Hyp normative) and alkaline (Ol-Nph normative) geochemical affinity, have slight or explicit depletion in Nb, Ta, Ti, and P anomalies that are consistent with a subduction signature (Figs. 10B and 10D). The enrichment in LILEs and LREEs is also evident as in the Pliocene–Quaternary Anatolian basalts (Fig. 10F). The possible source and geodynamic implications of both Miocene–Pliocene and Pliocene–Quaternary Anatolian basalts are discussed in detail in the following sections.

Isotope Geochemistry

Sr-Nd-Pb isotope compositions of selected Eğrikuyu monogenetic field basalts (10 samples) are presented in Table 4 and plotted in Figure 11. The published isotope data from Pliocene–Quaternary Anatolian basalts (Pearce et al., 1990; Notsu et al., 1995; Deniel et al., 1998; Kürkçüoğlu et al., 1998; Alıcı et al., 2002; Alıcı-Şen et al., 2004; Lustrino et al., 2010; Chakrabarti et al., 2012; Aydin et al., 2014; Ekici et al., 2014; Reid et al., 2017), Aegean arc basalts (Santorini and Nisyros; Buettner et al., 2005; Bailey et al., 2009; Klaver et al., 2016), and Eastern Mediterranean Sea (Klaver et al., 2015) sediments were also included in the diagrams to attain a better source characterization of the Eğrikuyu monogenetic field basalts.

The Eğrikuyu monogenetic field basalts are represented by less radiogenic Sr (87Sr/86Sr = 0.704411–0.705368) and more radiogenic Nd (143Nd/144Nd = 0.512704–0.512815) compositions, similar to other Anatolian and Aegean arc basalts, and they are close to OIB composition (Fig. 11A). However, the Eğrikuyu monogenetic field basalts are clearly distinguished from the CiMACI and Kula basalts, which were derived from a mainly depleted mantle source with a strong OIB-like affinity (Lustrino and Wilson, 2007; Aldanmaz et al., 2015). The heterogeneity in the mantle source of the Eğrikuyu monogenetic field basalts (i.e., depleted and enriched mantle end members) and the contribution of Eastern Mediterranean Sea sediments are also apparent in Figure 11A, which bears similarities to the evolution of Aegean arc basalts (Klaver et al., 2016). The Pb isotope compositions of Eğrikuyu monogenetic field basalts also yield a limited range, with 206Pb/204Pb = 18.7191–18.8821, 207Pb/204Pb = 15.5934–15.6762, and 208Pb/204Pb = 38.6843–39.0990 ratios, except three samples that have relatively high Pb-isotope ratios (Table 4) owing to either source heterogeneity or crustal/lithospheric contamination. On the other hand, most of the Eğrikuyu basalts sampled in this study have distinct 207Pb/204Pb ratios that resemble the Kula and Eastern Anatolian volcanic province basalts, but they are lower than the previously published ratios of other Eğrikuyu monogenetic field and Central Anatolian volcanic province basalts (Aydin et al., 2014; Reid et al., 2017). The Eğrikuyu basalts, in general, have a comparable Pb-isotope composition with the Kula basalts, but they have a higher Sr-isotope composition (Fig. 11B). They are isotopically enriched (Δ7/4 ∼9.5 and Δ8/4 ∼47.3) compared to depleted mantle with respect to the Northern Hemisphere reference line (NHRL; Figs. 11C and 11D; Hart, 1984). Except for some of the Eastern Anatolian volcanic province, Kula, and Eğrikuyu monogenetic field basalts, all remaining basalts have slightly to very high 207Pb/204Pb ratios (Fig. 11C), also supporting the role of subducted sediments (Eastern Mediterranean Sea; Klaver et al., 2016; Reid et al., 2017) in their evolution.

The Pb-isotope compositions of the Eğrikuyu monogenetic field basalts do not strictly resemble the common component (“C”) mantle composition Hanan and Graham, 1996), but most of the Anatolian basalts are close to the “C” mantle, given their higher 207Pb/204Pb contents (Fig. 11C). Although the contribution of sediments in melting is requirement for most Anatolian basalts that resulted in elevated 207Pb/204Pb ratios, the idea of possible “C” mantle-like composition of the regional asthenosphere may not be neglected (Gall et al., 2014, and references therein). On the other hand, most CiMACI basalts (Lustrino and Wilson, 2007) are prominently similar to “C” mantle-like isotopic compositions (Figs. 11C and 11D), probably due to their deep asthenospheric sources.

Source Characteristics

The possible source(s) for the generation of Quaternary Central Anatolian volcanic province basalts (including the Eğrikuyu monogenetic field) have been discussed in the literature for decades (Ercan et al., 1992; Notsu et al., 1995; Kürkçüoğlu et al., 1998; Alıcı-Şen et al., 2004; Aydin, 2008; Gençalioğlu-Kuşcu and Geneli, 2010; Gençalioğlu-Kuşcu, 2011; Aydin et al., 2014; Reid et al., 2017). The strong subduction signature (depletion in Nb, Ta, P, and Ti; Fig. 10) and the transitional (Ol-Hyp normative) and mildly alkaline affinity (Ol-Nph normative) of the Central Anatolian volcanic province basalts elicit contentious ideas regarding their mantle source and possible geodynamic origin (e.g., Gençalioğlu-Kuşcu, 2011; Aydin et al., 2014). The common idea on the evolution of these basalts is that they were mainly derived from subcontinental lithospheric mantle metasomatized by the fluids of earlier subduction, with a contribution from upwelling asthenospheric mantle (e.g., Deniel et al., 1998; Alıcı-Şen et al., 2004; Gençalioğlu-Kuşcu and Geneli, 2010). Recently, Reid et al. (2017) proposed three distinct source components for the Central Anatolian volcanic province (especially for the Eğrikuyu monogenetic field) basalts, namely, subduction-modified mantle (SMM), Central Anatolia ambient upper mantle (AUM), and an intraplate-like component (IPC). SMM was also previously defined as “inherited subduction-enrichment” from a previous subduction event in the history of Central Anatolia for the genesis of Central Anatolian volcanics. The AUM component was identified as enriched MORB-like mantle beneath Central Anatolia (Reid et al., 2017). On the other hand, IPC is represented by the Kula basalts (Reid et al., 2017), which have melts of intraplate-like asthenosphere unmodified by subduction enrichment, and possibly raised through a slab tear (Gans et al., 2009; Biryol et al., 2011; Aldanmaz et al., 2015). In this section, we will discuss the relative importance and role of these three components in the generation of monogenetic basaltic volcanism within the Eğrikuyu monogenetic field by using mineral, trace-element, and isotope geochemistry in comparison to other Pliocene–Quaternary basalts in and around Anatolia.

Geochemical Indications of Mantle Enrichment

The Sr anomaly (Sr* = SrN/√[PrN × NdN], where N stands for normalization to PM of Sun and McDonough, 1989; McGee et al., 2013) of some Eğrikuyu monogenetic field basalts displays positive trends with K/La, possibly indicating interaction with metasomatized sources or fluids (Fig. 12A). This is in accord with the role of Eastern Mediterranean Sea sediments (Klaver et al., 2015), an important metasomatization agent in the evolution of the Central Anatolian volcanic province and Eğrikuyu monogenetic field basalts (Reid et al., 2017). In addition, the contribution of amphibole in the source of Central Anatolian volcanic province basalts (Reid et al., 2017) can be traced by the higher K/La ratios of some Eğrikuyu monogenetic field basalts (Fig. 12A) and also lower La/Sm ratios (Table 3). However, Eğrikuyu monogenetic field basalts have higher Al2O3/TiO2 (∼11–19) ratios compared to alkali basalts (OIB-like) derived from melting of amphibole-bearing metasomatic veins (e.g., Pilet, 2015). Some of the Eğrikuyu monogenetic field basalts (especially Ol-Nph normative ones) have higher Th/Yb ratios, implying a residual garnet-rich source for magma genesis due to either heterogeneity in the mantle source or subduction enrichment (Fig. 12B). MREE/HREE ratios of the Eğrikuyu monogenetic field basalts (e.g., higher Sm/Yb ∼2.1, Tb/Yb ∼0.3, and Dy/YbN ∼1.2 with respect to normal [N] MORB) also suggest that garnet remains in the residual phase. The ratios of Nb/Y or Nb/Yb (not shown) can be used as a proxy for mantle enrichment, and the Central Anatolian volcanic province basalts, including some of the Eğrikuyu monogenetic field basalts, seem to be derived from an enriched source, as illustrated in the diagram with respect to Ti anomaly (Ti* = TiN/√[HfN × SmN], where N stands for normalization to PM; Fig. 12C; McGee and Smith, 2016). The Central and Eastern Anatolian volcanic province basalts are clearly distinguished from Kula basalts, which have a strong within-plate signature (Fig. 12C). The Zr/Hf and Nb/Ta ratios are important proxies for understanding crust-mantle differentiation (i.e., enriched mantle source via subduction and crustal recycling; Figs. 12D and 12E; Pfänder et al., 2007, 2012; McGee and Smith, 2016). The Eğrikuyu monogenetic field basalts have Zr/Hf ratios between 38.6 and 49.4, and variable Ti*, possibly suggesting a heterogeneity and/or lower-degree melting in the source (Fig. 12D; e.g., Pfänder et al., 2007). The Eğrikuyu monogenetic field basalts also have rather depleted Nb (5.8–13.3 ppm) and Ta (0.2–0.7 ppm) contents compared to continental basalts and OIBs of Pfänder et al. (2007). In addition, Nb/Ta ratios decrease with Zr/Nb ratios in a negative trend (Fig. 12E), indicating different degrees of partial melting. Some samples with high Ba and Th contents (i.e., GU-3, GU-7, and GU-16; Table 3) are also characterized by elevated Nb/Ta ratios (Fig. 12E), and these might reflect the role of crust-mantle differentiation (e.g., Pfänder et al., 2007). High Zr/Y ratios (Pearce, 1983) of the Eğrikuyu monogenetic field and Central Anatolian volcanic province basalts also favor enrichment by subduction. The common location of most of the Eğrikuyu monogenetic field (except the three mentioned above) and Central Anatolian volcanic province basalt data in the PM field in Figures 12E and 12F might be indicative of earlier PM-like compositions before any subduction-related enrichment, similar to late Miocene mafic lavas of western Anatolia (Ersoy et al., 2012).

On the other hand, lower Zr/Nb and elevated Nb/Ta and Th/Yb ratios for the most primitive samples (GU-3 and GU-16; Figs. 12B and 12E) plotted in the Eastern Mediterranean Sea sediment field (Fig. 13B) can be explained by either strong subduction enrichment or crustal/lithospheric contamination. Fractionation of Cpx can also increase the Nb contents and Nb/Ta and Zr/Hf ratios in basalts (Pfänder et al., 2007), but a slight Cpx fractionation of the Eğrikuyu monogenetic field and Central Anatolian volcanic province basalts (Fig. 9) cannot result in such elevated anomalies. In addition, bulk mixing of PM-like composition (Sun and McDonough, 1989) with Eastern Mediterranean Sea sediments (even if it is an unrealistic model due to the involvement of subducted sediments in the melt as either sediment-released fluid or melt) may explain the source enrichment in Eğrikuyu monogenetic field basalts (Fig. 13A).

Possible Mantle Source Agents for Eğrikuyu Monogenetic Field Basalts

In Figure 13, we modified the diagrams proposed by Klaver et al. (2016) to show an incongruent batch melting model for Aegean arc basalts with a contribution from Eastern Mediterranean Sea sediments in their evolution. Th/Nb and Nb/Yb ratios are also important proxies for understanding enrichment by subduction or other processes (Pearce, 2008). The Eğrikuyu monogenetic field and Central Anatolian volcanic province basalts have elevated Th/Yb with Nb/Yb ratios, suggesting subduction-zone enrichment (Fig. 13A). Likewise, mixing of lower-degree melting (2%–6%) of subslab enriched mantle (SSEM) and higher-degree melting of average depleted mantle (ADM) with a contribution from Eastern Mediterranean Sea sediments (≥0.5) may explain the trace-element characteristics of the Eğrikuyu monogenetic field and Central Anatolian volcanic province basalts (Figs. 13A and 13B). Klaver et al. (2016) derived the SSEM composition assuming that Kula basalt (sample 26-wav of Chakrabarti et al., 2012) is consistent with 2% partial melting of a lherzolitic mantle that almost represents the subslab African lithospheric mantle. The ADM composition, on the other hand, is based on the depleted MORB mantle of Workman and Hart (2005), which was slightly modified by Klaver et al. (2016) (increased Zr and Hf contents). Details about the melting model and sediment contribution to the mixed source can be found in the supplementary file of Klaver et al. (2016).

The trace-element characteristics of Eğrikuyu monogenetic field basalts mostly resemble those of Nisyros basalts, which can be derived from 4% partial melting of an enriched source, including 9% SSEM component with the addition of 0.15% Eastern Mediterranean Sea sediments (∼0.15; Figs. 13A and 13B). The observed linear array in Figure 13B may indicate the continuity between the higher-degree melting of the ADM and lower-degree melting of the SSEM components. Contrary to the similarity in trace-element characteristics of the Eğrikuyu monogenetic field and Nisyros basalts, isotope compositions are not correlated, particularly in terms of 207Pb/204Pb (Figs. 13C and 13D). The inferred source for the Nisyros basalts that have enriched mantle type 1 (EM-I) affinity, representing the subcontinental lithospheric mantle of the African continent (Klaver et al., 2016), can partly explain the isotope composition of the Eğrikuyu monogenetic field basalts, with a lower contribution from Eastern Mediterranean Sea sediments (Fig. 13C). However, the Eğrikuyu monogenetic field basalts have distinct 207Pb/204Pb contents that are comparable to some Kula basalts (also comparable to the western Anatolian mantle “WAM” of Aldanmaz, 2002; Aldanmaz et al., 2006, 2015). Most of the Eğrikuyu monogenetic field basalts are clearly distinct from other Central Anatolian volcanic province basalts (Reid et al., 2017) that have elevated 207Pb/204Pb contents (Fig. 13D). Therefore, an inferred Nisyros source with any contribution of Eastern Mediterranean Sea sediments cannot form lower 207Pb/204Pb ratios, as seen in the Eğrikuyu monogenetic field basalts presented in this study (Fig. 13D).

Considering the trace-element and isotope characteristics of Eğrikuyu monogenetic field and Central Anatolian volcanic province basalts compared to Eastern Anatolian volcanic province and Kula basalts, we agree with the proposed SMM, AUM, and IPC components (Reid et al., 2017) for the generation of Eğrikuyu monogenetic field and Central Anatolian volcanic province basalts. Trace-element fingerprints of SMM (high La/Sm and Nb/Ta ratios) and AUM (low La/Sm and Nb/Ta ratios) components were also detected (Table 3; Fig. 12E). Pressure-dependent variations (not shown), especially in TiO2 (0.79–1.51 wt%), Zr/Hf (38.6–49.4), and Ba/Nb (23.4–56.9), also support a contribution of IPC to the generation of the Eğrikuyu monogenetic field basalts. Moreover, Reid et al. (2017) stated that isotopic covariations (e.g., lower 207Pb/204Pb) between Central Anatolian volcanic province basalts (mainly the Eğrikuyu monogenetic field) and IPC could not be demonstrated owing to the stronger imprint of mixed AUM-SMM arrays. However, the lower 207Pb/204Pb contents of our samples (except the three that have an SMM-dominated source) possibly represent the missing IPC signature of Reid et al. (2017). Therefore, it can be proposed that trace-element signature derived from IPC is diluted by enriched AUM + SMM, whereas their distinct isotopic signatures are still preserved in some of the Eğrikuyu monogenetic field basalts (Fig. 13D). Plotting of these Eğrikuyu monogenetic field basalts together with some of the Kula basalts (Alıcı et al., 2002; Chakrabarti et al., 2012; Aldanmaz et al., 2015) and Eastern Anatolian volcanic province basalts (Ekici et al., 2014) on isotope diagrams (Figs. 11C and 13D) also supports this interpretation.

In addition, traces of these components can also be followed in various mineral compositions (especially Ol and Cpx) of Eğrikuyu monogenetic field basalts. For instance, some of the Cpx compositions (including both ferroan and ferrian diopside) resemble alkali basalts derived from heterogeneous mantle sources (e.g., Duda and Schmincke, 1985; Albert et al., 2015; Jankovics et al., 2015). In particular, the presence of titanian augite and ferrian diopsides in the Eğrikuyu monogenetic field basalts might also reflect the presence of IPC in their sources.

Geodynamic Implications of Quaternary Volcanism in the Central Anatolian Volcanic Province

The recent geodynamic models, which are mainly based on geophysical and some petrological studies (e.g., Bartol and Govers, 2014; Delph et al., 2017; Reid et al., 2017; Schleiffarth et al., 2018), have provided new insight into the understanding of widespread volcanism in the Central Anatolian volcanic province. However, there are some contradictory interpretations (i.e., SW younging of Central Anatolian volcanism, a contribution from a deep-hot asthenosphere in the geochemistry of Central Anatolian volcanic province basalts) that the results from this study may help to clarify.

Volcanism in Central Anatolia (sensu lato) was initiated in the Late Cretaceous (ca. 76 Ma, Galatian volcanic province; Wilson et al., 1997; Koçyiğit et al., 2003, and references therein) during the closure of the northern Neo-Tethys ocean along the İzmir-Ankara-Erzincan suture zone and Inner Tauride suture zone (Fig. 1A). After the initiation of collision along the Bitlis suture zone around the early Miocene (ca. 23 Ma; Okay et al., 2010), volcanism continued in the Galatia volcanic province (ca. 19–10 Ma; Wilson et al., 1997; Varol et al., 2014), and also initiated in the western (Karacadağ, ca. 21–14 Ma; Asan and Kurt, 2011), southwestern (Erenlerdağ-Alacadağ-Sulutaş, ca. 22–3 Ma; Keller et al., 1977; Temel et al., 1998; Gençoğlu-Korkmaz et al., 2017), and eastern (Sivas volcanics, ca. 23–4 Ma; Platzman et al., 1998; Kürkçüoğlu et al., 2015; Kocaarslan and Ersoy, 2018) parts of the Kırşehir block (Fig. 1A). The start of Central Anatolian volcanic province volcanism, on the other hand, has been interpreted as late Miocene (ca. 13 Ma; Besang et al., 1977), and volcanism has continued until historical times (e.g., Schmitt et al., 2014). Considering all available information, it can be deduced that there is no clear evidence of a migration in time and space throughout Central Anatolia (cf. Schleiffarth et al., 2018). This is also supported by the disparate patterns of volcanism, even in the small monogenetic fields (Table 1; e.g., Eğrikuyu monogenetic field, 0.07–2.57 Ma; Notsu et al., 1995; Reid et al., 2017). However, while the lack of clear volcanic migration cannot totally reject recently proposed geodynamical models (e.g., Delph et al., 2017), the timing of slab rollback and break-off should be reconsidered.

Contrary to the OIB-like affinity of basalts in Galatia (ca. 10 Ma to Pliocene?; Wilson et al., 1997; Varol et al., 2014) and Sivas (especially Pliocene; Kürkçüoğlu et al., 2015; Kocaarslan and Ersoy, 2018), Central Anatolian volcanic province basalts (including mildly alkaline ones) have strong arc-type trace-element patterns in the multi-element diagram (Fig. 10), probably due to melting of heterogeneous mantle sources (enriched lithosphere with a contribution from asthenosphere e.g., Gençalioğlu-Kuşcu and Geneli, 2010; Reid et al., 2017). The main strike-slip fault zones and related normal faults behaved as pathways for the Galatia and Sivas basalts, which have been interpreted to be derived from the asthenosphere, while there is a more complex geodynamic scenario for widespread volcanism in the Central Anatolian volcanic province (drip-like tectonism?; Reid et al., 2017; Göğüş et al., 2017). The alignment of volcanoes in the Central Anatolian volcanic province parallel to the Ecemiş fault zone and Tuz Gölü fault zone also indicates the role of tectonism in volcanism, but there is no clear evidence of an OIB-like asthenospheric source in the geochemical characteristics of Central Anatolian volcanics (e.g., Gençalioğlu-Kuşcu and Geneli, 2010, and references therein). Therefore, we totally disagree with some attempts (e.g., Aydin, 2008; Güçtekin and Köprübaşı, 2009) that suggest an OIB-like geochemical affinity for recent volcanics in the Central Anatolian volcanic province and a transition from orogenic to anorogenic magmatism, as in the case of many CiMACI provinces (Lustrino and Wilson, 2007). However, geochemical fingerprints of deep-hot upwelling asthenosphere (IPC of Reid et al., 2017) in the Central Anatolian volcanic province volcanics are only observed in some trace-element ratios (e.g., Ba/Nb, Zr/Hf; Reid et al., 2017), as well as lower 207Pb/204Pb ratios and distinct mineral compositions (e.g., titanian augite and ferrian diopsides) of Eğrikuyu monogenetic field basalts presented in this study (Figs. 8 and 11).

All these indications call into question whether Eğrikuyu monogenetic field basalts were derived from IPC or from a depleted part of an AUM component. In this case, the geodynamic model for the mid-Miocene to Pliocene Eastern Anatolian volcanics (Keskin, 2007) can be a good proxy for better understanding of the geochemical and geodynamic implications of Central Anatolian volcanic province basalts. Upwelling of asthenosphere through a slab tear (Gans et al., 2009; Biryol et al., 2011; Reid et al., 2017; Delph et al., 2017; Abgarmi et al., 2017) has been mainly characterized by low seismic velocities (Gans et al., 2009; Biryol et al., 2011; Delph et al., 2017; Abgarmi et al., 2017). However, as opposed to Quaternary Karacadağ basalts (Eastern Anatolian volcanic province; e.g., Lustrino et al., 2012; Ekici et al., 2014), the geochemical characteristics of upwelling asthenosphere (i.e., IPC) are somehow chemically diluted due to the interaction with enriched, but isotopically depleted, Anatolian lithosphere ((Fig. 13D; Reid et al., 2017). If this is the case, then we may conclude that some portions of Eğrikuyu monogenetic field basalts (presented in this study) isotopically represent the deeper upwelling asthenosphere, similar to sources of the Kula and Karacadağ basalts (Fig. 13D). Alternatively, lower 207Pb/204Pb ratios of some of the Eğrikuyu monogenetic field basalts represent the depleted portion of AUM, and therefore the contribution of chemically diluted IPC may not be clearly observed (or even absent) in the geochemistry of Central Anatolian volcanic province basalts (Reid et al., 2017).

The similarity in Eğrikuyu monogenetic field and Kula basalts in terms of isotope compositions is especially clear in Figure 13D, where some of the Eğrikuyu monogenetic field basalts are plotted close to the “western Anatolian mantle” of Aldanmaz et al. (2015, and references therein). On the other hand, possible reasons for the difference in trace-element characteristics between Eğrikuyu monogenetic field (i.e., arc-like signature with strong subduction enrichment) and Kula basalts (i.e., OIB-like with minor subduction enrichment) could be the different thicknesses of subduction-modified Anatolian lithosphere beneath western (i.e., thin) and central (i.e., relatively thick) Anatolia, differences in length, depth, and location of tears/break-off (Biryol et al., 2011) with respect to the basaltic field (e.g., Aegean vs. Kula basalts; Klaver et al., 2016), and the possible existence of drip tectonism in central Anatolia (Göğüş et al., 2017; Reid et al., 2017). In addition, the role of the Anatolian lithosphere is displayed in the Eastern Anatolian volcanic province basalts (except Karacadağ; e.g., Oyan et al., 2016, and references therein), which are similar to Eğrikuyu monogenetic field basalts in terms of both trace-element and isotope geochemistry (Figs. 11 and 13). Therefore, considering the initiation of the volcanism (ca. 13 Ma; Innocenti et al., 1976; Besang et al., 1977) and some similarities in the geochemical (i.e., existence of both primitive Ol-Hyp and Ol-Nph normative basalts, except for the OIB-like signature in some of the Eastern Anatolian volcanic province basalts; cf. Reid et al., 2017) and geodynamic (Keskin, 2007; Delph et al., 2017) characteristics of Central Anatolian and Eastern Anatolian volcanic province basalts, the idea of a single-slab delamination model proposed by Bartol and Govers (2014) should not be completely neglected.

CONCLUDING REMARKS

Eğrikuyu monogenetic field is one of the best examples of a monogenetic basaltic field in Anatolia in terms of not only spectacular morphology of the scoria cones and maars, but also petrological and geochemical diversity. The Eğrikuyu monogenetic field basalts are composed mainly of olivine (Fo64–90) phenocrysts with clinopyroxene (mainly ferroan diopside with subordinate titanian augite and ferrian diopside) and plagioclase (labradorite to bytownite) microphenocrysts. They are mostly Ol-Nph normative alkali-olivine basalts (maximum normative Nph = 5.8%), but there are also two Ol-Hyp normative transitional basalts that are comparatively evolved (50–51 wt% SiO2) relative to other basalts (45–50 wt% SiO2), possibly owing to slight fractionation processes recognized in mafic mineral phases (mainly Ol plus minor Cpx). Eğrikuyu monogenetic field basalts are one of the most primitive basalts among the Quaternary Anatolian basalts, as shown by higher Mg# (63–72) and Ni (up to 226 ppm) contents. High LREE relative to HREE contents and a significant subduction signature on multi-element diagrams (depletion of Nb, Ta, P, and Ti) are diagnostic trace-element characteristics of the Eğrikuyu monogenetic field and other basalts in the Central Anatolian volcanic province.

Despite the similarity in multi-element patterns of all the Central Anatolian volcanic province basalts, there are at least two distinct source components in terms of mineral composition (e.g., including both ferroan and ferrian diopsides) and some trace-element (variable La/Sm, Nb/Ta, Zr/Hf, Th/Yb, and Ba/Nb) and isotope ratios (lower 207Pb/204Pb only in some of the Eğrikuyu monogenetic field basalts). One of the distinct components is either deep-hot upwelling asthenosphere (IPC of Reid et al., 2017) through the Cyprian slab tear (Biryol et al., 2011), which is chemically enriched in the Eastern Mediterranean Sea sediment signature of Klaver et al. (2015), but isotopically depleted (lower 207Pb/204Pb), or it was derived from an isotopically depleted portion of the AUM component (Reid et al., 2017). The other source component can be represented by subcontinental lithospheric mantle (SMM of Reid et al., 2017), which has been chemically and isotopically enriched by successive subduction events that have occurred in Central Anatolia since the Late Cretaceous. In addition, the high Nb/Ta ratios and depletion in Y and Yb observed in some of the Eğrikuyu monogenetic field basalts suggest a metasomatized enriched source that includes residual amphibole (plus rutile) and garnet.

Decompression melting (2%–6%) of mixed mantle components (with a greater proportion of SMM) may explain the diverse geochemical characteristics of the Eğrikuyu monogenetic field and other Central Anatolian volcanic province basalts. The small degree of crustal contamination and mafic mineral fractionation may also contribute to the trace-element compositions within these basalts. Most recent geophysical and geodynamical studies also have supported the existence of upwelling asthenosphere at shallow levels beneath Central Anatolia, and the Eğrikuyu monogenetic field is located within the region in the Central Anatolian volcanic province where low Pn velocity anomalies (Gans et al., 2009; Biryol et al., 2011) and also thinner crust (Abgarmi et al., 2017) are observed. Although it is geochemically difficult to constrain the geodynamic processes responsible for the evolution of the Central Anatolian volcanic province, a role can be proposed for deep, hot, and slightly enriched asthenospheric mantle that rose through a slab tear and interacted with either dripping (Reid et al., 2017; Göğüş et al., 2017) or overlying (Kadıoğlu and Dilek, 2010) previously enriched subcontinental lithospheric mantle. Alternatively, melting of mixed SMM and AUM components (Reid et al., 2017) triggered by lithospheric-scale transtensional fault systems in the Central Anatolian volcanic province could have formed the Central Anatolian volcanic province basalts without a contribution from the deep asthenospheric mantle (e.g., Gençalioğlu-Kuşcu and Geneli, 2010). Differences in the residence time of the Eğrikuyu monogenetic field basalts while rising to the surface can also explain the record of chemical heterogeneity (Gençalioğlu-Kuşcu and Geneli, 2010). Drip-tectonism (Göğüş et al., 2017) and crustal variations throughout the Central Anatolian volcanic province (Uluocak et al., 2016; Abgarmi et al., 2017) may have fostered uplift of the region. Therefore, rising of deep asthenosphere through a slab tear would not be required to explain the generation of volcanism in the Central Anatolian volcanic province.

Our findings indicate that the Eğrikuyu monogenetic field displays mantle source heterogeneity in Quaternary basalts of the Central Anatolian volcanic province, and it also represents a distinct mantle source isotopically similar to the western Anatolian mantle (WAM) of Aldanmaz et al. (2015). In addition, we reveal the importance of a detailed petrological investigation in a small monogenetic basaltic field with increased geochemical sampling.

ACKNOWLEDGMENTS

This study was the M.Sc. dissertation of G. Uslular, and it was supported by grants from the Scientific Research Projects of Muğla Sıtkı Koçman University (project numbers 13/95 and 15/211) and the “Continental Dynamics: Central Anatolian Tectonics (CD-CAT)” project funded by National Science Foundation (grant EAR 1109762 to Donna Whitney). We thank Mary R. Reid for supporting some parts of the field trips. We would like to thank Donna Whitney and Andreas Kronz for the mineral chemistry analyses. We also thank Serhat Köksal for the isotope analyses. Uslular is especially thankful to İlkay Kuşcu for his pertinent and helpful comments. We kindly acknowledge the editorial handling by Raymond M. Russo and Guest Associate Editor Christian Teyssier and also comments by Karoly Németh and one anonymous reviewer. Primitive versions (or templates before vector modifications) of the geochemical diagrams were drawn using GCDkit (Janoušek et al., 2006).

1Supplemental Material. Supplemental File: Analytical procedures. Supplemental Data: Whole data set of mineral chemistry analyses by EMPA. Please visit https://doi.org/10.1130/GES01682.S1 or access the full-text article on www.gsapubs.org to view the Supplemental Material.
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