Ophiolitic rocks derived from Tethyan seaways are abundant in Anatolia; many are in arrays that mark sutures between Eurasia, Gondwana, and continental ribbons and island arcs. Ophiolitic fragments also occur dispersed between sutures, indicating tectonic transport of possibly hundreds of kilometers. Scattered fragments of the Central Anatolian Ophiolite (CAO) have been interpreted as originating in oceans to the north, west, and/or south of their current locations, with implications for the magnitude and direction of transport and the relation of ophiolite obduction to regional metamorphism of the underlying continental-margin terrane (Central Anatolian Crystalline Complex [CACC]). Ophiolitic clasts (primarily gabbro) are widespread in sedimentary basins and alluvial terraces, particularly in the southern CACC. Petrologic and geochemical data from (meta)gabbro outcrops, gabbro clasts in conglomerates, and gabbro cobbles on alluvial terraces near the Niğde Massif, a metamorphic dome at the southern tip of the CACC, indicate paleosources and can be used to reconstruct the history of ophiolite emplacement, metamorphism, erosion, and dispersal. (Meta)gabbro at the northern margin of the Niğde Massif is geochemically similar to CAO gabbro: both have low Ti/V and depleted high field strength elements, typical of boninitic (forearc) magma, although Niğde mafic and associated ultramafic rocks were metamorphosed at middle to upper amphibolite facies, and the rest of the CAO were metamorphosed at (sub)greenschist-facies conditions. Amphibolite-facies mafic and ultramafic rocks near the contact with underlying CACC metasedimentary rocks have been ductilely deformed in mylonitic high-strain zones with top-to-south kinematics likely related to tectonic interleaving of ophiolitic and continental margin rocks at depth. The confinement of high-grade metaophiolite to the southern tip of the CACC may indicate oblique and diachronous obduction from south to north.
Whole-rock trace-element data for gabbro clasts indicate that early to middle Miocene sediments were derived from mixed sources (CAO and Tauride ophiolites), whereas later Miocene sediments were sourced entirely from the CAO, even those on the opposite side (south) of the Niğde topographic high. These results may indicate that late Miocene uplift and arching of the Tauride Mountains at the southern margin of the Central Anatolian plateau drove reorganization of sediment dispersal and topographic disconnection of Miocene depocenters from their CAO sources.
Ophiolite belts mark the locations of suture zones that represent the sites of oceans that have closed during subduction and collision. Ophiolites also occur as dispersed fragments between suture zones, indicating long-distance tectonic transport (obduction), in some cases possibly driving high-grade metamorphism of the underlying continental margin (e.g., Coleman, 1981; Yalınız et al., 1996; Searle and Cox, 1999; Floyd et al., 2000; Searle and Treloar, 2010).
Anatolia is festooned with ophiolites derived from various strands of Tethyan seaways and now exposed in suture zones (Fig. 1). Other Anatolian ophiolites, such as the Central Anatolian Ophiolite (CAO), occur as isolated fragments dispersed between suture zones (Fig. 1). Based on paleotectonic reconstructions, obducted Anatolian ophiolite is inferred to have been “quite continuous over a vast region,” on the scale of the Semail ophiolite in Oman (Robertson et al., 2009).
Late Cretaceous ophiolites occur throughout central and southern Anatolia and include the Tauride ophiolites emplaced on the Tauride carbonate platform in the south (now comprising the Tauride Mountains), ophiolites marking the Izmir-Ankara-Erzincan suture zone (IAESZ) to the north, and the Central Anatolian Ophiolite (CAO) within the interior of the Central Anatolian Crystalline Complex (CACC) (e.g., Dilek et al., 1999; Rolland et al., 2012; Parlak et al., 2013a). Middle Jurassic ages have been reported for ophiolitic mélange from the IAESZ where it forms the northern boundary of the CACC, although the obduction has been dated as Late Cretaceous (Çelik et al., 2011; Rolland et al., 2012).
Under debate is the paleogeographic origin of the Late Cretaceous ophiolites exposed in Central Anatolia. Obduction of a large nappe composed of Jurassic and Cretaceous oceanic crust and derived from the Izmir-Ankara-Erzincan strand of the Neotethys Ocean has been proposed for the origin of both the CAO and Tauride ophiolites (e.g., Hassig et al., 2016a, 2016b; van Hinsbergen et al., 2016). Others suggest that the Tauride ophiolites were emplaced from a subduction zone located between the CACC and Tauride terranes and now represented by the Inner Tauride suture zone (ITSZ) (e.g., Dilek et al., 1999; Parlak et al., 2013a).
The Central Anatolian Ophiolite occurs as fragments of gabbro and related rocks dispersed among exposures of high-grade metasedimentary rocks and associated granitoids that comprise the CACC (Göncüoğlu et al., 1996–1997) (Fig. 2). Field relations in the CACC require that the Central Anatolian Ophiolite was emplaced prior to calc-alkaline intrusion and regional metamorphism, and ophiolite obduction has been proposed as the cause of high-grade metamorphism of the CACC (e.g., Akıman et al., 1993). U-Pb ages for most calc-alkaline intrusions and high-grade metamorphic rocks in the CACC are ca. 84–74 Ma (Whitney and Hamilton, 2004; Köksal et al., 2012). However, the age of peak metamorphism and crystallization of the earliest intrusions in the CACC is ca. 91 Ma (Whitney et al., 2003, 2007), similar to ages determined for ophiolitic rocks in Central Anatolia (ca. 90 Ma; e.g., Yalınız et al., 1996; Yaliniz et al., 2000; van Hinsbergen et al., 2016). The occurrence of oceanic plagiogranite with magmatic ages that are almost identical to the age of anatexis in the underthrust continental margin units raises questions about timing and dynamics of ophiolite obduction and the implications for regional metamorphism.
Abundant gabbro clasts occur in basins surrounding the southern CACC; these sediments provide insight into the timing of denudation of the now highly fragmented Central Anatolian Ophiolite. Therefore, to better understand the tectonic and metamorphic history of ophiolitic gabbro in Central Anatolia and to track gabbro from source (ophiolites) to sinks (basins and terraces), we use petrologic, geochemical, and structural data to document and interpret (1) the origin and tectonic-metamorphic evolution of (meta)gabbro and related rocks that crop out at the northern margin of the Niğde Massif; and (2) gabbro clasts in basins and gabbro cobbles on alluvial terraces to the north, east, and south of the Niğde Massif (Fig. 3). We integrate these results to discuss the tectonic, metamorphic, and landscape evolution of Central Anatolia from the Late Cretaceous to the Mio-Pliocene.
OVERVIEW OF CENTRAL ANATOLIAN GEOLOGY
Central Anatolia consists largely of a triangular-shaped microcontinent, ∼200 km long on each side, bordered by suture zones and major strike-slip and/or normal faults (Fig. 2). The microcontinent is the Central Anatolian Crystalline Complex, which is also known as the Kırşehir Block, Kırşehir Massif, or Niğde-Kırşehir Massif, and is composed dominantly of high-grade metasedimentary rocks (marble, quartzite, and schist) and granitoid intrusions (Fig. 2). Amphibolite also occurs in the CACC and is interlayered with metasedimentary rocks, commonly with calc-schist. The metasedimentary basement is considered to be the metamorphosed equivalent of Gondwanan continental margin sequences exposed in the Tauride Belt to the south (Kocak and Leake, 1994) (Fig. 2).
Paleomagnetic data from the CACC indicate significant internal rotation along strike-slip faults since the Late Cretaceous (e.g., Görür et al., 1984; Lefebvre et al., 2013). Before faulting, the CACC may originally have been an elongate, north-south–oriented ribbon or magmatic arc (Lefebvre et al., 2013). In this reconstruction, the calc-alkaline intrusive suite in the CACC is interpreted as a continental arc associated with east-dipping subduction along the Inner Tauride Ocean to the west of the CACC.
The major geologic components that are important for this study of Central Anatolia are the Niğde Massif (part of the CACC), ophiolites (Central Anatolian Ophiolite and Tauride Ophiolites), and basins surrounding the Niğde Massif (Ecemiş to the east, Ulukışla to the south, and Tuz Gölü-Konya to the WSW) (Fig. 2). In the first part of the study, we focus on determining whether the mafic complex on the northern margin of the Niğde Massif (here termed the Niğde Mafic Complex) (Fig. 2) was part of the CAO (i.e., oceanic gabbro) or part of the Niğde Massif (i.e., gabbro intrusions in CACC metasedimentary rocks). We then use this information to track gabbro from sources to sedimentary basins and alluvial deposits in the region. In the following sections, we briefly describe each of these geologic components of Central Anatolia.
The Niğde Massif is a dome of high-grade metamorphic rocks at the southern end of the CACC (Figs. 2 and 3) and has been interpreted as a metamorphic core complex (Whitney and Dilek, 1997). The age of the metamorphic peak is indicated by ca. 91 Ma zircon (U-Pb) rims in sillimanite–K-feldspar migmatite from the core of the dome (Whitney et al., 2003). Monazite U-Pb ages are slightly younger (ca. 85 Ma). Peak metamorphism was followed soon after by intrusion of the crustally derived (S-type) Üçkapılı granite during extension and/or transtension and initial rapid cooling of metamorphic and intrusive rocks (e.g., hornblende 40Ar/39Ar ages are as old as 88 Ma, followed by 81–79 Ma muscovite and 79–74 Ma biotite ages) (Whitney et al., 2003, 2007).
Following Late Cretaceous exhumation and erosion into adjacent basins, the massif records a second cycle of burial and exhumation that also affected the deepest levels of the overlying sedimentary basin and that occurred from ca. 45–50 Ma (start of reburial) to <15–20 Ma (final cooling below ∼100 °C), with a peak (maximum P-T ∼10 km, 300 °C) at ca. 25 Ma (Fayon and Whitney, 2007; Umhoefer et al., 2007; Idleman et al., 2014). This second cycle was possibly a result of transpression driven by a late Oligocene–early Miocene phase of the Arabia-Eurasia collision in SE Anatolia (Clark and Robertson, 2005; McQuarrie and van Hinsbergen, 2013). Final exhumation was completed by mid-late Miocene time (Fayon et al., 2001); high-grade metamorphic rocks in the dome are unconformably overlain by 4.9 Ma ignimbrite (Whitney et al., 2008). Final exhumation occurred in a transtensional to extensional setting during the transition to the modern tectonic escape system.
Sedimentary Basins and Alluvial Deposits
The Ulukıșla Basin is located on the Inner Tauride suture, between the Niğde Massif and the Tauride (Bolkar) Mountains, which form the southern margin of the Central Anatolian Plateau (Fig. 2) and were uplifted >1.5–2 km since the late Miocene (<8 Ma; Schildgen et al., 2012). Tauride platform-carbonates are structurally overlain by ophiolites interpreted to have been derived from the Inner Tauride Ocean (e.g., Dilek and Whitney, 1997; Parlak et al., 2013a), including the Pozantı-Karsantı (Aladağ) and Alihoca ophiolites (Figs. 1 and 2), and the Ulukıșla basin is underlain by ophiolitic rocks (Clark and Robertson, 2002, 2005; Gürer et al., 2016). The basin records a complex history of sedimentation and deformation dominated by Late Cretaceous to Paleocene extension and Eocene to Oligocene contraction (Clark and Robertson, 2002, 2005; Gürer et al., 2016).
The upper Ulukıșla Basin comprises late Miocene–early Pliocene deposits of the adjacent Tuz Gölü-Konya Basin to the west (Cihanbeyli conglomerates, volcanic deposits, and lacustrine sediments; Huvaz, 2009; Fernandez-Blanco et al., 2013) and unconformably overlies folded Paleogene sedimentary rocks. The Ecemiş Basin, which is contiguous with the Ulukıșla Basin to the NE, is located between the Niğde Massif and the Tauride (Aladağ) Mountains, along the southern (Ecemiş) segment of the Central Anatolian fault zone (Figs. 2 and 3). The Ecemiş Basin shares the Paleogene marine sedimentation and contraction to transpression history with the Ulukıșla Basin and also records a transition to transtension in the early to middle Miocene. The Ecemiş fault lies along the eastern margin of the CACC and separates this terrane from the Tauride platform.
Gabbro-bearing fluvial conglomerates in the basins surrounding the Niğde Massif occur within the Oligo-Miocene Çukurbağ Formation and in the Mio-Pliocene Cihanbeyli Formation of the Tuz Gölü Basin (a unit designated on MTA maps as Mio-Pliocene “m3pl,” Fig. 3). The Çukurbağ Formation is composed of fluvial and lacustrine deposits, including alluvial-fluvial conglomerates containing a wide variety of clast types. The Çukurbağ Formation is typically tilted (locally steeply) and/or moderately to openly folded (Fig. 4A), likely by early to mid-Miocene strike-slip and transtensional deformation along the Ecemiş fault (Jaffey and Robertson, 2001).
In the study area, the Cihanbeyli Formation is gently tilted toward the WNW by 2.5° owing to the arching of the Tauride Mountains and is affected by minor, N40°-striking undulations and N110°-striking normal faults, parallel to the normal faults that more significantly affect sedimentation of the Tuz Gölü depocenter farther west (Fernandez-Blanco et al., 2013). The formation comprises lacustrine sediments intercalated with tuff and conglomerate (Fig. 4B) and thickens westward, from 60 m near Postallı to more than 120 m near Havuzlu (Fig. 3). Meters-thick conglomerates dominated by gabbro and marble cobbles (up to boulder-size) are interfingered with lacustrine travertine.
Boulders and cobbles litter the surface of a large mesa of cap-travertine that covers most of the Cihanbeyli (m3pl) Formation SW of the Niğde Massif (Fig. 4C). The cap does not exist in the east, allowing for erosion to incise deeply into the underlying Paleogene to Miocene sedimentary rocks of the Ulukıșla Basin. There, the Cihanbeyli Formation is only preserved on a few hilltops along the southern margin of the Niğde Massif.
Central Anatolian Ophiolite
The Central Anatolian Ophiolite (CAO) refers to a number of petrologically and structurally similar outcroppings of ophiolitic rocks that occur throughout the CACC. Although disaggregated, the CAO bears the hallmark qualities of a Tethyan ophiolite; i.e., it has a suprasubduction zone origin and was obducted onto a continental margin. The dispersed fragments of the CAO (Figs. 1 and 2) are in tectonic contact with CACC metasedimentary units and consist of ultramafic rocks, gabbro, plagiogranite, mafic and felsic volcanic rocks (including mafic sheeted-dikes and pillow lava), and pelagic sedimentary rocks (Yalınız et al., 1996; Floyd et al., 2000; Yalınız et al., 2000; Toksoy-Köksal and Göncü;oğlu, 2001; Yalınız, 2008). Plagiogranite in the CAO crystallized at ca. 90 Ma (zircon U-Pb; van Hinsbergen et al., 2016). Calc-alkaline to alkaline granitic plutons of the CACC crosscut ophiolitic and metasedimentary rocks, indicating that obduction pre-dated continental arc magmatism (Yalınız et al., 1996; Yılmaz and Boztuğ, 1998). The oldest of these calc-alkaline granitoids are 84–82 Ma (e.g., zircon U-Pb: Köksal et al., 2012).
The most complete exposures of the CAO are the Sarıkaraman and Çiçekdağ localities (Fig. 2), where gabbro is associated with pillow lavas and pelagic sedimentary rocks (Yalınız et al., 1996; Yılmaz and Boztuğ, 1998). In addition, isolated exposures of gabbro have also been attributed to the CAO based on their geochemical characteristics (Floyd et al., 2000; Kocak et al., 2005; İlbeyli, 2008) (Fig. 2). Ultramafic rocks are not well exposed in the CAO compared to other Tethyan ophiolites.
Different localities of the CAO record different degrees of metamorphism, although most are interpreted to have experienced little to no effect from the regional metamorphism that characterizes the structurally lower metasedimentary rocks (Table 1). The volcanic portions of the CAO typically contain greenschist-facies assemblages that have in some cases been attributed to ocean-floor hydrothermal metamorphism (İlbeyli, 2008) or to contact metamorphism by postobduction intrusion of granitoid magma (Yılmaz and Boztuğ, 1998). Higher-grade conditions are observed in some gabbro (Table 1), as indicated by hornblende rims on pyroxene and hornblende pseudomorphs after pyroxene; in some cases, these have been attributed to late magmatic processes (Kocak et al., 2005).
Gabbro in Central Anatolia also occurs locally with granitic plutons in the CACC metasedimentary massifs (e.g., in the Ağaçören region, Fig. 2) and has been interpreted as intrusive based on petrology, geochemistry, and field relations (Kadıoğlu et al., 2003). Gabbro at the northern margin of the Niğde Massif has also been mapped as intrusions (e.g., Atabey, 1989; Atabey et al., 1990), although it has many features of an ophiolitic gabbro (see next section).
In the following sections, we describe the petrology, structure, and geochemistry of the Niğde Mafic Complex, other outcroppings of mafic and ultramafic rocks in neighboring regions, and gabbro clasts and cobbles in conglomerate and on alluvial terraces near the Niğde Massif. We use these data to discuss the tectonic and metamorphic history of the mafic and ultramafic rocks and their subsequent erosion and dispersal during regional Miocene uplift in Central Anatolia.
Nigğde Mafic Complex and Neighboring Mafic and Ultramafic Rocks
The Niğde Mafic Complex (NMC, Figs. 1–3) is dominated by gabbro and gabbronorite that is crosscut by plagiogranite and diabase dikes, all of which have been variably metamorphosed and deformed (Figs. 5A and 5B). Evidence for intense deformation (isoclinal folds and mylonite zones) occurs in all NMC lithologies (e.g., Fig. 5C). The mafic complex is also crosscut by undeformed dikes of muscovite-garnet-tourmaline granitoids (Fig. 5C), similar to those associated with the Üçkapılı granite, which outcrops mainly in the central Niğde Massif (Whitney and Dilek, 1998) (Fig. 3).
The contact of the NMC with structurally underlying Niğde metasedimentary rocks (primarily marble) is a north-dipping high-strain zone (Figs. 3A, 3B, 6A, and 6B) with top-to-S kinematics. Metaperidotite occurs locally near the NMC-metasediment contact (Figs. 6B and 6D).
Mafic and ultramafic rocks also crop out in the Ecemiş fault zone (east of the Niğde Massif) and in isolated outcrops north of the massif (Figs. 2 and 3). Ecemiş gabbro is compositionally layered, undeformed, and lacks plagiogranite and diabase dikes. Ultramafic rocks are highly serpentinized.
North of the NMC, Neogene volcanic rocks of Cappadocia are the most widely exposed lithology (Fig. 2). In this northern region, gabbro monadnocks dot the landscape up to ∼40 km beyond the northern boundary of the Niğde Massif (Figs. 2 and 3). In some locations, serpentinite, plagiogranite, and diabase occur with gabbro. Near Yıldıztepe (Fig. 3), large quantities of basalt occur with pegmatitic gabbro and massive hornblende gabbro.
PETROGRAPHY AND MINERAL COMPOSITIONS: OUTCROP SAMPLES
Eighty-four samples were collected from mafic complexes: 65 from the NMC, 11 from the Ecemiş area, and 8 from isolated gabbro exposures north of the Niğde Massif (Fig. 3; Supplemental Table S11). Unless specified, samples are referred to with the suffix ND15- omitted.
Mineral compositions were analyzed in 19 representative thin sections using a JEOL JXA-8900R electron microprobe at the University of Minnesota. The accelerating voltage was 15 kV with a beam current of 20 nA and a spot size of 5 μm for beam-sensitive minerals (e.g., hornblende) and 1 μm for other minerals (e.g., olivine and pyroxene). In the following sections, we describe mineral assemblages, textures, and compositions (Table 2) for gabbro, diabase, basalt, plagiogranite, and ultramafic rocks. More detailed descriptions and mineral composition documentation are in Ray (2016). Mineral and component abbreviations follow the scheme of Whitney and Evans (2010).
Mineral assemblages and textures vary significantly between the NMC, Ecemiş, and northern parts of the study area. In the NMC, gabbroic rocks are clinopyroxene (Cpx) gabbro and two-pyroxene gabbro (gabbronorite) that exhibit variable degrees of amphibolitization and deformation (Figs. 7A–7C). Mineral compositions vary between samples: pyroxene Mg# ranges from 64 to 79 for orthopyroxene (Opx) and 37–47 for Cpx, and plagioclase is zoned from more calcic cores (An74–90) to more sodic rims (An49–71) (Table 2). Some gabbro has cumulate layering of alternating pyroxene-rich and plagioclase-rich bands.
Hornblende occurs in all NMC gabbro observed and exhibits a variety of textures, including as interstitial igneous grains and as metamorphic rims on Cpx and Opx (Fig. 7B). The most common amphibole composition is magnesiohornblende (Table 2).
In deformed gabbro, hornblende defines the foliation and is flattened or fish-shaped (Fig. 7C). Plagioclase in hornblende-rich gabbro also typically shows evidence for deformation; in one mylonitic gabbro (ND14-02B), plagioclase porphyroclasts (An78–87) are mantled by less anorthitic wings (An49–68) (Fig. 7C and Table 2).
Gabbro in the Ecemiş region is distinct from that of the NMC in that some Ecemiş gabbro contains olivine (not observed in the NMC), and hornblende is absent (abundant in the NMC) (Fig. 7D). All gabbro observed in the Ecemiş area is undeformed. Mineral compositions in olivine gabbro ND14-07A are Mg# = 83 (Ol), Mg# = 47 (Cpx), and An85 (unzoned plagioclase [Pl]) (Table 2). Some gabbro exhibits magmatic textures; in others, low-grade secondary minerals such as serpentine (after olivine) and uralite and/or actinolite (after Cpx) have obliterated the original texture.
North of the NMC, a diverse range of gabbro types occurs, including olivine gabbronorite (34B) in which Cpx contains blebs of hornblende with variable composition (Fig. 7E); gabbro (53A) containing poikilitic tschermakitic hornblende and plagioclase (An85–91) overprinted by aggregates of actinolite needles; and pegmatitic hornblende-gabbro (53C).
Plagiogranite dikes occur in NMC and northern gabbro. Plagiogranite contains subequal amounts of quartz and plagioclase, with lesser amounts of hornblende or cummingtonite, biotite, and accessory zircon. Plagioclase has normal zoning (Ca-richer cores and Na-richer rims); e.g., An43–55 in cores and An27–35 near rims (sample 40A) (Table 2). In the NMC, mylonitic zones are commonly concentrated along plagiogranite dike boundaries (Fig. 5B). Mylonitic plagiogranite displays foliation-parallel myrmeckite.
Diabase and Basalt
Diabase in the NMC occurs as aphanitic to porphyritic dikes or sills in gabbro. The mineral assemblage in diabase is similar to that of (meta)gabbro but has a smaller matrix grain size (∼500 μm in diabase, compared to 0.5–3 mm in non-mylonitized gabbro) and strongly zoned plagioclase phenocrysts. A typical mineral assemblage (e.g., sample 40B) is magnesiohornblende, zoned plagioclase (cores: An74–91; rims: An45–67), K-feldspar, biotite, and quartz. One diabase analyzed (sample 20) also preserves relict Cpx. Many diabase dikes and sills have a foliation defined by hornblende and textural features that indicate subsolidus recrystallization of plagioclase and hornblende (Fig. 7F).
In the region north of the NMC, undeformed hornblende diabase occurs near Doğanlı (sample 32, Fig. 3). Low-grade metabasalt (sample 53B) occurs at Yıldıztepe; it is aphanitic and composed of zoned plagioclase (cores: An74–80; rims: An29–51), quartz, and hornblende partially replaced by actinolite and accessory Fe-Ti oxides.
Ultramafic rocks in the field area include hornblendite, clinopyroxenite, and metaperidotite and/or serpentinite; in this paper, we describe only the metaperidotite. Peridotite in the region is typically serpentinized and appears to be mostly harzburgite based on the presence of mesh and bastite textures, rarely with relict olivine and Opx (Figs. 8A and 8B). In the Ecemiş and northern areas, metaperidotite occurs as rarely outcropping serpentinite and—based on observations from float—likely underlies a fairly large region.
In the NMC, metaperidotite occurs as meter-scale blocks that are typically near contacts with metasedimentary rocks (Göncüoğlu et al., 1991). Near Özyurt (Fig. 6D), an olivine-talc-tremolite rock (48A and 48D) contains elongate “jackstraw” or spinifex-textured olivine. This texture is known from meta-ultramafic rocks in other orogens (e.g., Evans and Trommsdorff, 1974; Hietanen, 1977; Snoke and Calk, 1978; Bakke and Korneliussen, 1986; Trommsdorff et al., 1998; Padrón-Navarta et al., 2011; Evans and Cowan, 2012). Elongate olivine (and mesh-textured serpentine pseudomorphs after olivine) occurs in a groundmass of talc and tremolite, in which tremolite has partially replaced talc (Fig. 8C).
NIGĞDE MAFIC COMPLEX DEFORMATION
Although previously described as “massive” gabbro or gabbro displaying magmatic fabrics (e.g., Gautier et al., 2008), ductile deformation features are prevalent in the NMC. Foliation defined by amphibole (± biotite) is observed throughout the NMC, but not in the Ecemiş or northern areas. The NMC also contains folded dikes and veins, compositional layering, boudinage, and mylonite zones. Foliation is typically north dipping (Fig. 6), consistent with the orientation of foliation in the underlying metasedimentary rocks. Shear bands in the NMC indicate top-to-south dextral motion.
Intensely deformed (mylonitic) rocks occur near contacts between the NMC and underlying Niğde Massif rocks (high-strain zones in Figs. 6A and 6B). Away from the ∼20–30-m-wide high-strain zones, deformation is highly variable; some gabbro outcrops vary from apparently undeformed to foliated within a few meters. In the high-strain zones, mafic and felsic dikes are transposed into the plane of foliation (Fig. 5A). Mylonitization affected all rock types of the NMC. Microstructures indicate ductile deformation and synkinematic mineral growth. The main deformation phase postdated intrusion of plagiogranite and diabase dikes but predated intrusion of undeformed, crosscutting pegmatite and aplite dikes (Fig. 5D) inferred from their Al-rich mineral assemblages (e.g., Ms, Tur, and Grt) to be related to the Late Cretaceous Üçkapılı Granite, which intruded Niğde metasedimentary units (Fig. 3).
The thermal history of (meta)gabbro and diabase in the Niğde Mafic Complex has been partially reconstructed using mineral compositions (Table 2) and two geothermometers that provide information about conditions of igneous crystallization, metamorphism, and deformation (two-pyroxene and hornblende-plagioclase). Application of the two-pyroxene geothermometer to four gabbronorites from the NMC and the northern region yielded temperatures (T) ∼840–950 °C (average ∼880 °C) (Wells, 1977), interpreted as the magmatic crystallization temperature (Table 3).
The Holland and Blundy (1994) geothermometer was applied to NMC samples containing hornblende and plagioclase. Multiple hornblende-plagioclase pairs were evaluated for each sample, leading to a spread of temperatures. We report results for temperatures at 0.5 GPa, but calculations were also made using pressures higher and lower by 0.5 GPa. This pressure range affects calculated temperature by ±50 °C, which is within the uncertainty of this technique.
Calculated temperatures at 0.5 GPa are ∼740–785 °C for a metagabbronorite (sample 15, Aktaş locality, Fig. 6A) and ∼680–760 °C for a metadiabase from the same outcrop (sample 40B; Table 3). A protomylonite from the Aktaş high-strain zone (sample 14-02B; Fig. 6A) yielded T ∼680–740 °C. Protomylonites from the western and eastern parts of the Uluağaç high-strain zone (Fig. 6C) yielded T ∼560–620 °C (sample 51F) and ∼730–745 °C (sample 22C), respectively. These results indicate igneous crystallization of gabbroic rocks at T >840 °C and metamorphism and deformation at ∼700 ± 50 °C, with possibly some deformation and metamorphism at lower T (∼550 °C).
The assemblage talc + forsterite + tremolite observed in metaperidotite from the NMC is stable over a relatively narrow range of amphibolite-facies temperatures (550–650 °C), in agreement with thermometric results for metamorphism of NMC mafic rocks. Microstructural evidence from deformed metaplagiogranite, including foliation-parallel myrmeckite, dynamic recrystallization of plagioclase, and subgrain rotation, suggests high-T deformation conditions (>600 °C), also in agreement with results of hornblende-plagioclase thermometry from the metagabbroic rocks.
Overall, the evidence overwhelmingly indicates middle-upper amphibolite-facies temperatures for metamorphism and deformation of the NMC. Depending on geothermal gradient, this implies a depth of metamorphism of >16–18 km (for gradients of 30–35 °C/km).
GABBRO CLAST CHARACTERISTICS
In order to understand the history of gabbro erosion, transport, and deposition in basins, gabbro cobbles were sampled from the Oligocene–Miocene Çukurbağ Formation of the Ecemiş Basin and the upper Miocene Cihanbeyli Formation (m3pl unit) where it overlies the Ulukışla Basin (Fig. 3). Seventeen gabbro clasts were sampled from the Çukurbağ Formation of the Ecemiş area; eight gabbro clasts and one diabase clast were collected from the Cihanbeyli Formation capping the Ulukıșla Basin (two from in-place conglomerates and the remaining from surface deposits); and five gabbro and two diabase cobbles were sampled from “m3pl” conglomerates north of the Niğde Massif (Fig. 3).
Çukurbagğ Conglomerate and Ecemisş Basin
In the Ecemiş Basin, the lithologic assemblage of gabbro-bearing conglomerate was analyzed in outcrop at four sites: near the village of Hacıbeyli (NE of the Niğde Massif), two sites near Bademdere (E of the massif), and one site near Çamardı (E of the massif) (Figs. 3 and 9). These sites each contained a substantial proportion of gabbroic clasts; other lithologies observed were granite, marble, quartzite, limestone, sandstone, and various volcanic rocks (Fig. 9 and Table 4). Some of these rock types indicate a particular source area. Metasedimentary rocks likely indicate a source area in the Niğde Massif, unmetamorphosed limestone was likely derived from the Aladağ (Tauride) Mountains, and volcanic rocks were likely derived from the Ulukışla Basin. Outcropping sources of gabbro occur to the north (CAO and/or NMC) and south and/or east (Alihoca and Pozantı-Karsanti ophiolites) (Fig. 2).
Seventeen gabbro clasts and one plagiogranite clast from six sites in the Ecemiş Basin were analyzed petrographically. Gabbro types include clinopyroxene gabbro, olivine gabbro, and hornblende gabbro (e.g., Figs. 10A and 10B). All clasts examined from the Çukurbağ Formation lack evidence for ductile deformation and all record a greenschist-facies overprint of magmatic textures (Figs. 10A and 10B). Metamorphic hornblende was not observed in any of the Çukurbağ samples, although in some cases textural relationships were unclear because of extensive retrogression.
Cihanbeyli Conglomerate–Tuz Gölü Basin and Conglomerates North of Nigğde Massif
At the top of the Ulukıșla Basin, the Cihanbeyli Formation unconformably overlies Paleogene and Miocene rocks and is composed of lacustrine sediments with intercalations of volcanic tuff and fluvial conglomerate (MTA, 2002) (Fig. 4B). A volcanic tuff near sample locality 43 (Fig. 3) was dated at 6.2 ± 0.12 Ma (40Ar/39Ar hornblende; M. Meijers and G. Brocard, personal commun., 2016), and is possibly related to a 6.81 ± 0.24 Ma ignimbrite that is widespread across the eastern Tuz Gölü Basin (Özsayin et al., 2013) and that crops out as far east as Havuzlu (Fig. 3). A poorly lithified conglomerate, locally containing abundant gabbro, occurs ∼25 m above the 6.2 Ma tuff and is locally interfingered with travertine.
Two gabbro samples were obtained from intact Cihanbeyli outcrops displaying stratigraphy (localities 43 and 46); three samples were surface clasts from hilltops representing incised remnants of the Cihanbeyli paleosurface (localities 42, 44, and 45), and four samples were float presumably derived from the nearby Cihanbeyli hillocks (17A, 17B, 18, and 41) (Figs. 3 and 4C). Cobbles litter hilltops along the southern margin of the Niğde Massif, in the lateral continuation of the travertine mesa farther west. Float cobbles found locally at lower elevations may have been transported by downslope processes. Gabbro cobbles on top of surfaces are typically >50 cm, whereas those from outcrops within the Cihanbeyli Formation (e.g., Figs. 4B and 4C) range from a few cm up to 50 cm (Table 4). The occurrence of such coarse cobbles and boulders on terrace surfaces as opposed to in outcrops may be an effect of weathering, whereby smaller clasts were transported by runoff or broken down and incorporated into soil, thus concentrating large boulder-sized particles on terrace surfaces and in adjacent gullies.
Nine gabbro cobbles were analyzed petrographically from the Cihanbeyli unit capping the Ulukıșla Basin: five are igneous-textured olivine gabbro (17A, 18, and 44) or clinopyroxene gabbro (17B and 45) with accessory green spinel and greenschist-facies overprint (chlorite, uralite, and/or epidote). Three (42, 43, and 46) are hornblende metagabbro with relict igneous clinopyroxene, and one (41) is a hornblende-bearing metadiabase with similar texture to metadiabase of the NMC (Figs. 10C and 10D). Of these, samples 41 and 46 also have a greenschist-facies overprint of chlorite and/or epidote.
In several locations north of the Niğde Massif (localities 33, 35, and 49; Fig. 3), gabbro-rich conglomerates were observed to form small hills locally capped by ignimbrite (Figs. 3 and 4D). The capping ignimbrite is variably mapped (Le Pennec et al., 1994, 2005) as the Kızılkaya (5.2 ± 0.1 Ma, Aydar et al., 2012; 5.5 ± 0.2 Ma, Lepetit et al., 2014), Gordeles-Sofular (6.3 ± 0.1 Ma, Aydar et al., 2012; 6.2 ± 0.1 Ma, Lepetit et al., 2014), or Cemilköy ignimbrite (7.6–8.4 Ma, Le Pennec et al., 2005; 6.6–7.2 Ma, Aydar et al., 2012). These northern conglomerates are tentatively correlated with the Cihanbeyli conglomerate south of the Niğde Massif because of the similarity in age (inferred from the capping ignimbrite) and appearance (poorly lithified and intercalated with red sandstone). Northern deposits, however, are dominated by conglomerate (at least 10 m thick) with minor sandstone, and the southern deposits have relatively thin (2–4 m) conglomerate and are associated with lacustrine deposits. Despite these differences, and the observation that late Miocene or early Pliocene ignimbrite caps conglomerate in the north and that the late Miocene ignimbrite in the south is stratigraphically below most of the conglomerates, the close relationship of these units indicates they are all likely late Miocene in age.
Clast type and size were analyzed at a site near Çayırlı-Yarhisar (Fig. 3): clasts range from a few cm to >1 m in diameter, and lithologies observed are similar to those in the NMC: gabbro, diabase, and plagiogranite. Another “conglomerate hill” was observed near Doğanlı (sample locality 33; Fig. 3), in which boulders up to 50 cm in diameter are present. Lithologies observed in this conglomerate include ultramafic rocks, gabbro, granite, and basalt.
Five gabbro and two diabase cobbles were sampled from conglomerates north of the Niğde Massif (Fig. 3). One diabase (49B) has a porphyritic texture with plagioclase and hornblende phenocrysts and is unmetamorphosed; the other diabase (ND14-05B) has a porphyritic texture and an overprint of secondary chlorite. Clinopyroxene-gabbro sample 49A has been ∼70% altered to uralite. The remaining four gabbro samples are hornblende metagabbro in which hornblende is interpreted to be metamorphic based on its textures relative to relict igneous minerals.
Whole-rock compositions were obtained for gabbro, diabase, and plagiogranite, as well as amphibolite interlayered with Niğde Massif metasedimentary rocks, in order to determine their geochemical characteristics and evaluate their tectonic affinities (Table 5). Whole-rock data for clasts in conglomerate and cobbles on alluvial terraces were also determined (Table 6).
The least altered samples from each lithologic group were analyzed and include: 14 “outcrop” gabbro (ten from the NMC, three from the Ecemiş fault zone, and one from the northern region), six diabase and/or basalt (four NMC and two northern), four plagiogranite (three NMC and one northern), four fine-grained amphibolites from the Niğde Massif (three from the structurally highest, marble-dominated unit; one from the structurally lowest, migmatitic core area); and 25 cobbles: ten from the Çukurbağ Formation, nine from the Cihanbeyli Formation south of the Niğde Massif, and six from conglomerates north of the Niğde Massif (sample locations in Table S1[footnote 1]). Samples were analyzed at Macalester College using a Philips PW-2400 X-ray fluorescence (XRF) spectrometer and Super-Q analytical software. A detailed description of the preparation procedure and analytical methods is given in Vervoort et al. (2007).
A subset of samples was further analyzed for rare-earth element (REE) and other trace elements using inductively coupled plasma mass spectrometry (ICP MS) at the University of Wisconsin–Eau Claire (Tables 4 and 5): seven outcrop-gabbro (four NMC, two Ecemiş, and one northern), two diabase (NMC), one basalt (northern), and two Niğde Massif amphibolite (one from upper unit and one from lower). The tungsten-carbide container used for grinding samples resulted in contamination of Nb; thus interpretation of this element is made with caution. Incomplete zircon dissolution is reflected by anomalously low Zr and Hf values for the two amphibolite samples (ND01-32B and ND15-47B); therefore, Zr and Hf values obtained using XRF are reported for these two samples.
Gabbro, Diabase, and Amphibolite (Outcrop Samples)
Despite the range of textures and mineral assemblages, all analyzed non-cumulate gabbro (screened using the parameters Ni < 250 ppm, Sc < 50 ppm, and Al2O3 < 20 wt%) of the NMC, Ecemiş area, and northern region have similar whole-rock major-element compositions: 46–49 wt% SiO2, very low TiO2 (0.13–0.45 wt%), and high Mg# 61–87 (Table 5). Compared to gabbro, most diabase has slightly higher SiO2 (49–55 wt%) and similar TiO2 (0.23–0.49 wt%) and Mg# = 63–71. Amphibolite from the Niğde Massif differs from gabbro and diabase by its lower Mg# (48–60) and higher TiO2 (0.50–2.91 wt%) (Table 5). Gabbro and diabase are also characterized by Zr < 30 ppm and low Ti/V ratios (6–11), whereas Niğde Massif amphibolite has much higher Zr (58–295 ppm) and higher Ti/V (27–72) (Fig. 11A and Table 5).
Gabbro and diabase have variably enriched large ion lithophile elements (LILE) relative to the high field strength elements (HFSEs), which are depleted relative to mid-ocean ridge basalt (MORB) and are positively sloping (Fig. 12A). The strong depletion in HFSE (Hf, Zr, and Ti) relative to MORB and variable enrichment in LILE (Sr, Rb, and Ba) displayed by gabbro, diabase, and basalt are typical of suprasubduction zone magmas (Pearce et al., 1984a) (Fig. 12A). Amphibolite of the Niğde Massif (47B and ND01-32B) has a humped pattern enriched in LILE and with MORB-like HFSE (Fig. 12A).
Chondrite-normalized rare-earth element (REE) patterns are similar for gabbro from the NMC, Ecemiş fault zone, and northern region despite the differences in lithologic association and metamorphic overprint of each group (Fig. 12B). The gabbros all display positively sloping light rare-earth element (LREEs) (LaN/YbN = 0.40–0.89), a positive Eu anomaly, and flat or slightly negative heavy rare-earth elements (HREEs). Of the three diabase and basalt samples analyzed for REE (Fig. 12B), two (40B from the NMC and 53B from the northern region) show distinct patterns, with nearly flat or gently negative sloping patterns (LaN/YbN = 0.96–2.19) and no significant Eu anomaly. The diabase sample (28) has a pattern identical to gabbro and was likely co-magmatic with the main suite of gabbroic rocks. Niğde Massif amphibolite is distinct from NMC, Ecemiş, and northern mafic rocks, with higher REE abundance and a negatively sloping pattern enriched in LREE (LaN/YbN = 4.37–15.80; Fig. 12B). The negative slope of the amphibolite REE pattern indicates a fertile (undepleted) mantle source, whereas the positive-humped slopes of NMC, Ecemiş, and northern gabbro indicate a depleted normal (N)-MORB source and a high degree of partial melting. Hornblende gabbro, sheeted dikes, and basalt from CAO rocks north and northwest of the study area have similar REE patterns including LaN/YbN < 1, flat HREE, and positive Eu anomaly interpreted as indicating derivation from a previously depleted N-MORB source in a suprasubduction zone setting (Yaliniz et al., 1996; Yalniz, 2008; Kocak et al., 2005). The flatter profiles of the diabase (40B) and basalt (53B) samples may indicate a lower degree of partial melting or a change in melt source with time (Fig. 12B).
Compositions of the NMC mafic rocks are compared to published compositions for intrusive (gabbro and diabase) and extrusive (basalt) mafic rocks of neighboring ophiolites to evaluate the likely origin of the NMC (Figs. 11A and 11B). Literature data from the CAO, İzmir-Ankara, and central Tauride ophiolites were screened to eliminate cumulate compositions (Al2O3 > 20%, Ni > 250 ppm, and Sc > 50 ppm) and highly altered samples (loss on ignition [LOI] > 4%). Mafic rocks from the CAO typically plot within or near the field of boninites on discrimination diagrams (Figs. 11A and 11B). Nearby ophiolites of the Tauride Mountains typically have gabbro and diabase that plot in the field of island-arc tholeiite (IAT) to MORB, although lesser amounts of boninitic rocks also occur in the Alihoca ophiolite (Sarıfakıoğlu et al., 2013) (Figs. 11A and 11B). The composition of boninitic gabbro from the NMC and from the Alihoca ophiolite of the Taurides can be distinguished on a plot of Mg# versus a compatible element such as Cr (Fig. 11C). The Alihoca boninitic rocks in general have significantly lower Cr at a given Mg#. Overall, the geochemical features of NMC, Ecemiş, and northern region gabbro and diabase are most similar to their lithologic counterparts in the CAO, and geochemical differences between ophiolite belts in the central Taurides and CACC are significant enough to be used as a tool for evaluating provenance of gabbro cobbles.
Plagiogranite has 73–79 wt% SiO2, 0.09–0.25 wt% K2O, and 3.7–4.5 wt% Na2O (Table 5). Plagiogranite dikes are calcic (Fig. 13A) and borderline metaluminous-peraluminous (A/CNK = 0.94–1.01). Niğde Mafic Complex and northern-region plagiogranite resembles oceanic plagiogranite in their low K (<1 wt% K2O), low Rb (<20 ppm), and calcic character (Pearce et al., 1984b; Maniar and Piccoli, 1989). Analyzed plagiogranite also geochemically resembles similar rocks from the suprasubduction zone Troodos Ophiolite (Cyprus) and the Sarıkaraman locality of the CAO (Figs. 13A and 13B).
Clasts and Cobbles—Çukurbagğ and Cihanbeylı
Ten gabbro clasts from the Çukurbağ Formation were analyzed for whole-rock composition (Table 6); of these, three (3, 5, and 19B) were not plotted on discrimination diagrams because they exceeded the cumulate or LOI screening parameters. Two compositional groups can be distinguished among the Çukurbağ samples (Figs. 14A and 14B): (1) gabbro clasts with strongly depleted HFSE, including TiO2 (0.1–0.17 wt%), Zr (1–2 ppm), and Y (2–4 ppm), and Ti/V ratios of 4–8—typical of boninitic magmas; and (2) gabbro clasts with intermediate TiO2 (0.68–0.98 wt%), Zr (45–63 ppm), and Y (16–22 ppm), and Ti/V ratio (15–23) typical of island-arc tholeiite (IAT). There is no clear geographic separation of the two geochemical groups: at Hacıbeyli (Fig. 3), both boninitic (sample 1) and IAT (sample ND14-04B) type clasts occur in the same conglomerate horizon. In the Bademdere region, samples from four outcrops include both boninitic (samples 08A and 8614) and IAT-MORB (PU-13B, PU-14, and PU-17) type clasts.
Nine cobbles from the Cihanbeylı unit of the Ulukıșla Basin were analyzed for whole-rock composition (Figs. 14A and 14B and Table 6). Of these, eight have basaltic compositions (47–52 wt% SiO2)—one of which (42) has Sc > 50 ppm and is not plotted on discrimination diagrams—and one (sample 41) has an andesitic composition (57% SiO2). All analyzed Cihanbeyli cobbles have boninitic composition: low TiO2 (0.12–0.49 wt%), Zr (1–23 ppm), and Y (3–13 ppm), and Ti/V ratios of 7–11 (Figs. 14A and 14B and Table 6).
Six cobbles were also analyzed from the “m3pl” unit north of the Niğde Massif (Table 6). These gabbro cobbles show diverse trace-element concentrations: two samples (33B and 49A) have boninitic compositions characterized by low Zr (4–7 ppm), Y (5 ppm), and Ti/V (9–10), and the other four have intermediate TiO2 (0.5–0.98 wt%), Zr (20–70 ppm), and Y (15–23), and Ti/V (13–28), reflecting IAT-MORB compositions (Figs. 14A and 14B).
Cobbles from the Çukurbağ and Cihanbeyli formations that plot within the “boninite” field on Figures 14A and 14B were plotted on Figure 11C to determine whether they were more similar to boninitic gabbros of the Niğde area or to those in the Alihoca ophiolite in their Cr content and Mg#. The Çukurbağ cobbles show similarity to both groups; sample 8A plots within the field represented by the Alihoca boninites, while samples 8614 and 1 are more similar to the NMC and EFZ gabbros. The Cihanbeyli cobbles show more scatter but are generally more similar to the NMC and EFZ gabbros (Fig. 11C).
Metagabbro of the NMC records a history from subduction zone magmatism to obduction and metamorphism and ultimately denudation and dispersal to sedimentary basins. As such this unit can be evaluated in order to better understand the tectonic and geomorphic evolution of this region; key questions are (1) What was the origin and evolution of the Niğde Mafic Complex? Is it part of the Central Anatolian Ophiolite? If so, why did the NMC apparently experience significantly higher metamorphic grade and degree of ductile deformation (including mylonitization) compared to the rest of the CAO, including geochemically indistinguishable rocks outcropping <20 km to the north (e.g., localities 34 and 53; Fig. 3)?; and (2) What was the source (or sources) and evolution of abundant gabbro clasts in Oligocene–Miocene conglomerates and cobbles on alluvial terraces? What were the dispersal routes of gabbro-bearing sediment, and what are the implications of these transport routes for the evolution of topography during plateau uplift, exhumation, and faulting in Central Anatolia?
Origin of the Nigğde Mafic Complex and Central Anatolian Ophiolite
In Central Anatolia, gabbro occurs as part of ophiolitic fragments and as plutons that intruded as part of the magmatic history of the Central Anatolian Crystalline Complex. Therefore, the first question to address about the origin of NMC gabbro is whether it is part of an ophiolite or whether the gabbro represents ensialic intrusions. In the field, direct contact between NMC gabbro and Niğde Massif metasedimentary rocks was not observed. In some locations, however, metasedimentary and gabbroic rocks occur within tens of meters of each other; in all cases, the mafic complex is structurally higher than the metasedimentary rocks, and foliation dips away from the core of the Niğde Massif, concordant with foliation of the underlying metasedimentary rocks (Fig. 6). High-strain zones and metamorphosed ultramafic rocks commonly occur at the NMC-Niğde contact (Figs. 3 and 6), indicating that it is tectonic (Göncüoğlu et al., 1991).
The lack of intrusive field relations between NMC and metasedimentary rocks suggests that these units were not in contact during the magmatic development of the NMC. In the Niğde Massif, mafic rocks occur as amphibolite interlayered with metasedimentary rocks and have very different chemical composition (Figs. 11 and 12; Table 5) and mineral assemblage and mineral composition (Whitney and Dilek, 1998) than metagabbro and metadiabase of the NMC (Floyd et al., 2000; this study). The NMC was therefore originally a distinct tectonic and petrologic unit from the underlying Niğde Massif and not a continental intrusion.
The lithologic association of peridotite, gabbro, diabase, and plagiogranite is characteristic of ophiolites. Whole-rock compositions of NMC plagiogranite, gabbro, and diabase document that these have a suprasubduction zone tectonic affinity, similar to the CAO (Floyd et al., 2000; Yalınız et al., 2008) (Figs. 11A, 11B, 13A, and 13B). Petrologic features of NMC gabbro, such as magmatic hornblende, Mg-rich orthopyroxene, and calcic plagioclase, as well as a high-Mg, low-Ti whole-rock composition, are typical of boninitic magma commonly ascribed to an oceanic forearc tectonic setting (Pearce et al., 1984a; Bloomer and Hawkins, 1987). Field observations and petrologic and geochemical data support the interpretation that the NMC is part of the Central Anatolian Ophiolite, further indicating that the now-scattered CAO outcrops once formed a much larger ophiolite sheet that has been intensely fragmented and eroded.
Previous studies of the CAO have interpreted the ophiolite as derived from an intra-oceanic subduction zone corresponding to the Izmir-Ankara-Erzincan (IAE) suture zone to the north of the CACC (Yalınız et al., 1996; Yılmaz and Boztuğ, 1998; Floyd et al., 2000; Yalınız et al., 2000; Yalınız, 2008). Reconstructions based on paleomagnetic data indicate that large-scale block rotations within the CACC since latest Cretaceous time may have reorganized an originally elongate north-south continent into its present wedge shape and that a subduction zone dipped eastward beneath the CACC microcontinent, leading to Andean-type magmatism during Campanian–Maastrichtian time (Görür et al., 1984; Kadıoğlu et al., 2006; Lefebvre et al., 2013). Based on the paleomagnetic reconstruction of Lefebvre et al. (2013) and paleomagnetic data from the Central Anatolian Ophiolites, van Hinsbergen et al. (2016) have suggested that the intraoceanic subduction zone within the İzmir-Ankara-Erzincan ocean was not strictly E-W oriented and that emplacement of the CAO occurred by east-directed underthrusting of the elongate CACC beneath a N-S–oriented jog in the IAE subduction zone. Such a scenario is more consistent with the distribution of CAO outcrops, which form an elongate N-S band in the reconstruction of Lefebvre (2011). After ophiolite emplacement, a jump of the subduction zone could have generated the active margin along the western CACC (Fig. 15A).
The ophiolitic rocks of the IAESZ, Tauride belt, and CAO can be compared in order to evaluate whether the CAO is a distinct unit or simply represents a continuation of these larger ophiolite belts. Geographically, the IAESZ and Tauride ophiolites define two roughly E-W–trending belts (Figs. 1 and 2). The CAO has a more random geographic distribution; however, when tectonic rotations are accounted for—as modeled in Lefebvre et al. (2013)—a N-S belt is more apparent.
Structurally, the CAO is mostly represented by crustal sections of gabbro, plagiogranite, and volcanic rocks (Yalınız et al., 1996; Yılmaz and Boztuğ, 1998) with relatively sparse and highly serpentinized ultramafic rocks (Yalınız and Göncüöğlu, 1998; İlbeyli, 2008). Parts of the CAO have been metamorphosed along with the underlying continental margin at amphibolite facies. Both the Tauride and IAESZ ophiolites display more complete sections that include mélange, metamorphic sole, mantle, and crustal sections but often lack the uppermost volcanic sections (Dilek et al., 1999; Parlak et al., 2013b; Parlak, 2016). The ophiolites are mostly unmetamorphosed with the exception of amphibolite metamorphic soles, some of which have a blueschist-facies overprint (Dilek and Whitney, 1997; Dilek et al., 1999; Parlak, 2016).
The majority of mafic rocks from the CAO plot within or near the boninite field on discrimination diagrams (Figs. 11A and 11B), reflecting their high-Mg, low-HFSE compositions. Rare-earth element and trace-element contents of the NMC, Ecemiş, and northern gabbro, diabase, and basalt analyzed for this study further indicate that the source for these rocks was highly depleted mantle (Figs. 12A and 12B). The geochemical features of the CAO suggest a high degree of partial melting of a depleted and hydrous mantle, possibly suggestive of a short-lived, pre-arc suprasubduction zone origin (Pearce et al., 1984a; Yalınız, 2008). Tauride ophiolitic gabbro and diabase mostly plot within or near the island-arc tholeiite field on discrimination diagrams (Figs. 11A and 11B). Compared to the CAO, the Tauride ophiolites appear to represent a more mature stage of suprasubduction zone magmatism or a less depleted source. Gabbro and diabase of the IAESZ ophiolites span a wide range of compositions on tectonic discrimination diagrams (Figs. 11A and 11B). Mafic compositions including alkaline ocean-island basalts, MORB-like backarc basalts, island-arc tholeiites, and boninites have been reported, and some ages from the IAESZ are Jurassic, suggesting that the oceanic lithosphere here represents a longer, multi-phased magmatic history (Sarıfakıoğlu et al., 2009; Çelik et al., 2011; Parlak et al., 2013b; Hassig et al., 2016a, 2016b). These differences indicate that the CAO is a distinct petrologic unit from other Anatolian ophiolites.
Age constraints suggest that emplacement of the CAO onto the CACC occurred at approximately the same time as emplacement of the IAESZ ophiolites. Emplacement of the Tauride ophiolites must have been a distinct event to have occurred later as indicated by the abundant evidence in these ophiolites for subduction-related magmatism after 90 Ma by which time the CAO must have been already emplaced (Parlak et al., 2013a).
Metamorphism and Exhumation of the Nigğde Mafic Complex
The geochemical similarities of the mafic and ultramafic rocks of the NMC, Ecemiş, and northern region suggest that they were all derived from the Central Anatolian Ophiolite, but the NMC had a distinct metamorphic history from the other parts of the CAO. Niğde Mafic Complex mineral assemblages in gabbro, diabase, plagiogranite, and ultramafic rocks reveal amphibolite-facies conditions for metamorphism and deformation, in contrast to (sub)greenschist-facies metamorphism for other parts of the CAO. Alteration of igneous minerals and textures in CAO units has been attributed to hydrothermal alteration at the seafloor or late magmatic processes (Kocak et al., 2005; İlbeyli, 2008). High-grade metamorphism—synkinematic with ductile deformation—of the NMC requires a different explanation.
The high metamorphic grade and other characteristics of the NMC indicate regional metamorphism at conditions similar to those of the underlying metasedimentary rocks at the northern margin of the Niğde Massif (Whitney and Dilek, 1998; Whitney et al., 2001, 2007). A possible explanation for the tectono-metamorphic similarities of the NMC and Niğde Massif is that part of the obducting ophiolite was tectonically sliced into the CACC prior to peak metamorphism (Fig. 15B). This could account for the difference in metamorphic grade between ophiolitic rocks in the NMC (amphibolite facies) and those in the Ecemiş corridor and north of the massif (i.e., Edikli and Yıldıztepe), which are greenschist facies (Fig. 3).
The proposed tectonic model is based on the determination that the NMC is a fragment of the CAO but, unlike the rest of the CAO, was metamorphosed and deformed at mid-crustal conditions similar to those experienced by the underlying Niğde Massif metasedimentary rocks. To account for the tectonic evolution of the CAO and metamorphism of the NMC, we have used the paleogeographic interpretation of Görür et al. (1984)—including the presence of an Inner Tauride ocean that subducted along an E-dipping zone below the CACC—and the reconstruction of Lefebvre et al. (2013), in which the CACC had an originally ∼N-S–elongate orientation. According to this model, the CAO is derived from a N-S jog in an intraoceanic subduction zone of the IAE ocean. After underthrusting of the CACC, the subduction zone jumped westward to form the Inner Tauride subduction zone, which would later emplace the Tauride ophiolites to the south (Fig. 15A).
The timing of crystallization of gabbro-diabase-plagiogranite in the oceanic realm relative to emplacement is potentially problematic for this model because of the similarity of igneous crystallization ages of ophiolitic rocks with peak metamorphic ages for high-grade metamorphic rocks. An age of ca. 90 Ma (U-Pb zircon) has been determined for CAO plagiogranite (Sarıkaraman locality; van Hinsbergen et al., 2016), consistent with fossil ages for associated oceanic sediments (Yalınız et al., 2000). This is similar to the ca. 91 Ma U-Pb zircon rim age determined for sillimanite-Kfs migmatite in the core of the Niğde Massif (Whitney et al., 2003). The field relations throughout the CACC require that the CAO was obducted prior to regional metamorphism and intrusion of continental granites. Diachronous emplacement of the ophiolite may account for the overlap in ages since the oldest metamorphic and granite ages come from the Niğde Massif, in the southernmost CACC, while the CAO has so far only been dated at the Sarıkaraman locality (∼100 km to the northwest; Fig. 2).
The proposed tectonic model accounts for the age and metamorphic grade of Niğde Massif and NMC metamorphism relative to the rest of the CAO by diachronous, oblique obduction of ophiolite (Fig. 15A) and tectonic interleaving of ophiolite (generated not long before obduction) and continental margin metasedimentary rocks in the southern CACC (Fig. 15B). There was likely also an unmetamorphosed to low-grade ophiolitic sheet above the Niğde Massif–NMC, represented today by the mafic and ultramafic rocks N and E of the Niğde Massif–NMC (Fig. 3). Because kinematic indicators in the high-strain zones between the NMC and Niğde Massif contact are consistent with high-grade, ductile top-to-S thrusting, these zones are likely related to tectonic interleaving of the Niğde Massif and NMC rather than exhumation. During Late Cretaceous extension that produced the Niğde Massif as a core complex and/or during an Oligo-Miocene episode of transtension along the Ecemiş fault (Umhoefer et al., 2007; Whitney et al., 2008), metasedimentary rocks and NMC were exhumed. Any exhumation-related structures on the northern margin of the Niğde Massif–NMC are likely not exposed, because they are buried under Neogene volcanic and alluvial deposits.
Provenance Analysis of Gabbro in Conglomerates and on Alluvial Terraces
Gabbro clasts and cobbles are abundant in sedimentary basins surrounding the Niğde Massif and therefore provide a link between the Late Cretaceous ophiolites and landscape development in the Cenozoic. Gabbro cobbles are particularly abundant in two Miocene conglomerate units (Çukurbağ and Cihanbeyli) of the Ecemiş and Ulukışla Basins, as well as in upper Miocene conglomerates north of the Niğde Massif (Fig. 3). Possible sources of gabbro are ophiolites N and NE of the Niğde Massif (e.g., CAO and NMC) and the Tauride ophiolites to the south (Figs. 1–3). In some cases, significant topographic barriers presently separate possible sources from the basins where gabbro clasts occur (Fig. 16). Determining clast and/or cobble sources is therefore applicable to interpretations of paleodrainage, paleotopography, and the extent and timing of major erosion of ophiolite nappe(s), all of which are of interest to understanding the tectonic and landscape evolution of Central Anatolia in the Cenozoic.
A key question to be assessed is whether the NMC, north of the Niğde Massif, could have been a source area for cobbles in the Çukurbağ and/or Cihanbeyli units despite their present-day topographic separation. The NMC is a distinctive source because of its high-grade metamorphic and deformation features. In contrast, gabbro from elsewhere in the CAO, including in the Ecemiş area and scattered outcrops in the northern part of the field area (Fig. 3), have a greenschist-facies overprint or are unmetamorphosed (Table 1) (Kocak and Leake, 1994; Yılmaz and Boztuğ, 1998; Toksoy-Köksal and Göncüoğlu, 2001, 2009; İlbeyli, 2008). Similarly, gabbro of the Alihoca and Pozantı-Karsanti ophiolites of the Tauride Mountains has a low-grade overprint and is undeformed, although higher-grade metamorphic rocks occur in the metamorphic soles (Parlak et al., 2000; Sarıfakıoğlu et al., 2013).
Jaffey and Robertson (2005) assessed paleocurrent features and provenance of sandstone in the Çukurbağ Formation in the Ecemiş Basin and suggested that it was deposited in an elongate, inwardly draining basin with a depocenter near Bademdere (Fig. 3). In the northern part of the Ecemiş Basin, clast counts, sandstone petrography, and mudstone geochemistry suggest a shift in provenance from Tauride-dominated in the lower sections of the Çukurbağ to Niğde-dominated in upper sections, as well as a diminished ophiolite influence up-section (Jaffey and Robertson, 2005). Paleogene volcanic rocks of the Ulukıșla Basin were also a major source of detritus for the Çukurbağ Formation. These data may indicate that the Tauride ophiolites were progressively eroded to expose the carbonate platform rocks and that the Niğde Massif was unroofed during this time (Jaffey and Robertson, 2005). Because CAO ophiolite was presumably more widely outcropping on and around the Niğde Massif in Oligo-Miocene time, it is possible that some ophiolite-derived sediment could have come from the north rather than the south (Taurides); we therefore consider petrographic and geochemical features of gabbro clasts to evaluate possible source ophiolites.
Whole-rock composition is an important tool in the assessment of source areas (Deaton and Burkart, 1984). Both the CAO and Tauride ophiolites are of suprasubduction origin but have distinct geochemical characteristics (Figs. 11C, 14A, and 14B). Because no single source area is entirely homogeneous in petrologic or geochemical characteristics, we also consider the possibility that multiple source areas have contributed to sites with heterogeneous gabbro characteristics.
Four of seven noncumulate, low LOI (i.e., relatively unaltered) clasts analyzed from Çukurbağ Formation fluvial conglomerates have IAT or MORB-like affinities, and the other three have boninitic characteristics (Figs. 14A and 14B). Gabbro clasts with IAT and boninitic compositions occur in the same conglomerate horizon. Within the Bademdere area (Figs. 3 and 9), three (PU15-13B, -14, and -17) of five analyzed cobbles have Ti/V ranging from 15 to 23, indicating IAT-MORB, and the other two (08A and 8614) have boninitic compositions. The four IAT-MORB–like cobbles were very likely derived from the Taurides and fall within or near the field of the Alihoca, Aladağ (Pozantı-Karsanti), and Mersin ophiolites (Figs. 1, 14A, and 14B).
The difference in Mg#, Cr, and Ni content of boninitic gabbro in different locations in the region can be used to further distinguish sources of gabbro clasts (Fig. 11C). Two of the three boninitic Çukurbağ clasts (1 and 8614) have similar composition to boninitic gabbro from the NMC and Ecemiş area gabbro northwest of the basins in which they were deposited, and the fourth (8A) is more similar to the Alihoca boninitic gabbro, located to the south. The two samples in the first group come from localities in the northern (Hacıbeyli) and central (Bademdere) parts of the Ecemiş Basin (Figs. 3 and 9). The primitive composition of two of the Çukurbağ clasts (1 and 8614) and the texture of sample 8614 (Fig. 10A) resemble the low-grade Ecemiş area gabbro, which is also geographically close to where the cobbles were deposited. Based on these limited data, we conclude that the Çukurbağ conglomerates were derived from multiple sources—from the Taurides and from an unmetamorphosed or low-grade part of the CAO that may have covered much of the Niğde Massif in Oligocene–early Miocene time.
Gabbro clasts from the Cihanbeyli conglomerates above the Ulukıșla Basin have characteristics that indicate a CAO source. Five of the gabbro cobbles (41–44 and 46) are texturally similar to the NMC in that they have polygonal textured, poikilitic, or foliated hornblende with or without relict clinopyroxene (Figs. 10C and 10D). The other four (17A, 17B, 18, and 45) are igneous-textured gabbro with a greenschist-facies overprint, possibly indicating derivation from the Ecemiş area gabbro or other low-grade parts of the CAO. All analyzed Cihanbeyli gabbro cobbles are boninitic, similar to NMC and CAO gabbro (Figs. 11C, 14A, and 14B).
The northern m3pl conglomerates are also most likely predominantly from the CAO, because they are composed exclusively of ophiolitic lithologies (gabbro, diabase, plagiogranite, basalt, and ultramafic rocks), and they occur in close proximity to exposed ophiolite bedrock (Fig. 3). All but two of the seven analyzed northern conglomerate cobbles have metamorphic hornblende, suggesting the NMC or another metamorphosed region of the CAO was the source area. With the exception of two float samples (05A and 05B), which have MORB-like composition, all northern m3pl samples collected from conglomerate outcrops overlap in composition with rocks from the NMC and CAO.
Based on these results, we conclude that the CAO-NMC was the dominant source of gabbro clasts during the uppermost Miocene, with some Tauride influence during deposition of the Çukurbağ Formation (Oligocene–Miocene) and little to no Tauride influence during deposition of the Cihanbeyli (uppermost Miocene). Owing to the differences between the Çukurbağ and Cihanbeyli gabbro clasts, including different compositions and average clast sizes (Table 4), we further conclude that gabbro cobbles at the surface of the Cihanbeylı outlying hills and lower elevation terraces do not represent reworked Çukurbağ conglomerates.
Evolution of the Miocene Landscape
Gabbro-bearing fluvial conglomerates record widespread denudation of the CAO. The presence of gabbro-dominated deposits—including some with boulder-sized gabbro blocks—to the north, east, and south of the Niğde Massif has implications for the tectonic and geomorphic evolution of Central Anatolia. Three possible scenarios that explain CAO-derived cobbles in upper Miocene deposits in all these locations are: (1) Cihanbeyli/m3pl gabbro was derived from the CAO-NMC north of the Niğde Massif and transported via fluvial systems that either went through or around the massif (Fig. 16); (2) the relatively thin gabbro conglomerates in the Cihanbeyli Formation south of the Niğde Massif were recycled from thicker “m3pl” alluvial fan deposits north of the Niğde Massif–NMC (i.e., conglomerate hills at Çayırlı-Yarhisar, Fig. 3); or (3) an ophiolitic nappe covered the entire Niğde Massif but has been completely eroded in the south, in which case the Cihanbeylı clasts were locally derived (Fig. 16). The presence of a 4.9 ± 0.2 Ma ignimbrite flooring a valley in the core of the Niğde Massif (Whitney et al., 2008) indicates that little erosion has taken place over the massif in the past ∼5 m.y. (Fig. 16). Considering that the volcanic deposits interlayered with conglomerate north and south of the massif are 5–7 m.y. old, little to no time is left to erode a vast covering ophiolite sheet, making a local gabbro source (scenario 3) unlikely. Scenarios 1 and 2 invoke a fluvial system connecting the northern and southern sides of the Niğde Massif, requiring either uplift of the southern Cihanbeyli Formation (presently at an elevation of ∼2 km along the southern margin of the massif) or subsidence of the northern gabbro conglomerates (presently at 1.3–1.5 km).
Vertical displacements around the Niğde Massif have two identifiable origins: (1) the broad arching of the Tauride Mountains to the south, which was likely responsible for the gentle 500 m rise of the Cihanbeylı Formation over 12 km from Halaç-Havuzlu (1200 m) to Postallı (1700 m), projecting at the 2000 m elevation of the Cihanbeylı Formation outliers farther east; and (2) normal faulting along the NW flank of the massif, of possible early to middle Miocene age (Toprak and Göncüoğlu, 1993). The elevation of the m3pl deposits around the massif is consistent with the amplitude of WNW warping of the Cihanbeylı Formation along the southern flank of the Niğde Massif and implies that the massif has also experienced similar warping since the deposition of the m3pl layers.
The Taurides emerged from the Mediterranean Sea after 8 Ma (Cosentino et al., 2012) and reached their current elevation by 5 Ma (Meijers et al., 2015). The close timing between the deposition of the m3pl units and the growth of the arch implies that the north-south–flowing dispersal system that conveyed the NMC gabbros to the southern side of the Niğde Massif stopped flowing southwards soon after deposition of the gabbro-rich conglomerates along the southern side of the massif. The north-south–directed streams that deposited their alluvial load, either through the massif and/or around it, drained into the inward-draining Tuz Gölü–Konya Basin. Today, these sediments are dissected by an aggressively incising drainage system connected to the Mediterranean Sea through a breach in the Tauride Mountains along the Ecemiş fault zone (Figs. 2 and 16). This drainage is currently extending northward into the Anatolian plateau interior and has integrated the drainage of the southern half of the Niğde Massif.
Field, petrologic, structural, and geochemical (whole-rock major- and trace-element) data from gabbro outcrops, gabbro clasts in conglomerates, and gabbro cobbles on alluvial terraces document the origin and dispersal of gabbro and related rocks near the Niğde Massif in Central Anatolia. The Niğde Mafic Complex (NMC) is a fragment of the Central Anatolian Ophiolite (CAO), but, unlike the rest of the CAO, it was buried and heated at mid-crustal conditions along with the underlying continental margin rocks. The difference between the NMC and the rest of the ophiolite indicates oblique and diachronous obduction that started in the south and migrated north along the Central Anatolian microcontinent. Central Anatolian Ophiolite–NMC mafic rocks have geochemical signatures of boninitic (forearc) magma and can be distinguished from ophiolitic rocks of the Tauride Mountains to the south using a combination of petrography and geochemistry. This difference allows tracking of gabbro source regions and Miocene deposition. Upper Miocene gabbro-bearing conglomerates had mixed sources (CAO and Taurides), whereas later Miocene sediments were derived entirely from the CAO, despite present-day topographic barriers to transport. Miocene uplift related to arching of the Tauride Mountains drove reorganization of fluvial systems and disconnected depocenters from their sources.
This research was funded by National Science Foundation grants EAR-1109762, “Continental Dynamics: Central Anatolian Tectonics” (CD-CAT) to Donna Whitney and Christian Teyssier, EAR-1109762 to Paul Umhoefer, and EAR-1109703 to Jane Willenbring. This project represents the M.S. thesis work of Molly Radwany (née Ray) at the University of Minnesota. We thank Bülent Tokay and Ekrem Tosun for assistance with field work and Başar Özşafak for hospitality in the Çamardı area.