Geodynamic evolution of the Karakaya Mélange Complex, Turkey: A review of geological and petrological constraints

Journal of Geodynamics 65 (2013) 56–65

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Geodynamic evolution of the Karakaya Mélange Complex, Turkey: A review of geological and petrological constraints Kaan Sayıt a,∗ , M. Cemal Göncüoglu b a b

Department of Geological Sciences, San Diego State University, San Diego, CA 92182-1020, USA Department of Geological Engineering, Middle East Technical University, 06531 Ankara, Turkey

a r t i c l e

i n f o

Article history: Received 14 December 2011 Received in revised form 8 April 2012 Accepted 12 April 2012 Available online 11 May 2012 Keywords: Palaeotethys Mélange OIB Mantle plume Geological evolution

a b s t r a c t The Karakaya Complex in the pre-Liassic basement of the Sakarya Composite Terrane includes relicts inherited from the closure of the Palaeotethys. The interpretations regarding its origin are still controversial. The main reason for these controversies is largely due to the mélange character of the complex and the misidentification and misinterpretation of the tectonostratigraphic units that make up the complex. Kocyigit (1987) subdivided the complex into the Upper and Lower Karakaya Nappes. This tentative division assumes that the Upper Karakaya Nappe (Lower Karakaya Complex) is composed of rock assemblages metamorphosed under greenschist/blueschist facies. In contrast, the Lower Karakaya Nappe (Upper Karakaya Complex) is thought to be composed of relatively unmetamorphosed, yet deformed rock lithologies. In this study, our detailed geological and petrological observations show that subdivision of the Karakaya Complex based on differences in metamorphism does little to decipher the geological evolution of the complex. Supported by our recent findings (Sayit et al., 2010), however, we suggest that the mafic rocks in both Karakaya Nappes are actually similar to each other in terms of metamorphism and tectonic setting. We also propose that in classifying such chaotically mixed units, tectonomagmatic origin (together with age data) should be taken as the primary criterion, rather than metamorphism and deformation. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction The term “melange” simply refers to chaotic rock bodies in which the primary relationships have been disturbed owing to some physical processes (e.g. Raymond, 1984; Festa et al., 2010). Of these processes, fragmentation and mixing appear to be the predominant processes in generating mélanges (e.g. Hsü, 1968; Cloos, 1984). Triggered by the tectonic and/or sedimentary events, mélanges are commonly characterized by block-in-matrix structures where variable sized blocks/fragments are embedded in a finer-grained matrix. Mélanges are commonly associated with subduction/accretion complexes (e.g. Raymond, 1984; Cloos, 1984). The high pressure/low temperature (HP/LT) mineral assemblages are ubiquitous in mélanges formed in subduction/accretion complexes, which develop as a result of cold burial of subducting oceanic slab (e.g. Coleman and Lanphere, 1971; Baldwin and Harrison, 1992). During closure of an oceanic basin, the oceanic lithosphere is consumed through subduction. Some pieces of the subducting

∗ Corresponding author. Tel.: +1 619 594 1241; fax: +1 619 594 4372. E-mail addresses: [email protected] (K. Sayıt), [email protected] (M.C. Göncüoglu). 0264-3707/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jog.2012.04.009

slab are incorporated into the subduction/accretion complex along with continental material derived from the active margin. Therefore, subduction/accretion complexes and their mélange units hold invaluable information regarding the geodynamic evolution of the ancient oceanic realms and associated continental fragments (e.g. Isozaki, 1997). One such ancient seaway is Palaeotethys, which is believed to have existed between the Middle/Late Paleozoic and Early Mesozoic (Sengör, 1979; Sengör et al., 1984; Stampfli and Borel, 2002). The closure of this oceanic basin during the Early Mesozoic led to accretion of several oceanic and continental fragments. In Anatolia, the relicts of this event (the Cimmeride Orogeny; Sengör et al., 1984), are found within the Karakaya Complex that lies within the pre-Liassic basement of the Sakarya Composite Terrane (Göncüoglu et al., 1997) (Fig. 1). In this paper, we focus on understanding the origin of the Karakaya Complex which is a critical location to study the Palaeotethyan evolution in the Eastern Mediterranean region. However, the origin of the Karakaya Complex is a highly debated subject (e.g. Okay and Göncüoglu, 2004; Sayit and Göncüoglu, 2009a; Sayit et al., 2010), which results from the chaotic or “mélange” nature of the complex. In this regard, our aim here is to identify the geological units making up the Karakaya Complex, which will allow us to place constraints on its geodynamic evolution. We use geochemistry as a tool to interpret the tectonomagmatic origin of the mafic

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Fig. 1. Distribution of the Karakaya Complex in northern Turkey (based on Göncüoglu et al., 1997). Also shown is geological map of the Ankara region. Modified from Kocyigit (1991).

assemblages within the Karakaya Complex. We emphasize here that magmatic suites with distinct metamorphic signatures could actually be relicts of the same magmatism, depending on their geochemical characteristics and ages, as also proposed by Sayit et al. (2010). We also suggest that the Upper and Lower Karakaya Complexes, which have been treated as two distinct entities (e.g. Kocyigit, 1987; Okay et al., 1991), have, in fact, similar origins. 2. The Karakaya Complex: definitions and subdivisions Bingöl et al. (1973) introduced the term “Karakaya Formation” to define the very low-grade metamorphic lithologies found in NW Anatolia. They suggested that these lithologies were formed in a continental-rift setting, which was triggered by extensional tectonics during the Early Triassic. In contrast to the rift-related origin proposed by Bingöl et al. (1973), Tekeli (1981) proposed that the Karakaya Complex represents a subduction/accretion prism, which he called “the North Anatolian Belt”. This view was based on the mélange character of the unit and the existence of high-pressure lithologies. These different interpretations, i.e. continental-rift vs subduction/accretion, have led to development of two main classes of thought, regarding the geodynamic evolution of the Karakaya Complex (e.g. Kocyigit, 1987; Altiner and Kocyigit, 1993; Genc and Yilmaz, 1995; Pickett and Robertson,

1996; Göncüoglu et al., 2000; Okay, 2000; Sayit and Göncüoglu, 2009b). As we noted before, the chaotic nature of the Karakaya Complex has been one of the major obstacles in correlating and interpreting its sub-units, which in turn resulted in creation of new divisions and/or formations/units that are actually of the same characteristics. The large-scale subdivision of the Karakaya Complex was first proposed by Kocyigit (1987) who separated the complex, for the Ankara region, into two tectonic units called “the Lower Karakaya Nappe” and “the Upper Karakaya Nappe”. This tectonic division was in contrast with that of Akyürek et al. (1984) who suggested that the units within the Karakaya Complex (their Ankara Group) were transitional. According to the subdivision of Kocyigit (1987), the Lower Karakaya Nappe is characterized by a volcano-sedimentary mixture comprising variable-sized blocks of distinct origins. The Upper Karakaya Nappe, on the other hand, is represented by lowgrade metamorphics including metabasites. Kocyigit et al. (1991) assigned these metamorphics to the Late Paleozoic, thus separating them from the Triassic-aged Karakaya lithologies (their Karakaya Group consisting of the Olukman, Bahcecik and Kendirli Formations). Kocyigit (1992) renamed the low-grade metamorphic assemblages, which he previously termed the Upper Karakaya Nappe, as “the Eymir Complex”. Altiner and Kocyigit (1993) also kept the same classification for the Karakaya Complex; a Paleozoic

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Table 1 Comparison of the various sub-units of the Karakaya Complex and their places in the “Upper” and “Lower” Karakaya context. Unit/formation

Ref.

Upper Karakaya Complex (Lower Karakaya Nappe) Erol (1956) Elmada˘g Blocky Series Norman (1973) Limestone Blocky Mélange Bingöl et al. (1973) Karakaya Akyürek et al. (1984) Elmada˘g, Ortaköy Kaya et al. (1986) Dıs¸kaya Okay et al. (1991) Hodul, C¸al, Orhanlar Kocyigit et al. (1991) Olukman, Kendirli, Bahc¸ecik Pickett and Robertson (1996) Ortaoba Göncüoglu et al. (2000) So˘gukkuyu Metamorphics Lower Karakaya Complex (Upper Karakaya Nappe) Norman (1973) Metamorphic Blocky Mélange C¸avdartepe Akyürek and Soysal (1981) Emir Akyürek et al. (1984) I˙ znik Metamorphics Göncüoglu et al. (1987) Nilüfer Okay et al. (1991) Kocyigit (1992) Eymir Complex Genc and Yilmaz (1995) Yenis¸ehir Metamorphics Göncüoglu et al. (2000) Tepeköy

basement (the Eymir Complex) overlain by the Triassic Karakaya basin assemblages (the Karakaya Group). In NW Anatolia, the study of Okay et al. (1991) brought a different subdivision of the Karakaya Complex. In their classification, these workers defined four tectonostratigraphic units termed the C¸al Unit, Orhanlar Greywacke, Hodul Unit and Nilüfer Unit, in which the Nilüfer Unit was the lower unit tectonically overlain by the others. Pickett and Robertson (1996) also adopted this division except for the Hodul Unit, which they used the Ortaoba Unit instead. Based on the classification scheme introduced by Okay et al. (1991), Okay and Göncüoglu (2004) suggested another large-scale subdivision of the Karakaya Complex, which was somewhat similar to that of Kocyigit (1987). These authors used the terms the “Upper” and “Lower” Karakaya Complexes, where “Upper” and “Lower” refers to the tectonic position of the units. Consequently, the C¸al, Hodul and Orhanlar units were considered to belong to the Upper Karakaya Complex, whereas the Nilüfer Unit by itself was regarded to represent the Lower Karakaya Complex (see Table 1 for a comparison of the various sub-units of the Karakaya Complex as well as their positions in the “Upper” and “Lower” Karakaya context). The subdivision of Okay and Göncüoglu (2004), therefore, differs from that of Kocyigit (1987) in that the Nilüfer Unit (the Lower Karakaya Complex or the Upper Karakaya Nappe) is the tectonically upper unit in the Ankara region, whereas in NW Anatolia the situation is reversed (the Nilüfer Unit is the tectonically lower unit). The recent studies performed in Central and NW Anatolia (Sayit and Göncüoglu, 2009b; Sayit et al., 2010, 2011), however, showed that the Nilüfer Unit of Okay et al. (1991) bear some similarities to those characterizing the Upper Karakaya Complex. It appeared, then, that basaltic lithologies and associated oceanic-derived sediments are also found in some of the Upper Karakaya units as well. On the basis of this fact, Sayit et al. (2010, 2011) redefined these assemblages on the basis of their age, lithology and geochemical characteristics rather than their structural setting and metamorphic grade. 2.1. Redefined Nilüfer Unit The redefined Nilüfer Unit (sensu Sayit et al., 2010) consists primarily of metabasaltic lithologies intercalated with recrystallized neritic and pelagic limestone, mudstone and minor chert. The important criterion in defining the metabasaltic rocks within the redefined Nilüfer Unit is that they display OIB- and E-MORB-type geochemical signatures (Fig. 2) (Capan and Floyd, 1985; Pickett and Robertson, 1996, 2004; Genc, 2004; Sayit and Göncüoglu, 2009b;

Sayit et al., 2010). Sayit et al. (2010) noted that the redefined Nilüfer-type assemblages are ubiquitous in central to western sectors of the Karakaya Complex. In NW Anatolia the redefined Nilüfer Unit is mainly characterized by variably deformed metabasic rocks interbedded with recrystallized limestone and phyllite (Genc, 1987; Okay et al., 1991). The redefined Nilüfer-type metabasic rocks have been largely metamorphosed under greenschist-facies conditions; though high-pressure equivalents, such as blueschists and eclogites, can also be encountered (Okay and Monie, 1997; Okay et al., 2002). In NW Anatolia the redefined Nilüfer Unit includes the originally defined Nilüfer and C¸al Units of Okay et al. (1991), the Bahc¸ecik Formation (Kocyigit et al., 1991) and a part of the Ortaoba Unit (Pickett and Robertson, 1996). The Karakaya Formation (Bingöl et al., 1973), C¸avdartepe Formation (Akyürek and Soysal, 1981), I˙ znik Metamorphics (Göncüoglu et al., 1987), and Yenis¸ehir Metamorphics (Genc and Yilmaz, 1995) are also partly included within the redefined Nilüfer Unit. In Central Sakarya, the metabasaltic portion of the Tepeköy Metamorphics (Göncüoglu et al., 2000) displays the same geochemical signatures with those from Ankara and NW Anatolia (Sayit, 2010). Sayit (2010) suggested that these basic rocks may occur as blocks embedded in the clastic material within the Tepeköy Metamorphics, thus excluding the terrigenous material and metafelsic tuffs that are uncommon to the redefined Nilüfer Unit (Okay et al., 1996; Pickett and Robertson, 1996; Sayit and Göncüoglu, 2009b). In that case, the redefined Nilüfer Unit partly consists, in the Central Sakarya region, of the Tepeköy Metamorphics and the So˘gukkuyu Metamorphics of Göncüoglu et al. (2000). Therefore, the Tepeköy Metamorphics that was originally interpreted to be pre-Permian (Göncüoglu et al., 2000) may actually be Triassic in age. In the Ankara region, the redefined Nilüfer Unit consists of blocks of metabasic rocks alternating largely with limestones, which are embedded in a low-grade metaclastic matrix represented by the Eymir Unit (Kocyigit, 1987; Sayit and Göncüoglu, 2009b; Sayit et al., 2011). In this region, the redefined Nilüfer Unit includes the Ortaköy Formation (Akyürek et al., 1984) and the Bahc¸ecik Formation (Altiner and Kocyigit, 1993; Sayit and Göncüoglu, 2009b). The Eymir Complex (Kocyigit, 1992) and the Emir Formation (Akyürek et al., 1984), on the other hand, are partly included. Although some studies (Okay, 2000; Genc, 2004) correlate these units with the Nilüfer Unit (sensu Okay et al., 1991), the Eymir Complex consists mainly of metaclastics intruded by diabase dikes (Sayit and Göncüoglu, 2009b), which contrasts with the predominantly basaltic nature of the redefined Nilüfer Unit. There are, however, rare mafic and ultramafic blocks within the Eymir Complex, some of which appear to have experienced elevated pressure conditions (Sayit and Göncüoglu, 2009b; Sayit, 2010). These metaigneous blocks with/without high-pressure imprints show enriched geochemical signatures similar to OIB and E-MORB (Sayit, 2010), thus included in the redefined Nilüfer Unit. Such a result suggests that only a small portion of the Eymir Complex, in fact, can be considered within the definition of the redefined Nilüfer Unit. In the eastern sector of the Karakaya Complex, the metabasic rocks with N-MORB-like signatures from the Pulur Complex (Topuz et al., 2004) are not included in the redefined Nilüfer Unit (Sayit et al., 2010), at least on the basis of the available data. More detailed works are needed to discover the geochemical nature of the metabasic rocks in this area. The Tokat Massif is another place where the geochemical identification of the metabasic rocks is unknown (e.g. Rojay and Göncüoglu, 1997), yet the geological relationships have been studied in detail. Yilmaz et al. (1997) differentiated two main units within their Yes¸ilırmak Group, which relate to each other with an unconformity; a Triassic upper unit and a Permo-Carboniferous lower unit. Based on the work of Yilmaz et al. (1997), it seems for now that only some part of Triassic upper

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Fig. 2. (a) Trace element and (b) REE patterns of the metabasaltic rocks from the Nilüfer Unit. Normalization values for N-MORB from Pearce (1983). Chondrite values from Sun and McDonough (1989). (c) Nb/Yb vs Th/Yb plot of the Nilüfer-type metabasic rocks (after Pearce and Peate, 1995). Also shown for comparison are the SSZ-type diabases intruding into the Eymir-type clastics. Average values of N-MORB, E-MORB and OIB from Sun and McDonough (1989). (a) and (b) Data from Capan and Floyd (1985), Pickett and Robertson (1996, 2004), Genc (2004), Sayit and Göncüoglu (2009b), and Sayit et al. (2010). (c) Data from Sayit and Göncüoglu (2009b) and Sayit et al. (2010).

unit can be considered within the context of the redefined Nilüfer Unit. The age of the redefined Nilüfer Unit is based on the paleontological findings. In NW Anatolia, Kozur et al. (2000) suggested an Early Triassic age on the basis of conodont fauna found in the marbles alternating with metabasites. Genc (1987) found an Early-Middle Triassic age on the basis of conodont-bearing limestones intercalated with metabasalts. Kaya and Mostler (1992) ascribed a Middle Triassic age based on the conodonts from the limestones interbedded with the metabasaltic rocks. Recently, Sayit and Göncüoglu (2009b) proposed a Middle-Late Triassic age on the basis of a conodont-bearing chert band associated with metabasalts, thus placing an upper age limit on the Nilüfer-type assemblages. Therefore, in NW Anatolia, the redefined Nilüfer Unit appears to span an age range from the Early to Late Triassic. Regarding the Ankara region, Akyürek et al. (1984) obtained a Middle Triassic age from the neritic limestones. Altiner and Kocyigit (1993) suggested a narrower interval with a Late Anisian (Middle Triassic) age on the basis of the shallow-water limestones alternating with pillow basalts. Sayit and Göncüoglu (2009b) also provided a tight constraint based on the foraminiferal fauna found in the neritic limestones associated with metabasalts, ascribing a Middle Anisian (Middle Triassic) age to the unit. Therefore, in the Ankara region, overall paleontological evaluation suggests a Middle-Late Triassic age for the redefined Nilüfer Unit.

known as the “Eymir Unit” (Sayit, 2010). The Eymir Unit consists mainly of greywackes and arkosic sandstones intercalated with shales (or their metamorphic equivalents), and it does not include any metabasic rocks with E-MORB and OIB-type signatures, i.e. the redefined Nilüfer Unit (Sayit et al., 2010, 2011). In general, the Eymir Unit composes the matrix material in which the variably sized blocks of the redefined Nilüfer Unit are embedded (Sayit, 2010; Sayit et al., 2011) (Fig. 3). In some studies these clastics were assumed to be primarily associated with the metabasaltic lithologies of the redefined Nilüfer Unit (e.g. Bingöl et al., 1973; Akyürek et al., 1984; Kocyigit, 1987; Altiner and Kocyigit, 1993;

2.2. Eymir Unit Variably deformed and metamorphosed clastic rock assemblages commonly outcropping within the complex are collectively

Fig. 3. Field photograph from the Ankara region depicting the block–matrix relationship between the Nilüfer and Eymir Units.

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Fig. 4. Continental-(back-arc) rift model of Altiner and Kocyigit (1993) regarding the development of the Karakaya Rift Basin. 1. Pre-Permian low-grade metamorphic rocks, 2. Uppermost Carboniferous granitoid, 3. Carboniferous to Triassic shallow-water carbonates, and 4. Permian (?) to Triassic rift basin successions of the Karakaya Group, which includes (a) Kendirli Formation, (b) Bahc¸ecik Formation, and (c) Olukman Formation.

Genc and Yilmaz, 1995), whereas some others interpreted them as distinct entities (e.g. Okay, 2000; Sayit and Göncüoglu, 2009b; Sayit et al., 2010). These clastic lithologies were previously defined under various names in different areas (e.g. Okay and Göncüoglu, 2004). On the basis of the study of Sayit (2010), in the Ankara region, the Eymir Unit partially include the Eymir Complex (Kocyigit, 1992) or the Emir Formation (Akyürek et al., 1984), the Elmada˘g Formation (Akyürek et al., 1984), the Elmada˘g Blocky Series (Erol, 1956), the Limestone Blocky Mélange (Norman, 1973), and the Kulm Flysch Formation (Erk, 1977). In NW Turkey, the Eymir Unit comprises the Dıs¸kaya Formation (Kaya et al., 1986; Kaya, 1991) and the Orhanlar Greywacke (Okay et al., 1991). The Hodul Unit of Okay et al. (1991), the Ortaoba Unit of Pickett and Robertson (1996) and the Kendirli Formation of Kocyigit et al. (1991), are partly included in the Eymir Unit. The Halobia macrofauna obtained from a number of studies ascribes the age of the Eymir clastics to the Late Triassic in NW Anatolia (Kaya et al., 1986; Okay et al., 1991; Wiedmann et al., 1992; Leven and Okay, 1996; Okay and Altiner, 2004). In the Ankara region, a Carnian-Rhaetian age (Late Triassic) has been assigned to the unit on the basis of the fossil fauna in the limestones (Özgül, 1993). On the other hand, Akyürek et al. (1984) suggested an EarlyLate Triassic age for the unit. Recently, a Carnian-Norian age (Late Triassic) was evidenced by Sayit et al. (2011) based on the radiolarian fauna found in the red chert band within clastics of the Eymir Unit. 3. Discussion The Karakaya Complex can be largely considered as a tectonic/sedimentary mélange that comprises several tectonostratigraphic units (e.g. Sayit et al., 2010). In some places, it displays a block–matrix relationship, where the block size can reach up to km-scale. As mentioned at the beginning, in such places the key criterion for differentiating units is their tectonomagmatic origin, not the degree of metamorphism. In a subduction/accretion prism, rock packages that come from the same tectonic setting may experience different degrees of metamorphism, depending on how deep the burial is. Thus, it is inevitable to encounter rock assemblages that reflect different type of metamorphism, but with the same geochemical signatures. Therefore, the type/degree of

metamorphism should not be taken as a key factor in the first place to define or correlate the mélange units. 3.1. A similar origin for the Upper and Lower Karakaya Complexes By the redefinition proposed by Sayit et al. (2010), the redefined Nilüfer-type metabasic rocks display only OIB- and E-MORBtype signatures (Fig. 2). The terrigenous metaclastic assemblages defined as the Eymir Unit form the matrix material in which the redefined Nilüfer Unit is embedded (Sayit et al., 2011). Therefore, the relationship between these two units is not primary, but it may have resulted from their later incorporation during mélangeforming processes. However, regarding the Ankara region, Akyürek et al. (1984) argued that these units are primarily related and transitional, and that they represent the lithologies derived from the same tectonic environment, namely a continental-rift. Although Kocyigit (1987) pointed out the chaotic nature of the units, he similarly suggested that these units were also originally formed in a continental-rift. This idea was also adopted by the later studies of Kocyigit et al. (1991) and Altiner and Kocyigit (1993) (Fig. 4). However, as shown by Sayit et al. (2010, 2011), such a primary relationship between the metabasaltic assemblages and metaclastics is not plausible for the following reasons: (1) the presence of HP/LT lithologies within the Nilüfer Unit (e.g. Okay et al., 1991) certainly constitutes the strongest argument in this regard, because such conditions are typically achieved by the cold subduction of an oceanic slab (e.g. Thompson and Ridley, 1987; Bucher and Frey, 1994). This result further indicates that the metabasic rocks of the redefined Nilüfer Unit should have been generated in an oceanic setting. We must also note that the occurrence of high-pressure metamorphism is not restricted only to the parts including schistose metabasics, but also found in the non-deformed varieties (Sayit, 2010); (2) the redefined Nilüfer Unit does not contain any continent-derived material (Pickett and Robertson, 1996, 2004; Okay et al., 1996; Sayit and Göncüoglu, 2009b), thus in agreement with the idea of distinct origins regarding clastics and metabasic rocks; (3) the redefined Nilüfer-type metabasaltic rocks reflect no continental input on the basis of their geochemistry (Sayit and Göncüoglu, 2009b; Sayit et al., 2010); and (4) in the Ankara region, the Eymir-type continent-derived clastic lithologies (Late Triassic; Sayit et al., 2011) are definitely younger than the redefined Nilüfer-type metabasic rocks (Middle Triassic; Sayit and

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Göncüoglu, 2009b), therefore precluding a primary relationship between the Nilüfer and Eymir Units. The clastic sequences along with metabasic blocks outcropping in the Ankara region play an integral part in the discussion regarding the origin of the Lower and Upper Karakaya Complexes. Are they two distinct units of different origin and ages, or are they actually similar? Resolving this problem also has important implications in terms of the general character of the clastic units found all along the complex. The studies of Sayit (2010) and Sayit et al. (2011) suggest that there is no distinction between the clastics of the Olukman Formation (a part of the Lower Karakaya Nappe of Kocyigit, 1987) and the Eymir Complex (the Upper Karakaya Nappe). Thus, both units are part of the Eymir Unit. This interpretation is somewhat consistent with the results of Akyürek et al. (1984) who suggest that these two units (corresponding to their Elmada˘g and Emir Formations) are transitional. Kocyigit (1987) and Sayit and Göncüoglu (2009b), however, argue that these units are tectonically related (via thrust faults). However, no distinct tectonic contact has been found between these metaclastic units (e.g. Sayit et al., 2011). Furthermore, the degree of metamorphism and deformation reflected by these metaclastic assemblages appear to be highly variable, thus it is not possible to make a distinction between the Olukman (or Elmada˘g) Formation and the Eymir Complex (or Emir Formation). Therefore, the terms like “less or no-metamorphosed” commonly assigned to the Upper Karakaya Complex (or Lower Karakaya Nappe) assemblages or “more metamorphosed” ascribed to the Lower Karakaya Complex (or the Upper Karakaya Nappe) are misleading. The study of Sayit (2010) indicates that the part of the mélange previously interpreted as “less-metamorphic” (Kocyigit, 1987) or “limestone blocky mélange” (Norman, 1973) contains elevated-pressure imprints as evidenced by the presence of metamorphic Na-amphibole within the redefined Nilüfer-type metabasic blocks. Sayit (2010) also argued that the same situation is observed in the other part too, which is named “metamorphic blocky mélange” (Norman, 1973) that was interpreted as “more metamorphic” (Kocyigit, 1987). This result clearly shows that metabasaltic sections of both parts have been variably influenced by HP/LT metamorphism. Therefore, the usage of these terms is ambiguous, and it is not possible to make a distinction on the basis of metamorphism in the Ankara region and also in the other parts of the Karakaya Complex. As mentioned before, the redefined Nilüfer Unit includes (partially or completely) the units of both Upper and Lower Karakaya Complexes, and these units show the similar geochemical signatures (OIB- and E-MORB-type). We must also note that they all reflect oceanic origin and display similar primary lithologies (e.g. Pickett and Robertson, 1996; Okay, 2000; Sayit and Göncüoglu, 2009b). Even though the original Nilüfer Unit of Okay et al. (1991) has been interpreted to be a metabasite–marble–phyllite association, the same unit is apparently composed of basaltic lithologies (basalt flows, volcaniclastics, and hyaloclastic breccias) which are intercalated with limestone, and to a lesser extent with mudstone and chert, in less deformed places (Pickett and Robertson, 2004). The same lithologies also characterize the less deformed C¸al Unit or the Bahcecik Formation, which belong to the Upper Karakaya Complex. Besides, these metabasic assemblages interpreted previously as distinct units also reflect similar ages. For example, the originally defined Nilüfer Unit (sensu Okay et al., 1991; the Lower Karakaya Complex) displays the Early-Middle Triassic ages (Kaya and Mostler, 1992; Kozur et al., 2000). Similarly, the basaltic lithologies representing the Upper Karakaya Complex have also been found to be the Early-Late Triassic in age (Genc, 1987; Sayit and Göncüoglu, 2009b). Therefore, their similar geochemical characteristics, ages and primary lithological content may imply that these so-called distinct units actually represent derivation from a similar tectonic setting.

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3.2. Origin of the Karakaya Complex As shown by a number of studies, the age of the Karakaya Complex in Central and NW Anatolia is clearly pre-Liassic. The mélange units of the Karakaya Complex are unconformably overlain by the Early Jurassic clastics, which limits the uppermost age of the complex to the latest Triassic (Kocyigit, 1987; Altiner et al., 1991; Okay et al., 1991; Kocyigit et al., 1991). The latest Triassic age also marks the age of the common deformation phase that resulted from the Cimmeride Orogeny. The trace of this orogenic event is evidenced by the presence of Nilüfer-type metabasic rocks with high-pressure overprints, which indicates consumption of the Palaeotethyan oceanic lithosphere. The most widespread of these rocks are high-pressure greenschists (Okay et al., 1991; Sayit and Göncüoglu, 2009b; Sayit, 2010). The Ar–Ar phengite ages of the eclogites and blueschist metabasites from the Nilüfer Unit give a Latest Triassic age (Okay and Monie, 1997; Okay et al., 2002), which is consistent with the geological results. The oldest ages acquired from the Karakaya Complex imply the existence of an oceanic basin on which the Karakaya units were formed (e.g. Bingöl et al., 1973; Okay and Mostler, 1994; Okay et al., 2011). The nature of this basin, however, is a matter of debate. Does it represent a relatively small basin separate from Palaeotethys or does it represent Palaeotethys itself? This problem mainly stems from how the Karakaya-related entities have been interpreted in the Palaeotethyan framework. In the continental-rift model (e.g. Bingöl et al., 1973; Sengör and Yilmaz, 1981; Kocyigit, 1987; Altiner and Kocyigit, 1993; Genc and Yilmaz, 1995), the Karakaya lithologies are thought to have been formed by opening of a rather short-lived back-arc basin that was isolated from the Palaeotethys. According to this view, the Karakaya basin persisted during the Late Permian-Late Triassic interval. However, the metabasic rocks from the Pulur Complex give the Late Permian metamorphism ages, indicating that the subduction was still in process during that period (Topuz et al., 2004). This, in turn, requires that the oceanic basin should have already been present before the Late Permian. This idea is in agreement with the recent discovery of ribbon cherts of Devonian age, which suggests the presence of a deep basin as old as Devonian (Okay et al., 2011). The geodynamic evolution of the Karakaya Complex has been argued on the basis of two main ideas, namely the rift model and subduction/accretion model (see also Okay and Göncüoglu, 2004). The rift model, originally suggested by Bingöl et al. (1973), proposes that the Karakaya Units were formed in a basin that opened through back-arc extension during the Early Triassic (Fig. 4) (Kocyigit, 1987; Altiner and Kocyigit, 1993; Genc and Yilmaz, 1995). On the other hand, the subduction/accretion model includes, at least, some ocean-derived assemblages that developed on the Palaeotethyan oceanic crust itself, which were then incorporated into an accretionary prism during the latest Triassic (Fig. 5). Apart from these two distinct classes of thought (rift vs subduction/accretion) regarding the geodynamic evolution of the Karakaya Complex, another approach that unifies these two ideas was suggested by Göncüoglu et al. (2000) and Sayit et al. (2011), where the subduction–accretion prism material of Palaeotethys were emplaced on a Late Triassic extensional basin that had been formed on the pre-Triassic basement of the Sakarya Composite Terrane of Göncüoglu et al. (1997). 3.3. Origin of the metabasic rocks within the Karakaya Complex The tectonic setting of the metabasic rocks within the Karakaya Complex is a controversial subject, since these rocks are directly related to the overall geodynamic picture. Some studies suggest that they were formed in a continental-rift setting that failed to evolve into an ocean floor spreading stage (e.g. Bingöl et al., 1973;

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Fig. 5. Subduction/accretion model of Okay (2000), which relates the geodynamic evolution of the Karakaya Complex to a Middle Triassic-aged oceanic plateau that later were incorporated into the Laurasian active continental margin.

Akyürek et al., 1984; Kocyigit, 1987; Altiner and Kocyigit, 1993; Genc and Yilmaz, 1995; Kozur et al., 2000) or that advanced into the mature oceanic stage (Sengör and Yilmaz, 1981; Sengör et al., 1984; Stampfli and Borel, 2002; Moix et al., 2008), while others argue that they represent a seamount and/or oceanic island (Capan and Floyd, 1985; Pickett and Robertson, 1996, 2004; Yaliniz and Göncüoglu, 2002), an oceanic plateau (Okay, 2000), a large igneous province including an oceanic plateau as well as seamounts (Genc, 2004) or mantle plume-related seamounts and/or oceanic islands associated with a spreading ridge (Sayit and Göncüoglu, 2009b). A third hypothesis is that these rock assemblages were generated in an intra-oceanic subduction zone (Okay et al., 1996). An oceanic plateau origin, which was initally proposed by Okay (2000) (Fig. 5) and followed by Genc (2004) (with some modifications), is not supported by the geochemical character of the redefined Nilüfer-type metabasic rocks (Fig. 2). Sayit et al. (2010) argued that oceanic plateaus are largely made up of basaltic rocks of tholeiitic composition (e.g. Mahoney et al., 1993; Frey et al., 2000), whereas the redefined Nilüfer Unit is predominantly of alkaline character (Capan and Floyd, 1985; Pickett and Robertson, 1996, 2004; Sayit and Göncüoglu, 2009b). Their other argument was that oceanic plateaus mainly display flat REE patterns (e.g. Floyd, 1989; Mahoney et al., 1993), which contrast with the redefined Nilüfer-type metabasic rocks that show variable enrichment in LREE relative to HREE (Fig. 2) (Sayit and Göncüoglu, 2009b; Sayit et al., 2010). As discussed in detail by Sayit et al. (2010), a continental-rift setting is also not supported in terms of the available geological or geochemical data for the following reasons. First, the redefined Nilüfer Unit does not include any continental crustal material. Second, the Nilüfer-type metabasaltic rocks show no crustal contamination in terms of their geochemical character. A continental back-arc or intra-oceanic arc option also does not seem likely, as the Nilüfer Unit metabasic rocks do not show any subduction-related signatures. In contrast, they appear to be predominantly OIB-like (Capan and Floyd, 1985; Pickett and Robertson, 2004; Sayit and Göncüoglu, 2009b; Sayit et al., 2010). In addition to the OIB- and E-MORB-type metabasic rocks predominantly found in the Karakaya Complex, which characterize the redefined Nilüfer Unit, SSZ-type mafic rocks are also encountered. However, the latter type is not widespread and mainly restricted to the Küre and Ankara regions (Ustaömer and Robertson, 1994; Sayit and Göncüoglu, 2009b) (Fig. 2). We must note, however, that there is no precise age constraint on the BABB-type diabases in the Ankara region. Although the geological evidence suggests a preJurassic (Karakaya-related) origin, there is still a possibility that these diabases were generated during a younger, i.e. Neotethyan, event.

Fig. 6. Petrological reconstruction by Sayit and Göncüoglu (2009b) (slightly modified), in which the Late-Middle alkaline-dominated magmatism is represented by seamounts/oceanic islands fed by a mantle plume located nearby the Palaeotethyan spreading ridge.

Considering both geological and geochemical findings together, the most likely interpretation is that the Middle-Late Triassic OIBand E-MORB-like magmatism represent oceanic islands and/or seamounts formed on Palaeotethyan oceanic crust during this time interval (Sayit and Göncüoglu, 2009b; Sayit et al., 2010). According to Sayit and Göncüoglu (2009b) the seamounts/oceanic islands were created nearby a mid-ocean ridge in association with a mantle plume (Fig. 6). They suggested that the OIB-like magmatism results from the partial melts directly derived from the mantle plume. The E-MORB-type melts, on the other hand, are the products of the melt-mixing that has occurred as a result of mixing of enriched plume derived melts coming from garnet-stability field with the depleted spinel-facies melts. 3.4. Tectonomagmatic evolution of the Karakaya Complex Based on the constraints mentioned above, a brief summary of the tectonomagmatic evolution of the Karakaya Melange Complex is given in Fig. 7. According to this geodynamic scenario combining both rift and subduction/accretion models (Sayit et al., 2011), the southward subduction of the Palaeotethyan oceanic lithosphere during the Late Permian led to rifting of the northern margin of Tauride-Anatolide platform. Even though a northward subduction polarity has been adopted by some studies (e.g. Okay et al., 2006; Stampfli and Kozur, 2006; Moix et al., 2008), we favor here a southerly dipping subduction model mainly due to the relative location and the lithological content of the Istanbul Terrane (Göncüoglu et al., 1997). The presence of the syn-orogenic flysch deposits that overlie the platform-slope-type carbonates suggests that the I˙ stanbul Terrane was located at an area of passive margin development. There is a general agreement that the I˙ stanbul Terrane was lying to the north of the Palaeotethys (e.g. Sengör et al., 1984; Okay and Tüysüz, 1999; Göncüoglu et al., 2000). Therefore, this suggests that the polarity of subduction of the Palaeotethys should have been directed toward the south, at least starting from the Carboniferous. Also supporting such a model is the rifting event that occurred during the Missisipian (Early Carboniferous) along the northern border of the Tauride-Anatolide Platform (Göncüoglu et al., 2007). The other point we would like to point out is that in the geodynamic model we assume that the Sakarya Terrane is attached to the Taurid-Anatolide Platform as evidenced and outlined by Turhan et al. (2004) and Okuyucu and Göncüoglu (2010). This continental back-arc rifting process created the “Karakaya Rift Basin” (sensu Sayit et al., 2011). We must, however, note that this rift basin is different than previously suggested (e.g. Kocyigit, 1987; Altiner and Kocyigit, 1993). This is because the

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tectonic activity in this period resulted in the deposition of continental sediments in piggy-back-type basins (Sayit et al., 2011). As mentioned before, the metabasaltic rocks from the Lower and Upper Karakaya Complexes reflect a common origin. We think that this situation can be explained by a “flow melange” origin (Cloos, 1982, 1984) of the Karakaya Mélange Complex. In such a scenario, the thermal gradients are at the peak during the initial stages of the subduction. At this time period, the upper part of the subducting slab is hotter due to conducted heat from the overriding plate. Therefore, the basaltic seamount material interleaved with sediments will undergo metamorphic recrystallization at most in this region, and schistose texture will develop. The metamorphic grade will be higher with the increasing depth along the slab owing to increase in temperature. In contrast, less schistose or less metamorphic rocks of the mélange form away from the base of overriding plate, or in the lower layer, where the temperatures are lower. 4. Conclusions

Fig. 7. Geodynamic reconstruction for the evolution of the Karakaya Complex (based on Sayit et al., 2011). Metamorphic structure of the slab based on Cloos (1984).

metabasic lithologies interpreted as reflecting a continental-rift setting in the previous studies have been shown by Sayit et al. (2010) as parts of oceanic islands/seamounts that they called the “redefined Nilüfer Unit”. Hence, the Karakaya Rift Basin of Sayit et al. (2011) is a completely different entity and does not include any mafic material characterizing the redefined Nilüfer Unit. Another alternative is that this rift basin was opened by the action of a mantle plume that penetrated into the Palaeotethyan mantle. However, the lack of any evidence of voluminous continental flood magmatism, which would be expected during decompression melting of a plume-head (e.g. North Atlantic Igneous Province, Parana-Etendeka Province; Saunders et al., 1997; Peate, 1997), makes this option less likely. During the Early?-Middle Triassic, the relative plate motion caused by the ongoing oceanic spreading brought the Palaeotethyan oceanic lithosphere over the mantle plume. The resulting OIB- and E-MORB-type magmatism formed oceanic islands and seamounts as represented by the redefined Nilüfer Unit (Sayit et al., 2010). This oceanic within-plate magmatism probably lasted until Early-Late Triassic as indicated by the metabasalts interbedded with the Ladinian?-Carnian cherts (Sayit and Göncüoglu, 2009b). During the Late Triassic, the oceanic islands and seamounts began accreting into the mélange prism and become incorporated with the continental material derived from the active margin (the Sakarya platform). Some pieces of the oceanic islands should have experienced cold, deep burial as evidenced by the development of metamorphic sodic amphibole (high-pressure greenschists and blueschists). The presence of amphibolites and eclogites suggests that even higher pressures were attained during the subduction of oceanic slab (Topuz et al., 2004; Okay and Monie, 1997; Okay et al., 2002). The pauses in

The Karakaya Complex from northern Turkey, which holds the records inherited from the closure of Palaeotethys, comprises a number of sedimentary/tectonic melange units formed in a subduction zone setting. One of these units, the redefined Nilüfer Unit, represents the oceanic islands/seamounts formed on the Palaeotethyan oceanic crust. Another mélange unit, the Eymir Unit, includes continent-derived clastics and forms the matrix material in which the Nilüfer-type blocks are embedded. Combined geological and geochemical evidence obtained from the Nilüfertype metabasic rocks suggests that the Lower and Upper Karakaya Complexes, which were previously treated as distinct entities, are actually of similar origins. Furthermore, the so-called “non- or less metamorphic” Lower Karakaya Complex is found to have experienced greenschist facies conditions, and in some places elevated pressure conditions are also observed. We think that the difference in the thermal structure within the subduction zone and distance to the contact zone (base of the overriding mantle) is the main source of the textural and metamorphic differences between the rocks of the same magmatic origin within the Karakaya Mélange Complex. We also note that in classifying such chaotically mixed units, tectonomagmatic origin (together with age data) should be used as the primary criterion rather than metamorphism and deformation. Acknowledgement The handling editor Erdin Bozkurt and two anonymous reviewers are gratefully acknowledged for their comments and helpful suggestions. References Akyürek, B., Soysal, Y., 1981. Biga Yarımadası Güneyinin (Savas¸tepe-Kırka˘gac¸Bergama-Ayvalık) temel jeoloji özellikleri. Bulletin of the Mineral Research and Exploration 95/96, 1–13. Akyürek, B., Bilginer, E., Akbas, B., Hepsen, N., Pehlivan, S., Sunu, O., Soysal, Y., Dager, Z., Catal, E., Sözeri, B., Yıldırım, H., Hakyemez, Y., 1984. Basic geological features of Ankara–Elmadag–Kalecik region. Jeol Mühendisligi 20, 31–46 (in Turkish with English abstract). Altiner, D., Kocyigit, A., 1993. Third remark on the geology of the Karakaya basin. An Anisian megablock in northern central Anatolia: micropaleontologic, stratigraphic and tectonic implications for the rifting stage of Karakaya basin, Turkey. Revue de Paléobiologie 12, 1–17. Altiner, D., Kocyigit, A., Farinacci, A., Nicosia, U., Conti, M.A., 1991. Jurassic-Lower Cretaceous stratigraphy and paleogeographic evolution of the southern part of north-western Anatolia. Geologica Romana 27, 13–80. Baldwin, S.L., Harrison, T.M., 1992. The P–T–t history of blocks in serpentinite-matrix mélange, west-central Baja California. Geological Society of America Bulletin 104, 18–31. Bingöl, E., Akyürek, B., Korkmazer, B., 1973. The geology of the Biga Peninsula and some features of the Karakaya Formation. In: Proceedings of the

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