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Articles

Diverse tectonic settings of formation of the metaigneous rocks in the Jurassic metamorphic accretionary complexes (Refahiye, NE Turkey) and their geodynamic implications

, , , &
Pages 294-310 | Published online: 11 Dec 2013

Abstract

Two isolated metamorphic accretionary complexes of Jurassic age, the Refahiye and Kurtlutepe metamorphic rocks, crop out as tectonic slices within the coeval suprasubduction-zone ophiolite at the southern margin of the Eastern Pontides (NE Turkey), close to the İzmir-Ankara-Erzincan suture. The Refahiye metamorphic rocks are made up of greenschist, marble, serpentinite, phyllite and minor garnet amphibolite, garnet micaschist and metachert. The whole unit was metamorphosed under garnet-amphibolite-facies conditions and strongly retrogressed during exhumation. The Kurtlutepe metamorphic rocks consist of subgreenschist-facies metavolcanics, metavolcaniclastics, marble, calc-phyllite, and minor serpentinite and metachert. Metabasites in the Refahiye metamorphic rocks are represented by four distinct geochemical affinities: (i) cumulate “flavor,” (ii) alkaline oceanic island basalt (OIB), (iii) enriched mid-ocean ridge basalt (E-MORB) and (iv) tholeiitic island arc basalt (IAB). On the other hand, the Kurtlutepe metavolcanic rocks display only tholeiitic to calc-alkaline island arc geochemical affinities. The metabasic rocks with OIB affinities were interpreted as parts of the accreted oceanic islands, and those with E-MORB affinities as parts of accreted ridge segments close to oceanic islands and/or plume-distal mid-ocean ridges with a mantle previously metasomatized by plume components. The metabasic rocks with IAB affinities might have been derived from the overlying suprasubduction ophiolite and/or arc domain by a number of tectonic or sedimentary processes including tectonic slicing of accretionary complex and overlying fore-arc ophiolite, juxtaposition of the magmatic arc with subduction zone by strike slip faults, submarine gravity sliding and debris flows or subduction erosion. However, totally recrystallized nature of the metabasic rocks together with field relations does not allow any inference on the processes involved. The Kurtlutepe metavolcanic rocks might represent collided and accreted oceanic island arc with the subduction zone. Attempted subduction of an intraoceanic island arc may also explain the magmatic lull during Late Jurassic–Early Cretaceous in the Eastern Pontides.

1. Introduction

Accretionary complexes are essential components of modern and fossil accreting plate boundaries (e.g. Clift & Vannucchi, Citation2004; Karig, Citation1980; Karig & Sharman, Citation1975; Shervais et al., Citation2011; von Huene & Lallemand, Citation1990; von Huene & Scholl, Citation1991), and contain materials of diverse origin such as (i) offscraped material from the topographic highs on the subducting oceanic floor (seamounts, oceanic islands, oceanic plateaus, oceanic island arcs and associated sedimentary rocks; e.g. Cloos, Citation1993; Isozaki, Maruyama, & Furuoka, Citation1990; Kimura & Ludden, Citation1995), (ii) materials derived from the subducting oceanic plate by faulting and subduction accretion (Karig & Sharman, Citation1975, Shervais et al., Citation2011, and references therein), (iii) materials derived from forearc and magmatic arc by gravity sliding, and debris flows (e.g. Draut & Clift, Citation2013, and references therein) and (iv) materials derived from the overriding plate through tectonic slicing. Deeply subducted parts of the accretionary complexes are mostly subjected to high-pressure metamorphism whereby primary structures of the accreted materials are mostly obliterated (e.g. Agard, Yamato, Jolivet, & Burov, Citation2009; Ernst, Citation2003; Topuz, Okay, Altherr, Satır, & Schwarz, Citation2008). Geochemistry of the metabasic rocks provides the best means to determine the nature of the accreted materials in ancient metamorphic accretionary complexes provided that the fluid flow during the accretionary processes have not strongly modified rock composition.

In this paper, we present field geological, petrographic and bulk-geochemical data on the meta-igneous rocks from two metamorphic oceanic accretionary complexes of Early to Middle Jurassic age from the Eastern Pontides (NE Turkey), and discuss these data in the context of the nature of the accreted material and their geodynamic implications.

2. Geological framework

The Eastern Mediterranean region within the Tethyan realm is a tectonic collage of Laurasia (Ukrainian shield) to the north and Gondwana (Arabian platform) in the south and intervening continental blocks (e.g. Menderes-Taurus, Kırşehir, Sakarya and Istanbul) (Figure , e.g. Okay & Tüysüz, Citation1999; Şengör, Citation1987, Citation1990; Şengör & Yılmaz, Citation1981). Main continents and intervening continental blocks are separated by sutures representing traces of the former oceanic domains such as İzmir-Ankara-Erzincan (İAE), Bitlis-Zagros (BZ), Intra-Tauride (IT) and Intra-Pontide (IP). These sutures are marked by accretionary complexes, ophiolites and high-pressure metamorphic rocks. The İAE suture represents the main Tethyan suture, separating the Sakarya zone in the north and the Menderes-Taurus block and the Kırşehir massif in the south. Final collage of these continental blocks along the İAE suture occurred during the early Cenozoic (e.g. Okay & Şahintürk, Citation1997; Topuz et al., Citation2011). The closed oceanic domain dates back to Devonian times, and the northward subduction under the Sakarya zone led to episodic accretionary growth from late Paleozoic to end-Mesozoic time (e.g. Okay et al., Citation2013; Topuz, Göçmengil et al., Citation2013).

Figure 1. Main Tethyan sutures and continental blocks in the Eastern Mediterranean region together with the locations of different accretionary complexes (modified after Topuz, Göçmengil, et al., Citation2013). For clarity, only accretionary complexes along the İzmir-Ankara-Erzincan suture (IAES) and South Azerbaijan suture (SAS) are displayed. ITS Intra-Tauride suture; BZS Bitlis-Zagros suture; MT Mirdita; PD Pindos; AG Agoriani; EV Evia; KH Khoy.

Figure 1. Main Tethyan sutures and continental blocks in the Eastern Mediterranean region together with the locations of different accretionary complexes (modified after Topuz, Göçmengil, et al., Citation2013). For clarity, only accretionary complexes along the İzmir-Ankara-Erzincan suture (IAES) and South Azerbaijan suture (SAS) are displayed. ITS Intra-Tauride suture; BZS Bitlis-Zagros suture; MT Mirdita; PD Pindos; AG Agoriani; EV Evia; KH Khoy.

The known oceanic accretionary complexes in the northern Turkey are represented by four different age groups (Figure ): (i) Carboniferous accretionary complexes (e.g. Robertson, Parlak, & Ustaömer, Citation2009; Robertson & Ustaömer, Citation2009a, Citation2009b; Zanchi, Garzanti, Larghi, Angiolini, & Gaetani, Citation2003) (ii) Permo-Triassic metamorphic and volcano-sedimentary accretionary complexes (e.g. Okay, Citation1984; Okay & Göncüoğlu, Citation2004; Okay, Monod, & Monié, Citation2002; Okay, Noble, & Tekin, Citation2011; Pickett & Robertson, Citation1996; Robertson & Ustaömer, Citation2012; Sayit, Göncüoglu, & Furman, Citation2010; Topuz, Altherr, Satır, & Schwarz, Citation2004; Topuz, Okay et al., Citation2013), (iii) Early to Middle Jurassic metamorphic accretionary complexes (Topuz, Çelik et al., Citation2013; Topuz, Göçmengil et al., Citation2013) and (iv) Early to Late Cretaceous mainly non-metamorphic accretionary complexes (e.g. Bailey & McCallien, Citation1950, Citation1953; Bragin & Tekin, Citation1996; Çapan & Floyd, Citation1985; Eyüboğlu, Bektaş, & Pul, Citation2007; Gökten & Floyd, Citation2007; Göncüoğlu, Sayit, & Tekin, Citation2010; Norman, Citation1985; Okay et al., Citation2006; Rice, Robertson, Ustaömer, İnan, & Taslı, Citation2009; Robertson et al., Citation2009; Rojay, Citation2013; Rojay, Yalınız, & Altıner, Citation2001; Rolland, Billo, Corsini, Sosson, & Galoyan, Citation2009; Tekin, Göncüoğlu, & Turhan, Citation2002). Among these, Jurassic accretionary complexes are rare and are known only from two locations: (i) the Refahiye area in NE Turkey (Topuz, Çelik, et al., Citation2013; Topuz, Göçmengil et al., Citation2013; topic of this paper) and (ii) the Khoy area in NW Iran (e.g. Khalatbari-Jafari et al., Citation2004). However, fragments of Jurassic suprasubduction-zone ophiolites, ophiolite-related metamorphic and deep sedimentary rocks are widespread in the Cretaceous accretionary complexes in Turkey and farther east (e.g. Çelik et al., Citation2011; Çelik, Chiaradia, Marzoli, Billor, & Marschik, Citation2013; Dilek & Thy, Citation2006; Göncüoğlu et al., Citation2012; Göncüoğlu, Yalınız, & Tekin, Citation2006; Hässig et al., Citation2013; Okay et al., Citation2013; Rolland, Galoyan, Sosson, Melkonyan, & Avagyan, Citation2010). In clear distinction to Turkey and farther east, Jurassic accretionary complexes are common in the Balkans, occurring beneath the Jurassic ophiolites (e.g. Bortolotti et al., Citation2004; Danelian & Robertson, Citation2001; Robertson, Citation2012 and references therein; Saccani & Photiades, Citation2005; Saccani, Photiades, & Padoa, Citation2003). Rarity of the Jurassic accretionary complexes and intact ophiolites in Turkey is ascribed to the later removal by subduction erosion between Middle Jurassic and Late Cretaceous time and dismemberment by tectonic processes from the upper plate of the subduction zone and incorporation into Cretaceous ophiolitic mélanges (Topuz, Çelik et al., Citation2013).

3. Local geology

The Refahiye and Kurtlutepe Jurassic metamorphic rocks occur in the Eastern Pontides, close to the İAE suture, and are interleaved with the coeval suprasubduction-zone ophiolite (Figure ; cf. Parlak et al., Citation2013; Rice et al., Citation2006, Citation2009; Topuz, Çelik et al., Citation2013; Topuz, Göçmengil et al., Citation2013; Yılmaz, 1985; Yılmaz & Yılmaz, 2004). The Refahiye ophiolite is tectonically underlain by the Late Cretaceous accretionary complex to the south, and is bound by the right-lateral North Anatolian Fault to the north. All the units were discordantly overlain by Middle Eocene and younger sedimentary sequences, suggesting that the tectonic interleaving occurred before the Middle Eocene (Topuz, Çelik et al., Citation2013).

Figure 2. Geological map of the Refahiye and Kurtlutepe meta-igneous areas together with the sample locations (modified after Topuz, Çelik et al., Citation2013).

Figure 2. Geological map of the Refahiye and Kurtlutepe meta-igneous areas together with the sample locations (modified after Topuz, Çelik et al., Citation2013).

The Refahiye metamorphic rocks are exposed over an area of ~100 km2, and comprise predominantly metabasite (~40% of the outcrop area), marble (~30%), serpentinite (~20%), phyllite (~10%), and subordinately metachert, micaschist, and garnet-amphibolite (Figure ). Garnet-amphibolite and garnet-micaschist occur at two locations where garnet-amphibolite is surrounded by garnet-free amphibolite and garnet-micaschist. Greenschist is characterized by a dark bluish green color and well-developed foliation (Figure (a)). On the other hand, garnet-amphibolite is massive to feebly foliated (Figure (b)). Marble is massive and forms large exposures, especially to the NE of the Refahiye metamorphics (Figures and (c)). In contrast to the serpentinite within the Refahiye ophiolite, the serpentinite within the metamorphic rocks displays locally well-developed foliation and is intimately associated with marble and greenschist. Crenulation cleavage is common in the phyllite. In general, all rock types are crosscut by up to 5 cm thick late veins of calcite and quartz. Timing of metamorphism is constrained by stepwise Ar–Ar phengite and U–Pb rutile ages as 175 ± 5 Ma (2σ; Early to Middle Jurassic) (Topuz, Göçmengil et al., Citation2013). The Refahiye metamorphic rocks were interpreted as a metamorphic accretionary complex on the basis of (i) widespread presence of oceanic rock types (e.g. serpentinite, metachert), (ii) locally preserved evidence for high-pressure metamorphism (e.g. local presence of Na-Ca amphibole) and (iii) common presence of the metabasic rocks with ocean island basalt (OIB) and enriched mid-ocean ridge basalt (E-MORB) signatures (Topuz, Çelik et al., Citation2013; Topuz, Göçmengil et al., Citation2013; this paper).

Figure 3. Field and hand-sample pictures of the meta-igneous rocks from the Refahiye and Kurtlutepe areas: (a) well-foliated greenschist (Refahiye), (b) unfoliated garnet amphibolite in hand sample (Refahiye), (c) large massive marble body (Refahiye), (d) volcanic fragments in the metavolcaniclastic rock (Kurtlutepe), and (e) metavolcaniclastic rock (Kurtlutepe), and (f) marble and calc-phyllite association (Kurtlutepe).

Figure 3. Field and hand-sample pictures of the meta-igneous rocks from the Refahiye and Kurtlutepe areas: (a) well-foliated greenschist (Refahiye), (b) unfoliated garnet amphibolite in hand sample (Refahiye), (c) large massive marble body (Refahiye), (d) volcanic fragments in the metavolcaniclastic rock (Kurtlutepe), and (e) metavolcaniclastic rock (Kurtlutepe), and (f) marble and calc-phyllite association (Kurtlutepe).

The Kurtlutepe metamorphic rocks are exposed over an area of ~70 km2, and made up of metavolcaniclastic and metavolcanic rocks (~65% of the outcrop area), marble (~20%), calc-phyllite (~15%), minor carbonated serpentine and metachert. In general, primary igneous and sedimentary structures are well-preserved in clear distinction to the Refahiye metamorphic rocks: In the metavolcaniclastic rocks, volcanic fragments and sedimentary layering are discernible (Figure (d)–(e)). Volcanic fragments are mostly angular, ranging in size from 1 to 2 mm to 40 cm. Marble is poorly foliated and occurs in close association with calc-phyllite (Figure (f)). Metachert has a fine lamination. Carbonated serpentine is encountered only in the 1 km north of Kırbulak village (Figure ). No geochronological data are available on the Kurtlutepe metamorphic rocks. A Jurassic age of metamorphism is regarded likely, because the Kurtlutepe metamorphic rocks also occur as a tectonic sheet within the Jurassic ophiolite comparable to the Refahiye ones. However, marked differences in metamorphic grade and rock assemblage, and tectonic position of the metamorphic domains indicate that both domains are unrelated to each other (Topuz, Çelik et al., Citation2013).

4. Petrography

Over 100 thin sections from both the Refahiye and Kurtlutepe metamorphic rocks are petrographically investigated. Mineral constituents and their estimated abundances of the investigated samples are given in Table . As indicated above, the metabasic rocks in the Refahiye metamorphic rocks are represented by greenschist, amphibolite and garnet amphibolite, and those in Kurtlutepe metamorphic rocks by intermediate to basic metavolcanic rocks.

Table 1. Estimated modal abundances in the metabasic and metavolcanic rocks from the Refahiye and Kurtlutepe areas.

4.1. The Refahiye metabasic rocks

Greenschist is fine- to medium-grained (~0.1–0.5 mm), and contains actinolite, Na–Ca-amphibole, epidote, albite, chlorite, sphene and quartz, calcite, rutile and apatite (Table ; Figure (a)). Na–Ca amphibole is found in samples 87A, 113A, 130, 140 and 415a. In some samples, hornblende, ilmenite and rutile are present (Figure (b)). Hornblende is replaced by actinolite. Rutile and ilmenite occur as grains rimmed by sphene and are therefore regarded as relics.

Figure 4. Microtextural features of the meta-igneous rocks in the Refahiye and Kurtlutepe metamorphic areas: (a) A well-recrystallized greenschist with epidote (Ep), actinolite (Act) and albite (Ab) (sample # 83a), (b) A greenschist with Na–Ca amphibole (Na/Ca Amp), chlorite (Chl), sphene (Sph) and ilmenite (Ilm). Note that the sphene grains have relic cores of ilmenite (Ilm) (sample #87A), (c) Relic rutile is rimmed by sphene in a amphibolite sample (sample #109a), (d) A garnet amphibolite: Garnet (Grt), hornblende (Hbl), and clinopyroxene (Cpx) (sample #514E), (e) Well-preserved microgranular porphyric texture in a metavolcanic sample (sample #105B) and (f) Local growth of metamorphic epidote, albite and stilpnomelane (Stp) in a metavolcanic sample from the Kurtlutepe area (sample #351a).

Figure 4. Microtextural features of the meta-igneous rocks in the Refahiye and Kurtlutepe metamorphic areas: (a) A well-recrystallized greenschist with epidote (Ep), actinolite (Act) and albite (Ab) (sample # 83a), (b) A greenschist with Na–Ca amphibole (Na/Ca Amp), chlorite (Chl), sphene (Sph) and ilmenite (Ilm). Note that the sphene grains have relic cores of ilmenite (Ilm) (sample #87A), (c) Relic rutile is rimmed by sphene in a amphibolite sample (sample #109a), (d) A garnet amphibolite: Garnet (Grt), hornblende (Hbl), and clinopyroxene (Cpx) (sample #514E), (e) Well-preserved microgranular porphyric texture in a metavolcanic sample (sample #105B) and (f) Local growth of metamorphic epidote, albite and stilpnomelane (Stp) in a metavolcanic sample from the Kurtlutepe area (sample #351a).

Amphibolite is feebly foliated, medium-grained (~0.1–2 mm) and comprise hornblende, epidote, albite, titanite, rutile, quartz, and accessory apatite and calcite. Hornblende grains contain inclusions of sphene, epidote and rutile. Locally rutile is rimmed by sphene (Figure (c)).

Garnet amphibolite is medium to coarse-grained and comprises garnet, clinopyroxene, hornblende, epidote, quartz, and accessory rutile and sphene (Figure (d)). Foliation is absent to feeble. Garnet forms porphyroblasts (up to 7 mm in diameter) with inclusions of quartz, clinopyroxene, epidote and rutile. Up to 2 mm thick veins of albite are present.

4.2. The Kurtlutepe metavolcanic rocks

The Kurtlutepe metavolcanic rocks are poorly foliated, and comprise albite, epidote, chlorite, stilpnomelane, pumpellyite, quartz, together with secondary calcite, sericite and opaque minerals. Microgranular porphyric textures are well-preserved. Metamorphic minerals such as epidote, stilpnomelane and pumpellyite are locally developed (Figure (e) and (f)). The metamorphic mineral assemblage is characteristic of the subgreenschist-facies.

5. Bulk rock geochemistry

5.1. Analytical technique

Whole-rock analyses of 25 meta-igneous samples were performed at Acme Analytical Laboratories Ltd. in Vancouver (Canada), and are given in Tables and . Analyses of major elements and the trace elements Ba, Nb, Ni, Sr, Sc, Y and Zr, sample solutions were aspirated into an ICP emission spectrograph (Jarrel Ash AtomComb 975). Determination of other trace elements together with rare earth elements (REE), the solutions were aspirated into an ICP mass spectrometer (Perkin-Elmer Elan 6000). Analytical procedures and levels of uncertainty are the same as outlined in Topuz et al. (Citation2010).

Table 2. Whole rock compositions of metabasic rocks from the Refahiye metamorphic area, NE Turkey.

Table 3. Whole rock compositions of metavolcanic rocks from the Kurtlutepe metamorphics. NE Turkey.

5.2. General considerations

Overall, the basic rocks in the Refahiye and Kurtlutepe metamorphic domains are variably hydrated, as reflected in relatively high and variable loss on ignition (LOI) values (1.3–5.1 wt.%) (Tables and ). These LOI values are in line with the presence of abundant chlorite, epidote, pumpellyite and amphibole. As the rocks are metamorphosed, we use high field strength (HFSE: Nb, Ta, Zr, Ti and Y) and REEs in the petrogenetic considerations because these elements are regarded generally immobile during low-grade metamorphism and alteration (e.g. Floyd & Winchester, Citation1978; Pearce & Cann, Citation1973). In order to circumvent the problem of compositional modification in absolute abundances of elements due to the possible volume change during metamorphism, we will mostly rely on the ratios of the fluid immobile elements. In the sample selection for chemical analysis, we bestowed great care to avoid samples of possible volcaniclastic origin. In case of the Kurtlutepe metamorphic rocks, it is easy because the primary textures in all the samples are well-preserved. In case of the Refahiye metamorphic rocks, the selection is performed on the basis of presence of primary calcite and irregularities in texture. As some metabasic rocks could have been derived from former cumulates, recognition of the metabasic rocks as cumulate has been done merely on the geochemical ground for the Refahiye metamorphic rocks (anomalously high Al2O3 and CaO contents, positive Eu anomalies; e.g. Arculus, Lapierre, & Jaillard, Citation1999; Liu et al., Citation2007; Liu, Zong, Keleman, & Gao, Citation2008; Seifert, Gibson, Weis, & Brunotte, Citation1996; Spandler, Hermann, Arculus, & Mavrogenes, Citation2004). Bulk composition of cumulate rocks does not represent the primary melt composition. Notwithstanding numerous studies dealing with petrogenetic modeling on the metabasic rocks from the accretionary complexes (e.g. Aldanmaz, Yalınız, Güçtekin, & Göncüoğlu, Citation2008; Dupuis et al., Citation2005; Göncüoğlu et al., Citation2010; Saccani, Beccaluva, Photiades, & Zeda, Citation2011; Yang et al., Citation2012) we evade such modeling because genetically unrelated rocks are mostly found next to each other in accretionary complexes, and even co-magmatic relationship within the same geochemical group cannot be proven.

5.3. The Refahiye metabasic rocks

The Refahiye metabasic rocks display a wide-ranging compositional variation (normalized on a volatile-free basis: SiO2 ~37–47 wt.%; MgO ~3–14 wt.%; CaO ~8–21 wt.%; Fe2O3tot ~8–21 wt.%; Table ). On the Nb/Y vs. Zr/TiO2 of Winchester & Floyd (Citation1977), the samples plot in the fields of alkaline basalt, subalkaline basalt, basaltic andesite and basalt (Figure ).

Figure 5. Nb/Y vs. Zr/TiO2*0.0001 classification diagram (after Winchester & Floyd, Citation1977).

Figure 5. Nb/Y vs. Zr/TiO2*0.0001 classification diagram (after Winchester & Floyd, Citation1977).

Mainly four different geochemical types are distinguished: Type I, encompassing garnet amphibolites, is characterized by anomalously high CaO and Al2O3 contents, 13–21 and 17–21 wt.%, respectively, suggesting that they were derived from plagioclase-rich cumulates. Their REE patterns are variable, ranging from U-shaped one to the almost flat ones (Figure (a)). Only sample 440B displays a pronounced positive Eu anomaly (Eu/Eu*~1.8). On the multi-element variation diagrams, they show a marked Nb–Ta anomaly and positive Th anomaly (not shown). Type II, petrographically greenschist with Na–Ca amphibole, has high TiO2 (1.7–5.2 wt.%), Zr (107–359 ppm) and Nb (23–91 ppm) contents and resulting low Zr/Nb ratios (Table ; Figure (a) and (b)). Their REE patterns are strongly fractionated with (La/Yb)cn values of 8 and 18, and display no discernible Eu anomaly (Figure (b)). On the multi-element variation diagrams, they lack any Nb–Ta anomaly (not shown). With these geochemical features, Type-II metabasites resemble alkaline oceanic island basalts (OIB), what is also suggested on the Th–Zr–Nb discrimination diagram of Wood, Joron, and Treuil (Citation1979) (Figure (a)). Type III metabasites, petrographically greenschists, are characterized by low Th, Nb and Zr contents, and resulting low Zr/Nb ratios (Figure (a) and (b)). They display two different REE patterns (Figure (c)): while sample 454B is characterized by a slightly fractionated REE pattern with (La/Yb)cn ~3, the other samples display spoon-shaped patterns with no obvious Eu anomaly. They plot into the fields of enriched middle ocean ridge basalt (E-MORB) (Figure (a)). On the other hand, Type IV metabasites, petrographically amphibolites, differ from the other types by their extremely low Nb and Th contents, and resulting high Zr/Nb ratios (Figure (a) and (b)). Their REE patterns range from the nearly flat one ([(La/Yb)cn~1.1), to slightly LREE-depleted ones [(La/Yb)cn~0.6], and display no Eu anomaly (Figure (d)). On the multi-element variation diagrams they display a marked Nb-Ta anomaly, contrary to Type II and III metabasic rocks (not shown). With these geochemical characteristics, they are classified as tholeitic island arc basalts (IAB) (Figure (a)).

Figure 6. Chondrite-normalized REE earth element patterns of metabasic and metavolcanic rocks from Refahiye and Kurtlutepe metamorphic areas. Normalization values are from Boynton (Citation1984).

Figure 6. Chondrite-normalized REE earth element patterns of metabasic and metavolcanic rocks from Refahiye and Kurtlutepe metamorphic areas. Normalization values are from Boynton (Citation1984).

Figure 7. (a) Th vs. Zr/Nb and (b) Nb vs. Zr/Nb plots (modified after Sayit et al., Citation2010) for the meta-igneous rocks from the Refahiye and Kurtlutepe metamorphic areas.

Figure 7. (a) Th vs. Zr/Nb and (b) Nb vs. Zr/Nb plots (modified after Sayit et al., Citation2010) for the meta-igneous rocks from the Refahiye and Kurtlutepe metamorphic areas.

Figure 8. (a) Th–Zr–Nb (after Wood et al., Citation1979) and (b) Nb/Yb vs. Th/Yb (after Pearce, Citation2008) tectonic discrimination diagrams for the meta-igneous rocks from the Refahiye and Kurtlutepe metamorphic areas.

Figure 8. (a) Th–Zr–Nb (after Wood et al., Citation1979) and (b) Nb/Yb vs. Th/Yb (after Pearce, Citation2008) tectonic discrimination diagrams for the meta-igneous rocks from the Refahiye and Kurtlutepe metamorphic areas.

Summarizing, the metabasic rocks in the Refahiye metamorphic rocks were derived from three distinct tectonic settings of formation: (i) OIBs (Type II), (ii) E-MORB (Type III) and (iii) island arc basalts (IAB, Type IV). The Th/Yb–Nb/Yb diagram of Pearce (Citation2008) substantiate the above deductions (Figure (b)).

5.4. The Kurtlutepe metavolcanic rocks

The Kurtlutepe metavolcanic rocks display a wide-compositional variation (SiO2~49–60 wt.%; Al2O3~14–19 wt.%; Fe2O3*~7–14 wt.%; CaO~2–9 wt.%), spanning subalkaline basalt, basaltic andesite and andesite (Table ; Figure ). Their REE patterns range from unfractionated to slightly fractionated ones ((La/Yb)cn~0.7–4.1) (Figure (e) and (f)). The Eu a feeble Eu anomaly is either absent to feeble (Eu/Eu*~0.8–1.1). All the Kurtlutepe metavolcanic rocks display low Nb and Th contents and high Zr/Nb ratios (Figure (a) and (b)). Overall the Kurtlutepe metavolcanic rocks resemble tholeiitic to calc-alkaline island arc rocks (Figure (a) and (b)).

6. Discussion

The Refahiye metabasic rocks formed in three different tectonic settings such as ocean island, enriched mid-ocean ridge and island arc. On the other hand, the Kurtlutepe metavolcanic rocks were derived from a single tectonic setting, an island arc setting. In both metamorphic areas, no clear N-MORB type rocks were found. Commonly, topographic highs on the ocean floor (i.e. seamounts, oceanic islands and oceanic plateaus and fracture zones) represent the most favorable domains to be accreted in the subduction zones (e.g. Isozaki et al., Citation1990; Kimura & Ludden, Citation1995). Anorogenic alkaline to tholeiitic rocks are common components of oceanic islands. Enriched mid ocean ridge-type basalts commonly form essential part of the ridge segments close to the oceanic islands (e.g. Donnely, Goldstein, Langmuir, & Spiegelman, Citation2004; Hémond, Hofmann, Vlastélic, & Nauret, Citation2006) or plume-distal-mid-ocean ridges whose mantle source was previously metasomatized by a plume (e.g. Haase & Devey, Citation1996; Le Roex et al., Citation1985). Consequently, we interpret the metabasic rocks with anorogenic alkaline affinities (Type II) as parts of accreted oceanic islands, and those with E-MORB affinities (Type III) as accreted ridge segments that were located close to oceanic islands, and/or plume-distal-mid-ocean ridge segments with mantle source previously metasomatized by plume components. Thick massive marbles were probably derived from neritic limestone deposited on top of the topographic highs on the ocean floor or continental platforms (e.g. Robertson, Citation2012). Based on the intra-oceanic nature of the accretionary complex, and close association of the marbles with OIB-type metabasites, we favor deposition on an oceanic environment rather than on continental margin, before arrival at the subduction zone (Figure ).

Figure 9. Schematic drawing for the possible tectonic settings of formation of accreted materials in the Jurassic metamorphic accretionary complexes.

Figure 9. Schematic drawing for the possible tectonic settings of formation of accreted materials in the Jurassic metamorphic accretionary complexes.

The main problem is how the basic rocks with island arc affinity were incorporated into an accretionary complex. An appraisal of the geochemical data from the non-metamorphic and metamorphic accretionary complexes of different ages in Turkey (e.g. Aldanmaz et al., Citation2008; Eyüboğlu et al., Citation2007; Genç, Citation2004; Göncüoğlu et al., Citation2006; Göncüoğlu et al., Citation2010; Polat, Casey, & Kerrich, Citation1996; Sayit et al., Citation2010; Tankut, Dilek, & Önen, Citation1998) and around the world (e.g. Franciscan complex, Saha, Basu, Wakabayashi, & Wortman, Citation2005; Wakabayashi, Ghatak, & Basu, Citation2011; Río San Juan metamorphic complex; Escuder-Viruete, Friedman, Castillo-Carrión, Jabites, & Pérez-Estaún, Citation2011; the Central Asian Orogenic Belt, Windley, Alexeiev, Xiao, Kröner, & Badarch, Citation2007) reveals that the incorporation of the basic rocks with island arc affinity into the accretionary complexes is a common phenomena. Several possible processes can be considered: (i) forearc magmatic material (e.g. fore-arc ophiolite) can be transported into the subduction zone by gravity sliding, and debris flows (e.g. Draut & Clift, Citation2013 and references therein), (ii) collision of an oceanic island arc with the active continental margin (e.g. Brown et al., Citation2011; Clift, Schouten, & Vannucchi, Citation2009) and (iii) juxtaposition of the magmatic arc with subduction zone by strike slip faults (e.g. Karig, Citation1980; Şengör, Citation2004) or tectonic slicing of the accretionary complex and overlying forearc ophiolite. In addition, subduction erosion is thought to be an important process occurring in most convergent plate boundaries (e.g. Clift & Vannucchi, Citation2004; Kukowski & Oncken, Citation2006; Stern, Citation2011; von Huene, Ranero, & Vannucchi, Citation2004; von Huene & Scholl, Citation1991), and removes material from the accretionary complex and forearc domain. Removed material is transported into the mantle or can be underplated below the wedge. Nonetheless, the effectiveness of the subduction erosion in the incorporation of the forearc material into the accretionary complex is uncertain.

In the Refahiye area, the Jurassic accretionary complexes are interleaved with Jurassic ophiolite with suprasubduction-zone affinity, and both were bound by the Triassic accretionary complex in the north and Late Cretaceous accretionary complex in the south (Figure ; cf. Parlak et al., Citation2013; Rice et al., Citation2009; Rice, Robertson, & Ustaömer, Citation2006; Topuz, Göçmengil et al., Citation2013; Topuz, Çelik et al., Citation2013; Topuz, Okay et al., Citation2013; Yılmaz, 1985; Yılmaz & Yılmaz, 2004). In contrast to the accretionary complexes, the Refahiye ophiolite was not subducted, and represents the entrapped fore-arc lithosphere above its own subduction zone (Topuz, Çelik et al., Citation2013). So, it seems feasible that the metabasic rocks (e.g. gabbroic dikes and cumulate stocks) with island arc affinity might have been derived from the overlying ophiolite body (Figure ). However, the field relations and thoroughly recrystallized nature of the metabasic rocks do not allow any inference on the processes responsible for the incorporation of the island arc material into the accretionary complex. However, presence of the well-preserved volcanic and volcaniclastic textures in the Kurtlutepe metavolcanic rocks rules out derivation from the overlying suprasubduction-zone ophiolite. Transport from the fore-arc to arc domain by gravity flows or collision of an intraoceanic island arc with the subduction zone represents two possible processes. In the Eastern Pontides, Early to Middle Jurassic volcanic and volcaniclastic rocks are mostly concordantly overlain by Late Jurassic neritic platform carbonates with a magmatic lull (Okay & Şahintürk, Citation1997; Şen, Citation2007). Only in the Artvin region, a local unconformity is documented between Middle Jurassic and Upper Jurassic units (Konak et al., Citation2001; Ustaömer & Robertson, Citation2010). This unconformity is hypothetically ascribed to the collision of a topographic edifice with the southern margin of the Eastern Pontides (Ustaömer & Robertson, Citation2010; Ustaömer, Robertson, Ustaömer, Gerdes, & Peytcheva, Citation2013). For subduction and collision of an intraoceanic arc with the subduction zone, colliding intraoceanic arc must be relatively small because collision of large island arcs would lead to a collisional orogeny with uplift above sea level (e.g. Cloos, Citation1993). So the colliding putative intraoceanic island arc should be small (≤17 km thick, according to the calculations of Cloos, Citation1993). Collision of a relatively small intraoceanic island arc (Kurtlutepe metamorphics) with the subduction zone can also explain why widespread arc magmatism of Early to Middle Jurassic age (e.g. Şen, Citation2007; Ustaömer et al., Citation2013) was succeeded by a magmatic lull during Late Jurassic time in the Eastern Pontides (Figure ).

Figure 10. Schematic tectonic model for subduction and collision of a putative intraoceanic island arc.

Figure 10. Schematic tectonic model for subduction and collision of a putative intraoceanic island arc.

7. Conclusions

Two metamorphic oceanic accretionary complexes (Refahiye and Kurtlutepe) of early to middle Jurassic age are tectonically interleaved with coeval ophiolite in the Eastern Pontides close to the İAE suture. The accretionary complexes differ from each other in terms of their metamorphic grade and type of accreted igneous rocks. The Refahiye metamorphic rocks were subjected to garnet-amphibolite/eclogite facies metamorphism and strongly retrograded to greenschist facies during exhumation. High-grade domains are locally preserved. On the other hand, the Kurtlutepe metamorphic rocks were metamorphosed in subgreenschist-facies whereby the primary structures and mineral assemblages are recognizable. The metabasic rocks in the Refahiye metamorphic rocks display four distinct geochemical affinities: (i) cumulates, (ii) alkaline OIBs, (iii) enriched middle ocean ridge basalts and (iv) tholeiitic island arc basalts. The metabasic rocks with OIB and E-MORB affinities most probably represent accreted material from the topographic highs on the sea floor (e.g. seamount, oceanic island, oceanic plateau, fracture zones). Origin of the metabasic rocks with island arc affinity is unclear, but they might have been derived from (i) the tectonic erosion of the basal portions of the suprasubduction-zone ophiolite or (ii) transport of the arc material by submarine canyons to the subduction zone. On the other hand, the metabasic rocks in the Refahiye metamorphic rocks and the Kurtlutepe metavolcanic rocks display only tholeiitic to calc-alkaline island arc affinities, and might represent the vestiges of an oceanic island arc entered into the subduction zone. This could also explain why Early to Middle Jurassic arc magmatism was succeeded by a magmatic lull during the Late Jurassic time in the Eastern Pontides.

Acknowledgments

Over years, we have greatly benefited from the discussions with Aral Okay on the development of accretionary complexes and geology of the Eastern Mediterranean region. This paper would have been not possible without these discussions. This paper resulted from the parts of the master theses of G. Göçmengil and İ. E. Altıntaş, which was supported by a research grant (#109Y059) from the TÜBİTAK, and the research fund of the Istanbul Technical University (İTÜ-BAP). G. Topuz acknowledges support from the TUBA-GEBIP program. Careful reviews by T. Ustaömer and E. Saccani, and discussions with A.M.C. Şengör and Boris Natal’in are gratefully acknowledged.

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