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Evidence of Precambrian sedimentation/magmatism and Cambrian metamorphismin the Bitlis Massif, SE Turkey utilising whole-rock geochemistry and U–PbLA-ICP-MS zircon dating
P. Ayda Ustaömer a,⁎, Timur Ustaömer b, Axel Gerdes c, Alastair H.F. Robertson d, Alan S. Collins e
a Yıldız Teknik Üniversitesi, Doğa Bilimleri Araştırma Merkezi, TR-34210 Davutpaşa-Esenler, İstanbul, Turkeyb İstanbul Üniversitesi, Mühendislik Fakültesi, Jeoloji Mühendisliği Bölümü, TR-34850 Avcılar-Istanbul, Turkeyc Goethe Universität, Institut für Geowissenschaften, Altenhöferallee 1, D-60438 Frankfurt am Main, Germanyd University of Edinburgh, School of GeoSciences, Grant Institute, West Mains Road, Edinburgh EH9 3JW, Scotland, UKe Tectonics, Resources and Exploration (TRaX), School of Earth and Environmental Sciences, University of Adelaide, Adelaide, SA 5005, Australia
The Bitlis Massif is a regional-scale, south-vergent allochthon that was finally emplaced by collision of theEurasian and Afro-Arabian plates during Miocene time. The Bitlis Massif includes a large outcrop of Precambriancontinental crust, the closest counterpart to which is the Arabian–Nubian Shield ~1000 km to the south. TheMassif is sub-divided into two rock associations: a pre-Middle Devonian high-grade basement and a MiddleDevonian-Triassic low-grade cover. The pre-Devonian basement comprisesmeta-granitic plutons emplaced intohigh-grademetamorphic rocks, including schist, paragneiss, amphibolite and eclogite. New laser-ablation zirconages obtained for zircons separated from a meta-granite body and its host paragneiss provide constraints onmagmatism, sedimentation and metamorphism. Whole-rock geochemical data indicate that the plutoncrystallised from peraluminous I-type melts from an arc-type subduction influenced source. Sm–Nd isotopesystematics suggest crustal contamination. Zircon dating yielded a 206Pb/238U age of 572±4.8 Ma, interpreted asthe time of crystallisation. Igneous zircons exhibit metamict metamorphic domains dated at 529 Ma (EarlyCambrian), interpreted as the time of latest Pan-African metamorphism. Nine detrital zircon grains from hostparagneiss yielded Neoproterozoic ages (992–627 Ma). Combinedwith the crystallisation age data, this suggeststhat the sedimentary protolith of the paragneiss was deposited from ~627 to ~572 Ma (Ediacaran). The lateNeoproterozoic ages suggest the BitlisMassif is a peri-Gondwanan terranewith a likely origin in northeast Africawhere similar early Neoproterozoic (0.9–1.0 Ga) ages have been reported.
High-quality radiometric age data from the basement units of thevarious continental terranes that make up Turkey and the Balkanregion generally suggest an origin in the Neoproterozoic continentalmargin of peri-Gondwana. The main evidence is: (1) the presence ofEdiacaran–Cambrian arc-type intrusive and extrusive rocks (Ustaömeret al., 2005; 2009; Sunal et al., 2006; Gessner et al., 2004; Gürsu et al.,2004; Gürsu and Göncüoğlu, 2005, 2006), and (2) provenance analysisof sedimentary units that have yieldedNeoproterozoic ages (Ustaömeret al., 2011a; Sunal et al., 2006). The Ediacaran–Cambrian arc-typeassemblages are interpreted as fragments of an Andean-type activecontinental margin, represented by the Cadomian–Avalonian Belt(Ustaömer et al., 2009 and references therein). Arc magmatism is
dated at 650 to 540 Ma, with episodes at 640–650, 610, 580–570 and550 Ma (Gerdes and Zeh, 2006;Murphy et al., 2002, 2004, 2009; Nanceet al., 2008).
The Cadomian–Avalonian Belt rifted from the north-Gondwanamargin during Cambrian–Ordovician and later accreted to Laurussiaduring Palaeozoic-Cenozoic (Stampfli et al., 2002; Murphy et al.,2004; Linnemann et al., 2007; Nance et al., 2008, 2010; Balintoni et al.,2011). As a result, small pieces of the Cadomian–Avalonian Belt arenow dispersed within Alpine orogens extending from North Americato India.
To restore the Cadomian–Avalonian Belt fully it is necessary toidentify terrane fragments within younger orogens and to determinethe ages of any related tectonic, magmatic or metamorphic events.This paper focuses on new high-quality radiometric and supportinggeochemical data from one such dispersed terrane, namely the BitlisMassif, SE Turkey.
Several of the continental blocks in Turkey (Fig. 1) includeCadomian arc-type basement with granitic or related volcanic rocks,as follows: (1) İstanbul Fragment (575 Ma granites; Chen et al., 2002;
Fig. 1. Tectonicmap showing the locations of Cadomian–Avalonian basement units in Europe and the EasternMediterranean area. Suture zones of Turkey are indicated. The Cadomianorogenic belt includes an Andean-typemargin along the peri-Gondwanamargin. This crust was later dispersed as exotic terranes throughout Palaeozoic–Cenozoic orogenic belts. Redbox (larger rectangle) indicates the location of Fig. 2 and the blue box (smaller rectangle) the study area. Data sources: Quésada (1990), Abramovitz et al. (1999), Guterch et al. (1999),Miller et al. (1999), Unrug et al. (1999), Chantraine et al. (2001), Savov et al. (2001), Bandres et al. (2002), Chen et al. (2002), Dörr et al. (2002), Gubanov (2002), Linnemann andRomer (2002), Murphy et al. (2002), Neubauer (2002), Pin et al. (2002), Romano et al. (2004), Gürsu and Göncüoğlu (2005), Okay et al. (2008); Ustaömer et al. (2005, 2009). ATAArmorican Terrane Assemblage, GTZ Gavrovo–Tripolitza–Ionian Zone, P Pindos Zone, VA Vardar–Axios Zone, WT Western Taurides. The base map uses the Lambert projection.
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Ustaömer et al., 2005); (2) Armutlu Metamorphics (565–585 Magranites; Okay et al., 2008); (3) Anatolide–Tauride Platform (570–540 Ma granites and volcanics; Kröner and Şengör, 1990; Hetzel and
Fig. 2. Simplified geological map of the Bitlis Massif, modified from the
Reichmann, 1996; Gessner et al., 2004; Gürsu et al., 2004; Gürsu andGöncüoğlu, 2005, 2006); and (4) Bitlis Massif (543–533 Ma; Ustaömeret al., 2009). In addition, detrital zircon ages obtained from several
1/500.000-scale Van and Erzurum sheets of Turkey (MTA, 2002).
Fig. 4. Field photographs of the Doğruyol meta-granite. A) Close-up of the meta-granite fabric. Note parallel alignment of mica defining foliation; B) Intrusive contact of themeta-granitewith amphibolite host rocks; C) Complex network of granitic dykes near the contact of the meta-granite with the host rocks; D) Basic dykes (now amphibolite) within themeta-granite.
Fig. 5. Q-A-P classification diagram of the Doğruyol pluton, using modal compositions.
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Palaeozoic sedimentary units (e.g. Istranca Massif; Palaeozoic ofİstanbul; Central Sakarya Basement; Pulur Massif) suggest Gondwanansource areas (Sunal et al., 2006; Ustaömer et al., 2010a,b; 2011a,b).
The Bitlis Massif in SE Turkey is a regional-scale allochthon thatwas emplaced southwards over the Arabian continental margin andits related foreland basin during the Early Miocene (Rigo de Righi andCortesini, 1964; Perinçek, 1979, 1980; Dean et al., 1981; Aktaş andRobertson, 1984; Perinçek et al., 1991; Yılmaz et al., 1993; Robertsonet al., 2006). The Massif is underlain by a south-vergent fold-and-thrust belt that includes Upper Cretaceous dismembered ophiolites,coloured mélange and high-pressure/low-temperature (HP/LT)blueschist facies rocks that record the closure of the southern branchof aMesozoic Tethyan ocean (South Neotethys) (Hall, 1976; Aktaş andRobertson, 1984; Yılmaz et al., 1993; Robertson et al., 2006).
The Bitlis Massif is situated ~1000 km north of the Arabian–NubianShield, which is characterized by juvenile continental crust with Nddepleted mantle model (TDM) ages of~0.55–0.83 Ga. The shield isinterpreted as a collage of juvenile volcanic arc terranes and ophiolitesthat amalgamated during the East African Orogeny (870–550 Ma)(Collins and Pisarevsky, 2005; Cox et al., 2011). Associated pre-Neoproterozoic crust has TDM ages of 1.1 to 2.48 Ga (Johnson andWoldehaimanot, 2003; Be'eri-Shlevin et al., 2009).
The exposure between the Bitlis Massif and the Arabian–NubianShield is dominated by Phanerozoic sediments, mostly Mesozoic–Cenozoic platform-type sediments (Beydoun, 1991; Guiraud and Bos-worth, 1999). In SE Turkey the basement of the Arabian continentalmargin is only exposed in a small area south of the BitlisMassif (Mardin-Derik) where it is represented by a low-grade volcanogenic succession(Telbesmi Formation) of inferred Late Neoproterozoic–Cambrian age.Unmetamorphosed shallow-marine shelf-type sediments dominate theunconformably overlying Cambrian–Cenozoic succession.
The Bitlis Massif is divided into two stratigraphic rock associationsthat are separated by an angular unconformity. The lower association
(Hizan Group) comprises medium- to high-grade metamorphic rocksincluding mica schist, paragneiss, amphibolite and eclogite (Göncüo-ğlu and Turhan, 1984; Şengün et al., 1991; Şengün, 1993; Erdoğan andDora, 1983; Helvacı and Griffin, 1983; Çağatay et al., 1984; Genç, 1985,1990; Okay et al., 1985; Oyan and Tolluoğlu, 2006). A number ofgranitic plutons intrude the high-grade basement. The crystallisationages of two of these granites were recently dated using the U–Pbzircon method at 546–532 Ma (Ustaömer et al., 2009), near the
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Precambrian–Cambrian boundary using the timescale of Gradsteinet al. (2004).
The upper association (Mutki Group) is a greenschist-faciessedimentary succession represented by quartzite, recrystallisedlimestone, metaclastics and metavolcanics (Göncüoğlu and Turhan,1984; Şengün et al., 1991; Genç, 1985, 1990; Oyan and Tolluoğlu,2006). Fossils from the metasediments indicate a Mid-Devonianage for the lower part and a Triassic age for the upper part of the
Fig. 6. Photomicrographs of the Doğruyol pluton: A) Biotite, the main mafic phase, defines thare well developed in quartz. Deformation by grain-boundary migration is visible. QC) Recrystallisation of orthoclase to microcline is common in the pluton. Deformation byfeldspars. CPL; D) Common myrmekite texture in the Doğruyol pluton. This developed at tmyrmekite; E) Biotites are locally chloritised. PPL. Bi biotite; Mu muscovite; F) Kink folds in bpresent. PPL. Tour tourmaline; Amp amphibole; H) Xenocrystic garnets in the Doğruyol plu
succession (Göncüoğlu and Turhan, 1984). The lower-grade coverwas metamorphosed after deposition of the youngest dated rocks inthis cover sequence (Late Triassic) but prior to the Upper Cretaceousage of the oldest unconformably overlying unmetamorphosedsediments (Upper Maastrichtian Kinzu Formation; Göncüoğlu andTurhan, 1984). An Upper Cretaceous age is generally assumed for thelower-grade metamorphism on regional grounds (Yazgan andChessex, 1991).
e foliation in the Doğruyol pluton. PPL (plane polarised light); B) Sub-grain boundariesquartz; AF alkali feldspar; SGB sub-grain boundary. CPL (cross-polarised light);
grain-boundary-migration and undulose extinction are visible within unaltered alkalihe expense of alkali feldspars. Note local straight grain boundaries in quartz. CPL. Myriotite. PPL; G) Barroisitic amphibole was seen in only one section. Tourmaline is locallyton (PPL).
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Here,we report newU–Pb LA-SF-ICP-MS (laser ablation-sectorfield-inductive coupled plasma-mass spectrometry) age data to help con-strain the timing of magmatism, sedimentation and metamorphism.New geochemical and Sm–Nd isotope data are also used to help con-strain the magmatism.
2. Local geological setting
The area investigated is to the east and southeast of Bitlis city(Fig. 2), where a thrust sheet of Mesozoic ophiolite and ophioliticmélange is sandwiched between internally imbricated high-grademetamorphic rocks (Göncüoğlu and Turhan, 1984; Şengün et al.,1991). Our study area is located in the footwall of one large thrustimbricate located east of Bitlis city (Fig. 3). A large, E–W trendinganticline exposes high-grade metamorphic rocks in its core, whereaslow-grade metamorphic rocks are exposed on the limbs of thisstructure (Şengün et al., 1991).
The high-grade basement (~Yolcular Group of Şengün et al. (1991),or Hizan Group of Göncüoğlu and Turhan (1984) is divided intometasediments (paragneiss, mica schist and garnet-mica schist),metabasic rocks (amphibolite) and meta-silicic volcanic rocks (leptitegneiss; meta-rhyolite/rhyodacite) (Şengün et al., 1991; Şengün, 1993;
Table 1Major-element and trace-element analyses of the Doğruyol meta-granite. 1–4:meta-tonalitethe University of Adelaide (Australia) and the rest of the samples by ICP OS and ICP MS at
Sample 1 2 3 4 5 6 7 8 9
BIT2 BIT4 BIT5B BIT7 BIT 19 BIT 20 BIT 21 BIT 9 BI
Genç, 1985, 1990). Theparagneisses aremainly quartzo-feldpathic rockswith subordinate mafic mineral-rich bands. Individual dark metamor-phic layers are typically 20–30 cm thick although occasional dark greyamphibolite layers reach several metres in thickness. Contrasting greenamphibolite forms laterally discontinuous lenses, orientated parallel tothe layering of adjacent paragneiss. Individual lenses are up to a fewmetres thick in the centre of outcrops but pinch out laterally. The lensesprobably represent attenuated layers of meta-volcanogenic sedi-ments, sills or dykes. The amphibolites contain hornblende+quartz+oligoclase+alkali feldspar+garnet±sphene±biotite±magnetite±apatite, together with a retrograde mineral assemblage of actinolite+quartz+albite+epidote+chlorite±sphene±biotite±magnetite(Şengün, 1993;Genç, 1985, 1990). Themica schists are rich in biotite andmuscovite but are otherwise similar to the paragneisses (i.e. quartz+oligoclase+alkali feldspar+biotite+muscovite+garnet+kyanite±sillimanite±staurolite±zircon±sphene±epidote±opaques). Theoverall assemblage indicates amphibolite faciesmetamorphism(Şengünet al., 1991).
The sequence as a whole exhibits strong ductile deformation, withisoclinal folding and the development of LS-tectonites. Later cross-cutting brittle deformation is represented by closely spaced shearfractures and small faults with b20 cm offsets.
; 5–7:meta-granodiorite, 8–17:meta-leucogranite. Sample BIT 7was analysed by XRF atACME Laboratories (Canada).
10 11 12 13 14 15 16 17
T 12 BIT 15 BIT 16 BIT 18 BIT 22 BIT 24 BIT 25 BIT 26 BIT 23
Fig. 7. Geochemical classification diagrams for the Doğruyol meta-granite: A) P-Q(P=K−(Na+Ca); Q=Si/3−(K+Na+2Ca/3)) classification diagram (Debon and LeFort, 1983); B) Shand Index. Note that the pluton is classified as an I-type intrusion onthis diagram since A/CNK (molar Al2O3/CaO+Na2O+K2O)b1.1.
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Several meta-granitic plutons, stocks and dykes cut the high-grademetasedimentary and meta-igneous rocks (Fig. 3) of which theDoğruyol meta-granitoid (Şengün et al., 1991; Oyan and Tolluoğlu,2006) is the largest in the study area.
The high-grade basement is unconformably overlain by a lowergrade meta-sedimentary succession (Bitlis and Çadırdağı Groups ofŞengün et al. (1991); Mutki Group of Göncüoğlu and Turhan (1984)).Greenschist facies quartzite, calc-schist, marble and phyllite predom-inate (Şengün et al., 1991). Marble in the stratigraphically higherlevels yielded mid-Devonian fossils (Bitlis Group; Göncüoğlu andTurhan, 1984). The upper part of the Permian–Triassic succession(Çadırdağı Group) is mostly meta-carbonate rocks, phyllite, meta-chert and metabasic lava (Göncüoğlu and Turhan, 1984; Şengün et al.,1991).
3. Meta-granite
The Doğruyol meta-granite (Oyan and Tolluoğlu, 2006), which westudied in detail, is a small (5 km×2 km), mushroom-shaped body ofleucocratic rocks exposed between Doğruyol and Bölükyazı villages(Fig. 3). The pluton is fine to medium grained and foliated with parallelalignment of biotite flakes (Fig. 4A). An irregular magmatic contact iswell exposed between the meta-granite and the host paragneiss-amphibolite (Fig. 4B). The abundance of biotite, the main mafic phase,decreases from east to west. Three facies exist from east to west withinthe pluton (Oyan and Tolluoğlu, 2006): meta-tonalite, meta-granodi-orite and meta-leucogranite. The meta-leucogranite predominates(Fig. 3) whereas the meta-granodiorite occupies an area of 700 m longby 300 m wide between the meta-leucogranite and meta-tonalite. Themeta-tonalite forms an elongate, 1.5 km-wide, N–S trending outcrop inthe eastern part of the pluton. This thins westwards to a few tens ofmetres ending up as a narrow east–west-trending body near thesouthernmargin of thepluton (Fig. 3). The crystal size generally reducestowards the host rocks. The pluton is cut by a swarm of meta-graniticdykes (each averaging 40–60 cm thick byb2.5 m thick). The meta-dykes extend into the paragneiss and amphibolite at the margins of thepluton (Fig. 4C). Themeta-dykes typically cut thegneissic layeringof themeta-granite at a low angle. However, further away individual meta-granite dykes (~30 cm thick) cut the gneissic layering at higher angles(up to 60°).
Thepluton is also cut byoccasional steeply inclined, dark-greenbasicdykes (Fig. 4D) (40–150 cm thick), which occasionally taper until theydisappear. The margins of the dykes are mainly planar but occasionallyirregular. The meta-granite and also the meta-granitic dykes are cut byaplitic dykes (b10 cm wide). Pegmatitic dykes of granitic compositionare seen within the country rocks.
3.1. Petrography of the granitic rocks
The mineral assemblages of the different lithologies are as fol-lows. For themeta-leucogranite: quartz (36–45%)+alkali feldspar (17–32%)+plagioclase (27–35%)+biotite (1–6%)±muscovite±amphibole(barroisite)±sphene±garnet±zircon±apatite± tourmaline±opaque minerals; for the meta-granodiorite: quartz (38–41%)+alkali feldspar (17–20%)+plagioclase (35–38%)+biotite (6–7%)±sphene±zircon±apatite±tourmaline±opaque minerals and forthe meta-tonalite: quartz (32–44%)+plagioclase (42–51%)+biotite(8–12%)+alkali feldspar (2–10%)±garnet±tourmaline±sphene±zircon±apatite±opaque minerals. A retrograde mineral assemblage(epidote-chlorite-sericite) is locally developed. Epidote is well devel-oped along foliation planes together with recrystallised biotite. Inthe QAP (Quartz-Alkali feldspar-Plagioclase) ternary diagram (Fig. 5;Streickeisen, 1976), the meta-leucogranites plot in the granite field, themeta-granodiorites near the granodiorite-granite boundary and themeta-tonalites in the granodiorite and tonalite fields.
The meta-granite as a whole exhibits a grano-lepidoblasticstructure (Fig. 6A) defined by parallel alignment of biotite ormuscovite, whereas felsic minerals such as quartz and feldsparsform a granular structure. Quartz forms equidimensional crystalaggregates surrounding larger feldspar crystals. The contacts of theadjacent quartz crystals are often straight but sutured contacts arelocally present suggesting that the quartz was deformed by grain-boundary migration (Fig. 6B). Sub-grain boundaries are well devel-oped within recrystallised quartz grains (chess-board patterns werenot observed). Plagioclase commonly forms albite twins but defor-mation twins are also seen. Extinction angles indicate that theplagioclases are in the albite-oligoclase compositional range. Grainsare locally sericitised along twin planes. Alkali feldspar is mostlyorthoclase. Recrystallisation of orthoclase to microcline is common inthe meta-leucogranites and meta-granodiorites. Orthoclase crystalsappear to be unaltered without sericiticisation (Fig. 6C). However,locally alkali feldspars show some sericite formation. Some coarsemuscovite laths are associated with retrograde epidote and chlorite.Alkali feldspars exhibit undulose extinction. Myrmekite texture is alsowidespread (Fig. 6D).
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Biotite is dominantly green but occasionally brown-green andchloritised along crystal margins (Fig. 6E). Elongate crystal aggre-gates are intergrown with muscovite. Folding is associated withrecrystallisation of biotite throughout the pluton (Fig. 6F). Ananhedral blue-green variety of amphibole (barroisite) was observedin a single sample (BİT 25; Fig. 6G).
Garnet forms small subhedral to anhedral crystal aggregates withoccasional quartz inclusions (Fig. 6H). Tourmaline is occasionally pres-ent in the meta-tonalite and meta-leucogranites. Retrograde epidoteand chlorite replace plagioclase and biotite. Sphene, zircon, apatite andundetermined opaque minerals are present as accessory phases.
4. Geochemistry of the basement meta-igneous rocks
4.1. Meta-granite
Seventeen samples from the Doğruyol pluton were selected forwhole-rockmajor- and trace-element chemical analyses. Three sampleswere from the meta-granodiorite, four from the meta-tonalite and tensamples from the meta-leucogranite. The analyses (Table 1) werecarried out using ICP-MS and ICP-OS at ACME Laboratories (Canada).
Themeta-leucogranites plots in the granite and adamellitefields, themeta-granodiorites in the granodiorite field and the meta-tonalites inthe tonalite field when plotted on the P versus Q (P=K−(Na+Ca);Q=Si/3−(K+Na+2Ca/3)) lithology diagram (Fig. 7A; Debon and
Fig. 8. Harker-type major-element variation diagrams for the Doğruyol pluton.
LeFort (l983). All of the granitic rocks plot in the peraluminous fieldon a Shand diagram (Shand, 1943; Fig. 7B). The A/CNK (molar Al2O3/CaO+Na2O+K2O) values ofb1.1 (except for one sample) suggestan origin as an I-type melt (Fig. 7B). Normative corundum values aredominantly b1 indicating that the pluton is weakly peraluminous. Thethree pluton lithologies exhibit similar compositional trends (linear orcurved) onHarker diagrams (Figs. 8, 9), ranging frommeta-tonalite-themost basic to meta-leucogranite-the most acidic (Fig. 8). SiO2 and K2Odisplay positive trends with increasing K2O/Na2O, whereas Na2O,TiO2, CaO,MgO, Fe2O3 and P2O5 decrease. Al2O3 remains nearly constant(12–12.7%).
There is marked co-variance of the trace elements with respect toincreasing K2O/Na2O (Fig. 9). Zr and Nb decrease with increasingK2O/Na2O (Fig. 9), but with an inflection in the rate of depletion at ~1%K2O/Na2O. Hf shows a negative trend, whereas Ga and Th displayweak negative and positive trends, respectively. Rb increases rapidlyfrom meta-tonalites to meta-granodiorite until an inflection at 0.5%K2O/Na2O. Sr and Eu show positive and then negative trends, with aninflection at 0.5% K2O/Na2O.
Elements that are considered to be immobile during greenshist andamphibolite faciesmetamorphism(Pearce, 1980) providea useful guideto the tectonic setting of formation. In the Y versus Nb diagram (Pearceet al., 1984), themeta-granodiorites andmost of themeta-leucogranitesplot in the VAG (volcanic arc granite)+syn-COLG (syn-collisionalgranites)field,with somemeta-leucogranite plotting in the ORG (ocean
K2O/Na2O ratio is used as a differentiation index. See text for explanation.
Fig. 9. Harker-type variation diagrams for selected trace- and rare earth-elements of the Doğruyol meta-granite. K2O/Na2O ratio is used as a differentiation index. See textfor explanation.
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Fig. 10. Tectonic discrimination diagrams for the Doğruyol pluton: A) Y versus Nb (Pearce et al., 1984); B) Y+Nb versus Rb (Pearce et al., 1984); C) Hf versus Rb/30 versus Ta*3ternary diagram (Harris et al., 1986); D) Ta/Hf versus Th/Hf diagram (Schandl and Gorton, 2002).
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ridge granite) field (Fig. 10A). The meta-tonalites plot in the WPG(within plate granite) field. The Y+Nb versus Rb diagram (Pearce et al.,1984) can be used to distinguish VAG and syn-COLG granites (Fig. 10B).The meta-tonalites and four meta-leucogranites plot in the WPG fieldwith the remainder in the VAG field (Fig. 10B). Most samples plot in thevolcanic arc granite field on the Hf versus Rb/30 versus Ta*3 ternarydiagram (Fig. 10C), with one meta-leucogranite in the syn-collisionalgranitefield and one in the late to post-collisional granitefield. All of thesamples lie in the active continental margin field on the Ta/Hf versusTh/Hf diagram (Fig. 10D).
On ocean-ridge granite (ORG)-normalised spider grams (Fig. 11)the granitic rocks exhibit LIL (large ion lithophile) element enrich-ment, Nb depletion relative to LREE (light rare earth elements) and Zrdepletion relative toHf and Sm, all features of arcmagmas. However, themeta-tonalite (Fig. 11A) differs from the meta-granodiorite and themeta-leucogranite (Fig. 11C, E); two samples of meta-tonalite show Nbdepletion relative to Ce, whereas the other two display flat patternsfromNb toYb. Zr depletion relative toHf and Sm is notably absent in themeta-tonalites.
Chondrite-normalised REE patterns display LREE enrichment overMREEs and HREEs (Fig. 11B, D, F). All of the samples display a negativeEu anomaly, which increases from the meta-tonalite to the meta-leucogranite. The MREEs and HREEs display flat patterns in the meta-tonalite andmeta-granodiorite, whereas a negative slope fromMREEsto HREEs is present in the meta-granodiorite.
4.2. Amphibolites
Four samples of amphibolite were analysed by whole-rock major-elements and trace elements by X-ray fluorescence (XRF) at thechemical laboratories of the Department of Geology, University of
Adelaide, Australia, using the method reported in Ustaömer et al.(2009). Two of the samples were also analyzed for REE by ICP-MS atACME Laboratories, Canada (Table 2).
The amphibolite samples are classified as basaltic-andesite in theNb/Y versus Zr/TiO2 diagram (Winchester and Floyd, 1977; Fig. 12A).These display a tholeiitic character on an AFM diagram (not shown). Inseveral tectonic discrimination diagrams the samples plot in the MORB(mid-ocean ridgebasalt) andVAB (volcanic arc basalt)fields (Fig. 12B, C,D, E). On MORB-normalised spidergrams, LIL elements show enrichedpatterns, whereas HFSE (high field strength elements) display flatpatterns, similar to MORB (Fig. 13A). There is a marked Nb-depletionrelative to LREE (i.e. Ce). Chondrite-normalised REE patterns of theamphibolites are flat, similar to the patterns of lavas from mid-oceanridges or back-arc basins (Fig. 13B).
5. Nd isotope chemistry
Two samples of the meta-granite were analysed for Nd isotopes atthe University of Adelaide, using the method described in Wade et al.(2005) (Table 3).
The εNd572 values of the samples Bit 16 and Bit 21 of the meta-granite (on thebasis of 143/144Nd572) are−5.13and−4.11, respectively.Negative εNd572 values suggest contamination of arc-type melts bycrustal material (Faure, 1989). TDM ages were calculated for two meta-granite samples as 1.3 and 2.14 Ga, suggesting variable degrees ofcontamination. The 1.3 Ga model age is similar to that obtained fromlatest Ediacaran–Cambrian meta-granite of the Bitlis Massif (TDM=1.1to 1.4 Ga;Ustaömer et al., 2009). Similar TDM valueswere obtained fromthe Khida terrane of the Arabian Nubian Shield (Hargrove et al., 2006).The TDM value of 2.14 Ga suggests that older crust was also involved inthe genesis of the Doğruyol pluton.
Fig. 11. ORG-normalised (A, C, E) and chondrite-normalised (B, D, F) spidergrams of meta-tonalite, meta-granodiorite and meta-leucogranite, respectively. Normalising values fromPearce et al. (1984) and from Boynton (1984). The shaded area represents the field of 2500 Andean granites in c and e (www.gla.ac.uk/gcdkit/).
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6. Zircon dating by LA–SF-ICP-MS
6.1. Analytical techniques
Zircon U–Pb analysis was carried out at Goethe-UniversityFrankfurt using a Thermo-Scientific Element II SF-ICP-MS coupled toa NewWaveUP213 ultraviolet laser system equippedwith a teardrop-shaped, low-volume ablation cell (Gerdes and Zeh, 2006, 2009). Priorto the U–Pb dating, the internal structure of the zircon grains wasinvestigated by cathodoluminescence (CL) using back-scattered
electron (BSE) imaging. Zircons were separated using conventionaltechniques (crushed to reduce grain size, sieved, gravitationallyseparated using bromoform and methylene iodide heavy liquids, andmagnetically separated using Frantz Iso-dynamicmagnetic separator).Twenty grains from the meta-leucogranite pluton and nine grains fromthe host metasandstones were then hand picked under a binocularmicroscope. Zircons were set in synthetic resin mounts, polished andcleaned in a warm HNO3 ultrasonic bath. The results of the U–Pb zirconanalyses from the meta-granite and the host paragneiss are given inTables 4 and 5.
Table 2Major-element and trace-element analyses of the amphibolites. 1–2 were analysed byXRF at the University of Adelaide (Australia) and 3–4 by ICP OS and ICP MS at ACMELaboratories (Canada).
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Although the number of grains dated is relatively low the resultsare considered to bemeaningful because themeta-granite shows littleage inheritance and the metasandstone exhibit a reasonable age span(see below).
6.2. Zircon ages of the meta-granite
Cathodoluminescence images of euhedral zircons from the plutonshow typical magmatic oscillatory zoning, with evidence of distur-bance, zircon resorbtion and partial recrystallisation (e.g. resorbtion(a, b, d); metamictization (g and rim of f); white/bright luminescentdomains (d) (Fig. 14).
The zircon U–Pb analysis yielded two different age popula-tions: 572±4.8 Ma from oscillatory-zoned domains, and 529.1±5.3 Ma (Fig. 15) from recrystallised domains. The age of 572±4.8 Mais interpreted as the time of crystallisation of the Doğruyol Pluton andthe younger age as the time of zircon recrystallisation/resetting dueto metamorphism.
6.3. Zircon ages of the paragneiss
Nine brown-pink zircon grains were separated from paragneissclose to the contact between the granite and host rocks (Fig. 3). Thezircons range from subhedral to euhedral, suggesting differing degreesof sedimentary rounding and thus different source areas, althoughthe more rounded grains could have been reworked from an oldersedimentary unit. The CL images exhibit complex growth patterns,suggesting that the source areas experienced several tectono-thermalevents (Fig. 16). Th/U ratios of the zircons are mostly N0.1, consistentwith an igneous origin, whilst two of the zircon grains have a Th/Uratio of 0.04 (grains b and c; Fig. 16) suggesting a metamorphic origin.
The ages of the detrital zircons range from 627 to 999 Ma (Figs. 17,18). These ages are N95% concordant. The sub-rounded grains yieldedthe oldest ages while the euhedral igneous grains yielded youngerages (~646–670 Ma). Four grains yielded 627–670 Ma and threegrains gave 903–992 Ma. Neoproterozoic igneous and metamorphicrocks therefore dominated the provenance.
7. Discussion
7.1. Interpretation of age and chemical data
The pre-Mid-Devonian basement of the Bitlis Massif includesparagneiss, mica schist and garnet schist (meta-sedimentary),amphibolite (meta-basic igneous), leptite gneiss (meta-rhyolite anddacite) and crosscutting meta-granitic intrusives. The basement wasmetamorphosed at amphibolite facies before deposition of thePalaeozoic cover succession. Rare eclogite lenses have been reportedfrom the paragneiss (Okay et al., 1985). Greenschist facies metamor-phism was superimposed during the Cretaceous (Göncüoğlu andTurhan, 1984; Yazgan and Chessex, 1991).
Harker variation diagrams for the threemapped units of the pluton(Figs. 8, 9) showed that these lie on the same fractional crystallisationtrend, suggesting differentiation within a single magma chamber.The subhedral to anhedral garnet crystals in the meta-leucograniteand the meta-tonalite are interpreted as xenocrysts derived from themetamorphic host rocks. These xenocrysts may be responsible for thegenerally ‘enriched’ composition (but with flat MREE and HREEpatterns) on the chondrite-normalised REE spidergrams (Fig. 11).
The Doğruyol pluton is weakly peraluminous I-type granite, dis-playing geochemical features similar to arc-type granites (Figs. 7, 10).However, two of the biotite-rich samples of the meta-tonalite yieldedflat patterns in the ORG-normalised spidergrams (Fig. 10). Biotitecrystallisation could have controlled Nb enrichment since the partitioncoefficient of Nb is high for biotites in acidic melts (Nash and Crecraft,1985). As a result these samples could have been displaced into non-orogenic WPG field in tectonic discrimination diagrams involvingniobium.
The arc-related Doğruyol meta-granite was emplaced into thepre-Mid-Devonian basement of the Bitlis Massif around 572 Ma. TheMutki Granite (545 Ma) and nearby granitic dykes (533 Ma) that areexposed to the west of Bitlis city (Ustaömer et al., 2009; Fig. 3) couldrepresent younger products of this Cadomian arc magmatism.
The recrystallisation of feldspar, the extensive myrmekite, theevidence of recrystallisation and the grain boundary migration inquartz and feldspar, coupled with the presence of barroisite andgreen biotite (Fig. 6) point to metamorphism of the Doğruyol plutonunder upper greenschist facies conditions. The absence of chessboardpatterns in the quartz opposes any high-temperature metamorphismand deformation. The combined evidence strongly suggests thatthe Doğruyol pluton was metamorphosed to greenschist facies afteramphibolite facies metamorphism of the pre-Devonian basementrocks. The greenschist facies metamorphic events is therefore, con-sidered to Cambrian, rather than Alpine, as previously considered(Şengün et al., 1991).
Fig. 12. Geochemical discrimination diagrams for amphibolites: A) Nb/Y versus Zr/Ti diagram (Winchester and Floyd, 1977); B) Ti versus V diagram (Shervais, 1982); C) Zr versusZr/Y diagram (Pearce and Norry, 1979); D) Zr/4-2Nb-Y ternary diagram (Meschede, 1986); E) Nb/16 versus Th versus Hf/3 ternary diagram (Wood, 1980).
Fig. 13. MORB- and REE-normalised spidergram of amphibolites. Normalising valuesfrom Pearce (1983) and from Boynton (1984).
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Zircon ages can also be used to document episodes of fluid-rockinteraction during various types of metamorphism, including contactmetamorphismandmid- tohigh-grade regionalmetamorphism(Ayers,2006; Chen et al., 2010). Th/U ratios of metamictic/recrystalliseddomains in some of the meta-granite zircons range from 0.09 to 0.14suggesting that recrystallization took place during regional metamor-phism. The domains yielded a U–Pb age of 529.1±5.3 Ma (Terreneu-vian; i.e. Lower Cambrian), which is interpreted as the age of the uppergreenschist facies metamorphism. No further metamorphism affectedthe Bitlis Massif until Alpine (i.e. pre-latest Cretaceous) regional meta-morphism related to closure of Neotethys.
The youngest zircon age of 627 Ma from the paragneiss suggests itsmaximum possible depositional age. Since the paragneiss is cut by ameta-granite dated at 572 Ma, the sediment was deposited between627 and 572 Ma.
The detrital zircons from the paragneiss yielded an age distribu-tion ranging from 627 Ma to 999 Ma. A density-probability diagramfor the Bitlis paragneiss (Fig. 18) shows age clusters at 640 and 675 Mawith minor peaks at 820, 915 and 975 Ma. This suggests that the mainCadomian–Avalonian arc magmatism commenced between 730 Maand 650 Ma with the main activity from 640 to 575 Ma. Detrital zirconages similar to those obtained from the basement paragneiss occur inother Cadomian–Avalonian terranes (Linnemann et al., 2007, 2008;Gerdes and Zeh, 2006; Nance et al., 2008; Ustaömer et al., 2011a). Theolder Neoproterozoic ages (727 Ma-825 Ma) may correspond toextensive early Neoproterozoic magmatism as recorded in the Arabi-an–Nubian Shield (Johnson andWoldehaimanot, 2003; Unrug, 1997). Anotable feature of the detrital zircon ages is an age of ~975 Ma. Similarzircon ages characterise the peri-Gondwanan units, termed Minoanterranes (Zulauf et al., 2007). These include the Menderes Massif
Notes:1. High pressure digestions.2. Static/dynamic Nd measurement on Finnigan MAT262 mass spectrometer.3. Dynamic Sm concentration measurement on Finnigan MAT261 mass spectrometer.4. Nd concentrations corrected for 200 pg Nd blank.5. Sm concentrations corrected for 150 pg Sm blank.MAT262 mass spectrometer measurement statistics6. Neodymium standard, La Jolla 143/144Nd=.511826 .000005 (sd !) for 2 analyses (25/1/2006).6a. Internal standard (Johnson Matthey Nd2O3), 143/144Nd=.511602 .000005 (sd) for 4 analyses (31/1/2006).
Table 4U/Pb LA-SF-ICP-MS data for the Doğruyol meta-granite.
Diameter of laser spot=20 μm; depth of crater 10–15 μm.a Within run background-corrected mean 207Pb signal in counts per second.b U and Pb content and Th/U ratio were calculated relative to GJ-1 reference (LA-ICP-MS values, Gerdes, unpublished).c Corrected for background, common Pb and within-run Pb/U fractionation and subsequently normalised to GJ-1 (ID-TIMS value/measured value). 207Pb/235U calculated using
207Pb/206Pb/(238U/206Pbx1/137.88). Uncertainties propagated following Gerdes & Zeh (2006, 2008).d Rho is the error correlation defined as err206Pb/238U/err207Pb/235U.
Diameter of laser spot=20 μm; depth of crater 10–15 μm.a Within run background-corrected mean 207Pb signal in counts per second.b U and Pb content and Th/U ratio were calculated relative to GJ-1 reference (LA-ICP-MS values, Gerdes, unpublished).c Corrected for background, common Pb and within-run Pb/U fractionation and subsequently normalised to GJ-1 (ID-TIMS value/measured value). 207Pb/235U calculated using
207Pb/206Pb/(238U/206Pbx1/137.88). Uncertainties propagated following Gerdes & Zeh (2006, 2008).d Rho is the error correlation defined as err206Pb/238U/err207Pb/235U.
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Fig. 14. Cathodoluminescence images of selected zircon grains from the Doğruyol meta-granite (sample no. Bit23). The small areas ablated during analysis and the agesobtained are shown. Note the oscillatory zoning and metamictic/recrystalliseddomains. A density-probability age diagram is shown in the lower right. See text forexplanation.
Fig. 16. Cathodoluminescence images of selected zircon grains from the paragneiss(Yolcular Group; sample number Bit 33).
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(W Turkey), the Sfaka paragneiss (Crete) and the Ordovician Elatsandstone (Israel). The source of these zircons is unclear but could befrom northeast Africa (Zulauf et al., 2007; Be'eri-Shlevin et al., 2009).Similar ages have also been reported from southern Chad (DeWit et al.,2005).
7.2. Summary of geological development
A clastic sedimentary succession was deposited during Ediacarantimes. The meta-rhyolites (leptinites) and metabasic rocks (amphibo-lites) associated with the basement of the Bitlis Massif remain undated,although field relations suggest that bimodal volcanism accompaniedsedimentation. The geochemical data for the amphibolites suggest asupra-subduction zone tectonic setting of magmatism. The clastic infillof the basin was shed from a source area dominated by Tonian andCryogenian (~975–645 Ma) igneous rocks. The euhedral to slightly
Fig. 15. U–Pb concordia plot of zircon ages from the metagranite sample. See text forexplanation.
rounded zircon grain in the meta-sediments suggest a relatively localsource area. The sedimentary basin was subsequently deformed andintruded by ~572 Ma granites. This was followed by the intrusion of theMutki Granite (546 Ma) and granitic dykes (533 Ma), as exposed westof Bitlis city (Ustaömer et al., 2009). The area underwent metamor-phism during Early Cambrian time (529 Ma). This was possibly relatedto a collision during the final amalgamation of exotic terranes withnortheastGondwana, or to thedevelopmentof a subsequent subductionzone along the margin Gondwana (Collins and Pisarevsky, 2005). Asimilar age of metamorphism (502±10Ma) was obtained from Rb/Srdating of migmatites in the Menderes Massif and interpreted as thetiming of peak metamorphism (Satır and Friedrichsen, 1986).
The metamorphic basement was exhumed and transgressed bymarine clastic and carbonate rocks (Meydan Formation) during Mid-Devonian time (Göncüoğlu and Turhan, 1984). There is no definiteevidence of Cambrian–Ordovician sedimentary rocks in the BitlisMassif. However, lithological correlation with the Arabian Platform(Çağatay et al., 1984; Perinçek, 1990; Şengün, 1993) suggests that thelower part of the cover sequence could be Early Palaeozoic.
Fig. 17. U–Pb concordia plot of detrital zircon ages from the paragneiss sample. See textfor explanation.
Fig. 18. Density-probability diagram of the detrital zircons. See text for explanation.
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8. Conclusions
1. Leucocratic magmatism represented by the Doğruyol meta-graniteintruded the pre-Mid-Devonian basement of the Bitlis Massif at~572 Ma.
2. The meta-granite crystallised from calc-alkaline, I-type, peralumi-nous arc-type melts.
3. Nd isotope data of the meta-granite provide evidence for contam-ination by crustal rocks. The inferred depleted mantle model age forthis mixing is ~1.3 to 2.2 Ga.
4. Preliminary zircondata yield amaximumdepositional age of ~627 Mafor paragneiss. The gneiss is crosscut by the Doğruyol meta-graniteindicating that its depositionmust be Ediacaran (~627 and ~572 Ma).The paragneiss is thus the oldest known rock unit in Turkey.
5. Recrystallised metamorphic zircon domains in the Doğruyol meta-granite yielded an age of ~529 Ma, interpreted as the age ofregional metamorphism.
6. The presence of ~1 Ga zircons in the detrital zircon populationof the paragneiss suggests that the Bitlis Massif originated as a peri-Gondwanan, (Minoan) terrane, possibly located in the NE Africa.
7. Comparisons of the stratigraphy and tectono-thermal events in theBitlis Massif and the Arabian Platform suggest that the basement ofthe Bitlis Massif was amalgamated to Gondwana during Ediacaran–Cambrian time. It remained until Triassic rifting of the SouthernNeotethys and final re-amalgamation during latest Cretaceous-early Cenozoic.
Acknowledgements
This study was supported by Yıldız Technical University ScientificResearch Project Coordinatorship (No. 24.13.02.01) and a TÜBİTAK(The Scientific and Technical Research Council of Turkey)-DFG(Deutsche Forschungsgemeinschaft) joint grant to the first author.We thank Profs. G. Zulauf, D. Nance, M. Santosh and an anonymousreviewer for their constructive comments. The Mineral Research andExploration Institute in Ankara is thanked for the preparation of thinsections. We would also like to thank the staff of the University ofAdelaide (S Australia) and of the Goethe University Frankfurt(Germany) for their help during the laboratory studies. Logisticalsupport from the Bitlis City Governorship is gratefully acknowledged.ASC's contribution forms TRaX Record #176.
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