Cadomian (Ediacaran–Cambrian) arc magmatism in the Bitlis Massif, SE Turkey: Magmatism along the developing northern margin of Gondwana

Tectonophysics 473 (2009) 99–112

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Tectonophysics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t e c t o

Cadomian (Ediacaran–Cambrian) arc magmatism in the Bitlis Massif, SE Turkey: Magmatism along the developing northern margin of Gondwana P. Ayda Ustaömer a,⁎, Timur Ustaömer b, Alan S. Collins c, Alastair H.F. Robertson d a

Yıldız Teknik Üniversitesi, Doğa Bilimleri Araştırma Merkezi, 34349 Beşiktaş-Istanbul, Turkey İstanbul Üniversitesi, Mühendislik Fakültesi, Jeoloji Mühendisliği Bölümü, 34850 Avcılar-Istanbul, Turkey Continental Evolution Research Group, Geology and Geophysics, School of Earth and Environmental Sciences, University of Adelaide, Adelaide SA5005, Australia d University of Edinburgh, Grant Institute, School of GeoSciences, West Mains Road, Edinburgh EH9 3JW, UK b c

A R T I C L E

I N F O

Article history: Received 7 January 2008 Accepted 5 June 2008 Available online 17 June 2008 Keywords: LA-ICP-MS U/Pb zircon dating Ediacaran–Cambrian magmatism Granite Bitlis Massif Turkey Northern margin of Gondwana

A B S T R A C T Small granitic plutons and associated granitic dykes that intrude the pre-Devonian basement of the Bitlis Massif were previously inferred to have a broadly Late Palaeozoic crystallisation age related to the Hercynian orogeny; this was tested during this work. The brittle–ductile-deformed Mutki granite pluton and nearby granitic dykes comprise mainly quartz, alkali feldspar, plagioclase, subordinate biotite, muscovite and rare amphibole. Based on the results of whole-rock major-element and trace-element analysis, the Mutki pluton and associated dykes are inferred to have crystallised from metaluminous, to peraluminous subduction influenced I-type melts. Sm–Nd isotope systematics indicate melting of a mantle source (of notional 1.3 Ga age), with increasing amounts of crustal contamination through time. U/Pb zircon dating of the Mutki granite and a nearby granitic dyke by laser inductively coupled plasma mass spectrometry (LA-ICP-MS) yielded 238U/206Pb crystallisation ages of 545.5 ± 6.1 Ma and 531.4 ± 3.6 Ma, respectively (Ediacaran–Early Cambrian). This shows for the first time that the regionally extensive Bitlis Massif was affected by Cadomian arc-type magmatism. The Ediacaran–Early Cambrian granitic rocks of the Bitlis Massif can be compared with similar-aged metagranitic and metavolcanic rocks within basement units exposed in the Tauride–Anatolide Platform (Menderes–Taurus Block) in western Anatolia and also in NW Turkey. Similar-aged rocks are also exposed in the basement of Iran. All of these magmatic units and their host rocks are interpreted as fragments of a Cadomian active margin bordering the northern margin of Gondwana after its final amalgamation. Formation of the Bitlis Massif granites and contemporaneous granitic units elsewhere in Turkey as fragments of an Andean-type margin adjacent to the Arabian–Nubian Shield is favoured over an alternative explanation as exotic terranes transported N2000 km eastwards from a Cadomian active margin near West Africa–Amazonia (now NW Africa). © 2008 Elsevier B.V. All rights reserved.

1. Introduction Basement terranes of Greece, Turkey and the Middle East that were amalgamated to the southern, active margin of Eurasia during the Cenozoic originally formed along the northern margin of Gondwana (Robertson and Dixon, 1984; Şengör, 1984). Many of these terranes preserve evidence of latest Ediacaran (latest Precambrian)–Early Cambrian magmatism, although an important gap in the record exists between western Anatolia and Iran, which we address in this paper. Ediacaran–Early Cambrian magmatism is suggested to have formed a widespread continental arc along the northern margin of a newlyformed Gondwanan supercontinent (Ramezani and Tucker, 2003; Gessner et al., 2004; Hassanzadeh et al., 2008). Gondwana was assembled by the collision of about 7–8 Australia-sized Neoproterozoic

⁎ Corresponding author. E-mail address: [email protected] (P.A. Ustaömer). 0040-1951/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2008.06.010

continents during two main periods, first at ~ 650–600 Ma and secondly at ~ 570–520 Ma (Collins and Pisarevsky, 2005). Soon after final amalgamation, Andean-type active margins formed along the Australia–Antarctica–South America sector of the Gondwana supercontinental margin (Cawood, 2005; Foden et al., 2006; Cawood and Buchan, 2007). In the Himalayas an arc formed along the northern margin of the India sector of the Gondwana margin soon after the amalgamation of Gondwana (Cawood et al., 2007). This margin can be traced eastwards into Iran where similar-age magmatism is documented from the basement (Ramezani and Tucker, 2003; Hassanzadeh et al., 2008). Further west, the Tauride–Anatolian Platform (Menderes– Tauride Block) of Aegean Turkey exposes granites of Ediacaran– Cambrian age that are also of active-margin type (Gessner et al., 2004). At present, the extensive Bitlis Massif of south-east Turkey (Figs. 1 and 2) is largely unknown in terms of the age and tectonic significance of its basement. By investigating the chemistry and age of the granites exposed in the Bitlis Massif, our main aim is to better understand the nature of

100 P.A. Ustaömer et al. / Tectonophysics 473 (2009) 99–112 Fig. 1. Suture map of Europe and the Eastern Mediterranean region showing Cadomian, Avalonian and Pan-African tectonic units within the Caledonian, Variscan and Tethyan belts (modified after Ustaömer et al., 2005). The Cadomian orogenic belt is thought to represent an Andean-type margin located along the peri-Gondwana margin; this was later dispersed as exotic terranes throughout Palaeozoic–Mesozoic and Cenozoic orogenic belts. The box marks the location of the study area. See Ustaömer et al. (2005) for data sources. Abbreviations: P Pindos Zone; VA Vardar-Axios Zone.

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Fig. 2. Simplified geological map of the Bitlis Massif (modified from 1/500,000-scale geological map (MTA, 2004).

the north-Gondwana margin and to fill the ~ 2000-kilometre gap in our knowledge of the margin between western Turkey and Iran. We present new field, petrographic and geochemical evidence (majorelement, trace-element and Sm–Nd isotope data) with the objective of characterising the intrusive setting of granitic bodies and associated granitic dykes that cut the metamorphic basement of the Bitlis Massif. We test a widespread view that these granitic rocks are of Late Palaeozoic age (Göncüoğlu and Turhan, 1981, 1984; Göncüoğlu, 1983, 1998) with the aid of new U–Pb LA-ICP-MS zircon age data. Our results have important implications for the nature and extent of the northGondwana margin in Ediacaran to earliest Palaeozoic times. 2. Tectonic setting of the Bitlis Massif The Bitlis Massif forms a part of the Tethyan suture zone that was assembled during Late Mesozoic–Early Cenozoic time (Yılmaz et al., 1993; Robertson, 1998). It is a regional-scale allochthonous unit with a high-grade metamorphic basement and a lower-grade cover sequence (Fig. 2; Göncüoğlu and Turhan, 1984). The Massif is underlain by the Bitlis suture zone, a south-vergent fold and thrust belt that includes Upper Cretaceous dismembered ophiolites, coloured melange and high-pressure/low-temperature (HP/LT) blueschist facies rocks (Hall, 1976; Aktaş and Robertson, l990). The Bitlis suture zone is widely interpreted to record the closure of the Southern Neotethyan ocean (Aktaş and Robertson, 1984; Yılmaz et al., 1993; Robertson et al., 2007). The entire allochthonous assemblage was finally emplaced southwards over the Arabian continental margin and its related foreland basin during the Early Miocene (Rigo de Righi and Cortesini, 1964; Perinçek, 1979,1980; Perinçek et al., 1991; Yılmaz et al., 1993; Aktaş and Robertson, 1984; Robertson et al., 2007). Collision of the Bitlis Massif and related allochthonous units with the Arabian continent to the south was followed by Plio-Quaternary post-collisional left-lateral strike-slip along the East Anatolian Transform Fault (Şengör et al., 1985; Dewey et al., 1986). In the north, the Bitlis Massif is overlain by Eocene volcanic-sedimentary units (Maden Complex) and ophiolitic melange (Göncüoğlu and Turhan, 1984), and more widely by Upper Miocene–Quaternary volcanics of the East Anatolian Plateau (Şengör and Yılmaz, 1981; Pearce et al., 1984a,b; Keskin, l998, 2003). The tectono-stratigraphy of the Bitlis Massif begins with a highgrade (amphibolite facies) metamorphic basement (Hizan Group);

this includes rare eclogitic blocks (Okay et al., 1985; Göncüoğlu and Turhan, 1984, 1985; Tolluoğlu, 1988). An unconformably overlying lower-grade (greenschist facies) cover sequence (Upper Mutki Group) begins with Devonian–Carboniferous shallow-marine meta-sediments and metavolcanics (greenschist facies) that are unconformably overlain by Permian–Early Mesozoic meta-sedimentary and sparse metavolcanic rocks. Neotethyan ophiolites and Upper Cretaceous blueschist facies metamorphic rocks were emplaced over the Upper Palaeozoic–Lower Mesozoic cover sequence during latest Cretaceous time (Göncüoğlu and Turhan, 1984). Stratigraphically overlying units of Cenozoic age are all unmetamorphosed (Göncüoğlu and Turhan, 1984). The metamorphism of the high-grade basement of the Bitlis Massif predates the Devonian age of the oldest dated cover sediments. The lower-grade cover was metamorphosed after deposition of the youngest dated rocks in this cover sequence (Late Triassic) but prior to the Upper Cretaceous age of the oldest unconformably overlying unmetamorphosed sediments (Upper Maastrichtian Kinzu Formation; Göncüoğlu and Turhan, 1984). An Upper Cretaceous age is generally assumed for the lower-grade metamorphism on regional grounds (Yazgan and Chessex, 1991). A number of granitic plutons, collectively termed the Muş Granite, occur within the Bitlis Massif (Fig. 2; Göncüoğlu and Turhan, 1984). Most of these plutons (Cacas metagranite, Avnik metagranite, Yayla metagranite, Mutki metagranite) have been described as intruding the high-grade metamorphic basement (Hizan Group; Yılmaz, 1971; Yılmaz et al., 1981; Helvacı and Griffin, 1983, 1984; Göncüoğlu and Turhan, 1984, 1985). In addition, the Muş-Kızılağaç metagranite has been reported to intrude Devonian dolomites and associated skarns (Göncüoğlu, 1998). Unfortunately, these critical relationships could not be checked as the outcrops were not accessible during this work. In the past various units within the Bitlis Massif have been radiometrically dated. Country rock gneisses and amphibolites near Cacas (Fig. 2) were dated at 505 ± 37 Ma to 920 ± 224 Ma using the Rb/Sr whole-rock method (Yılmaz, 1971). Associated metagranite (Cacas metagranite) was dated at 325 ± 3 Ma by the same method. An Rb/Sr age of 97 ± 8 Ma was reported for chloritised biotites within the metagranite of this area. In view of these differing reported ages Yılmaz et al. (1981) suggested that granitic magmatism in the Cacas area spanned 570 Ma–100 Ma. Later, whole-rock samples and mineral

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separates from another granite (Yayla metagranite; Fig. 2) were dated at 347 ± 52 Ma, based on a reasonably well-defined Rb/Sr isochron (Helvacı, 1983). However, another granite (Avnik granite; Fig. 3) showed severe disturbance of its Rb/Sr system and so could not be dated (Helvacı and Griffin, 1983). Similarly, the Muş-Kızılağaç metagranite (Fig. 3) failed to provide a Rb/Sr isochron after wholerock analysis. However, phengites could be dated by the Rb/Sr method at 95 ± 4 Ma, and by K/Ar method at 75 ± 1.2 Ma (Göncüoğlu, 1998). Despite the variable age data, Göncüoğlu (1998) inferred a Middle Devonian–Late Permian crystallisation age (i.e. Hercynian) for the granitic rocks as a whole. The main reason was the reported field

evidence that some of the granites cut Devonian meta-sediments but not the Mesozoic cover (Göncüoğlu, and Turhan, 1984). However, these key field relations remain unconfirmed. Göncüoğlu (1998) also suggested that the Cretaceous radiometric ages reflected thermal resetting associated with Upper Cretaceous ophiolite emplacement. 2.1. Local geology The area sampled during this work is located west of Bitlis city, near the town of Mutki (Fig. 3). Four large south-vergent thrust sheets are present in this area. The structurally lowest thrust sheet consists of

Fig. 3. Geological map of the study area (modified from Göncüoğlu and Turhan, 1985).

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Upper Cretaceous ophiolitic melange (Kinzu Formation), whereas the overlying three thrust sheets are metamorphic rocks assigned to the high-grade metamorphic basement and the lower-grade cover units of the Bitlis Massif. The high-grade metamorphic basement rocks in this area are divided into two units: the Andok gneiss and the Ohink schists (Göncüoğlu and Turhan, 1984, 1985). During this work it was observed that small granitic plutons and dykes occur within the high-grade metamorphic rocks. Parts of the perimeter of the largest granitic pluton, the Mutki granite, are bounded by a low-angle thrust or by a high-angle strike-slip fault zone. However, a primary intrusive contact is exposed along the western margin of the pluton. Internally, the Mutki granite is pinkish to beige-coloured and is dominated by equigranular, medium-grained quartz, feldspar and biotite. The pluton is locally cut by aplite veins, quartz veins and also by rare mafic dykes. A weak foliation is developed near the southern margin of the pluton. Country rocks (Andok gneiss) are well exposed along the Bitlis–Mutki road where they are cut by granitic dykes. These dykes are generally b5 m wide and exhibit chilled margins that are a few centimetres thick. The mineralogical composition of the dykes is similar to the Mutki granite, although the dykes are greyish rather than pinkish in colour. 3. Petrography and microfabrics of the Mutki granites Study of representative samples of the Mutki granite and the nearby dykes in thin sections with an optical microscope showed that both are composed of quartz + alkali feldspar + plagioclase + biotite ± amphibole ± muscovite ± zircon ± titanite ± opaque minerals. The granite exhibits a sheared fabric resulting from brittle–ductile deformation. Quartz shows undulose extinction and evidence of recrystallisation. The presence of sutured quartz grains suggests that strain-induced grain-boundary migration was a significant mechanism of deformation and recrystallisation. Feldspars by contrast are

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deformed cataclastically and exhibit grain-size reduction. Tectonically rounded feldspar crystals are commonly enveloped in ductiledeformed quartz or foliated biotite. Periclinal twinning occurs in feldspars owing to cataclastic-plastic deformation. The granitic dykes contain orthoclase perthite, formed by exsolution of albite-rich feldspar from potassium-rich feldspar. Plagioclase minerals are dominantly albite, with more calcic cores. Post-crystallisation alteration includes breakdown of biotite to chlorite, epidote and titanite, and also alteration of feldspars to sericite and clay minerals. 4. Chemical analysis of the granitic rocks 4.1. Major-elements and trace-elements The main objective of whole-rock geochemical analysis by X-ray fluorescence (XRF) was to characterise the magmatic affinities of the granitic rocks and to use the results to test published interpretations of the tectonic setting of formation. It was previously suggested that the granitic rocks formed in an arc setting (Şengör, 1991), or a back-arc setting (Göncüoğlu, 1998) related to southward subduction during Late Palaeozoic time (Şengör, 1991; Göncüoğlu, 1998). A total of sixteen samples were selected for whole-rock majorelement and trace-element analysis. Nine were from the Mutki granite and seven from associated granitic dykes. The samples were analysed at the School of Earth and Environmental Sciences of the University of Adelaide, using the method given below. Whole-rock chemical compositions of the rocks were determined by a Philips PW 1480 X-ray Fluorescence Spectrometer, using an analysis programme calibrated against several international and local Standard Reference Materials (SRM's). A dual-anode (Sc-Mo) X-ray tube was used, operating at 40 kV, 75 mA. The samples were dried in an oven at 110 °C for over 2 hours to remove the absorbed moisture. They were then weighed into alumina

Table 1

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 SO3 LOI Total Zr Nb Y Sr Rb U Th Pb Ga Cu Zn Ni Ba Sc Co V Ce Nd La Cr

1

2

3

4

5

6

7

8

9

10

11

BIT54

BIT54B

BIT56

BIT57

BIT60a

BIT63a

BIT66a

BIT67

BIT70a

BIT71a

BIT71b

12 BIT72

76.44 0.19 13.26 0.99 0.03 0.25 1.50 5.38 0.77 0.02 0.01 0.48 99.32 138.7 5.6 19.7 239.8 28.7 2.3 11.2 10.5 15.2 0 26 2 185 4.6 4 11 54 18 30 6

76.40 0.20 12.70 1.29 0.03 0.38 1.43 4.96 0.82 0.03 0.00 0.85 99.08 153.5 6.5 18.6 215.2 31.5 2.5 12.1 8.8 15.2 8 27 2 202 4.7 86 14 61 21 32 5

78.33 0.14 11.49 1.00 0.03 0.19 1.01 4.33 2.28 0.02 0.02 0.44 99.28 103.3 8.1 40.7 134.2 45.0 2.9 11.8 10.3 14.6 9 21 2 741 5.7 4 11 48 17 20 2

74.99 0.23 13.92 1.51 0.04 0.30 2.00 5.00 2.32 0.05 0.02 0.38 100.75 203.2 6.1 26.9 164.3 55.3 4.0 7.7 24.0 16.0 2 21 3 615 9.2 5 14 39 15 19 5

76.54 0.18 12.85 0.88 0.02 0.16 1.22 4.36 2.69 0.03 0.01 0.44 99.37 135.3 3.1 11.1 192.8 62.5 1.9 10.5 21.4 14.3 0 20 1 1049 5.3 3 9 75 24 49 5

76.88 0.20 13.45 0.56 0.01 0.41 1.47 7.09 0.18 0.07 0.02 0.27 100.61 184.2 6.2 27.8 127.4 4.1 3.0 13.5 0.9 15.6 0 5 1 44 5.7 60 12 58 25 26 0

76.53 0.11 12.62 0.99 0.03 0.21 0.83 4.52 2.88 0.02 0.01 0.44 99.19 112.7 6.7 23.8 66.3 39.4 2.7 12.1 3.6 17.3 8 3 1 276 6.5 2 10 38 17 15 4

77.90 0.03 12.48 0.91 0.03 0.08 0.58 3.81 4.59 0.01 0.01 0.22 100.64 61.4 6.1 30.6 43.6 112.9 3.7 13.6 7.5 15.0 9 6 1 339 2.6 2 4 23 12 9 4

67.97 0.88 14.31 4.40 0.08 1.00 4.35 5.54 0.37 0.22 0.01 0.27 99.39 616.7 22.7 75.5 231.7 3.3 3.5 9.9 7.2 23.7 13 34 5 144 25.0 6 36 74 42 28 9

74.92 0.17 13.15 1.69 0.03 0.22 1.48 4.16 2.99 0.04 0.01 0.78 99.63 176.9 5.5 28.9 135.9 60.3 3.0 9.5 5.7 17.0 8 13 2 792 5.1 3 9 57 21 23 3

74.74 0.15 13.36 2.00 0.03 0.19 1.36 4.32 3.12 0.03 0.01 0.43 99.74 147.6 5.5 24.2 100.9 78.6 2.8 9.4 7.7 16.3 0 12 2 696 5.6 3 10 36 12 13 4

74.11 0.27 13.14 3.04 0.04 0.48 2.17 5.74 0.63 0.05 0.01 0.35 100.01 244.2 7.6 13.2 151.2 14.3 2.7 15.1 5.5 17.7 18 12 11 179 12.0 3 15 31 8 13 5

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crucibles and ignited overnight in a furnace at 960 °C to yield Loss on Ignition (LOI) values. For the major oxide contents, 1 g of the ignited material was then accurately weighed with nominally 4 g of flux (commercially available as type 12:22, comprising 35.3% lithium tetraborate and 64.7% lithium metaborate). The sample-flux mixture was fused using a propane-oxygen flame at a temperature of ~ 1150 °C using Pt–Au crucibles, and cast into a preheated mould to produce a glass disc suitable for analysis. The results are presented on a “dry basis” in tabular form as oxides. For trace-element contents, about 5– 10 g of sample powder was mixed with nominally 1 ml of binder solution (polyvinyl alcohol) and pressed to form a pellet. This was allowed to dry in air and was heated for a further 1 to 2 hours in an over at 60 °C to ensure that the pellet was completely dry before analysis. The samples were analysed using a Phillips PW 1480 XRF Spectrometer, using several analysis programs covering suites of from 1 to 7 trace elements, with conditions optimised for the elements being analysed. The programs were calibrated against thirty or more local and international SRM's (Standard Reference Materials). The dualanode Sc-Mo tube (operated at a sufficiently high voltage to excite the Mo) and an Au tube were used for the analysis. Matrix corrections were made using either the Compton scatter peak, or mass absorption coefficients calculated from the major-element data (analytical results are given in Table 1). When plotted on the normative Q' [100Q/(Q+ Or + Ab + An)] versus ANOR [100An/(An + Or)] diagram (Streckeisen and Le Maitre, 1979) the Mutki granite and the associated dykes can be mainly classified as monzogranite, and to some extent as granite, granodiorite or tonalite (Fig. 4a). On the SiO2 versus MALI (Modified Alkali-Lime Index: Na2O + K2O − CaO) diagram (Fig. 4b) the samples are subalkaline and classified as calcic and calc-alkaline (Frost et al., 2001). The A/ CNK values (molecular Al2O3/CaO + Na2O + K2O) of the samples vary from 0.8 to 1.1, indicating that the granites range from metaluminous to peraluminous in composition. The SiO2 values of the granitic rocks range from 67.97 to 78.33%. The relatively high SiO2 values (N73%), above those typical of granites, are suggestive of secondary silicification. For this reason, preference was given to the relatively immobile major-element oxide, TiO2 when assessing fractional crystallisation and differentiation processes. On Harker diagrams (Fig. 5), the Mutki granite and the granitic dykes plot on a common differentiation trend. With increasing TiO2, negative trends of SiO2 and K2O are observed, whereas the other major oxides show positive trends. On the 10,000 ⁎ Ga/Al versus Zr diagram (Whalen et al., 1987), used to distinguish A-type granites from I- and S-type granites, the Bitlis granitic rocks plot in the overlap of the I-, S- and M-type granite fields (Fig. 6a). On the SiO2 versus A/CNK diagram (Clarke, 1992) most samples plot in the I-type field, with one in the S-type granite field (Fig. 6b). On N-MORB-normalised spidergrams (Sun and McDonough, 1989) the Mutki granite and the associated granitic dykes display similar patterns, with enrichments of most elements relative to NMORB, other than P, Ti and Y (Fig. 7). Nb depletion relative to LREE and LIL-elements is clearly demonstrated, whereas Pb is relatively enriched. In the Y versus Nb diagram, used as an indication of tectonic settings (Pearce et al., 1984a,b), all of the samples plot in the VAG (Volcanic Arc Granite) + Syn-COLG (Syn-Collisional Granite) field (Fig. 8a). On the Y + Nb versus Rb diagram (Pearce et al., 1984a,b) the samples plot in the VAG field (Fig. 8b). We therefore infer that the Mutki granitic rocks and associated dykes are likely to be co-magmatic and that they crystallised in a tectonic setting influenced by subduction. The possible source of the granitic rocks can be inferred from experimental data on the melting of pelitic, psammitic, andesitic and basaltic rocks. On a molar (CaO/FeOt + MgO) versus molar (K2O/Na2O) diagram, and also on a molar (CaO/FeOt + MgO) versus Na2O diagram (Altherr and Siebel, 2002) most of the granitic rock data fall into the metabasite-derived melt field (Fig. 9a, b). Low magnesium numbers of

Fig. 4. Geochemical classification diagrams of the granitic rocks. a) Q' [100Q/(Q+Or+Ab+ An)] versus ANOR [100An/(An+Or)] diagram, b) SiO2 versus MALI (Modified Alkali-Lime Index) diagram.

all of the granitic rocks are compatible with melting of a metabasaltic source. Dehydration melting of metabasaltic sources normally produces strongly peraluminous granites (Aluminum Saturation Index — ASIN 1.1), dissimilar to the analysed Bitlis granitic rocks (ASIb 1.1). The generation of metaluminous melts from metabasaltic sources in the lower crust requires a high degree of melting at elevated temperatures (N1000 °C; Rapp and Watson, 1995, Altunkaynak, 2007). 4.2. Nd isotope chemistry Nd isotopes were analysed from the Mutki granite and the associated granitic dykes to help determine whether their melts were derived from asthenospheric (mantle) or crustal sources. The isotopic analysis was carried out at the University of Adelaide, using the method described in Wade et al. (2005). Two samples from the Mutki granite and two samples from the granitic dykes were analysed for Nd isotopes by determining static/

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dynamic Nd concentrations using a MAT261 spectrometer. In addition, dynamic Sm concentrations were determined using a Finnigan MAT262 spectrometer. During this study, analysis of several Nd internal standards gave the following results: Johnson Matthey standard 143Nd/144Nd = 0.511602 ± 0.000005 (to one standard deviation): La Jolla standard 143Nd/144Nd = 0.511826b ± 0.000005 (to one standard deviation). εNdt values of the samples and TDM (depleted mantle model age) ages were calculated using the equations given in Faure (1989) (see Table 2). When plotted against SiO2, the initial 143/144Nd values (Fig. 10) define a pronounced negative trend (r2 = 0.99), suggesting the existence of similar sources for both the Mutki granite and the associated dykes. The Mutki granite yielded εNdt values of −1.88 and − 1.237

Fig. 6. Magmatic affinity diagrams of the granitic rocks. a) 10,000 ⁎ Ga/Al versus Zr diagram; b) SiO2 versus A/CNK diagram. See text for explanation.

(Table 3) (on the basis of (143/144Nd)546), whereas the granitic dykes gave εNdt values of −2.88 and −2.03 (on the basis of (143/144Nd)532). The small negative εNdt values in all of the analysed granitic rocks suggest the presence of an enriched mantle source or a basic crustal source derived from a depleted mantle. Given that the major-element and trace-element chemistry is suggestive of a subduction-related, arc-type setting it is probable that the negative εNdt values reflect the existence of a basic crustal source derived from mantle that had experienced subduction-related depletion. The trend of increasing εNdt values from −2.88 to −1.23 (Fig. 10) suggests that increasing amounts of crustal rock contamination have occurred through time. A notional “depleted mantle age” of the granite source rocks can be calculated as ~1.3 Ga. 5. Laser Ablation ICP-MS dating of granite and granitic dykes Fig. 5. Harker variation diagram of the granitic rocks. Note that TiO2 is used as an index of screening the fractional crystallisation and differentiation index.

U–Pb analysis of zircons from samples BIT 57 and BIT 67, a granitic dyke and the Mutki grantite, respectively was conducted using the

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Fig. 7. Spidergrams for the granitic rocks. a) Dykes, b) Mutki granite.

laser inductively coupled plasma mass spectrometer (LA-ICP-MS) at the University of Adelaide. Zircons were separated using conventional methods that include crushing, sieving, magnetic separation and floatation. More than twenty zircon grains were handpicked under a binocular microscope. The zircons were then set in synthetic resin mounts, polished and cleaned in a warm HNO3 ultrasonic bath. Cathodolumuniscence (CL) and back-scattered electron (BSE) imaging were carried out to help characterise any compositional variation within individual zircons. Equipment and operating conditions for zircon analysis were identical to those reported by Payne et al. (2006).

A spot size of 30 μm and repetition rate of 5 Hz was used for U–Pb data acquisition, producing a laser power density of ~8 J cm− 2. Zircon ages were calculated using the GEMOC GJ-1 zircon standard to correct for U–Pb fractionation (TIMS normalisation data 207Pb/206Pb = 608.3 Ma, 206 Pb/238U = 600.7 Ma and 207Pb/235U = 602.2 Ma — Jackson et al. (2004)), and the GLITTER software for data reduction (Van Achterbergh et al., 2001). Over the duration of this study the reported average normalised ages for GJ-1 were 609 ± 10, 600.2 ± 2.7 and 601.9 ± 2.4 Ma for the 207Pb/206Pb, 206Pb/238U and 207Pb/235U ratios, respectively (n = 24). Accuracy was monitored by repeat analyses of the

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Concordia. The analyses that lie to the right of the Concordia are interpreted to be due to the presence of common Pb, which appears not to have been fully resolved. Twenty near-concordant analyses define a bimodal 238U/ 206Pb age distribution when plotted on a probability density distribution plot (Fig. 11a). The twelve youngest data (younger than the 545 Ma infection point) yield a weighted mean 206 Pb/238U age of 531.4 ± 3.6 Ma (95% confidence, MSWD = 0.77), which is interpreted as the age crystallisation of the granitic dykes. The eight older analyses provide a weighted mean 206Pb/238U age of 552.9 ± 5.0 Ma (95% confidence, MSWD = 0.44). This age is within error of the interpreted crystallisation age of the Mutki granite (see below) and may reflect the presence of xenocrysts derived from this slightly earlier intrusion. 5.1.2. Mutki granite (BIT 67) The analysed zircon grains range in 238U/ 206Pb age from ~620 Ma to ~380 Ma (Table 2, Fig. 11b). The 18 analyses cluster close to the Concordia, with a number of analyses plotting beneath and to the right

Fig. 8. Tectonic discrimination of the granitic rocks. a) Y versus Nb diagram, b) Y + Nb versus Rb diagram (Pearce et al., 1984a,b). See text for explanation.

Sri Lankan in-house internal standard (BJWP-1, 207Pb/206Pb = 720.9 Ma, 206Pb/238U = 720.4 Ma and 207Pb/235U = 720.5 Ma, unpublished Massachusetts Institute of Technology TIMS data). Over the duration of this study the reported average ages for BJWP-1 were 734 ± 21, 711.8 ± 6.1 and 717.3 ± 8.1 Ma for the 207Pb/206Pb, 206Pb/238U and 207 Pb/235U ratios, respectively (n = 7). 5.1. Zircon ages The following ages were determined. 5.1.1. Granitic dyke: dyke (BIT 57) The analysed zircon grains range in 238U/206Pb age from ~498 Ma to ~ 612 Ma (Table 2; Fig. 11a). The 29 analyses cluster close to the

Fig. 9. Diagrams used to identify possible source rocks of the granitic rocks. a) Molar (CaO/MgO + FeOt) versus molar (K2O/Na2O diagram, b) molar (CaO/MgO + FeOt) versus Na2O diagram. See text for explanation.

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Table 2 Sample

−1

Nd ugg

Sm ugg

−1

143/144

Nd

147

Sm/ Nd

143/144

εNdt

TDM

0.5118338 0.5118496 0.511838 0.5118709

− 2.34 − 2.03 − 1.88 − 1.20

1.3 Ga 1.13 Ga 1.2 Ga 1.4 Ga

Ndi

144

BIT56 BIT62 BIT66 BIT70A BLANK#984 TASBAS#161

18.2 26.9 26.1 47.6 39.2

3.8 4.4 4.8 11.4 171 pg 7.7

0.512268 0.512192 0.512239 0.51239

0.1248 0.0984 0.112 0.1451

0.512916

0.1191

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 statistics. 6. Neodymium standard, La Jolla 143/144Nd = 0.511826 ⁎ 0.000005 (SD !) for 2 analyses (25/1/2006). 6a. Internal standard (Johnson Matthey Nd2O3), 143/144Nd = 0.511602 ⁎ 0.000005 (SD) for 4 analyses (31/1/2006).

of the Concordia (Fig. 11b). These discordant analyses are interpreted to be due to Pb-loss, and/or the presence of undetected common Pb. Nine near-concordant analyses (Fig. 11b inset) yield a 206Pb/238U age of 545.5 ± 6.1 Ma (95% confidence, MSWD = 1.4), which is interpreted here as the age of crystallisation of the granite. 6. Discussion The new zircon crystallisation ages of 545.5 ± 6.1 Ma and 531.4 ± 3.6 Ma (Ediacaran–Early Cambrian) constitute the first U/Pb radiometric dates of granitic rocks from the Bitlis Massif in SE Turkey. The combined major-element, trace-element and Nd isotopic chemical analyses suggest that these rocks formed in a subduction-related, arctype setting and were derived from mantle of notionally ~1.3 Ga age, with increasing contamination by continent crust through time. We now consider the regional tectonic implications of these new results. A key issue is where the Bitlis Massif was located during the Ediacaran–Early Cambrian. It is widely believed that the Bitlis Massif, together with the Pütürge Massif and related allochthonous units further west (Figs. 1, 2), rifted from the northern margin of Arabia during Early Mesozoic time and were re-amalgamated to Arabia by the Miocene (Yılmaz et al., 1993; Robertson, 1998; Robertson et al., 1996; Robertson et al., 2007). Strictly, therefore, the Bitlis Massif is an exotic terrane and few workers now believe that the massif is simply a relatively autochthonous northward extension of the Arabian continent (cf. Çağlayan et al., 1984). Despite being allochthonous, the Bitlis Massif is likely to restore as part of the northern margin of Gondwana during the Palaeozoic, probably close to its present position along the Arabian margin (Robertson et al., 2007). A less probable alternative is that it represents a “Cimmerian” terrane (Şengör, 1984) that rifted from Gondwana and drifted across proto-Tethys to be amalgamated with the Eurasian margin during Late Palaeozoic time, and later reassembled with Gondwana during a latest Triassic “Cimmerian” orogeny (Stampfli and Borel, 2000). Significantly, there is no known record of Early Mesozoic “Cimmerian” compressional deformation within the Bitlis Massif (e.g. Göncüoğlu and Turhan, 1984; Aktaş and Robertson, 1984; Robertson et al., 2007). Locally exposed Early Mesozoic metasedimentary and metavolcanic rocks of the Bitlis and Pütürge massifs include Triassic alkaline metavolcanic rocks that are interpreted to reflect the Triassic rifting of Neotethys (Perinçek, 1979, 1980). Subduction-related magmatism bordering the northern Gondwana margin is reported from east (present coordinates) of the Bitlis Massif, in basement rocks of Iran (Ramezani and Tucker, 2003; Hassanzadeh et al., 2008) and also further east in rocks involved in the Himalayan orogen (Cawood et al., 2007). West of the Bitlis Massif, subduction zones are inferred to have bordered Gondwana in the

region of West Africa–West Amazonia (present NW Africa), where related magmatism ranged from Neoproterozoic to Ordovician in age (D'Lemos et al., 1990; Fig. 12). In all of these areas subduction zones were initiated soon after the final amalgamation of the Gondwanan supercontinent (Cawood, 2005; Foden et al., 2006; Cawood and Buchan, 2007). Existing palaeogeographic maps of Gondwana during the Late Precambrian–Cambrian indicate a passive margin setting for the northern margin of Arabia (Meert and Torsvik, 2003; Collins and Pisarevsky, 2005). This is surprising given our evidence of subductionrelated arc magmatism in the Bitlis Massif of SE Turkey. Cambrian A-type (rift related) granitic magmatism is documented from the Arabian–Nubian Shield ~ 1000 km south of the Bitlis Massif (Mushkin et al., 2003) but this is dissimilar to the I-type granitic rocks of the Bitlis Massif. Our new results can also be compared with previous evidence of dated Late Precambrian magmatic rocks in Turkey (Fig. 1). The nearest of these units to the Mutki granitic rocks are the Ediacaran–Cambrian metagranites and metavolcanic rocks of the Tauride–Anatolide Platform (Menderes–Taurus Block), known as the Sandıklı Porphyroids (700 km to the west). Kröner and Şengör (1990) obtained a 207Pb/206Pb single-zircon age of 543 ± 4 Ma from meta-quartz porphry in the Sandıklı area. Metavolcanics in this area have been dated by the singlezircon 207Pb/206Pb method at 541.1 ± 9.3 to 555 ± 14.3 Ma (Gürsu et al., 2004). Based mainly on geochemical evidence, these rocks were inferred to have formed in a back-arc setting related to southward subduction beneath the northern margin of Gondwana (Gürsu and Göncüoğlu, 2005). Late Precambrian–Cambrian metagranitic rocks dated as ~ 570 to ~ 520 Ma are well documented from within the high-grade metamorphic basement of the Menderes Massif slightly further west (Hetzel and Reischmann, 1996; Dannat, 1997; Loos and Reischman, 1999, 2001; Gessner et al., 2004). Hetzel and Reischman (1996) obtained a 207Pb/206Pb age of 546.2 ± 1.2 Ma from metagranites of the Çine submassif, southern Menderes Massif, similar to the age of the Mutki granite. These data were later confirmed by ion microprobe analysis (Gessner et al., 2004). The Menderes Massif intrusive rocks are generally attributed to Cadomian arc-type magmatism (Kröner and Şengör, 1990; Neubauer, 2002). The Precambrian country rocks include leptite gneiss and schists, with local eclogitic metagabbro lenses (Candan and Dora, 1998). The Palaeozoic cover units of the Menderes Massif are represented by metapelites and metacarbonates, without metavolcanic rocks (Candan and Dora, 1998). In addition, metagranitic rocks occur within high-grade metamorphic basement rocks in northwest Turkey beneath cover rocks known as the Palaeozoic of Istanbul (Ustaömer, 1999; Chen et al., 2002; Ustaömer et al., 2005). These metagranitic rocks exhibit arctype chemical characteristics and have been dated at ~ 575 Ma (Late

Fig. 10. SiO2 versus 143/144Ndi diagram of the granitic rocks. Note that all the samples define a strong negative linear trend. See text for explanation.

P.A. Ustaömer et al. / Tectonophysics 473 (2009) 99–112

109

Table 3 LA-ICP-MS data. Spot name

Isotope ratios Pb/206Pb

BIT57-1 BIT57-2 BIT57-3 BIT57-4 BIT57-5 BIT57-6 BIT57-7 BIT57-8 BIT57-9 BIT57-10 BIT57-11 BIT57-12 BIT57-13 BIT57-14 BIT57-15 BIT57-16 BIT57-17 BIT57-18 BIT57-19 BIT57-20 BIT57-21 BIT57-22 BIT57-23 BIT57-24 BIT57-26 BIT57-27 BIT57-28 BIT57-29 BIT57-30 BIT67-1 BIT67-2 BIT67-3 BIT67-4 BIT67-5 BIT67-6 BIT67-7 BIT67-8 BIT67-9 BIT67-10 BIT67-11 BIT67-12 BIT67-13 BIT67-14 BIT67-15 BIT67-17 BIT67-18 BIT67-19

Ages

207

1σ error

206

Pb/238U

1σ error

207

Pb/235U

1σ error

207

0.0750 0.0675 0.0979 0.0708 0.0841 0.0613 0.0655 0.1307 0.0647 0.0587 0.0608 0.0833 0.0590 0.0611 0.0764 0.0593 0.0602 0.0609 0.0596 0.0598 0.0593 0.0658 0.0600 0.0650 0.0594 0.0605 0.0753 0.1022 0.0636 0.0609 0.0623 0.0765 0.0776 0.0715 0.0590 0.0648 0.0691 0.0604 0.0614 0.0681 0.0591 0.0600 0.0620 0.0657 0.0706 0.0684 0.0602

0.0010 0.0026 0.0013 0.0010 0.0013 0.0012 0.0008 0.0024 0.0011 0.0007 0.0010 0.0029 0.0013 0.0009 0.0013 0.0012 0.0008 0.0025 0.0011 0.0007 0.0011 0.0019 0.0010 0.0010 0.0011 0.0012 0.0013 0.0019 0.0009 0.0007 0.0009 0.0011 0.0011 0.0010 0.0006 0.0008 0.0011 0.0008 0.0008 0.0010 0.0007 0.0008 0.0008 0.0009 0.0010 0.0010 0.0007

0.0905 0.0891 0.0932 0.0903 0.0874 0.0872 0.0866 0.0995 0.0883 0.0856 0.0861 0.0814 0.0864 0.0850 0.0829 0.0852 0.0840 0.0852 0.0862 0.0945 0.0899 0.0900 0.0874 0.0804 0.0912 0.0866 0.0889 0.0908 0.0892 0.0879 0.0870 0.0845 0.0900 0.0831 0.0872 0.1009 0.0978 0.0887 0.0880 0.0867 0.0885 0.0869 0.0911 0.0897 0.0607 0.0879 0.0899

0.0011 0.0017 0.0011 0.0011 0.0012 0.0012 0.0010 0.0016 0.0012 0.0010 0.0011 0.0017 0.0011 0.0011 0.0011 0.0012 0.0010 0.0016 0.0012 0.0010 0.0012 0.0015 0.0011 0.0009 0.0013 0.0012 0.0012 0.0013 0.0012 0.0011 0.0011 0.0011 0.0011 0.0011 0.0011 0.0014 0.0014 0.0012 0.0011 0.0011 0.0011 0.0012 0.0012 0.0012 0.0009 0.0012 0.0012

0.9354 0.8272 1.2585 0.8816 1.0138 0.7365 0.7822 1.7931 0.7868 0.6927 0.7217 0.9351 0.7021 0.7161 0.8734 0.6965 0.6977 0.7156 0.7080 0.7786 0.7343 0.8159 0.7232 0.7203 0.7456 0.7213 0.9224 1.2780 0.7814 0.7366 0.7464 0.8901 0.9620 0.8183 0.7085 0.8998 0.9313 0.7382 0.7452 0.8134 0.7219 0.7184 0.7773 0.8114 0.5897 0.8288 0.7461

0.0131 0.0313 0.0174 0.0128 0.0165 0.0151 0.0103 0.0347 0.0138 0.0086 0.0130 0.0347 0.0171 0.0126 0.0165 0.0152 0.0102 0.0359 0.0140 0.0086 0.0140 0.0233 0.0131 0.0105 0.0147 0.0149 0.0168 0.0242 0.0128 0.0097 0.0114 0.0138 0.0146 0.0127 0.0092 0.0134 0.0161 0.0109 0.0107 0.0126 0.0101 0.0107 0.0120 0.0127 0.0100 0.0133 0.0100

1068 854 1585 952 1295 648 792 2108 763 556 632 1277 566 642 1106 578 612 637 589 595 576 801 605 776 581 620 1076 1665 730 634 684 1108 1137 971 568 766 903 618 654 870 572 603 673 796 946 881 612

Precambrian) by the 238U/206Pb and 207Pb/206Pb TIMS zircon dating method (Ustaömer, 1999; Ustaömer and Rogers, 1999; Ustaömer et al., 2005). We, therefore, infer that the Ediacaran–Early Cambrian magmatic units so far known from Turkey (i.e. Bitlis, SE Turkey; Sandıklı and Menderes, W Turkey; Palaeozoic of Istanbul, NW Turkey) represent dispersed fragments of a regionally extensive Cadomian active margin that extended N5000 km from present NW Africa to northern India. The length of this active margin is similar to that of the modern Andean active margin. However, the ages in the Bitlis Massif (~545 and 531 Ma) are significantly younger than those obtained from the Amazonian end of the active continental margin (555 Ma in the Avalonian basement of England; Compston et al., 2002). A possible explanation or this difference is that the locus of magmatism migrated eastward along the active margin with time. In the Turkish region, as further west in southern and central Europe (D'Lemos et al., 1990; Murphy et al., 2002; Dörr et al., 2004), outboard areas of the inferred active margin bordering oceanic crust were later dispersed throughout younger orogenic belts. However, more inboard areas including A-type back-arc magmatic rocks

Pb/206Pb

1σ error

206

1σ error

207

27 79 24 27 29 42 25 31 34 24 39 26 31 31 33 26 38 27 30 29 40 58 37 30 40 43 35 33 30 24 29 27 28 29 23 26 32 28 28 29 27 28 28 29 28 30 24

558.3 550.0 574.6 557.4 540.3 538.9 535.2 611.5 545.2 529.5 532.5 504.4 534.0 526.1 513.5 527.0 520.0 527.0 532.9 581.9 554.9 555.7 540.2 498.3 562.7 535.2 548.9 560.3 550.8 543.0 537.9 522.7 555.2 514.8 538.7 619.6 601.7 548.1 543.9 536.1 546.8 537.4 561.8 553.7 380.0 543.2 554.8

Pb/238U

6.3 10.1 6.6 6.6 6.8 7.1 6.0 9.3 7.0 6.0 6.7 6.1 6.2 6.6 6.8 6.2 6.7 6.1 6.1 6.5 7.0 9.0 6.7 5.4 7.5 6.9 6.9 7.7 7.2 6.5 6.7 6.8 6.7 6.4 6.5 8.0 8.0 6.8 6.5 6.5 6.5 6.9 7.2 7.0 5.5 6.9 6.8

670.5 612.0 827.3 641.9 710.8 560.4 586.7 1043.0 589.3 534.4 551.7 670.3 540.0 548.4 637.4 536.7 537.4 548.1 543.6 584.7 559.1 605.7 552.6 550.8 565.7 551.4 663.6 836.0 586.3 560.4 566.1 646.4 684.3 607.1 543.8 651.6 668.3 561.3 565.4 604.3 551.8 549.7 583.9 603.2 470.6 612.9 566.0

Pb/235U

1σ error 6.9 17.4 7.8 6.9 8.3 8.8 5.8 12.6 7.9 5.2 8.0 7.1 6.3 6.9 8.6 5.7 7.8 6.0 6.1 6.2 8.2 13.0 7.7 6.2 8.5 8.8 8.9 10.8 7.3 5.7 6.6 7.4 7.5 7.1 5.5 7.2 8.5 6.4 6.2 7.0 5.9 6.3 6.8 7.1 6.4 7.4 5.8

remained with their parent Gondwanan continent, as now exposed in the Arabian–Nubian Shield. Clues to possible tectonic dispersal history are provided by cover sequences. The stratigraphy of the Tauride–Anatolide Platform (Menderes–Taurus Block), and that of the associated relatively allochthonous Tauride units (e.g. Aladağ (Hadim) and Bolkar nappes) documents an overall intact stratigraphic succession from Ediacaran to Early Cenozoic. Pre-Devonian sedimentary rocks are absent from the Bitlis Massif but are known further west in the Taurides, including within the Sandıklı area (Gürsu and Göncüoğlu, 2005, 2006) and also within the autochthonous Arabian platform (e.g. Derik area) (Dean et al., 1981). The Arabian platform succession begins with slightly metamorphosed “Infracambrian” rhyolitic volcanics and volcaniclastic sediments, followed by shales, sandstones, microbial and siliceous carbonates, and then by andesitic–rhyolitic lavas and tuffs (Dean et al., 1981; Kozlu and Göncüoğlu, 1997). Unconformably overlying Middle Cambrian sedimentary rocks begin with basal conglomerates, followed by shallow-marine sandstones and limestones, deeper-marine limestones, and then later-Palaeozoic mixed carbonate-clastic successions that accumulated on a continental shelf (Özgül et al., 1991).

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Mesozoic times, whereas other Cadomian units (e.g. in NW Turkey) were dispersed away from Gondwana during Early Palaeozoic time. The Bitlis, Sandıklı and Menderes units rifted from Gondwana during the Triassic but were reassembled with Gondwana prior to the Neogene. Assuming that these units were reassembled near their original Late Precambrian positions, Gondwana was bordered by a subduction-related magmatic arc along its entire northern margin from present northwest Africa to the Himalayas. Acknowledgements This study was supported by a Scientific Research Project from Yıldız Technical University (Project No. 24.13.02.01), and a TÜBİTAK (Scientific and Technical Research Council of Turkey) post-doctoral overseas research grant to P.A.U. The Bitlis Central Jandarma (Turkish Army) is thanked for logistical support during fieldwork. We thank John Stanley, David Bruce, Justin Payne and Ben Wade of the University of Adelaide for help during the laboratory studies. Mr. Gürkan Tunay, vice president of the MTA (Mineral Research and Exploration Institute), Ankara is also thanked for providing us with thin sections. Constructive comments by J. Winchester and G. Zulauf on the manuscript are gratefully acknowledged. References

Fig. 11. U/Pb Concordia graphs of the granitic rocks. a) Age graph of the granitic dykes. b) Age graph of the Mutki granite.

In the Sandıklı area, the Palaeozoic sequence begins with a basal metaconglomerate, made up of fragments of felsic metavolcanics and black cherts, followed by metasandstone and metashales, together with spilitic lava flows and pyroclastic deposits. Trace fossils suggest an Early Cambrian (Tommotian) age for this lower part of the succession (Erdoğan et al., 2004). Overlying quartzite and limestones are dated as Mid-Cambrian (Çaltepe Formation; Dean and Özgül, 1994; Gürsu et al., 2004). The top of the exposed succession is represented by the Late Cambrian–Late Ordovician Seydişehir Shales (Özgül et al., 1991). There are, therefore, close similarities between the “Infracambrian” sequences of the Menderes–Taurus basement and those of the Arabian continent to the south, which is consistent with mutual formation adjacent to the Arabian–Nubian Shield. 7. Conclusions Granitic rocks of the Bitlis Massif that were previously assumed to be of Late Palaeozoic (‘Hercynian’) age are here shown to be of Ediacaran–Early Cambrian age. Geochemical evidence suggests that these granitic rocks formed in a subduction-related setting. The Bitlis Massif, together with the Sandıklı and Menderes Massif units of similar age further west, remained as part of Gondwana until Early

Aktaş, G., Robertson, A.H.F., 1984. The Maden Complex, S E Turkey: evolution of a Neotethyan continental margin. In: Dixon, J.E., Robertson, A.H.F. (Eds.), The Geological Evolution of the Eastern Mediterranean. Geological Society of London Special Publication, London, vol. 17, pp. 375–402. Aktaş, G., Robertson, A.H.F., 1990. Tectonic evolution of the Tethys suture zone in S.E. Turkey: evidence from the petrology and geochemistry of Late Cretaceous and Middle Eocene extrusives. In: Moores, E.M., Panayiotou, A., Xenophontos, C. (Eds.), Ophiolites–Oceanic Crustal Analogues. Proceedings International. Symposium “Troodos 1987”. Geological Survey Department, Cyprus, pp. 311–329. Altherr, R., Siebel, W., 2002. I-type plutonism in a continental back-arc setting: Miocene granitoids and monzonites from the central Aegean Sea, Greece. Contributions to Mineralogy and Petrology 143 (4), 397–415. Altunkaynak, Ş., 2007. Collision-driven slab breakoff magmatism in northwestern Anatolia, Turkey. Journal of Geology 115, 63–82. Candan, O., Dora, Ö., 1998. Granulite-eclogite and blue schists relicts in the Menderes Massif: an approach to Pan-African and Tertiary Metamorphic Evolution. Türkiye Jeoloji Bülteni 41 (1), 1–35 (in Turkish). Cawood, P.A., 2005. Terra Australis Orogen: Rodinia breakup and development of the Pacific and Iapetus margins of Gondwana during the Neoproterozoic and Paleozoic. Earth-Science Reviews 69, 249–279. Cawood, P.A., Buchan, C., 2007. Linking accretionary orogenesis with supercontinent assembly. Earth-Science Reviews 82, 217–256. Cawood, P.A., Johnson, M.R.W., Nemchin, A.A., 2007. Early Palaeozoic orogenesis along the Indian margin of Gondwana: tectonic response to Gondwana assembly. Earth and Planetary Science Letters 255, 70–84. Chen, F., Siebel, W., Satır, M., Terzioğlu, N., Saka, K., 2002. Geochronology of the Karadere basement (NW Turkey) and implications for the geological evolution of the Istanbul zone. International Journal of Earth Sciences 91, 469–481. Clarke, D.B., 1992. Granitoid Rocks. Chapman & Hall, New York. 283 pp. Collins, A.S., Pisarevsky, S.A., 2005. Amalgamating eastern Gondwana: the evolution of the Circum-Indian Orogens. Earth Science Reviews 71, 229–270. Compston, W., Wright, A.E., Toghill, P., 2002. Dating the Late Precambrian volcanicity of England and Wales. Journal of the Geological Society, London 159, 323–339. Çağlayan, M.A., Ünal, R.N., Şengün, M., Yurtsever, A., 1984. Structural setting of Bitlis Massif. In: Tekeli, O., Göncüoğlu, M.C. (Eds.), Geology of the Taurus Belt, Proceedings of International Symposium. Mineral Research and Exploration Institute of Turkey (MTA), Ankara, Turkey, pp. 245–255. Dannat, C., 1997. Geochemie, Geochronologie und Nd–Sr-isotopie der granitoiden Kerngneise des Menderes-Massivs, SW-Türkei, PhD thesis, Johannes GutenbergUniversität Mainz, Germany. Dean, W.T., Özgül, N., 1994. Cambrian Rocks and Faunas, Hüdai Area, Taurus Mountains, Southwestern Turkey. Bull. Inst. Royal Sci. Natur. Belgique, Sci. de la Terre, 64, pp. 5–20. Dean, W.T., Monod, O., Perinçek, D., 1981. Correlation of Cambrian and Ordovician rocks in SE Turkey. General Directory Petroleum Affairs Bulletin 25, 269–291. D'Lemos, R.S., Strachan, R.A., Topley, C.G., 1990. The Cadomian Orogeny in the north Armorican Massif: a brief review. In: D'Lemos, R.S., Strachan, R.A., Topley, C.G. (Eds.), The Cadomian Orogeny. Geological Society of London Special Publication, vol. 51, pp. 3–12. Dewey, J.F., Hempton, M.R., Kidd, W.S.F., Şaroğlu, F., Şengör, A.M.C., 1986. Shortening of continental lithosphere: the neotectonics of Eastern Anatolia — a young collision zone. In: Coward, M.P., Ries, A.C. (Eds.), Collision Tectonics. Geological Society of London Special Publication, vol. 19, pp. 3–36.

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Fig. 12. Palaeogeographical map of Gondwana during Late Precambrian time (modified from Meert and Torsvik, 2003). The new chemical and dating evidence presented here show that granitic rocks within the Bitlis Massif, SE Turkey formed part of a Late Cambrian magmatic arc. A position north of the Arabian-Shield is preferred to one near West Africa– Amazonia, implying that subduction extended around the periphery of Gondwana from present NW Africa to the Himalayas.

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