Early Cretaceous migmatitic mafic granulites from the Sabzevar range (NE Iran): implications for the closure of the Mesozoic peri-Tethyan oceans in central Iran Federico Rossetti, 1 Mohsen Nasrabady, 2 Gianluca Vignaroli, 1 Thomas Theye, 3 Axel Gerdes, 4 Mohammad Hossein Razavi 2 and Hosein Moin Vaziri 2 1 Dipartimento di Scienze Geologiche, Universita ` Roma Tre, 00146 Roma, Italy; 2 Department of Geology, Tarbiat Moalem University, Tehran, Iran; 3 Institut fu ¨r Mineralogie und Kristallchemie, Universita ¨t Stuttgart, 70569 Stuttgart, Germany; 4 Institut fu ¨r Geowissenschaften, J. W. Goethe Universita ¨t, D-60438 Frankfurt, Germany Introduction The Iranian ophiolites are part of the orogenic sutures marking the diachro- nous closure of the Tethyan oceanic realms (Palaeotethys and Neotethys) along the Alpine–Himalayan conver- gent front running from the Mediter- ranean through East Europe, Middle East to Asia (Fig. 1a). In particular, various ophiolitic sutures surround the Central East Iranian Microconti- nent (CEIM, Fig. 1b). These are rem- nants of the Mesozoic peri-Tethyan oceanic basins formed in the upper- plate of the Neothethyan subduction and document a polyphase tectonic evolution during its Mesozoic–Ceno- zoic consumption along the Sana- ndaj–Sirjan Zone (Sto¨cklin, 1974; Sengo¨r et al., 1988; McCall, 1997; Stampfli and Borel, 2002; Bagheri and Stampfli, 2008). Although data from these ophiolites might provide key elements to better assess the pal- aeotectonic scenario along the Eurasia convergent margin, few modern pet- rological and geochronological studies exist. In this paper, we document the first report of migmatitic mafic granulites from the Palaeogene ophiolitic me´l- ange of the Sabzevar Range, located at the northern edge of the CEIM (Figs 1b–c). We asses their peak ther- mo-baric conditions and constrain timing of metamorphic climax by in situ laser ablation (LA)-ICPMS U–Pb dating of zircon and titanite occurring in felsic melt segregations. These data document an unknown episode of Early Cretaceous (c. 107 Ma) high-grade metamorphism linked to dehydratation melting of amphibole-bearing mafic protoliths. Results from this study impose recon- sideration of the current geodynamic reconstructions of the Neotethyan pal- aeo-convergent margin in the region. Regional Geology The NW–SE trending ophiolitic belt of the Sabzevar Range formed at the expenses of the Late Cretaceous Sab- zevar ocean, a part of the marginal basins that originally segmented the CEIM northward of the active mar- gin of Neotethys (McCall, 1997) (Figs 1b–c). The structural architec- ture of the Sabzevar Range consists of a ductile-to-brittle, S ⁄ SW-verging accretionary complex, made of a dis- membered ophiolitic suite with a tec- tonised and partially serpentinised mantle section and a volcano-sedi- mentary sequence, upper Late Creta- ceous (Campanian; c. 84 Ma Baroz et al., 1984) to Palaeocene in age (Shojaat et al., 2003). These rock types occur dispersed as centimetre- to kilometre-size blocks into a highly sheared serpentinite matrix to form a major ophiolitic tectonic me´ lange. Variably-sized, foliated metabasic rocks (blueschists, greenschists and amphibolites) are also involved in the tectonic me´lange (Lench et al., 1977; Macaudier, 1983; Baroz et al., 1984). A further me´lange unit underlies the serpentinite me´lange and consists of SW-verging embricated thrust slices of red limestones, cherts, and volcanic- volcaniclastic rocks forming the fron- tal part of the range. Late tectonic, sheeted granites intrude the ophiolitic me´lange in the inner sector of the chain. Available radiometric data, derived from K–Ar (muscovite) and Rb–Sr (whole rock and muscovite) methods, constrain the tectono-meta- morphic structure of the Sabzevar Range to the Early Eocene (at about 50–55 Ma; Baroz et al., 1984). The Sabzevar granulites Two exposures of km-scale (c. 10 km long and 1 km wide), variably retrogressed (amphibolitised) mafic granulitic bodies were recognized in ABSTRACT The ophiolitic me ´ lange of the Sabzevar Range (northern Iran) is a remnant of the Mesozoic oceanic basins on the northern margin of the Neotethys that were consumed during the Arabia–Eurasia convergence history. Occurrence of km-scale, dismembered mafic HP granulitic slices is reported in this study. Granulites record an episode of amphibole-dehydratation melting and felsic (tonalite ⁄ throndhjemite) melt segregation at c. 1.1 GPa and 800 °C. In situ U(-Th)–Pb geochronology of zircon and titanite grains hosted in melt segregations points to an Early Cretaceous (Albian) age for the metamorphic climax. Results of this study (i) impose reconsideration of the current palaeotectonic models of the Neothetyan convergent margin during the Early Cretaceous and (ii) argue that punctuated events of subduction of short-lived back-arc oceanic basins accompanied the long-lasting history of the Neotethyan subduction in the region. Terra Nova, 22, 26–34, 2010 Correspondence: F. Rossetti, Dipartimento di Scienze Geologiche, Universita` Roma Tre, Largo S. L. Murialdo, 1, 00146 Rome, Italy. Tel.: +390657338043; fax: +390657338201; e-mail: rossetti@uniroma3. it 26 Ó 2009 Blackwell Publishing Ltd doi: 10.1111/j.1365-3121.2009.00912.x
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Early Cretaceous migmatitic mafic granulites from the Sabzevarrange (NE Iran): implications for the closure of the Mesozoicperi-Tethyan oceans in central Iran
Federico Rossetti,1 Mohsen Nasrabady,2 Gianluca Vignaroli,1 Thomas Theye,3 Axel Gerdes,4
Mohammad Hossein Razavi2 and Hosein Moin Vaziri21Dipartimento di Scienze Geologiche, Universita Roma Tre, 00146 Roma, Italy; 2Department of Geology, Tarbiat Moalem University, Tehran,
Iran; 3Institut fur Mineralogie und Kristallchemie, Universitat Stuttgart, 70569 Stuttgart, Germany; 4Institut fur Geowissenschaften, J. W.
Goethe Universitat, D-60438 Frankfurt, Germany
Introduction
The Iranian ophiolites are part of theorogenic sutures marking the diachro-nous closure of the Tethyan oceanicrealms (Palaeotethys and Neotethys)along the Alpine–Himalayan conver-gent front running from the Mediter-ranean through East Europe, MiddleEast to Asia (Fig. 1a). In particular,various ophiolitic sutures surroundthe Central East Iranian Microconti-nent (CEIM, Fig. 1b). These are rem-nants of the Mesozoic peri-Tethyanoceanic basins formed in the upper-plate of the Neothethyan subductionand document a polyphase tectonicevolution during its Mesozoic–Ceno-zoic consumption along the Sana-ndaj–Sirjan Zone (Stocklin, 1974;Sengor et al., 1988; McCall, 1997;Stampfli and Borel, 2002; Bagheriand Stampfli, 2008). Although datafrom these ophiolites might providekey elements to better assess the pal-aeotectonic scenario along the Eurasiaconvergent margin, few modern pet-rological and geochronological studiesexist.
In this paper, we document the firstreport of migmatitic mafic granulitesfrom the Palaeogene ophiolitic mel-ange of the Sabzevar Range, locatedat the northern edge of the CEIM(Figs 1b–c). We asses their peak ther-mo-baric conditions and constraintiming of metamorphic climax byin situ laser ablation (LA)-ICPMSU–Pb dating of zircon and titaniteoccurring in felsic melt segregations.These data document an unknownepisode of Early Cretaceous (c.107 Ma) high-grade metamorphismlinked to dehydratation melting ofamphibole-bearing mafic protoliths.Results from this study impose recon-sideration of the current geodynamicreconstructions of the Neotethyan pal-aeo-convergent margin in the region.
Regional Geology
The NW–SE trending ophiolitic beltof the Sabzevar Range formed at theexpenses of the Late Cretaceous Sab-zevar ocean, a part of the marginalbasins that originally segmented theCEIM northward of the active mar-gin of Neotethys (McCall, 1997)(Figs 1b–c). The structural architec-ture of the Sabzevar Range consistsof a ductile-to-brittle, S ⁄SW-vergingaccretionary complex, made of a dis-membered ophiolitic suite with a tec-tonised and partially serpentinised
mantle section and a volcano-sedi-mentary sequence, upper Late Creta-ceous (Campanian; c. 84 Ma Barozet al., 1984) to Palaeocene in age(Shojaat et al., 2003). These rocktypes occur dispersed as centimetre-to kilometre-size blocks into a highlysheared serpentinite matrix to form amajor ophiolitic tectonic melange.Variably-sized, foliated metabasicrocks (blueschists, greenschists andamphibolites) are also involved in thetectonic melange (Lench et al., 1977;Macaudier, 1983; Baroz et al., 1984).A further melange unit underlies theserpentinite melange and consists ofSW-verging embricated thrust slices ofred limestones, cherts, and volcanic-volcaniclastic rocks forming the fron-tal part of the range. Late tectonic,sheeted granites intrude the ophioliticmelange in the inner sector of thechain. Available radiometric data,derived from K–Ar (muscovite) andRb–Sr (whole rock and muscovite)methods, constrain the tectono-meta-morphic structure of the SabzevarRange to the Early Eocene (at about50–55 Ma; Baroz et al., 1984).
The Sabzevar granulites
Two exposures of km-scale (c. 10 kmlong and 1 km wide), variablyretrogressed (amphibolitised) maficgranulitic bodies were recognized in
ABSTRACT
The ophiolitic melange of the Sabzevar Range (northern Iran) isa remnant of the Mesozoic oceanic basins on the northernmargin of the Neotethys that were consumed during theArabia–Eurasia convergence history. Occurrence of km-scale,dismembered mafic HP granulitic slices is reported in this study.Granulites record an episode of amphibole-dehydratationmelting and felsic (tonalite ⁄ throndhjemite) melt segregationat c. 1.1 GPa and 800 �C. In situ U(-Th)–Pb geochronology ofzircon and titanite grains hosted in melt segregations points to
an Early Cretaceous (Albian) age for the metamorphic climax.Results of this study (i) impose reconsideration of the currentpalaeotectonic models of the Neothetyan convergent marginduring the Early Cretaceous and (ii) argue that punctuatedevents of subduction of short-lived back-arc oceanic basinsaccompanied the long-lasting history of the Neotethyansubduction in the region.
Terra Nova, 22, 26–34, 2010
Correspondence: F. Rossetti, Dipartimento
di Scienze Geologiche, Universita Roma
Tre, Largo S. L. Murialdo, 1, 00146
Rome, Italy. Tel.: +390657338043; fax:
+390657338201; e-mail: rossetti@uniroma3.
it
26 � 2009 Blackwell Publishing Ltd
doi: 10.1111/j.1365-3121.2009.00912.x
the frontal part of the range, to thenorthwest of the Sabzevar city(Fig. 1c). They occur as dismembered,NW ⁄SE-striking tectonic sliversembedded within the ophiolitic mel-ange. Contacts with the surroundingrocks are obscured by intense brittledeformation because of the late Neo-gene to Quaternary faulting.
Texture, petrography and mineralcompositions
The granulite bodies are dark, med-ium to fine-grained rocks showinggranoblastic groundmass or weakfoliation. Texture is characterized byoccurrence of submillimetric to milli-
metric leucocratic patches interlayeredwithin the granoblastic mineral matrixmade of Am + Grt + Cpx + Pl ±Qtz, with Ilm, Rt, Ap, Zr (abbrevia-tion after Bucher and Frey, 2002) asmain accessory phases (Fig. 2a). Bothgarnet and clinopyroxene form por-phyroblasts, which typically occurwithin the leucocratic domains; gar-nets are poikiloblastic, hosting multi-phase and single inclusionassemblages made of Am, Pl, Qtz,Rt, Ilm, Ttn (Figs 2a–c). The leuco-cratic patches invariably consist ofQtz + Pl-rich segregations of broadlytonalitic ⁄ trondhjemitic composition(Qtz ⁄Pl modal proportions 50 ⁄35–50 ⁄65). They show a systematic intra-
granular connectivity, with Pl and Qtzshowing a coarse granoblastic and,usually, strain-free texture (Fig. 2d).The Pl + Qtz associations also formfilm-like intergrowths surroundingmatrix amphibole, with quartz usuallyshowing xenomorphic habit (Fig. 2e).Titanite and zircon are the mainaccessory phases in the leucocraticsegregations (Fig. 2f).Representative mineral composi-
tions are shown in Table 1. Garnetis essentially almandine-grossular-pyrope and spessartine poor (Alm53–48
Prp21–12Sps6–3Grs21–31), commonlycharacterized by flat chemical profiles.Zoning is only seldom evident at thegarnet rim, with a general increase in
(a) (b)
(c)
Fig. 1 (a) Distribution of the remnants of the Tethyan oceanic realm along the Alpine–Himalayan convergence zone. (b) Simplifiedgeological map showing the main tectonic domains in Iran, with the main ophiolitic belts (in white) indicated (modified afterShojaat et al., 2003; Bagheri and Stampfli, 2008). CEIM: Central East Iranian Microcontinent. (c) Geological map of the SabzevarRange (modified and readapted after Lench et al., 1977), with location of the granulite-facies rocks. The location of the sampleNG353 studied for U–Pb geochronology together with its geographical coordinates are also indicated.
Terra Nova, Vol 22, No. 1, 26–34 F. Rossetti et al. • Closure of the mesozoic tethyan oceans, Iran
Fig. 2 Textures and mineral assemblages from the Sabzevar mafic granulites (sample NG353) (a) Rock slab showing the overalltexture of the rock. Coarse-grained, dark green amphibole forms the main matrix assemblage with porphyroblastic garnet andclinopyroxene. Leucocratic Qtz–Pl segregations enclose garnet porphyroblasts. The dashed white circle indicates the rock slab usedfor the in situ U–Pb dating. (b) Thin section showing microstructures. Garnet is typically euhedral, but also xenoblastic grains areobserved. Straight boundaries occur with the matrix amphibole and the leucocratic segregations (plane polarized light). (c)Poikiloblastic garnet hosting Ilm–Am–Qtz–Pl composite inclusions and rutile needles (plane polarized light). (d) Leucocratic Qtz–Pl segregation showing well preserved igneous texture (crossed polars). (e) Interstitial Qtz–Pl segregations and xenomorphic quartzsurrounding matrix amphibole (crossed polars). (f) Back scattered electron (BSE) image showing coexisting zircon and titanite inthe Qtz–Pl segregations.
Closure of the mesozoic tethyan oceans, Iran • F. Rossetti et al. Terra Nova, Vol 22, No. 1, 26–34
Ca and Fe ⁄ (Fe+Mg). Clinopyroxeneis diopside-rich with minor heden-bergite, orthopyroxene andCa-Tschermaks components (Di49–60Hd10–30Opx7–15Ca-Ts2–18). Core-to-rim decrease of the Ca-ts componentis systematically observed. Plagioclaseis andesine (An43–51Ab50–56Or0–1),either occurring in Pl–Qtz segrega-tions or as inclusion in garnet. Amphi-bole (either inclusion in garnet or inthe matrix) shows Mg ⁄ (Mg + Fe2+)
values ranging between 0.6 and 0.8,with Si4+between 6.0 and 6.7 a.p.f.u.It can be classified as tschermakitetransitional to Mg-hornblende (Leakeet al., 2004).
Peak P–T estimates
The peak mineral assemblage (Grt +Cpx+Pl±Am±Qtz) is indicative ofthe Opx-free, high-pressure (HP) gran-ulite facies (Pattison, 2003). Textures
such as those described above showstrong similarities with those reportedin Hartel and Pattison (1996), whointerpreted the Qtz + Pl segregationsas remnants of melt. In particular, theskeletal nature of the quartz and theQtz+Plfilms aroundamphibole arguefor an in situ origin of such melts (e.g.Brown, 2002), and hence product ofmigmatisation of a basic protolith. Alikely scenario is amphibole dehydra-tation melting during prograde granu-lite faciesmetamorphism according thefollowing generalized reaction (Harteland Pattison, 1996):
Amþ Pl ¼ Grtþ Cpx
þ TtnþmeltðtrondhjemiteÞð1Þ
The flat chemical profiles inporphyroblastic garnets are hereinterpreted as the effect of the high-temperature chemical homogeniza-tion attained at the metamorphicclimax (e.g. Spear, 1993; Ganguly,2002). The rimward zoning can beinterpreted as due garnet growth inpresence of a Ca-rich melt phase,coupled with partial post-peak P–Tre-equilibration (e.g. Spear andKohn, 1996; Kohn and Spear, 2000).Mineral core composition of largecrystals showing textural equilibriaare then combined with those ofmineral inclusions (amphibole andplagioclase) hosted in garnet and usedfor estimating peak conditions. TheP–T estimates and phase reactioncalculations were obtained using theTHERMOCALC3.26 software (Powelland Holland, 2008). Considering thecoexisting phases Grt + Cpx + Pl +Am + Qtz, results running THERMO-
CALC in the average P–T mode are811 ± 81 �C and 1.09 ± 0.13 GPa(Table 2). Phase reaction calculationsconsidering the Grt-Cpx Fe–Mg ex-change thermometry and the equilib-ria 2Grs + Prp +3Qtz = 3An + 3Di (GADS) and 2Grs + Alm +3Qtz= 3An + Hd (GAHS) for barometryprovided an intersection at 800 �Cand 1.1 GPa. These data were com-plemented with the zirconium-inrutile thermometry, which yields con-sistent results ranging between 721 to810 �C (Table 2). Calculated peakP–T conditions thus provide furtherevidence for partial melting ofamphibolite as they locate above theH2O-saturated basaltic solidus(Fig. 3).
Table 1 Representative microprobe analyses and structural formulae of equilibrium
mineral phases at the metamorphic peak in the Sabzevar granulites*.
Sample NG353 (see Fig. 1c for samplelocation) was chosen for U–Pb geo-
chronology as it provides the bestpreserved example of the peak granu-lite metamorphism. The same rocksection used for the petrographical
study (Fig. 2a) was used for in situdating of zircon and titanite occurringin melt segregations (Fig. 4a). Aftercareful petrographical investigation, acircular (1 inch in diameter), 100 lmthick rock slab was cored from thesection and prepared for both back-scattered electron (BSE) and cathodo-luminescence (CL) imaging. Zirconsare typically rounded in shape andfine-grained (20 lm as average). BSEand cathodoluminescence images re-veal a homogeneous zircon popula-tion characterized by oscillatory- tosector zoning (Fig. 4b), a texture thatis typical of magmatic crystallization(e.g. Harley et al., 2007). Titanite isinstead relatively coarse grained (com-monly 60–100 lm) with no composi-tional zoning (Fig. 4c). Selected spotsof 16–40 lm in diameter were thenanalysed for U–Th–Pb isotopic com-positions using a laser ablationICPMS system. Plots and age calcu-lations were made using the ISOPLOT
software (Ludwig, 2003). Results areshown Fig. 4d and listed in Table 3,which also provides details on theanalytical methods. Six spots on fivezircon grains yielded a monomodalage distribution with a concordia ageof 107.4 ± 2.4 Ma. Ten spots on sixtitanite grains provided a nearly iden-tical well defined concordant cluster at105.9 ± 2.3 Ma. These results pointto an Early Cretaceous (Albian) agefor felsic melt segregation and peakmetamorphism in the Sabzevar gran-ulites.
A working hypothesis for theclosure of the Mesozoicperi-Tethyan oceans in central Iran
The HP granulite facies metamorphicconditions such those documented inthis study are diagnostic of crustalthickening in collisional belts (O�Brienand Rotzler, 2003). Peculiarity here isthe fact that granulitemetamorphism isreported from a basic protolith andboth the mineral assemblages and thehigh-grade conditions are in principlecompatible with those reported fromsubophiolitic dynamothermal soles(e.g. Williams and Smyth, 1973; Jamie-son, 1986). Textures suggest deepmelt-ing of mafic rocks to form felsic melt(tonalite ⁄ trondhjemite) and garnet-clinopyroxene residues (HP granulite).Formation environments of maficgarnet granulite residues in orogenic
Table 2 Mineral assemblages, activity models and average P–T results as obtained
from the THERMOCALC calculations (average P–T mode), together with results from
Zr-in rutile thermometry.
THERMOCALCv3.26 (+ Ttn, Rt, Ilm)
Mineral Grt Cpx Pl Am P (GPa ± 1r) T (�C ± 1r) corr.
Mineral
activities
aGrs 0.033
aPrp 0.089 aDi 0.490 aAn 0.570 aTs 0.001
aAlm 0.110 aHd 0.310 aAb 0.560 afact 0.001
aSps 0.001 aTr 0.036 1.09 ± 0.13 811 ± 81 0.869
aparg 0.015
Zr-in rutile thermometry
Calibration Zr(n) (ppm) W06 F&W07 T07
n = 28 777–1606 727–803 �C 721–802 �C 716-810 �C*
n, number of analysed rutile grains; Zr(n): range of the Zr-in rutile content for the n analysed grains; W06,
Watson et al. (2006); F&W07, Ferry and Watson (2007); T07, Tomkins et al. (2007); *At 1.1 GPa.
Fig. 3 Peak P–T estimates for the Sabzevar granulites as obtained from the THER-
MOCALC software. The ellipse quotes the errors at 1r level (average P–T modecalculations). Metamorphic facies boundaries are after Bucher and Frey (2002). Thegrid showing regimes of melting for basaltic system is after Vielzeuf and Schmidt(2001). Key to symbols: A, amphibolite facies; EA, epidote amphibolite facies; G,granulite facies; Ecl, eclogite facies.
Closure of the mesozoic tethyan oceans, Iran • F. Rossetti et al. Terra Nova, Vol 22, No. 1, 26–34
settings have been generally ascribed totwo-end member processes: arc matu-ration, i.e. formation in consequence ofmagmatic loading at the mature arcstage (e.g. Garrido et al., 2006; Bergeret al., 2008), or slab melting in highheat-flow subduction settings, andhence remnants of a former oceaniccrust (e.g. Garcıa-Casco et al., 2008).Most of reconstructions on the
closure of the Neotethys propose thatformation of the active margin alongthe Eurasian margin started since theLate Triassic–Early Jurassic (e.g. Ber-berian and King, 1981; Besse et al.,1998; Stampfli and Borel, 2002; Arvinet al., 2007). This oceanic subductionwas accompanied by formation of acordilleran-type margin along theSanandaj–Sirjan Zone during theJurassic–Cretaceous (e.g. Berberianand Berberian, 1981; Ghasemi andTalbot, 2006) and by formation ofvarious marginal oceans in the back-arc domain (Inner Mesozoic Oceansof McCall, 1997). In particular, recentgeochronological studies from theCEIM ophiolites have documentedthat such oceanic basins formed in
(a)
(b)
(c)
Fig. 5 Tentative palaeotectonic reconstruction of the sequence of events linked to theNeotethyanclosurealongtheEurasianmarginofIran(sourceofdataandreadaptedafterGhasemi andTalbot, 2006;Agard et al., 2007;Moghadam et al., 2009).Relativemotionof the various crustal blocks making up the upper-plate of the Neothetyan subductionshouldbe eventually contemplated.Not to scale; locationof structures is only indicative.
(a) (b)
(c)
(d)
Fig. 4 (a) Rock slab used for the U–Pb geochronology and localization of the dated zircon and titanite grains. (b) Representativeback-scattered electron (BSE) (top) and cathodoluminescence (bottom) images of zircons. The grains showhomogeneous growthwithoscillatory zoning. Locations of the LA-ICPMS spots (white circles) are also indicated. (c) Representative BSE images of some of thedated titanite grains, with laser spots (white circles) and U–Pb isotope data indicated. (d) Conventional concordia diagrams showingall data. All ages are concordia ages, with errors quoted at 2r level.
Terra Nova, Vol 22, No. 1, 26–34 F. Rossetti et al. • Closure of the mesozoic tethyan oceans, Iran
two major periods, during the LateJurassic–Early Cretaceous (Sistan andFannuj ophiolites; Fotoohi Rad et al.,2009) and the Late Cretaceous (Sab-zevar and Naien-Baft ophiolites; Sho-jaat et al., 2003; Moghadam et al.,2009). Closure of these basins oc-curred diachronously, during theMesozoic and the Palaeogene times,concomitantly with the Arabia–Eur-asia collision at a late stage (Stampfliand Borel, 2002; Shojaat et al., 2003;Bagheri and Stampfli, 2008; FotoohiRad et al., 2009; Moghadam et al.,2009).The in situU–Pb dating of felsic melt
segregation in the Sabzevar granulitesconstrains timing of peak metamor-phism to the Early Cretaceous, thusc. 20–25 Ma before of the Late Creta-ceous opening of the Naien and Sabze-var back-arc oceans. We then arguethat the Sabzevar granulites formedduring subduction of a branch of theearly formed, Late Jurassic–Early Cre-taceous back-arc oceanic system (here-after referred as Sistan Ocean todistinguish it from the lately-formedSabzevar-Naien Ocean). The Sistansuture can be now tentatively tracednorthward to Sabzevar structural zone,bounding the eastern margin of theCEIM towards the Eurasian plate(Fig. 1b).Further studies are necessary to elu-
cidate the tectonic evolution of thevarious ophiolitic melanges surround-ing the CEIM, but argument is pro-vided for polyphase opening andclosure of short-lived back-arc basinsin the overriding plate of the Neoteth-yan subduction. Figure 5 shows a pos-sible geodynamic scenario to accountfor such a complex history. This recon-struction starts in Early Cretaceous,when the north-dipping oceanic sub-duction of the Neotethys was activealong the Sanandaj–Sirjan Zone (Gha-semi and Talbot, 2006; Agard et al.,2007). Northward, this major subduc-tion zone was associated with theconsumption of the Sistan Ocean,accommodated by a synthetic subduc-tion zone active along the Eurasianmargin (cfr. Tirrul et al., 1983)(Fig. 5a). Closure of the Sistan oceanoccurred along a diachronous suturethat conformed to two different geo-thermal gradient conditions, from<10 �C km)1 (the older, c. 125 Ma,Sistan eclogites; Fotoohi Rad et al.,2009) to 20–25 �C km)1 (the younger
Sabzevar granulites), and hence from alow to a relatively high heat-flow sub-duction setting. We speculate that itwas consequence of an along-striketransition from a mature (Sistan) toan infant (Sabzevar) stage of oceanicsubduction (cfr. Peacock, 1996) alongthe Sistan active margin. Although it isdifficult to decipher the precise forma-tion environment of the Sabzevar gran-ulites, we then favour a scenario of slabmelting in the course of subduction of ayoung (and hence hot) oceanic litho-spere (e.g. Peacock et al., 1994; Ueharaand Aoya, 2005). During the LateCretaceous, intraoceanic subductiondeveloped within the Neotethys, fol-lowed by island arc-continent suturingand ophiolite obduction along theArabian margin (Agard et al., 2007).Later on, renewed back-arc extensioncaused formation of the Sabzevar-Naien Ocean in the upper-plate of theNeotethyan subduction, concurrentlywith the main phase of magmatism inthe Sanandaj–Sirjan Zone (Omraniet al., 2008). Back-arc extension over-printed the early orogenic sutureformed after consumption of the SistanOcean, leading to CEIM fragmenta-tion (Fig. 5b). During the Palaeocene–Eocene, suturing between Iran andArabia occurred. This event wasaccompanied by closure of the LateCretaceous Sabzevar back-arc ocean,with formation of major suture zonesbetween the CEIM and the Eurasianmargin concomitantly with renewedmagmatism (Berberian and Berberian,1981; Arvin et al., 2007; Omrani et al.,2008) (Fig. 5c).To conclude, our study suggests
that multi-stage extensional-compres-sional events accompanied consump-tion of the Neotethys since LateJurassic. Evidence of these eventsshould be preserved along the marginsof the CEIM, although this requires afull assessment of the evolving palaeo-tectonic configuration of the Eurasianconvergent margin during progress ofthe Neothetyan subduction, includingthe pattern of tectonic rotations incentral Iran during the Mesozoic–Cenozoic times (cfr. Bagheri andStampfli, 2008).
Acknowledgements
This paper is dedicated to the memory ofR. Funiciello. We thank Amir and Habibfor assistance during field work. D. Coz-
zupoli participated in the field work and isthanked for useful discussion together withH. J. Massonne, C. Faccenna and M.Mattei. Comments from S. Bagheri on anearly version of the manuscript areacknowledged. Constructive criticism andadvice from G. Stampfli are greatly appre-ciated.
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Received 14 June 2009; revised versionaccepted 25 September 2009
Closure of the mesozoic tethyan oceans, Iran • F. Rossetti et al. Terra Nova, Vol 22, No. 1, 26–34