early cretaceous migmatitic maﬁc granulites from the

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

.............................................................................................................................................................

� 2009 Blackwell Publishing Ltd 27

(d)

(b)

(a)

(c)

(e) (f)

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*.

Mineral Grt Grt Cpx Cpx Amp Amp Pl Pl Ttn Ttn

Analysis #36-c #42-r #107-i #28-m #44-i #24-m #46-i #48-m #72-i #33-m

SiO2 37.55 37.47 51.89 49.61 41.88 42.61 56.51 56.25 29.70 29.81

TiO2 0.12 0.20 0.23 0.40 1.38 1.53 0.01 0.00 38.93 38.00

Al2O3 21.99 21.51 2.93 4.11 11.62 12.25 27.63 28.77 1.12 1.04

Cr2O3 0.01 0.01 0.01 0.00 0.02 0.03 0.00 0.00 0.01 0.02

FeOTot 25.82 25.12 10.38 12.31 18.09 16.53 0.00 0.00 0.90 0.98

MnO 2.12 1.98 0.31 0.40 0.14 0.21 0.02 0.02 0.09 0.02

MgO 4.74 3.71 14.00 10.29 8.89 10.42 0.02 0.01 0.04 0.01

CaO 8.56 10.38 20.32 21.25 10.96 11.58 8.89 9.36 28.07 28.46

Na2O 0.04 0.02 0.58 0.84 1.90 1.70 6.43 6.48 0.00 0.09

K2O 0.00 0.00 0.02 0.01 0.70 0.73 0.22 0.16 0.00 0.04

Total 100.95 100.40 100.67 99.51 95.58 97.59 99.73 101.05 98.81 98.47

12 O 12 O 6 O 6 O 23 O 23 O 8 O 8 O

Cations

Si 2.91 2.93 1.92 1.89 6.43 6.35 2.54 2.50 1.00 1.00

Ti 0.01 0.01 0.01 0.01 0.16 0.17 0.00 0.00 0.99 0.96

Al 2.01 1.98 0.13 0.18 2.10 2.15 1.46 1.51 0.04 0.04

Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Fe3+ 0.17 0.14 0.07 0.08 0.26 0.34 0.02 0.01 0.02 0.02

Fe2+ 1.51 1.50 0.25 0.31 2.07 1.72 0.00 0.00

Mn 0.14 0.13 0.01 0.01 0.02 0.03 0.00 0.00 0.00 0.00

Mg 0.55 0.43 0.77 0.58 2.04 2.31 0.00 0.00 0.00 0.00

Ca 0.71 0.87 0.80 0.87 1.80 1.85 0.43 0.45 1.01 1.02

Na 0.01 0.00 0.04 0.06 0.57 0.49 0.56 0.56 0.00 0.01

K 0.00 0.00 0.00 0.00 0.14 0.14 0.01 0.01

Sum 8.00 8.00 4.00 4.00 15.67 15.67 5.01 5.03 3.06 3.05

X_Grs 0.24 0.30

X_Prp 0.19 0.15

X_Alm 0.52 0.51

X_Sps 0.05 0.04

X_Di 0.54 0.49

X_Hd 0.18 0.26

X_Ts 0.07 0.10

X_Opx 0.15 0.07

X_An 0.43 0.44

X_Ab 0.56 0.55

X_Or 0.01 0.01

*Mineral compositions were measured with a CAMECA SX100 electron probe at the Institut fur Mineralogie,

Universitat Stuttgart (15 kV, 15 nA beam conditions; WDS mode).

Structural formula normalization (to number of oxygen (O) or to Si4+= 1 for Ttn) and estimate of the

Fe3+content are derived from the AX2000 software included in the THERMOCALC package. Am-i, amphibole

inclusion in garnet; Am-m, amphibole in matrix; Cpx-i, clinopyroxene inclusion in garnet; Cpx-m,

clinopyroxene in matrix; Grt-c, garnet core; Grt-r, garnet rim; Pl-i, plagioclase inclusion in garnet; Pl-m,

plagioclase in matrix; Ttn-i, titanite inclusion in amphibole; Ttn-m, titanite in matrix.


.............................................................................................................................................................


In situ U–Pb geochronology

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.


.............................................................................................................................................................


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.


.............................................................................................................................................................


Tab

le3

ResultsofU–Th–Pbin

situ

LA-ICP-M

Sanalysesonzirconsandtitanitegrainsfrom

theSabzevargranulites*.

Ag

e(M

a)

Min

eral

Spo

tN

o.

207P

b†

(cp

s)U

(pp

m)‡

Pb

(pp

m)‡

Th/U

‡206P

b/2

04P

b206P

b/2

38U

§±

2r

(%)

207P

b/2

35U

§±

2r

(%)

207P

b/2

06P

b§

±2r

(%)

Rh

o*

*2

06P

b/2

38U

±2r

(Ma)

20

7P

b/2

35U

±2r

(Ma)

Zra1

539

310

51.

70.

0133

640.

0171

54.

30.

1061

9.6

0.04

498.

60.

4511

05

102

9

Zra1

742

310

61.

70.

0113

650.

0163

44.

70.

1022

9.8

0.04

538.

60.

4810

55

999

Zra1

833

015

62.

50.

0221

880.

0169

04.

50.

1087

9.8

0.04

678.

60.

4710

85

105

10

Zra2

038

818

12.

90.

0125

090.

0166

84.

60.

1164

9.3

0.05

068.

00.

4910

75

112

10

Zra2

139

886

1.4

0.01

2780

0.01

724

4.0

0.11

8210

0.04

979.

20.

4011

04

113

11

Zra2

332

423

73.

90.

0212

970.

0164

14.

70.

1114

9.8

0.04

928.

60.

4810

55

107

10

Ttn

a25

915

601.

80.

2575

20.

0155

83.

90.

1069

8.8

0.04

987.

90.

4510

04

103

9

Ttn

a26

724

721.

40.

3014

140.

0163

93.

60.

1094

8.7

0.04

847.

90.

4210

54

105

9

Ttn

a27

529

861.

60.

0492

90.

0175

05.

00.

1086

9.7

0.04

508.

30.

5211

26

105

10

Ttn

a28

725

107

1.8

0.05

2118

0.01

685

3.4

0.11

379.

00.

0490

8.4

0.38

108

410

99

Ttn

a29

1062

108

3.3

0.14

625

0.01

596

3.1

0.10

725.

10.

0487

4.0

0.61

102

310

35

Ttn

a30

1069

108

2.7

0.22

613

0.01

700

3.2

0.10

985.

10.

0468

3.9

0.64

109

310

65

Ttn

a31

924

130

2.9

0.66

1914

0.01

661

3.2

0.10

965.

00.

0479

3.8

0.64

106

310

65

Ttn

a32

591

135

2.4

0.14

3690

0.01

728

4.7

0.12

009.

70.

0504

8.4

0.49

110

511

511

Ttn

a34

457

107

2.0

0.17

1068

0.01

675

3.6

0.11

065.

20.

0479

3.8

0.69

107

410

65

Ttn

a35

593

133

2.6

0.20

1328

0.01

694

4.4

0.11

228.

10.

0480

6.9

0.54

108

510

88

Dia

met

erof

lase

rsp

otw

as16

,20

or30

l Mfo

rzi

rcon

(Zr)

and

30or

40l

mfo

rtit

anite

(Ttn

);de

pth

ofcr

ater

10–1

5l

M.

*Ura

nium

,th

oriu

man

dle

adis

otop

esw

ere

anal

ysed

usin

ga

Ther

mo-

Scie

ntifi

cEl

emen

t2

sect

orfie

ldIC

P-M

Sco

uple

dto

aN

ewW

ave

Res

earc

hU

P-21

3ul

trav

iole

tla

ser

syst

emat

Goe

the

Uni

vers

ityFr

ankf

urt

(Ger

des

and

Zeh,

2006

,20

09).

Dat

aw

ere

acqu

ired

intim

ere

solv

ed–

peak

jum

ping

–pu

lse

coun

ting

mod

eov

er81

0m

ass

scan

s,w

itha

16s

back

grou

ndm

easu

rem

ent

follo

wed

by28

ssa

mpl

eab

latio

n.La

ser

spot

-siz

esva

ried

from

16to

30l

M

with

aty

pica

lpen

etra

tion

dept

hof�

15–2

0l

M.S

igna

lwas

tune

dfo

rm

axim

umse

nsiti

vity

for

Pban

dU

whi

leke

epin

gox

ide

prod

uctio

n,m

onito

red

as2

54U

O⁄2

38U

,wel

lbel

ow1%

.Ate

ardr

op-s

hape

d,lo

wvo

lum

e(<

2.3

cm3)

lase

r

cell

was

used

(Fre

ian

dG

erde

s,20

09an

dre

fere

nces

ther

ein)

.Th

isce

llen

able

sde

tect

ion

and

sequ

entia

lsa

mpl

ing

ofhe

tero

gene

ous

grai

ns(e

.g.,

grow

thzo

nes)

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gtim

ere

solv

edda

taac

quis

ition

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eto

itsre

spon

setim

eof

<1

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ime

until

max

imum

sign

alst

reng

thw

asac

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ed)

and

was

h-ou

t(<

99%

ofpr

evio

ussi

gnal

)tim

eof

<5s

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itha

dept

hpe

netr

atio

nof�

0.6

lm

s)1

and

a0.

9sin

tegr

atio

ntim

e(=

15m

ass

scan

s=

1ra

tio)

any

sign

ifica

nt

varia

tion

ofth

ePb

⁄Pb

and

U⁄P

bin

the

lm

scal

eis

dete

ctab

le.

Raw

data

wer

eco

rrec

ted

offli

nefo

rba

ckgr

ound

sign

al,

com

mon

Pb,

lase

rin

duce

del

emen

talf

ract

iona

tion,

inst

rum

enta

lmas

sdi

scrim

inat

ion,

and

time-

depe

nden

t

elem

enta

lfr

actio

natio

nof

Pb⁄U

usin

gan

in-h

ouse

MS

Exce

l�sp

read

shee

tpr

ogra

m(G

erde

san

dZe

h,20

06).

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mm

on-P

bco

rrec

tion

base

don

the

inte

rfer

ence

-an

dba

ckgr

ound

-cor

rect

ed2

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sign

alan

da

mod

elPb

com

posi

tion

(Sta

cey

and

Kra

mer

s,19

75)

was

carr

ied

out,

whe

rene

cess

ary.

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nece

ssity

ofth

eco

rrec

tion

was

judg

edon

whe

ther

the

corr

ecte

d2

07Pb

⁄20

6Pb

layo

utw

ithth

ein

tern

aler

rors

ofth

em

easu

red

ratio

s.Th

ein

terf

eren

ce

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gon

mas

s20

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ases

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edus

ing

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g⁄2

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gra

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99an

dm

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ser-

indu

ced

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enta

lfra

ctio

natio

nan

din

stru

men

talm

ass

disc

rimin

atio

nw

ere

corr

ecte

dby

norm

aliz

atio

nto

the

refe

renc

ezi

rcon

GJ-

1fo

rea

chan

alyt

ical

sess

ion.

Prio

rto

this

norm

aliz

atio

n,th

edr

iftin

inte

r-el

emen

tal

frac

tiona

tion

(Pb

⁄U)

durin

g28

sof

sam

ple

abla

tion

was

corr

ecte

dfo

rth

ein

divi

dual

anal

ysis

.Th

eco

rrec

tion

was

done

byap

plyi

nga

linea

r

regr

essi

onth

roug

hal

lm

easu

red

ratio

s,ex

clud

ing

the

outli

ers

(±2

stan

dard

devi

atio

n;2S

D),

and

usin

gth

ein

terc

ept

with

the

y-ax

isas

the

initi

alra

tio.

The

tota

lof

fset

ofth

em

easu

red

drift

-cor

rect

ed2

06Pb

⁄23

8U

ratio

from

the

�true

�ID

-TIM

Sva

lue

(0.0

986

±0.

0004

;ID

-TIM

SJW

Gva

lue)

ofth

ean

alys

edG

J-1

grai

nw

asty

pica

llyar

ound

3–9%

.R

epor

ted

unce

rtai

ntie

s(2

r)

ofth

e2

06Pb

⁄23

8U

ratio

wer

epr

opag

ated

byqu

adra

ticad

ditio

nof

the

exte

rnal

repr

oduc

ibili

ty(2

SD%

)ob

tain

edfr

omth

est

anda

rdzi

rcon

GJ-

1( n

=12

;2

SD�

1.3%

)du

ring

the

anal

ytic

alse

ssio

nan

dth

ew

ithin

-run

prec

isio

nof

each

anal

ysis

(2SE

%;

stan

dard

erro

r).

Inca

seof

the

20

7Pb

⁄20

6Pb

we

used

a2

07Pb

sign

alde

pend

ent

unce

rtai

nty

prop

agat

ion

(Ger

des

and

Zeh,

2009

).Th

eac

cura

cyof

the

met

hod

was

verifi

edby

anal

yses

ofre

fere

nce

zirc

on91

500

(106

4.8

±4.

3M

a,M

SWD

ofco

ncor

danc

ean

deq

uiva

lenc

e=

0.86

),

Ples

ovic

e(3

37.7

±1.

6M

a,M

SWD

C+

E=

0.84

),an

dTe

mor

a(4

16.6

±2.

5M

a,M

SWD

C+

E=

0.9)

.

�With

inru

nba

ckgr

ound

-cor

rect

edm

ean

20

7Pb

sign

alin

cps

(cou

nts

per

seco

nd).

�Uan

dPb

cont

ent

and

Th⁄U

ratio

wer

eca

lcul

ated

rela

tive

toG

J1re

fere

nce

zirc

on.

§Cor

rect

edfo

rba

ckgr

ound

(20

4H

g=

447

±8,

20

6Pb

=65

±5,

20

7Pb

=52

±4,

20

8Pb

=11

1±

11cp

s),

with

in-r

unPb

⁄Ufr

actio

natio

n(2

06Pb

⁄23

8U

ratio

)an

dco

mm

onPb

and

subs

eque

ntly

norm

aliz

edto

GJ1

(ID

-TIM

S

valu

e⁄m

easu

red

valu

e);2

07Pb

⁄23

5U

calc

ulat

edus

ing

20

7Pb

⁄20

6Pb

⁄(23

8U

⁄20

6Pb

*1⁄1

37.8

8).2

06Pb

⁄23

8U

erro

ris

the

quad

ratic

addi

tions

ofth

ew

ithin

run

prec

isio

n(2

SE)

and

the

exte

rnal

repr

oduc

ibili

ty(2

SD)

ofth

eG

J1re

fere

nce

zirc

on.

20

7Pb

⁄20

6Pb

erro

rpr

opag

atio

n(2

07Pb

sign

alde

pend

ent)

follo

win

gG

erde

san

dZe

h(2

009)

.2

07Pb

⁄23

5U

erro

ris

the

quad

ratic

addi

tion

ofth

e2

07Pb

⁄206

Pban

d2

06Pb

⁄23

8U

unce

rtai

nty.

**R

hois

the

erro

rco

rrel

atio

nde

fined

aser

r20

6Pb

⁄23

8U

⁄err

20

7Pb

⁄23

5U

.


.............................................................................................................................................................


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


.............................................................................................................................................................


early cretaceous migmatitic maﬁc granulites from the

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