Journal of Asian Earth Sciences 41 (2011) 238–249

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Journal of Asian Earth Sciences

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U–Pb dating and emplacement history of granitoid plutons in the northern Sanan-daj–Sirjan Zone, Iran

Shahryar Mahmoudi a, Fernando Corfu b,⇑, Fariborz Masoudi c, Behzad Mehrabi a, Mohammad Mohajjel d

a Geology Department, Tarbiat Moallem University, 49 Mofateh Avenue, P.O. Box 15614, Tehran, Iranb Department of Geosciences, University of Oslo, Postbox 1047, Blindern, N-0316 Oslo, Norwayc Faculty of Earth Sciences, Shahid Beheshti University, Tehran, Irand Department of Geology, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Iran

a r t i c l e i n f o a b s t r a c t

Article history:Received 7 December 2009Received in revised form 5 February 2011Accepted 3 March 2011Available online 13 March 2011

Keywords:Sanandaj–Sirjan ZoneIranGraniteU–Pb ageZirconID-TIMS

1367-9120/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.jseaes.2011.03.006

⇑ Corresponding author. Tel.: +47 22 85 66 80; fax:E-mail addresses: shahryar.mahmoudi@gmail.com

corfu@geo.uio.no (F. Corfu).

The Sanandaj–Sirjan Zone (SSZ), which runs parallel to the Zagros fold and thrust belt of Iran, underwenta multistage evolution starting with Neotethys initiation, its subsequent subduction below the Iraniancontinental crust, and eventual closure during convergence of Arabia towards central Iran. Plutonic com-plexes are well developed in the northern part of the SSZ and we have dated a number of them by ID-TIMS U–Pb on zircon. The new data record the following events: a Mid Jurassic period that formed theBoroujerd Plutonic Complex (169 Ma), the Astaneh Pluton (168 Ma) and the Alvand Pluton (165 Ma); LateJurassic emplacement of the Gorveh Pluton (157–149 Ma); Mid Cretaceous (109 Ma) formation of a I-typephase in the Hasan Salary Pluton near Saqqez, followed by Early Paleocene (60 Ma) intrusion of A-typegranite in the same pluton; and the youngest intrusive event recorded so far in the SSZ with the intrusionof granite in the Gosheh–Tavandasht Complex near Boroujerd at 34.9 Ma. These different events reflectspecific stages of subduction-related magmatism prior to the eventual Miocene collision between thetwo continental blocks.

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1. Introduction

The NW-trending Zagros Mountains in Iran are a member of theAlpine–Himalayan orogenic belt and represent one of the youngestcontinental collision zones on Earth. Their development was char-acterized by a protracted magmatic history in an Andean-type set-ting culminated by collision during closure of the Neotethys Oceanbetween Arabia and Eurasia, a process that is still active today (e.g.Berberian, 1995; Golonka, 2004).

The Zagros Orogen is divided into the external Zagros Fold andThrust Belt, and the internal Sanandaj–Sirjan Zone (SSZ) flanked bythe Tertiary Urumieh-Dokhtar volcanic arc (inset in Fig. 1). The SSZis located on the foreland side of the Zagros Mountains and evolvedthrough processes related to the Neotethys initiation and closureaccompanied by Late Jurassic to Eocene calc-alkaline magmatism(Berberian and King, 1981; Alavi, 1994; Mohajjel et al., 2003; Agardet al., 2005; Ghasemi and Talbot, 2006). The intrusions are welldeveloped, and it is possible to determine a relative chronologywhich in turn allows us to estimate the duration of the orogeny(Witt and Davy, 1997; Mendes et al., 2006).

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+47 22 85 42 15.(S. Mahmoudi), fernando.

Previous radiometric age constraints on the timing of plutonismin the SSZ are few and in part controversial. This TIMS U–Pb zirconstudy was therefore undertaken in order to reduce the gaps in theage coverage and to address some of the most pressing questionson the genesis of the intrusive in SSZ.

2. Sanandaj–Sirjan Zone

The SSZ is characterized by dextral transpression (Mohajjel andFergusson, 2000; Mohajjel et al., 2003; Sarkarinejad and Azizi,2008) and represents the metamorphic belt of the Zagros orogen(Fig. 1) extending for 1500 km from Sirjan in the southeast to San-andaj in the northwest of Iran (Mohajjel and Fergusson, 2000).

The northeast boundary of the SSZ, which separates it fromUrumieh-Dokhtar volcanic arc, is recognized by several youngdepressions and locally Mesozoic steep dextral strike-slip faults(Stöcklin, 1974). The Zagros main thrust, referred to as the locationof the Neotethys suture zone (e.g. Berberian and King, 1981;Mohajjel et al., 2003; Agard et al., 2005), forms the south-westernboundary separating the SSZ from the Zagros Fold and Thrust Belt.

The SSZ consists of Paleozoic continental sediments, subse-quently overlain by sedimentary and volcanic rocks during theTriassic–Jurassic opening of the Neotethys (Alavi, 1994; Mohajjelet al., 2003) and variously metamorphosed (Baharifar et al.,2004). The SSZ is subdivided into two parts. The southeastern part

Fig. 1. Simplified geological map of the northern Sanandaj–Sirjan Zone with sample locations. Inset highlights the main tectonic features of the Zagros orogen (based onStöcklin and Setudinia, 1972); the Sanandaj–Sirjan Zone (SSZ) is indicated by the grey domain and the magmatic complexes of the Urumieh-Dokhtar volcanic arc (UDva) areshown in black.

S. Mahmoudi et al. / Journal of Asian Earth Sciences 41 (2011) 238–249 239

comprises Paleozoic metamorphic rocks of relatively high meta-morphic grade that were deformed and metamorphosed in theMiddle to Late Triassic (Berberian, 1995).

The northwestern SSZ consists mostly of metamorphosed andcomplexly deformed sedimentary rocks associated with granitoidplutons, both deformed and undeformed, as well as widespreadMesozoic volcanic rocks. The oldest rocks are medium and high-grade metamorphic rocks, such as garnet schist, staurolite schist,andalusite schist and sillimanite schist, and low- to very low-grademetamorphic rocks, slates and phyllites (Baharifar et al., 2004).Ammonites found in the slates at the top of the metamorphic se-quence in the Hamadan area indicate early Mid-Jurassic ages forthe low-grade metamorphic rocks. The northwestern SSZ was de-formed in the Late Jurassic–Cretaceous.

The Iran micro-continent was part of Gondwana before its sep-aration in the Late Paleozoic by intra-continental rifting thatformed the Neotethys along the Zagros suture (Stöcklin, 1974;Berberian and King, 1981; Sengör, 1990; Mohajjel et al., 2003).These events caused continental rift volcanism, sin-extensionsedimentation (Berberian and King, 1981), Mid Triassic volcanismand volcano-sedimentary deposition and extrusion of Late Triassic

pillow lavas in the SSZ. Subduction of the Neotethys oceanic crustunder the south-western part of the Iranian micro-continentstarted in the Late Triassic (Berberian, 1983), Late Triassic–Early Jurassic (Berberian and King, 1981; Arvin et al., 2007;Davoudzadeh and Schmidt, 1984), Middle Jurassic (Agard et al.,2005) or Late Jurassic to Cretaceous time (Mohajjel et al., 2003).Subduction caused Late Jurassic–Early Cretaceous volcanism andplutonism, greenschist facies metamorphism and the developmentof tight folds with S-SW dipping axial planes (Mohajjel et al., 2003).In the Late Cretaceous, the collision of the passive margin of theArabian plate with an island arc in the SW of the SSZ caused obduc-tion and emplacement of ophiolites on the northeastern margin ofthe Arabian plate (Berberian, 1983; Mohajjel et al., 2003; Agardet al., 2005). Dextral transpressional movements and obliquesubduction characterize the Late Cretaceous convergence his-tory (Mohajjel and Fergusson, 2000; Sheikholeslami et al., 2008;Sarkarinejad and Azizi, 2008; Agard et al., 2005). In the LateCretaceous, subduction-related magmatism shifted 300 km to theNE of the SSZ and Paleocene–Eocene volcanism started in theUrumieh-Dokhtar volcanic arc (Berberian, 1981; Berberian et al.,1982; Omrani et al., 2008). Final closure of the Neotethys Ocean

240 S. Mahmoudi et al. / Journal of Asian Earth Sciences 41 (2011) 238–249

occurred in the Miocene (Berberian, 1983; Mohajjel et al., 2003), orLate Mesozoic (Sheikholeslami et al., 2008) or Late Eocene (Agardet al., 2005) by collision of the central Iranian plateau with theNE margin of the Arabian plate.

The major structures in the SSZ formed during three separateevents: (1) subduction along the active margin of Central Iran atthe northeastern margin of the Neotethys, (2) ophiolite obductionalong the northeastern margin of Neotethys, and (3) continentalcollision of Arabia and Central Iran. Structures associated with sub-duction are characterized by the development of intense foldingwith south–southwest vergence and greenschist facies metamor-phism. The main effects of the collision have been thrusting,including out-of sequence thrusting, along the older obductionzone with incorporation of Cenozoic successions into the imbricatesystem and development of blind thrusting and folding in theZagros Fold and Thrust Belt (Alavi, 1994; Berberian, 1995; Mohajjelet al., 2003). The collision deformed Cambrian to Holocene rocks(e.g., Sepehr and Cosgrove, 2005). The Phanerozoic sequences arefolded and faulted above the crystalline Precambrian basement(Fig. 1). Cenozoic sedimentary rocks, with a stratigraphic thicknessof up to 10 km, are folded into simple kilometer-scale anticlinesand synclines (Stöcklin, 1968).

A number of granitoid complexes occur along the northwesternSSZ, the major ones near Boroujerd and Hamadan and furthernorth around Sanandaj and south of Gorveh (Fig. 1). Previousgeochronology using the K–Ar and Rb–Sr methods on the Astaneh,Alvand, Gorveh and Boroujerd granitoid complexes produced datesranging from 135 to 38 Ma (Bellon and Braud, 1975; Valizadeh andCantagrel, 1975; Masoudi, 1997; Masoudi et al., 2002; Baharifaret al., 2004) whereas zircon U–Pb dating yielded uniform olderages of around 170 Ma for the Boroujerd Complex (Ahmadi Khalajiet al., 2007). Other parts of the SSZ at Sanandaj and Saqqez lackradiometric ages altogether.

2.1. Saqqez, Sanandaj and Gorveh

The northernmost parts of the SSZ are dominated by a 2000–3000 m thick sequence of shallow-marine detrital sediments over-lain by mud- and siltstones interbedded with volcanic rocks(Eftekharnejad, 1981; Zahedi et al., 1985). Permian schist and marbleare the oldest exposed rocks (Azizi and Moinevaziri, 2009). TheMesozoic sequence consists of sandy limestone, shale, and crystal-line limestone intercalated with Jurassic volcanic rocks, and Creta-ceous basic to intermediate volcanic rocks interbedded with darkgrey shale (Fig. 1). The largest (50 km2) plutonic mass in the Saqqezarea is the Hasan Salary Pluton (Hariri, 2003), which has a well devel-oped contact metamorphic aureole to the surrounding volcano-sed-imentary rocks. Sepahi and Athari (2006) distinguished two phases:G1 are granites to syenites of peralkaline affinity, showing within-plate characteristics on discrimination diagrams, and fitting the gen-eral classification of A-type granites; G2 comprises monzogranite,granodiorite and tonalite, containing abundant microgranular en-claves, and of subalkaline and metaluminous affinity fitting the clas-sification of volcanic arc I-type granites.

The area west of Sanandaj comprises sandy shale with lime-stone lenses and sandy limestone interbedded with Cretaceous–Paleocene volcanics, and is locally unconformably overlain by ared basal conglomerate of Eocene age, which, in turn, is conform-ably overlain by Oligocene limestone. The Gorveh region comprisesTriassic–Jurassic low-grade metamorphic rocks, including spilite,basalt, andesite and rarely rhyolite with interbedded sedimentarylenses of mudstone and silt. In this sequence there are no diagnos-tic fossils. These rocks are intruded by gabbroic to granitic igneousrocks, which are weakly sheared and metamorphosed along NW-trending faults. The Gorveh Plutonic Complex comprises a seriesof bodies trending from south to west of Gorveh and ranging from

gabbro to granite. Hossiny (1999) indicated that these plutons be-long to the alkaline to tholeiitic series. Torkian et al. (2008) de-scribed a section of this complex and presented geochemical datawhich indicate that the rocks formed in a subduction setting frommixed mantle and crustal sources.

Eocene non-metamorphic sedimentary rocks and Neogene vol-canic rocks (Hossiny, 1999) locally cover this area. Their sedimentsinclude conglomerate with good rounding and sorting, calcareousnummulitic sandstone and volcanic breccia.

2.2. Hamadan

The Hamadan area is underlain by Mesozoic meta-sedimentaryrocks and has been affected by a complex metamorphic and tec-tonic history. Regional metamorphism reached its peak beforethe Upper Cretaceous (Majidi and Amidi, 1980; Baharifar et al.,2004). Low grade metamorphic rocks are mainly located east ofHamadan, the metamorphic grade increasing toward the west withslates and phyllites giving way to staurolite ± garnet schist, andalu-site ± sillimanite ± garnet schists and paragneiss.

The more than 400 km2 Alvand batholith (Fig. 2a) includesmainly porphyroid and fine-grained granitoid rocks with minorgranite, tonalite, diorite and gabbro, pegmatite and quartz veins(Sepahi, 1999; Baharifar et al., 2004). The granitic rocks show acalc-alkaline nature, while gabbros are rather tholeiitic. This differ-ence suggests that there could be two magmatic endmembers: (1)crustal anatectic magmas generating S-type granite and relatedrocks, and (2) M-type magmas generating a gabbro–diorite–tona-lite suite and mafic-intermediate dykes. There are many NE-trend-ing syn-plutonic microdioritic dykes that locally are disaggregatedto mafic microgranular enclaves (Sepahi, 1999). The plutons aresurrounded by a well developed contact aureoles with andalu-site–sillimanite–cordierite hornfels, cordierite hornfels, garnetandalusite hornfels and cordierite schist.

2.3. Boroujerd

The area consists of regionally metamorphosed supracrustalrocks, locally covered by young Quaternary sediments. The leastmetamorphosed rocks are meta-volcanic flows and tuffs, meta-cherty limestone, meta-sandstone, slate and phyllite of pre-Jurassic age (Mohajjel, 1997).

Granitic bodies are widespread throughout the area and threedifferent units can be identified: the Astaneh Pluton, the BoroujerdPlutonic Complex and the Tavandasht–Gosheh Complex.

The Astaneh Pluton is composed mainly of fine-grained equi-granular granodiorite, commonly hosting xenoliths (Fig. 2b) andcut by aplite dykes (e.g., Tahmasbi et al., 2010).

The Boroujerd Plutonic Complex is the biggest intrusion in thearea. It is a NW-trending elongated body covering an area of200 km2. The dominant phase is granodiorite (Ahmadi Khalajiet al., 2007), showing a distinct foliation related to the orientationof minerals, especially biotite. The granodiorite is generally alteredsuch that biotite is deformed and altered to chlorite, or to aggre-gates of titanite, prehnite, muscovite, opaques and quartz, andgreen amphibole is transformed into biotite, chlorite, epidote andprehnite. Subsidiary monzogranite occurs as scattered bodies andtends to be less altered. Quartz–diorite forms small stocks withgenerally sharp contacts to the granodiorite (Fig. 2c). They standout topographically because they are much less altered than grano-diorite. Pegmatites are abundant, especially in the contact aureole(Masoudi et al., 2002; Esmaeily et al., 2009). They vary from a fewmeters to tens of meters in length and a few meters in width. Thecontact metamorphic aureole, best developed along the northernmargin of the complex, includes cordierite–andalusite and cordier-ite–sillimanite hornfelses.

a

b

c

Fig. 2. (a) Overview of the Alvand Pluton at Hamadan. (b) Angular mafic xenolithsin Astaneh Pluton. (c) Sharp contact between quartz–diorite dykes and hostgranodiorite in the Boroujerd Plutonic Complex.

S. Mahmoudi et al. / Journal of Asian Earth Sciences 41 (2011) 238–249 241

The Tavandasht–Gosheh Complex comprises three main smallbodies of monzogranite to monzodiorite. The rocks are calc-alkaline, light in color, fine to coarse-grained, and with a granulartexture. They are also fresh, unaltered and used as building stone.

3. Sample descriptions and U–Pb geochronology

3.1. Analytical procedure

Major and trace element analyses of most of the dated samples(Table 1) were obtained at Activation Laboratories Ltd., Ancaster,

ON, Canada, by fusion ICP-MS. The accuracy of major elements is0.01%, for most trace elements it is better than 10 ppm and forthe rare earth elements (REE) it is 0.1 ppm or better.

The U–Pb analyses (Table 2) were carried out by ID-TIMS at theUniversity of Oslo. The rocks were crushed and pulverized in a jawcrusher and hammer mill and the heavy minerals concentratedwith a succession of Wilfley table flotation, free fall and high gra-dient magnetic separation and methylene iodide density separa-tion. Further selection was carried out by hand-picking under abinocular microscope and mechanical abrasion (Krogh, 1982) to re-move discordant domains. Some fractions were also subjected tochemical abrasion based on Mattinson (2005) but followingapproximately the procedure of Schoene et al. (2006) with anannealing stage of 3 days at 900 �C, a partial dissolution step withHF (+HNO3) at 194 �C overnight, and a 2 h hot plate step in 6 N HClafter removal of the solution and some rinsing. The dissolution wascarried out following Krogh (1973) as described in Corfu (2004) butusing a mixed 202Pb–205Pb–235U spike. The data are calculatedusing the decay constants of Jaffey et al. (1971) and the Isoplot pro-gram of Ludwig (2003).

3.2. Hasan Salary Pluton, Saqqez: samples SS-100 and SS-101

The Hasan Salary Pluton is composed mainly of granodiorite togranite, generally coarse-grained (>2 mm), and characterized by agranular texture with perthitic alkali feldspar (35–40%), plagio-clase (albite-oligoclase) (30–40%), quartz (15–25%), biotite (5%)and amphibole (<5%) plus accessory zircon. The two samples se-lected for U–Pb dating include SS-100, an altered leucocratic gran-ite collected about 1 km north of Hasan Salary Village, some 20 kmsouth of Saqqez, and SS-101, taken about 3.5 km further south, rep-resenting a fresher, less altered granite, with accessory titanite andfluorite. The two rocks are calc-alkalic and peraluminous but theyhave quite distinct trace element characteristics with crossingpatterns in REE and spider diagrams (Figs. 3 and 4). This reflectshigher abundances of LILE but lower abundances of HREE and Pin SS-101 than in SS-100. The latter also has a volcanic arc affinity(Fig. 5) in contrast to the within-plate character of sample SS-101.

Zircons of sample SS-100 are mostly pale brown and euhedral,although some grains show marginal pitting. They have numerousscattered inclusions of other minerals. Three of the four zircon frac-tions (Table 2) yield overlapping concordant data points defining aconcordia age of 108.76 ± 0.30 Ma (Fig. 6a).

The zircon crystals of sample SS-101 are colorless and euhedral,but occur mostly as fractured or broken fragments. Three analysesare identical and concordant yielding a concordia age of59.78 ± 0.18 Ma (Fig. 6b).

3.3. Gorveh Plutonic Complex: samples SS-107, SS-109 and SS-112

Three samples were analyzed from this plutonic complex, andchemical analyses are available for two of them. Sample SS-107is a granite collected approximately 45 km west of Gorveh fromthe westernmost extension of the Gorveh Plutonic Complex. Therock is composed of alkali feldspar (30–40%), plagioclase (albite-andesine) (25–30%), quartz (30–45%) and biotite (10%). The secondsample SS-109 is a monzonite collected 2 km WNW of Mayhamvillage. The rock has a granular texture and a simple mineralogyof alkali feldspar (30–35%), plagioclase (45–50%), amphibole (10–15%) and biotite (<5%). The third sample SS-112 is a gabbro taken5 km WNW of Mayhem village (some 10 km south of Gorveh)and consisting mainly of plagioclase (50–55%), clinopyroxene (5–10%), amphibole (5–10%), and intercumulus micrographic inter-growths and quartz (5–15%). The monzonite is calc-alkalic andthe gabbro tholeiitic, both are peraluminous and have moderatelyfractionated parallel REE patterns, but with a marked decrease in

Table 1Major (%) and trace element abundances for plutonic rocks of the Sanandaj–Sirjan Zone.

Sample SS-100 SS-101 SS-109 SS-112 SS-119 SS-130 SS-135 SS-136 Detection limit

SiO2 67.64 71.81 71.56 51.24 65.12 60.77 60.02 63 0.01Al2O3 14.6 14.16 14.76 17.39 16.31 17.69 15.41 15.7 0.01Fe2O3(T) 4.25 2.47 1.88 9.68 5.16 5.41 6.74 6.13 0.01MnO 0.047 0.016 0.034 0.153 0.07 0.08 0.113 0.108 0.001MgO 1.05 0.33 0.57 5.47 1.16 1.71 4.18 2.57 0.01CaO 4.43 0.73 3.26 9.78 1.71 2.96 4.57 4.76 0.01Na2O 3.07 4.05 5.15 3.61 2.84 3.65 2.75 2.52 0.01K2O 1.55 5.27 1.34 0.69 5.51 3.05 2.17 2.78 0.01TiO2 0.471 0.363 0.371 1.517 0.668 0.631 0.644 0.533 0.001P2O5 0.13 0.06 0.08 0.25 0.15 0.15 0.1 0.1 0.01LOI 2.15 0.56 0.72 0.06 0.91 3.36 2.35 1.5

Total 99.38 99.81 99.73 99.82 99.62 99.45 99.05 99.7 0.01

Sc 9 2 9 29 11 14 21 20 1Be 1 5 3 1 2 3 2 2 1V 69 29 21 206 75 85 153 105 5Cr 260 380 270 370 300 120 400 360 20Co 8 4 4 30 11 11 19 16 1Ni 140 190 150 150 160 60 170 170 20Cu 30 10 10 50 20 20 20 20 10Zn <30 50 40 80 70 60 80 70 30Ga 14 18 20 17 20 19 17 16 1Ge 1.7 1 1.1 1.6 1.8 1.6 1.9 2 0.5As <5 8 <5 <5 7 8 7 10 5Rb 36 212 38 15 198 131 88 120 1Sr 340 399 294 303 160 378 293 176 2Y 19.4 8.4 33.5 24.5 27.3 13.2 21.4 23.7 0.5Zr 156 295 348 152 257 272 140 118 1Nb 15.6 93.1 40.8 16.1 28.3 16.6 15.1 12.2 0.2Mo 3 4 3 3 3 <2 3 3 2Ag <0.5 0.5 0.6 <0.5 <0.5 0.5 <0.5 <0.5 0.5In <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 0.1Sn 2 2 5 2 6 2 4 3 1Sb <0.2 <0.2 <0.2 <0.2 <0.2 0.4 0.4 <0.2 0.2Cs 1.1 4.4 0.5 0.4 10.4 8.7 6.3 12.4 0.1Ba 730 542 129 181 800 693 270 300 3La 42 69.4 17 17.4 56.5 14 88.8 31.9 0.05Ce 71.4 118 44.8 36.8 113 27.2 198 65.9 0.05Pr 7.56 11.6 6.49 4.73 12.9 3.43 24 7.46 0.01Nd 24.1 30.1 25.4 18.3 42.5 12.8 81.2 24.7 0.05Sm 4.62 4.12 6.24 4.69 8.78 3.21 14.4 5.21 0.01Eu 1.02 0.686 1.16 1.46 1.27 1.37 3.11 0.79 0.005Gd 4.16 2.58 6.14 5.47 7.53 3.03 9.62 4.92 0.01Tb 0.58 0.33 0.99 0.82 0.99 0.42 1.08 0.73 0.01Dy 3.37 1.67 5.89 4.9 5.19 2.43 5.28 4.35 0.01Ho 0.74 0.34 1.28 1.03 1.03 0.52 0.96 0.91 0.01Er 2.34 1.03 3.84 3.14 3.01 1.63 2.73 2.79 0.01Tm 0.359 0.161 0.58 0.472 0.44 0.243 0.393 0.432 0.005Yb 2.49 1.11 3.87 3.08 2.84 1.56 2.54 2.93 0.01Lu 0.431 0.184 0.607 0.477 0.435 0.245 0.4 0.47 0.002Hf 4.2 8.3 9.3 3.6 6.9 6.7 4.2 3.5 0.1Ta 1.14 7.92 2.81 1.05 1.71 0.84 1.21 1.15 0.01W <0.5 0.9 <0.5 <0.5 <0.5 <0.5 3.5 1.6 0.5Tl 0.13 0.47 0.12 0.06 0.74 0.54 0.39 0.49 0.05Pb <5 11 6 <5 29 11 14 17 5Bi <0.1 <0.1 <0.1 <0.1 0.1 0.2 0.1 <0.1 0.1Th 12.1 92.9 21.6 2.51 24.1 2.79 41.1 10.2 0.05U 1.95 5.51 3.25 0.66 2.57 1.72 3.01 3.05 0.01

242 S. Mahmoudi et al. / Journal of Asian Earth Sciences 41 (2011) 238–249

La and a much more pronounced negative Eu anomaly in the mon-zonite. In the spider diagram (Fig. 4) the gabbro (SS-112) shows arelatively regular, gently decreasing pattern interrupted only by apositive anomaly for Pb whereas the monzonite has distinct nega-tive anomalies in Ba, P and Ti and positive anomalies especially inTh, U, Nb, Pb, Zr and Sm. The monzonite plots in the within-platefield in Fig. 5 in contrast to the volcanic arc position of the gabbro.

The zircon population in sample SS-107 comprises mainly cleareuhedral prisms. Seven analyses were carried out but six of themwere variously affected by inherited components (Table 2). Theconcordant analysis, supported by a second nearly concordantone yields a Concordia age of 156.5 ± 0.6 Ma (Fig. 6c). Regressionlines calculated through different groups of data project towards

upper intercept ages of between 484 and 1786 Ma (Fig. 6d). Oneof the analyses suggests a Devonian age (Table 2, not plotted),but because of the extremely low content of U its precision is verypoor. A fraction of titanite yields a younger age of about 136 Ma(Fig. 6d), but the position of the data point may just reflect partialresetting at some later point in time.

Zircon of monzonite sample SS-109 occurs as colorless euhedralcrystals either long or short prismatic and also as broken frag-ments. Six analyses are clustered close to the Concordia curveyielding a 206Pb/238U age of 151.0 ± 0.2 Ma (Table 2; Fig. 6e). Thefact that these analyses have mostly higher 207Pb/206Pb ages ofaround 158 Ma, and one additional analysis plots above the maingroup, could be taken to indicate that the actual age is somewhat

Table 2U–Pb data for plutonic rocks of the Sanandaj–Sirjan Zone.

Characteristicsa Weight(ug)

U(ppm)

Th/Ub

Pbcc

(pg)

206Pb/204Pbd 207Pb/235Ue ±2r[abs]

206Pb/238Ue ±2r[abs]

rho 207Pb/206Pbe ±2r[abs]

206Pb/238Ue ±2r[abs]

207Pb/235Ue ±2r 207Pb/206Pbe

[age in Ma]

SS-100 Leucogranite, Hasan Salary Pluton, Hasan Salary Village, Saqqez (36�03020’’/46�1700000)Z sp[1] 1 351 0.73 1.0 400 0.1147 0.0023 0.016710 0.000095 0.37 0.04978 0.00094 106.8 0.6 110.2 2.1 185Z sp-eq [4] 10 122 0.74 1.4 971 0.1132 0.0014 0.017072 0.000171 0.76 0.04809 0.00040 109.1 1.1 108.9 1.3 104Z sp-eq [11] 10 252 0.90 2.5 1075 0.1134 0.0007 0.017000 0.000047 0.45 0.04838 0.00028 108.7 0.3 109.1 0.7 118Z sp-eq [3] 2 273 0.58 1.0 586 0.1136 0.0019 0.017106 0.000150 0.58 0.04817 0.00064 109.3 0.9 109.3 1.7 108

SS-101 Granite, Hasan Salary Pluton, Lakzy Dam, Saqqez (36�0102500/46�1700000)Z fr[5] 4 258 1.71 1.7 375 0.0611 0.0013 0.009329 0.000041 0.40 0.04751 0.00097 59.86 0.26 60.23 1.27 75Z sp-eq [5] 26 106 1.04 2.1 786 0.0605 0.0007 0.009302 0.000051 0.44 0.04720 0.00048 59.68 0.33 59.68 0.65 59Z Ip [3] 11 286 1.22 2.1 891 0.0607 0.0006 0.009312 0.000062 0.49 0.04729 0.00044 59.75 0.40 59.86 0.61 64

SS-107 Fine-grained white granite, Gorveh Pluton, Sofy Abad Village (35�0503000/47�1600000)Z spCA[1] 1 4 0.93 1.4 31 0.5025 0.3159 0.066119 0.002771 0.77 0.05512 0.03292 413 17 413 194 —Z sp in [1] 31 73 0.74 2.2 3007 0.3493 0.0012 0.046609 0.000104 0.70 0.05436 0.00013 293.7 0.6 304.2 0.9 386Z sp CA [4] 40 238 0.55 12.9 1774 0.2765 0.0026 0.037798 0.000342 0.97 0.05306 0.00012 239.2 2.1 247.9 2.0 331Z spCA[14] 22 922 0.55 2.7 17588 0.3579 0.0009 0.036743 0.000078 0.96 0.07064 0.00005 232.6 0.5 310.6 0.6 947Z sp in [12] 7 1491 0.82 2.2 9079 0.2188 0.0005 0.030173 0.000064 0.90 0.05259 0.00006 191.6 0.4 200.9 0.4 311Z spCA[1] 1 1054 0.75 0.9 1924 0.1660 0.0010 0.024580 0.000096 0.62 0.04897 0.00023 156.5 0.6 155.9 0.9 146Z sp CA [1] 7 526 0.53 1.6 3491 0.1683 0.0006 0.024688 0.000074 0.69 0.04945 0.00013 157.2 0.5 158.0 0.5 169T Ibr [6] 20 219 0.87 26.0 243 0.1451 0.0022 0.021291 0.000057 0.21 0.04943 0.00072 135.8 0.4 137.6 1.9 168

SS-109 Monzonite, Gorveh Pluton, Mayham Sofla Village (35�0305100/47�5204900)Z Ip [12] 9 101 0.81 1.5 920 0.1625 0.0015 0.023941 0.000100 0.48 0.04922 0.00040 152.5 0.6 152.9 1.3 158Z fr[15] 19 353 0.73 2.4 4154 0.1608 0.0005 0.023694 0.000057 0.77 0.04921 0.00009 151.0 0.4 151.4 0.4 158Z sp [21] 29 384 0.74 4.2 3990 0.1609 0.0004 0.023707 0.000052 0.83 0.04922 0.00008 151.0 0.3 151.5 0.4 159Z lpin CA[13] 54 225 0.60 2.2 8205 0.1604 0.0004 0.023648 0.000050 0.88 0.04921 0.00006 150.7 0.3 151.1 0.4 158Z IpinCA [11] 22 311 0.60 2.1 4892 0.1609 0.0005 0.023751 0.000064 0.73 0.04913 0.00011 151.3 0.4 151.5 0.5 154Z Ip [6] 9 308 0.59 3.7 1141 0.1596 0.0010 0.023652 0.000090 0.60 0.04893 0.00023 150.7 0.6 150.3 0.8 145Z Ip [12] 22 232 0.69 3.9 1955 0.1608 0.0006 0.023710 0.000056 0.67 0.04918 0.00013 151.1 0.4 151.4 0.5 156

SS-112 Gabbro, Gorveh Pluton, Mayham Olya Village (35�0304000/47�5004600)Z Ip [6] 143 147 0.74 2.1 14874 0.1587 0.0004 0.023433 0.000051 0.93 0.04912 0.00004 149.3 0.3 149.6 0.3 153Z fr[1] 66 77 0.56 1.5 5050 0.1586 0.0005 0.023382 0.000066 0.71 0.04919 0.00011 149.0 0.4 149.5 0.5 157Z fir [10] 89 106 0.78 3.8 3683 0.1586 0.0004 0.023409 0.000054 0.82 0.04913 0.00008 149.2 0.3 149.5 0.4 154

SS-119 Porphyritic granite to granodiorite, Alvand Pluton, dominant phase, Hamedan, Gang namh Village (34�4504500/48�2602100)Z lp CA [20] 53 243 0.14 1.6 12919 0.1771 0.0004 0.025979 0.000055 0.92 0.04943 0.00005 165.3 0.3 165.5 0.4 168Z lp in CA [33] 34 908 0.27 2.7 18794 0.1769 0.0005 0.025976 0.000061 0.95 0.04939 0.00004 165.3 0.4 165.4 0.4 167Z lp in CA [16] 34 291 0.14 1.6 10296 0.1763 0.0005 0.025866 0.000073 0.89 0.04942 0.00007 164.6 0.5 164.8 0.5 168Z lp CA [14] 127 114 0.15 2.1 11041 0.1765 0.0005 0.025935 0.000058 0.90 0.04935 0.00006 165.1 0.4 165.0 0.4 165Z lp CA [12] 30 226 0.31 1.7 6343 0.1777 0.0005 0.026012 0.000058 0.83 0.04956 0.00007 165.5 0.4 166.1 0.4 174

SS-124 Diorite, Alvand Pluton, Hamedan, Abas Abad Village (34�4503200/48�2603100)Z fr [14] 98 703 0.92 19.1 5887 0.1767 0.0005 0.025925 0.000062 0.94 0.04943 0.00005 165.0 0.4 165.2 0.4 168Z fr [33] 106 1075 1.25 24.8 7441 0.1760 0.0005 0.025834 0.000069 0.96 0.04940 0.00004 164.4 0.4 164.6 0.5 167Z fr [15] 108 927 1.27 26.6 5939 0.1711 0.0004 0.025106 0.000057 0.94 0.04943 0.00005 159.8 0.4 160.4 0.4 168

SS-130 Altered hornblende granodiorite, Boroujerd Plutonic Complex (33�4103800/49�0604400)Z lp [44] 127 526 0.38 5.6 19809 0.1824 0.0005 0.026685 0.000062 0.97 0.04959 0.00003 169.8 0.4 170.2 0.4 176Z sp CA [12] 61 469 0.36 1.9 24618 0.1821 0.0005 0.026625 0.000061 0.94 0.04959 0.00004 169.4 0.4 169.8 0.4 176Z lp [15] 123 815 0.38 10.2 16394 0.1814 0.0005 0.026570 0.000066 0.97 0.04952 0.00004 169.0 0.4 169.3 0.4 173Z sp CA [24] 44 651 0.40 4.3 11035 0.1810 0.0005 0.026501 0.000064 0.93 0.04954 0.00005 168.6 0.4 168.9 0.4 173Z lp CA [11] 37 579 0.34 6.5 5438 0.1801 0.0005 0.026291 0.000063 0.92 0.04968 0.00006 167.3 0.4 168.1 0.4 180

SS-135 Monzogranite, Boroujerd Plutonic Complex, near mobile antenna, Nezam Abad (33�4300800/49�1201600)Z lp CA [17] 55 414 0.29 1.8 21448 0.1819 0.0005 0.026627 0.000062 0.95 0.04954 0.00004 169.4 0.4 169.7 0.4 174Z sp CA [15] 78 339 0.30 2.1 21554 0.1820 0.0005 0.026661 0.000063 0.89 0.04951 0.00006 169.6 0.4 169.8 0.4 172

(continued on next page)

S.Mahm

oudiet

al./Journalof

Asian

EarthSciences

41(2011)

238–249

243

high

erth

an151

Ma,

but,

onth

eoth

erh

and,

the

generally

goodqu

alityof

the

analyzed

zircongrain

san

dth

efact

that

these

in-

cluded

bothm

echan

icallyan

dch

emically

abradedfraction

s,su

g-gest

that

the

positionof

the

datapoin

tsis

afu

nction

of2

31Pa

excessrath

erth

anPb

loss(e.g.

Parrishan

dN

oble,2003).

Hen

ce151.0

±0.2

Ma

iscon

sideredto

beth

etim

eof

crystallizationof

the

mon

zonite.

The

zirconpopu

lationin

the

gabbrosam

pleSS-112

consists

mostly

ofstubby,pale

brown

euh

edralcrystals,locallyw

ithh

ollowcavities

and

alson

um

erous

darkin

clusion

s.Three

analyses

overlap,alth

ough

asin

the

previous

caseth

eyten

dto

plotto

the

right

ofC

oncordia

and

yielda

mean

20

6Pb/ 23

8Uage

of149.2

±0.2

Ma

(Fig.6f).

3.4.Alvand

Pluton,Ham

adan:sam

plesSS-119

andSS-124

Sample

SS-119represen

tsfresh

porphyritic

granite

togran

odi-orite

with

2–4cm

long

crystalsof

feldsparin

afi

ner

grained

crys-tallin

em

atrix.It

stems

fromth

eh

istoricalplace

Gan

gN

ameh

inA

lvand.

The

rockcon

sistsof

plagioclase(20–40%

),qu

artz(10–

15%),

alkalifeldspar

(30–40%)

and

biotite10–20%

with

zircon,

tourm

aline

and

kyanite

asaccessory

min

erals.Th

erock

is

Table 2 (continued)

Characteristicsa Weight(ug)

U(ppm)

Th/Ub

Pbcc

(pg)

206Pb/204Pbd 207Pb/235Ue ±2r[abs]

206Pb/238Ue ±2r[abs]

rho 207Pb/206Pbe ±2r[abs]

206Pb/238Ue ±2r[abs]

207Pb/235Ue ±2r 207Pb/206Pbe

[age in Ma]

Z eq CA [24] 86 399 0.30 2.5 23305 0.1821 0.0005 0.026656 0.000060 0.96 0.04954 0.00004 169.6 0.4 169.8 0.4 173

SS-136 Granite, Astaneh Pluton (33�4800400/49�1900800)Z lp in [4] 29 447 0.38 21.4 2136 0.5797 0.0017 0.055795 0.000124 0.84 0.07535 0.00013 350.0 0.8 464.3 1.1 1078Z lp [4] 10 503 0.33 3.2 2796 0.1943 0.0006 0.027794 0.000068 0.73 0.05070 0.00011 176.7 0.4 180.3 0.5 227Z sp [7] 20 692 0.31 86.1 287 0.1840 0.0023 0.026717 0.000067 0.22 0.04995 0.00061 170.0 0.4 171.5 2.0 193Z Ip [4] 16 448 0.25 12.7 954 0.1809 0.0008 0.026445 0.000058 0.57 0.04961 0.00017 168.3 0.4 168.8 0.7 177

SS-128 Quartz monzodiorite, Gosheh-Tavandasht Complex, near Abshar village (33�4105800/49�0905000)Z lp CA [7] 10 788 0.67 1.6 1669 0.0350 0.0002 0.005421 0.000017 0.57 0.04677 0.00024 34.85 0.11 34.89 0.21 38Z lp CA [4] 7 849 0.62 1.1 1804 0.0350 0.0002 0.005425 0.000021 0.57 0.04681 0.00023 34.88 0.13 34.94 0.21 39Z sp CA [3] 4 818 0.55 1.3 891 0.0350 0.0004 0.005422 0.000022 0.48 0.04680 0.00041 34.86 0.14 34.92 0.34 39

a Z = zircon; T = titanite; Ip = long prismatic (l/w = >4); sp = short prismatic; eq = equant; fr = fragments, broken prisms; Ibr = light-brown; in = inclusions; CA = treated with chemical abrasion; all the others mechanicallyabraded; [1] number of grains in fraction.

b Th/U model ratio inferred from 208/206 ratio and age of sample.c Total amount of common Pb (initial + blank).d Raw data corrected for fractionation.e Corrected for fractionation, spike, blank and initial common Pb; error calculated by propagating the main sources of uncertainty.

10

10010

10010

10010

100

LaCePr

NdPm

SmEu

GdTb

DyHoEr

TmYb

etirdnohc/kcoR

SS-100

SS-101Hasan

SalaryPluton

SS-109

SS-112

Gorveh

PlutonicComplex

SS-119Alvand

Pluton,Hama

SS-130

SS-136

SS-135

AstanehPluton

BoroujerdPlutonic

Complex

BoroujerdPlutonic

Complex

Fig.3.

REE-pattern

sof

datedgran

itoidrocks

ofth

eSSZ.

The

REE

aren

usin

gch

ondritic

values

ofSu

nan

dM

cDon

ough

(1989).

244S.M

ahmoudi

etal./Journal

ofA

sianEarth

Sciences41

(2011)238–

249

Lu

dan

ormalized

1

10

100

1000

SS-100

SS-101 Hasan Salary Pluton

10

100

SS-109

SS-112

Gorveh Plutonic Complex

1

10

100

1000 SS-119 Alvand Pluton, Hamadan

1

10

100

1000

CsRbBaTh U Nb K LaCePbPr Sr P NdZrSmEu Ti Dy Y YbLu

SS-130SS-136

SS-135

Astaneh PlutonBoroujerd Plutonic Complex}

eltnaM

evitimirP/

kcoR

Fig. 4. Spider diagram with trace element data of dated granitoid and gabbroicrocks of the SSZ. Normalization to Primitive Mantle according to Sun andMcDonough (1989).

10 100 10001

10

100

1000

Rb

Y+Nb

syn-COLG WPG

VAG

ORG

100

101

109

112

130

135136

119

Fig. 5. Diagram of Rb vs. Y + Nb (Pearce et al., 1984) illustrating the geochemicalaffinity of dated plutonic rocks of the SSZ. VAG = volcanic arc granites, ORG = oce-anic ridge granites, WPG = within-plate granites, COLG = collisional granites.

S. Mahmoudi et al. / Journal of Asian Earth Sciences 41 (2011) 238–249 245

calc-alkalic and peraluminous, plots in the volcanic arc field ofFig. 5, it exhibits a fractionated REE pattern with a strong Eu anom-aly (Fig. 3) and, in the spider diagram (Fig. 4) it shows distinct neg-ative anomalies for Ba, Nb, Sr, P and Ti, together with a positiveanomaly for Pb. The second sample SS-124 from the Abas AbadValley is a finer grained hornblende-rich (30–50%) diorite also con-taining plagioclase (35–45%), alkali feldspar (10–15%) and biotite(<5%) with rare zircon and apatite.

Zircon in sample SS-119 occurs as colorless and euhedral pris-matic crystals commonly with inclusions of apatite. Four analyses

yield a concordia age of 165.2 ± 0.2 Ma with an additional pointslightly off presumably because of some inheritance (Fig. 7a).

The zircon population in SS-124 is similar to that in sample SS-119 but they tend to have higher U-contents. The three analysesare spread along a discordia line anchored at 0 Ma yielding anupper intercept age of 167.9 ± 1.2 Ma. This age, however, is identi-cal to the 207Pb/206Pb ages of the analyses of sample SS-119, sug-gesting that the 206Pb/238U age of 165.0 ± 0.4 Ma of the mostconcordant analysis may be more correct (Table 2; Fig. 7b).

3.5. Boroujerd Plutonic Complex: samples SS-130 and SS-135

Sample SS-130 is an altered granodiorite from the BoroujerdPlutonic Complex. It is medium to coarse-grained with plagioclase(30–40%), biotite (10–20%), quartz (25–30%) and alkali feldspar(<20%). Apatite, zircon, allanite and opaques are common accessoryminerals. Plagioclase is sericitised and quartz occurs as anhedralisolated grains and aggregates displaying recrystallisation withundulatory extinction, typical of incipient solid-state deformation.Monzogranite sample SS-135 was collected close to Nezam Abadvillage from the highest point in the Boroujerd Complex. Thisphase occurs as widely scattered small outcrops through thesouthern part of the Boroujerd Complex. The rocks are light in col-or, fine to coarse-grained, with a granular texture in the centre ofthe unit and a porphyritic texture with feldspar megacrysts atthe margin. Mineral assemblages include quartz (30–35%), alkalifeldspar (30–35%), plagioclase (25– 35%), biotite (5–10%) and somesecondary muscovite. Zircon, allanite and apatite are commonaccessory minerals. Quartz intergrowths with K-feldspar and pla-gioclase form micrographic textures and myrmekite, respectively.Large phenocrysts of quartz show undulatory extinction. Perthiticalkali feldspar occurs as euhedral to subhedral crystals. Plagioclaseis commonly zoned. These rocks are calc-alkalic, peraluminous,plot in the volcanic arc field of the Rb vs. Y + Nb diagram (Fig. 5),and show variously fractionated REE patterns with small positive(SS-130) and negative (SS-135) Eu anomalies (Fig. 3). Besides thevariable abundance of the REE, the main distinction observed inthe spider diagram concerns Th and to some degree Zr which have,respectively, negative and positive anomalies in SS-130 and the

149.8

149.4

149.0

0.02332

0.02340

0.02348

0.158 0.159

150

151

152

153

154

0.0236

0.0240

0.1630.159

280

240

200

160

0.020

0.028

0.036

0.044

0.340.260.18

156.2

157.0

157.8

0.0245

0.0247

0.165 0.167 0.169

59.2

59.6

60.0

0.00926

0.00934

0.0620.060

107

108

109

110

111

0.0164

0.0168

0.0172

0.1160.112

SS-100Hasan Salary Pluton

SS-101Hasan Salary Pluton

SS-107Gorveh Plutonic Complex

SS-107Gorveh Plutonic Complex

SS-109Gorveh Plutonic Complex

SS-112Gorveh Plutonic Complex

207Pb/235U

206Pb238U

Three points: Concordia age108.8 ± 0.3 Ma

(2 , decay-const. errors included)MSWD (of conc.) = 0.47

One point: Concordia age

Six points: average 206 238Pb/ U age

156.5 ± 0.6 Ma

151.0 ± 0.2 Ma 149.3 ± 0.2 Ma

(2 , decay-const. errors included)

(2 ) (2 )

MSWD (of conc.) = 2.9

MSWD (of conc.) = 1.6 MSWD (of conc.) = 3.8

Three points: Concordia age

Three points: Concordia age

59.78 ± 0.18 Ma(2 )

MSWD (of conc.) = 0.14

484 Ma

titanite (ca. 136 Ma)

1786 Ma

a b

c d

e f

Fig. 6. Concordia diagrams displaying U–Pb data from the Saqqez and Gorveh regions. Ellipses represent 2r errors. See text for discussion of the data.

246 S. Mahmoudi et al. / Journal of Asian Earth Sciences 41 (2011) 238–249

opposite in SS-135. Both have strong negative anomalies in Nb, Pand Ti (Fig. 4).

Zircon in sample SS-130 is mostly colorless and generally elon-gated crystals with local fracture and inclusions. The five analysesplot close to the Concordia curve but show some dispersion both interms of U–Pb and also of 207Pb/206Pb age. It appears as if there areeffects both from Pb loss as well as from some inheritance. Such ef-fects are less evident in the three intermediate analyses, whichprovide an average 206Pb/238U age of 169.0 ± 1.0 Ma (Fig. 7c).Zircon grains in SS-135 are morphologically very similar to those

in samples SS-130, but the three analyses are more coherent andconcordant defining a concordia age of 169.6 ± 0.3 Ma (Fig. 7d).

3.6. Astaneh Pluton: sample SS-136

The mineral assemblage in this granite includes quartz (25–30%), alkali feldspar (30–35%), plagioclase (commonly zoned, 35–40%) and biotite (5–10%). Zircon, allanite and apatite are commonaccessory minerals. The rock is calc-alkalic and peraluminous,showing a volcanic arc affinity (Fig. 5). The REE have a distinct

35.2

35.0

34.8

34.60.00538

0.00546

0.0346 0.0350 0.0354

166

168

170

172

174

176

178

0.0264

0.0272

0.0280

0.180 0.188 0.196

170.2

170.0

169.8

169.6

169.4

0.0266

0.0267

4281.06181.0

206Pb238U

167

168

169

170

0.0263

0.0267

5181.05971.0

160

162

164

166

0.0252

0.0256

0.0260

571.0171.0

One point: 206 238Pb/ U age165.0 ± 0.4 Ma

(2 )164

166

0.0258

0.0260

871.0671.0

165

SS-119Alvand Pluton

SS-130Boroujerd Plutonic Complex

SS-135Boroujerd Plutonic Complex

SS-136Astaneh Pluton

SS-128Gosheh-Tavandasht Complex

SS-124Alvand Pluton

206Pb238U

207Pb/235U

165.2 ± 0.2 Ma

169.6 ± 0.3 Ma

34.87 ± 0.08 Ma

MSWD (of conc.) = 0.49

MSWD (of conc.) = 2.7

MSWD (of conc.) = 0.74

combined Concordia ageand lower intercept ages

Four points: Concordia age

Three points: Concordia age

Three points: Concordia age

(2 , decay-const. errors included)

(2 , decay-const. errors included)

(2 , decay-const. errors included)

Three points: average 206 238Pb/ U age

169.0 ± 1.0 Ma(2 )

MSWD = 3.9

167.5 ± 1.0 Ma

870 Ma

1500 Ma

a b

c d

e f

Fig. 7. Concordia diagrams displaying U–Pb data from the Hamadan and Boroujerd regions. Ellipses represent 2r errors. See text for discussion of the data.

S. Mahmoudi et al. / Journal of Asian Earth Sciences 41 (2011) 238–249 247

negative Eu anomaly and flat heavy REE (Fig. 3). In the spiderdiagram the sample shows characteristics that for most elementsare intermediate between those of SS-130 and SS-135 of theBoroujerd Plutonic Complex (Fig. 4).

The zircon population consists of well developed prisms likethose of samples SS-130, but they contain xenocrystic cores withonly two analyses plotting within error of the concordia curve(Fig. 7e). A regression through three of the analyses yields inter-cepts at 166.7 ± 1.7 and 870 Ma whereas the two youngest analy-ses together with the oldest one define intercept ages of

167.7 ± 1.7 and 1500 Ma. The youngest analysis alone has a concor-dia age of 168.3.0 ± 0.3 Ma. These variations result from not exactlyquantifiable amounts of inheritance, Pb loss, and/or perhaps 207Pbexcess. These effects are taken into account in the rounded esti-mate of 167.5 ± 1.0 Ma.

3.7. Gosheh–Tavandasht Complex: samples SS-128

Sample SS-128 is a quartz–monzodiorite representing theGosheh–Tavandasht Complex. In outcrop scale, the rock is whitish

248 S. Mahmoudi et al. / Journal of Asian Earth Sciences 41 (2011) 238–249

with an average grain size of 2 mm, a granular to porphyritic tex-ture with plagioclase megacrysts, and a mineral assemblage of pla-gioclase (40–50%), biotite (15–20%), green amphibole (10–15%),quartz (<15%) and alkali feldspar (<5%). Plagioclase occurs as zonedeuhedral to subhedral plates, locally altered to sericite, epidote andcalcite. Zircon, titanite and apatite are conspicuous accessory min-erals. Three analyses of zircon yield a concordia age of34.87 ± 0.08 Ma (Fig. 7f).

4. Discussion

The results from this study record events related to the evolu-tion of the SSZ from a Andean-type environment during subduc-tion of the Neotethys to a collisional zone between Arabia andEurasia. The data complement the geological record provided firstof all by the lithologies and structures, and both expands and cor-rects the information accumulated during the previous geochrono-logical studies.

Much of the existing geochronological information is based onK–Ar and Rb–Sr ages, both systems very susceptible to resetting.Because of the complex tectonic and associated thermal evolutionof the Zagros Orogen, several suites of such ages have been par-tially overprinted and reset providing dates of no specific geologi-cal significance, which then have the potential to introducedistortions into tectonic reconstructions (e.g. Fig. 12 in Agardet al., 2005). One example of such partial resetting is the studyby Masoudi et al. (2002) on the Boroujerd Plutonic Complex thatdemonstrates variable resetting of the Rb–Sr systems creating agesof no direct geological significance. In this case the resetting couldbe related to local alteration and faulting, and/or to the emplace-ment of the nearby Late Eocene Gosheh–Tavandasht Complex.Resetting has also severely affected the Rb–Sr and K–Ar dates fromthe Alvand Pluton (Valizadeh and Cantagrel, 1975; Baharifar et al.,2004). Another example is the Gorveh Pluton, which on the basis ofearlier K–Ar dates (Bellon and Braud, 1975, quoted by Torkian et al.(2008)) was considered to represent a Late Eocene (38–40 Ma)intrusion.

The earliest major events recorded in this study, and perhapsthe defining time in the early Mesozoic evolution of the SSZ, isthe intense episode of Jurassic arc magmatism at around 170–165 Ma. This event is reflected by the ages of the Boroujerd Plu-tonic Complex (Ahmadi Khalaji et al., 2007; this study) and the Alv-and Plutonic Complex (this study). In the southern SSZ a gabbro-anorthosite complex was emplaced during the same event (Fazlniaet al., 2007). Mid-Jurassic magmatism was also responsible for thegenesis of granites intruding into and metamorphosing the DehSalm Metamorphic Complex of the Lut Block of East Iran (Mahmo-udi et al., 2010). This synchronous activity all along the SSZ andsurrounding regions can be related to initiation of subduction ofthe Neotethys oceanic crust below the Iranian continental crust(Bagheri and Stampfli, 2008).

Emplacement of the Gorveh Plutonic Complex at 157–149 Mareflects a later stage in the arc magmatic activity, apparently ofmore modest significance. The monzonite is geochemically similarto the younger phase of the Hasan Salary Pluton (SS-101), indicat-ing an A-type affinity. Hence, the gabbros and alkalic rocks of theGorveh Plutonic Complex suggest a genesis by melting of lowercrust, possibly in a back-arc setting.

The oldest phase of the Hasan Salary Pluton is 109 Ma, i.e.Albian, and has been shown by Sepahi and Athari (2006) and ourpresent data to have I-type geochemical characteristics consistentwith continued subduction-related magmatism. This episode ofgranitic formation post-dates deposition of the regionally wide-spread Barremian–Aptian (ca. 125 Ma) Orbitolina limestones,which overlies unconformably earlier Jurassic strata (Agard et al.,

2005), likely reflecting some readjustment in the subductiongeometry and speed. The I-type Urumieh Plutonic Complex, witha somewhat younger K–Ar age of about 100 Ma, is present furthernorth and is also explained in terms of subduction-related magma-tism (Ghalamghash et al., 2009).

The youngest phase of the Hasan Salary Pluton is much youngerat 60 Ma, i.e., Middle Paleocene. It has a geochemical compositionindicating an A-type affinity (Sepahi and Athari, 2006 and ourdata), generally linked to a provenance from melting of lower con-tinental crust. In the present context it is not quite evident whattriggered the formation of these particular rocks which are typicalneither for subduction nor for collisional settings. A possibility isthat it reflects mafic underplating in an extensional back-arc set-ting. In fact the subsequent Eocene volcanic activity in the Uru-mieh-Dokhtar volcanic arc has again arc-type characteristics(Omrani et al., 2008). Ghalamghash et al. (2009) also report theoccurrence of A-type granite with an amphibole K–Ar age of about80 Ma in the Urumieh Plutonic Complex, at the northern end of theSSZ, They suggest a provenance from melting of felsic lower crustalmaterial, suggesting, however, a collisional setting.

The youngest unit dated in this study is the 35 Ma quartz mon-zonite from the Gosheh–Tavandasht Complex near Boroujerd. Thefact that this is the youngest age for magmatism found so far in theSSZ suggests that this was an event of local significance, although italso correlates with the late Eocene volcanism that is widespreadacross much of central and eastern Iran (Omrani et al., 2008).Mazhari et al. (2009) reported a U–Pb age of about 41 Ma for bimo-dal Piranshahr massif in the northern SSZ, and interpret them asindicating a post-collisional setting, a view that is not uncontrover-sial as the main collision is regarded by others to have started onlyin the Miocene (Allen, 2009; Agard et al., 2005.

5. Conclusions

The new ID-TIMS U–Pb data from the SSZ document several ma-jor phases in the evolution of the zone. Mid Jurassic subduction-re-lated magmatism formed the Alvand Plutonic Complex(165.2 ± 0.2 Ma and 165.0 ± 0.4 Ma), the Boroujerd Plutonic Com-plex (169.0 ± 1.0 Ma and 169.6 ± 0.3 Ma), and the neighbouringAstaneh Pluton (167.5 ± 1.0 Ma). A younger Late Jurassic event,possibly in a back-arc setting, formed the Gorveh Plutonic Complex(156.5 ± 0.6 Ma, 151.0 ± 0.2 Ma and 149.3 ± 0.2 Ma). Mid Creta-ceous plutonism is recorded in the I-type part of the Hasan SalaryPluton near Saqqez (108.8 ± 0.3 Ma), whereas the A-type phase ofthe same pluton has an Early Paleocene age of 59.78 ± 0.18 Ma.The youngest, Late Eocene, plutonic rock so far dated in the SSZwas found in the Gosheh–Tavandasht Complex near Boroujerd(34.87 ± 0.8 Ma).

The results are consistentt with the process of subduction ofNeotethys oceanic crust in the Jurassic and subsequent conti-nent–continent collision in the Miocene.

Acknowledgements

We acknowledge financial support by the Iranian GovernmentResearch Agency to the first author during his research at Oslo Uni-versity. Constructive reviews and comments by S.L. Chung and edi-tor Bor-Ming Jahn are gratefully acknowledged.

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