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

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

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U–Pb dating and emplacement history of granitoid plutons in the northern Sanandaj–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, Iran Department of Geosciences, University of Oslo, Postbox 1047, Blindern, N-0316 Oslo, Norway c Faculty of Earth Sciences, Shahid Beheshti University, Tehran, Iran d Department of Geology, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Iran b

a r t i c l e

i n f o

Article history: Received 7 December 2009 Received in revised form 5 February 2011 Accepted 3 March 2011 Available online 13 March 2011 Keywords: Sanandaj–Sirjan Zone Iran Granite U–Pb age Zircon ID-TIMS

a b s t r a c t The Sanandaj–Sirjan Zone (SSZ), which runs parallel to the Zagros fold and thrust belt of Iran, underwent a multistage evolution starting with Neotethys initiation, its subsequent subduction below the Iranian continental crust, and eventual closure during convergence of Arabia towards central Iran. Plutonic complexes are well developed in the northern part of the SSZ and we have dated a number of them by IDTIMS U–Pb on zircon. The new data record the following events: a Mid Jurassic period that formed the Boroujerd Plutonic Complex (169 Ma), the Astaneh Pluton (168 Ma) and the Alvand Pluton (165 Ma); Late Jurassic emplacement of the Gorveh Pluton (157–149 Ma); Mid Cretaceous (109 Ma) formation of a I-type phase in the Hasan Salary Pluton near Saqqez, followed by Early Paleocene (60 Ma) intrusion of A-type granite in the same pluton; and the youngest intrusive event recorded so far in the SSZ with the intrusion of granite in the Gosheh–Tavandasht Complex near Boroujerd at 34.9 Ma. These different events reflect specific stages of subduction-related magmatism prior to the eventual Miocene collision between the two continental blocks. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction The NW-trending Zagros Mountains in Iran are a member of the Alpine–Himalayan orogenic belt and represent one of the youngest continental collision zones on Earth. Their development was characterized by a protracted magmatic history in an Andean-type setting culminated by collision during closure of the Neotethys Ocean between 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 and Thrust Belt, and the internal Sanandaj–Sirjan Zone (SSZ) flanked by the Tertiary Urumieh-Dokhtar volcanic arc (inset in Fig. 1). The SSZ is located on the foreland side of the Zagros Mountains and evolved through processes related to the Neotethys initiation and closure accompanied by Late Jurassic to Eocene calc-alkaline magmatism (Berberian and King, 1981; Alavi, 1994; Mohajjel et al., 2003; Agard et al., 2005; Ghasemi and Talbot, 2006). The intrusions are well developed, and it is possible to determine a relative chronology which in turn allows us to estimate the duration of the orogeny (Witt and Davy, 1997; Mendes et al., 2006).

⇑ Corresponding author. Tel.: +47 22 85 66 80; fax: +47 22 85 42 15. E-mail addresses: [email protected] (S. Mahmoudi), fernando. [email protected] (F. Corfu). 1367-9120/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2011.03.006

Previous radiometric age constraints on the timing of plutonism in the SSZ are few and in part controversial. This TIMS U–Pb zircon study was therefore undertaken in order to reduce the gaps in the age coverage and to address some of the most pressing questions on the genesis of the intrusive in SSZ. 2. Sanandaj–Sirjan Zone The SSZ is characterized by dextral transpression (Mohajjel and Fergusson, 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 Sanandaj in the northwest of Iran (Mohajjel and Fergusson, 2000). The northeast boundary of the SSZ, which separates it from Urumieh-Dokhtar volcanic arc, is recognized by several young depressions and locally Mesozoic steep dextral strike-slip faults (Stöcklin, 1974). The Zagros main thrust, referred to as the location of the Neotethys suture zone (e.g. Berberian and King, 1981; Mohajjel et al., 2003; Agard et al., 2005), forms the south-western boundary separating the SSZ from the Zagros Fold and Thrust Belt. The SSZ consists of Paleozoic continental sediments, subsequently overlain by sedimentary and volcanic rocks during the Triassic–Jurassic opening of the Neotethys (Alavi, 1994; Mohajjel et al., 2003) and variously metamorphosed (Baharifar et al., 2004). The SSZ is subdivided into two parts. The southeastern part

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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 on Stöcklin 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) are shown in black.

comprises Paleozoic metamorphic rocks of relatively high metamorphic grade that were deformed and metamorphosed in the Middle to Late Triassic (Berberian, 1995). The northwestern SSZ consists mostly of metamorphosed and complexly deformed sedimentary rocks associated with granitoid plutons, both deformed and undeformed, as well as widespread Mesozoic volcanic rocks. The oldest rocks are medium and highgrade metamorphic rocks, such as garnet schist, staurolite schist, andalusite schist and sillimanite schist, and low- to very low-grade metamorphic rocks, slates and phyllites (Baharifar et al., 2004). Ammonites found in the slates at the top of the metamorphic sequence in the Hamadan area indicate early Mid-Jurassic ages for the low-grade metamorphic rocks. The northwestern SSZ was deformed in the Late Jurassic–Cretaceous. The Iran micro-continent was part of Gondwana before its separation in the Late Paleozoic by intra-continental rifting that formed 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-extension sedimentation (Berberian and King, 1981), Mid Triassic volcanism and volcano-sedimentary deposition and extrusion of Late Triassic

pillow lavas in the SSZ. Subduction of the Neotethys oceanic crust under the south-western part of the Iranian micro-continent started 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 and plutonism, greenschist facies metamorphism and the development of tight folds with S-SW dipping axial planes (Mohajjel et al., 2003). In the Late Cretaceous, the collision of the passive margin of the Arabian plate with an island arc in the SW of the SSZ caused obduction and emplacement of ophiolites on the northeastern margin of the Arabian plate (Berberian, 1983; Mohajjel et al., 2003; Agard et al., 2005). Dextral transpressional movements and oblique subduction characterize the Late Cretaceous convergence history (Mohajjel and Fergusson, 2000; Sheikholeslami et al., 2008; Sarkarinejad and Azizi, 2008; Agard et al., 2005). In the Late Cretaceous, subduction-related magmatism shifted 300 km to the NE of the SSZ and Paleocene–Eocene volcanism started in the Urumieh-Dokhtar volcanic arc (Berberian, 1981; Berberian et al., 1982; Omrani et al., 2008). Final closure of the Neotethys Ocean

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occurred in the Miocene (Berberian, 1983; Mohajjel et al., 2003), or Late Mesozoic (Sheikholeslami et al., 2008) or Late Eocene (Agard et al., 2005) by collision of the central Iranian plateau with the NE margin of the Arabian plate. The major structures in the SSZ formed during three separate events: (1) subduction along the active margin of Central Iran at the northeastern margin of the Neotethys, (2) ophiolite obduction along the northeastern margin of Neotethys, and (3) continental collision of Arabia and Central Iran. Structures associated with subduction are characterized by the development of intense folding with south–southwest vergence and greenschist facies metamorphism. The main effects of the collision have been thrusting, including out-of sequence thrusting, along the older obduction zone with incorporation of Cenozoic successions into the imbricate system and development of blind thrusting and folding in the Zagros Fold and Thrust Belt (Alavi, 1994; Berberian, 1995; Mohajjel et al., 2003). The collision deformed Cambrian to Holocene rocks (e.g., Sepehr and Cosgrove, 2005). The Phanerozoic sequences are folded and faulted above the crystalline Precambrian basement (Fig. 1). Cenozoic sedimentary rocks, with a stratigraphic thickness of up to 10 km, are folded into simple kilometer-scale anticlines and synclines (Stöcklin, 1968). A number of granitoid complexes occur along the northwestern SSZ, the major ones near Boroujerd and Hamadan and further north around Sanandaj and south of Gorveh (Fig. 1). Previous geochronology using the K–Ar and Rb–Sr methods on the Astaneh, Alvand, Gorveh and Boroujerd granitoid complexes produced dates ranging from 135 to 38 Ma (Bellon and Braud, 1975; Valizadeh and Cantagrel, 1975; Masoudi, 1997; Masoudi et al., 2002; Baharifar et al., 2004) whereas zircon U–Pb dating yielded uniform older ages of around 170 Ma for the Boroujerd Complex (Ahmadi Khalaji et al., 2007). Other parts of the SSZ at Sanandaj and Saqqez lack radiometric ages altogether.

gabbro to granite. Hossiny (1999) indicated that these plutons belong to the alkaline to tholeiitic series. Torkian et al. (2008) described a section of this complex and presented geochemical data which indicate that the rocks formed in a subduction setting from mixed mantle and crustal sources. Eocene non-metamorphic sedimentary rocks and Neogene volcanic rocks (Hossiny, 1999) locally cover this area. Their sediments include conglomerate with good rounding and sorting, calcareous nummulitic sandstone and volcanic breccia. 2.2. Hamadan The Hamadan area is underlain by Mesozoic meta-sedimentary rocks and has been affected by a complex metamorphic and tectonic history. Regional metamorphism reached its peak before the Upper Cretaceous (Majidi and Amidi, 1980; Baharifar et al., 2004). Low grade metamorphic rocks are mainly located east of Hamadan, the metamorphic grade increasing toward the west with slates and phyllites giving way to staurolite ± garnet schist, andalusite ± sillimanite ± garnet schists and paragneiss. The more than 400 km2 Alvand batholith (Fig. 2a) includes mainly porphyroid and fine-grained granitoid rocks with minor granite, tonalite, diorite and gabbro, pegmatite and quartz veins (Sepahi, 1999; Baharifar et al., 2004). The granitic rocks show a calc-alkaline nature, while gabbros are rather tholeiitic. This difference suggests that there could be two magmatic endmembers: (1) crustal anatectic magmas generating S-type granite and related rocks, and (2) M-type magmas generating a gabbro–diorite–tonalite suite and mafic-intermediate dykes. There are many NE-trending syn-plutonic microdioritic dykes that locally are disaggregated to mafic microgranular enclaves (Sepahi, 1999). The plutons are surrounded by a well developed contact aureoles with andalusite–sillimanite–cordierite hornfels, cordierite hornfels, garnet andalusite hornfels and cordierite schist.

2.1. Saqqez, Sanandaj and Gorveh 2.3. Boroujerd The northernmost parts of the SSZ are dominated by a 2000– 3000 m thick sequence of shallow-marine detrital sediments overlain by mud- and siltstones interbedded with volcanic rocks (Eftekharnejad, 1981; Zahedi et al., 1985). Permian schist and marble are the oldest exposed rocks (Azizi and Moinevaziri, 2009). The Mesozoic sequence consists of sandy limestone, shale, and crystalline limestone intercalated with Jurassic volcanic rocks, and Cretaceous basic to intermediate volcanic rocks interbedded with dark grey shale (Fig. 1). The largest (50 km2) plutonic mass in the Saqqez area is the Hasan Salary Pluton (Hariri, 2003), which has a well developed contact metamorphic aureole to the surrounding volcano-sedimentary rocks. Sepahi and Athari (2006) distinguished two phases: G1 are granites to syenites of peralkaline affinity, showing withinplate characteristics on discrimination diagrams, and fitting the general classification of A-type granites; G2 comprises monzogranite, granodiorite and tonalite, containing abundant microgranular enclaves, and of subalkaline and metaluminous affinity fitting the classification of volcanic arc I-type granites. The area west of Sanandaj comprises sandy shale with limestone lenses and sandy limestone interbedded with Cretaceous– Paleocene volcanics, and is locally unconformably overlain by a red basal conglomerate of Eocene age, which, in turn, is conformably overlain by Oligocene limestone. The Gorveh region comprises Triassic–Jurassic low-grade metamorphic rocks, including spilite, basalt, andesite and rarely rhyolite with interbedded sedimentary lenses of mudstone and silt. In this sequence there are no diagnostic fossils. These rocks are intruded by gabbroic to granitic igneous rocks, which are weakly sheared and metamorphosed along NWtrending faults. The Gorveh Plutonic Complex comprises a series of bodies trending from south to west of Gorveh and ranging from

The area consists of regionally metamorphosed supracrustal rocks, locally covered by young Quaternary sediments. The least metamorphosed rocks are meta-volcanic flows and tuffs, metacherty limestone, meta-sandstone, slate and phyllite of preJurassic age (Mohajjel, 1997). Granitic bodies are widespread throughout the area and three different units can be identified: the Astaneh Pluton, the Boroujerd Plutonic Complex and the Tavandasht–Gosheh Complex. The Astaneh Pluton is composed mainly of fine-grained equigranular granodiorite, commonly hosting xenoliths (Fig. 2b) and cut by aplite dykes (e.g., Tahmasbi et al., 2010). The Boroujerd Plutonic Complex is the biggest intrusion in the area. It is a NW-trending elongated body covering an area of 200 km2. The dominant phase is granodiorite (Ahmadi Khalaji et al., 2007), showing a distinct foliation related to the orientation of minerals, especially biotite. The granodiorite is generally altered such that biotite is deformed and altered to chlorite, or to aggregates of titanite, prehnite, muscovite, opaques and quartz, and green amphibole is transformed into biotite, chlorite, epidote and prehnite. Subsidiary monzogranite occurs as scattered bodies and tends to be less altered. Quartz–diorite forms small stocks with generally sharp contacts to the granodiorite (Fig. 2c). They stand out topographically because they are much less altered than granodiorite. Pegmatites are abundant, especially in the contact aureole (Masoudi et al., 2002; Esmaeily et al., 2009). They vary from a few meters to tens of meters in length and a few meters in width. The contact metamorphic aureole, best developed along the northern margin of the complex, includes cordierite–andalusite and cordierite–sillimanite hornfelses.

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a

b

241

ON, Canada, by fusion ICP-MS. The accuracy of major elements is 0.01%, for most trace elements it is better than 10 ppm and for the rare earth elements (REE) it is 0.1 ppm or better. The U–Pb analyses (Table 2) were carried out by ID-TIMS at the University of Oslo. The rocks were crushed and pulverized in a jaw crusher and hammer mill and the heavy minerals concentrated with a succession of Wilfley table flotation, free fall and high gradient magnetic separation and methylene iodide density separation. Further selection was carried out by hand-picking under a binocular microscope and mechanical abrasion (Krogh, 1982) to remove discordant domains. Some fractions were also subjected to chemical abrasion based on Mattinson (2005) but following approximately the procedure of Schoene et al. (2006) with an annealing stage of 3 days at 900 °C, a partial dissolution step with HF (+HNO3) at 194 °C overnight, and a 2 h hot plate step in 6 N HCl after removal of the solution and some rinsing. The dissolution was carried out following Krogh (1973) as described in Corfu (2004) but using a mixed 202Pb–205Pb–235U spike. The data are calculated using the decay constants of Jaffey et al. (1971) and the Isoplot program of Ludwig (2003). 3.2. Hasan Salary Pluton, Saqqez: samples SS-100 and SS-101

c

The Hasan Salary Pluton is composed mainly of granodiorite to granite, generally coarse-grained (>2 mm), and characterized by a granular texture with perthitic alkali feldspar (35–40%), plagioclase (albite-oligoclase) (30–40%), quartz (15–25%), biotite (5%) and amphibole (<5%) plus accessory zircon. The two samples selected for U–Pb dating include SS-100, an altered leucocratic granite collected about 1 km north of Hasan Salary Village, some 20 km south of Saqqez, and SS-101, taken about 3.5 km further south, representing a fresher, less altered granite, with accessory titanite and fluorite. The two rocks are calc-alkalic and peraluminous but they have quite distinct trace element characteristics with crossing patterns in REE and spider diagrams (Figs. 3 and 4). This reflects higher abundances of LILE but lower abundances of HREE and P in 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 numerous scattered inclusions of other minerals. Three of the four zircon fractions (Table 2) yield overlapping concordant data points defining a concordia 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 analyses are identical and concordant yielding a concordia age of 59.78 ± 0.18 Ma (Fig. 6b). 3.3. Gorveh Plutonic Complex: samples SS-107, SS-109 and SS-112

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

The Tavandasht–Gosheh Complex comprises three main small bodies of monzogranite to monzodiorite. The rocks are calcalkaline, light in color, fine to coarse-grained, and with a granular texture. 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,

Three samples were analyzed from this plutonic complex, and chemical analyses are available for two of them. Sample SS-107 is a granite collected approximately 45 km west of Gorveh from the westernmost extension of the Gorveh Plutonic Complex. The rock is composed of alkali feldspar (30–40%), plagioclase (albiteandesine) (25–30%), quartz (30–45%) and biotite (10%). The second sample SS-109 is a monzonite collected 2 km WNW of Mayham village. The rock has a granular texture and a simple mineralogy of alkali feldspar (30–35%), plagioclase (45–50%), amphibole (10– 15%) and biotite (<5%). The third sample SS-112 is a gabbro taken 5 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 intergrowths and quartz (5–15%). The monzonite is calc-alkalic and the gabbro tholeiitic, both are peraluminous and have moderately fractionated parallel REE patterns, but with a marked decrease in

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Table 1 Major (%) 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 Al2O3 Fe2O3(T) MnO MgO CaO Na2O K2O TiO2 P2O5 LOI

67.64 14.6 4.25 0.047 1.05 4.43 3.07 1.55 0.471 0.13 2.15

71.81 14.16 2.47 0.016 0.33 0.73 4.05 5.27 0.363 0.06 0.56

71.56 14.76 1.88 0.034 0.57 3.26 5.15 1.34 0.371 0.08 0.72

51.24 17.39 9.68 0.153 5.47 9.78 3.61 0.69 1.517 0.25 0.06

65.12 16.31 5.16 0.07 1.16 1.71 2.84 5.51 0.668 0.15 0.91

60.77 17.69 5.41 0.08 1.71 2.96 3.65 3.05 0.631 0.15 3.36

60.02 15.41 6.74 0.113 4.18 4.57 2.75 2.17 0.644 0.1 2.35

63 15.7 6.13 0.108 2.57 4.76 2.52 2.78 0.533 0.1 1.5

0.01 0.01 0.01 0.001 0.01 0.01 0.01 0.01 0.001 0.01

Total

99.38

99.81

99.73

99.82

99.62

99.45

99.05

99.7

0.01

Sc Be V Cr Co Ni Cu Zn Ga Ge As Rb Sr Y Zr Nb Mo Ag In Sn Sb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W Tl Pb Bi Th U

9 1 69 260 8 140 30 <30 14 1.7 <5 36 340 19.4 156 15.6 3 <0.5 <0.1 2 <0.2 1.1 730 42 71.4 7.56 24.1 4.62 1.02 4.16 0.58 3.37 0.74 2.34 0.359 2.49 0.431 4.2 1.14 <0.5 0.13 <5 <0.1 12.1 1.95

2 5 29 380 4 190 10 50 18 1 8 212 399 8.4 295 93.1 4 0.5 <0.1 2 <0.2 4.4 542 69.4 118 11.6 30.1 4.12 0.686 2.58 0.33 1.67 0.34 1.03 0.161 1.11 0.184 8.3 7.92 0.9 0.47 11 <0.1 92.9 5.51

9 3 21 270 4 150 10 40 20 1.1 <5 38 294 33.5 348 40.8 3 0.6 <0.1 5 <0.2 0.5 129 17 44.8 6.49 25.4 6.24 1.16 6.14 0.99 5.89 1.28 3.84 0.58 3.87 0.607 9.3 2.81 <0.5 0.12 6 <0.1 21.6 3.25

29 1 206 370 30 150 50 80 17 1.6 <5 15 303 24.5 152 16.1 3 <0.5 <0.1 2 <0.2 0.4 181 17.4 36.8 4.73 18.3 4.69 1.46 5.47 0.82 4.9 1.03 3.14 0.472 3.08 0.477 3.6 1.05 <0.5 0.06 <5 <0.1 2.51 0.66

11 2 75 300 11 160 20 70 20 1.8 7 198 160 27.3 257 28.3 3 <0.5 <0.1 6 <0.2 10.4 800 56.5 113 12.9 42.5 8.78 1.27 7.53 0.99 5.19 1.03 3.01 0.44 2.84 0.435 6.9 1.71 <0.5 0.74 29 0.1 24.1 2.57

14 3 85 120 11 60 20 60 19 1.6 8 131 378 13.2 272 16.6 <2 0.5 <0.1 2 0.4 8.7 693 14 27.2 3.43 12.8 3.21 1.37 3.03 0.42 2.43 0.52 1.63 0.243 1.56 0.245 6.7 0.84 <0.5 0.54 11 0.2 2.79 1.72

21 2 153 400 19 170 20 80 17 1.9 7 88 293 21.4 140 15.1 3 <0.5 <0.1 4 0.4 6.3 270 88.8 198 24 81.2 14.4 3.11 9.62 1.08 5.28 0.96 2.73 0.393 2.54 0.4 4.2 1.21 3.5 0.39 14 0.1 41.1 3.01

20 2 105 360 16 170 20 70 16 2 10 120 176 23.7 118 12.2 3 <0.5 <0.1 3 <0.2 12.4 300 31.9 65.9 7.46 24.7 5.21 0.79 4.92 0.73 4.35 0.91 2.79 0.432 2.93 0.47 3.5 1.15 1.6 0.49 17 <0.1 10.2 3.05

1 1 5 20 1 20 10 30 1 0.5 5 1 2 0.5 1 0.2 2 0.5 0.1 1 0.2 0.1 3 0.05 0.05 0.01 0.05 0.01 0.005 0.01 0.01 0.01 0.01 0.01 0.005 0.01 0.002 0.1 0.01 0.5 0.05 5 0.1 0.05 0.01

La and a much more pronounced negative Eu anomaly in the monzonite. In the spider diagram (Fig. 4) the gabbro (SS-112) shows a relatively regular, gently decreasing pattern interrupted only by a positive anomaly for Pb whereas the monzonite has distinct negative anomalies in Ba, P and Ti and positive anomalies especially in Th, U, Nb, Pb, Zr and Sm. The monzonite plots in the within-plate field in Fig. 5 in contrast to the volcanic arc position of the gabbro. The zircon population in sample SS-107 comprises mainly clear euhedral prisms. Seven analyses were carried out but six of them were variously affected by inherited components (Table 2). The concordant analysis, supported by a second nearly concordant one yields a Concordia age of 156.5 ± 0.6 Ma (Fig. 6c). Regression lines calculated through different groups of data project towards

upper intercept ages of between 484 and 1786 Ma (Fig. 6d). One of the analyses suggests a Devonian age (Table 2, not plotted), but because of the extremely low content of U its precision is very poor. A fraction of titanite yields a younger age of about 136 Ma (Fig. 6d), but the position of the data point may just reflect partial resetting at some later point in time. Zircon of monzonite sample SS-109 occurs as colorless euhedral crystals either long or short prismatic and also as broken fragments. Six analyses are clustered close to the Concordia curve yielding a 206Pb/238U age of 151.0 ± 0.2 Ma (Table 2; Fig. 6e). The fact that these analyses have mostly higher 207Pb/206Pb ages of around 158 Ma, and one additional analysis plots above the main group, could be taken to indicate that the actual age is somewhat

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

Weight (ug)

U (ppm)

Th/ Ub

Pbcc (pg)

206

Pb/204Pbd

207

Pb/235Ue

±2r [abs]

206

Pb/238Ue

±2r [abs]

rho

207

Pb/206Pbe

±2r [abs]

206

Pb/238Ue

±2r [abs]

207

Pb/235Ue

±2r

207

Pb/206Pbe

[age in Ma] 0

0

00

SS-100 Leucogranite, Hasan Salary Pluton, Hasan Salary Village, Saqqez (36°03 20’’/46°17 00 Z sp[1] 1 351 0.73 1.0 400 0.1147 Z sp-eq [4] 10 122 0.74 1.4 971 0.1132 Z sp-eq [11] 10 252 0.90 2.5 1075 0.1134 Z sp-eq [3] 2 273 0.58 1.0 586 0.1136 SS-101 Granite, Hasan Salary Pluton, Lakzy Z fr[5] 4 258 Z sp-eq [5] 26 106 Z Ip [3] 11 286

0.016710 0.017072 0.017000 0.017106

0.000095 0.000171 0.000047 0.000150

0.37 0.76 0.45 0.58

0.04978 0.04809 0.04838 0.04817

0.00094 0.00040 0.00028 0.00064

106.8 109.1 108.7 109.3

0.6 1.1 0.3 0.9

110.2 108.9 109.1 109.3

2.1 1.3 0.7 1.7

185 104 118 108

0.0013 0.0007 0.0006

0.009329 0.009302 0.009312

0.000041 0.000051 0.000062

0.40 0.44 0.49

0.04751 0.04720 0.04729

0.00097 0.00048 0.00044

59.86 59.68 59.75

0.26 0.33 0.40

60.23 59.68 59.86

1.27 0.65 0.61

75 59 64

0.3159 0.0012 0.0026 0.0009 0.0005 0.0010 0.0006 0.0022

0.066119 0.046609 0.037798 0.036743 0.030173 0.024580 0.024688 0.021291

0.002771 0.000104 0.000342 0.000078 0.000064 0.000096 0.000074 0.000057

0.77 0.70 0.97 0.96 0.90 0.62 0.69 0.21

0.05512 0.05436 0.05306 0.07064 0.05259 0.04897 0.04945 0.04943

0.03292 0.00013 0.00012 0.00005 0.00006 0.00023 0.00013 0.00072

413 293.7 239.2 232.6 191.6 156.5 157.2 135.8

17 0.6 2.1 0.5 0.4 0.6 0.5 0.4

413 304.2 247.9 310.6 200.9 155.9 158.0 137.6

194 0.9 2.0 0.6 0.4 0.9 0.5 1.9

— 386 331 947 311 146 169 168

0.1625 0.1608 0.1609 0.1604 0.1609 0.1596 0.1608

0.0015 0.0005 0.0004 0.0004 0.0005 0.0010 0.0006

0.023941 0.023694 0.023707 0.023648 0.023751 0.023652 0.023710

0.000100 0.000057 0.000052 0.000050 0.000064 0.000090 0.000056

0.48 0.77 0.83 0.88 0.73 0.60 0.67

0.04922 0.04921 0.04922 0.04921 0.04913 0.04893 0.04918

0.00040 0.00009 0.00008 0.00006 0.00011 0.00023 0.00013

152.5 151.0 151.0 150.7 151.3 150.7 151.1

0.6 0.4 0.3 0.3 0.4 0.6 0.4

152.9 151.4 151.5 151.1 151.5 150.3 151.4

1.3 0.4 0.4 0.4 0.5 0.8 0.5

158 158 159 158 154 145 156

0.1587 0.1586 0.1586

0.0004 0.0005 0.0004

0.023433 0.023382 0.023409

0.000051 0.000066 0.000054

0.93 0.71 0.82

0.04912 0.04919 0.04913

0.00004 0.00011 0.00008

149.3 149.0 149.2

0.3 0.4 0.3

149.6 149.5 149.5

0.3 0.5 0.4

153 157 154

(34°450 4500 /48°260 2100 ) 0.025979 0.000055 0.025976 0.000061 0.025866 0.000073 0.025935 0.000058 0.026012 0.000058

0.92 0.95 0.89 0.90 0.83

0.04943 0.04939 0.04942 0.04935 0.04956

0.00005 0.00004 0.00007 0.00006 0.00007

165.3 165.3 164.6 165.1 165.5

0.3 0.4 0.5 0.4 0.4

165.5 165.4 164.8 165.0 166.1

0.4 0.4 0.5 0.4 0.4

168 167 168 165 174

Dam, Saqqez (36°010 2500 /46°170 0000 ) 1.71 1.7 375 0.0611 1.04 2.1 786 0.0605 1.22 2.1 891 0.0607

Gorveh Pluton, Mayham 9 101 19 353 29 384 54 225 22 311 9 308 22 232

Sofla Village (35°030 5100 /47°520 4900 ) 0.81 1.5 920 0.73 2.4 4154 0.74 4.2 3990 0.60 2.2 8205 0.60 2.1 4892 0.59 3.7 1141 0.69 3.9 1955

SS-112 Gabbro, Gorveh Pluton, Mayham Olya Village (35°030 4000 /47°500 4600 ) Z Ip [6] 143 147 0.74 2.1 14874 Z fr[1] 66 77 0.56 1.5 5050 Z fir [10] 89 106 0.78 3.8 3683

SS-119 Porphyritic granite to granodiorite, Alvand Pluton, dominant phase, Hamedan, Gang namh Village Z lp CA [20] 53 243 0.14 1.6 12919 0.1771 0.0004 Z lp in CA [33] 34 908 0.27 2.7 18794 0.1769 0.0005 Z lp in CA [16] 34 291 0.14 1.6 10296 0.1763 0.0005 Z lp CA [14] 127 114 0.15 2.1 11041 0.1765 0.0005 Z lp CA [12] 30 226 0.31 1.7 6343 0.1777 0.0005 SS-124 Diorite, Alvand Pluton, Hamedan, Abas Abad Z fr [14] 98 703 0.92 Z fr [33] 106 1075 1.25 Z fr [15] 108 927 1.27

Village (34°450 3200 /48°260 3100 ) 19.1 5887 0.1767 24.8 7441 0.1760 26.6 5939 0.1711

0.0005 0.0005 0.0004

0.025925 0.025834 0.025106

0.000062 0.000069 0.000057

0.94 0.96 0.94

0.04943 0.04940 0.04943

0.00005 0.00004 0.00005

165.0 164.4 159.8

0.4 0.4 0.4

165.2 164.6 160.4

0.4 0.5 0.4

168 167 168

0.0005 0.0005 0.0005 0.0005 0.0005

0.026685 0.026625 0.026570 0.026501 0.026291

0.000062 0.000061 0.000066 0.000064 0.000063

0.97 0.94 0.97 0.93 0.92

0.04959 0.04959 0.04952 0.04954 0.04968

0.00003 0.00004 0.00004 0.00005 0.00006

169.8 169.4 169.0 168.6 167.3

0.4 0.4 0.4 0.4 0.4

170.2 169.8 169.3 168.9 168.1

0.4 0.4 0.4 0.4 0.4

176 176 173 173 180

SS-135 Monzogranite, Boroujerd Plutonic Complex, near mobile antenna, Nezam Abad (33°430 0800 /49°120 1600 ) Z lp CA [17] 55 414 0.29 1.8 21448 0.1819 0.0005 0.026627 Z sp CA [15] 78 339 0.30 2.1 21554 0.1820 0.0005 0.026661

0.000062 0.000063

0.95 0.89

0.04954 0.04951

0.00004 0.00006

169.4 169.6

0.4 0.4

169.7 169.8

0.4 0.4

174 172

SS-130 Altered hornblende granodiorite, Boroujerd Plutonic Complex (33°410 3800 /49°060 4400 ) Z lp [44] 127 526 0.38 5.6 19809 0.1824 Z sp CA [12] 61 469 0.36 1.9 24618 0.1821 Z lp [15] 123 815 0.38 10.2 16394 0.1814 Z sp CA [24] 44 651 0.40 4.3 11035 0.1810 Z lp CA [11] 37 579 0.34 6.5 5438 0.1801

243

(continued on next page)

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

0.0023 0.0014 0.0007 0.0019

SS-107 Fine-grained white granite, Gorveh Pluton, Sofy Abad Village (35°050 3000 /47°160 0000 ) Z spCA[1] 1 4 0.93 1.4 31 0.5025 Z sp in [1] 31 73 0.74 2.2 3007 0.3493 Z sp CA [4] 40 238 0.55 12.9 1774 0.2765 Z spCA[14] 22 922 0.55 2.7 17588 0.3579 Z sp in [12] 7 1491 0.82 2.2 9079 0.2188 Z spCA[1] 1 1054 0.75 0.9 1924 0.1660 Z sp CA [1] 7 526 0.53 1.6 3491 0.1683 T Ibr [6] 20 219 0.87 26.0 243 0.1451 SS-109 Monzonite, Z Ip [12] Z fr[15] Z sp [21] Z lpin CA[13] Z IpinCA [11] Z Ip [6] Z Ip [12]

)

244

Table 2 (continued) Characteristicsa

±2r [abs]

rho

207

0.026656

0.000060

0.96

0.04954

0.00004

0.0017 0.0006 0.0023 0.0008

0.055795 0.027794 0.026717 0.026445

0.000124 0.000068 0.000067 0.000058

0.84 0.73 0.22 0.57

0.07535 0.05070 0.04995 0.04961

SS-128 Quartz monzodiorite, Gosheh-Tavandasht Complex, near Abshar village (33°410 5800 /49°090 5000 ) Z lp CA [7] 10 788 0.67 1.6 1669 0.0350 0.0002 Z lp CA [4] 7 849 0.62 1.1 1804 0.0350 0.0002 Z sp CA [3] 4 818 0.55 1.3 891 0.0350 0.0004

0.005421 0.005425 0.005422

0.000017 0.000021 0.000022

0.57 0.57 0.48

0.04677 0.04681 0.04680

Weight (ug)

U (ppm)

Th/ Ub

Pbcc (pg)

206

Pb/204Pbd

399

0.30

2.5

23305 2136 2796 287 954

207

Pb/235Ue

±2r [abs]

206

0.1821

0.0005

0.5797 0.1943 0.1840 0.1809

Pb/238Ue

Pb/206Pbe

±2r [abs]

207

±2r

207

169.6

0.4

169.8

0.4

173

0.00013 0.00011 0.00061 0.00017

350.0 176.7 170.0 168.3

0.8 0.4 0.4 0.4

464.3 180.3 171.5 168.8

1.1 0.5 2.0 0.7

1078 227 193 177

0.00024 0.00023 0.00041

34.85 34.88 34.86

0.11 0.13 0.14

34.89 34.94 34.92

0.21 0.21 0.34

38 39 39

±2r [abs]

206

Pb/238Ue

Pb/235Ue

Pb/206Pbe

[age in Ma] Z eq CA [24]

86

SS-136 Granite, Astaneh Pluton (33°480 0400 /49°190 0800 ) Z lp in [4] 29 447 0.38 21.4 Z lp [4] 10 503 0.33 3.2 Z sp [7] 20 692 0.31 86.1 Z Ip [4] 16 448 0.25 12.7

100

10

100

10

100

10

100

10

SS-100

SS-112 SS-101

SS-109

SS-119 Hasan Salary Pluton

Gorveh Plutonic Complex

Alvand Pluton, Hamadan

Astaneh Pluton

SS-130 Boroujerd Plutonic Complex

SS-136

SS-135 Boroujerd Plutonic Complex

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Fig. 3. REE-patterns of dated granitoid rocks of the SSZ. The REE are normalized using chondritic values of Sun and McDonough (1989).

higher than 151 Ma, but, on the other hand, the generally good quality of the analyzed zircon grains and the fact that these included both mechanically and chemically abraded fractions, suggest that the position of the data points is a function of 231Pa excess rather than Pb loss (e.g. Parrish and Noble, 2003). Hence 151.0 ± 0.2 Ma is considered to be the time of crystallization of the monzonite. The zircon population in the gabbro sample SS-112 consists mostly of stubby, pale brown euhedral crystals, locally with hollow cavities and also numerous dark inclusions. Three analyses overlap, although as in the previous case they tend to plot to the right of Concordia and yield a mean 206Pb/238U age of 149.2 ± 0.2 Ma (Fig. 6f).

3.4. Alvand Pluton, Hamadan: samples SS-119 and SS-124

Sample SS-119 represents fresh porphyritic granite to granodiorite with 2–4 cm long crystals of feldspar in a finer grained crystalline matrix. It stems from the historical place Gang Nameh in Alvand. The rock consists of plagioclase (20–40%), quartz (10– 15%), alkali feldspar (30–40%) and biotite 10–20% with zircon, tourmaline and kyanite as accessory minerals. The rock is

Rock / chondrite

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

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 mechanically abraded; [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.

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

SS-101

1000

Hasan Salary Pluton

1000

syn-COLG

WPG 101

100

130 100

119

136 135

10

Rb

SS-100

1

ORG

Gorveh Plutonic Complex

VAG

100

1

Rock / Primitive Mantle

112

10 SS-109

109

100

10

10

100

1000

Y+Nb

SS-112

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

1000

SS-119

Alvand Pluton, Hamadan

yield a concordia age of 165.2 ± 0.2 Ma with an additional point slightly off presumably because of some inheritance (Fig. 7a). The zircon population in SS-124 is similar to that in sample SS119 but they tend to have higher U-contents. The three analyses are spread along a discordia line anchored at 0 Ma yielding an upper intercept age of 167.9 ± 1.2 Ma. This age, however, is identical to the 207Pb/206Pb ages of the analyses of sample SS-119, suggesting that the 206Pb/238U age of 165.0 ± 0.4 Ma of the most concordant analysis may be more correct (Table 2; Fig. 7b).

100

10

1 1000

SS-136 SS-130 SS-135

Astaneh Pluton

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

} Boroujerd Plutonic Complex

100

10

1 CsRbBaTh U Nb K LaCePb Pr Sr P Nd ZrSmEu Ti Dy Y YbLu Fig. 4. Spider diagram with trace element data of dated granitoid and gabbroic rocks of the SSZ. Normalization to Primitive Mantle according to Sun and McDonough (1989).

calc-alkalic and peraluminous, plots in the volcanic arc field of Fig. 5, it exhibits a fractionated REE pattern with a strong Eu anomaly (Fig. 3) and, in the spider diagram (Fig. 4) it shows distinct negative anomalies for Ba, Nb, Sr, P and Ti, together with a positive anomaly for Pb. The second sample SS-124 from the Abas Abad Valley is a finer grained hornblende-rich (30–50%) diorite also containing 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 prismatic crystals commonly with inclusions of apatite. Four analyses

Sample SS-130 is an altered granodiorite from the Boroujerd Plutonic 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 accessory minerals. Plagioclase is sericitised and quartz occurs as anhedral isolated grains and aggregates displaying recrystallisation with undulatory extinction, typical of incipient solid-state deformation. Monzogranite sample SS-135 was collected close to Nezam Abad village from the highest point in the Boroujerd Complex. This phase occurs as widely scattered small outcrops through the southern part of the Boroujerd Complex. The rocks are light in color, fine to coarse-grained, with a granular texture in the centre of the unit and a porphyritic texture with feldspar megacrysts at the margin. Mineral assemblages include quartz (30–35%), alkali feldspar (30–35%), plagioclase (25– 35%), biotite (5–10%) and some secondary muscovite. Zircon, allanite and apatite are common accessory minerals. Quartz intergrowths with K-feldspar and plagioclase form micrographic textures and myrmekite, respectively. Large phenocrysts of quartz show undulatory extinction. Perthitic alkali feldspar occurs as euhedral to subhedral crystals. Plagioclase is 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 the variable abundance of the REE, the main distinction observed in the spider diagram concerns Th and to some degree Zr which have, respectively, negative and positive anomalies in SS-130 and the

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

111

SS-100

SS-101

Hasan Salary Pluton 0.0172

Hasan Salary Pluton 110

a

b 109

60.0

0.00934

108 59.6

0.0168

107 Three points: Concordia age

0.00926

Three points: Concordia age

59.78 ± 0.18 Ma

108.8 ± 0.3 Ma

(2 ) MSWD (of conc.) = 0.14

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

0.0164

0.112

59.2 0.060

0.116

157.8

SS-107

0.044

Gorveh Plutonic Complex

SS-107

280

Gorveh Plutonic Complex

484 Ma

d

c

0.0247

0.062

240

157.0 0.036

206

Pb

238

U

200

1786 Ma

156.2 One point: Concordia age

0.0245

0.028

156.5 ± 0.6 Ma

160

(2 , decay-const. errors included) MSWD (of conc.) = 2.9

0.165

0.167

0.169

titanite (ca. 136 Ma) 0.020

154

SS-109

e

0.26

0.02348

Gorveh Plutonic Complex

f

153

0.34

149.8

SS-112

Gorveh Plutonic Complex 0.0240

0.18

149.4

152 0.02340

149.0 151 Six points: average

0.0236

150

206

238

Pb/ U age

Three points: Concordia age

151.0 ± 0.2 Ma

(2 ) MSWD (of conc.) = 1.6

0.159

149.3 ± 0.2 Ma

0.02332

(2 ) MSWD (of conc.) = 3.8

0.158

0.163

0.159

207

Pb/235U

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.

opposite in SS-135. Both have strong negative anomalies in Nb, P and Ti (Fig. 4). Zircon in sample SS-130 is mostly colorless and generally elongated crystals with local fracture and inclusions. The five analyses plot close to the Concordia curve but show some dispersion both in terms of U–Pb and also of 207Pb/206Pb age. It appears as if there are effects both from Pb loss as well as from some inheritance. Such effects are less evident in the three intermediate analyses, which provide 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 and concordant 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 common accessory minerals. The rock is calc-alkalic and peraluminous, showing a volcanic arc affinity (Fig. 5). The REE have a distinct

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SS-119

166

Alvand Pluton

0.0260

166

SS-124

0.0260

Alvand Pluton

a

164

b 0.0256

165 162

Four points: Concordia age

164

One point:

160

206

238

Pb/ U age

165.0 ± 0.4 Ma (2 )

(2 , decay-const. errors included) MSWD (of conc.) = 0.49

0.176

0.171

0.178

0.175

170.2

SS-130 0.0267

0.0252

165.2 ± 0.2 Ma

0.0258

SS-135

170

Boroujerd Plutonic Complex

c

Boroujerd Plutonic Complex 0.0267

169

d

170.0

169.8

206

Pb

169.6

238

U

206

Pb

168

169.4

238

U 0.0266

Three points: Concordia age

0.0263 Three points: average

167

206

238

Pb/ U age

169.6 ± 0.3 Ma

169.0 ± 1.0 Ma

(2 , decay-const. errors included) MSWD (of conc.) = 2.7

(2 ) MSWD = 3.9

0.1795

0.0280

0.1815

SS-136

0.1816

178

0.1824

SS-128

870 Ma

Gosheh-Tavandasht Complex

Astaneh Pluton 176

e

0.00546

174

35.2

f 35.0

1500 Ma 0.0272

172

34.8 170

168

0.0264

167.5 ± 1.0 Ma

0.00538

34.6

combined Concordia age and lower intercept ages

Three points: Concordia age

34.87 ± 0.08 Ma (2 , decay-const. errors included) MSWD (of conc.) = 0.74

166 0.180

0.188

0.0346

0.196 207

0.0350

0.0354

Pb/235U

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.

negative Eu anomaly and flat heavy REE (Fig. 3). In the spider diagram the sample shows characteristics that for most elements are intermediate between those of SS-130 and SS-135 of the Boroujerd Plutonic Complex (Fig. 4). The zircon population consists of well developed prisms like those of samples SS-130, but they contain xenocrystic cores with only two analyses plotting within error of the concordia curve (Fig. 7e). A regression through three of the analyses yields intercepts at 166.7 ± 1.7 and 870 Ma whereas the two youngest analyses together with the oldest one define intercept ages of

167.7 ± 1.7 and 1500 Ma. The youngest analysis alone has a concordia age of 168.3.0 ± 0.3 Ma. These variations result from not exactly quantifiable amounts of inheritance, Pb loss, and/or perhaps 207Pb excess. These effects are taken into account in the rounded estimate of 167.5 ± 1.0 Ma. 3.7. Gosheh–Tavandasht Complex: samples SS-128 Sample SS-128 is a quartz–monzodiorite representing the Gosheh–Tavandasht Complex. In outcrop scale, the rock is whitish

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with an average grain size of 2 mm, a granular to porphyritic texture with plagioclase megacrysts, and a mineral assemblage of plagioclase (40–50%), biotite (15–20%), green amphibole (10–15%), quartz (<15%) and alkali feldspar (<5%). Plagioclase occurs as zoned euhedral to subhedral plates, locally altered to sericite, epidote and calcite. Zircon, titanite and apatite are conspicuous accessory minerals. Three analyses of zircon yield a concordia age of 34.87 ± 0.08 Ma (Fig. 7f).

4. Discussion The results from this study record events related to the evolution of the SSZ from a Andean-type environment during subduction of the Neotethys to a collisional zone between Arabia and Eurasia. The data complement the geological record provided first of all by the lithologies and structures, and both expands and corrects the information accumulated during the previous geochronological studies. Much of the existing geochronological information is based on K–Ar and Rb–Sr ages, both systems very susceptible to resetting. Because of the complex tectonic and associated thermal evolution of the Zagros Orogen, several suites of such ages have been partially overprinted and reset providing dates of no specific geological significance, which then have the potential to introduce distortions into tectonic reconstructions (e.g. Fig. 12 in Agard et al., 2005). One example of such partial resetting is the study by Masoudi et al. (2002) on the Boroujerd Plutonic Complex that demonstrates variable resetting of the Rb–Sr systems creating ages of no direct geological significance. In this case the resetting could be related to local alteration and faulting, and/or to the emplacement of the nearby Late Eocene Gosheh–Tavandasht Complex. Resetting has also severely affected the Rb–Sr and K–Ar dates from the Alvand Pluton (Valizadeh and Cantagrel, 1975; Baharifar et al., 2004). Another example is the Gorveh Pluton, which on the basis of earlier 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 perhaps the defining time in the early Mesozoic evolution of the SSZ, is the intense episode of Jurassic arc magmatism at around 170– 165 Ma. This event is reflected by the ages of the Boroujerd Plutonic Complex (Ahmadi Khalaji et al., 2007; this study) and the Alvand Plutonic Complex (this study). In the southern SSZ a gabbroanorthosite complex was emplaced during the same event (Fazlnia et al., 2007). Mid-Jurassic magmatism was also responsible for the genesis of granites intruding into and metamorphosing the Deh Salm Metamorphic Complex of the Lut Block of East Iran (Mahmoudi et al., 2010). This synchronous activity all along the SSZ and surrounding regions can be related to initiation of subduction of the Neotethys oceanic crust below the Iranian continental crust (Bagheri and Stampfli, 2008). Emplacement of the Gorveh Plutonic Complex at 157–149 Ma reflects a later stage in the arc magmatic activity, apparently of more modest significance. The monzonite is geochemically similar to the younger phase of the Hasan Salary Pluton (SS-101), indicating an A-type affinity. Hence, the gabbros and alkalic rocks of the Gorveh Plutonic Complex suggest a genesis by melting of lower crust, 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 our present data to have I-type geochemical characteristics consistent with continued subduction-related magmatism. This episode of granitic formation post-dates deposition of the regionally widespread Barremian–Aptian (ca. 125 Ma) Orbitolina limestones, which overlies unconformably earlier Jurassic strata (Agard et al.,

2005), likely reflecting some readjustment in the subduction geometry and speed. The I-type Urumieh Plutonic Complex, with a somewhat younger K–Ar age of about 100 Ma, is present further north and is also explained in terms of subduction-related magmatism (Ghalamghash et al., 2009). The youngest phase of the Hasan Salary Pluton is much younger at 60 Ma, i.e., Middle Paleocene. It has a geochemical composition indicating an A-type affinity (Sepahi and Athari, 2006 and our data), generally linked to a provenance from melting of lower continental crust. In the present context it is not quite evident what triggered the formation of these particular rocks which are typical neither for subduction nor for collisional settings. A possibility is that it reflects mafic underplating in an extensional back-arc setting. In fact the subsequent Eocene volcanic activity in the Urumieh-Dokhtar volcanic arc has again arc-type characteristics (Omrani et al., 2008). Ghalamghash et al. (2009) also report the occurrence of A-type granite with an amphibole K–Ar age of about 80 Ma in the Urumieh Plutonic Complex, at the northern end of the SSZ, They suggest a provenance from melting of felsic lower crustal material, suggesting, however, a collisional setting. The youngest unit dated in this study is the 35 Ma quartz monzonite from the Gosheh–Tavandasht Complex near Boroujerd. The fact that this is the youngest age for magmatism found so far in the SSZ suggests that this was an event of local significance, although it also correlates with the late Eocene volcanism that is widespread across much of central and eastern Iran (Omrani et al., 2008). Mazhari et al. (2009) reported a U–Pb age of about 41 Ma for bimodal Piranshahr massif in the northern SSZ, and interpret them as indicating a post-collisional setting, a view that is not uncontroversial as the main collision is regarded by others to have started only in the Miocene (Allen, 2009; Agard et al., 2005. 5. Conclusions The new ID-TIMS U–Pb data from the SSZ document several major phases in the evolution of the zone. Mid Jurassic subduction-related magmatism formed the Alvand Plutonic Complex (165.2 ± 0.2 Ma and 165.0 ± 0.4 Ma), the Boroujerd Plutonic Complex (169.0 ± 1.0 Ma and 169.6 ± 0.3 Ma), and the neighbouring Astaneh 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 Cretaceous plutonism is recorded in the I-type part of the Hasan Salary Pluton near Saqqez (108.8 ± 0.3 Ma), whereas the A-type phase of the same pluton has an Early Paleocene age of 59.78 ± 0.18 Ma. The youngest, Late Eocene, plutonic rock so far dated in the SSZ was found in the Gosheh–Tavandasht Complex near Boroujerd (34.87 ± 0.8 Ma). The results are consistentt with the process of subduction of Neotethys oceanic crust in the Jurassic and subsequent continent–continent collision in the Miocene. Acknowledgements We acknowledge financial support by the Iranian Government Research Agency to the first author during his research at Oslo University. Constructive reviews and comments by S.L. Chung and editor Bor-Ming Jahn are gratefully acknowledged. References Agard, P., Omrani, J., Jolivet, L., Mouthereau, F., 2005. Convergence history across Zagros (Iran): constraints from collisional and earlier deformation. International Journal of Earth Sciences 94, 401–419. Ahmadi Khalaji, A., Esmaeily, D., Valizadeh, M.V., Rahimpour-Bonab, H., 2007. Petrology and geochemistry of the granitoid complex of Boroujerd, Sanandaj– Sirjan Zone, Western Iran. Journal of Asian Earth Sciences 29, 859–877.

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