Petrochemistry of the Siah-Kuh granitoid stock southwest of Kerman, Iran: Implications for initiation of Neotethys subduction

Journal of Asian Earth Sciences 30 (2007) 474–489 www.elsevier.com/locate/jaes

Petrochemistry of the Siah-Kuh granitoid stock southwest of Kerman, Iran: Implications for initiation of Neotethys subduction Mohsen Arvin

a,b

, Yuanming Pan

b,*

, Sara Dargahi

a,b

, Azita Malekizadeh a, Abbed Babaei

c

a

c

Department of Geology, College of Sciences, Shahid Bahonar University of Kerman, P.O. Box 133-76175 Kerman, Iran b Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK, Canada S7N 5E2 Department of Biological, Geological and Environmental Sciences, Cleveland State University, Cleveland, OH 44115, USA Received 30 September 2005; received in revised form 18 October 2006; accepted 6 November 2006

Abstract The Upper Triassic Siah-Kuh granitoid stock is part of a series of Triassic-Cretaceous intrusions in the Phanerozoic Sanandaj-Sirjan zone in the Zagros orogenic belt, south-central Iran. The Siah-Kuh stock intrudes Lower Paleozoic amphibolites of the Abshur metamorphic complex and Upper Devonian-Lower Carboniferous greenschists of the Sargaz metamorphic complex. The Siah-Kuh granitoid stock consists mainly of coarse to medium grained leucogranodiorite, leucomonzogranite and alkali granite with subordinate syenite. It also contains a number of mafic enclaves of different sizes and is intruded by alkali granite and granophyre dikes along with monzonitic and a few dioritic dikes. The Siah-Kuh granitoid rocks are characterized by enrichment in large ion lithophile elements (LILE) such as Rb, Ba, K, Ce and depletion in high field strength elements (HFSE) such as Y, Nb and Zr. The chondrite normalized REE patterns are characterized by moderate LREE enrichment [(La/Yb)N = 1.31–4.82] and unfractionated HREE [(Gd/Yb)N = 0.50 to 1.33]. Granodiorites with the least fractionated HREE and a general absence of Eu anomalies suggest the involvement of plagioclase and absence of garnet during the melting processes. Alkali granites with the highest (La/Yb)N values and pronounced negative Eu anomalies are more differentiated products. Nd isotope analyses of the Siah-Kuh granitoids yield an ‘‘errorchron’’ age of 199 ± 30 Ma (MSWD = 1.30), consistent with the field relationships and give Nd model ages (TDM) from 0.81 to 1.64 Ga and eNd(T) values from +1.81 to +2.45 at 200 Ma. These geochemical data suggest that the Siah-Kuh granitoid stock has characterisitics of metaluminous to slightly peraluminous, calc-alkaline, I-type granite formed in a volcanic arc setting. The onset of subduction of the Neotethys oceanic crust beneath the Central Iranian microcontinent in Triassic time could have accounted for the continental arc volcanism. Dehydration of subducted oceanic crust and partial melting of mantle wedge caused partial melting of subcontinental lithosphere, which resulted in the formation of metasomatised and enriched mafic arc magmas, at variable water fugacity and led to the formation of the Siah-kuh granitoid rocks. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Siah-Kuh stock; Zagros orogenic belt; Iran; I-type granite; Upper Triassic; Neotethys

1. Introduction The Zagros orogenic belt of Iran consists of three parallel tectonic subdivisions from northwest to southeast: (1) the Urumieh-Dokhtar magmatic assemblage, (2) the Sanandaj-Sirjan zone (SSZ) and (3) the Zagros folded-thrust belt (Alavi, 2004) (Fig. 1a). This orogenic belt has been proposed to have resulted from the opening and subduction *

Corresponding author. E-mail addresses: [email protected] (M. Arvin), yuanming.pan@ usask.ca (Y. Pan). 1367-9120/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2007.01.001

of the Neotethyan oceanic realm and subsequent oblique collision of Afro-Arabia (Gondwana) with the Iranian microcontinent in the Late Cretaceous–Early Tertiary (Berberian and King, 1981; Alavi, 1994, Alavi, 2004; Mohajjel and Fergussen, 2000). The northwest–southeast trending Sanandaj-Sirjan zone (Stocklin, 1968; or the Zagros imbricate zone in Alavi, 1994) consists of Paleozoic-Triassic metamorphic rocks that are overlain by Phanerozoic shallow water sedimentary rocks of a passive continental margin and intruded by large-scale Mesozoic plutons ranging from

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Fig. 1. Geological maps illustrating (a) distribution of Mesozoic intrusions in conjunction with Mesozoic ophiolitic rocks in Iran and (b) the Siah-Kuh granitoid stock, southwest of Kerman.

gabbro to granite. The distribution of Mesozoic plutonic bodies in Iran is mostly restricted to regions close to the eventual active plate margins marked by ophiolitic-melange belts (Fig. 1a). They appear to have been generated extensively along and above the early Mesozoic subduction zone of the Sanandaj-Sirjan zone. Apart from a

few age determinations (Sabzehei et al., 1970; Valizadeh and Cantagrel, 1975) and field reports (Sabzehei and Berberian, 1973; Berberian and Nogol, 1974; Sabzehei, 1974; Berberian and Berberian, 1981), no detailed studies have been carried out on any of the Mesozoic plutonic rocks in the Sanandaj-Sirjan zone.

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Accordingly, this contribution reports on petrographic, whole-rock geochemical and Sm–Nd isotope characteristics of the Siah-kuh granitoid stock (a probably Upper Triassic intrusion) in the southeastern part of the SanandajSirjan zone (Fig. 1a). These petrographic and geochemical data are then used to shed light on the origin and tectonic setting of these rocks, particularly in relation to the initiation of Neotethys subduction. 2. Geological settings The Sanandaj-Sirjan zone of the Zagros orogenic belt is bordered to the northeast by the Urumieh-Dokhtar magmatic assemblage or the ‘‘Urmiah-Dukhtar’’ of Schroeder, 1944 and to the southwest by the Zagros folded-thrust belt (Alavi, 2004). The Urumieh-Dokhtar magmatic assemblage, 150 km wide, forms a subduction related, distinctively linear and voluminous magmatic arc composed of tholeiitic, calc–alkaline, and K-rich alkaline intrusive and extrusive rocks (with associated pyroclastic and volcaniclastic successions) along the active margin of the Iranian plate. The oldest rocks in the Urumieh-Dokhtar magmatic assemblage are Pre-Jurassic calc-alkaline intrusive rocks that are exposed in the south eastern margin of Central Iran. The youngest rocks in this area consist of lava flows and pyroclastics that belong to Quaternary to Pliocene volcanic cones of alkaline and calc–alkaline composition (Berberian and Berberian, 1981). The Zagros folded-thrust belt forms the less strained external part of the orogen. This belt consists of folded and faulted rocks composed of 4–7 km of mainly Paleozoic and Mesozoic successions overlain by 3–5 km of Cenozoic siliciclastic and carbonate rocks resting on highly metamorphosed Proterozoic Pan–African basement that was affected by Late Neoproterozoic–Cambrian Najd strike slip faults (Alavi, 2004). The Sanandaj-Sirjan zone is a zone of thrust faults. They have transported numerous slices of metamorphosed and non-metamorphosed Phanerozoic stratigraphic units of the Afro-Arabian passive continental margin, as well as obducted ophiolites, from the collisional suture zone on the northeast toward interior parts of the Arabian craton to the southwest (Fig. 1a). The metamorphosed units were intruded by gabbroic and granitoid plutons. 3. Geology of the Siah-Kuh granitoid stock The Siah-Kuh granitoid stock (80 km2), located 245 km southwest of Kerman in southern Iran, is situated in the southeastern part of the Sanandaj-Sirjan zone (Fig. 1a). The Siah-Kuh stock is among the best exposed intrusions in the region. This massive granitoid complex forms an elongated, east-west trending stock-like intrusive mass (Fig. 1b). The southern and northeastern parts of this stock intruded Lower Paleozoic amphibolites of the Abshur metamorphic complex and Upper Devonian–Lower Carboniferous greenschists of the Sargaz metamorphic

complex, respectively. The Abshur metamorphic complex separates the Siah-Kuh granitoid stock from the Lower Paleozoic Sikhoran mafic and ultramafic complex. The contact between the granitoid rocks and the metamorphic complexes is marked by a thin zone (30 cm) of recrystallized amphiboles and other minerals. The northwest end and the northern part of the stock are unconformably overlain by Upper Jurassic Calpionella limestone and Upper Triassic to Lower Middle Jurassic flysch turbidites and conglomerates that include fragments of the Siah-Kuh granitoids. The Siah-Kuh granitoid rocks range from leucogranodiorite (in a few places showing sign of differentiation to leucomonzogranite) to alkali granite, with subordinate amounts of syenite (nomenclature after Le Maitre, 1989). However, contacts between leucogranite and alkali granite are generally not exposed. This granitoid stock is cut by a series of NW-SE trending dikes that are 1–2 m wide and over a hundred metres long. These dikes are mostly monzonitic in composition, including monzonite, monzodiorite, and quartz monzonite, although scattered dioritic dikes are also observed. In addition, centimeter to decimeter-wide dikes of fine grained, grey alkali granite and granophyre cut through the Siah-Kuh stock. Also mafic enclaves of various types and sizes (centimetres to metres) are found throughout the stock, being particularly abundant in the eastern part where they form massive clusters. Most rocks of the Siah-Kuh granitoid stock have been subject to extensive hydrothermal alteration, including argillation of K–feldspar, sericitization and saussuritization of plagioclase and chloritization of biotite. Syenite, composed of largely alkali feldspar and plagioclase, is almost completely altered to chlorite-rich rocks. Monzonitic and dioritic dikes are also affected by late hydrothermal alteration. 4. Samples and analytical methods A total of 55 samples have been collected from the SiahKuh granitoid stock and associated dikes. Polished thin sections were prepared for all of these samples for petrographic examination. On the basis of petrographic observations, twenty four samples with minimal effects of late hydrothermal alteration were selected for whole-rock geochemical analysis (Table 1). Twenty samples (Tables 2 and 3) were analyzed for major and minor elements on a Varian inductively coupled plasma atomic emission spectrometer (ICP-AES) and trace elements (including rare-earth elements, REE) by inductively coupled plasma mass spectrometry (ICP-MS) at the College of Oceanic and Atmospheric Sciences, Oregon State University, Oregon, USA. ICP-AES analysis used approximately 0.1 g rock powder that was first mixed with 0.9 g lithium borate in a carbon crucible and then melted in a furnace at 1100 °C for 30 min. The fused glass samples were dissolved (using a magnetic stirring device) in 100 ml of 1% HNO3 solution with Ge as an internal standard. International granite standards such as G2 and

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Table 1 Summary of samples of the Siah-Kuh granitoid stock and associated dikes Sample

Lithology

Mineral assemblage

MZ-2 MZ-6 MZ-32 MZ-72 MZ-8 MZ-12 MZ-13 MZ-36 MZ-15 MZ-26 MZ-30 MZ-50 MZ-3 MZ-37 MZ-48 MZ-21 MZ-23 MZ-22 MZ-52 MZ-16 MZ-29 MZ-33 MZ-35 MZ-40

Leucogranodiorite Leucogranodiorite Leucogranodiorite Leucogranodiorite Leucomonzogranite Leucomonzogranite Leucomonzogranite Leucomonzogranite Alkali granite Alkali granite Alkali granite Alkali granite Syenite Diorite Diorite Quartz diorite Quartz diorite Monzonite Monzonite Quartz monzonite Quartz monzonite Quartz monzonite Quartz monzonite Quartz monzonite

Pl + Kfs + Hbl + Bt + Qtz + Opq + Apt + (Chl + Epi + Tnt) Pl + Kfs + Hbl + Bt + Qtz + Opq + Apt + (Chl + Epi + Tnt) Pl + Kfs + Hbl + Bt + Qtz + Opq + (Chl + Epi + Tnt) Pl + Kfs + Hbl + Bt + Qtz + Opq + Apt + (Chl + Cal + Ser) Pl + Kfs + Bt + Qtz + Opq + Apt + (Chl + Cal + Epi + Tnt) Pl + Kfs + Hbl + Bt + Qtz + Opq + Apt + (Chl + Cal + Epi + Tnt + Ser) Pl + Kfs + Bt + Qtz + Opq + Apt + (Chl) Pl + Kfs + Bt + Qtz + Apt + (Chl) Pl + Kfs + Bt + Qtz + (Chl + Epi + Tnt + Ser) Pl + Kfs + Bt + Qtz + (Chl) Pl + Kfs + Bt + Qtz + (Chl + Epi + Ser) Pl + Kfs + Bt + Qtz + (Chl) Pl + Kfs + Bt + Apt + (Chl + Cal + Opq + Ser) Pl + Hbl + Opq + Apt + Tnt + (Chl + Cal + Epi) Pl + Hbl + Opq + Apt + (Chl + Tnt + Ser) Pl + Kfs + Hbl + Qtz + Opq + Apt + (Chl + Cal + Epi) Pl + Kfs + Hbl + Qtz + Opq + (Chl) Pl + Kfs + Hbl + Bt + Cpx + Opq + (Chl) Pl + Kfs + Hbl + Bt + Cpx + Opq + (Chl) Pl + Kfs + Hbl + Qtz + Opq + Apt + (Chl + Cal + Epi + Ser) Pl + Kfs + Hbl + Bt + Cpx + Qtz + Opq + (Chl) Pl + Kfs + Hbl + Bt + Cpx + Qtz + Opq + (Chl) Pl + Kfs + Hbl + Bt + Cpx + Qtz + Opq + Apt + (Chl + Cal) Pl + Kfs + Hbl + Cpx + Qtz + Opq + (Chl + Cal)

Mineral abbreviations: Apt, Apatite; Bt, Biotite; Cal, Calcite; Chl, Chlorite; Cpx, Clinopyroxene; Epi, Epidote; Hbl, Hornblende; Kfs, K–feldspar; Opq, Opaque; Pl, Plagioclase; Qtz, Quartz; Ser, Sericite; Tnt, Titanite; and Ur, Uralite. Secondary minerals are in parentheses. The minerals are listed in order of decreasing modal abundance.

RGM1 were also prepared and analyzed for using the same protocol for comparison. ICP-MS analysis of trace elements used 0.1 g rock powders that were dissolved in a mixture of HF, HCl and HNO3 in screw-top teflon vials. An internal standard solution containing indium was then added and the spiked sample dissolution was diluted with 1% HNO3. The internal standard was used for monitoring drift in mass response during measurements. Similarly, international standards including G2, RGM1 and GSP1 (Govindaraju and Mevelle, 1987) were analyzed for comparison. Four samples (MZ2, MZ3, MZ6 and MZ8; Table 2) were analyzed for major and minor elements by X-ray fluorescence spectrometry at the SGS Canada Inc. Ontario, Canada, and trace elements by inductively coupled plasma mass spectrometry (ICP-MS) at the University of Saskatchewan ICP-MS Laboratory. ICP-MS trace element analyses were made on a Perkin-Elmer Elan 5000 instrument after a standard acid-digestion procedure using a mixture of HF/HNO3 or a sinter procedure using Na2O2 (for REE). The accuracy and precision of the analytical results were monitored by using USGS standards (G2, RGM1 and GSP1) and found to be in the range of 0.1 wt% for major oxides and 5–10% for trace elements. Seven representative samples (two leucogranodiorites, one leucomonzogranite, one alkali granite, one syenite, one diorite dike and one monzonite dike; Table 4) were selected for whole-rock Sm–Nd isotope analysis at the Saskatchewan Isotope Laboratory. All samples were dis-

solved in Parr microwave digestion vessels using HF/ HNO3 acid mixtures, followed by redigestion with HCl. Samples for isotope dilution analysis were spiked with a mixed 150Nd–149Sm tracer prior to microwave digestion. Bulk REE were purified using standard ion exchange techniques with AG50Wx8 resin in HCl media. Sm and Nd were purified using Eichrom LN resin and dilute HCl media. Sm and Nd analyses were performed using double Re filaments with a tantalum oxide activator and run on a Thermo Finnigan Triton thermal ionization mass spectrometer. Internal normalization for mass fractionation was to a value of 0.7219 for 146Nd/144Nd. Repeat analyses of an internal Nd standard (Ames metal) run during the period of the analyses reported was 0.512130 ± 8 (1r on 7). This standard has previously been calibrated relative to the La Jolla neodymium standard and all samples are quoted relative to a value of 0.511842 for the La Jolla standard. The tNd and TDM values were calculated for the following reference values: 143 Nd/144NdCHUR (present) = 0.512638, 147Sm/144NdCHUR (present) = 0.1967, 143Nd/144Nddepleted mantle (present) = 0.513153 and 147 Sm/144Nddepleted mantle (present) = 0.2137. 5. Results 5.1. Petrography The granitoid rocks of the Siah-Kuh stock are generally coarse to medium grained with a hypidiomorphic inequi-

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Table 2 Whole-rock geochemical compositions of Siah-Kuh granitoid stock Lithology

Leucogranodiorite

Leucomonzogranite

Alkali granite

Syenite

Sample

MZ-2

MZ-6

MZ-32

MZ-72

MZ-8

MZ-12

MZ-13

MZ-36

MZ-15

MZ-26

MZ-30

MZ-50

MZ-3

SiO2 (wt%) TiO2 Al2O3 Fe2O3T MnO MgO CaO Na2O K2O P2O5 LOI

70.1 0.35 14.4 4.25 0.09 0.88 4.39 3.55 0.80 0.06 1.23

72.1 0.12 15.4 1.88 0.03 0.22 4.49 4 0.96 0.01 1.00

69.4 0.38 14.1 3.98 0.09 0.70 3.04 3.99 0.51 0.08 2.34

74.7 0.27 11 2.24 0.05 0.78 1.95 4.83 0.75 0.05 2.53

76.5 0.10 12 1.45 0.03 0.24 1.95 4.19 1.38 0.01 2.2

72.3 0.40 12.8 4.65 0.11 0.78 2.40 4.37 1.68 0.08 1.15

75.7 0.21 12.4 2.58 0.05 0.01 1.12 5.32 1.05 0.04 0.65

75.1 0.15 12.6 2.04 0.04 0.07 1.66 4.98 1.62 0.03 0.77

77.5 0.07 10.5 1.71 0.05 0.06 0.44 5.04 4.73 0.1 0.42

76 0.11 11.4 1.82 0.08 0.01 0.54 4 4.41 0.02 0.62

77.2 0.15 11.1 1.70 0.05 0.01 0.44 3.61 4.53 0.01 0.18

77.8 0.06 11.7 1.01 0.02 0.04 0.38 3.55 3.68 0.02 0.26

44.1 0.51 17.9 8.52 0.28 7.74 6.84 5.49 0.07 0.01 8.6

Total V (ppm) Cr Ni Cu Rb Sr Y Zr Nb Ba Zn Sc Th U Hf Ta La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu (La/Yb)N (Dy/Yb)N Eu/Eu*

100.2 52 25 nd 16 20 211 29 120 3.99 140 86 9 .35 2.05 0.48 0.76 0.24 8.3 17.01 2.07 8.69 2.51 0.78 3.55 0.57 4.19 0.85 2.57 0.45 2.58 0.36 2.33 1.09 0.81

100.2 21 13.6 3.24 11 15 211 13.77 41 4 172 53 2.43 2.67 0.8 0.91 0.14 7.35 13.81 1.52 5.33 1.14 0.86 1.2 0.22 1.46 0.33 1.08 0.21 1.13 0.18 4.42 0.82 2.50

98.6 27 6 29 nd 10 80 26 57 3 92 44 5.2 1.56 0.37 2.16 0.37 7 15 2 9 3 1 2.9 0.6 3.9 1.23 3.2 0.63 3 0.45 1.67 0.87 1.01

99.2 14 4 9 nd nd 52 33 151 nd 119 nd 16 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd

100.1 5.01 22 2.73 14 32 53 58 121 8 335 51 3.27 5.46 1.63 3.97 0.59 12.27 27.04 3.27 12.91 3.42 0.54 4.29 0.75 5.34 1.19 3.88 0.64 4.38 0.64 2.01 0.82 0.43

100.7 24 12 27 nd 30 167 49 102 5 275 29 5.7 3.34 1.05 4.42 0.37 10 21 3.2 14 4.6 0.98 4.9 1.1 7.13 2.4 5.9 1.16 5.5 0.83 1.3 0.87 0.63

99.2 8 30 57 nd nd 79 29 66 nd 201 nd 8 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd

99.1 3.14 14 99 nd 13 111 19 136 3 250 10 4.7 3.88 1.35 4.97 0.7 10 19 2.3 9 2 0.67 1.9 0.4 2.5 0.83 2.3 0.56 3 0.5 2.39 0.56 1.11

100.6 0.01 7 30 nd 62 35 52 129 7 695 8 4.7 9.25 2.41 6.35 0.98 20 43 5.8 22 6 0.41 5.4 1.16 8 2.5 6.8 1.44 7.3 1.16 1.96 0.73 0.22

99.1 5.15 22 43 nd 22 35 35 95 5 235 9 4.6 6.98 2.07 4.65 0.83 17 33 4.5 17 4.6 0.41 4 0.9 5.7 1.7 4.9 1.06 5.5 0.88 2.22 0.69 0.29

99.0 3.3 nd nd 0.07 31 44 37 135 6 364 12 5.6 5.67 1.77 5.5 0.73 24 49 6 24 6 0.6 4.5 0.8 6 1.8 5 1.08 5.5 0.86 3.13 0.73 0.34

98.5 0.57 nd nd 0.25 49 25 33 79 4 635 6 5.5 9 2.3 3.9 0.6 32 63 7 24 4.5 0.27 3.6 0.64 4 1.3 4 0.9 4.7 0.81 4.88 0.69 0.22

100.2 87 44 2.9 9.99 2 39 39 97 4 70 147 15.61 1.88 0.56 0.94 0.26 4.57 10.65 1.42 6.28 2.01 0.55 3.59 0.65 4.39 0.96 3.04 0.41 2.74 0.39 1.2 1.07 0.62

LOI, loss on ignition; T, total iron as Fe2O3; wt%, major and minor element oxides in weight percent; ppm, trace elements in part per million; nd, not determined.

granular texture. They are composed mainly of quartz (20–35% modal), plagioclase (10–35%), alkali feldspar (25–45%) and ferromagnesian minerals (hornblende and biotite; 5–10%) (Table 1). Sericite, chlorite, calcite and epidote are secondary phases. Fine (mm-size) accessory fluorapatite and zircon occur as inclusions in biotite and hornblende. Other accessory minerals are titanite and magnetite. Magnetite occurs both as discrete grains and as inclusions in hornblende and biotite. Most grains of magnetite are euhedral in shape, suggesting their primary nature (Clarke, 1992). Quartz occurs as anhedral to subhe-

dral grains, clustered between feldspar grains and locally forms micrographic, granophyric and worm-like myrmekitic intergrowths. Micrographic and granophyric textures indicate rapid and simultaneous crystallization of quartz and K–feldspar from an under cooled liquid at shallow depth (Barker, 1983; Clarke, 1992). Myrmekite usually forms by metasomatism, exsolution (Cox et al., 1979; Pitcher, 1993) or direct crystallization during deformation (Pitcher, 1993). The Siah-Kuh granitoid rocks are variably strained; typically with weak subgrain development in quartz and broken feldspars. Plagioclase varies from oligo-

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Table 3 Whole-rock geochemical compositions of associated dikes in the Siah-Kuh stock Lithology

Diorite

Sample

MZ-37

SiO2 (wt%) TiO2 Al2O3 Fe2O3T MnO MgO CaO Na2O K2O P2O5 LOI

51.2 0.71 15.6 10.6 0.2 7.25 10.24 2.53 0.36 0.06 1.56

Total V (ppm) Cr Ni Cu Rb Sr Y Zr Nb Ba Zn Sc Th U Hf Ta La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu (La/Yb)N (Dy/Yb)N Eu/Eu*

100.3 378 72 38 90 5 200 16 13 0.7 88 77 7.9 0.18 0.09 0.63 0.1 1.8 4.5 0.8 4 1.4 0.61 1.7 0.4 2.5 0.7 1.95 0.38 1.7 0.27 0.76 0.98 1.21

Quartz diorite

Monzonite

Quartz monzonite

MZ-48

MZ-21

MZ-23

MZ-22

MZ-52

MZ-16

MZ-29

MZ-33

MZ-35

MZ-40

49.7 0.62 16.5 10.7 0.23 6.66 8.11 4.1 1.17 0.04 1.92

55.2 0.67 15 9.9 0.24 3.98 7.11 5.13 0.98 0.07 1.66

54.3 0.71 16.4 11.4 0.27 4.04 7.53 2.81 0.57 0.1 2.17

55.9 0.96 14.9 10.7 0.21 3.36 6.11 3.09 1.53 0.35 2.11

57.6 0.95 15.8 8.2 0.21 2.05 4.64 4.03 2.51 0.41 2.21

61.1 0.67 14.8 7.2 0.38 3.23 4.59 4.46 1.11 0.07 1.65

56.8 0.99 15.8 8.2 0.22 2.59 4.59 4.38 2.81 0.41 2.15

55.8 1.01 15.6 10.9 0.22 3.13 6.40 2.67 0.61 0.1 2.33

60.9 0.95 15.2 9.7 0.2 2.09 5.41 2.59 0.44 0.12 2.19

58.8 0.95 15.8 8.5 0.23 2.13 4.88 3.42 1.46 0.42 2.19

99.8 238 49 2.8 0.39 19 168 58 32 2.5 111 104 8.9 0.76 0.27 1.5 0.15 5 13 2 12 5 1.2 5.7 1.2 9 2.7 6.6 1.36 5.9 0.87 0.61 1.02 0.69

99.9 nd nd nd nd nd 136 118 48 nd 212 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd

100.3 nd nd 66 35 nd 132 37 51 nd 135 113 53 nd nd nd nd 10 nd nd nd nd nd nd nd nd nd nd nd nd nd

99.2 nd nd 18 98 nd 420 37 168 nd 277 1.14 27 nd nd nd nd 25.5 nd nd nd nd nd nd nd nd nd nd nd nd nd

98.6 nd nd nd 19 nd 368 40 234 nd 332 133 15 nd nd nd nd 30.2 nd nd nd nd nd nd nd nd nd nd nd nd nd

99.3 133 27 nd nd 25 206 26 56 1.9 182 28 6 2.77 0.65 2.3 0.19 7 16 2.25 9 2.8 0.78 2.7 0.6 4 1.25 3.3 0.67 3.2 0.49 1.57 0.84 0.87

98.9 nd nd nd nd nd 228 39 220 nd 247 146 15 nd nd nd nd 26.7 nd nd nd nd nd nd nd nd nd nd nd nd nd

98.8 nd nd nd nd nd 175 27 61 nd 179 nd nd nd nd nd nd 8 nd nd nd nd nd nd nd nd nd nd nd nd nd

99.8 nd nd nd nd nd 186 33 78 nd 188 nd nd nd nd nd nd 11 nd nd nd nd nd nd nd nd nd nd nd nd nd

98.8 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd

LOI, loss on ignition; T, total iron as Fe2O3; wt%, major and minor element oxides in weight percent; ppm, trace elements in part per million; nd, not determined.

clase to labradorite in composition, displays varying degrees of saussuritization, and is characterized by oscillatory zoning in some samples. Oscillatory zoning indicates variations in the crystallization environment (Holton et al., 1999). The grains of plagioclase commonly exhibit corroded boundaries and in some samples are replaced by muscovite, epidote and calcite. Subhedral K–feldspar (orthoclase and rarely microcline) occurs as phenocrysts, perthitic crystals and intergrowth with quartz. K–feldspar commonly shows evidence of sericitization and kaolinitization. Hornblende is present as subhedral prisms and is

often partly altered to chlorite, epidote and calcite. Biotite forms subhedral flakes of ribbon appearance and is commonly altered to chlorite, titanite, epidote, muscovite and opaque minerals. 5.2. Major and trace element compositions of the Siah-Kuh granitoid rocks Results of whole-rock geochemical analysis of all samples are presented in Table 2. The Siah-Kuh granitoid samples display SiO2 and MgO contents ranging

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Table 4 Nd isotope compositions of the Siah-Kuh granitoid stock Sample lithology

Sm (ppm)

Nd (ppm)

147

Sm/144Nd

143

Nd/144Nd (2r)

TDM(Ma) eNd(T)

Siah-Kuh main phase MZ-2 granodiorite MZ-6 granodiorite MZ-8 monzogranite MZ-15 alkali granite MZ-3 syenite

2.604 1.439 4.764 6.00 2.659

9.121 7.382 18.16 22.00 6.852

0.1726 0.1178 0.1586 0.1650 0.2346

0.512709 ± 9 0.512645 ± 8 0.512713 ± 11 0.512689 ± 14 0.512798 ± 7

1640 + 2.00 810 + 2.15 1220 + 2.45 1450 + 1.81 + 2.15

Siah-Kuh dikes MZ-16 quartz monzonite MZ-48 diorite

2.80 5.00

9.00 12.0

0.1882 0.2521

0.512709 ± 12 2640 0.512863 ± 11

+ 1.60 + 2.98

147 Sm/144Nd values of samples Mz-15, -16 and -48 are calculated from the Sm and Nd contents from ICP-MS analyses. TDM values were calculated for the following reference values 143Nd/144Nd = 0.513153 and 147Sm/144Nd = 0.2137 in a present depleted mantle reservoir. tNd values were based on 143 Nd/144NdCHUR (present) = 0.512638 and 147Sm/144NdCHUR (present) = 0.1967 at an emplacement age of 200 Ma.

from 69.4 to 77.8 wt% and from 0.04 to 0.88 wt%, respectively. With increasing SiO2 there is a general increase in K2O (to some extent Na2O as well), Zr, Y, Nb, Ba, Hf, Ta and REE and a corresponding decrease in Al2O3, Fe2O3T, TiO2, CaO, MgO, MnO and Sr (Fig. 2). These variations indicate that plagioclase, hornblende and magnetite have played important roles during the crystallization of the granitoid rocks.These correlations confirm our petrographic observations that the selected samples were not significantly affected by late hydrothermal alteration. Moreover, these correlations may suggest that the diverse lithologies in the Siah-Kuh stock are most likely related by fractionation processes but is inconsistent with geochemical data (see below). Also, the presence of mafic enclaves indicates that magma mixing may also played a role. Low MgO content suggests separation of mafic minerals and petrographic evidence favors a probable fractionation involving both hornblende and biotite. Fig. 3a and b show that the Siah-Kuh granitoid rocks are subalkaline and plot in the calc–alkaline field on an AFM diagram, except for three samples (MZ2, MZ12 and MZ32) that straddle the calc–alkaline/tholeiitic boundary owing to their elevated Fe2O3T contents. The Siah-Kuh granitoid rocks can be assigned to the medium and high potassium rock series (Table 2). Classification of these rocks by the aluminum saturation index (ASI, Zen, 1986) indicates that most of the granitic rocks are metaluminous and that only a few are slightly peraluminous (i.e. Al index <1.1; Fig. 3b). The peraluminous nature of those rocks may be attributable to differentiation of hornblende (Zen, 1986) or heterogeneity of water content in the protolith (Waight et al., 1998). Overall, the mineralogy of the Siah-kuh granitoid rocks, which include biotite, hornblende, magnetite, fluorapatite and zircon, strongly suggests a metaluminous source. The SiO2 and Na2O contents, molecular A/CNK ratio, K2O/Na2O ratio, abundance of Cr and Ni, microgranitoid enclaves, key modal minerals (such as hornblende, titanite, and zircon) all suggest that Siah-Kuhe granitoids show I-type characteristics on the basis of the Chappel and White, 1974.

5.3. Rare-earth elements of the Siah-Kuh granitoids There is an apparent increase in the total REE contents from leucogranodiorite through leucomonzogranite to alkali granite in the Siah-Kuh stock (Table 2). Fig. 4 shows that the samples of leucogranodiorite and leucomonzogranite from the Siah-Kuh stock are characterized by relatively flat chondorite-normalized REE patterns with only slight enrichment in LREE over HREE [(La/ Yb)N = 1.31–4.42]. The chondrite-normalized REE patterns of two leucogranodiorite samples are characterized by the absence of any Eu anomalies, whereas that of the third sample (MZ32) has a positive Eu anomaly (Fig. 4b). This positive Eu anomaly is consistent with the anomalously high abundance of plagioclase in this sample, confirming an accumulation of this mineral. The chondrite-normalized REE patterns of all leucomonzogranite samples have weak negative to no Eu anomalies [(Eu/Eu*)N = 0.43–1.11; Fig. 4c]. Similarly, the alkali granites are characterized by slightly LREE-enriched chondrite-normalized REE patterns [(La/Yb)N = 1.95–4.82} with pronounced negative Eu anomalies [(Eu/ Eu*)N = 0.22–0.34] and a general lack of fractionation among HREE (Fig. 4d). In general the chondrite normalized REE patterns of all Siah-Kuh granitoid rocks are characterized by moderate LREE enrichment [(La/ Yb)N = 1.31–4.82] and unfractionated HREE [(Gd/ Yb)N = 0.50–1.33]. The negative Eu anomalies in leucomonzogranite and alkali granite are indicative of feldspar involvement during fractionation and/or melting (Rollinson, 1993). The unfractionated HREE (and Y) patterns and low Sr and Eu contents suggest that the magma was produced outside the garnet stability field (i.e., plagioclase stable without garnet; Cullers and Graf, 1989; Drummond and Defant, 1990; Rapp et al., 1991; Springer and Seck, 1997; Martin, 1999; Pe-Piper et al., 2002). 5.4. Tectonic setting of the Siah-Kuh granitoids Trace element discrimination diagrams (Fig. 5) depict the probable tectonic settings of intrusive rocks contain-

M. Arvin et al. / Journal of Asian Earth Sciences 30 (2007) 474–489

Fig. 2. Binary plots of selected major element oxides and trace element versus SiO2 content.

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Fig. 2 (continued)

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483

Fig. 3. Classification of the Siah-Kuh stock: (a) alkali versus silica diagram (Irvine and Baragar, 1971), (b) AFM diagram (Kuno, 1968), and (c) plot of A/ NK versus A/CNK [A/NK = molar Al2O3/(Na2O + K2O) and A/CNK = molar Al2O3/(CaO + Na2O + K2O)].

ing 56–80% SiO2 (Pearce et al., 1984; Pearce, 1996). These diagrams, based on immobile elements, are effective at discriminating different tectonic environments in the for-

mation of granitoids. The Siah-Kuh granitoid samples are plotted mainly in the volcanic arc + syncollisional granite fields (Fig. 5a). Fig. 5b shows that the Siah-Kuh

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granitoid rocks plot within the volcanic arc granite field. Fig. 5c shows that the Siah-Kuh granitoids plot mainly in the field of island arc felsic magmas. Comparing the data with the field of arc-type granitoids (Fig. 5d), the Siah-Kuh granitoids plot mainly in the field of primitive island arc/continental margin arc. In the Sr/Y versus Y diagram the Siah-Kuh granitoid rocks have low Sr/Y values and plot in the field of mantle-derived arc magmas (Fig. 5e). Therefore, trace element compositions (i.e., Rb, Y, Nb, Th, Ta, Zr and Yb) suggest that the Siah-Kuh granitoid stock is similar to intrusive rocks from Phanerozoic arcs (Fig. 5a–e). The ocean ridge granite (ORG)-normalized multi element diagram of the Siah-Kuh granitoids shows selective enrichment in large ion lithophile elements (LILE) and depletion in high field strength elements (HFSE) (Fig. 6b–d). This type of multi-element patterns shows that the Siah-Kuh granitoids are similar to volcanic arc granites (Pearce et al., 1984). Moreover, the distinctly negative Nb anomalies (Fig. 6) are typical of magmas derived from a subduction-metasomatized mantle (Wilson, 1989). 5.5. Sm–Nd isotope compositions of the Siah-Kuh granitoids The five representative samples of the Siah-Kuh granitoids have Nd contents from 7.38 to 22 ppm and 147 Sm/144Nd values from 0.1178 to 0.1796, except for the highly altered syenite having only 6.58 ppm Nd and a 147 Sm/144Nd value of 0.2346 (Table 4). This REE composition of the syenite sample is different from that expected from fractional crystallization and is probably attributable to late hydrothermal remobilization. Nevertheless, the Sm– Nd isotope data of these five samples yield an ‘‘errorchron’’ age of 199 ± 30 Ma (MSWD = 1.30) [or 184 ± 82 Ma (MSWD = 1.81) excluding the altered syenite] (Fig. 7). These ages with significant uncertainties are consistent with the field relationships for an emplacement during Late Triassic. These samples are characterized by Nd model ages relative to a depleted mantle reservoir (TDM) of 0.81– 1.64 Ga and a narrow range of eNd(T) values from +1.81 to +2.45 at 200 Ma (Table 4). These isotopic results confirm other geochemical data that leucogranodiorite, leucomonzogranite, alkali granite and syenite of the Siah-Kuh stock were comagmatic and must have a major mantle input. Unfortunately, an evaluation of possible mixing between mantle sources and crustal components (cf. DePaolo, 1981; Anthony, 2005) is not possible owing to (1) the limited variation of etNd values of the Siah-Kuh stock and (2) possible isotopic variations in the Tethyan upper mantle in the Mesozoic (Mahoney et al., 1998; Zhang et al., 2005 and references therein). 5.6. Geochemistry of the dikes in the Siah-Kuh stock Fig. 4. Chondrite-normalized REE plots of the Siah-Kuh stock. Normalization Irvine and Baragar, 1971factors from Taylor and McLennan, 1985.

Results of whole-rock geochemical analysis of 11 dikes are presented in Table 3. In contrast to the main phases of the Siah-Kuh stock, the dikes are subalkaline (Fig. 3a)

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485

Fig. 5. Tectonic discriminant diagrams for the Siah-Kuh granitoids: (a) Y versus Nb, (b) Rb versus Y + Nb (after Pearce et al., 1984), (c) La/Yb versus Th/Yb diagram (after Condie, 1989), (d) Rb/Zr versus Nb diagram (after Brown et al., 1984) and (e) Sr/Y versus Y after Martin, 1993. VAG, volcanic arc granites; ORG, ocean ridge granites; WPG, within-plate granites; COLG, syn-collision granites; and post-COLG, post-collision granites.

and are transitional between tholeiitic and calc-alkaline (Fig. 3b). Fig. 2 shows that Na2O, K2O (though not well defined) and TiO2 of the dikes exhibit positive correlations with SiO2, whereas Al2O3, Fe2 OT3 , MgO, MnO, and CaO show negative correlations with SiO2. Similarly Ba and to some extent Zr and Sr show positive correlations with

SiO2, whereas Y shows a negative correlation with SiO2. The scatter in Na2O versus SiO2, to some extent, K2O and Ba (Fig. 2g, h, and n) may be attributed to combine effects of fractionation, assimilation, late hydrothermal alteration or in part may be an artifact of insufficient sampling. Fig. 4a shows that the diorite and monzonite dikes

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Fig. 7. Whole-rock ‘‘isochron’’ illustrating the Nd isotope compositions of all seven representative samples from the Siah-Kuh granitoid stock. Regression was made for the five samples from the main phase (solid squares) including the altered syenite (MZ-3; see text for discussion), but not the two dike samples (open squares).

are characterized by relatively flat chondrite-normalized REE patterns with slight depletion in LREE over HREE [(La/Yb)N = 0.61–1.57], similar to those of leucogranodiorite except for slight depletion in LREE. The dikes define almost a common trend with the main phase granitoids, suggesting a possible genetic relationship between them, although there is a distinct compositional gap between 60 and 70 wt% SiO2 (Fig. 2). This suggestion is supported by broadly similar eNd(T) values between these dikes and the main phase granitoids (Table 4). The LILE contents of dioritic are enriched above MORB level (Fig. 6a). This along with LILE enrichment above ORG level (Fig. 6b–d), probably indicating contribution of the mantle wedge from dehydrated subducted oceanic crust (Pearce, 1983; Pearce et al., 1984).

6. Discussion and conclusion 6.1. Petrogenesis of the Siah-Kuh granitoid stock

Fig. 6. Ocean ridge- basalt and -granite normalized geochemical patterns for the Siah-Kuh stock (normalization factors from Pearce, 1983 and Pearce et al., 1984).

Granitoid rocks are an important component of continental crust, and different model envisaged for their generation (e.g. direct partial melting of the mantle, fractional crystallization from basaltic magma, etc.). The geological setting and the chemical composition of the rock types within the Siah-kuh granitoid stock, all support the hypothesis that the igneous activity occurred above an active subduction zone. The relatively low contents of Zr, Y, Nb, and La and the Nd isotope data strongly suggest that Siah-Kuh stock is an I-type granitoid formed in an arc environment. Normally for continental arc granitoids a fundamental role is assigned to mantle derived mafic magmas. They may be parental magmas, end members in mixing or assimilation processes, material for lower crustal source regions, and/or heat sources that derived crustal

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melting (Tepper et al., 1993 and references therein). Furthermore, there is an emphasis on the fractional crystallization, crustal anatexis and role of open system processes such as magma mixing and assimilation on the origin of granitic magmas (Tepper et al., 1993), which probably contributed to generation of the Siah-Kuh granitoids. The LILE contents of dioritic dikes and granitoid rocks are enriched above MORB and ORG level (Fig. 6), probably indicating the contribution of the mantle wedge from dehydrated subducted oceanic crust (Pearce, 1983; Pearce et al., 1984). Such a source is commonly invoked for subduction related magmas (e.g. Pearce and Peate, 1995) and is presumably located in the convecting asthenospheric mantle wedge above subducting slab (Stopler and Newman, 1994). On the variation diagrams of SiO2 versus some major and trace elements (Fig. 2), the samples of the felsic intrusion fall on the same trend as samples of the dikes; this might imply that they have a genetic relationship. This may be supported by broadly similar eNd(T) values between dikes and the main phase granitoids. Though a model of fractionation crystallization from a mafic parental magma may be possible in the Siah-Kuh, but is inconsistent with geochemical data: e.g., lack of Eu anomalies in leucogranodiorite and high Sr contents in leucogranodiorite and leucomonzogranite relative to diorites. The lack of Eu anomalies in the least fractionated granodiorite of the felsic intrusion reflects the absence of differentiation in terms of plagioclase extraction in mafic parent melt. In order to evolve to a granodiorite from mantle-derived basic parental magma would require a prominent fractionation (Dokuz et al., 2006). Low MgO concentrations of the samples of the felsic intrusion and other geochemical parameters rule out a direct derivation from mantle wedge. All these indicate a later and new parental melt for the felsic part rather than generation from the mafic rocks by fractional crystallization. This indicates necessity of an additional process in the generation of the felsic rocks. On the other hand, partial melts, particularly from tholeiitic basalts and to lesser extent calc-alkaline basaltic amphibolites, yield parent magmas that have lower K2O and higher SiO2 contents (Dokuz et al., 2006), similar to Siah-Kuh granitoid stock. Their low Al2O3 (<15 wt%), low Sr contents, lack of any prominent Eu anomaly, flat to slightly LREE enrichment and flat HREE patterns suggest their formation at low pressure (65 kbar) and presence of plagioclase and lack of garnet as a residue in their genesis (Rapp et al., 1991; Beard and Lofgren, 1989; Springer and Seck, 1997). Experimental studies have shown that granitoid magmas were produced at different water fugacity during partial melting of basaltic compositions or an amphibolitic source and that water fugacity strongly influences melt composition and residuum mineralogy (Tepper et al., 1993 and references therein). They stated that lithologic diversity among the granitoids is primarily the result of variable water fugacity during melting and variations in the water contents of mantle derived basalts may reflect variability in the amount of water given off by the subducted

487

slab. The negative Eu anomalies and slightly or no Eu anomalies in granitoid rocks, such as Siah-Kuh stock, can be the result of fH2O variations during the melting of subcontinental lithosphere and unlikely to result from crystal fractionation (Tepper et al., 1993). Furthermore, field and petrological observations (e.g., oscillatory zoning in plagioclase and presence of microgranular enclaves), low Rb/Sr values (average = 0.62) indicate that the sub-solvus Siah-Kuh granitoid stocks evolved in an open system and that the magma mixing may also contribute to its generation, though its extent is unclear. 6.2. Implications for initiation of Neotethys subduction It is generally believed that the detachment of Central Iran from Arabia during Late Permian and its northwestward movement led to the formation of a new ocean (Neotethys) along the present main Zagros folded-thrust belt (Berberian and Berberian, 1981; Berberian and King, 1981). The Middle Triassic orogeny in Iran is interpreted as the result of subduction of the Neotethys oceanic crust underneath Central Iran, which initiated regional metamorphism and magmatism along the Sanandaj-Sirjan zone (Berberian and King, 1981). Plutonic bodies in Iran appear to have been generated extensively during 150 m.y. of Early Mesozoic subduction (220–65 Ma) along the Sanandaj-Sirjan zone (Fig. 1a). During this period, plutonic activity was episodic, probably due to episodic plate motions and changes in the consumption rate of the oceanic crust, with climaxes around the Middle Triassic, Late Jurassic and Late Cretaceous (Berberian and Berberian, 1981). The Triassic plutonic rocks are well exposed in the southeastern part of the Sanandaj-Sirjan zone in the Sirjan and Esfandagheh area (Fig. 1a). The absence of these rocks in the central and northwestern sections of this zone is probably either due to their being covered by Jurassic and Cretaceous sedimentary rocks or because subduction of Neotethys started from the southeast and propagated northward (Berberian and Berberian, 1981). Berberian and Berberian, 1981 suggested that the Triassic plutonic rocks might form as a result of steeply dipping Neotethys oceanic slab (Mariana-type) underneath southeastern Central Iran. The distribution of Mesozoic plutons such as the Late Triassic Siah-Kuh granitoid stock is mostly restricted to regions close to the active plate margin and close to the coeval trench, the position of which is represented by the ophiolites emplaced along the Main Zagros reverse fault (Fig. 1a). The geochemical compositions (Table 2) and tectonomagmatic discrimination diagrams (Fig. 5a–e) of the Siah-Kuh granitoid rocks suggest their affinity with arc magmas formed in a primitive island arc/continental margin volcanic arc setting, perhaps close to a trench zone. The theoleiitic or transitional nature of the Siah-Kuh samples suggests that they probably belong to the early tholeiitic stage of arc development (Ringwod, 1974). As in many

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I-type granitoids, basaltic magmatism plays an important role (Pitcher, 1987). The onset of subduction of the Neotethys oceanic crust beneath the southeast part of Central Iranian microcontinent in Triassic could account for the arc volcanism. With increased convergence rate, the supply of arc-related basaltic magmas would increase as larger amounts of slab-derived aqueous fluid are added to the mantle wedge. As the subduction and underplating continued, mafic arc magmas with significant fluids were produced as the consequence of dehydration of the oceanic crust and partial melting of mantle wedge which in turn initiated partial melting of the subcontinental lithosphere which is considerably metasomatised and enriched (Huppert and Sparks, 1988; Pe-Piper et al., 2002; Takagi, 2004). Partial melting of this subcontinental lithosphere by mafic arc magmas at variable water fugacity probably led to the formation of the Siah-kuh granitoid rocks (Tepper et al., 1993). However, the low contents of Nb and Mg may point to the fact that contamination did not play an important role in the formation of Siah-Kuh granitoids. This and the fact that the Siah-Kuh stock and associated dikes have largely juvenile source [eNd(T) = +1.81 to +2.45] may suggest that the island arc did not develop on a continental crust but on a foundation of oceanic crust not far from an active continental margin (Wilson, 1989). Furthermore, the lack of voluminous plutonism in Triassic may point to either fast subduction into the central and northwestern part of the Sanandaj-Sirjan zone or slowing convergence rate. Volcanic arc granitoids related to subduction of the Neotethys have also been reported from Turkey, where their genesis and emplacement are related to either single or double subduction or multi-phase convergence (Parlak, 2006). Also, further east along the Neotethyan belt, an intraoceanic accretion and later marginal arc magmatism of Mesozoic age has been suggested in the Kohistan– Karakorum–Himalayas area (Rolland et al., 2002). To test these models in the Sanadaj-Sirjan zone, additional studies on Mesozoic plutonic rocks and other related rocks are needed. Acknowledgements We wish to thank Dr. B. Eglington (Nd isotope) and Dr. J. Fan (ICPMS) for analytical assistance, and the Natural Science and Engineering Research Council (NSERC) of Canada for financial support. M. Arvin is also grateful to Dr. S. Bloomer of Geology Department of Oregon State University for facilitating and financial support for part of the analytical work. We also thank Dr. J. Tepper and Dr. G. Pe-Piper for their valuable comments and critical reviews of this manuscript. References Alavi, M., 1994]. Tectonics of the Zagros orogenic belt of Iran: new data and interpretations. Tectonophysics 229, 211–238.

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