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Isotopic dating of the Khoy metamorphic complex (KMC), northwestern Iran: A significant revision of the formation age and magma source
Precambrian Research 185 (2011) 87–94
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Isotopic dating of the Khoy metamorphic complex (KMC), northwestern Iran: A significant revision of the formation age and magma source Hossein Azizi a,∗ , Sun-Lin Chung b , Tsuyoshi Tanaka c , Yoshihiro Asahara d a
Mining Department, Faculty of Engineering, University of Kurdistan, Pasdaran, Sanandaj, Iran Department of Geosciences, National Taiwan University, P.O. Box 13-318, Taipei 106, Taiwan Center for Chronological Research, Nagoya University, Chikusa, Nagoya 464-8602, Japan d Department of Earth and Environmental Sciences, Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8601, Japan b c
a r t i c l e
i n f o
Article history: Received 9 September 2010 Received in revised form 17 November 2010 Accepted 2 December 2010 Available online 13 December 2010 Keywords: Iran Khoy metamorphic complex Granite Zircon U–Pb dating Sr–Nd isotopes
a b s t r a c t The Khoy metamorphic complex (KMC) consists of metabasite, metagranite and some metasediments, which are overlain by the Oligo-Miocene Qom formation. Because there are no reliable stratigraphic sequences or isotope dating data available, discordant ages for the formation and tectonic setting of the KMC have been proposed by many studies. For example, various studies have suggested a Jurassic age, Cretaceous age or younger age. In the present study, new isotopic information based on U–Pb zircon and Rb–Sr mineral isochron dating has revealed the origin and timing of metamorphism for the KMC. These new results do not confirm the previously proposed ages. The results of U–Pb dating from 65 points in the core and rings of zircon minerals from the metagranite and metabasite rocks suggests a late Proterozoic (550–590 Ma) consolidation of granitic and basaltic magma. In addition, Rb–Sr mineral isochron dating indicates that the metamorphism occurred 146 Ma ago. Finally, mylonitic foliation overprinted the metamorphic and granitic complex, probably occurring in the pre-Oligocene. Furthermore, initial 143 Nd/144 Nd and 87 Sr/86 Sr ratios strongly suggest that the original basic magma formed in a depleted mantle in the subduction zone, with some contamination from recycled sediments. Published by Elsevier B.V.
1. Introduction The Iranian Plateau is divided into eight major parts based on geological structure (Fig. 1). The majority of the parts in the western, northern and eastern regions of Iran are separated by colored mélange zones (Stocklin and Nabavi, 1972). The geology of Iran is immensely complicated because of the collision between the micro-continents and the overprinting of many metamorphic and tectonic events (Stocklin and Nabavi, 1972). The absence of reliable isotope ages has further complicated the interpretation of the tectonic setting of volcanic and granitic rocks. However, the study of Iranian geology has entered a new phase in the past decade. Recently, researchers (e.g., Hassanipak and Ghazi, 2000; Masoudi et al., 2002; Khalatbari-Jafari et al., 2003, 2004; Hassanzadeh et al., 2008; Ghalamghash et al., 2009; Karimpour et al., 2010) have reported isotope ages (e.g., Rb–Sr, Sm–Nd, U–Pb, K–Ar and Ar–Ar ages) for the igneous rocks in Iran. Most of the geochronological data, however, do not clarify the previous “relative age” of the igneous rocks. For an improved understanding of the geology of Iran, and particularly the plutonic and volcanic rocks, the relative
∗ Corresponding author. Tel.: +98 871 6660073; fax: +98 871 6660073. E-mail address: [email protected] (H. Azizi). 0301-9268/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.precamres.2010.12.004
ages need to be reassessed and/or revised based on reliable isotope dating. The Khoy metamorphic complex (KMC) is one of the more complicated geological areas of Iran. Many studies have investigated the KMC complex, and various papers have been published suggesting different ages and different tectonics for the Khoy area. The results of these previous studies will be outlined in this paper. Due to the data deficiency of reliable isotope dating for the KMC rocks, the tectonic models are variable and based on different hypotheses. Many discordant models have been proposed, including the Neotethyan arc, back-arc and mid-ocean ridge models. We collected several samples of metagranites and metabasites (metabasalt and metagabbro) from the KMC. In this paper, we report new geochronological data for the metagranite and metabasic rocks based on zircon U–Pb dating and Rb–Sr minerals isochron dating, and show that there are additional differences between the source origin and tectonic setting of the KMC and the Khoy ophiolite complex (KOC). 2. Regional geology The Khoy region is located between 38◦ 30 –38◦ 50 north and 44◦ 37 –45◦ 00 east in northwestern Iran, approximately 650 km northwest of Tehran (Fig. 1). According to the tec-
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H. Azizi et al. / Precambrian Research 185 (2011) 87–94
Fig. 1. Geological map of the Khoy area (modified from Azizi et al., 2006). The sample locations are shown and the prefix “IRK” from the sample names is abbreviated. The inset map is a simplified geological map of Iran (Stocklin and Nabavi, 1972).
Fig. 2. Zircon grains in metagranites and metabasite: (a) IRK-GR-1, (b) IRK-GR-2 and (c) IRK-AM-1. Each number indicates the type of analysis and location. Data are shown in Table 1.
H. Azizi et al. / Precambrian Research 185 (2011) 87–94
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tonic and magmatic map of Iran (Stocklin and Nabavi, 1972; Aghanabati, 2004), this region encompasses the junction between the Alborz–Azarbaijan (AA) Block (Stocklin and Nabavi, 1972) in the northeast and the Arabian Plate in the southwest. A simplified geological map of the study area (Fig. 1) shows that the KMC is positioned between the Khoy ophiolite complex (KOC) in the west, that is overlain on the Arabian plate, and Paleozoic unmetamorphosed sedimentary rocks of the AA Block in the east (Ghoraishi and Arshadi, 1987; Radfar and Amini, 1999; Azizi et al., 2006). West of the KMC, an unmetamorphic ophiolite suite from the late Cretaceous or earlier period is outcropped (Hassanipak and Ghazi, 2000; Khalatbari-Jafari et al., 2004; Azizi et al., 2006; Monsef et al., 2010). Recent studies by Azizi and Jahangiri (2008) and Azizi and Moinevaziri (2009) suggest that the Khoy ophiolite may be a part of the Northern Anatolian Fault (NAF) dismembered ophiolite. They suggested a new branch of Tethyan oceanic crust, called the KhoyZanjan branch, trending NW–SE parallel to the Zagros fault. The remnant of this oceanic crust is observed in the Tabriz fault, which may be a suture zone between the Sanandaj–Sirjan Zone (SSZ) and the AA Block in the northwestern Iran. In this regard, Azizi and Moinevaziri (2009) proposed an accordion tectonic model in this part of Iran. The KMC is comprised two rock assemblages: metabasites and metagranites. The metabasites generally consist of greenschist and amphibolites, with interbedded marble, quartzite, meta-arkoses, and graphitic schist. The metabasite protolith consists of mafic to intermediate volcanic rocks with tholeiitic and calc-alkaline magmatic signatures in an island arc tectonic setting (Azizi et al., 2002). Geothermobarometric analysis of these rocks suggested peak metamorphic conditions of 450–680 ◦ C at 5.5–7.5 kbar (Azizi et al., 2006). The P–T–t path of these activities was clockwise, with an average geothermal gradient of 27–37 ◦ C/km (Azizi et al., 2006), concordant with the temperature regime of collision zones. Hassanipak and Ghazi (2000) proposed that the KMC rocks were the result of ophiolite emplacement (sole metamorphism) in the late Cretaceous, and suggested that the KMC rocks show an inverse metamorphism grade, which increases toward the KOC. Some Ar–Ar ages of the KMC rocks have been reported, suggesting that the amphibole minerals from the hornblende gabbroic rocks were generated 158.6–154.9 ± 1.0 Ma (Hassanipak and Ghazi, 2000). Additionally, Khalatbari-Jafari et al. (2003) showed that the rocks were generated approximately 110–104 Ma. 3. Analytical techniques Zircons were separated from ∼2 kg samples using conventional heavy-liquid and magnetic separation techniques. Cathodoluminescence (CL) images (Fig. 2) were taken at the Institute of Earth Sciences at Beijing University. Zircon U–Pb isotopic analyses were obtained by the laser ablation inductively coupled plasma mass spectrometer (LA-ICPMS) at the Department of Geosciences, National Taiwan University (NTU). The LA-ICPMS techniques were conducted using an Agilent 7500s quadrupole ICPMS and a New Wave UP213 laser ablation system. The laser ablation was performed using a helium carrier gas that substantially reduces the deposition of ablated material onto the sample surface and greatly improves transport efficiency, thus increasing the signal intensities compared to “conventional” ablation using argon as the carrier gas (Eggins et al., 1998; Günther and Heinrich, 1999; Jackson et al., 2004). During the experiments, approximately 1 min was spent measuring the gas blank. The results indicated sensitivities less than 1000 counts per second (cps) for all measured isotopes. Calibration was performed using the GJ-1 zircon standard (provided by the Australian Research Council National Key Centre for Geochemi-
Fig. 3. These diagrams show an age of 566.0 ± 9.3 Ma (a) U–Pb concordia diagram for zircons (core and rim) from the metagranite sample IRK-GR-1. (b) The distribution of zircon ages, with a peak at approximately 566 Ma. (c) Most of the ages were close to the average value, with slight differences.
cal Evolution and Metallogeny of Continents (GEMOC) at Macquarie University, Sydney). For Sm–Nd and Rb–Sr dating, the rock sample was crushed to smaller than 60 m, and 0.3–0.5 g of the rock powder was chemically decomposed with 5 ml HF, 1 ml HNO3 and 2 ml HClO4 . Conventional column chemistry was conducted to isolate Rb, Sr and Rare Earth Elements (REEs) using cation exchange resin (BioRad AG50W-X8, 200–400 mesh) with an HCl eluent. Nd and Sm were separated from the extracted REE fraction by another cation exchange column with ␣-hydroxy isobutyric acid (␣-HIBA) as eluent.
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Table 1 LA-ICPMS data for zircon from IRK-AM-1 (amphibolite), IRK-GR-1 (metagranite) and IRK-GR-2 (metagranite). ±1
206
0.0594 0.0603 0.0594 0.0591 0.0599 0.0592 0.0596 0.0613 0.0591 0.0602 0.0585 0.0629 0.0589 0.0582 0.0593 0.0586 0.0587 0.0586 0.0605 0.0598 0.0584 0.0581 0.0582 0.0583 0.0592
0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0005 0.0005 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006
0.0889 0.0805 0.0886 0.0892 0.0836 0.0886 0.0882 0.0913 0.0793 0.0894 0.0888 0.0833 0.0894 0.0894 0.0887 0.0884 0.0903 0.0903 0.0834 0.0885 0.0877 0.0901 0.0833 0.0831 0.0888
0.0017 0.0016 0.0017 0.0018 0.0016 0.0017 0.0017 0.0018 0.0016 0.0018 0.0018 0.0017 0.0018 0.0018 0.0017 0.0018 0.0018 0.0018 0.0017 0.0018 0.0017 0.0018 0.0017 0.0017 0.0018
0.78 0.40 0.35 0.32 1.22 0.97 0.57 0.54 0.36 0.38 0.42 0.35 0.48 0.34 0.31 0.65 0.22 0.04 0.69 0.42 0.24 0.39 0.53 0.79 0.58 0.56 0.16 0.48
0.0533 0.0606 0.0607 0.0640 0.0565 0.0667 0.0596 0.0610 0.0613 0.0632 0.0599 0.0610 0.0592 0.0617 0.0593 0.0597 0.0597 0.0563 0.0735 0.0607 0.0568 0.0690 0.1594 0.0602 0.0593 0.1104 0.0549
0.0007 0.0006 0.0007 0.0007 0.0008 0.0008 0.0006 0.0006 0.0007 0.0007 0.0006 0.0007 0.0006 0.0007 0.0007 0.0006 0.0007 0.0006 0.0008 0.0007 0.0011 0.0007 0.0017 0.0007 0.0006 0.0012 0.0016
0.0353 0.0828 0.0883 0.0945 0.0144 0.0886 0.0917 0.0891 0.0893 0.0956 0.0966 0.0919 0.0904 0.0974 0.0929 0.0862 0.0951 0.0716 0.1802 0.0924 0.0813 0.0697 0.4594 0.0923 0.0938 0.3203 0.0593 0.0897
0.63 0.52 0.76 0.32 1.09 0.44 0.99 0.53 0.54 0.25 1.23 0.17 0.79 0.19 0.21
0.0641 0.0627 0.0715 0.0611 0.0696 0.0616 0.0616 0.0632 0.0624 0.1413 0.0870 0.0607 0.0577 0.0660 0.0602
0.0006 0.0006 0.0006 0.0006 0.0008 0.0007 0.0006 0.0007 0.0006 0.0013 0.0014 0.0006 0.0013 0.0006 0.0006
0.1186 0.1233 0.1626 0.0977 0.1469 0.0981 0.0997 0.0962 0.0976 0.4108 0.1326 0.0920 0.0543 0.1268 0.0974
Spot
Th/U
207
IRK-AM-1 AM-1-01a AM-1-02 AM-1-03a AM-1-04a AM-1-05 AM-1-05Ba AM-1-06a AM-1-07a AM-1-08 AM-1-08Ba AM-1-09a AM-1-10 AM-1-10Ba AM-1-11a AM-1-12a AM-1-13a AM-1-14a AM-1-15a AM-1-16 AM-1-17a AM-1-18a AM-1-19a AM-1-20 AM-1-21 AM-1-22a Wt. mean age (2, n = 18)
1.39 0.78 1.00 1.03 1.01 0.99 1.01 1.09 1.14 1.00 1.11 1.15 1.16 1.20 1.41 0.84 1.09 1.10 0.88 1.08 1.15 1.22 1.64 0.94 1.43
IRK-GR-1 GR-1-01 GR-1-02 GR-1-03a GR-1-04a GR-1-05 GR-1-06a GR-1-07a GR-1-08a GR-1-09a GR-1-10a GR-1-11a GR-1-12a GR-1-13a GR-1-14a GR-1-15a GR-1-16a GR-1-17a GR-1-18 GR-1-19 GR-1-20a GR-1-21 GR-1-22 GR-1-22C GR-1-23a GR-1-24a GR-1-25 GR-1-25R GR-1-26a wt. mean age (2, n = 18) IRK-GR-2 GR-2-01 GR-2-01C GR-2-02 GR-2-03a GR-2-04 GR-2-05a GR-2-06a GR-2-07a GR-2-08a GR-2-09 GR-2-10 GR-2-11a GR-2-12 GR-2-13 GR-2-14a
Pb/206 Pb
Pb/238 U
±1
±1
Error corr.
0.7270 0.6692 0.7254 0.7268 0.6901 0.7224 0.7249 0.7718 0.6460 0.7417 0.7163 0.7228 0.7257 0.7173 0.7248 0.7141 0.7302 0.7297 0.6956 0.7300 0.7057 0.7214 0.6679 0.6685 0.7244
0.0154 0.0146 0.0156 0.0158 0.0153 0.0158 0.0156 0.0168 0.0143 0.0165 0.0160 0.0163 0.0163 0.0153 0.0152 0.0156 0.0157 0.0160 0.0157 0.0162 0.0154 0.0157 0.0144 0.0150 0.0159
0.9198 0.9026 0.9105 0.9054 0.8870 0.8999 0.9135 0.9021 0.8878 0.8874 0.8850 0.8780 0.8795 0.9210 0.9333 0.9074 0.9161 0.9019 0.8798 0.8960 0.9095 0.9125 0.9201 0.8917 0.9022
549 499 547 551 517 547 545 563 492 552 548 516 552 552 548 546 557 557 517 547 542 556 516 515 548 550.1
10 9 10 10 10 10 10 11 9 10 10 10 10 10 10 10 11 11 10 10 10 11 10 10 10 4.7
0.0008 0.0019 0.0020 0.0022 0.0004 0.0021 0.0021 0.0021 0.0021 0.0022 0.0023 0.0022 0.0021 0.0023 0.0022 0.0020 0.0022 0.0017 0.0043 0.0022 0.0020 0.0016 0.0109 0.0022 0.0022 0.0075 0.0015 0.0021
0.2597 0.6911 0.7383 0.8339 0.1121 0.8139 0.7540 0.7501 0.7546 0.8323 0.7972 0.7722 0.7375 0.8282 0.7596 0.7087 0.7825 0.5557 1.8265 0.7730 0.6368 0.6630 10.0964 0.7663 0.7658 4.8755 0.4488 0.7419
0.0072 0.0166 0.0181 0.0209 0.0036 0.0218 0.0181 0.0178 0.0181 0.0204 0.0192 0.0191 0.0179 0.0201 0.0191 0.0170 0.0196 0.0134 0.0457 0.0189 0.0245 0.0158 0.2408 0.0190 0.0183 0.1150 0.0211 0.0176
0.8428 0.9608 0.9427 0.9312 0.7509 0.8776 0.9647 0.9774 0.9648 0.9515 0.9695 0.9479 0.9671 0.9636 0.9374 0.9763 0.9397 0.9745 0.9481 0.9648 0.6305 0.9901 0.9904 0.9547 0.9887 0.9981 0.5245 0.9906
224 513 545 582 92 547 566 550 551 588 595 566 558 599 572 533 586 446 1068 570 504 434 2437 569 578 1791 372 554 566.0
5 11 12 13 2 12 13 12 12 13 13 13 13 13 13 12 13 10 23 13 12 10 48 13 13 37 9 12 9.3
0.0023 0.0024 0.0031 0.0019 0.0029 0.0019 0.0019 0.0019 0.0019 0.0080 0.0028 0.0018 0.0011 0.0025 0.0019
1.0473 1.0660 1.6015 0.8234 1.4093 0.8337 0.8467 0.8382 0.8393 8.0002 1.5903 0.7704 0.4321 1.1538 0.8079
0.0218 0.0229 0.0329 0.0179 0.0362 0.0198 0.0182 0.0211 0.0195 0.1694 0.0509 0.0169 0.0171 0.0246 0.0168
0.9270 0.9029 0.9409 0.8926 0.7774 0.8273 0.9004 0.7882 0.8414 0.9138 0.6623 0.8857 0.5253 0.9116 0.9274
722 749 971 601 884 603 613 592 600 2219 802 567 341 769 599
13 14 17 11 16 11 11 11 11 36 16 11 7 14 11
207
Pb/235 U
206 Pb/238 U 207 Pb/206 Kp age (Ma±±1) (Ma ± 1)
1028
21
2450
17
1806
18
2243
16
H. Azizi et al. / Precambrian Research 185 (2011) 87–94
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Table 1 (Continued) Spot
Th/U
207
GR-2-15a GR-2-16 GR-2-17a GR-2-18 GR-2-19a GR-2-20a GR-2-21 GR-2-22a GR-2-23 wt. mean age (2, n = 12)
0.50 1.09 0.40 0.76 0.23 0.51 0.01 0.44 0.83
0.0590 0.0653 0.0599 0.0680 0.0606 0.0687 0.1204 0.0602 0.0718
a
Pb/206 Pb
±1
206
0.0006 0.0006 0.0006 0.0008 0.0006 0.0022 0.0011 0.0008 0.0007
0.0953 0.1278 0.0964 0.1368 0.0956 0.1010 0.3448 0.0952 0.1659
Pb/238 U
±1
0.0019 0.0025 0.0019 0.0027 0.0019 0.0023 0.0067 0.0019 0.0033
207
Pb/235 U
0.7755 1.1511 0.7958 1.2821 0.7992 0.9573 5.7227 0.7899 1.6412
±1
Error corr.
0.0177 0.0249 0.0174 0.0325 0.0184 0.0483 0.1207 0.0216 0.0387
0.8533 0.8993 0.8906 0.7870 0.8530 0.4433 0.9199 0.7329 0.8384
206 Pb/238 U 207 Pb/206 Kp age (Ma±±1) (Ma ± 1)
587 776 593 827 589 620 1910 586 990 595.2
11 14 11 15 11 13 32 11 18 8.4
1962
16
Choosen for mean age calculation.
For natural isotope analysis of Sr and Nd, 100–200 ng of Sr were loaded with 2 M-H3 PO4 on a Ta-single filament, and 100–200 ng of Nd were loaded with 2 M-H3 PO4 on Re triple filaments. Procedure blanks were 20 ng for Rb and 3 ng for Sr and were considered negligible amounts for our samples. Strontium and Nd isotope ratios were measured with a thermal ionization mass spectrometer (TIMS; VG Sector 54-30 of Nagoya University). The mass fractionations during the Sr and Nd isotope measurements were corrected based on the following ratios: 86 Sr/88 Sr = 0.1194 and 146 Nd/144 Nd = 0.7219. The abundance of Rb, Sr, Nd and Sm based on the ID method was measured by a thermal ionization quadrupole mass spectrometer (Finnigan MAT Thermoquad THQ of Nagoya University). In this study, the natural Sr and Nd isotope ratio standards NBS987 and JNdi-1 (Tanaka et al., 2000) were used. Averages and 2 errors for the repeated analyses of the standards during this study were as follows: NBS987: 87 Sr/86 Sr = 0.710238 ± 28 (n = 13) and JNdi-1: 143 Nd/144 Nd = 0.512097 ± 19 (n = 14). To check the reliability of the Rb–Sr and Sm–Nd analyses, the Rb, Sr, Nd and Sm fractions were assessed for the Geological Survey of Japan (GSJ) reference rock sample, JG-1a, and were measured parallel to our samples.
4. Zircon morphology and Th/U ratios The majority of zircons in the two metagranite samples IRKGR1 and IRK-GR2 (Fig. 1) had a zoning shape, but some of them had a prismatic form (Fig. 2a and b). In these rocks, some zircons exhibited older ages, such as 1890–2450 Ma, and these results suggested that they might be xenocrysts. Most of the zircons in the metabasite IRK-AM1 (Fig. 2c) were rounded and were not distinctly crystalline in shape. In general, when the shape and morphology are problematic for classifying zircons, their compositions yield information about their origin. Additionally, the Th/U ratio can be used to determine the origin of zircon. The Th/U ratio is usually higher in magmatic zircon than in metamorphic zircon. For example, low ratios of <0.31 indicate a metamorphic origin, and very low ratios, in the range of 0.02–0.11, indicate a metamorphic origin (Chen et al., 2007). Fuping et al. (2007) used the Th/U ratio to discriminate metamorphic and igneous zircons in the basement metamorphic rocks in the Songliao Basin. They showed that zircons with high Th/U ratios (0.15–1.37) imply magmatic events, and zircons with low Th/U ratios (0.06–0.34) indicate metamorphic origin. The chemical composition of the zircons in the amphibolite IRK-AM-1 and the metagranite IRK-GR-1 and IRK-GR-2 imply that most of the zircons had high Th/U ratios and were magmatic in origin. The majority of Th/U ratios from the metagranites in this study were >0.6 (IRK-GR-1 = 0.16–1.22 and IRK-GR-2 = 0.20–1.23) although some samples had low values of 0.04. The ratios for IRK-AM-2 varied from 0.78 to 1.65.
5. U–Pb dating We chose one amphibolite (IRK-AM-1) and two metagranite (IRK-GR-1 and IRK-GR-2) samples, since these are the two groups representing the main composition of KMC rocks. In general, the morphology and Th/U ratio of zircon grains infer the magmatic origin. Sixty-five points were measured from the center and rim of the zircon grains (analyzed locations and names are shown in Fig. 2). The results of the LA-ICPMS analyses are listed in Table 1. The U–Pb age data from this study clearly indicated that the zircon crystallization occurred 500–600 Ma ago. However, a few zircons, especially those from the granitic rocks (IRK-GR-1-22C and IRKGR-1-25), showed older ages of 1890–2440 Ma, probably because the zircon originated from S-type granite. These atypical ages were omitted from the calculation of age average (median or mean), but were included in the analysis of the mode, which is the value occurring most frequently in the distribution (Allegre, 2008). The concordia diagrams for the samples IRK-GR-1, IRK-GR-2 and IRK-AM-1 are provided in Figs. 3–5, which show that the U–Pb data for all samples fell below the concordia line. This indicates that a portion of the Pb was lost after zircon crystallization. The ages of the granitic rock were calculated to be 566.0 ± 9.3 Ma (IRK-GR-1) and 595.2 ± 8.4 Ma (IRK-GR-2), and the age of the amphibolite was calculated as 550.1 ± 4.7 Ma (IRK-AM-1). These ages are consistent with zircon U–Pb ages of granitoids in the north and northwest region of Iran (Hassanipak and Ghazi, 2000).
6. Rb–Sr mineral isochron The metabasite IRK-AM-3 was used for mineral separation (Table 2) and was composed of hornblende with minor biotite and pyroxene as a mafic group, plagioclase, plagioclase + alkali feldspar + quartz and fine-grained components (smaller than 125 m). Apart from the mafic group (mainly hornblende), the other three groups displayed high correlation and seemed to form an isochron. The mafic group also showed a small difference in the 87 Sr/86 Sr ratio, and this was probably caused by mineral disequilibrium during metamorphic events. It is possible that some of the original minerals (e.g., clinopyroxene) were retained as an inclusion in the hornblende. The other three groups in the IRK-AM-3 rock yielded a good Rb–Sr isochron date of 146 Ma with a low Mean Square Weighted Deviate (MSWD) of 0.00054 (Fig. 6). This age is much younger than the zircon U–Pb age of 550 Ma, but is similar to the K–Ar and Ar–Ar ages of hornblende and whole rock as reported by Hassanipak and Ghazi (2000) and Khalatbari-Jafari et al. (2003). During metamorphism, Rb–Sr and/or Sm–Nd isotope systems in a rock are generally reset (e.g., Dickin, 2005; Allegre, 2008). Therefore, the mineral isochron age of Rb–Sr strongly suggests a metamorphic event occurred in this area.
92
Table 2 Sm–Nd, and Rb–Sr isotopic data. Sample (rock type)
Rb (ppm)
Sr (ppm)
87
IRK-AM-1 (metabasite) IRK-AM-2 (metabasite) IRK-AM-3 (metabasite) IRK-AM-4 (metabasite) IRK-G-1 (metagabbro) IRK-G-2 (metagabbro) IRK-GR-1 (metagranite) IRK-GR-2 (metagranite) IRK-GR-4 (metagranite) Minerals of IRK-AM-3 WR < 125 m Pl > 70 Hbl > 80b Pl + Kf
55.70 16.19 13.78 1.51 1.57 1.48 135.5 24.00 52.50
300.1 328.6 192.5 149.5 174.8 188.8 102.1 155.5 524.8
11.35 16.55 11.46 10.93
151.2 394.1 162.5 161.7
Rb/86 Sr
(87 Sr/86 Sr)p
2SEa
(87 Sr/86 Sr)i
Nd (ppm)
Sm (ppm)
147
Sm/144 Nd
(143 Nd/144 Nd)p
0.537 0.143 0.207 0.029 0.026 0.023 3.85 0.447 0.289
0.709741 0.706819 0.705968 0.70364 0.704904 0.704306 0.730294 0.715769 0.704404
±16 ±14 ±13 ±13 ±15 ±14 ±17 ±14 ±15
0.7055 0.7057 0.7043 0.7036 0.7047 0.7043 0.7001 0.7123 0.7021
20.59 13.88 10.87 1.66
5.470 3.850 3.760 0.750
0.1606 0.1678 0.2091 0.2714
0.512510 0.512402 0.512869 0.513044
0.85
0.310
0.2210
0.512703 0.512314 0.512404 0.512685
0.217 0.122 0.226 0.196
0.705417 0.705221 0.705717 0.705383
±16 ±16 ±16 ±14
(143 Nd/144 Nd)i
ε0 Nd (T = 0 Ma)
εt Nd (T = 550 Ma)
±8 ±8 ±8 ±8
0.5119 0.5118 0.5121 0.5121
−2.5 −4.6 4.5 7.9
−0.3 −2.3 3.6 3.6
±10 ±8 ±8 ±8
0.5119
1.3 −6.3 −4.6 0.9
−0.3
2SE
143
144
T
143
144
T
εt Nd = (( Nd/ Nd)sample /( Nd/ Nd) CHUR − 1) × 10, 000. a Standard error. b Data for this mineral are not used for age calculation.
Fig. 4. U–Pb concordia diagram for the metagranite sample IRK-GR-2 (a, b, c). These diagrams show the age of 595.2 ± 8.4 Ma. Please refer Fig. 3 for details.
7. Discussion
The initial ratios of 87 Sr/86 Sr and 143 Nd/144 Nd were calculated based on the zircon U–Pb age of 550 Ma. In the metabasites, the initial 87 Sr/86 Sr ratios varied from 0.7034 to 0.7057, and the initial ratios of 143 Nd/144 Nd varied from 0.5117 to 0.5121 (Table 2). The ε0 Nd values for granitic rocks varied from −6.3 to 0.9, suggesting that these rocks originated from the melting of the continental crust. The values of ε0 Nd (present) and εt Nd (t = 550 Ma) were calculated based on the value of (143 Nd/144 Nd)present , the value of CHUR (Chondritic uniform reservoir, 0.512638), (143 Nd/144 Nd)t , and the CHUR value at t = 550 Ma (Depaolo and Wasserburg, 1976;
H. Azizi et al. / Precambrian Research 185 (2011) 87–94
p = present at time; i = initial ratio for T = 0 and p = primary ratio which is calculated based on T = 550 Ma. The natural ratios were normalized based on 146 Nd/144 Nd = 0.7219 and 87 Sr/86 Sr = 0.1194. The average and 1 for standards samples for JNdi-1 and NBS987 Sr are =0.512097 ± 0.000010 (n = 13) and 0.71024 ± 0.00001 (n = 17), respectively. CHUR (Chondritic uniform reservoir) values, 147 Sm/144 Nd = 0.1967 and 143 Nd/144 Nd = 0.512638, were used to calculate εNd. JG-1a as reference sample was measured as follows: 87 Sr/86 Sr = 0.710980 ± 14(2SE), 143 Nd/144 Nd = 0.512375 ± 8 (2SE), Rb = 172.6 ppm, Sr = 188.9 ppm, Nd = 18.48, Sm = 4.68 ppm. εNd was calculated based on the ε0 Nd = ((143 Nd/144 Nd)sample /(143 Nd/144 Nd)CH UR) − 1) × 10,000 and
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Fig. 6. The mineral isochron age was 146 Ma, indicating the age of a metamorphic event in the late Jurassic to early Cretaceous Period. This age was calculated using Isoplot v3.7 (Ludwig, 2009) with = 6.54 × 10−12 y−1 (Lugmair and Marti, 1978; Begemann et al., 2001).
Fig. 7. A 143 Nd/144 Nd–87 Sr/86 Sr diagram for the metabasite rocks for T = 0 Ma. This shows that the change in 87 Sr/86 Sr ratios is not related to an alteration caused by seawater, and further implies a mixing of source magma with subduction sediments (Rollinson, 2007), since seawater would not significantly change the Nd-isotope ratio. The trend seen in the metabasites is similar to the volcanic rocks of the Lesser Antilles island arc and infers the mixing of source magma with slab sediments. The average Atlantic sediments were from Davidson (1983). GLOSS is the average composition of global subducting sediment from Plank and Langmuir (1998).
Fig. 5. U–Pb concordia diagram for the metabasite sample IRK-AM-1 (a, b, c). The diagrams show the age 550.1 ± 4.7 Ma. Please refer Fig. 3 for details.
Jacobsen and Wasserburg, 1980). The value of ε0 Nd in the metabasite rocks varied between −4.6 and 7.9, and the εt Nd values ranged between −2.3 and 3.6 (Table 2). From these results, the metabasites appear to have heterogeneous origins. The εNd values suggest that the metabasites are not compatible with a mid-ocean ridge basaltic (MORB) origin, although the 147 Sm/144 Nd ratios (0.16–0.27) in the metabasites were compatible with the MORB 147 Sm/144 Nd ratios (0.15–0.30; Allegre, 2008). The values of εt Nd suggest that the metabasite originated from a depleted mantle. In the Nd–Sr isotope diagram (Rollinson, 2007), all of the Khoy metabasites show a negative correlation between the Nd and Sr ratios, and also showed mixing of the MORB source and recycled (subducted) sediments, as is observed in the Lesser Antilles (Figs. 7 and 8). The isotope char-
Fig. 8. The Sr–Nd diagram for metabasite rocks for T = 550 Ma. This diagram shows the trends of sedimentary rock recycling and magmatic differentiation during magmatic evolution. The Khoy metabasite rocks confirm the sedimentary affinity to the source magma. Sao Miguel data was obtained from Dosso and Murthy (1980).
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acteristics confirm an island arc origin of the Khoy amphibolite, as suggested by the chemical analyses of major and trace elements (Azizi et al., 2002). Our U–Pb values suggest that the age of the original rocks is 550–590 Ma and they belong to the late Proterozoic. Hassanzadeh et al. (2008) applied U–Pb dating to many granitic bodies in the central north and northwest regions of Iran and suggested that most of the granitic rocks actually formed in Neoproterozoic to lower Paleozoic. Additionally, Gass (1977) indicated an intra-oceanic island arc environment for the mainly volcanoclastic sediments based on the northeastern African and Arabian basements, and suggested that magmatic products were generated at or above easterly-inclined Benioff zones in the late Proterozoic (∼600 Ma). Kröner et al. (1979) reported an age of 586 ± 43 Ma for metagranitic rocks in the Saudi Arabia shield with Pan African ages, based on the Rb–Sr isochron. The age of granitic rocks in the Saudi Arabia shield increases toward the west and center of this shield (e.g., 760–850 Ma; Kröner and Stern, 2004). These dates suggest the accretion-type growth of Gondwanaland from the northern and eastern parts. As a result, our data suggest that the KMC was part of Gondwanaland and originated from an active zone in the late Proterozoic during the Pan African orogen. This suggestion is compatible with new U–Pb dating results from granitic rocks in the Iranian Plateau (Hassanzadeh et al., 2008). Thus, most of the older granitic bodies in the Iranian plateau are not related to the Arabian and Iranian plate collision in the Mesozoic Age or earlier, but rather were generated in the late Proterozoic and early Paleozoic and were a part of the Pan-African orogenic between 500 and 800 Ma. 8. Conclusions Zircon U–Pb dating revealed that the Khoy metamorphic protolith does not belong to the Jurassic or Cretaceous, as was previously proposed. The age data strongly suggests that the Khoy metagranite and original basalt was generated in the late Proterozoic, and the rocks are not related to Neotethyan oceanic crust or the collision between the Arabian Plate and the Alborz–Azarbaijan Block. The age of 550 Ma also demonstrates that the granitic rocks in the Khoy area have the same age as most of the older granitic rocks in Iran. Furthermore, the Rb–Sr mineral isochron age for high-grade metamorphic minerals (amphibolite facies) and the previously reported Ar–Ar and K–Ar ages imply that the metamorphism occurred in the late Jurassic or early Cretaceous after a shear zone developed and mylonitic foliation (Sm ) overprinted the S1 foliation. Acknowledgements H. Azizi would like to thank to University of Kurdistan for financing a sabbatical period at Nagoya University. The authors would like to thank Beijing University for zircon separation analysis. We are also grateful to. P.A. Cawood and H. Rollinson and anonymous reviewers for critical and constructive comments, S. Razyani and A. Khani for laboratory assistance and R. Koole for assistance during the submission of this work. References Aghanabati, A., 2004. Major Sedimentary-Structural Units of Iran. Geological Survey of Iran. Allegre, C.J., 2008. Isotope Geology. Cambridge University Press, 512p. Azizi, H., Jahangiri, A., 2008. Cretaceous subduction-related volcanism in the northern Sanandaj-Sirjan Zone. Iran. J. Geodyn. 45, 178–190. Azizi, H., Moinevaziri, H., Mohajjel, M., Yagobpoor, A., 2006. PTt path in metamorphic rocks of the Khoy region (northwest Iran) and their tectonic significance for Cretaceous–Tertiary continental collision. J. Asian Earth Sci. 27, 1–9. Azizi, H., Moinevaziri, H., Noghreayan, M., 2002. Geochemistry of metabasites in the north of Khoy. J. Sci. (in Farsi), University of Isfahan, Iran 15, 1–20.
Azizi, H., Moinevaziri, H., 2009. Review of the tectonic setting of Cretaceous to Quaternary volcanism in northwestern Iran. J. Geodyn. 47, 167–179. Begemann, F., Ludwig, K.R., Lugmair, G.W., Min, K., Nyquist, L.E., Patchett, P.J., Renne, P.R., Shih, C.Y., Villa, I.M., Walker, R.J., 2001. Call for and improved set of decay constants for geochronological use. Geochim. Cosmochim. Acta 65, 111–121. Chen, R.X., Zheng, Y.F., Zhao, Z.F., Tang, J., Wu, F.Y., Liu, X.M., 2007. Zircon U–Pb age and Hf isotope evidence for contrasting origin of bimodal protoliths for ultrahigh-pressure metamorphic rocks from the Chinese Continental Scientific Drilling Project. J. Metamorp. Geol. 25, 873–894. Dickin, A.P., 2005. Radiogentic Isotope Geology. Cambridge University Press, UK, 492p. Davidson, J.P., 1983. Lesser Antilles isotopic evidence of the role of subducted sediment in island arc magma genesis. Nature 306, 253–256. Depaolo, D.J., Wasserburg, G.J., 1976. Nd isotopic variations and petrogenetic models. Geophys. Res. Lett. 3, 249–252. Dosso, L., Murthy, V.R., 1980. A Nd isotopic study of the Kerguelen Islands: inferences on enriched oceanic mantle sources. Earth Planet. Sci. Lett. 48, 268–276. Eggins, S.M., Kinsley, L.P.J., Shelley, J.M.M., 1998. Deposition and elemental fractionation processes during Atmospheric pressure laser sampling for analysis by ICPMS. Appl. Surf. Sci. 127–129, 278–286. Fuping, P., Wenliang, X., Debin, Y., Quanguo, Z., Xiaoming, L., Zhaochu, H., 2007. Zircon U–Pb geochronology of basement metamorphic rocks in the Songliao Basin. Chin. Sci. Bull. 52, 942–948. Gass, I.G., 1977. The evolution of the Pan African crystalline basement in NE Africa and Arabia. J. Geol. Soc. 134 (2), 129–138. Ghalamghash, J., Nédélec, A., Bellon, H., Vousoughi-Abedini, M., Bouchez, J.L., 2009. The Urumieh plutonic complex: a magmatic record of the geodynamic evolution of the Sanandaj–Sirjan zone (NW Iran) during Cretaceous times – Part II: petrogenesis and 40 K/40 Ar dating. J. Asian Earth Sci. 5, 401–415. Ghoraishi, M., Arshadi, S., 1987. Geology Map of Khoy Quadrangle (1:250,000). Geology Survey of Iran. Günther, D., Heinrich, C.A., 1999. Enhanced sensitivity in laser ablation–ICP mass spectrometry using helium–argon mixtures as aerosol carrier. J. Anal. Atom. Spectrom. 14, 1363–1368. Hassanipak, A.A., Ghazi, M., 2000. Petrology, geochemistry and tectonic setting of the Khoy ophiolite, northwest Iran: implications for Tethyan tectonics. J. Asian Earth Sci. 18, 109–121. Hassanzadeh, J., Stockli, D.F., Horton, B.K., Axen, G.J., Stockli, L.D., Grove, M., Schmitt, A.K., 2008. U–Pb zircon geochronology of late Neoproterozoic–Early Cambrian granitoids in Iran: implications for paleogeography, magmatism, and exhumation history of Iranian basement. Tectonophysics 451, 71–96. Jacobsen, S.B., Wasserburg, G.J., 1980. Sm–Nd isotopic evolution of chondrites. Earth Planet. Sci. Lett. 50, 139–155. Jackson, S.E., Pearson, N.J., Griffin, W.L., Belousova, E.A., 2004. The application of laser ablation-inductively coupled plasma-mass spectrometry to in situ U–Pb zircon geochronology. Chem. Geol. 211, 47–69. Karimpour, M.H., Stern, C.R., Farmer, G.L., 2010. Zircon U–Pb geochronology, Sr–Nd isotope analyses, and petrogeneticstudy of the Dehnow diorite and Kuhsangi granodiorite (Paleo-Tethys), NE Iran. J. Asian Earth Sci. 37, 384–393. Khalatbari-Jafari, M., Juteau, T., Bellon, H., Emami, H., 2003. Discovery of two ophiolite complexes of different ages in the Khoy area (NW Iran). CR Geosciences 335. Académie des Sciences, Paris, pp. 917–929. Khalatbari-Jafari, M., Juteau, T., Bellon, H., Whitechurch, H., Cotten, J., Emami, H., 2004. New geological, geochronological and geochemical investigations on the Khoy ophiolites and related formations, NW Iran. J. Asian Earth Sci. 23, 507–535. Kröner, A., Roobol, M.J., Ramsay, C.R., Jackson, N.J., 1979. Pan African ages of some gneissic rocks in the Saudi Arabian Shield. J. Geol. Soc. 136 (4), 455–461. Kröner, A., Stern, R.J., 2004. Pan-African Orogeny. Encycl. Geol 1, 1–12. Ludwig, 2009. ISOPLOT: a plotting and regression program for radio-isotope data. U.S. Geol. Survey Open-File Report, 91-445, 39p. Lugmair, G.W., Marti, K., 1978. Lunar initial 143 Nd/144 Nd: differential evolution of the Lunar crust and mantle. Earth Planet. Sci. Lett. 39, 349–357. Masoudi, F., Yardley, B.W.D., Clif, R.A., 2002. Rb–Sr geochronology of pegmatites, plutonic rocks and a hornfels in the region south-west of Arak, Iran. J. Sci., Islamic Republic of Iran 13, 249–254. Monsef, I., Rahgoshay, M., Mohajjel, M., Shafaii Moghadam, H., 2010. Peridotites from the Khoy ophiolite complex, NW Iran: evidences of mantle dynamics in a Supra-Subduction-zone context. J. Asian Earth Sci. 38, 105–120. 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Precambrian Research journal homepage: www.elsevier.com/locate/precamres
Isotopic dating of the Khoy metamorphic complex (KMC), northwestern Iran: A significant revision of the formation age and magma source Hossein Azizi a,∗ , Sun-Lin Chung b , Tsuyoshi Tanaka c , Yoshihiro Asahara d a
Mining Department, Faculty of Engineering, University of Kurdistan, Pasdaran, Sanandaj, Iran Department of Geosciences, National Taiwan University, P.O. Box 13-318, Taipei 106, Taiwan Center for Chronological Research, Nagoya University, Chikusa, Nagoya 464-8602, Japan d Department of Earth and Environmental Sciences, Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8601, Japan b c
a r t i c l e
i n f o
Article history: Received 9 September 2010 Received in revised form 17 November 2010 Accepted 2 December 2010 Available online 13 December 2010 Keywords: Iran Khoy metamorphic complex Granite Zircon U–Pb dating Sr–Nd isotopes
a b s t r a c t The Khoy metamorphic complex (KMC) consists of metabasite, metagranite and some metasediments, which are overlain by the Oligo-Miocene Qom formation. Because there are no reliable stratigraphic sequences or isotope dating data available, discordant ages for the formation and tectonic setting of the KMC have been proposed by many studies. For example, various studies have suggested a Jurassic age, Cretaceous age or younger age. In the present study, new isotopic information based on U–Pb zircon and Rb–Sr mineral isochron dating has revealed the origin and timing of metamorphism for the KMC. These new results do not confirm the previously proposed ages. The results of U–Pb dating from 65 points in the core and rings of zircon minerals from the metagranite and metabasite rocks suggests a late Proterozoic (550–590 Ma) consolidation of granitic and basaltic magma. In addition, Rb–Sr mineral isochron dating indicates that the metamorphism occurred 146 Ma ago. Finally, mylonitic foliation overprinted the metamorphic and granitic complex, probably occurring in the pre-Oligocene. Furthermore, initial 143 Nd/144 Nd and 87 Sr/86 Sr ratios strongly suggest that the original basic magma formed in a depleted mantle in the subduction zone, with some contamination from recycled sediments. Published by Elsevier B.V.
1. Introduction The Iranian Plateau is divided into eight major parts based on geological structure (Fig. 1). The majority of the parts in the western, northern and eastern regions of Iran are separated by colored mélange zones (Stocklin and Nabavi, 1972). The geology of Iran is immensely complicated because of the collision between the micro-continents and the overprinting of many metamorphic and tectonic events (Stocklin and Nabavi, 1972). The absence of reliable isotope ages has further complicated the interpretation of the tectonic setting of volcanic and granitic rocks. However, the study of Iranian geology has entered a new phase in the past decade. Recently, researchers (e.g., Hassanipak and Ghazi, 2000; Masoudi et al., 2002; Khalatbari-Jafari et al., 2003, 2004; Hassanzadeh et al., 2008; Ghalamghash et al., 2009; Karimpour et al., 2010) have reported isotope ages (e.g., Rb–Sr, Sm–Nd, U–Pb, K–Ar and Ar–Ar ages) for the igneous rocks in Iran. Most of the geochronological data, however, do not clarify the previous “relative age” of the igneous rocks. For an improved understanding of the geology of Iran, and particularly the plutonic and volcanic rocks, the relative
∗ Corresponding author. Tel.: +98 871 6660073; fax: +98 871 6660073. E-mail address: [email protected] (H. Azizi). 0301-9268/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.precamres.2010.12.004
ages need to be reassessed and/or revised based on reliable isotope dating. The Khoy metamorphic complex (KMC) is one of the more complicated geological areas of Iran. Many studies have investigated the KMC complex, and various papers have been published suggesting different ages and different tectonics for the Khoy area. The results of these previous studies will be outlined in this paper. Due to the data deficiency of reliable isotope dating for the KMC rocks, the tectonic models are variable and based on different hypotheses. Many discordant models have been proposed, including the Neotethyan arc, back-arc and mid-ocean ridge models. We collected several samples of metagranites and metabasites (metabasalt and metagabbro) from the KMC. In this paper, we report new geochronological data for the metagranite and metabasic rocks based on zircon U–Pb dating and Rb–Sr minerals isochron dating, and show that there are additional differences between the source origin and tectonic setting of the KMC and the Khoy ophiolite complex (KOC). 2. Regional geology The Khoy region is located between 38◦ 30 –38◦ 50 north and 44◦ 37 –45◦ 00 east in northwestern Iran, approximately 650 km northwest of Tehran (Fig. 1). According to the tec-
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Fig. 1. Geological map of the Khoy area (modified from Azizi et al., 2006). The sample locations are shown and the prefix “IRK” from the sample names is abbreviated. The inset map is a simplified geological map of Iran (Stocklin and Nabavi, 1972).
Fig. 2. Zircon grains in metagranites and metabasite: (a) IRK-GR-1, (b) IRK-GR-2 and (c) IRK-AM-1. Each number indicates the type of analysis and location. Data are shown in Table 1.
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tonic and magmatic map of Iran (Stocklin and Nabavi, 1972; Aghanabati, 2004), this region encompasses the junction between the Alborz–Azarbaijan (AA) Block (Stocklin and Nabavi, 1972) in the northeast and the Arabian Plate in the southwest. A simplified geological map of the study area (Fig. 1) shows that the KMC is positioned between the Khoy ophiolite complex (KOC) in the west, that is overlain on the Arabian plate, and Paleozoic unmetamorphosed sedimentary rocks of the AA Block in the east (Ghoraishi and Arshadi, 1987; Radfar and Amini, 1999; Azizi et al., 2006). West of the KMC, an unmetamorphic ophiolite suite from the late Cretaceous or earlier period is outcropped (Hassanipak and Ghazi, 2000; Khalatbari-Jafari et al., 2004; Azizi et al., 2006; Monsef et al., 2010). Recent studies by Azizi and Jahangiri (2008) and Azizi and Moinevaziri (2009) suggest that the Khoy ophiolite may be a part of the Northern Anatolian Fault (NAF) dismembered ophiolite. They suggested a new branch of Tethyan oceanic crust, called the KhoyZanjan branch, trending NW–SE parallel to the Zagros fault. The remnant of this oceanic crust is observed in the Tabriz fault, which may be a suture zone between the Sanandaj–Sirjan Zone (SSZ) and the AA Block in the northwestern Iran. In this regard, Azizi and Moinevaziri (2009) proposed an accordion tectonic model in this part of Iran. The KMC is comprised two rock assemblages: metabasites and metagranites. The metabasites generally consist of greenschist and amphibolites, with interbedded marble, quartzite, meta-arkoses, and graphitic schist. The metabasite protolith consists of mafic to intermediate volcanic rocks with tholeiitic and calc-alkaline magmatic signatures in an island arc tectonic setting (Azizi et al., 2002). Geothermobarometric analysis of these rocks suggested peak metamorphic conditions of 450–680 ◦ C at 5.5–7.5 kbar (Azizi et al., 2006). The P–T–t path of these activities was clockwise, with an average geothermal gradient of 27–37 ◦ C/km (Azizi et al., 2006), concordant with the temperature regime of collision zones. Hassanipak and Ghazi (2000) proposed that the KMC rocks were the result of ophiolite emplacement (sole metamorphism) in the late Cretaceous, and suggested that the KMC rocks show an inverse metamorphism grade, which increases toward the KOC. Some Ar–Ar ages of the KMC rocks have been reported, suggesting that the amphibole minerals from the hornblende gabbroic rocks were generated 158.6–154.9 ± 1.0 Ma (Hassanipak and Ghazi, 2000). Additionally, Khalatbari-Jafari et al. (2003) showed that the rocks were generated approximately 110–104 Ma. 3. Analytical techniques Zircons were separated from ∼2 kg samples using conventional heavy-liquid and magnetic separation techniques. Cathodoluminescence (CL) images (Fig. 2) were taken at the Institute of Earth Sciences at Beijing University. Zircon U–Pb isotopic analyses were obtained by the laser ablation inductively coupled plasma mass spectrometer (LA-ICPMS) at the Department of Geosciences, National Taiwan University (NTU). The LA-ICPMS techniques were conducted using an Agilent 7500s quadrupole ICPMS and a New Wave UP213 laser ablation system. The laser ablation was performed using a helium carrier gas that substantially reduces the deposition of ablated material onto the sample surface and greatly improves transport efficiency, thus increasing the signal intensities compared to “conventional” ablation using argon as the carrier gas (Eggins et al., 1998; Günther and Heinrich, 1999; Jackson et al., 2004). During the experiments, approximately 1 min was spent measuring the gas blank. The results indicated sensitivities less than 1000 counts per second (cps) for all measured isotopes. Calibration was performed using the GJ-1 zircon standard (provided by the Australian Research Council National Key Centre for Geochemi-
Fig. 3. These diagrams show an age of 566.0 ± 9.3 Ma (a) U–Pb concordia diagram for zircons (core and rim) from the metagranite sample IRK-GR-1. (b) The distribution of zircon ages, with a peak at approximately 566 Ma. (c) Most of the ages were close to the average value, with slight differences.
cal Evolution and Metallogeny of Continents (GEMOC) at Macquarie University, Sydney). For Sm–Nd and Rb–Sr dating, the rock sample was crushed to smaller than 60 m, and 0.3–0.5 g of the rock powder was chemically decomposed with 5 ml HF, 1 ml HNO3 and 2 ml HClO4 . Conventional column chemistry was conducted to isolate Rb, Sr and Rare Earth Elements (REEs) using cation exchange resin (BioRad AG50W-X8, 200–400 mesh) with an HCl eluent. Nd and Sm were separated from the extracted REE fraction by another cation exchange column with ␣-hydroxy isobutyric acid (␣-HIBA) as eluent.
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Table 1 LA-ICPMS data for zircon from IRK-AM-1 (amphibolite), IRK-GR-1 (metagranite) and IRK-GR-2 (metagranite). ±1
206
0.0594 0.0603 0.0594 0.0591 0.0599 0.0592 0.0596 0.0613 0.0591 0.0602 0.0585 0.0629 0.0589 0.0582 0.0593 0.0586 0.0587 0.0586 0.0605 0.0598 0.0584 0.0581 0.0582 0.0583 0.0592
0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0005 0.0005 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006 0.0006
0.0889 0.0805 0.0886 0.0892 0.0836 0.0886 0.0882 0.0913 0.0793 0.0894 0.0888 0.0833 0.0894 0.0894 0.0887 0.0884 0.0903 0.0903 0.0834 0.0885 0.0877 0.0901 0.0833 0.0831 0.0888
0.0017 0.0016 0.0017 0.0018 0.0016 0.0017 0.0017 0.0018 0.0016 0.0018 0.0018 0.0017 0.0018 0.0018 0.0017 0.0018 0.0018 0.0018 0.0017 0.0018 0.0017 0.0018 0.0017 0.0017 0.0018
0.78 0.40 0.35 0.32 1.22 0.97 0.57 0.54 0.36 0.38 0.42 0.35 0.48 0.34 0.31 0.65 0.22 0.04 0.69 0.42 0.24 0.39 0.53 0.79 0.58 0.56 0.16 0.48
0.0533 0.0606 0.0607 0.0640 0.0565 0.0667 0.0596 0.0610 0.0613 0.0632 0.0599 0.0610 0.0592 0.0617 0.0593 0.0597 0.0597 0.0563 0.0735 0.0607 0.0568 0.0690 0.1594 0.0602 0.0593 0.1104 0.0549
0.0007 0.0006 0.0007 0.0007 0.0008 0.0008 0.0006 0.0006 0.0007 0.0007 0.0006 0.0007 0.0006 0.0007 0.0007 0.0006 0.0007 0.0006 0.0008 0.0007 0.0011 0.0007 0.0017 0.0007 0.0006 0.0012 0.0016
0.0353 0.0828 0.0883 0.0945 0.0144 0.0886 0.0917 0.0891 0.0893 0.0956 0.0966 0.0919 0.0904 0.0974 0.0929 0.0862 0.0951 0.0716 0.1802 0.0924 0.0813 0.0697 0.4594 0.0923 0.0938 0.3203 0.0593 0.0897
0.63 0.52 0.76 0.32 1.09 0.44 0.99 0.53 0.54 0.25 1.23 0.17 0.79 0.19 0.21
0.0641 0.0627 0.0715 0.0611 0.0696 0.0616 0.0616 0.0632 0.0624 0.1413 0.0870 0.0607 0.0577 0.0660 0.0602
0.0006 0.0006 0.0006 0.0006 0.0008 0.0007 0.0006 0.0007 0.0006 0.0013 0.0014 0.0006 0.0013 0.0006 0.0006
0.1186 0.1233 0.1626 0.0977 0.1469 0.0981 0.0997 0.0962 0.0976 0.4108 0.1326 0.0920 0.0543 0.1268 0.0974
Spot
Th/U
207
IRK-AM-1 AM-1-01a AM-1-02 AM-1-03a AM-1-04a AM-1-05 AM-1-05Ba AM-1-06a AM-1-07a AM-1-08 AM-1-08Ba AM-1-09a AM-1-10 AM-1-10Ba AM-1-11a AM-1-12a AM-1-13a AM-1-14a AM-1-15a AM-1-16 AM-1-17a AM-1-18a AM-1-19a AM-1-20 AM-1-21 AM-1-22a Wt. mean age (2, n = 18)
1.39 0.78 1.00 1.03 1.01 0.99 1.01 1.09 1.14 1.00 1.11 1.15 1.16 1.20 1.41 0.84 1.09 1.10 0.88 1.08 1.15 1.22 1.64 0.94 1.43
IRK-GR-1 GR-1-01 GR-1-02 GR-1-03a GR-1-04a GR-1-05 GR-1-06a GR-1-07a GR-1-08a GR-1-09a GR-1-10a GR-1-11a GR-1-12a GR-1-13a GR-1-14a GR-1-15a GR-1-16a GR-1-17a GR-1-18 GR-1-19 GR-1-20a GR-1-21 GR-1-22 GR-1-22C GR-1-23a GR-1-24a GR-1-25 GR-1-25R GR-1-26a wt. mean age (2, n = 18) IRK-GR-2 GR-2-01 GR-2-01C GR-2-02 GR-2-03a GR-2-04 GR-2-05a GR-2-06a GR-2-07a GR-2-08a GR-2-09 GR-2-10 GR-2-11a GR-2-12 GR-2-13 GR-2-14a
Pb/206 Pb
Pb/238 U
±1
±1
Error corr.
0.7270 0.6692 0.7254 0.7268 0.6901 0.7224 0.7249 0.7718 0.6460 0.7417 0.7163 0.7228 0.7257 0.7173 0.7248 0.7141 0.7302 0.7297 0.6956 0.7300 0.7057 0.7214 0.6679 0.6685 0.7244
0.0154 0.0146 0.0156 0.0158 0.0153 0.0158 0.0156 0.0168 0.0143 0.0165 0.0160 0.0163 0.0163 0.0153 0.0152 0.0156 0.0157 0.0160 0.0157 0.0162 0.0154 0.0157 0.0144 0.0150 0.0159
0.9198 0.9026 0.9105 0.9054 0.8870 0.8999 0.9135 0.9021 0.8878 0.8874 0.8850 0.8780 0.8795 0.9210 0.9333 0.9074 0.9161 0.9019 0.8798 0.8960 0.9095 0.9125 0.9201 0.8917 0.9022
549 499 547 551 517 547 545 563 492 552 548 516 552 552 548 546 557 557 517 547 542 556 516 515 548 550.1
10 9 10 10 10 10 10 11 9 10 10 10 10 10 10 10 11 11 10 10 10 11 10 10 10 4.7
0.0008 0.0019 0.0020 0.0022 0.0004 0.0021 0.0021 0.0021 0.0021 0.0022 0.0023 0.0022 0.0021 0.0023 0.0022 0.0020 0.0022 0.0017 0.0043 0.0022 0.0020 0.0016 0.0109 0.0022 0.0022 0.0075 0.0015 0.0021
0.2597 0.6911 0.7383 0.8339 0.1121 0.8139 0.7540 0.7501 0.7546 0.8323 0.7972 0.7722 0.7375 0.8282 0.7596 0.7087 0.7825 0.5557 1.8265 0.7730 0.6368 0.6630 10.0964 0.7663 0.7658 4.8755 0.4488 0.7419
0.0072 0.0166 0.0181 0.0209 0.0036 0.0218 0.0181 0.0178 0.0181 0.0204 0.0192 0.0191 0.0179 0.0201 0.0191 0.0170 0.0196 0.0134 0.0457 0.0189 0.0245 0.0158 0.2408 0.0190 0.0183 0.1150 0.0211 0.0176
0.8428 0.9608 0.9427 0.9312 0.7509 0.8776 0.9647 0.9774 0.9648 0.9515 0.9695 0.9479 0.9671 0.9636 0.9374 0.9763 0.9397 0.9745 0.9481 0.9648 0.6305 0.9901 0.9904 0.9547 0.9887 0.9981 0.5245 0.9906
224 513 545 582 92 547 566 550 551 588 595 566 558 599 572 533 586 446 1068 570 504 434 2437 569 578 1791 372 554 566.0
5 11 12 13 2 12 13 12 12 13 13 13 13 13 13 12 13 10 23 13 12 10 48 13 13 37 9 12 9.3
0.0023 0.0024 0.0031 0.0019 0.0029 0.0019 0.0019 0.0019 0.0019 0.0080 0.0028 0.0018 0.0011 0.0025 0.0019
1.0473 1.0660 1.6015 0.8234 1.4093 0.8337 0.8467 0.8382 0.8393 8.0002 1.5903 0.7704 0.4321 1.1538 0.8079
0.0218 0.0229 0.0329 0.0179 0.0362 0.0198 0.0182 0.0211 0.0195 0.1694 0.0509 0.0169 0.0171 0.0246 0.0168
0.9270 0.9029 0.9409 0.8926 0.7774 0.8273 0.9004 0.7882 0.8414 0.9138 0.6623 0.8857 0.5253 0.9116 0.9274
722 749 971 601 884 603 613 592 600 2219 802 567 341 769 599
13 14 17 11 16 11 11 11 11 36 16 11 7 14 11
207
Pb/235 U
206 Pb/238 U 207 Pb/206 Kp age (Ma±±1) (Ma ± 1)
1028
21
2450
17
1806
18
2243
16
H. Azizi et al. / Precambrian Research 185 (2011) 87–94
91
Table 1 (Continued) Spot
Th/U
207
GR-2-15a GR-2-16 GR-2-17a GR-2-18 GR-2-19a GR-2-20a GR-2-21 GR-2-22a GR-2-23 wt. mean age (2, n = 12)
0.50 1.09 0.40 0.76 0.23 0.51 0.01 0.44 0.83
0.0590 0.0653 0.0599 0.0680 0.0606 0.0687 0.1204 0.0602 0.0718
a
Pb/206 Pb
±1
206
0.0006 0.0006 0.0006 0.0008 0.0006 0.0022 0.0011 0.0008 0.0007
0.0953 0.1278 0.0964 0.1368 0.0956 0.1010 0.3448 0.0952 0.1659
Pb/238 U
±1
0.0019 0.0025 0.0019 0.0027 0.0019 0.0023 0.0067 0.0019 0.0033
207
Pb/235 U
0.7755 1.1511 0.7958 1.2821 0.7992 0.9573 5.7227 0.7899 1.6412
±1
Error corr.
0.0177 0.0249 0.0174 0.0325 0.0184 0.0483 0.1207 0.0216 0.0387
0.8533 0.8993 0.8906 0.7870 0.8530 0.4433 0.9199 0.7329 0.8384
206 Pb/238 U 207 Pb/206 Kp age (Ma±±1) (Ma ± 1)
587 776 593 827 589 620 1910 586 990 595.2
11 14 11 15 11 13 32 11 18 8.4
1962
16
Choosen for mean age calculation.
For natural isotope analysis of Sr and Nd, 100–200 ng of Sr were loaded with 2 M-H3 PO4 on a Ta-single filament, and 100–200 ng of Nd were loaded with 2 M-H3 PO4 on Re triple filaments. Procedure blanks were 20 ng for Rb and 3 ng for Sr and were considered negligible amounts for our samples. Strontium and Nd isotope ratios were measured with a thermal ionization mass spectrometer (TIMS; VG Sector 54-30 of Nagoya University). The mass fractionations during the Sr and Nd isotope measurements were corrected based on the following ratios: 86 Sr/88 Sr = 0.1194 and 146 Nd/144 Nd = 0.7219. The abundance of Rb, Sr, Nd and Sm based on the ID method was measured by a thermal ionization quadrupole mass spectrometer (Finnigan MAT Thermoquad THQ of Nagoya University). In this study, the natural Sr and Nd isotope ratio standards NBS987 and JNdi-1 (Tanaka et al., 2000) were used. Averages and 2 errors for the repeated analyses of the standards during this study were as follows: NBS987: 87 Sr/86 Sr = 0.710238 ± 28 (n = 13) and JNdi-1: 143 Nd/144 Nd = 0.512097 ± 19 (n = 14). To check the reliability of the Rb–Sr and Sm–Nd analyses, the Rb, Sr, Nd and Sm fractions were assessed for the Geological Survey of Japan (GSJ) reference rock sample, JG-1a, and were measured parallel to our samples.
4. Zircon morphology and Th/U ratios The majority of zircons in the two metagranite samples IRKGR1 and IRK-GR2 (Fig. 1) had a zoning shape, but some of them had a prismatic form (Fig. 2a and b). In these rocks, some zircons exhibited older ages, such as 1890–2450 Ma, and these results suggested that they might be xenocrysts. Most of the zircons in the metabasite IRK-AM1 (Fig. 2c) were rounded and were not distinctly crystalline in shape. In general, when the shape and morphology are problematic for classifying zircons, their compositions yield information about their origin. Additionally, the Th/U ratio can be used to determine the origin of zircon. The Th/U ratio is usually higher in magmatic zircon than in metamorphic zircon. For example, low ratios of <0.31 indicate a metamorphic origin, and very low ratios, in the range of 0.02–0.11, indicate a metamorphic origin (Chen et al., 2007). Fuping et al. (2007) used the Th/U ratio to discriminate metamorphic and igneous zircons in the basement metamorphic rocks in the Songliao Basin. They showed that zircons with high Th/U ratios (0.15–1.37) imply magmatic events, and zircons with low Th/U ratios (0.06–0.34) indicate metamorphic origin. The chemical composition of the zircons in the amphibolite IRK-AM-1 and the metagranite IRK-GR-1 and IRK-GR-2 imply that most of the zircons had high Th/U ratios and were magmatic in origin. The majority of Th/U ratios from the metagranites in this study were >0.6 (IRK-GR-1 = 0.16–1.22 and IRK-GR-2 = 0.20–1.23) although some samples had low values of 0.04. The ratios for IRK-AM-2 varied from 0.78 to 1.65.
5. U–Pb dating We chose one amphibolite (IRK-AM-1) and two metagranite (IRK-GR-1 and IRK-GR-2) samples, since these are the two groups representing the main composition of KMC rocks. In general, the morphology and Th/U ratio of zircon grains infer the magmatic origin. Sixty-five points were measured from the center and rim of the zircon grains (analyzed locations and names are shown in Fig. 2). The results of the LA-ICPMS analyses are listed in Table 1. The U–Pb age data from this study clearly indicated that the zircon crystallization occurred 500–600 Ma ago. However, a few zircons, especially those from the granitic rocks (IRK-GR-1-22C and IRKGR-1-25), showed older ages of 1890–2440 Ma, probably because the zircon originated from S-type granite. These atypical ages were omitted from the calculation of age average (median or mean), but were included in the analysis of the mode, which is the value occurring most frequently in the distribution (Allegre, 2008). The concordia diagrams for the samples IRK-GR-1, IRK-GR-2 and IRK-AM-1 are provided in Figs. 3–5, which show that the U–Pb data for all samples fell below the concordia line. This indicates that a portion of the Pb was lost after zircon crystallization. The ages of the granitic rock were calculated to be 566.0 ± 9.3 Ma (IRK-GR-1) and 595.2 ± 8.4 Ma (IRK-GR-2), and the age of the amphibolite was calculated as 550.1 ± 4.7 Ma (IRK-AM-1). These ages are consistent with zircon U–Pb ages of granitoids in the north and northwest region of Iran (Hassanipak and Ghazi, 2000).
6. Rb–Sr mineral isochron The metabasite IRK-AM-3 was used for mineral separation (Table 2) and was composed of hornblende with minor biotite and pyroxene as a mafic group, plagioclase, plagioclase + alkali feldspar + quartz and fine-grained components (smaller than 125 m). Apart from the mafic group (mainly hornblende), the other three groups displayed high correlation and seemed to form an isochron. The mafic group also showed a small difference in the 87 Sr/86 Sr ratio, and this was probably caused by mineral disequilibrium during metamorphic events. It is possible that some of the original minerals (e.g., clinopyroxene) were retained as an inclusion in the hornblende. The other three groups in the IRK-AM-3 rock yielded a good Rb–Sr isochron date of 146 Ma with a low Mean Square Weighted Deviate (MSWD) of 0.00054 (Fig. 6). This age is much younger than the zircon U–Pb age of 550 Ma, but is similar to the K–Ar and Ar–Ar ages of hornblende and whole rock as reported by Hassanipak and Ghazi (2000) and Khalatbari-Jafari et al. (2003). During metamorphism, Rb–Sr and/or Sm–Nd isotope systems in a rock are generally reset (e.g., Dickin, 2005; Allegre, 2008). Therefore, the mineral isochron age of Rb–Sr strongly suggests a metamorphic event occurred in this area.
92
Table 2 Sm–Nd, and Rb–Sr isotopic data. Sample (rock type)
Rb (ppm)
Sr (ppm)
87
IRK-AM-1 (metabasite) IRK-AM-2 (metabasite) IRK-AM-3 (metabasite) IRK-AM-4 (metabasite) IRK-G-1 (metagabbro) IRK-G-2 (metagabbro) IRK-GR-1 (metagranite) IRK-GR-2 (metagranite) IRK-GR-4 (metagranite) Minerals of IRK-AM-3 WR < 125 m Pl > 70 Hbl > 80b Pl + Kf
55.70 16.19 13.78 1.51 1.57 1.48 135.5 24.00 52.50
300.1 328.6 192.5 149.5 174.8 188.8 102.1 155.5 524.8
11.35 16.55 11.46 10.93
151.2 394.1 162.5 161.7
Rb/86 Sr
(87 Sr/86 Sr)p
2SEa
(87 Sr/86 Sr)i
Nd (ppm)
Sm (ppm)
147
Sm/144 Nd
(143 Nd/144 Nd)p
0.537 0.143 0.207 0.029 0.026 0.023 3.85 0.447 0.289
0.709741 0.706819 0.705968 0.70364 0.704904 0.704306 0.730294 0.715769 0.704404
±16 ±14 ±13 ±13 ±15 ±14 ±17 ±14 ±15
0.7055 0.7057 0.7043 0.7036 0.7047 0.7043 0.7001 0.7123 0.7021
20.59 13.88 10.87 1.66
5.470 3.850 3.760 0.750
0.1606 0.1678 0.2091 0.2714
0.512510 0.512402 0.512869 0.513044
0.85
0.310
0.2210
0.512703 0.512314 0.512404 0.512685
0.217 0.122 0.226 0.196
0.705417 0.705221 0.705717 0.705383
±16 ±16 ±16 ±14
(143 Nd/144 Nd)i
ε0 Nd (T = 0 Ma)
εt Nd (T = 550 Ma)
±8 ±8 ±8 ±8
0.5119 0.5118 0.5121 0.5121
−2.5 −4.6 4.5 7.9
−0.3 −2.3 3.6 3.6
±10 ±8 ±8 ±8
0.5119
1.3 −6.3 −4.6 0.9
−0.3
2SE
143
144
T
143
144
T
εt Nd = (( Nd/ Nd)sample /( Nd/ Nd) CHUR − 1) × 10, 000. a Standard error. b Data for this mineral are not used for age calculation.
Fig. 4. U–Pb concordia diagram for the metagranite sample IRK-GR-2 (a, b, c). These diagrams show the age of 595.2 ± 8.4 Ma. Please refer Fig. 3 for details.
7. Discussion
The initial ratios of 87 Sr/86 Sr and 143 Nd/144 Nd were calculated based on the zircon U–Pb age of 550 Ma. In the metabasites, the initial 87 Sr/86 Sr ratios varied from 0.7034 to 0.7057, and the initial ratios of 143 Nd/144 Nd varied from 0.5117 to 0.5121 (Table 2). The ε0 Nd values for granitic rocks varied from −6.3 to 0.9, suggesting that these rocks originated from the melting of the continental crust. The values of ε0 Nd (present) and εt Nd (t = 550 Ma) were calculated based on the value of (143 Nd/144 Nd)present , the value of CHUR (Chondritic uniform reservoir, 0.512638), (143 Nd/144 Nd)t , and the CHUR value at t = 550 Ma (Depaolo and Wasserburg, 1976;
H. Azizi et al. / Precambrian Research 185 (2011) 87–94
p = present at time; i = initial ratio for T = 0 and p = primary ratio which is calculated based on T = 550 Ma. The natural ratios were normalized based on 146 Nd/144 Nd = 0.7219 and 87 Sr/86 Sr = 0.1194. The average and 1 for standards samples for JNdi-1 and NBS987 Sr are =0.512097 ± 0.000010 (n = 13) and 0.71024 ± 0.00001 (n = 17), respectively. CHUR (Chondritic uniform reservoir) values, 147 Sm/144 Nd = 0.1967 and 143 Nd/144 Nd = 0.512638, were used to calculate εNd. JG-1a as reference sample was measured as follows: 87 Sr/86 Sr = 0.710980 ± 14(2SE), 143 Nd/144 Nd = 0.512375 ± 8 (2SE), Rb = 172.6 ppm, Sr = 188.9 ppm, Nd = 18.48, Sm = 4.68 ppm. εNd was calculated based on the ε0 Nd = ((143 Nd/144 Nd)sample /(143 Nd/144 Nd)CH UR) − 1) × 10,000 and
H. Azizi et al. / Precambrian Research 185 (2011) 87–94
93
Fig. 6. The mineral isochron age was 146 Ma, indicating the age of a metamorphic event in the late Jurassic to early Cretaceous Period. This age was calculated using Isoplot v3.7 (Ludwig, 2009) with = 6.54 × 10−12 y−1 (Lugmair and Marti, 1978; Begemann et al., 2001).
Fig. 7. A 143 Nd/144 Nd–87 Sr/86 Sr diagram for the metabasite rocks for T = 0 Ma. This shows that the change in 87 Sr/86 Sr ratios is not related to an alteration caused by seawater, and further implies a mixing of source magma with subduction sediments (Rollinson, 2007), since seawater would not significantly change the Nd-isotope ratio. The trend seen in the metabasites is similar to the volcanic rocks of the Lesser Antilles island arc and infers the mixing of source magma with slab sediments. The average Atlantic sediments were from Davidson (1983). GLOSS is the average composition of global subducting sediment from Plank and Langmuir (1998).
Fig. 5. U–Pb concordia diagram for the metabasite sample IRK-AM-1 (a, b, c). The diagrams show the age 550.1 ± 4.7 Ma. Please refer Fig. 3 for details.
Jacobsen and Wasserburg, 1980). The value of ε0 Nd in the metabasite rocks varied between −4.6 and 7.9, and the εt Nd values ranged between −2.3 and 3.6 (Table 2). From these results, the metabasites appear to have heterogeneous origins. The εNd values suggest that the metabasites are not compatible with a mid-ocean ridge basaltic (MORB) origin, although the 147 Sm/144 Nd ratios (0.16–0.27) in the metabasites were compatible with the MORB 147 Sm/144 Nd ratios (0.15–0.30; Allegre, 2008). The values of εt Nd suggest that the metabasite originated from a depleted mantle. In the Nd–Sr isotope diagram (Rollinson, 2007), all of the Khoy metabasites show a negative correlation between the Nd and Sr ratios, and also showed mixing of the MORB source and recycled (subducted) sediments, as is observed in the Lesser Antilles (Figs. 7 and 8). The isotope char-
Fig. 8. The Sr–Nd diagram for metabasite rocks for T = 550 Ma. This diagram shows the trends of sedimentary rock recycling and magmatic differentiation during magmatic evolution. The Khoy metabasite rocks confirm the sedimentary affinity to the source magma. Sao Miguel data was obtained from Dosso and Murthy (1980).
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H. Azizi et al. / Precambrian Research 185 (2011) 87–94
acteristics confirm an island arc origin of the Khoy amphibolite, as suggested by the chemical analyses of major and trace elements (Azizi et al., 2002). Our U–Pb values suggest that the age of the original rocks is 550–590 Ma and they belong to the late Proterozoic. Hassanzadeh et al. (2008) applied U–Pb dating to many granitic bodies in the central north and northwest regions of Iran and suggested that most of the granitic rocks actually formed in Neoproterozoic to lower Paleozoic. Additionally, Gass (1977) indicated an intra-oceanic island arc environment for the mainly volcanoclastic sediments based on the northeastern African and Arabian basements, and suggested that magmatic products were generated at or above easterly-inclined Benioff zones in the late Proterozoic (∼600 Ma). Kröner et al. (1979) reported an age of 586 ± 43 Ma for metagranitic rocks in the Saudi Arabia shield with Pan African ages, based on the Rb–Sr isochron. The age of granitic rocks in the Saudi Arabia shield increases toward the west and center of this shield (e.g., 760–850 Ma; Kröner and Stern, 2004). These dates suggest the accretion-type growth of Gondwanaland from the northern and eastern parts. As a result, our data suggest that the KMC was part of Gondwanaland and originated from an active zone in the late Proterozoic during the Pan African orogen. This suggestion is compatible with new U–Pb dating results from granitic rocks in the Iranian Plateau (Hassanzadeh et al., 2008). Thus, most of the older granitic bodies in the Iranian plateau are not related to the Arabian and Iranian plate collision in the Mesozoic Age or earlier, but rather were generated in the late Proterozoic and early Paleozoic and were a part of the Pan-African orogenic between 500 and 800 Ma. 8. Conclusions Zircon U–Pb dating revealed that the Khoy metamorphic protolith does not belong to the Jurassic or Cretaceous, as was previously proposed. The age data strongly suggests that the Khoy metagranite and original basalt was generated in the late Proterozoic, and the rocks are not related to Neotethyan oceanic crust or the collision between the Arabian Plate and the Alborz–Azarbaijan Block. The age of 550 Ma also demonstrates that the granitic rocks in the Khoy area have the same age as most of the older granitic rocks in Iran. Furthermore, the Rb–Sr mineral isochron age for high-grade metamorphic minerals (amphibolite facies) and the previously reported Ar–Ar and K–Ar ages imply that the metamorphism occurred in the late Jurassic or early Cretaceous after a shear zone developed and mylonitic foliation (Sm ) overprinted the S1 foliation. Acknowledgements H. Azizi would like to thank to University of Kurdistan for financing a sabbatical period at Nagoya University. The authors would like to thank Beijing University for zircon separation analysis. We are also grateful to. P.A. Cawood and H. Rollinson and anonymous reviewers for critical and constructive comments, S. Razyani and A. Khani for laboratory assistance and R. Koole for assistance during the submission of this work. References Aghanabati, A., 2004. Major Sedimentary-Structural Units of Iran. Geological Survey of Iran. Allegre, C.J., 2008. Isotope Geology. Cambridge University Press, 512p. Azizi, H., Jahangiri, A., 2008. Cretaceous subduction-related volcanism in the northern Sanandaj-Sirjan Zone. Iran. J. Geodyn. 45, 178–190. Azizi, H., Moinevaziri, H., Mohajjel, M., Yagobpoor, A., 2006. PTt path in metamorphic rocks of the Khoy region (northwest Iran) and their tectonic significance for Cretaceous–Tertiary continental collision. J. Asian Earth Sci. 27, 1–9. Azizi, H., Moinevaziri, H., Noghreayan, M., 2002. Geochemistry of metabasites in the north of Khoy. J. Sci. (in Farsi), University of Isfahan, Iran 15, 1–20.
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