The Shir-Kuh pluton (Central Iran): Magnetic fabric evidences for the coalescence of magma batches during emplacement

Journal of Asian Earth Sciences 46 (2012) 39–51

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The Shir-Kuh pluton (Central Iran): Magnetic fabric evidences for the coalescence of magma batches during emplacement M. Sheibi a,⇑, J.L. Bouchez b, D. Esmaeily c, R. Siqueira b,1 a

Faculty of Earth Sciences, Shahrood University of Technology, Shahrood, Iran University of Toulouse, GET/OMP, 14 Ave. E. Belin, 31400 Toulouse, France c School of Geology, College of Science, University of Tehran, Tehran, Iran b

a r t i c l e

i n f o

Article history: Received 9 March 2011 Received in revised form 6 October 2011 Accepted 13 October 2011 Available online 26 November 2011 Keywords: Anisotropy of magnetic susceptibility (AMS) S-type granite Microstructures Restite Iran

a b s t r a c t The 136 Ma, NW–SE elongate Shir-Kuh pluton is one of the most poorly understood geological feature of Central Iran. It is composed of peraluminous rocks, corresponding to ilmenite-bearing S-type granites compositionally ranging from granodiorites to leucogranites. These rocks show a continuum in their chemistry attributed to progressive differentiation. This allows using the anisotropy of magnetic susceptibility technique to tempt establishing the relative chronology between emplacements of magma batches in the pluton. The rather low susceptibility magnitudes (Km < 400 lSI) depict a dominant paramagnetic behavior of the pluton. The magnetic fabrics data (magnetic lineation and foliation maps, K, P and T parameters), complemented by field and microstructural observations, reveal that two feeder zones at least, as characterized by areas having steep lineations ascribed to magma flow likely issued from the base of the brittle crust, served as conduits for the magmas. The early Cretaceous age of the pluton, the orientations of the feeding zones, the overall lineation directions throughout the pluton, as well as the S-type nature of the magmas call for a dextral transpressive regime which might have been active in the back-arc region located above the subducting eastern branch of the Neo-Tethys. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction The Central Iranian microcontinent is surrounded by fold-andthrust belts belonging to the Alpine–Himalayan orogenic system of Western Asia. This terrain is an area of continuous continental deformation, in response to the ongoing convergence between the Arabian (Gondwana) and Eurasian (Turan) plates. Closure of the Neo-Tethys occurred by subduction in the region of the present Zagros Mountains (Fig. 1; Stöcklin, 1968; Berberian and King, 1981; Alavi, 1994) and recorded in the southwest Sanandaj-Sirjan zone, led to the emplacement of some intrusive bodies such as the Esmail-Abad and Airakan granite massifs (Berberian, 1983), the Kolah-Ghazi and the Shir-Kuh S-type granitoids (Berberian and Berberian, 1981) in the Central Iranian micro-continent (Fig. 1). In spite of the importance of such intrusives on the understanding of the geodynamic evolution of this geologically interesting area, none of them has been so far studied in detail. The present study aims to give a better understanding of the processes responsible for the formation and emplacement of the Shir-Kuh pluton.

⇑ Corresponding author. Tel.: +98 273 3392205 2251; fax: +98 273 3396007. E-mail addresses: [email protected] (M. Sheibi), [email protected] (J.L. Bouchez), [email protected] (R. Siqueira). 1 Present address: USP, Sao Paulo, Brazil. 1367-9120/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2011.10.006

Construction of batholiths involves the growth and coalescence of magma batches formed by magma injection through different feeder conduits. The relationship between the relative chronology of emplacement of these magma batches and the regional tectonics raises several questions that are important to clarify. To answer these questions we used the tools of structural geology, essentially the anisotropy of magnetic susceptibility (AMS) to build up structural patterns, and microstructural observations in order to distinguish between fabrics acquired in the magmatic state from those acquired in the solid state. These data and observations provide information on how to better constrain the tectonic evolution of the Yazd block, which is one of the most poorly understood areas of the Central Iranian Terrain. 2. Geological setting The NW–SE elongate Shir-Kuh granite pluton belongs to the Yazd block and covers an area of 550 km2. It lies from 31°250 to 31°430 N and from 53°530 to 54°200 E (Fig. 2). The geology of the Shir-Kuh and surrounding formations has been already investigated by several researchers. Nabavi (1970) studied the stratigraphy of Precambrian-Triassic sediments in the Taft district and published the geological map of the Yazd quadrangle (1:250,000). The oldest exposed rocks in the study area are sandstones, shales and limestones from the Devonian-Carboniferous period. The

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M. Sheibi et al. / Journal of Asian Earth Sciences 46 (2012) 39–51

Fig. 1. Main geotectonic elements of the Central Iranian microcontinent and adjacent areas (compiled from Alavi, 1994; Berberian, 1983). Rectangle delineates the Shir-Kuh area.

Permian rocks are followed by Upper Triassic black shales, sandstones, quartzites and numerous thin-bedded limestones, referred to as the Nayband Formation (Stöcklin et al., 1972) and by Lower Jurassic sandstones and shales, known as the Shemshak Formation (Assereto, 1966). The pluton, that intrudes the Shemshak Formation, consists of three main units: granodiorites, monzogranites and leucogranites (Fig. 2). Granodiorites are located in the northern part of the area, metamorphosed the host sediments of the Shemshak Formation into spotted schist and cordierite-bearing hornfelses characterized by the presence of andalusite, cordierite and opaque minerals. In the western part of the Shir-Kuh, where leucogranites are predominant, the country rocks have undergone just a low-grade metamorphism, and only slight recrystallization has developed. To the east, the pluton is covered by the Lower Cretaceous Sangestan Formation composed of very thick sequences of red clastic sediments, comprising conglomerates and fine sandstones, with some carbonate intercalations locally terminated with a limy shale layer on its top. The conglomerates contain some pebbles from the Shir-Kuh pluton (Nabavi, 1970; Khosrotehrani and Vazirimoghadam, 1993). The U–Pb zircon age obtained for the Shir-Kuh is 136.5 ± 5.0 Ma (Jamshid Hassanzadeh, written communication) agrees with the stratigraphic relationships observed in the field. A recent petrological and geochemical study of the Shir-Kuh concludes that the magmas forming the pluton correspond to S-type granitoids that derived from a young crustal protolith (Sheibi et al., 2010). 2.1. The Yazd block The Central Iranian microcontinent, situated to the northeast of the Zagros-Makran Neo-Tethyan suture, is sub-parallel to the

Cenozoic magmatic arc (Urumieh–Dokhtar Magmatic Arc, Fig. 1). It consists, from east to west, of three major blocks (Lut, Tabas and Yazd) traversed by long, generally N–S, right-lateral, strike-slip faults, concave to the east (Sengör, 1990). The blocks are separated by a series of intersecting regional-scale faults. The Yazd block is limited by the Kuhbanan Fault to the east, the Biabanak Fault to the north and the Rafsanjan and Shahre Babak Faults to the south and west (Alavi, 1994) (Fig. 1). The stratigraphy of the Yazd Block is globally similar to the other parts of Central Iran; it contains sequences of Neoproterozoic to Middle Triassic sedimentary rocks, covered by extensive coal-bearing Upper Triassic to Mid-Jurassic sandstones, siltstones and shales (Shemshak Formation). Although some of the stratified cover rocks can be correlated between the different blocks, significant unit and/or thickness variations occur locally across the domain boundaries. 2.2. The Shir-Kuh pluton Based on field, petrographic and geochemical arguments, the three granite units that have been identified within the Shir-Kuh (Fig. 2) can be described as follows. The granodiorites at the northern margin of the batholith, are composed mainly of plagioclase, quartz, K-feldspar, biotite, and a lesser amount of muscovite, garnet, cordierite, ilmenite, zircon, apatite and monazite. The monzogranites, with the same mineral assemblage, form the principal unit, varying widely in composition from relatively mafic (cordierite-bearing) to felsic (muscovite-rich) rocks. The leucogranites, characterized by a white color due to the absence of dark minerals, outcrop locally as small stocks and dykes especially in the northwestern margin (Fig. 2). It is worth to mention that the observed contacts between these three units are always sharp.

M. Sheibi et al. / Journal of Asian Earth Sciences 46 (2012) 39–51

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Fig. 2. Simplified geological map of the Shir-Kuh pluton giving sampling stations (181 sites) used for the magnetic fabric study. Boxed: tectonic frame of the study area.

Sheibi et al. (2010) have concluded that the granites forming the pluton are peraluminous, typical of S-type, and were derived from a young crustal protolith. Small biotite clusters with higher XMg relative to host flaky biotites, surmicaceous enclaves, biotite cored by sillimanite, calcic-cored plagioclase and cordierite containing ragged biotites and quartz inclusions are the main restites which are widespread in the more mafic rocks of the pluton. In addition, inherited zircons, monazites and apatites enclosed in biotites are also considered as restites.

3. Magnetic fabrics Magnetic fabric measurements (Hrouda, 1982; Bouchez, 1997, 2000) have been performed in Toulouse, using a Kappabridge KLY-3S susceptometer (AGICO, Brno) operating at low field (4  104 T; 920 Hz). In-situ oriented core samples were collected according to a grid, as regular as possible in such a mountainous region giving 181 different sampling stations (Fig. 2) covering the different parts of the pluton. Each core, one-inch-in-diameter, was cut into 22 mm-long cylinders that were measured for their anisotropy of magnetic susceptibility (AMS). The orientations and magnitudes of the three principal axes of the AMS ellipsoids (K1 P K2 P K3) were obtained, for each sampling station, through the tensor average of four individual AMS measurements. The anisotropy percentage P% = 100 [(K1/K3)  1] and the shape param-

eter T = ln[K2/(K1/K3)]/ln[K1/K3] (Jelinek, 1978, 1981) were then calculated for each sampling station and reported in Table 1. Note that since our rocks have rather low mean susceptibilities (Km = (K1 + K2 + K3)/3; Table 1) the diamagnetic contribution of the rock matrix (D  14 lSI), usually considered as constant and isotropic (Hrouda, 1986; Rochette, 1987), has been subtracted from K1 and K3 in the calculation of the anisotropy percentage. Basic magnetic mineralogy, aimed at identifying the minerals responsible for the magnetic susceptibility, has been performed using the CS-2 furnace (also from AGICO) coupled to a KLY-2 susceptometer that gives the variation of the susceptibility (K) with temperature (from 20 °C to 700 °C). 3.1. Magnetic data The geographical locations and the magnetic data for all the sampling stations from the Shir-Kuh are reported in Table 1. They gather the directional aspects (declination/inclination of K1 and K3) and the main scalar parameters (mean K, T, P%) of the AMS ellipsoids. K1 is called the magnetic lineation, and the plane perpendicular to K3 is called the magnetic foliation. The orientation data were used to build up the structural maps in Figs. 3 and 4. In the granodiorite unit the magnetic foliations are often parallel to the border, with steep dips toward the NW or the SE, and the lineation plunges are gentle, to the E or the W. Besides the granodiorites, the general tendency of the fabrics is, to the east, the apparent curvature of the lineation and

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M. Sheibi et al. / Journal of Asian Earth Sciences 46 (2012) 39–51

Table 1 Anisotropy of magnetic susceptibility data. Locations (Long./Lat.); Km: mean magnetic susceptibility in 106 SI; Kmax: magnetic lineation; Kmin: normal to Foliation; P%: anisotropy percentage; T: shape parameter. Site

Long.

Lat.

Km

Kmax: Az/Pl

Kmin: Az/Pl

P (%)

T

Granodiorites SK11 SK12 SK14 SK147 SK148 SK149 SK15 SK150 SK151 SK152 SK153 SK154 SK155 SK175 SK177 SK21 SK22 SK23 SK24 SK25

54.01 54.02 54.02 53.99 53.98 53.97 53.98 53.95 53.94 53.94 53.92 53.92 53.91 54.05 53.90 54.08 54.07 54.06 54.03 54.01

53.99 54.00 54.02 54.03 53.99 54.00 54.02 54.04 54.03 54.04 54.03 53.96 53.95 53.99 53.98 53.97 53.96 53.96 53.96 53.97 54.13 54.13 54.12 54.12 54.23 54.23 54.34 54.34 54.31 54.30 54.34 54.31 54.30 54.28 54.27 54.27 54.26 54.25 54.24 54.25 54.24 54.23 54.22 54.22 54.21 54.19 54.19 54.18 54.16 54.15

Long.

Lat.

Km

Kmax: Az/Pl

Kmin: Az/Pl

P (%)

T

Monzogranite 31.67 31.67 31.67 31.70 31.70 31.70 31.67 31.69 31.68 31.69 31.68 31.67 31.68 31.54 31.66 31.71 31.72 31.71 31.71 31.70

231 118 183 254 239 268 82 197 247 260 271 176 244 203 198 305 247 332 235 307

216/13 209/9 193/26 280/22 261/37 63/31 238/15 76/9 22/6 39/19 17/7 43/20 177/34 83/14 173/49 49/26 255/8 225/6 76/52 251/1

326/56 104/56 330/57 2/11 141/33 157/7 342/43 336/49 281/62 280/55 287/1 309/10 299/39 182/34 312/33 311/16 160/30 316/7 182/13 161/2

1.52 1.27 1.92 5.17 2.84 3.57 1.63 4.88 2.90 2.10 2.62 0.84 1.98 2.28 1.01 3.66 3.85 4.69 5.54 3.58

0.59 0.26 0.94 0.22 0.38 0.14 0.41 0.32 0.06 0.68 0.03 0.55 0.20 0.58 0.96 0.32 0.60 0.30 0.56 0.73

SK35 SK36 SK37 SK38 SK39 SK40 SK41 SK42 SK43 SK44 SK45 SK46 SK47 SK48 SK49 SK50 SK51 SK52 SK53 SK54 SK55 SK56

54.12 54.11 54.13 54.11 54.14 54.13 54.14 54.13 54.12 54.09 54.11 54.16 54.17 54.16 54.17 54.16 54.18 54.18 54.19 54.20 54.19 54.19

31.59 31.60 31.63 31.62 31.64 31.61 31.59 31.60 31.58 31.59 31.61 31.60 31.60 31.59 31.59 31.58 31.60 31.60 31.59 31.52 31.56 31.56

185 113 64 209 211 216 238 189 218 220 232 103 116 182 190 210 211 192 159 189 222 242

272/2 344/13 298/19 360/14 266/1 168/19 127/1 48/20 226/28 223/24 299/26 321/3 128/18 132/19 161/6 291/1 168/23 229/30 99/81 146/16 157/9 276/23

3/34 186/77 94/70 195/76 173/69 59/43 37/25 210/70 38/62 12/63 115/64 56/56 10/55 26/38 61/56 21/14 45/52 90/52 334/5 39/45 40/71 26/38

0.41 0.97 2.08 0.43 0.88 1.05 0.65 0.57 1.04 1.37 0.88 1.17 2.94 1.24 0.87 0.69 0.97 1.42 1.07 1.72 0.57 1

0.31 0.04 0.75 0.18 0.38 0.78 0.64 0.25 0.48 0.81 0.11 0.18 0.81 0.04 0.16 0.21 0.89 0.33 0.64 0.11 0.36 0.53

31.68 31.66 31.64 31.62 31.63 31.61 31.58 31.57 31.55 31.54 31.65 31.66 31.65 31.65 31.61 31.60 31.61 31.59 31.58 31.57 31.60 31.59 31.57 31.57 31.52 31.53 31.41 31.43 31.44 31.44 31.46 31.43 31.43 31.44 31.44 31.45 31.45 31.47 31.49 31.45 31.45 31.46 31.45 31.46 31.46 31.45 31.46 31.47 31.47 31.47

213 187 185 186 207 171 197 197 164 215 193 189 191 212 45 171 128 167 189 66 179 184 193 327 207 195 154 160 192 194 171 183 204 204 187 190 208 208 202 207 240 245 266 186 188 233 272 219 198 197

46/24 146/86 158/43 292/14 202/38 201/27 159/16 318/61 223/1 278/33 255/0 103/57 126/38 177/12 220/43 297/67 11/41 344/39 32/39 22/62 298/4 146/15 156/38 171/8 156/40 224/5 277/74 26/49 109/30 242/60 221/42 332/60 18–5 238/49 248/17 292/15 41/16 160/6 224/53 37/57 128/78 216/48 281/64 169/62 240/21 238/83 97/79 256/56 266/66 37/66

285/50 22/2 323/46 122/76 84/32 83/42 61/27 118/28 130/74 187/2 345/19 353/13 326/51 273/24 88/36 104/23 152/42 111/37 174/44 228/25 31/36 25/63 39/30 66/59 20/41 321/50 119/15 228/39 219/30 17/4 111/21 158/30 117/49 106/30 140/44 179/57 163/61 254/37 335/15 274/19 293/12 107/16 47/17 280/11 119/54 87/6 273/11 148/12 120/20 255/20

2.57 0.94 0.65 0.78 0.84 1.12 0.89 0.93 1.13 2.12 1.00 0.96 1.11 0.76 1.24 0.72 0.61 1.03 0.84 1.58 0.40 0.81 0.95 2.53 0.75 0.70 3.62 0.53 1.19 0.97 1.27 1.22 0.87 1.07 1.27 0.83 1.70 0.84 1.22 2.53 3.27 1.99 3.23 3.78 2.22 3.90 5.71 1.76 5.33 3.20

0.51 0.07 0.13 0.57 0.21 0.37 0.56 0.39 0.30 0.38 0.03 0.32 0.60 0.48 0.54 0.11 0.09 0.18 0.57 0.48 0.09 0.48 0.12 0.03 0.35 0.29 0.47 0.37 0.28 0.61 0.48 0.23 0.09 0.16 0.40 0.79 0.39 0.23 0.55 0.76 0.34 0.07 0.17 0.18 0.48 0.11 0.70 0.81 0.52 0.35

SK57 SK58 SK59 SK60 SK61 SK62 SK63 SK64 SK65 SK66 SK67 SK68 SK69 SK70 SK71 SK72 SK73 SK74 SK75 SK76 SK77 SK78 SK79 SK80 SK128 SK129 SK130 SK131 SK132 SK133 SK134 SK135 SK136 SK137 SK138 SK139 SK140 SK141 SK142 SK143 SK144 SK145 SK146 SK156 SK157 SK158 SK159 SK160 SK161 SK162

54.20 54.21 54.21 54.22 54.21 54.20 54.21 54.21 54.21 54.20 54.21 54.20 54.21 54.21 54.24 54.24 54.23 54.22 54.22 54.22 54.22 54.23 54.23 54.23 54.02 54.02 53.96 53.97 53.98 53.96 53.99 54.00 54.00 54.00 53.96 54.14 54.15 54.13 54.13 54.14 54.08 54.08 54.07 53.93 53.91 54.00 53.99 54.01 54.02 53.95

31.57 31.57 31.57 31.58 31.57 31.56 31.54 31.54 31.53 31.53 31.53 31.53 31.51 31.51 31.56 31.55 31.54 31.54 31.54 31.52 31.52 31.51 31.50 31.52 31.55 31.56 31.53 31.54 31.54 31.52 31.52 31.51 31.52 31.54 31.53 31.50 31.51 31.55 31.52 31.53 31.51 31.51 31.51 31.65 31.66 31.55 31.57 31.55 31.55 31.54

206 213 174 32 314 189 219 216 265 171 213 192 166 201 110 182 142 195 207 221 194 177 184 195 168 207 192 118 215 205 199 194 208 184 87 185 207 176 170 184 212 205 210 177 192 110 152 130 193 175

161/0 152/4 327/14 298/11 157/19 286/50 308/1 344/38 111/6 119/64 171/47 246/3 139/21 200/38 174/11 117/3 309/13 175/22 152/8 83/10 171/44 108/21 97/26 248/71 315/51 323/19 251/24 219/63 283/39 103/12 253/14 311/20 91/2 112/24 280/10 289/49 8/49 251/57 288/30 154/15 177/64 46/70 289/34 82/16 48/2 313/44 324/17 3/19 7/49 252/9

71/64 59/36 229/26 152/77 357/70 54/28 39/46 190/49 17/38 324/24 307/34 338/30 269/59 74/37 345/79 212/65 51/42 48/57 52/51 327/68 333/45 216/38 346/35 34/16 166/35 120/69 140/40 348/18 44/32 198/20 350/28 50/24 360/21 16/13 17/33 19/0 201/41 109/27 93/59 274/62 331/24 175/13 25/9 322/60 312/70 51/8 219/40 257/40 247/29 350/39

1.37 1.74 1.54 1.67 1.48 1.30 0.59 0.97 0.74 0.52 0.64 1.22 0.90 0.91 1.44 1.24 0.81 0.72 1.42 1.38 1.35 0.92 1.29 0.76 2.02 0.88 1.85 3.52 2.65 1.51 1.72 2.69 3.42 3.84 1.66 1.85 0.80 0.53 0.92 0.92 3.31 2.08 4.28 1.49 1.84 2.98 1.22 1.89 2.33 1.40

0.45 0.56 0.28 0.57 0.41 0.09 0.12 0.37 0.40 0.36 0.41 0.23 0.63 0.84 0.20 0.14 0.67 0.55 0.21 0.50 0.02 0.40 0.43 0.44 0.42 0.31 0.29 0.45 0.14 0.12 0.35 0.52 0.62 0.87 0.64 0.12 0.19 0.22 0.03 0.29 0.73 0.02 0.43 0.13 0.06 0.22 0.09 0.16 0.25 0.19

Monzogranite SK1 SK2 SK3 SK4 SK5 SK6 SK7 SK8 SK9 SK10 SK13 SK16 SK17 SK18 SK19 SK20 SK27 SK28 SK29 SK30 SK31 SK32 SK33 SK34 SK81 SK82 SK83 SK84 SK85 SK86 SK87 SK88 SK89 SK90 SK91 SK92 SK93 SK94 SK95 SK96 SK97 SK98 SK99 SK100 SK101 SK102 SK103 SK104 SK105 SK106

Site

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M. Sheibi et al. / Journal of Asian Earth Sciences 46 (2012) 39–51 Table 1 (continued) Site

Long.

Lat.

Km

Kmax: Az/Pl

Kmin: Az/Pl

P (%)

T

Site

Long.

Lat.

Km

Kmax: Az/Pl

Kmin: Az/Pl

P (%)

T

SK107 SK108 SK109 SK110 SK111 SK112 SK113 SK114 SK115 SK116 SK117 SK118 SK119 SK120 SK121 SK122 SK123 SK124

54.14 54.14 54.13 54.13 54.13 54.11 54.10 54.09 54.08 54.07 54.06 54.04 54.05 54.05 54.05 54.06 54.04 54.03

31.48 31.50 31.51 31.50 31.48 31.49 31.50 31.49 31.50 31.51 31.52 31.54 31.54 31.54 31.51 31.52 31.51 31.51

224 207 266 209 207 226 193 189 179 262 250 254 225 174 205 172 197 198

23016 278/14 79/54 244/79 192/75 224/52 244/48 225/31 77/41 120/61 211/60 273/42 275/32 219/44 60/6 74/48 231/9 136/50

323/11 185/9 203/22 154/0 325/10 329/12 334/0 121/21 175/10 341/23 335/18 176/8 184/2 340/28 150/3 342/2 324/23 17/22

2.38 1.72 0.97 2.63 3.50 4.33 2.29 2.74 5.02 2.71 3.13 1.60 1.60 1.78 2.85 4.36 3.78 2.73

0.89 0.34 0.91 0.94 0.32 0.71 0.40 0.18 0.40 0.72 0.12 0.02 0.33 0.32 0.12 0.12 0.94 0.62

SK163 SK164 SK165 SK166 SK167 SK168 SK169 SK170 SK171 SK172 SK173 SK174 SK176 SK179 SK180 SK181

53.96 53.98 53.94 53.93 53.92 53.98 53.97 53.97 54.03 54.10 54.09 54.06 54.05 54.02 54.09 54.09

31.55 31.56 31.58 31.55 31.53 31.57 31.64 31.62 31.63 31.52 31.52 31.53 31.58 31.58 31.58 31.59

38 117 150 192 170 158 170 194 200 188 172 222 173 170 243 230

247/17 355/17 356/42 235/63 108/23 306/75 127/36 348/29 226/33 337/60 311/47 212/69 348/18 230/79 191/14 127/6

343/22 251/39 198/46 83/24 331/59 39/1 11933 109/44 336/28 149/30 132/43 345/15 94/41 79/9 320/68 33/30

0.87 3.47 0.92 0.88 1.14 1.10 0.94 0.80 0.79 1.50 1.26 3.17 0.80 0.74 0.73 0.76

0.53 0.30 0.62 0.15 0.58 0.31 0.08 0.51 0.49 0.33 0.67 0.75 0.92 0.86 0.17 0.31

SK125 SK126 SK127

54.03 54.03 54.02

31.52 31.54 31.54

207 182 163

267/5 250/28 249/61

160/73 149/20 12/1

2.60 3.26 2.74

0.15 0.13 0.47

SK178 SK26

31.64 31.63

20 174

159/11 77/8

68/0 200/76

1.06 0.90

0.15 0.66

foliation trajectories around the border of the leucogranite body; in the rest of the pluton, a dominance of NW–SE trends or strikes is observed at the exception of the steeply plunging lineations and vertically dipping foliations that appear all along the southern margin of the pluton, suggesting the presence of a magma feeding zone. Another possible, thin and ENE-elongate feeding zone appears to the north of the locality of Sange-sefid. To the south of this locality, subvertical and EW-striking foliations and shallowly plunging lineations could mark an area where transfer zones developed.

Leucogranite 53.89 53.96

The rather low susceptibilities (Km), ranging from 20 to 332 lSI, with a mean value of 180 lSI (Fig. 5a), call for the dominance (in volume) of iron-bearing silicates as magnetic carriers, biotite in the present case. Such weak susceptibilities, considered typical of the so-called paramagnetic granites (Bouchez, 1997), suggest that the ferrimagnetic fraction is subsidiary. This allows a first-order correlation between magnetic susceptibility and rock-type (Gleizes et al., 1993), namely: Km < 50 lSI for leucogranitic and aplitic types 50 < Km < 100 lSI for monzogranites and 100 < Km < 200 lSI for granodiorites. The suscepti-

Fig. 3. Magnetic lineation map of the Shir-Kuh.

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M. Sheibi et al. / Journal of Asian Earth Sciences 46 (2012) 39–51

Fig. 4. Magnetic foliation map of the Shir-Kuh.

bility map of the Shir-Kuh roughly correlates with the amount of biotite which globally, and excepted for the leucogranite, decreases from the northern periphery of the body to its center (Fig. 5a). The anisotropy percentage is rather low throughout the massif. It varies from 0.4% to 5.7%, with a mean value of 1.7% (Table 1). As usually observed in granites (Bouchez, 1997), P% values lower than 4% correspond to magnetite-free rocks (low Km values). In map view (Fig. 5b), the highest anisotropies are located along the southern and northwest borders of the Shir-Kuh body, calling for some solid-state overprints. As for the shape parameter (Fig. 5c), T greatly varies from 0 to 0.96 but is mostly highly oblate (T > 0). Gleizes et al. (2006) observed that in granites the positive values of T depend on both the intensity of deformation and the amount of biotite. The same is observed in the Shir-Kuh since the oblateness of the shape parameter is higher both at places showing the largest anisotropy degree (Fig. 5b) and in the granodiorite which is richer in biotite (Fig. 5e). It is worth noting that, beside these two areas (northern granodiorite and southern feeder zone), the T values do not correlate with the contacts between the structural domains as defined in Fig. 8, as if these contacts did not act as strain localization sites during or after emplacement.

bility starting from 300 °C and up to 500–520 °C, attributed to the neoformation of magnetite and/or maghemite. Neoformation is also evidenced by the sudden increase in susceptibility below 600–590 °C, the Curie temperature domain of magnetite. However, magnetite may be present in small amounts as revealed by the initial parts of the K versus T curves. Following Hrouda (1994) and Esmaeily et al. (2007), we used the hyperbola fit of these curves between room temperature and 150 °C. This fit, which depicts the paramagnetic contribution of the rock, should be asymptotic to the T axis in absence of a ferromagnetic fraction. The height of the curve above the T °C axis, at T °C much larger than the Curie temperatures of magnetite and hematite, gives the amount of the initial ferromagnetic contribution. Subtracting this value from the total susceptibility (at room temperature) gives the paramagnetic contribution. This procedure is illustrated in Fig 6b, for specimen SK81. As a result, the paramagnetic contributions for the studied specimens, selected from the full range of susceptibilities, vary from 79% (specimen SK116) to 46% (specimen SK180; Fig. 6c).

4. Microstructural observations 3.2. Refining the magnetic mineralogy A better knowledge of the magnetic mineralogy is required in order to interpret the magnetic fabrics, particularly if the susceptibility is due to the contribution of different minerals (Rochette et al., 1992). The low susceptibilities found in this pluton (Table 1) indeed suggest that the magnetic fabrics mainly reflect the contribution of the paramagnetic minerals. Susceptibility versus temperature curves coming from typical samples (#25, #81, #116 and #180; Fig. 6a) display a slight increase in suscepti-

In granitic rocks, the microstructures provide useful indications about the physical state of the magma at the time of acquisition of its fabric and, eventually, about the solid-state overprints it may have suffered. Several authors have defined microstructural criteria, essentially based on the observation of quartz, feldspar and biotite (Paterson et al., 1989; Bouchez et al., 1990; Passchier and Trouw, 1996; Vernon, 2000). Using them, four main microstructural types were defined: (i) magmatic; (ii) incipiently deformed at high temperature; (iii) moderately deformed at high temperature; and (iv) mylonitic (see Fig. 9).

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Fig. 5. Contour maps of the scalar magnetic parameters from the Shir-Kuh. (a) Magnetic susceptibility (K). (b) Anisotropy percentage (Ppara %); and (c) shape parameter (T). (d) Frequency histogram for bulk magnetic susceptibility (in 106 SI). (e) Anisotropy percentage (P) versus shape parameter (T) for the three units of the Shir-Kuh.

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less than 0.1 mm) mostly distributed along the boundaries of the original large quartz grains which display high-angle undulose extinctions and sub-boundaries (Fig. 7f). These features are typical of rather low temperatures of deformation and likely high stress conditions (Nicolas and Poirier, 1976; Passchier and Trouw, 1996). As can be seen in the map showing the microstructures (Fig. 8) the magmatic type is dominant throughout the pluton (90% of the stations), particularly in its eastern half and central parts. Incipient high temperature solid-state microstructures are also relatively abundant (3% of the stations) toward the contact between monzogranites and leucogranites and particularly along the southwestern margin, close to the village of Sange-sefid. There, a few stations display microstructures with intense solid-state overprints, that could point to the presence of shear zones. In conclusion, strain has remained low to modest in most of the sampling stations; the only station having mylonitic microstructures with high-angle undulose extinctions and sub-boundaries in quartz (Fig. 7f) is located in the center of the monzogranite (in between Sange-sefid and Deh-Bala, site 8). 5. Fabric patterns On the basis of the microstructural observations, together with the magnetic foliation and lineation maps, three main structural domains were distinguished, the second domain being subdivided into four sub-domains (Fig. 8). 5.1. Domain 1

Fig. 6. Thermomagnetic curves illustrating the variation of the magnetic susceptibility (K) as a function of the temperature (T) for selected samples from the ShirKuh. Both heating and cooling (dashed) curves are reported.

In the magmatic microstructure (i), quartz grains are euhedral, their boundaries being directly inherited from their crystallization. Some undulose extinction is present, but no recrystallized grain is observed. Microstructures resulting from incipent deformation at high temperature in the solid-state (ii) are characterized by subgrains frequently organized into squared arrays (also called chessboard patterns) and affecting large quartz grains (Fig. 7a). They result from the presence of two orthogonal families of sub-boundaries, namely basal {0 0 0 1} and prismatic (containing [0 0 0 1]). The prismatic sub-boundaries denote the activity of a [c]-slip that operates at temperatures close to the solidus (Blumenfeld et al., 1986). Due to the high mobility of dislocations at high temperature, quartz grains carrying basal sub-boundaries must have ceased their deformation at subsolidus temperatures. Vermicular intergrowths of plagioclase and quartz (myrmekites) occur where small K-feldspar grains are interspersed (Fig. 7b), attesting to exsolutions that formed at high temperature (Ashworth, 1972). Myrmekites are able to survive to deformation overprints, explaining why they are also observed in microstructures resulting from intense deformations. Microstructures moderately deformed at high temperature (iii) are marked by elongate quartz aggregates and substantial recrystallization into smaller new grains (Fig. 7c). The presence of ubiquitous recrystallized grains, bent plagioclases and kinked biotites (Fig. 7d and e), attest that a solid-state imprint was recorded at high to intermediate temperatures (Mainprice et al., 1986; Simpson and Wintsch, 1989; Kruhl, 1996). Finally, the mylonitic microstructure (iv) is characterized by fine-grained recrystallized grains (average size

Domain 1 is composed of 20 stations that include the northern border of the pluton mostly of granodioritic type. The fabric pattern of this unit shows a well-developed sub-horizontal NE–SW trending lineation, with mean at 50°/0°. The magnetic foliations are mostly parallel to the border, with steep dips. The highest susceptibilities (K < 250 lSI; mean at 238 lSI) and P% values (mean at 3.1%) encountered in the pluton belong to this domain. Microstructures showing a slight to strong deformation, the parallelism of the planar and linear structures with the border of the pluton, and the presence of contact metamorphism suggest that the nearby Jurassic Formation has played the role of a barrier along which some deformation took place during emplacement. 5.2. Domain 2 Although no sharp contact can be observed in the field within the monzogranites, our petrographic investigations as well as the trends and plunges of the lineations, allow to subdivide this domain into four subdomains. 5.2.1. Subdomain 2A (57 stations) In the northeastern part of the pluton, the foliation planes roughly follow the northern margin of the pluton, forming a curve generally dipping toward the pluton center. The mean trend and plunge of the lineation in this domain is 142°/10°. All stations have medium susceptibilities (mean at 193 lSI) and magmatic microstructures are ubiquitous. 5.2.2. Subdomain 2B (feeder zone: 47 stations) It is located to the south and mostly corresponds to monzogranitic rocks with magmatic or close to magmatic microstructures. The lineations steeply plunge to the south, hence call for an extension of the pluton to the south beneath the quaternary formations. The foliation strikes crosscut the southern margin and their dips, mostly steeper than 70°, are the steepest of the whole sampling. Since most lineations have steep plunges (mean at 204°/65°) it is

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Fig. 7. Contact relationships and microstructures in the Shir-Kuh. (a) Bent biotite with elongated prehnite lenses parallel to its cleavage. Intergrowths of plagioclase and quartz (myrmekite) pointing to high-temperature deformation. (b) Crystals of plagioclase showing microstructures of intracrystalline deformation by dislocation creep. (c) Chessboard pattern in quartz. (d) Conjugate kinked bands in a crystal of biotite. (e) Recrystallization in quartz. (f) Mylonitic texture. The abbreviations are from Kretz (1983).

suggested that this subdomain represents a feeder zone. The average susceptibility, with a mean at 205 lSI, is higher than in the rest of the monzogranitic body, and the P values are mostly higher than 2.2% with a mean at 2.4%.

5.2.3. Subdomain 2C (shear zone; 21 stations) In this subdomain the foliations are E–W in strike with moderate to strong dips to the south, roughly wrapping the convexity of the contact with subdomain 2D. The mean lineation trend and plunge is 263°/23°. The mean susceptibility of 180 lSI is less than the average in the monzogranitic body. Microstructures pointing to the imprint of a weak solid-state deformation are abundant in this domain where low temperature deformation features locally intensify, the quartz grains becoming entirely recrystallized into a mosaic of new grains. A concomitant increase of hydrothermal alteration is observed with sphene becoming abundant at the expense of biotite which is transformed into chlorite, and plagioclase into sericite.

5.2.4. Subdomain 2D (34 stations) Here, the lineation trends rotate clockwise from north to south along the subcircular contact with the leucogranites, and their plunges are mostly to the north in the southern part of the subdomain, and to the south in the northern part. Hence, the magnetic fabric roughly outlines the shape of saddle. The low to medium susceptibility (mean: 163 lSI) compositionally corresponds to the more fractionated monzogranites. A narrow and NE-elongate feeder zones could be delineated where lineation plunges are higher than 60° (Fig. 3). Note the discordant nature of the magnetic structures at the contact between subdomains 2C and 2D.

5.3. Domain 3: leucogranite The lack of accessibility to this part of the pluton, the broken nature of the rocks and their high degree of alteration, explain the scarcity of stations in the leucogranites. The dips toward

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Fig. 8. Microstructural map of the Shir-Kuh, showing the subdivision into four categories, from magmatic to mylonitic.

the west of the few foliations recorded at the eastern end of this body, as well as the wrapping lineations of domain 2D paralled to contact with the leucogranites is interpreted as due to the intrusion of the leucogranites into domain 2D while still in the magmatic state. 6. Discussion 6.1. Plutons’s anatomy and evolution A relative chronology of emplacement for the magma batches that form the Shir-Kuh has to be established. To the west, the clear differences that exist in both petrographic characters and susceptibility magnitudes between the monzogranites of subdomain 2D and the granodiorites (Domain 1) are confirmed by the well-marked contact in the field that separates these two magma types. The presence of enclaves made of granodiorite in the monzogranite ascertains the early emplacement of the granodiorite. Subdomain 2D, which shows a well-defined contact with the leucogranite originates from a distinct batch. The NE-elongate and narrow zone of steeply plunging lineations is viewed as a feeder zone for this subdomain. Representing the more fractionated parts of the monzogranite, subdomain 2D also differs from subdomain 2C (to the south) by its distinct fabric pattern. Distinction between subdomains 2D and 2A also results from the contrasted foliation and lineation patterns, and from sites of solid-state deformation that appear at the contact between these subdomains.

Subdomain 2C, with its microstructures almost everywhere affected by some deformation, its foliations close to vertical and EW in strike, its lineations close to horizontal and its rather high P values, call for an area where EW shear corridors were active by the end of pluton emplacement. However, since subdomain 2C abuts, to the east, against subdomains (2A and 2B) having almost exclusively magmatic microstructures, the end of magma feeding from subdomain 2B likely post-dated the activity of shearing in subdomain 2C. Subdomain 2B, already viewed as an elongate feeder at the southern limit of the pluton, may have fed the monzogranitic magmas of the eastern part of the pluton. Finally, the fabric patterns observed in subdomain 2A, where low plunge lineations, magmatic microstructures and low P values are ubiquitous, are consistent with an emplacement possibly fed by subdomain 2B. In conclusion, we speculate that a succession of batches took place during pluton emplacement, issued from at least two identified feeder zones. It is reasonable to think that another source might have produced the granodiorite to the northwest of the pluton. These successive batches were emplaced within not fully crystallized magmas since no solid-state deformation is recorded between domains or subdomains, excepted between 2C and 2D. Hence, accepting that the petrographic variations in the ShirKuh resulted from crystal fractionation (Sheibi et al., 2010), the first magmas to be emplaced were the granodiorites, followed by the monzogranites, possibly in two to three stages. The leucogranite was emplaced by the end, probably pushing aside the

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Fig. 9. The structural domains and subdomains in the Shir-Kuh; lower hemisphere diagrams of the lineations (K1) and poles to foliations (K3). n = Number of measurements.

monzogranitic magma of subdomain 2D. These successive magma pulses are also recorded by their internal structure marking either a continuous feeding, resulting in continuous structural trends, or a discontinuous feeding, resulting in abruptly more evolved magmas issued from different feeder zones, cross-cutting contacts and discordant structural patterns. Successions of magma batches in the infilling process of a pluton, using both petrographic and magnetic fabric data, is now frequently reported by authors such as, for example Razanatseheno et al. (2010).

6.2. The Shir-Kuh within its geodynamic frame The Shir-Kuh belongs to the western border of the Central Iranian block which, by the late Jurassic and early Cretaceous, was situated above the subducting eastern branch of the Neo-Tethys (Dercourt et al., 1985; Golonka, 2004; Fig. 10a). This subduction was directed to the northeast against a NW-directed plate boundary. Taking into account the S-type nature of the magmas composing the Shir-Kuh, i.e. issued mostly from the melting of sediments, a location close to the trench is difficult to envisage since it would

lead to rocks having a calc-alcaline nature. Hence, a remote location, possibly in back-arc position, would be easier to invoke. The main structural feature of the pluton is given by the mean lineation direction which is considered to represent the finite extension direction that the magma suffered during emplacement (see discussion and references in §6 of Ghalamghash et al., 2009). An EW-trending mean lineation can be easily derived from the ENE-, EW- and ESE-lineation trends that are recorded in Domain A, subdomain 2C and subdomain 2B, respectively. This average excludes subdomain 2D, which is largely affected by the emplacement of the leucogranite, and subdomain 2B which is the main feeder zone with steep lineations. Accepting such a rough approximation, it is reasonable to infer that the EW-stretching of the magmas in the pluton during its emplacement is due to NS-compression (pure shear), to simple shear or to a combination of both. Coming back to the geological context, a NS-compression, obtained with a NS slip vector of the eastern branch of the NeoTethys, would not be easy to argue since the plate boundary was close to NW–SE. For such an orientation of the plate boundary, an EW extension of the pluton could be easily argued with a NS to NNE slip vector resulting in a dextral shear along the plate

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Fig. 10. Sketch of the geodynamical context during emplacement of the Shir-Kuh pluton. (a) In section: SSJ: Sandandaj-Sirjan block. (b) In map view: sv: slip vector.

boundary with a component of compression (see discussion in Ghalamghash et al., 2009). In such a context the elongate feeders, NE-directed within subdomain 2D and ESE-directed in subdomain 2B, are tentatively attributed to the extension gashes that could have developped in the hypohetical back-arc environment of within which the Shir-Kuh was emplaced (Fig. 10b). 7. Conclusions The Shir-Kuh granite body is an example of felsic magmas emplaced in the upper crust. It illustrates some petro-structural features of coalescent magma batches that progressively built up a pluton. Successive pulses of magma are recorded in their structural and chemical evolutions. Considering the bulk shape of the pluton, its regional tectonic environment, its fabric patterns and microstructures, it is tentatively argued that it was emplaced within an extensional zone of the crust, in a back-arc environment above the subducting NeoTethys. Acknowledgment The authors thank A. Asna-Ashari for detailed fieldwork. The paper also benefited from discussions with A. Nédélec (Toulouse). References Alavi, M., 1994. Tectonics of Zagros Orogenic belt of Iran, new data and interpretation. Tectonophysics 229, 211–238. Ashworth, J.R., 1972. Myrmekites of exsolution and replacement origins. Geological Magazine 109, 45–62. Assereto, R., 1966. The Jurassic Shemshak formation in central Elburz (Iran). Rivista Italiana di Paleontologia e Stratigrafia 72, 1133–1182. Berberian, M., 1983. Generalized tectonic Map of Iran. Geological Survey of Iran, Report No. 52. Berberian, F., Berberian, M., 1981. Tectono-plutonic episodes in Iran. In: Gupta, H.K., Delany, F.M. (Eds.), Zagros-Hindu Kush-Himalaya Geodynamic Evolution. Geodynamics Series 3. American Geophysical Union, Washington, DC, pp. 5–32. Berberian, M., King, G.C.P., 1981. Towards a paleogeography and tectonic evolution of Iran. Canadian Journal of Earth Sciences 18 (2), 210–265. Blumenfeld, P., Mainprice, D., Bouchez, J.L., 1986. C-slip in quartz from subsolidus deformed granites. Tectonophysics 127, 95–115. Bouchez, J.L., 1997. Granite is never isotropic: an introduction to AMS studies in granitic rocks. In: Bouchez, J.L., Hutton, D.H.W., Stephens, W.E. (Eds.), Granite: from segregation of melt to emplacement fabrics. Kluver, Dordrecht, pp. 95–112.

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