Earth and Planetary Science Letters 230 (2005) 339–354 www.elsevier.com/locate/epsl

Alkaline syenites in eastern Cathaysia (South China): link to Permian–Triassic transtension

Qiang Wanga,*, Jian-Wei Lib, Ping Jianc, Zhen-Hua Zhaoa, Xiao-Lin Xionga, Zhi-Wei Baoa, Ji-Feng Xua, Chao-Feng Lid, Jin-Long Maa

aGuangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, PR China bFaculty of Earth Resources, China University of Geosciences, Wuhan 430074, PR China cGeological Institute, Chinese Academy of Geological Sciences, Beijing 100037, PR China dInstitute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, PR China Received 9 October 2003; received in revised form 15 June 2004; accepted 5 November 2004 Availale online 12 January 2005 Editor: V. Courtillot

Abstract

Two alkaline syenite plutons, the Tieshan and Yangfang plutons, have recently been recognized within NE-trending fault zones in eastern Cathaysia, South China. The rocks are very enriched in K2O (6.28–9.39 wt.%), rare earth elements (REE; particularly light REE) and large ion lithophile elements, but are relatively low in high field strength elements. Isotopically, they 87 86 are characterized by high initial Sr/ Sr (0.7093 to 0.7123) and low eNd(t) values (À5.64 to À10.63). The geochemical data suggest that the alkaline syenites most likely formed via fractional crystallization of enriched mantle-derived magmas. Sensitive High-Resolution Ion Microprobe zircon U–Pb dating indicates that these two intrusions have Late Permian (254F4 Ma) and Early Triassic (242F4 Ma) crystallization ages, respectively. Our data suggest that a tectonic regime dominated by transtension probably existed from at least the latest Permian into the Triassic and was responsible for the formation of the Tieshan and Yangfang alkaline syenites. When combined with previous paleomagnetic, structural, and sedimentology data, we suggest that the transtension along the NE-trending strike-slip fault zones was related to oblique subduction of the Pacific plate underneath South China. D 2004 Elsevier B.V. All rights reserved.

Keywords: Late Permian and Early Triassic; alkaline syenite; transtension; Cathaysia; South China

1. Introduction

The Permo-Triassic was an actively period con- * Corresponding author. Tel.: +86 20 8529 0277; fax: +86 20 cerning the tectonic evolution of South China and 8529 0130. neighboring areas as documented by a series of E-mail address: [email protected] (Q. Wang). significant events like the accretion of the Sibumasu

0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2004.11.023 340 Q. Wang et al. / Earth and Planetary Science Letters 230 (2005) 339–354

Block towards the Indochina–South China Block [1] Yangtze Craton was mainly built upon a stable or the initial collisions between Indochina and South Proterozoic basement. Cathaysia is separated from China Blocks [2] and between South China and North the Yangtze Craton by the Jiangshan–Shaoxing China Blocks [3–6]. Some workers have suggested suture (or fault zone; Fig. 1a), and consists of that South China was dominated by compression in Proterozoic basement and Sinian to Triassic sedi- the Late Permian–Early Triassic (P–T boundary) [7– mentary cover [18]. Late Permian–Triassic granites 9]. On the other hand, there are also arguments are widespread both in Cathaysia and the Yangtze addressing local extension in South China Block Craton (Fig. 1a). Majority of these granites has been (SCB) at the P–T boundary and a few models about classified as peraluminous granites and has been the local extension have been proposed (e.g., Panxi considered to have formed under a compressional rift and Youjiang trough) [10,11]. Lately, Gilder et al. tectonic setting [17–21]. [12] suggested that the Mesozoic tectonic regime in The study area is located in the west of Fujian SCB was dominated by transtension of the NE- Province, eastern Cathaysia (Fig. 1a). NE-trending trending strike-slip fault zones related to the oblique fault zones are well developed in the whole of subduction of the proto-Pacific plate underneath Cathaysia (Fig. 1a). From east to west in Fujian, South China. Obviously, the P–T boundary tectonics major regional structures are the Changle–Nan’ao, in SCB needs to be re-examined or clarified. Fu’an–Nanjing, Zhenghe–Dapu, Wuping–Pucheng, Alkaline igneous rocks are typically found in and Heyuan–Shaowu fault zones (Fig. 1a) [23]. extensional tectonic settings (e.g., post-collision, rift, The Changle–Nan’ao and Fu’an–Nanjing fault or anorogeny) [13–16]. From this point of view, a zones appear to have controlled the emplacement study of alkaline igneous rocks will provide insight of Late Cretaceous alkaline or A-type granites in into regional tectonic environments. Widespread Late eastern Fujian [24]. In Cathaysia, the Late Per- Permian–Triassic igneous rocks crop out in eastern mian–Triassic peraluminous granites occur primarily SCB, which are mainly of peraluminous granites and to the west of the Fu’an–Nanjing fault (Fig. 1a). In have been commonly interpreted in terms of com- addition, Late Paleozoic–Mesozoic sedimentary pressional tectonic settings [17–21]. Our recent work basins also developed to the west of Fu’an–Nanjing has led to recognition of two Late Permian to Early fault [12,23,25,26]. Triassic alkaline syenite plutons in eastern Cathaysia, The Tieshan and Yangfang alkaline syenite South China. In this paper, we present high-quality plutons occur in western Fujian (Fig. 1a). The trace elements, and Sr–Nd and U–Pb isotopic data Tieshan pluton has an exposed area of ~8 km2 and for these two syenites. These results were used to is located to the east of Zhenghe county. Its long axis place additional and important constraints on the trends northeast and parallel to the Zhenghe–Dapu tectonic evolution of South China in Permo-Triassic fault zone in which the pluton is found, where it times. intruded Proterozoic metamorphic rocks (Fig. 1a and b). It is locally overlain by Jurassic–Cretaceous volcanic rocks and is intruded by Jurassic–Creta- 2. Geological setting ceous granites to the east (Fig. 1b). The Yangfang pluton occurs in the northeast of Mingxi County and The SCB is composed of the Yangtze Craton in is tectonically controlled by the Wuping–Pucheng the west and Cathaysia in the east (Fig. 1a). The fault zone (Fig. 1a and c). It is composed of three

Fig. 1. (a) Schematic map showing the distribution of the late Permian–Triassic granites in the east of South China Block (SCB). The upper-left inset illustrates major tectonic unites in eastern Asia (after [1,4,6]). SCB=South China Block; NCB=North China Block; YC=Yangtze Craton; IC=Indochina Block; SI=Sibumasu Block; SG=Songpan-Ganze Accretionary Complex; WB=West Burma; Hi=Himalaya; LS=Lhasa;

QT=Qiangtang. Fault zones: F1=Jiangshan–Shaoxing fault zone; F2=Heyuan–Shaowu fault zone; F3=Wuping–Pucheng fault zone; F4=Zhenghe–Dapu fault zone; F5=Fu’an–Nanjing fault zone; F5=Changle–Nan’ao fault zone. The geochronological data are after [17–22], and references therein]. (b, c) Geological maps of the Tieshan and Yangfang plutons, respectively. 1—volcanic rocks; 2—Jurassic–Cretaceous granites; 3—alkaline syenite; 4—Siluric granites; 5—Jurassic–Cretaceous volcanic rocks; 6—early Paleozoic sedimentary rocks; 7— Carboniferous and Early Permian sedimentary rocks; 8—Proterozoic rocks; 9—fault. Q. Wang et al. / Earth and Planetary Science Letters 230 (2005) 339–354 341 342 Q. Wang et al. / Earth and Planetary Science Letters 230 (2005) 339–354 separate intrusions with NE-oriented long axes (Fig. 4. Analytical methods 1c). The total exposed area amounts to about 8 km2. They intruded late Paleozoic Carboniferous and Samples for SHRIMP zircon U–Pb dating were Permian sedimentary rocks and are partially overlain collected from the Yangfang aegiriteaugite syenite by Cenozoic volcanic rocks to the south (Fig. 1c). (sample 99FJ024) and the Tieshan melanite syenite The Tieshan and Yangfang alkaline syenites have (sample 99FJ031). Zircon grains were separated using been considered as late Mesozoic intrusive rocks for conventional heavy liquid and magnetic techniques. a long time [23,27,28] and many aspects regarding Representative zircon grains were handpicked under their petrogenesis remains unknown, largely due to binocular microscope and mounted in an epoxy resin the scarcity of precise geochemical and chronolog- disc, and then polished and coated with gold film. ical constraints. Internal morphology was examined using backscatter electron microscopy prior to U–Pb isotopic analyses. The U–Pb isotopic analyses were performed using the 3. Petrography Sensitive High-Resolution Ion Microprobe (SHRIMP- II) at the Chinese Academy of Geological Sciences The Tieshan pluton consists primarily of medium- (Beijing). Details of the analytical procedures of to-coarse-grained melanite syenite and melanite-bear- zircons using SHRIMP was described by Song et al. ing pyroxene syenite. The melanite syenite is gen- [29] and Jian et al. [30]. Inter-element fractionation erally light red to olive, and consists primarily of ion emission of zircon was corrected relative to the microcline (50–80%), melanite (10–40%), pyroxene RSES reference TEM (417 Ma). The uncertainties in (5–20%), minor plagioclase (2–5%), and accessory ages are cited as 1r, and the weighted mean ages are minerals (1%F) including magnetite, sphene, apatite, quoted at the 95% confidence level (2r). and zircon. Melanite is a characteristic alkali mafic Fresh rock samples from both the Tieshan and mineral of the Tieshan pluton, generally rounded, Yangfang plutons were also selected for elemental puce, and occasionally brown in the core and beige- and Sr–Nd isotopic analyses. Major elements were buff in the margin. Pyroxene is commonly columnar determined by gravimetry and AAS (wet chem- and colorless, but sometimes virescent due to alter- istry), following the analytical procedures of Gao et ation. The melanite-bearing pyroxene syenite is al. [31]. Trace elements were analyzed by a Perkin- celadon to puce, and petrographically similar to the Elmer ELAN 6000 ICP-MS at the Guiyang melanite syenite except that the former has higher Institute of Geochemistry, Chinese Academy of pyroxene (20–40%) and lower melanite (3–10%) Sciences, using the methods of Qi et al. [32]. contents. Analytical precision for most elements is better The Yangfang pluton is composed of aegiriteaugite than 3%. Sr and Nd isotopic compositions were syenite and aegiriteaugite-bearing syenite, which are determined using a Finnigan MAT-262 Mass gray and mainly fine-to-medium-grained, and consist Spectrometer operated in a static multi-collector of perthite (60–75%), aegiriteaugite (2–25%), and mode at the Institute of Geology and Geophysics, minor quartz (0–8%), albite (1–6%), aegirite (1–5%), Chinese Academy of Sciences, following the and biotite (1–3%). They also contain abundant and procedures of Zhang et al. [33]. The 87Sr/86Sr ratio diverse accessory minerals including sphene, magnet- of the NBS987 standard and 143Nd/144Nd ratio of ite, apatite, allanite, monazite, zircon, and pyrochlore. the La Jolla standard measured during this study Spectacular ribbon structures are developed in per- were 0.710234F7(2rm) and 0.511838F8(2jm), thite. Anhedral albite is commonly restricted between respectively. Overall blank contributions are about perthite grains, suggesting its occurrence as a terminal 0.2–0.5 ng for Rb and Sr, and about 50 pg for Nd phase in the crystallization of the magma. Columnar and Sm. The Rb, Sr, Sm, and Nd concentrations aegiriteaugite is the most important alkali mafic exhibit good agreement with those analyzed using mineral in the Yangfang plutons. It is characterized ICP-MS. The measured 143Nd/144Nd and 86Sr/88Sr by conspicuous zoning with a colorless core and green ratios are normalized to 143Nd/144Nd=0.7219 and margin. 86Sr/88Sr=0.1194, respectively. Q. Wang et al. / Earth and Planetary Science Letters 230 (2005) 339–354 343

5. Results yield a 206Pb/238U single age population of 235 to 267 Ma, with a weighted mean age of 254F4Ma(2r; 5.1. Zircon U–Pb geochronology Table 1 and Fig. 2a). Fifteen analyses of zircons from sample 99FJ031 give concordant ages of 233–249 Ma Backscatter electron images of the analyzed with a weighted mean of 242F4Ma(2r; Table 1 and zircons show that they are generally embayed with Fig. 2b). Age data of both samples are the best some cracks. A few grains clearly have micro-scale, estimates of crystallization ages of the Tieshan and magmatic oscillatory zonings (Fig. 2). The results of Yangfang alkaline syenites, suggesting these intru- the SHRIMP U-Pb zircon analyses are listed in Table sions formed in the latest Permian and Early Triassic, 1. Fifteen analyses of zircons from sample 99FJ024 respectively.

A17.1100µ m 500 0.08 99FJ024 D13.1 D3.1

400 0.06 U

238 300

D11.1 Pb / 0.04

D7.1 206 200 254 ± 4 Ma (n=15) MSWD=1.34 0.02 100

0 D9.1 D6.1 0.00 0.0 0.2 0.4 0.6

207 235 Pb / U (a)

100µm 500 0.08 99FJ031 A5.1 A18.1 400 0.06

U 300 A17.1 238 242 ± 4 Ma A13.1 0.04 Pb / (n=15) 200 206 MSWD=0.91

0.02 100 A7.1 A6.1

0 0.00 0.0 0.2 0.4 0.6 207 235 Pb / U (b)

Fig. 2. SHRIMP zircon U–Pb concordia diagram with backscatter electron images for samples 99FJ024 (Tieshan; a) and 99FJ031 (Yangfang; b). 344 Q. Wang et al. / Earth and Planetary Science Letters 230 (2005) 339–354

Table 1 SHRIMP zircon U–Pb isotopic analyses of the Tieshan and Yangfang alkaline syenites Spot 206Pbc U Th 232Th/238U 206Pb* 206Pb/238U 207Pb*/206Pb* F% 207Pb*/235U F% 206Pb*/238U F% [%] [ppm] [ppm] [ppm] [age, Ma] 99FJ024 D1.1 – 700 98 0.15 23.8 246F6 0.0485 6.5 0.260 7 0.0389 2.6 D2.1 – 811 32 0.04 27.8 250F6 0.0503 4.5 0.274 5.2 0.0395 2.6 D3.1 – 1177 33 0.03 39.9 249F6 0.0492 2.7 0.267 3.7 0.0393 2.5 D4.1 2.09 291 7 0.03 9.92 243F7 0.0419 21 0.222 21 0.0384 2.8 D5.1 0.5 797 47 0.06 25.6 235F6 0.0513 4.1 0.263 4.8 0.0371 2.6 D6.1 0.23 2293 149 0.07 81.5 261F7 0.0529 1.6 0.301 3.0 0.0413 2.5 D7.1 3.59 735 111 0.16 25.7 249F7 0.0556 8.7 0.302 9.1 0.0394 2.6 D8.1 0.03 3408 380 0.12 122 263F7 0.0511 1.7 0.293 3.1 0.0415 2.6 D9.1 0.36 1454 86 0.06 50.1 252F7 0.0516 2.3 0.284 3.4 0.0399 2.5 D10.1 0.13 2083 1164 0.58 73.2 258F7 0.0531 1.5 0.299 2.9 0.0408 2.5 D11.1 0.32 2171 106 0.05 77.3 261F7 0.0521 1.5 0.297 2.9 0.0413 2.5 D12.1 0.77 1087 55 0.05 37.2 251F7 0.0524 3.2 0.286 4.1 0.0396 2.5 D13.1 0.08 4158 340 0.08 149 263F7 0.0515 1.1 0.296 2.7 0.0417 2.5 D14.1 1.45 433 11 0.03 14.8 248F7 0.0512 7.9 0.277 8.4 0.0392 2.7 D15.1 0.38 1466 125 0.09 53.2 267F7 0.0543 2.4 0.316 3.5 0.0422 2.5

99FJ031 A4.1 0.45 1614 83 0.05 53.7 245F6 0.0549 1.9 0.293 3.1 0.0387 2.5 A5.1 0.22 3830 6708 1.81 126 242F6 0.0512 1.3 0.270 2.8 0.0383 2.5 A6.1 0.42 2792 3324 1.23 94.1 248F6 0.0510 1.3 0.281 2.8 0.0392 2.5 A7.1 – 3897 6995 1.85 130 245F6 0.0516 1.1 0.276 2.7 0.0388 2.5 A8.1 – 4876 8451 1.79 161 243F6 0.0524 1.1 0.277 2.7 0.0383 2.5 A9.1 0.23 3822 8914 2.41 124 239F6 0.0507 1.2 0.264 2.7 0.0378 2.5 A10.1 0.49 3313 1631 0.51 108 240F6 0.0515 1.5 0.270 2.9 0.0379 2.5 A11.1 0.05 5505 1306 0.25 192 256F6 0.0516 0.9 0.289 2.7 0.0406 2.5 A12.1 – 4136 8568 2.14 136 241F6 0.0500 1.2 0.263 2.8 0.0381 2.5 A13.1 0.10 5072 1115 0.23 172 248F6 0.0520 1.3 0.282 2.8 0.0393 2.5 A14.1 – 4955 9625 2.01 160 237F6 0.0510 1.1 0.264 2.7 0.0375 2.5 A15.1 0.13 3555 787 0.23 116 240F6 0.0510 1.3 0.267 2.8 0.0379 2.5 A16.1 – 4689 6405 1.41 152 238F6 0.0508 1.1 0.263 2.7 0.0376 2.5 A17.1 0.08 3290 6239 1.96 106 236F6 0.0524 1.9 0.270 3.1 0.0373 2.5 A18.1 0.28 5358 8159 1.57 170 233F6 0.0507 1.3 0.258 2.8 0.0368 2.5

(1) Errors are 1r;Pbc and Pb* indicate the common and radiogenic portions, respectively; (2) Error in Standard calibration was 0.33% (not included in above errors but required when comparing data from different mounts); (3) Common Pb corrected using measured 204Pb.

5.2. Major and trace elements trace elements with SiO2 display linear trends (Fig. 4a–j). Samples from the Tieshan and Yangfang Major and trace element compositions of the plutons have SiO2 concentrations ranging from 45– Tieshan and Yangfang plutons are tabulated in Table 55 wt.% and 58–65 wt.%, respectively. Contents of total 2, and major oxide concentrations are plotted in Fig. CaO, FeO (=FeO+Fe2O3Â0.9), P2O5,TiO2 and 3. All samples are plotted in the alkaline field in the MgO, and compatible elements (e.g., Co, Sc) total alkaline-silica (TAS) diagram (Fig. 3a) and in decrease, whereas Al2O3 and Sr contents increase, the shoshonitic field in the SiO2 (wt.%) versus K2O with increasing SiO2 concentrations. However, Na2O (wt.%) diagram (Fig. 3b). All samples of the concentrations of the Tieshan pluton are invariable Tieshan pluton and most of the Yangfang pluton with increasing SiO2 contents (Fig. 4g). Compatible samples fall within the ultra-potassic field on the elements (e.g., Ni, Co, Sc) show approximately total Na2O (wt.%) versus K2O (wt.%) diagram (Fig. 3c). positive correlations with CaO, FeO , and MgO Major oxide variations and correlations of some (Fig. 4k–o). Q. Wang et al. / Earth and Planetary Science Letters 230 (2005) 339–354 345

Table 2 Major oxides and trace elemental compositions of the Tieshan and Yangfang alkaline syenites Pluton sample Tieshan Yangfang 99FJ021-1 99FJ021-1-2 99FJ-023 99FJ-024 99FJ-030 99FJ-031 99FJ-033 99FJ033-2 99FJ033-3 Major oxides (%)

SiO2 48.75 50.97 54.99 51.80 64.39 58.53 62.07 61.50 61.93 TiO2 1.68 1.31 1.00 1.28 0.33 0.77 0.84 0.86 0.84 Al2O3 12.44 15.36 14.08 12.59 15.94 12.00 15.28 15.08 15.08 Fe2O3 5.05 3.83 4.89 5.06 1.55 1.73 2.57 2.67 2.62 FeO 3.33 2.53 1.77 3.35 1.43 2.77 1.33 1.37 1.40 MnO 0.23 0.16 0.17 0.21 0.09 0.11 0.09 0.09 0.09 MgO 3.27 2.36 0.92 2.61 0.72 4.60 1.15 1.16 1.16 CaO 14.06 10.58 9.68 12.42 2.23 6.38 1.97 2.31 2.15

Na2O 0.42 0.32 0.78 0.78 5.63 2.76 3.20 3.44 3.40 K2O 6.71 8.95 9.12 6.88 6.28 7.37 9.39 9.34 9.30 P2O5 1.83 1.42 0.41 0.85 0.14 1.07 0.27 0.28 0.27 CO2 0.08 0.03 0.03 0.06 0.03 0.06 0.06 0.14 0.11 HP2O 1.25 1.50 0.79 0.65 0.53 0.76 0.49 0.50 0.44 99.10 99.32 98.63 98.54 99.29 98.91 98.71 98.74 98.79

Trace elements (ppm) Sc 14.6 11.3 6.78 11.1 6.83 18.3 9.54 8.86 8.94 V 134 98.3 158 151 47.1 106 102 103 102 Cr 10.7 6.72 7.06 24.5 12.9 29.1 6.11 9.76 9.78 Co 13.9 10.4 6.86 14.7 2.93 19.3 8.43 7.74 7.96 Ni 12.0 9.15 5.18 12.7 4.85 21.6 4.30 6.11 6.28 Zn 152 108 86.5 137 109 77.4 79.3 85.5 85.1 Ga 23.2 19.8 24.8 21.0 29.5 20.9 24.4 23.2 22.7 Rb 364 663 220 169 365 342 434 439 425 Sr 1884 1345 3504 3237 1197 684 865 859 882 Y 139 114 111 70.7 159 34.9 59.9 61.2 61.8 Zr 635 480 625 408 228 128 499 473 492 Nb 62.1 52.9 25.6 5.63 47.4 21.8 58.9 57.9 55.2 Cs 6.27 4.41 1.79 3.49 4.77 4.33 8.28 8.34 8.14 Ba 4268 2666 7294 17849 2876 5913 7113 7138 6977 Hf 22.4 17.9 16.9 11.7 8.58 5.41 14.2 13.2 13.4 Ta 4.79 3.81 1.77 0.51 2.29 1.49 2.56 2.62 2.65 Pb 16.4 10.5 23.8 19.9 121 32.5 209 226 209 Th 60.4 50.3 62.3 45.4 92.3 45.0 95.9 127 135 U 5.59 4.24 9.30 5.41 10.2 6.79 14.6 15.1 15.2 La 263 233 162 195 1158 237 183 199 201 Ce 600 501 368 424 1946 440 366 344 404 Pr 82.3 67.7 47.5 51.4 132 48.7 38.8 42.4 42.9 Nd 351 288 202 211 405 170 145 157 160 Sm 71.9 60.7 43.3 40.0 57.6 26.0 27.8 30.5 31.1 Eu 17.6 14.8 10.5 9.67 13.2 5.54 6.86 7.41 7.53 Gd 56.9 49.0 33.0 30.8 46.9 18.4 22.9 24.3 25.2 Tb 7.21 6.06 4.42 3.51 6.28 1.85 2.73 3.04 3.11 Dy 34.6 29.1 22.4 16.6 30.7 7.7 12.7 13.8 14.5 Ho 5.40 4.57 3.91 2.50 5.23 1.12 1.90 2.13 2.13 Er 12.2 10.6 9.69 6.02 13.4 2.75 4.65 4.80 5.32 Tm 1.37 1.33 1.40 0.81 1.63 0.31 0.51 0.54 0.55 Yb 7.67 6.73 8.47 4.44 10.56 1.94 3.17 3.25 3.41 Lu 1.00 0.78 1.16 0.60 1.46 0.28 0.38 0.38 0.45 346 Q. Wang et al. / Earth and Planetary Science Letters 230 (2005) 339–354

15 (a) nificantly enriched light REE, strongly fractionated Tieshan heavy REE (La/Yb=19–43), and no obvious Eu Yangfang anomalies (Fig. 5a). Similarly, the Yangfang samples 10 also have very high LILE (Ba=2876–7138 ppm, Rb=342–438 ppm) and variable contents of high field

22 strength elements (HFSE; Y=35–159 ppm, Zr=128–

a+O(wt%) NaO+KO 5 499 ppm, and Nb=21.8–57.9 ppm). The primitive- mantle-normalized REE patterns of the Yangfang samples are essentially the same as the Tieshan 0 35 45 55 65 75 syenite (Fig. 5a), characterized by light REE enrich-

SiO2 (wt %) ment, strongly fractionated heavy REE (La/Yb=61– 10 (b) 122) and no obvious Eu anomalies (Fig. 5a). An exception is that sample 99FJ030 has much higher 8 P REE contents (P REE=3828 ppm) relative to the 6 other samples ( REE=816–962 ppm; Fig. 5a and Table 2). The primitive-mantle-normalized trace 2

K O (wt %) 4 shoshonitic elements profiles display very similar shapes for samples of both plutons, showing consistent enrich- High-K 2 ment in LILE (Rb, Ba, and Th) but depletion in HFSE Medium-K Low-K (Nb, Zr, Hf, and Ti; Fig. 5b). Samples from the 0 45 50 55 60 65 70 Yangfang pluton are depleted in Sr (Fig. 5b). It is

SiO2 (wt %) noted that LILE and HFSE contents are locally variable among distinct samples from both plutons. (c)ultra-potassic 10 The Zr+Nb+Ce+Y versus 10000Â Ga/Al diagram indicates that the Tieshan and Yangfang alkaline syenites are geochemically similar to A-type or

=2 within-plate igneous rocks (Fig. 6). O

2 5 /Na K O (wt %) 22 KO shoshonitic 5.3. Nd–Sr isotopic compositions /Na O=0.5 KO22 calc-alkaline Eight Nd–Sr isotopic measurements (Table 3) were 0 carried out on the Tieshan and Yangfang plutons. The 0 1 2 3 4 5 6 eNd(t) values range from À5.64 to À10.63 and the Na2 O (wt %) 87 86 ( Sr/ Sr)i ratios vary between 0.7093 and 0.7123. Fig. 3. (a) Chemical composition of the Tieshan and Yangfang These values are comparable to potassic and ultra- intrusive rocks plotted in the TAS classification diagram (after [34]). potassic rocks found elsewhere [37] (Fig. 7a). The bold solid line separates subalkaline and alkaline compositions. Notably, the Tieshan intrusive rocks have higher (b) SiO –K O variation diagram, showing shoshonitic character- 2 2 e values (À5.64 to À6.62) but lower (87Sr/86Sr) istics of Tieshan and Yangfang samples. (c) Na2O–K2O variation Nd(t) i diagram, showing ultrapotassic characteristics of Tieshan samples ratios (0.7093 to 0.7097) than the Yangfang intrusive 87 86 and most of the Yangfang samples. The major element oxides are rocks (eNd(t)=À9.15 to À10.63 and ( Sr/ Sr)i= from Table 2 and from [27,28]. 0.7107 to 0.7123). The Nd–Sr isotopic data plot between the contemporaneous Emeishan flood basalts All the Tieshan samples are very rich in large ion (western SCB), which are related to a mantle plume lithophile elements (LILE; e.g., Sr=1345–3504 ppm, [10,38], and peraluminous granites are generated by Ba=2666–17849 ppm) concentrations, and haveP var- partial melting of crustal rocks [17,18,20–22] (Fig. iable rare earth elements (REE) contents ( REE= 7a). The depleted mantle model ages (TDM) of the 918–1513 ppm; Table 2). They display similar syenites range from 1.37 to 1.86 Ga (Table 3), and the primitive-mantle-normalized REE patterns with sig- eNd(t=250 Ma) versus TDM diagram (Fig. 7b) demon- Q. Wang et al. / Earth and Planetary Science Letters 230 (2005) 339–354 347

20 2 (a)15 (b) (c) 15 10 wt %) 10 1 total 25 PO (wt%) CaO (wt %) FeO ( 5 5

0 45 50 55 60 65 70 45 50 55 60 65 70 45 50 55 60 65 70

SiO2 (wt %) SiO2 (wt %) SiO2 (wt %) 2 6 18 (d) (e) (f) 5 17 16 4 15 1 3 14 2

2 23 13 TiO (wt %) Al O (wt %) MgO (wt %) 12 1 11 0 0 10 45 50 55 60 65 70 45 50 55 60 65 70 45 50 55 60 65 70

SiO2 (wt %) SiO2 (wt %) SiO2 (wt %) 6 20 (g)20 (h) (i) 5

4 15 15

3 10 2 Sc (ppm) 2 Co (ppm)

Na O (wt %) 10 5 1 0 0 45 50 55 60 65 70 45 50 55 60 65 70 5 45 50 55 60 65 70 SiO (wt %) SiO (wt %) SiO2 (wt %) 2 2 4000 (j) (k)20 (l) 20 3200 15 2400 15 10 1600 10 Sr (ppm) Ni (ppm) Co (ppm)

800 5 5

0 0 45 50 55 60 65 70 0 5 10 15 0 2 3 4 5 6 7 8 9 10 total SiO2 (wt %) CaO (wt %) FeO (wt %) 20 (m)20 (n) (o) 20 15 15 15 10 10 Sc (ppm) Ni (ppm) Co (ppm) 10 5 5

0 0 0 1 2 3 4 5 6 0 1 2 3 4 5 6 5 0 1 2 3 4 5 6 MgO (wt %) MgO (wt %) MgO (wt %)

Fig. 4. Selected variation diagrams of major and trace elements for the Tieshan and Yangfang intrusive rocks. The major elements are from Table 2, [27] and [28]. The trace elements are from Table 2. Sample symbols are the same as those in Fig. 3. 348 Q. Wang et al. / Earth and Planetary Science Letters 230 (2005) 339–354

partial melting of crustal rocks, AFC process and 1000 Tieshan pluton magma mixing. Yangfang pluton Lack of inherited zircons (Fig. 2a and b) in our 100 samples clearly suggests that the alkaline syenites were not likely derived from partial melting of the Proterozoic–early Paleozoic rocks or via AFC process 10 [39]. The Nd isotopic compositions of the Tieshan and

Rocks/primitive mantle Yangfang alkaline syenites are distinct from those of (a) regional Proterozoic igneous and metamorphic rocks 1 (Fig. 7b), indicating that these alkaline syenites could La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu not have been generated by partial melting of ancient 10000 basement rocks. The Nd–Sr isotopic compositions (Fig. 7a), which are between the coeval mantle- 1000 derived Emeishan flood basalts [38] and crust-derived peraluminous granites [17,18,20–22], may lead to speculation that the alkaline syenites resulted from 100 magma mixing between basaltic magma and crustal melt. However, given the much lower Sr and Nd 10 concentrations in the Emeishan basalts and the Rocks/primitive mantle peraluminous granites [17,18,20–22,38], mixing of (b) these magmas would be incapable of forming high Sr 1 RbBa Th K Nb La Ce Sr Nd Hf Zr SmEuTi Gd Dy Er YbLu 2400 Fig. 5. Primitive-mantle-normalized rare earth elements patterns (a) Tieshan and multielement diagrams (b). The primitive-mantle values follow 2000 Yangfang Sun and McDonough [35]. 1600 strates that their Nd isotopic compositions are similar 1200 to the surrounding Proterozoic–Paleozoic sedimentary 800 A-type granites rocks rather than the Proterozoic igneous and meta- Zr+Nb+Ce+Y(ppm) morphic rocks in Cathaysia, which suggest that the 400 I- and S-type granites (a) sediments were likely involved in the formation of the 0 Tieshan and Yangfang syenites. 1 2 3 4 5 Ga/Al * 10000

1000 6. Discussion syn-COLG WPG

100 6.1. Petrogenesis post-COLG Rb (ppm) Several genetic models have been proposed to 10 interpret the formation of syenites, which mainly ORG include: (1) partial melting of crustal rocks [13,39]; VAG (b) (2) crustal assimilation and fractional crystallization 1 1 10 100 1000 (AFC) of basaltic magma [34]; (3) mixing of basaltic Y + Nb (ppm) magma with crustal melt [16,34]; and (4) fractional crystallization of mantle-derived magmas [15]. The Fig. 6. (a) Diagram of 10,000Â Ga/Al versus Zr+Nb+Ce+Y [13]. (b) Y+Nb versus Rb discrimination diagram [36]. VAG: volcanic present petrographical, geochemical, and zircon U–Pb arc granites; ORG: ocean ridge granites; WPG: within-plate data indicate that the Tieshan and Yangfang alkaline granites; syn-COLG and post-COLG: syn-and post-collision gran- syenites could not have been formed through directly ites. Sample symbols are the same as those in Fig. 3. Q. Wang et al. / Earth and Planetary Science Letters 230 (2005) 339–354 349

Table 3 Nd–Sr isotopic data of the Tieshan and Yangfang alkaline syenites Sample 99FJ021-1 99FJ021-1-2 99FJ023 99FJ024 99FJ030 99FJ031 99FJ033 99FJ033-2 Sm (ppm) 73.68 65.98 43.69 42.00 61.43 25.24 28.75 32.61 Nd (ppm) 366.9 314.5 205.6 224.0 439.9 165.7 148.8 172.5 147Sm/144Nd 0.1215 0.1269 0.1285 0.1134 0.0845 0.0921 0.1168 0.1144 143Nd/144Nd 0.512224 0.512183 0.512213 0.512183 0.511991 0.511945 0.511967 0.511971

2rm F5 F10 F5 F7 F7 F11 F6 F6 eNd(t) À5.64 À6.62 À6.09 À6.18 À9.16 À10.30 À10.63 À10.48 TDM (Ga) 1.53 1.69 1.67 1.47 1.37 1.51 1.86 1.80 Rb (ppm) 320.7 688.9 214.2 169.3 373.0 332.3 430.4 435.8 Sr (ppm) 1707 1227 2978 2946 1082 684.3 798.9 859.4 87Rb/86Sr 0.5439 1.626 0.2083 0.1664 0.9988 1.407 1.561 1.469 87Sr/86Sr 0.711701 0.715196 0.710134 0.710343 0.714107 0.715835 0.717501 0.717337

2rm F18 F7 F8 F9 F13 F10 F6 F10 87 86 ( Sr/ Sr)i 0.7097 0.7093 0.7094 0.7097 0.7107 0.7110 0.7121 0.7123

and Nd components observed in our syenites (Fig. 7c ever, Al2O3,Na2O, and Sr (Fig. 4f, g, and j) of the and d). In addition, the Nd isotopic systematics do not Yangfang intrusive rocks show positive correlation support crustal assimilation of basaltic magmas, with SiO2, indicating that albite in the residual magma which is further demonstrated in the SiO2 versus possibly increased with the fractional crystallization 87 86 ( Sr/ Sr)i and SiO2 versus eNd(t) diagrams (Fig. 7e of other minerals, which is indeed in accordance with and f). petrographical observation. A fractional crystalliza- Collectively, our data indicate that the Tieshan and tion model regarding the genesis of the syenites is also Yangfang alkaline syenites were most likely formed well documented by the Nd–Sr isotopic compositions via fractional crystallization of mantle-derived mag- (Fig. 7 e and f). mas with limited assimilation of older crust. The The Sr–Nd isotopic and trace elemental data of the Harker diagrams and compatible elements variations plutons indicate that the Tieshan and Yangfang with major oxides (Fig. 4) provide direct evidence for alkaline syenites originated most likely from an the fractional crystallization in the evolution of parent enriched mantle once interacted with crustal sedimen- magmas. It is known that CaO is mainly concentrated tary materials. The HFSE depletion may result from in plagioclase, pyroxene, and apatite; MgO in olivine mantle metasomatism related to subducted fluids or total 87 86 and pyroxene; P2O5 in apatite; FeO in olivine, sedimentary materials [35]. The initial Sr/ Sr pyroxene, and magnetite; and TiO2 in ilmenite. values (0.7093~0.7123) of the Tieshan and Yangfang Therefore, the decrease in CaO, MgO, FeOtotal, and alkaline syenites are higher than fluid components TiO2 with increasing SiO2 demonstrates that the (~0.7035) from subducted oceanic crust, but close to Tieshan and Yangfang alkaline syenites were possibly the calculated value for Global Subducting Sediments produced by fractional crystallization of plagioclase, (0.717 average) [40]. The Nd–Sr isotopic composi- pyroxene, olivine, apatite, magnetite, and ilmenite. tions of the two plutons lie between depleted mantle However, no olivine and rare plagioclase has been and EMII, and are similar to some potassic and observed in both syenites, suggesting that the major ultrapotassic rocks from South Africa and West oxide variations were not related to the fractional Australia that have been considered to be derived crystallization of plagioclase and olivine, as is seen by from enriched mantle metasomatised by subducted the positive correlations of Al2O3 and Sr with SiO2 sediments [37] (Fig. 7a). In addition, the U/Th versus 87 86 (Fig. 4f and j). For the Tieshan intrusive rocks, the Th and Sr/Th versus ( Sr/ Sr)i diagrams (Fig. 8) also positive correlation of K2O, Al2O3, and Sr with SiO2 show that subducted sediments have indeed played an indicate that K-feldspar (Figs. 3b and 4f–g) in the important role in the source region of the syenites residual magma had increasingly accumulated with [40]. Moreover, both syenites have high Th/Ce ratios the fractional crystallization of other minerals. How- (average of 0.12 and 0.22, respectively) that are 350 Q. Wang et al. / Earth and Planetary Science Letters 230 (2005) 339–354

87 86 87 86 Fig. 7. (a) Initial Sr/ Sr versus eNd(t) diagram. (b) Nd model age (TDM) versus eNd(t=250 Ma) diagram. (c) Sr versus initial Sr/ Sr value 87 86 diagram. (d) Nd versus eNd(t=250 Ma) diagram. (e) SiO2 versus initial Sr/ Sr diagram. (f) SiO2 versus eNd(t) values diagram. The data of the worldwide potassic–ultrapotassic rocks from South Africa, Central Italy, Australia, Tuscany, and Spanish are from [37]. The data of late Permian Emeishan basalts in the western Yangtze Craton are from [38]. The data of Late Permian–Triassic peralumious granites in eastern SCB are from [17,18,20–22]. The data of the Proterozoic igneous and metamorphic rocks and Proterozoic–Paleozoic sedimentary rocks in Cathaysia are from [18]. Sample symbols are the same as those in Fig. 3. similar to those of Global Subducting Sediments the source of the Tieshan and Yangfang alkaline (0.12) or Archean shale (0.22) [40], but clearly syenites is most compatible with enriched mantle different from those of MORB and OIB (0.016 and metasomatised by subducted sedimentary material. 0.052, respectively) [35]. Thus, it is concluded that The Nd isotopic compositions (Fig. 7b) also support Q. Wang et al. / Earth and Planetary Science Letters 230 (2005) 339–354 351

1.1 components of subducted sediments than that of the 1 Marianas, South Sandwich Fluids (a) IS., Tonga-Kermadecs, Tieshan syenites. Vanuatu, Japan-Kamchatka, Lesser Antilles Wedge Sediments 6.2. Tectonic control

U/Th Aeolian Is., Indonesia, Many NE-trending left-lateral strike-slip faults MORB Philippines have been recognized in the SCB [42]. Important strike-slip faults in the eastern Cathaysia are the Changle–Nan’ao, Fu’an–Nanjing, Zhenghe–Dapu, OIB Wuping–Pucheng, and Heyuan–Shaowu fault zones 0 0 20 40 60 80 100 120 140 (Fig. 1). These regional structures were commonly Th (ppm) interpreted in terms of the oblique subduction of the 1200 Marianas, South proto-Pacific plate underneath South China, which Fluids (b) Sandwich IS., initiated in the Triassic [12,26,42,43] or even the 1000 Tonga-Kermadecs, Vanuatu, Permian [7,8,44]. Recent paleomagnetic data docu- Japan-Kamchatka, Wedge Sediments 800 Lesser Antilles ments Late Permian–Triassic clockwise rotation within Cathaysia [12,26]. Blocks would be rotated

Sr/Th 600

400 Aeolian Is., Indonesia, 0.40 0.8 Philippines 200 0.6

La/Ba 0.4 0.30 0 0.702 0.704 0.706 0.708 0.710 0.712 0.714 0.2 La (ppm) 87 86 0 ( Sr/ Sr)i 180 210 240 270

La/Ba 0.20 Fig. 8. (a) Diagram of U/Th versus Th abundances for the studied alkaline syenites compared to basalt and andesites from selected arc 0.10 suites [40]. (b) Initial 87Sr/86Sr versus Sr/Th for the studied alkaline syenites compared to basalt and andesites from selected arc suites (a) [40]. Sample symbols are the same as those in Fig. 3. 0 0 200 400 600 800 1000 1200 La (ppm) this interpretation. The metasomatism during subduc- 0.025 tion was a possible cause for the formation of 0.004 phlogopiteFpotassic amphibole in their mantle 0.020 La/K source, which is evidenced by the La–La/K and La– 0.002 0.015 La/Ba correlations [41] (Fig. 9). La (ppm) 0 In addition, the Tieshan and Yangafang plutons are 180 210 240 270 La/K distinctly different in ages, mineral compositions, 0.010 major elements and isotopic geocheimistries. Their major and trace elements characteristics (Fig. 4c, e–o) 0.005 indicated that the Yangafang syenite magmas could (b) not have been derived from fractional crystallization 0 0 200 400 600 800 1000 1200 the Tieshan syenite magmas. Their different ages (Fig. La (ppm) 2) and Nd–Sr isotopic characteristics (Figs. 7 and 8) imply that they possibly originated from distinct Fig. 9. La versus La/Ba (a) and La/K (b) diagrams for studied alkaline syenites, showing positive correlations, indicating that the mantle sources which were metasomatised in various phlogopiteFpotassic amphibole was an important phase in their degrees by subducted sediments. The mantle source of enriched mantle source [41]. Sample symbols are the same as those the Yangafang syenite possibly contained more in Fig. 3. 352 Q. Wang et al. / Earth and Planetary Science Letters 230 (2005) 339–354 clockwise if they were bounded by sinistral strike slip emphasize that these granites had distinct sources faults. The geometric framework of the strike-slip and generating mechanism from the coeval alkaline faults in Cathaysia fits well with the rotation model of syenites and likely originated from Precambrian Fuh et al. [45] and Taylor et al. [46]. According to basement [12,17–22]. their models, the NNE-trending, sinistral strike-slip The well-known Shi-Hang zone in central SCB, faults (Changle–Nan’ao, Fu’an–Nanjing, Zhenghe– marked by the most prominent NE-trending Triassic– Dapu, Wuping–Pucheng, and Heyuan–Shaowu) Cretaceous extensional basins in SCB, and abundant behaved as the bounding faults and induced clockwise coeval Sm–Nd-enriched or A-type granites and some rotations of the central blocks. More recently, geo- lower Mesozoic bimodal volcanics, suggest that NE- logical mapping has identified several important trending transtension influenced by the movement of ductile strike-slip shear zones in eastern Cathaysia, the proto-Pacific plate may have persisted in the which were dated at 250–190 Ma (e.g., [47]; Y.J. Mesozoic in SCB [12]. The P–T boundary bimodal Wang, 2004, personal communication), providing volcanics were also reported in southern Jiangsu [49]. additional evidence on the timing of strike-slip Late Cretaceous alkaline or A-type granites distribut- faulting in Fujian and Cathaysia. ing along the Changle–Nan’ao and Fu’an–Nanjing There are widespread Late Permian–Early Triassic fault zones [24] further confirm long-term transten- fault depressions and pull-apart basins in western sional in Cathaysia. Therefore, we suggest that P–T Fujian, strictly defined by strike-slip faults [23,25,44]. transtension of the NE-trending strike-slip fault zones The geometry and lithological associations of these probably existed in western Fujian, presumably basins suggest transtensional nature of the boundary related to the change of the angle or rate of the faults during Late Permian–Early Triassic [23,25]. This proto-Pacific plate subduction underneath South is also best evidenced by the occurrence of approx- China. These changes probably triggered lithospheric imately coeval bimodal volcanics (e.g., Makeng extension and then transformation of the tectonic basalt–rhyolite association in SW Fujian [23]). The regime of the strike-slip fault zones from transpression fault-bounded Late Permian–Early Triassic basins also to transtension, which caused an extensional setting. provide an estimate on the timing of transtension Synchronously, asthenosphere likely upwelled and related to the regional strike-slip activity. The A-type or partial melting of enriched lithospheric mantle likely within-plate nature of the Tieshan and Yangfang took place. Magmas derived from the enriched mantle alkaline syenites indicates that their genesis was likely went through fractional crystallization during ascen- related to extension [12–16]. Field relationships and sion along the NE-trending fault zones and finally the zircon U–Pb dating demonstrate that the emplacements Tieshan and Yangfang alkaline syenite plutons of the Tieshan and Yangfang syenites were likely emplaced within the fault zones. controlled by the Zhenghe–Dapu and Wuping– Pucheng fault zones under a transtensional environ- ment during the Late Permian–Early Triassic. The 7. Conclusions approximately Triassic Taoxi metamorphic core com- plex occurring in western Fujian [48] is consistent SHRIMP zircon U–Pb dating yields precise crys- with such interpretations. Although the Late Permian– tallization ages of 254F4 Ma and 242F4 Ma for the Triassic peraluminous granites in Cathaysia (Fig. 1a) Tieshan and Yangfang alkaline syenites in eastern were considered as products of compressional setting Cathaysia, South China, respectively. 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