39Ar geochronology of the sheared metamorphic rocks in the Wuyishan: Constraints on the timing of Early Paleozoic and Early Mesozoic tectono-thermal events in SE China

Tectonophysics 501 (2011) 71–86

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La-ICP-MS U–Pb and 40Ar/39Ar geochronology of the sheared metamorphic rocks in the Wuyishan: Constraints on the timing of Early Paleozoic and Early Mesozoic tectono-thermal events in SE China Xu X.B. a, Zhang Y.Q. b,⁎, Shu L.S. a, Jia D. a a b

State Key Laboratory for Mineral Deposits Research, Nanjing University, Nanjing 210093, PR China Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, PR China

a r t i c l e

i n f o

Article history: Received 7 March 2010 Received in revised form 18 January 2011 Accepted 20 January 2011 Available online 27 January 2011 Keywords: Early Paleozoic Early Mesozoic Ductile shear zone 40 Ar–39Ar La-ICP-MS U–Pb SE China

a b s t r a c t Mylonite and ultramylonite have been well documented in Pre-Devonian meta-volcanic and metasedimentary rocks in the Wuyishan, SE China. Field observations and thin section analyses show dextral strike-slip shearing. Deformation temperature is estimated in 400 °C–500 °C and the pressure between 7 kbar and 10 kbar. La-ICP-MS U–Pb zircon and 40Ar/39Ar step-heating biotite–muscovite ages are used to constrain the timing of the dextral shear zone and its tectonothermal history. La-ICP-MS U–Pb dating of zircons from three gneissic granitic samples provides magmatic crystallization ages between 441 and 431 Ma, and dating of the growth rims of zircons from one ultramylonite sample provides metamorphic age of 433.8 ± 6.8 Ma. These dates provide additional constraint on the timing of the Early Paleozoic tectono-thermal event in the Wuyishan region. 40Ar/39Ar step-heating dating of biotite and muscovite separated from two mylonite samples yields ages of 235.3 ± 2.8 Ma and 238.5 ± 2.8 Ma, respectively; and an undeformed granitic dyke intruded into the shear zone was dated, 229.8 ± 2.2 Ma, by La-ICP-MS U–Pb zircon method. These dates reveal that metamorphic rocks in the Wuyishan suffered from dextral shearing at the time span between 230 Ma and 239 Ma, during the Triassic Indosinian tectono-thermal event. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The South China Block (SCB) plays an important role in the tectonic framework of the continental margin of southeastern Asia. Southeastern Asia underwent subduction and subsequent narrowing and closure of different branches of the Paleo-Tethys Ocean during the Early Paleozoic to Mesozoic, resulting in final suturing and collision of these blocks, including South China, Indochina, Simao, Sibumasu and West Burma. These blocks were assembled each other and accreted progressively to the North China Block during the Early Mesozoic (Carter et al., 2001; Chen and Wilson, 1996; Lepvrier et al., 2004, 2008; Meng and Zhang, 2000; Metcalfe, 1999; Roger et al., 2007). Early Paleozoic orogeny in SE China has been earlier recognized by a regional angular unconformity at the base of Upper Devonian conglomerates, with hiatus of Silurian deposits; it is characterized by folding of the Sinian to Early Paleozoic sedimentary sequences, widely distributed Ordovician–Silurian magmatism and high temperature metamorphism (Charvet et al., 1996, 2010; Faure et al., 2009; Li et al., 2010a; Ren, 1964, 1991; Shu et al., 2008a). Geographically, this orogenic belt covers the southeastern part of SCB, stretching for ca. ⁎ Corresponding author. Tel.: + 86 10 68412311; fax: +86 10 68422326. E-mail addresses: [email protected] (X. X.B.), [email protected] (Z. Y.Q.). 0040-1951/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2011.01.014

2000 km in the northeasterly direction and up to 600 km in width (Li et al., 2010a), and possibly extending to the Korean peninsula (Kim et al., 2006) and to Indochina region (Carter et al., 2001; Ren, 1991; Roger et al., 2007). With the accumulation of field observations and reliable geochronological data acquired during the last decade in the Wuyi-Yunkai region (Li et al., 2010a,b; Liu et al., 2010; Wan et al., 2007, 2010; Xu et al., 2009; Yu et al., 2005; Zeng et al., 2008), understanding of the tectonic evolution of this orogenic belt has been greatly advanced (Charvet et al., 2010; Faure et al., 2009; Li et al., 2010a; Shu et al., 2008a). It is now commonly accepted that the Early Paleozoic orogenic belt of SE China was an intracontinental or intraplate orogen formed by closing of a pre-existing Neoproterozoic rift basin (the Nanhua Rift basin). The orogenic belt was initiated in the Middle Ordovician before 460 Ma, and tectonic piling and peak metamorphism occurred at the time span between 460 and 440 Ma; this was followed by anatexis and granite emplacement at 440– 420 Ma (Charvet et al., 2010; Faure et al., 2009; Li et al., 2010a,b; Liu et al., 2010). This scenario of temporal evolution of the Early Paleozoic orogenic belt of SE China has been based mainly on the structural analysis and geochronological data of the metamorphic rocks mostly exposed in the Wuyishan-Yunkai region (Fig. 1). The Early Paleozoic orogenic belt of SE China suffered from intensive overprinting of the Mesozoic tectono-thermal events, particularly the Triassic Indosinian tectonic event (Charvet et al.,

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Fig. 1. Tectonic sketch map of the South China block and schematic structural section across the Wuyishan, compiled from Faure et al. (1996), Jia et al. (2006), Li et al. (2010c), Lin et al. (2008), Wang et al. (2005a, 2007a), Xiao and He (2005), Zhang and Cai (2009), Zhang et al. (2009). Box indicates the location of Fig. 2. YFFTB: Yangtze foreland fold-and-thrust belt; ZFTB: Northwestern Zhejiang fold-and-thrust belt; LFTB: Longmenshan fold-and-thrust belt; TF: Tan-Lu fault; NJF: Northeastern Jiangxi fault; ZDF: Zhenghe-Dapu fault; CCF: Chaling-Chenzhou fault; WSF: Wuchuan-Sihui fault; HHSZ: Hepu-Hetai shear zone; RRF, Red River Fault; XF: Xianshuihe Fault.

1996; Faure et al., 1996; Lin et al., 2008; Shu et al., 2008b; Wang et al., 2005a, 2007a; Xiao and He, 2005; Zhang and Cai, 2009; Zhang et al., 2009). The Indosinian tectonic event has been largely inferred from the Triassic magmatism in SE China (Wang et al., 2007b; Zhou et al., 2006); it is also well recorded by high temperature metamorphism in the Pre-Sinian metamorphic rocks exposed in the Wuyi-Yunkai region (Li and Li, 2007; Wang et al., 2007c; Xiang et al., 2008; Yang et al., 2010; Yu et al., 2009). But its kinematics and geodynamic setting have been largely debated (Faure et al., 1996; Li and Li, 2007; Lin et al., 2008; Shu et al., 2008b; Wang et al., 2005a,b, 2007a,b,c; Xiao and He, 2005; Zhang and Cai, 2009; Zhang et al., 2009). We report in this paper the strike-slip ductile shear deformation occurring in pre-Devonian metamorphic rocks and Devonian quartz conglomerates in the Wuyishan region. Its geometry, kinematics and deformation conditions have been analyzed based on field observation and laboratory analysis, and U–Pb and 40Ar/39Ar ages are used to constrain the timing of the ductile shear zones and its tectonothermal history.

2. Geological setting SCB consists of two main tectonic units: the Cathaysia block to the southeast and the Yangtze block to the northwest, which are welded by the Jiangnan orogenic belt (Fig. 1). The Wuyishan is located in the northeastern part of the Cathaysia block. Remnants of the Cathaysia block are represented by the Paleoproterozoic rock units of the Badu Group and the Tianjingping Formation. The Badu Group consists mainly of gneisses and schists, which deposited between 2.4 and 2.0 Ga (Hu et al., 1991, 1992). It underwent two stages high temperature metamorphism and anatexis: the early one occurring at 1888–1855 Ma, and the late one at 251–197 Ma (Li and Li, 2007; Xiang et al., 2008; Yu et al., 2009). The protolith age of amphibolites in Tianjingping Formation was 1766 ± 19 Ma (Li, 1997). This rock unit underwent anatexis at around 446– 425 Ma (Li et al., 2010a,b; Wan et al., 2007; Zeng et al., 2008). The Early Neoproterozoic Shuangxiwu and Shuangqiaoshan Groups are composed of a thick pile of pelitic and clastic sedimentary

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deposits interbedded with lesser volcanic rocks dated at around 880 Ma (Li et al., 2007, 2009; Wang et al., 2008). These rock units were strongly folded and unconformably overlain by the Neoproterozoic rifting succession. This unconformity has been interpreted as the result of the collision between the Cathaysia block and the Yangtze block, occurring at ca. 900–860 Ma, equivalent to the Grenville orogeny in the world (Li et al., 2007, 2008, 2009; Shu and Charevt, 1996; Wang et al., 2007d, 2008; Zhao and Cawood, 1999). Rifting began around 830 Ma in SCB, which resulted in the formation of the Nanhua Rift Basin infilled with a thick rift succession (Wang and Li, 2003). The rifting phase has been evidenced by the remnants of Neoproterozoic komatiitic, mafic and ultramafic rocks

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observed at several localities in SE China. Zircon SHRIMP U–Pb dating results show that these rocks formed at the time between 823 ± 6 Ma and 795 ± 7 Ma (Shu et al., 2008b; Wang et al., 2007e). It is also recorded by bimodal magmatism and volcano-sedimentary sequences represented by the Mayuan Group, Mamianshan Group, Wanquan Group, Longquan Group and Taoxi Formation in Cathaysia block, whose protolith ages are between 825 and 730 Ma (Li et al., 2005, 2010a,b; Liu et al., 2010; Wan et al., 2007; Xu et al., 2010; Yu et al., 2005). The Nanhua rifting ceased at around 730 Ma, it did not reach to the oceanic spreading stage (Charvet et al., 2010; Faure et al., 2009; Shu et al., 2008b; Wang and Li, 2003). This Neoproterozoic rifting phase is thought to be related to the break-up of the supercontinent

Table 1 List of reliable ages of the Early Paleozoic tectono-thermal event in SE China. Rock unit

Sample

Rock type

Age (Ma)

Method

Reference

SC831

Wu239

Micaschist Mylonite Mylonite Mylonite Mylonite Mylonite

453 ± 7 433 ± 3 430 ± 2 428 ± 2 427.7 ± 3.7 421 ± 8

EMP monazite 40 Ar/39 Ar biolite 40 Ar/39 Ar biolite 40 Ar/39 Ar biolite 40 Ar/39 Ar muscovite 40 Ar/39 Ar muscovite

Faure et al., 2009 Shu et al., 2008a Shu et al., 2008a Shu et al., 2008a Wang et al., 2007c Shu et al., 2008a

High-grade metamorphic ages Mayuan Group Mayuan Group Tianjingping Formation Taoxi Formation Yunkai Group Mayuan Group Taoxi Formation Mayuan Group Mayuan Group Mayuan Group Gaozhou Complex Anatectic granite Zhoutan Group Tianjingping Formation Taoxi Formation Neoproterozoic basement Mayuan Group

FJ0110 06FJ61-3 08WY-90 TX-105 G0104-1 01SC18 TX-104 06FJ65-1 06FI65-3 06FJ122-2 G0105-1 JC02-WU816 08WY-2 08WY-88 Wy-06 JC01-WU227 08WY-58

Gneiss Gneiss Amphibolites Gneiss Gneiss Amphibolites Gneiss Mesosome Leucosome Leucosome Gneiss Two-micas granite Amphibolites Amphibolites Ultramylonite Gneiss Amphibolites

458 ± 6 453 ± 3 446 ± 5 444 ± 5 443 ± 9 443 ± 6 443 ± 10 442 ± 8 439 ± 6 439 ± 4 438 ± 8 437 ± 5 435 ± 2 434 ± 4 433.8 ± 6.8 433 ± 9 423 ± 2

SHRIMP U–Pb zircon La-ICP-MS zircon U–Pb zircon La-ICP-MS zircon SHRIMP U–Pb zircon SHRIMP U–Pb zircon La-ICP-MS zircon La-ICP-MS zircon La-ICP-MS zircon La-ICP-MS zircon SHRIMP U–Pb zircon EMP monazite U–Pb zircon U–Pb zircon La-ICP-MS zircon EMP monazite U–Pb zircon

Wan et al., 2007 Liu et al., 2010 Li et al., 2010b Yu et al., 2005 Wan et al., 2010 Li et al., 2010a Yu et al., 2005 Liu et al., 2010 Liu et al., 2010 Liu et al., 2010 Wan et al., 2010 Charvet et al., 2010 Li et al., 2010b Li et al., 2010b This study Charvet et al., 2010 Li et al., 2010b

Retrograde metamorphic/cooling ages Anatectic granite Ductile décollements Ductile décollements Zhenghe-Dapu Fault

JC02-WU816 SC831 SC831 Wu209

Two-micas granite Micaschist Micaschist Marble

412 ± 5 405 ± 4 397 ± 4 391 ± 3

EMP monazite 40 Ar/39 Ar muscovite 40 Ar/39 Ar biolite 40 Ar/39 Ar muscovite

Charvet et al., 2010 Faure et al., 2009 Faure et al., 2009 Shu et al., 1999

Syn-orogenic granitoid ages Shangyou granite Granite in Baiyushan Tianjing Complex Tianjing Complex Weipu granite Granite in Baiyushan Xunwu Complex Granite in Yunkai Mayuan Group Gneissic granite in Wuping Weipu granite Granite in Yunkai Granite in Baiyushan Mayuan Group granite in Yunkai Weipu granite Weipu granite Sibao granite Gneissic granite in Wuping Granite in Yunkai Granite in Yunkai Granite in Yunkai Granite in Yunkai

04GN-02 BY005-1 01SC14-3 125-1 Jx-52 BY004-1 Jx-84 G0107-1 06FJ61-4 Wy-07 Jx-54 YK-60 BY022-3 06FJ59-2 G0102-1 Wy-16 04GD53 04GD68 Jx-70 G0108-1 YK-27 YK-42 YK-50

Granite Granite Granitic vein Granodiorite Granite Granite Migmatite Granite Granite Granite Granite Granite Granite Granite Granite Gneissic granite Granite Granite Granite Granite Granite Granite Granite

464 ± 11 453.5 ± 7.8 452 ± 8 447 ± 2 447.1 ± 4.7 446 ± 7 445.9 ± 3.8 443 ± 4 442 ± 4 440.8 ± 4.8 440.8 ± 3.4 440.7 ± 5.6 439 ± 9 438 ± 5 437 ± 5 435.6 ± 3.2 433 ± 5 432 ± 3 431.2 ± 3.2 430 ± 10 429.6 ± 5.2 427.1 ± 4.2 421.9 ± 9.8

SHRIMP U–Pb zircon SHRIMP U–Pb zircon SHRIMP U–Pb zircon La-ICP-MS zircon La-ICP-MS zircon SHRIMP U–Pb zircon La-ICP-MS zircon SHRIMP U–Pb zircon La-ICP-MS zircon La-ICP-MS zircon La-ICP-MS zircon SHRIMP U–Pb zircon SHRIMP U–Pb zircon La-ICP-MS zircon SHRIMP U–Pb zircon La-ICP-MS zircon SHRIMP U–Pb zircon SHRIMP U–Pb zircon La-ICP-MS zircon SHRIMP U–Pb zircon SHRIMP U–Pb zircon SHRIMP U–Pb zircon SHRIMP U–Pb zircon

Mao et al., 2008 Yang et al., 2010 Li et al., 2010a Zeng et al., 2008 Xu et al., 2009 Yang et al., 2010 Xu et al., 2009 Wan et al., 2010 Liu et al., 2010 This study Xu et al., 2009 Wang et al., 2007c Yang et al., 2010 Liu et al., 2010 Wan et al., 2010 This study Li et al., 2010a Li et al., 2010a This study Wan et al., 2010 Wang et al., 2007e Wang et al., 2007e Wang et al., 2007e

Ductile deformation/cooling ages Ductile décollements Mylonite in the Wuyishan Mylonite in the Wuyishan Jiangshan-Shaoxing Fault Nanfeng-Yintang shear zone Jiangshan-Shaoxing Fault

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Rodinia, caused by either a mantle plume (Li et al., 2003a,b; Wang et al., 2007e) or post-collision lithospheric extension (Zheng et al., 2008). Up to 7–8 km thick pile of clastic and carbonaceous rocks of Sinian to Early Paleozoic were deposited in a broad region in SCB,

which are thought to be under an intracontinental rifting setting (Bai et al., 2007; Wang and Li, 2003). These sequences and underlying basement which suffered from strong reworking by early Paleozoic orogeny are unconformably overlain by the Upper Devonian conglomerates. The Early Paleozoic tectonothermal event

Table 2 List of reliable ages of Early Mesozoic tectono-thermal events in SE China. Rock unit

Sample

Rock type

Age (Ma)

Method

Reference

Ductile deformation/cooling ages Ductile shear zone in Wuping Ductile shear zone in Wuping Mylonite in SW Zhejiang Yunkai tectonic belt Yunkai tectonic belt Yunkai tectonic belt Yunkai tectonic belt Yunkai tectonic belt Mylonite in SW Zhejiang Yunkai tectonic belt Yunkai tectonic belt Xuefengshan tectonic belt Xuefengshan tectonic belt Yunkai tectonic belt Xuefengshan tectonic belt Hepu-Hetai shear zone Hepu-Hetai shear zone Yunkai tectonic belt Yunkai tectonic belt Yunkai tectonic belt Yunkai tectonic belt Yunkai tectonic belt Xuefengshan tectonic belt Hepu-Hetai shear zone Hepu-Hetai shear zone Xuefengshan tectonic belt

Jx-66 Jx-63 LV-156 02YK27 02YK74 02YK30 02YK39 02YK38 LV-135 02YK56 02YK31 01HH-31 01XH-38 02YK26 01XH-36 Datong1 Datong2 02YK15 02YK64 02YK12 02YK80 02YK09 01HH-45 Hetai2 Hetai1 01HH-2

Mylonite Mylonite Mylonite Mylonite Mylonite Mylonite Mylonite Mylonite Mylonite Mylonite Mylonite Mylonite Mylonite Mylonite Mylonite Mylonite Mylonite Mylonite Mylonite Mylonite Mylonite Mylonite Mylonite Mylonite Mylonite Mylonite

238.5 ± 2.8 235.3 ± 2.8 237.6 ± 1.3 229.9 ± 0.5 227.9 ± 0.3 225.4 ± 0.3 224.7 ± 0.4 221.8 ± 0.4 221 ± 10 218.4 ± 0.3 216.9 ± 0.3 216.9 ± 0.3 215.3 ± 0.8 214.2 ± 0.4 213.5 ± 0.2 213 ± 4 211.6 ± 3.4 211.5 ± 0.5 211.1 ± 0.2 209 ± 0.2 208.9 ± 1.4 207.8 ± 0.2 207.2 ± 0.2 198.9 ± 1.2 195.2 ± 1.3 194.7 ± 0.3

40

Ar/39Ar muscovite Ar/39Ar biotite 40 Ar/39Ar muscovite 40 Ar/39Ar biotite 40 Ar/39Ar biotite 40 Ar/39Ar biotite 40 Ar/39Ar biotite 40 Ar/39Ar biotite 40 Ar/39Ar K-feldspar 40 Ar/39Ar biotite 40 Ar/39Ar biotite 40 Ar/39Ar biotite 40 Ar/39Ar muscovite 40 Ar/39Ar biotite 40 Ar/39Ar sericite 40 Ar/39Ar muscovite 40 Ar/39Ar muscovite 40 Ar/39Ar biotite 40 Ar/39Ar biotite 40 Ar/39Arsericite 40 Ar/39Arsericite 40 Ar/39Ar biotite 40 Ar/39Arsericite 40 Ar/39Ar muscovite 40 Ar/39Ar muscovite 40 Ar/39Ar whole rock

This study This study Zhu et al., 1997 Wang et al., 2007a Wang et al., 2007a Wang et al., 2007a Wang et al., 2007a Wang et al., 2007a Zhu et al., 1997 Wang et al., 2007a Wang et al., 2007a Wang et al., 2005a Wang et al., 2005a Wang et al., 2007a Wang et al., 2005a Zhang and Cai, 2009 Zhang and Cai, 2009 Wang et al., 2007a Wang et al., 2007a Wang et al., 2007a Wang et al., 2007a Wang et al., 2007a Wang et al., 2005a Zhang and Cai, 2009 Zhang and Cai, 2009 Wang et al., 2005a

High-grade metamorphic ages Badu Group Danzhu gneissic granite Gaozhou Complex Badu Group Yunkai tectonic belt Badu Group Lizhuang biotite granite Xiji two-mica monzogranite Wangyu biotite gneiss Danzhu biotite monzogranite Xikou biotite gneiss Yunkai tectonic belt

88-13 02SC35 L114 86-5 YK-42 112-1 zj06-39 zj06-31 zj06-23 zj06-21 zj06-15 BY004-1

Amphibolites Granite Gneiss Amphibolites Orthogneiss Amphibolites Granite Granite Gneiss Granite Gneiss Gneiss

251.1 ± 1.9 243 ± 5 242 ± 8 240 ± 2.8 236 ± 3.1 233.8 ± 2.8 233 ± 8 232 ± 5 230 ± 6 229 ± 12 226 ± 11 212 ± 12

La-ICP-MS U–Pb zircon SHRIMP U–Pb zircon SHRIMP U–Pb zircon La-ICP-MS U–Pb zircon SHRIMP U–Pb zircon La-ICP-MS U–Pb zircon La-ICP-MS U–Pb zircon La-ICP-MS U–Pb zircon La-ICP-MS U–Pb zircon La-ICP-MS U–Pb zircon La-ICP-MS U–Pb zircon SHRIMP U–Pb zircon

Xiang et al., 2008 Li and Li, 2007 Wan et al., 2010 Xiang et al., 2008 Wang et al., 2007c Xiang et al., 2008 Yu et al., 2009 Yu et al., 2009 Yu et al., 2009 Yu et al., 2009 Yu et al., 2009 Yang et al., 2010

99FJ024 2KGN32-1 2KGN50-1 02LSH05 02QSH06 99FJ031 02JSH03 Fch-1-1

Syenite Granite Granite Granite Granite Syenite Granite Granite Granite Granite Granite Granite Granite Granite Granite Granite Granite Granite Granite Granite Granite Granite Granite Granite Granite

254 ± 4 249 ± 6 247 ± 3 243 ± 4 243 ± 4 242 ± 4 241 ± 3 239 ± 17 239 ± 5 239 ± 3 237 ± 5 236 ± 6 236 ± 4 235.8 ± 7.6 234 ± 4 233 ± 5 231 ± 16 230 ± 4 229 ± 6.8 229.8 ± 2.2 224.6 ± 2.3 218 ± 3 214.1 ± 5.9 210.3 ± 4.7 205.3 ± 1.6

SHRIMP U–Pb zircon SHRIMP U–Pb zircon SHRIMP U–Pb zircon La-ICP-MS U–Pb zircon SHRIMP U–Pb zircon SHRIMP U–Pb zircon SHRIMP U–Pb zircon La-ICP-MS U–Pb zircon La-ICP-MS U–Pb zircon SHRIMP U–Pb zircon SHRIMP U–Pb zircon SHRIMP U–Pb zircon SHRIMP U–Pb zircon La-ICP-MS U–Pb zircon SHRIMP U–Pb zircon SHRIMP U–Pb zircon La-ICP-MS U–Pb zircon SHRIMP U–Pb zircon La-ICP-MS U–Pb zircon La-ICP-MS U–Pb zircon La-ICP-MS U–Pb zircon La-ICP-MS U–Pb zircon La-ICP-MS U–Pb zircon La-ICP-MS U–Pb zircon La-ICP-MS U–Pb zircon

Wang et al., 2005b Li and Li, 2007 Li and Li, 2007 Wang et al., 2007b Wang et al., 2007b Wang et al., 2005b Wang et al., 2007b Yu et al., 2007a Xu et al., 2003 Wang et al., 2007b Wang et al., 2007b Wang et al., 2007b Deng et al., 2004 Xu et al., 2003 Wang et al., 2007e Deng et al., 2004 Yu et al., 2007a Deng et al., 2004 Yu et al., 2007a This study Yu et al., 2007b Wang et al., 2007b Peng et al., 2006 Peng et al., 2006 Peng et al., 2006

Granitoid ages Tieshan melanite syenite Aigao biotite granite Sanbiao biotite granite Xiangzikou granite Tangshi granite Yangfang aegiriteaugite syenite Baimashan granite Fucheng–Hongshan Complex Luxi granite Guandimiao granite Yangmingshan granite Wufengxian granite Taima granite Xiazhuang granite Wanwutang granite Darongshan granite Fucheng–Hongshan Complex Jiuzhou granite Fucheng–Hongshan Complex Undeformated granite in Wuping Fucheng–Hongshan Complex Xiema Granite Xiema granite Xiangzikou granite Napeng granite

01GD09 01YM03 01WF09 2KD-171 01DM05 2KD-110a Fch-8-1 2KD-158a Fch-9 Jx-64 TX-121 01XM01 02XM-01 02LSH-05 02YK-83

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in SCB is characterized by widely distributed magmatism, high temperature metamorphism and extensive ductile deformation, which are summarized in Table 1. The Early Paleozoic orogenic belt of SCB was strongly overprinted by Mesozoic tectonothermal events comprising the Triassic Indosinian event and the Jurassic–Cretaceous Yanshanian events. The Upper Devonian to Middle Triassic sequence consists of a succession of shallow-marine and shore deposits that were deformed and are unconformably overlain by the Upper Triassic–Jurassic terrestrial deposits. The Triassic Indosinian tectono-thermal event in SCB has been recognized by magmatism, high temperature metamorphism and ductile deformation, which are referenced in Table 2.

3. Dextral shear zones in the Wuyishan Strike-slip shear deformation has been early recognized in the Proterozoic metamorphic rocks in the Wuyishan (Huang, 2003; Liu et al., 1993; Shu et al., 1999; Zhang et al., 2003). Because of massive intrusion of Mesozoic granitic plutons in this region, the shear zones observed in the pre-Devonian metamorphic rocks are discontinuously distributed in the area from Wuping County to Jiangle County (Fig. 2).

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3.1. Ductile shear zone at Taoxi Township in Wuping County Ductile shear zone at this locality develops in the high greenschist to low amphibolite facies metamorphic rocks of the Neoproterozoic Taoxi Formation (Fig. 3A, B, C). Field investigation and structural measurements show that the shear zone consists of 5–8 km wide, NNE–SSW trending mylonites and less deformed bands. Foliations of the mylonites steeply dip to SE, and lineations plunge 10° to 20° toward SW (Fig. 4A). The sub-horizontal lineations are marked by quartz elongation and mica arrangement. The best exposure of this mylonitic zone is observed across a section located at Taoxi Township of Wuping County (Fig. 5). At this locality, the shear zone is cut to the southeast by a normal fault that juxtaposes the Upper Triassic to Lower Jurassic sandstones with mylonitic rocks. Ultramylonite outcrops about six meters wide in the center of the shear zone; it consists of shear bands composed of quartz, feldspar and muscovite. Subhorizontal lineation well develops on the steeply dipping foliations. The asymmetric phenocrysts and S–C fabrics indicate dextral strikeslip shearing. Moreover, gneissic granitoids are locally seen in the shear zone. Asymmetric feldspar phenocrysts seen on “XZ” plane also indicate dextral strike-slip shearing in the northeastern direction. In addition, an undeformed biotite granitic dyke intruded into the shear zone and did not suffer from ductile deformation.

Fig. 2. Geological map of the Wuyishan and distribution of the ductile shear zones in the Wuyishan.

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In the area from Changting to Qingliu County, we observed shear deformation in the Cambrian slate and Upper Devonian quartz conglomerates (Fig. 3D, E, F), marked by N–S to NE-striking, steeply dipping foliations. The lineation plunges 1° to 15° toward NE and 30° to SSW (Fig. 5) and is marked by quartz and mica elongation. The asymmetric quartz phenocrysts indicate dextral strike-slip shearing.

thin-sections were cut parallel to “XZ” plane of the finite strain ellipsoid. Sigma-type rotations of feldspar phenocrysts observed on sample Wy-07 from the Taoxi site indicate dextral strike-slip shearing (Fig. 6A, B). The asymmetric quartz seen on sample Wy-15 from Qingliu and sample Wy-32 from Jiangle shows dextral strike-slip shearing (Fig. 6C, D). Microstructural analysis on thin sections is consistent with field observations, showing that the afore-mentioned ductile deformation in the Wuyishan is dominated by dextral strikeslip shearing. Electron microprobe analysis on muscovite from six mylonitic samples has been conducted to determine the pressure of the ductile deformation. The analysis was undertaken by using a JFOL JXA-800 Electron Microprobe, in the Electron Microprobe Laboratory, State Key Laboratory for Mineral Deposits Research at Nanjing University. The accelerated voltage is 15 kV while the electric current is 2E−8 A. The results listed in Table 3 show that the Si atoms (per 11 Oxygen) of muscovite in all samples are more than 3.27 pfu, some reach 3.37. These medium-Si phengite of mylonite formed under the pressure of 7–10 kbar (Massonne and Schreyer, 1987; Velde, 1965). Deformation temperature of the shear zone has been estimated by syn-mylonitization mineral assemblages and mineral deformation behavior in mylonitic rocks, as shown elsewhere, for an example along the Tan-Lu fault zone (Zhu et al., 2005). Deformation manner of the quartz and feldspar can be used to estimate the range of temperature (Kruhl, 1996; Mancktelow and Pennacchioni, 2004; Urai et al., 1986). Generally, subgrain rotation recrystallization (SR) transforms into grain boundary migration recrystallization (GBM) in quartz at the temperature of ca. 400 °C, and the coexistence of SR and GBM in the samples indicates the deformation temperature of 400 °C– 700 °C. Furthermore, feldspars with brittle fracturing formed at temperatures less than 400 °C; and plastic elongation, undulose extinction, subgrain and core–mantle structure occur in the temperature of 400 °C–500 °C, and dynamic recrystallization takes place dominantly with temperature over 500 °C. Table 3 lists the microscopical features and deformation manner of the minerals from the ductile shear zones in the Wuyishan. By comparison with the criteria mentioned above, we estimate that the deformation temperature of the mylonite from the ductile shear zones in the Wuyishan is between 400 °C and 500 °C, located in the upper greenschist facies.

3.3. Dextral shear zone in the Jiangle area

5. Zircon La-ICP-MS U–Pb geochronology

In the area of Jiangle County, ductile shear deformation occurred in greenschist to low amphibolite facies meta-volcano sedimentary rocks of the Wanquan Group (Fig. 3G). Field observations show two sets of foliation: one set striking NE and dipping to SE, with the lineation plunging either 20° to NE or about 40° to SW (Fig. 4C); another set striking nearly E–W and dipping to north, with the lineation plunging about 10° to west and locally about 40° to east (Fig. 4D). Moderate-angle plunging stretching lineation and asymmetric folds (Fig. 3H) indicate top-to-SE shearing. The cutting relationship between the east-trending foliations and the NE-striking foliations shows that top-to-SE shearing occurred earlier than the dextral strike-slip one (Zhang et al., 2003).

In order to date the ductile shear zones in the Wuyishan, we select one ultramylonite sample (Wy-06), three gneissic granite samples (Wy-07, Wy-16 and Jx-70) and one undeformed granite sample (Jx-64) taken from the shear zone for La-ICP-MS U–Pb zircon dating. Zircons were concentrated using standard gravimetric and magnetic separation techniques. Representative grains were then extracted by hand-picking under a binocular microscope. Zircons from the samples described below were cast, along with fragments of a standard zircon in an epoxy mount. Zircon grains were polished to reveal mid-sections, and all sectioned zircons were analyzed by transmitted and reflected light micrographs as well as cathodoluminescence (CL) images to identify their internal structures and select spots for analysis. The CL images were analyzed at the Beijing SHRIMP Center, Chinese Academy of Geological Sciences. Zircon La-ICP-MS U–Pb dating was carried out by using Agilent HP4500 and Merchantek/NWR 213 nm laser microprobe in the State Key Laboratory for Mineral Deposits Research at Nanjing University. Before analyzing

Fig. 4. Stereonet plots (Wulf net, lower hemisphere projection) of the foliation and lineation measured across the ductile dextral shear zone in the Wuyishan. A: data from shear zone across the Taoxi section in Wuping County; B: data from the area of Changting to Qingliu Counties; C and D: data from the area of Jiangle County. N: number of measured foliation; n: number of measured lineation.

3.2. Ductile shear zone in the area of Changting and Qingliu Counties

4. Microstructural analysis and estimates of P–T conditions Sixteen oriented samples were analyzed to determine the shear sense and to estimate the P–T conditions of shear deformation. These

Fig. 3. Field views of ductile dextral shear deformation in the Wuyishan. A, B and C: mylonite with steeply dipping foliation and nearly horizontal lineation in the metamorphic rocks of the Taoxi Formation, Taoxi Township, Wuping county; D: lineation on steeply dipping schistosity in the Cambrian slate, east of Changting County. E: steeply dipping foliation in Upper Devonian quartz conglomerates, south Qingliu; F: sigma-type phenocrystal on “XZ” plane indicating dextral strike-slip shearing, south Qingliu (high angle shot); G: nearly horizontal lineation on steeply dipping foliation, west Jiangle; H: asymmetric folds in gneiss indicating top-to-south shearing, west Shunchang.

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Fig. 5. Schematic structural section across the ductile shear zone at Taoxi, Wuping County (see Fig. 2 for location). 1: phyllite; 2: gneiss; 3: ultramylonite; 4: Silurian gneissic granite; 5: Triassic granite; 6: granulite; 7: Upper Triassic to Lower Jurassic sandstone; 8: normal fault; 9: location of rock samples for U–Pb and 40Ar/39Ar dating; 10: foliation and bedding.

every grain set, two standard GJ (207Pb/206Pb age of 608.5 ± 1.5 Ma, Jackson et al., 2004) and one Mud Tank (207Pb/206Pb age of 732 ± 5 Ma, Black and Gulson, 1978) samples were first dated. Then ten analyses were conducted and two standard GJ samples were analyzed. The diameter of analysis dot is 32 μm for magmatic zircons and 21 μm for metamorphic zircons. The analytical data are treated by GLITTER 4.4 software. The detailed analysis program, precision and veracity are referred to Yuan et al. (2004). The U–Pb isotope data are revised by common Pb offered by ComPbcorr#3-16f1 program (Andersen, 2002), and are treated by ISOPLOT program (Ludwig, 2001). In this investigation, zircons younger than 1.0 Ga were dated using the 206 Pb/238U age, whereas older ones used the 207Pb/206Pb age. Combined with zircon CL images, a number of different zircon fractions from these samples were targeted to constrain the origin,

age and tectonothermal overprinting. Efforts were made to avoid analyzing areas with cracks and inclusions. Figs. 7 and 8 show concordia diagrams, field outcrops of samples, morphologies and CL zoning patterns of some representative zircon crystals, as well as locations of the analytical pits and the resulting apparent ages. Analytic data of zircon La-ICP-MS U–Pb dating for each sample are listed in Table 4. 5.1. Sample Wy-07 (E116°10.314′, N25°19.306′): gneissic plagioclase granite in the Taoxi site, Wuping County The sample yields a single population of elongate (ca. 2:1), transparent and colorless, euhedral zircons. Zircons range in size from 100 μm to 150 μm. CL images present euhedral concentric zoning

Fig. 6. Photomicrographs of the mylonitic rocks taken from the ductile shear zone in the Wuyishan. Thin-sections oriented parallel to “XZ” plane, crossed polarizes (Q, quartz; Pl, plagioclase; Bi, biotite; Ms, muscovite). A: Sigma-type asymmetric feldspar phenocrystal from sample Wy-07 indicating dextral strike-slip shearing. B: Sigma-type asymmetric feldspar phenocrystal from sample Wy-07 indicating dextral strike-slip shearing. C: Asymmetric quartz phenocrystal from sample Wy-15 indicating dextral strike-slip shearing. D: Asymmetric quartz phenocrystal from sample Wy-32 indicating dextral strike-slip shearing.

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79

Table 3 Estimates of deformation temperatures and pressures for the shear zone in the Wuyishan. Sample

Locality Name

Long./lat.

Wy-06

Wuping

Wy-07

Wuping

Wy-08

Wuping

Wy-15

Qingliu

Wy-24

Jiangle

Wy-32

Jiangle

E116°07.495′ N25°19.206′ E116°10.314′ N25°19.306′ E116°09.336′ N25°22.766′ E116°42.419′ N26°00.103′ E117°11.352′ N26°40.569′ E117°28.624′ N26°41.219′

Rock type

Assemblage

Quartz recrystallisation

Feldspar deformation

Estimated T/°C

Si/kbar

Ultramylonite

P (5%): Fsp M (95%): Q + Ms P (5%): Fsp M (95%): Q + Ms P (5%): Fsp + Q M (95%): Q + Ms + Bi P (60%): Q M (40%): Q + Ms P (5%): Q + Bi M (95%): Q + Ms + Bi P (35%): Q M (65%): Q + Ms

SR + GBM

None

400

3.297/7

SR + GBM

Fracturing

450

3.271/7

BLG + SR

Elongated

400

3.307/7

BLG + SR

None

400

3.31/7

GBM

None

500

3.366/10

BLG + SR

None

400

3.371/8 3.405/9

Mylonite Mylonite Protomylonite Ultramylonite Mylonite

P: phenocryst; M: matrix; Fsp: feldspar; Q: quartz; Ms: muscovite; Bi: biotite; Ga: garnet. BLG: bulging recrystallization; SR: subgrain rotation recrystallization; GBM: grain boundary migration recrystallization.

common in crystals, indicative of magmatic origin. U concentrations range from 219 ppm to 954 ppm, and Th/U ratios from 0.26 to 0.86. A total of ten analyses were conducted on 10 different zircons, showing the results cluster on concordia (Fig. 7A). All of these analyses define a well-constrained mean 206Pb/238U age of 440.8 ± 4.8 Ma, which is considered the best estimate for the timing of crystallization of the gneissic granitic pluton. 5.2. Sample Jx-70 (E116°11.130′, N25°20.440′): gneissic K-feldspar granite in the Taoxi site, Wuping County This sample of gneissic K-feldspar granite contains predominantly elongate zircon grains with length to width ratios between 1:1 and 2:1. Zircons are euhedral to subhedral in shape, ranging in length from 50 to 150 μm. All zircons are transparent and colorless, with euhedral concentric zoning in CL images, indicative of igneous origin. U concentrations range from 62 ppm to 2082 ppm, and Th/U ratios from 0.19 to 1.43. Twenty analyses were conducted on 20 separate zircon grains. These analyses yield a weighted average age of 431.2 ± 3.2 Ma (Fig. 7B). We interpret this as the zircon crystallization age. The apparent scatter in concordant plots is likely due to a slight underestimation of analytical uncertainty and/or to Pb loss, which caused the calculated age slightly younger than that of the sample Wy-07. 5.3. Sample Wy-06 (E116°07.495′, N25°19.206′): ultramylonite in the Taoxi site, Wuping County Zircons in the sample are mainly colorless to light brown. The size of the crystals varies from less than 50 to nearly 150 μm, and with length to width ratios between 1:1 and 2:1. Relict cores of zircons are overgrown by metamorphic rims that were big enough to allow analysis and clearly identify their origin (Fig. 7C). The cores correspond to the relics of inherited zircons with fine internal zonation or homogeneous, while the surrounding rims are the growth part formed during the high-T metamorphism of the shear zone. Totally twenty-one analyses were performed on 11 separate zircon grains. Of the eleven rims, U concentrations range from 667 ppm to 1008 ppm, and Th/U ratios from 0.01 to 0.05, indicating that all rims are of metamorphic origin (Rubatto, 2002; Williams and Claesson, 1987; Zheng et al., 2005). These 11 analyses on the rim yield a mean 206 Pb/238U age of 433.8 ± 6.8 Ma (Fig. 7C). That is considered the best estimate for the timing of high temperature metamorphism of the shear zone. Of the 10 cores analyzed, U concentrations range from 37 ppm to 740 ppm, and Th/U ratios from 0.17 to 2.91. Seven ages of 9 cores are in 639–1620 Ma, whereas other two of them, 451 Ma and 512 Ma, are interpreted as mixed ages between the core and the rim. The older ages of the zircon cores cluster in the categories of

ca.1620 Ma, 1113 Ma, 707–732 Ma and 639–657 Ma, which possibly preserved evolutional information of sedimentary basin. 5.4. Sample Jx-64 (E116°09.350′, N25°22.686′): undeformed biotite granite in the Taoxi site, Wuping County All zircons are transparent and colorless as well as typical euhedral. The length of zircons ranges from 50 to 200 μm, and the length/width ratio varies from 1:1 to 3:1. Euhedral concentric zoning is clear on CL images. U concentrations range from 92 ppm to 1965 ppm, and Th/U ratios from 0.4 to 1.87, indicating magmatic origin. These analyses give a mean 206Pb/238U age of 229.8 ± 2.2 Ma (Fig. 7D), which represents the crystallization age of the granitic dyke. 5.5. Sample Wy-16 (E116°53.341′, N26°07.507′): gneissic K-feldspar granite in Qingliu County The majority of grains is between 50 μm and 350 μm in diameter and has euhedral to subhedral shape with well-developed concentric zoning. The transparent and colorless zircons have length to width ratios varying from 1:1 to 7:1. Twenty analyses were conducted on 20 zircons. U concentrations range from 199 ppm to 1868 ppm, and Th/U ratios from 0.11 to 1.09. Among them, seventeen analyses give a mean 206 Pb/238U age of 435.6 ± 6.2 Ma (Fig. 8), which is interpreted to be the best estimate of the zircon crystallization age. The other three ages, 606 Ma, 509 Ma, 488 Ma, may be either inherited from older zircons or caused by Pb loss. 6.

40

Ar/39Ar geochronology

In order to determine the timing of the dextral shear deformation in the Wuyishan, 40Ar/39Ar step-heating dating on biotite and muscovite has been conducted on two rock samples. High-purity biotite and muscovite were obtained from two mylonitic samples (Jx-63 and Jx66) taken from the Taoxi section in Wuping County, by crushing, washing in deionized water, sieving to 40 to 80 meshes, and handpicking. The amounts of biotite and muscovite used for analysis were 46.92 mg and 46.76 mg, respectively. The mineral concentrates were wrapped in aluminum foil packets, encapsulated in sealed quartz vials, and irradiated in the H8 position of the Swimming Pool Reactor at the Institute of Atomic Energy, Chinese Academy of Sciences (Beijing). The monitor used in this study is the Fangshan standard biotite (ZBH-25) with an age of 132.7 ± 1.2 Ma and potassium content of 7.6%. Following fast neutron irradiation, argon isotopes were measured with an MM-1200B mass spectrometer at the Ar–Ar Laboratory, Institute of Geology, Chinese Academy of Geological Sciences (Beijing). Mineral separates were step-heated in 9 to 10 steps at incrementally

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Fig. 7. Concordia plots of LA-ICP-MS U–Pb dating results for the samples collected from the Wuyishan, the corresponding field views of the rock samples and cathodoluminescence (CL) images. Sample Wy-07 and Jx-70 are taken from the gneissic plagioclase granite and K-feldspar granite within the shear zone (high angle shot, “XZ” plane); sample Wy-06 is ultramylonite (high angle shot, “XZ” plane); sample Jx-64 is granular biotite granite intruding to the ductile shear zone. White circles in the CL images indicate the spots of LA-ICP-MS U–Pb analysis.

higher power. The machine conditions and analytical methodology were described by Zhang et al. (2004). Ages for individual temperature steps were calculated by assuming an initial atmospheric 40Ar/36Ar

ratio equal to 295.5. Errors quoted on the ages and Ar isotope ratios are at 2σ levels. The 40Ar/39Ar dating results were treated and calculated by the program ISOPLOTS 2.49 (Ludwig, 2001).

X. X.B. et al. / Tectonophysics 501 (2011) 71–86

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Fig. 8. Concordia plots of LA-ICP-MS U–Pb dating results, mean ages (insets) and cathodoluminescence (CL) images of the gneissic granitic sample (Wy-16) collected from the northern part of the Weipu granitic pluton.

The 40Ar/39Ar full analytical data are listed in Table 5. Apparent age spectra and, where appropriate, corresponding isotope correlation diagrams (inverse isochron) are illustrated in Fig. 9. The standard criterion for identification of plateau (that is, undisturbed portions of the 40Ar/39Ar spectra) is commonly the existence of at least three contiguous steps with concordant ages which contain a significant proportion (N50%) of the 39Ar released (Dallmeyer and Lecorche, 1990; Dalrymple and Lanphere, 1974; Lee et al., 1991; McDougall and Harrison, 1999). However, a final criterion for defining a ‘true plateau’ age is based on concordance of inverse-isochron ages defined by isochronous (that is, low MSWD) data and plateau ages (Dalrymple and Lanphere, 1974). 6.1. Sample Jx-63 (E116°08.978′, N25°22.644′): mylonite in the Taoxi site, Wuping County Biotite from this sample gave a 6-step plateau age of 235.3 ± 2.8 Ma (Fig. 9A). The same temperature steps define an inverse isochron age of 232.4 ± 3.6 Ma (MSWD = 0.31) with a 40Ar/36Ar intercept of 371 ± 44. The isochron age overlaps with the plateau age, taking uncertainties into account, and thus dates the last Ar isotopic closure of the biotite. 6.2. Sample Jx-66 (E116°09.752′, N25°22.707′): mylonite in the Taoxi site, Wuping County Muscovite from this sample yields relatively flat 40Ar/39Ar apparent age spectra during intermediate-high temperature heating steps, giving a 6-step plateau age of 238.5 ± 2.8 Ma (Fig. 9B). The same temperature steps define an inverse isochron age of 238.1 ± 3.1 Ma (MSWD = 0.76) with a 40Ar/36Ar intercept of 297 ± 25. The inverse isochron age is consistent with the plateau age and therefore dates the last Ar isotopic closure of the muscovite. The plateau age agrees with its total gas ages (TGA) and inverse isochron ages (Ti) within errors, which indicate that it is of geological significance. 7. Age significance and tectonic implication It has been argued that the U–Pb isotope system in zircon and monazite, with closure temperature between 700 °C and 900 °C (Kalt et al., 2000; Suzuki et al., 1994), will give ages close to the deformation age of shearing under high temperature condition, while the 40Ar/39Ar isotope system in biotite and muscovite, with closure temperature of 350–450 °C (Hacker and Wang, 1995), represents cooling age of the shear zone. This has been demonstrated by the study on the Red River

shear zone in southwestern China, where mica 40Ar/39Ar dating gave ca. 25–17 Ma, whereas zircon and monazite U–Th–Pb dating yielded ca. 35 Ma of the sinistral shearing (Leloup et al., 2007 and references therein). In this study, we obtained seven ages to constrain the timing of the tectono-thermal events of the dextrally sheared metamorphic rocks in the Wuyishan, SE China. These ages fall into two categories: one is 431–441 Ma (Early Silurian), another one is 230–239 Ma (Middle Triassic). The La-ICP-MS U–Pb age of the growth rims of zircons obtained from the ultramylonite at Taoxi Township is of particular significance. This age (about 434 Ma) may represent the timing of high temperature metamorphism possibly associated with ductile shearing and anatexis, thus it may record the initiation of shear deformation in the Wuyishan. This age is consistent with the crystallization ages of the syn-deformational gneissic granites (U–Pb age of 441–431 Ma) from the shear zones in the Wuyishan, it is also consistent with the crystallization ages of the Weipu granitic pluton (U–Pb age of 447– 432 Ma, Li et al., 2010a; Xu et al., 2009). It is interesting to note that the above U–Pb ages are almost the same as 40Ar/39Ar plateau ages obtained from different localities in the Wuyishan, for example, 430 ± 2 Ma for the Ninghua gneissic granitic pluton (Shu et al., 2008a), 434.1 ± 2.2 Ma for dextral shear rocks from the Jiangle area (Zhang et al., 2003), and about 428 Ma for ductile shearing along the NNE-trending Yintan-Anyuan Fault (Wang et al., 2007c). These data indicated that ductile deformation under compression background occurred between 441 Ma and 428 Ma, whatever it is thrusting or strike-slip shearing. Thus, our geochronological study provides additional data allowing refinement of previously proposed tectonic evolution models of the Early Paleozoic orogeny in SE China (Charvet et al., 2010; Li et al., 2010a). We suggest that the pre-Sinian metamorphic rocks in the Wuyishan suffered from intensive shear deformation and anatexis at the time span of 441–430 Ma, during the late stage of Early Paleozoic orogeny. The mica 40Ar/39Ar ages of the shear zone might record Early Mesozoic tectonism, because temperature–pressure estimates of the shear zones indicate high greenschist facies condition (400–500 °C), similar to the closure temperature (350–450 °C) of mica 40Ar/39Ar isotope system (Hacker and Wang, 1995). The upper bound for the timing of the shear deformation can be restricted by the crystallization age (about 230 Ma) of an undeformed granitic dyke that intruded into mylonite belt at Taoxi Township of Wuping County. Thus, the dextral shear occurred at the time span between 239 Ma and 230 Ma. This timing of strike-slip shear deformation is roughly consistent with those reported in southwest Zhejiang province, where metamorphic

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Table 4 Analytical data of zircon La-ICP-MS U–Pb dating for granitoids and ultramylonite. Sample

207

Pb/206Pb

1s

206

Wy-07 Wy-07-01 Wy-07-02 Wy-07-03 Wy-07-04 Wy-07-05 Wy-07-06 Wy-07-07 Wy-07-08 Wy-07-09 Wy-07-10

Gneissic plagioclase granite 0.05449 0.00081 0.0547 0.0009 0.05405 0.00073 0.05628 0.00103 0.0554 0.00086 0.05491 0.00085 0.05542 0.00108 0.05544 0.00087 0.05426 0.00089 0.05767 0.0012

0.52488 0.53589 0.52474 0.55544 0.52927 0.54357 0.53095 0.55009 0.54049 0.55668

0.00871 0.0096 0.00813 0.01083 0.00898 0.00926 0.01085 0.00941 0.00956 0.01174

0.06987 0.07105 0.07041 0.07159 0.06929 0.07179 0.06948 0.07195 0.07223 0.07

0.00096 0.00098 0.00095 0.00101 0.00093 0.00098 0.00098 0.00098 0.001 0.00097

0.01026 0.01265 0.01036 0.0128 0.01613 0.01227 0.0139 0.01153 0.0103 0.01883

0.00031 0.00045 0.00034 0.00057 0.00081 0.00051 0.00068 0.0005 0.00049 0.0015

213 284 283 242 205 287 185 235 191 244

462 332 954 350 779 521 219 540 687 378

0.46 0.86 0.3 0.69 0.26 0.55 0.85 0.43 0.28 0.65

435 442 439 446 432 447 433 448 450 436

6 6 6 6 6 6 6 6 6 6

Jx-70 Jx-70-1 Jx-70-2 Jx-70-3 Jx-70-4 Jx-70-5 Jx-70-6 Jx-70-7 Jx-70-9 Jx-70-10 Jx-70-8 Jx-70-11 Jx-70-12 Jx-70-13 Jx-70-14 Jx-70-15 Jx-70-16 Jx-70-17 Jx-70-18 Jx-70-19 Jx-70-20

Gneissic K-feldspar granite 0.08866 0.01209 0.09125 0.0063 0.09807 0.01728 0.14759 0.01856 0.14383 0.00707 0.09897 0.00564 0.09312 0.00383 0.09924 0.00301 0.10836 0.0045 0.05672 0.00212 0.05767 0.00142 0.05836 0.00198 0.05651 0.00147 0.0564 0.00222 0.05805 0.00434 0.05661 0.00189 0.06969 0.00374 0.06057 0.00233 0.06104 0.00143 0.06677 0.00265

0.81595 0.86581 0.89088 1.34179 1.3643 0.94372 0.89262 0.94189 1.02614 0.53921 0.54823 0.56841 0.53971 0.54093 0.55816 0.53834 0.67074 0.58968 0.58043 0.63507

0.10921 0.05705 0.15486 0.17027 0.06457 0.05399 0.03666 0.02864 0.04412 0.02058 0.01304 0.01843 0.0135 0.02028 0.03985 0.01726 0.034 0.02182 0.01319 0.02422

0.06675 0.06832 0.06588 0.06818 0.06845 0.06906 0.06926 0.06893 0.06859 0.06888 0.06898 0.07076 0.06929 0.06956 0.06977 0.06901 0.06979 0.07062 0.069 0.06901

0.00175 0.00194 0.00192 0.00325 0.00163 0.00172 0.00134 0.00108 0.0014 0.0011 0.00088 0.00105 0.00089 0.00112 0.00187 0.00099 0.00151 0.00119 0.00088 0.00111

0.01974 0.03215 0.01928 −0.08811 0.03073 0.02746 0.02448 0.0274 0.03588 0.02037 0.01992 0.0245 0.0221 0.02473 0.02631 0.02454 0.02597 0.02459 0.02418 0.02486

0.00052 0.00905 0.00046 0.12319 0.00677 0.00721 0.00495 0.00489 0.0097 0.00556 0.0016 0.00471 0.0025 0.00473 0.00923 0.00331 0.00575 0.00513 0.00272 0.00336

144 143 235 1656 239 248 271 250 276 162 217 210 289 252 288 134 519 202 169 88

324 273 293 2082 243 484 323 269 376 187 234 570 370 338 411 164 2696 523 237 62

0.45 0.52 0.8 0.8 0.98 0.51 0.84 0.93 0.73 0.87 0.93 0.37 0.78 0.75 0.7 0.82 0.19 0.39 0.71 1.43

417 426 411 425 427 430 432 430 428 429 430 441 432 434 435 430 435 440 430 430

11 12 12 20 10 10 8 7 8 7 5 6 5 7 11 6 9 7 5 7

Wy-06 Wy-06-01 Wy-06-02 Wy-06-03 Wy-06-04 Wy-06-05 Wy-06-06 Wy-06-07 Wy-06-08 Wy-06-09 Wy-06-10 Wy-06-11 Wy-06-12D Wy-06-13 Wy-06-14 Wy-06-15 Wy-06-16 Wy-06-17 Wy-06-18 Wy-06-19 Wy-06-20 Wy-06-21

Ultramylonite 0.05592 0.05735 0.05688 0.05643 0.06577 0.05758 0.06374 0.0559 0.06061 0.05544 0.10267 0.05514 0.0631 0.05538 0.09004 0.05523 0.05914 0.05585 0.06434 0.05557 0.06366

0.00114 0.00108 0.00353 0.00098 0.00137 0.00099 0.0014 0.00093 0.0014 0.00117 0.00319 0.00084 0.00098 0.00089 0.00489 0.00101 0.00131 0.00096 0.00247 0.00104 0.00172

0.5461 0.55744 0.58673 0.54361 1.05041 0.5755 1.02256 0.52564 0.87137 0.55095 4.03599 0.53469 1.02585 0.53201 2.34063 0.50649 0.67372 0.54154 1.06646 0.52407 0.94217

0.01112 0.01053 0.03475 0.0096 0.02178 0.01028 0.02215 0.00904 0.01997 0.01159 0.12019 0.00864 0.0169 0.00903 0.12268 0.00938 0.01499 0.00966 0.03984 0.01006 0.02519

0.07085 0.0705 0.0756 0.06986 0.11583 0.07249 0.11636 0.0682 0.10428 0.07208 0.28574 0.07034 0.11792 0.06967 0.18855 0.06652 0.08263 0.07033 0.12022 0.0684 0.10735

0.00091 0.00089 0.00199 0.00086 0.00152 0.00094 0.00154 0.00085 0.00141 0.00095 0.00435 0.00088 0.00152 0.00089 0.00267 0.00084 0.00114 0.0009 0.00203 0.00089 0.00157

0.02077 0.02727 0.02022 0.02652 0.03439 0.02679 0.03452 0.02758 0.03659 0.02208 0.087 0.02766 0.02518 0.02226 0.05567 0.01984 0.02562 0.01715 0.0331 0.02574 0.03024

0.01112 0.01053 0.03475 0.0096 0.02178 0.01028 0.02215 0.00904 0.01997 0.01159 0.12019 0.00274 0.00131 0.00207 0.00071 0.00243 0.00177 0.00225 0.00305 0.00291 0.0026

11 4 46 16 313 126 376 10 255 10 859 8 537 10 236 12 112 8 82 11 176

893 667 1008 819 189 740 288 905 237 809 295 673 352 724 314 942 176 953 37 886 90

0.01 0.01 0.05 0.02 1.66 0.17 1.30 0.01 1.08 0.01 2.91 0.01 1.52 0.01 0.75 0.01 0.64 0.01 2.23 0.01 1.97

441 439 470 435 707 451 710 425 639 449 1620 438 719 434 1113 415 512 438 732 427 657

5 5 12 5 9 6 9 5 8 6 22 5 9 5 15 5 7 5 12 5 9

Jx-64 Jx-64-1 Jx-64-2 Jx-64-3 Jx-64-4 Jx-64-5 Jx-64-6 Jx-64-7 Jx-64-8 Jx-64-9 Jx-64-10 Jx-64-11 Jx-64-12 Jx-64-13 Jx-64-14 Jx-64-15 Jx-64-16 Jx-64-17

Granular biotite granite 0.06372 0.05304 0.04941 0.05118 0.05254 0.05111 0.05076 0.05273 0.05237 0.05478 0.05251 0.05194 0.05214 0.05132 0.05356 0.05283 0.05346

0.00236 0.00203 0.00408 0.001 0.00211 0.00258 0.00372 0.00123 0.00277 0.00846 0.0018 0.00448 0.00548 0.00862 0.00244 0.00145 0.00158

0.31925 0.27471 0.24787 0.25509 0.25842 0.25594 0.25699 0.26996 0.2606 0.27674 0.26422 0.26011 0.26598 0.25544 0.27074 0.25992 0.25567

0.01142 0.01024 0.01988 0.0051 0.01016 0.01259 0.01817 0.00633 0.01344 0.04095 0.00888 0.02159 0.027 0.04122 0.01194 0.00701 0.00748

0.03635 0.03759 0.03639 0.03615 0.03568 0.03632 0.03671 0.03715 0.0361 0.03668 0.03649 0.03621 0.03699 0.03602 0.03666 0.03568 0.03469

0.00062 0.00064 0.00087 0.00047 0.0006 0.00065 0.00091 0.00051 0.00066 0.0019 0.00056 0.001 0.00116 0.00181 0.00066 0.00051 0.0005

0.01644 0.0127 0.01359 0.01227 0.01105 0.01205 0.01022 0.01142 0.01167 0.01591 0.01351 0.01218 0.01021 0.01368 0.0109 0.0137 0.01154

0.00318 0.00245 0.00314 0.00175 0.00165 0.00226 0.00268 0.00225 0.00219 0.02752 0.00155 0.00236 0.00163 0.00452 0.00146 0.00197 0.00101

391 676 157 549 429 284 375 2783 266 538 176 324 168 260 453 385 356

610 835 92 1379 252 165 238 1965 142 1415 440 191 99 178 466 1122 338

0.64 0.81 1.7 0.4 1.7 1.72 1.58 1.42 1.87 0.38 0.4 1.7 1.69 1.46 0.97 0.34 1.05

230 238 230 229 226 230 232 235 229 232 231 229 234 228 232 226 220

4 4 5 3 4 4 6 3 4 12 3 6 7 11 4 3 3

1s

207

Pb/235U

Pb/238U

1s

208

Pb/232Th

1s

Th

U

Th/U

Age

1s

X. X.B. et al. / Tectonophysics 501 (2011) 71–86

83

Table 4 (continued) Sample

207

Pb/206Pb

1s

207

Jx-64 Jx-64-18 Jx-64-19 Jx-64-20

Granular biotite granite 0.0521 0.05261 0.05425

0.00151 0.00237 0.00266

Wy-16 Wy-16-01 Wy-16-02 Wy-16-03 Wy-16-04 Wy-16-05 Wy-16-06 Wy-16-07 Wy-16-08 Wy-16-09 Wy-16-10 Wy-16-12 Wy-16-13 Wy-16-14 Wy-16-15 Wy-16-16 Wy-16-17 Wy-16-18 Wy-16-19 Wy-16-20 Wy-16-11

Gneissic K-feldspar granite 0.05622 0.00083 0.05547 0.00089 0.05884 0.00118 0.06059 0.0011 0.05434 0.00117 0.05609 0.00113 0.05499 0.0008 0.06955 0.00147 0.06453 0.0011 0.05703 0.00107 0.05912 0.00212 0.06059 0.00147 0.05528 0.00111 0.05405 0.00204 0.05764 0.001 0.05576 0.00204 0.05662 0.0024 0.05724 0.00101 0.05577 0.00156 0.05823 0.00228

Pb/235U

1s

206

Pb/238U

0.26207 0.25846 0.28103

0.00751 0.01125 0.01327

0.03648 0.03561 0.03757

0.00052 0.00063 0.00072

0.01182 0.01214 0.01026

0.00109 0.00168 0.00178

417 315 366

417 314 881

1 1 0.41

231 226 238

3 4 4

0.55024 0.52388 0.54649 0.55271 0.5257 0.53188 0.53374 0.9438 0.73031 0.55714 0.57244 0.65635 0.54746 0.55166 0.54414 0.5495 0.56067 0.56581 0.55691 0.56249

0.00879 0.00887 0.01124 0.01029 0.01142 0.011 0.00849 0.02029 0.01327 0.01096 0.01994 0.0159 0.01142 0.02044 0.00987 0.01966 0.02306 0.0105 0.01558 0.02149

0.071 0.06851 0.06736 0.06621 0.0702 0.06879 0.07041 0.0985 0.08211 0.07088 0.07047 0.07866 0.07183 0.07401 0.06852 0.07151 0.07196 0.07168 0.07242 0.07007

0.00092 0.00089 0.00095 0.00085 0.00094 0.00093 0.00092 0.00135 0.00111 0.00097 0.00119 0.00112 0.00103 0.00136 0.0009 0.00132 0.00133 0.00095 0.00113 0.00124

0.01076 0.01056 0.00595 0.01935 0.02158 0.01804 0.00997 0.01857 0.01422 0.01548 0.01826 0.0246 0.00938 0.00796 0.01849 0.00629 0.02535 0.01779 0.0133 0.01802

0.00035 0.00035 0.00024 0.0016 0.00173 0.00107 0.00047 0.00176 0.00076 0.00102 0.00206 0.00203 0.00041 0.00082 0.00151 0.00043 0.00523 0.00148 0.001 0.00457

183 176 353 245 188 117 138 160 146 235 61 121 184 269 296 139 104 495 192 170

488 345 324 1182 312 232 1291 319 314 318 199 228 453 872 1868 308 287 1691 267 873

0.38 0.51 1.09 0.21 0.6 0.5 0.11 0.5 0.46 0.74 0.31 0.53 0.41 0.31 0.16 0.45 0.36 0.29 0.72 0.18

442 427 420 413 437 429 439 606 509 441 439 488 447 460 427 445 448 446 451 437

6 5 6 5 6 6 6 8 7 6 7 7 6 8 5 8 8 6 7 7

ages are between 251 and 212 Ma, concentrated in 240–230 Ma (Li and Li, 2007; Xiang et al., 2008; Yu et al., 2009), and ductile deformation dated at 238–221 Ma (Liu et al., 1993; Zhu et al., 1997). Whether or not the shear zone observed in southwest Zhejiang and those in this study belong to a same large shear zone needs careful mapping. In addition, a huge Triassic granitic pluton, the Fucheng– Hongshan granitic pluton outcrops to the west of Wuping County, with an area of about 450 km2. Dating results show that this granitic pluton was emplaced at the time of 239–225 Ma (Yu et al., 2007a,b). It could not be excluded that the argon isotope system of mica at Taoxi Township could be disturbed by the emplacement of this granitic pluton. Regionally, Triassic strike-slip shearing has been documented on the NNE–NE-striking faults in SE China, but the sense of shearing is controversy. It is sinistral strike-slip for Wang et al. (2005a, 2007a),

Table 5 Analytical data of step heated T (°C)

40

40

208

1s

Pb/232Th

1s

Th

U

Th/U

Age

1s

while dextral strike-slip for Lin et al. (2008) and Zhang and Cai (2009). Geodynamic settings for the Triassic shear deformation in SE China include the collision of the North and South China Blocks along the Qinling-Dabie range (e.g., Hacker and Wang, 1995; Li et al., 1993; Meng and Zhang, 2000), northward accretion of the Indo-China block to the South China Block (Carter et al., 2001; Lepvrier et al., 2004, 2008), and westward flat subduction of the paleo-Pacific plate under the South China Block (Li and Li, 2007; Wang et al., 2005b; Xiao and He, 2005). 8. Conclusion Ductile shear deformation was well documented in the preDevonian metamorphic rocks in the Wuyishan, SE China. Field observations and thin section analyses show dextral strike-slip shearing on NE-striking zones. Deformation temperature is estimated

Ar/39Ar dating for mylonites.

39

( Ar/ Ar)m

(36Ar/39Ar)m

(37Ar/39Ar)m

(38Ar/39Ar)m

40

40

Ar (%)

Ar*/39Ar

Jx-63, Biotite, W = 46.92 mg, J = 0.009823, TGA = 233.0 ± 6.4 Ma, Tp = 235.3 ± 2.8 Ma (71.8% of the 39Ar), Ti = 232.4 ± 3.6 Ma 700 35.2862 0.1081 0 0.0415 9.47 3.3429 800 19.6075 0.0466 0.1491 0.0383 29.82 5.8479 900 9.1223 0.0133 0.0511 0.0191 56.83 5.1841 1000 15.2209 0.0058 0.0099 0.0138 88.67 13.4968 1060 14.262 0.0008 0.0043 0.0129 98.31 14.0216 1120 14.4183 0.0009 0.0041 0.0125 98.2 14.1582 1180 14.5096 0.0015 0.0113 0.0129 96.97 14.0695 1260 16.0095 0.0054 0.0113 0.0125 89.98 14.4047 1320 15.2795 0.0021 0.0199 0.0136 95.95 14.6603 Jx-66, Muscovite, W = 46.76 mg, 700 19.4041 800 10.4526 900 12.4377 1000 14.8422 1060 14.8971 1120 14.6708 1180 14.7088 1230 14.8469 1300 14.7134 1400 18.8732

J = 0.009796, TGA = 238.0 ± 4.3 Ma, Tp = 238.5 ± 2.8 Ma (92.5% of the 0.0351 0.0494 0.0297 46.59 0.0121 0.0936 0.0241 65.68 0.0054 0.0264 0.0171 87.09 0.0065 0.0065 0.014 86.96 0.0021 0.0078 0.0132 95.83 0.0007 0.001 0.0127 98.65 0.0007 0.0032 0.0126 98.52 0.0014 0.0005 0.0128 97.18 0.0006 0.0022 0.0126 98.81 0.0149 0.0341 0.0174 76.65

39

39

Ar (%)

Age (Ma)

± 2σ (Ma)

0.37 0.76 5.27 25.55 52.3 66.68 71.19 97.31 100

58.3 100.8 89.6 224.6 232.8 234.9 233.5 238.7 242.7

9.9 6.8 1.6 2.3 2.9 3.2 3 2.5 2.7

Ar), Ti = 238.1 ± 3.1 Ma 9.0417 0.25 6.8653 0.92 10.8325 2.87 12.9076 7.51 14.2758 15.43 14.4725 58.57 14.4908 65.74 14.4284 73.55 14.5381 98.06 14.4665 100

153 117.4 181.9 214.8 236.1 239.2 239.5 238.5 240.2 239.1

17 5 2 3.1 2.7 3.2 4.7 3.9 3.2 3.9

84

X. X.B. et al. / Tectonophysics 501 (2011) 71–86

Fig. 9. Age spectra and inverse isochron plots of 40Ar/39Ar dating results for biotite and muscovite from the mylonitic rock samples Jx-63 and Jx-66 collected from the ductile shear zone at Taoxi township in Wuping County.

between 400 °C and 500 °C and deformation pressure is estimated about 7–10 kbar. La-ICP-MS U–Pb zircon and mica 40Ar–39Ar geochronological dating of the rock samples from the shear zones reveal two tectono-thermal events. The first one occurred at the time span of 431–441 Ma, which is interpreted to be associated with high temperature metamorphism and anatexis during the late stage of the Early Paleozoic orogeny in SE China. The late one developed in 230– 239 Ma, which is thought to be related to dextral strike-slip shearing during the Triassic Indosinian tectonic event. Acknowledgements We would express our gratitude to Professor Jacques Charvet for his careful reviewing and constructive suggestions. Thanks are given to the chief editor Mian Liu for the helpful advices. The field work was assisted by the Gannan Geological Bridge, Jiangxi Geological and Mineral Resource Bureau. Thanks are given to Doctor Jianxiang Xu for the logistic support, to Xiang Li and Bo Hu for their help in electron microprobe and microscopic analysis. This study was funded jointly by the National Natural Science Foundation of China (grant 40634022) and the project SinoProbe-08-01. References Andersen, T., 2002. Correction of common lead in U–Pb analyses that do not report 204 Pb. Chem. Geol. 192, 59–79. Bai, D.Y., Zhou, L., Wang, X.H., Zhang, X.Y., Ma, T.Q., 2007. Geochemistry of Nanhua– Cambrian sandstones in southeastern Hunan, and its constraints on Neoproterozoic–

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