Zircon U–Pb geochronology of the Mesozoic metamorphic rocks and granitoids in the coastal tectonic zone of SE China: Constraints on the timing of Late Mesozoic orogeny

Journal of Asian Earth Sciences 62 (2013) 237–252

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Zircon U–Pb geochronology of the Mesozoic metamorphic rocks and granitoids in the coastal tectonic zone of SE China: Constraints on the timing of Late Mesozoic orogeny Jianjun Cui a, Yueqiao Zhang a,⇑, Shuwen Dong a, Bor-ming Jahn b, Xianbing Xu c, Licheng Ma a a b c

Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, PR China Department of Geosciences, National Taiwan University, Taipei 106, Taiwan Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, Hubei, PR China

a r t i c l e

i n f o

Article history: Received 21 March 2012 Received in revised form 22 August 2012 Accepted 17 September 2012 Available online 11 October 2012 Keywords: Zircon U–Pb dating Changle-Nan’ao tectonic zone Late Mesozoic tectononthermal event SE China

a b s t r a c t The coastal Changle-Nan’ao tectonic zone of SE China contains important geological records of the Late Mesozoic orogeny and post-orogenic extension in this part of the Asian continent. The folded and metamorphosed T3–J1 sedimentary rocks are unconformably overlain by Early Cretaceous volcanic rocks or occur as amphibolite facies enclaves in late Jurassic to early Cretaceous gneissic granites. Moreover, all the metamorphic and/or deformed rocks are intruded by Cretaceous fine-grained granitic plutons or dykes. In order to understand the orogenic development, we undertook a comprehensive zircon U–Pb geochronology on a variety of rock types, including paragneiss, migmatitic gneiss, gneissic granite, leucogranite, and fine-grained granitoids. Zircon U–Pb dating on gneissic granites, migmatitic gneisses, and leucogranite dyke yielded a similar age range of 147–135 Ma. Meanwhile, protoliths of some gneissic granites and migmatitic gneisses are found to be late Jurassic magmatic rocks (ca. 165–150 Ma). The little deformed and unmetamorphosed Cretaceous plutons or dykes were dated at 132–117 Ma. These new age data indicate that the orogeny lasted from late Jurassic (ca. 165 Ma) to early Cretaceous (ca. 135 Ma). The tectonic transition from the syn-kinematic magmatism and migmatization (147–136 Ma) to the postkinematic plutonism (132–117 Ma) occurred at 136–132 Ma. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction The vast expanse of orogenic belts in SE China (>1000 km wide) is characterized by polycyclic orogenesis and multiple episodes of magmatism during the Mesozoic (Jahn et al., 1976, 1990; Lapierre et al., 1997; Davis et al., 1997; Chen, 1999; Chen et al., 2004; Li and Li, 2007; Sewell et al., 2012). However, the precise timing of all the tectonic processes and magmatic events remains much debated. Most workers considered that the widespread Jurassic to Cretaceous magmatism was related to the subduction of the paleoPacific plate beneath the Asian continent (Jahn et al., 1976, 1990; Lapierre et al., 1997; Zhou and Li, 2000; Li and Li, 2007; Chen et al., 2008a,b; He et al., 2010). In order to explain the impressive width of the magmatic belt, Zhou and Li (2000) proposed a model of low-angle subduction of the Pacific plate. In this model, the slab dip angle of the Pacific plate changed from a very low angle in early Jurassic to a medium angle in Cretaceous. However, most Late ⇑ Corresponding author. Address: Institute of Geomechanics, Chinese Academy of Geological Sciences, 11 Minzudaxue Nanlu, Beijing 100081, PR China. Tel.: +86 10 68412311; fax: +86 10 68422326. E-mail addresses: [email protected], [email protected] (Y. Zhang). 1367-9120/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jseaes.2012.09.014

Mesozoic magmas in SE China would have been derived from local sources, without contribution from the oceanic crust. On the other hand, considerable amount of igneous rocks of Jurassic to Cretaceous ages in the hinterland of SE China indicate their generation in an intraplate extension/rifting setting (Li, 2000; Li and Li, 2007; Zhu et al., 2010; He et al., 2010). Therefore, the processes of magmatism were more complicated than those depicted in the low-angle subduction model (Li et al., 2007). With some modification of Zhou and Li (2000), Li and Li (2007) suggested that the exceptionally-wide Mesozoic orogen and magmatic belt in SE China were produced by a flat-slab subduction during ca. 265– 190 Ma and followed by post-orogenic slab-foundering at ca. 190–155 Ma. Furthermore, the post-150 Ma magmatism was interpreted as a result of slab rollback. However, this model fails to explain the Late Mesozoic orogeny in the interior of SE China (Jahn, 1974; Jahn et al., 1976; BGMR, 1985; Chen, 1999; Chen et al., 2004, 2008a,b; Xing et al., 2008; Zhang et al., 2009). Regarding the orogenic processes, many authors considered that SE China has undergone two phases of tectonic contraction in Mesozoic (BGMR, 1985; Chen, 1999; Dong et al., 2008; Zhang et al., 2009). For example, Zhang et al. (2009) suggested that the nearly E–W trending fold zones were resulted from a far-field

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effect of the Indosinian collision or accretion alone the northern and southern margins of the South China Block, whereas the NNE-trending folds were originated from a low angle subduction of the paleo-Pacific plate under the SE China Block during Late Mesozoic. Some workers have suggested that the dynamic cause of intraplate orogeny (reworking) usually come from plate margins (Dyksterhuis and Müller, 2008). If so, the coastal Changle-Nan’ao zone, which documents polycyclic tectonothermal events and multiple phases of deformation and metamorphism in Mesozoic, could be a tectonic zone crucial to our understanding of the tectonic evolution in the coastal region of SE China. Until the present, the chronological study of the tectonothermal events in the coastal zone has been limited to some Rb/Sr and Ar/Ar dating (Jahn et al., 1976; Chen et al., 2002) and a few U–Pb zircon age determinations (Tong and Tobisch, 1996; Dong et al., 2006; Feng et al., 2011). Though the zircon age data suggest that while the early Cretaceous magmatism is the strongest, the entire scenario and precise ages of the thermal events remain obscure. In fact, controversy on the nature and evolution of the coastal zone still persists (Jahn et al., 1976; Tong and Tobisch, 1996; Wang and Lu, 1997; Chen et al., 2002). In order to resolve the controversy, we conducted a comprehensive zircon U–Pb geochronology on a variety of metamorphic rocks and granitoids from the Changle-Nan’ao tectonic zone of SE China. The new age data reveal that early Cretaceous is indeed a period of most important tectonothermal event. Important tectonic implications will be discussed.

2. Geology and previous geochronology of the coastal zone 2.1. General geology The coastal Changle-Nan’ao tectonic zone is located in the southeastern continental margin of SE China (Fig. 1). It extends >400 km in NE–SW direction and is 40–60 km wide. In the northwestern part of the coastal zone, Mesozoic sediments are separated by two regional unconformities into three stratigraphic units, from old to young, (1) T3–J coal-bearing sequences (e.g. Dakeng Formation and Lishan Formation); (2) Lower Cretaceous volcanogenic deposits (e.g. Nanyuan Formation), and (3) Upper Cretaceous volcanogenic deposits (e.g. Shimaoshan Group) (BGMR, 1985; Tong and Tobisch, 1996; Lapierre et al., 1997; Xing et al., 2008; He and Xu, in press). The Lower Cretaceous volcanogenic deposits are dominated by acid (or silicic) volcanic stuff, lava and minor basic tuff and dykes of ca. 145–117 Ma (Lapierre et al., 1997; Xing et al., 2008; He and Xu, in press). The typical mineral assemblage of the Lower Cretaceous volcanogenic deposits is quartz ± sericite ± muscovite ± biotite ± chlorite ± epidote ± plagioclase, indicating greenschist facies metamorphism. The Upper Cretaceous volcanogenic deposits (ca. 100–80 Ma) consist of gray tuffaceous siltstone/sandstone/glutenite and dacite vitric/crystal tuff, which show no signs of metamorphism (BGMR, 1985; Tong and Tobisch, 1996; He and Xu, in press). In the southeastern part of the coastal zone, the deformed and metamorphosed T3–J sedimentary rocks commonly occur as enclaves in late Jurassic to early Cretaceous gneissic granites (Jahn et al., 1976; Tong and Tobisch, 1996; Feng et al., 2011). Lithologically, the enclaves include paragneiss (Fig. 2A), migmatitic gneiss (Fig. 2B), leptynite, amphibolite, quartzite, and graphite-mica schist. The late Jurassic to early Cretaceous gneissic granites comprise multiple generations of magmatic rocks. The older gneissic granites (Fig. 2C) and the T3–J1 metasedimentary rocks were together deformed and migmatized during the formation of the younger gneissic granites (Fig. 2D). All the gneissic granites were intruded by little deformed Cretaceous plutons (Fig. 2E) and/or dykes, which occur throughout the coastal zone.

2.2. Previous geochronology Based on the Rb/Sr geochronological results of granitic gneisses, Jahn et al. (1976) proposed two Mesozoic tectonothermal events, at 165 ± 13 Ma and 90–120 Ma, in the coastal zone and defined the periods of two thermal episodes of the well-known Yanshan orogeny. Two decades later, Tong and Tobisch (1996) reported a single-zircon U–Pb age of 122 Ma for a migmatized granitic gneiss sample from the Dongshan Island. Because the zircons from the sample have a homogeneous igneous morphology (e.g. no core/ rim relation) and show no evidence of recrystallization, Tong and Tobisch (1996) proposed that at least part of these gneissic granites in the coastal zone were formed in Cretaceous. Recently, some late Jurassic gneissic granites have been identified in the Dongshan Island (Feng et al., 2011). Available U–Pb zircon ages of different rock types from the Changle-Nan’ao zone cluster at ca. 155 Ma, 140–120 Ma and 100–90 Ma (Tong and Tobisch, 1996; Dong et al., 2006; Feng et al., 2011). In addition, 40Ar/39Ar thermochronology on hornblende, muscovite and biotite has documented two episodes of cooling at ca. 130–110 Ma and ca. 100–80 Ma in the coastal zone (Chen et al., 2002). Nevertheless, the precise time framework and tectonic processes of the predominant Late Mesozoic magmatism and metamorphism are still controversial in the coastal zone of SE China. 3. Samples description In order to determine the time of metamorphism, deformation, migmatization, syn-kinematic magmatism, and post-kinematic plutonism in the coastal Changle-Nan’ao zone, twelve samples were chosen for zircon U–Pb geochronological study. 3.1. Two metamorphic rock samples (PD02-2 and PD23-2) Sample PD02-2 was collected from Nanwuli village, Pingtan County (Fig. 1). Penetrative shearing foliation (ca. 30°) in the paragneiss dip gently to the W, and stretching lineations (30 ± 5°) defined by quartz, muscovite, and biotite plunge to the W, with mineral grain sizes about 0.5–2 mm. The mineral assemblage of the paragneiss comprises garnet + sillimanite + muscovite + biotite + plagioclase + K-feldspar + quartz. Sample PD23-2 is a strongly migmatized gneiss (Fig. 2B) from Shenhu town, Shishi City. Migmatitic gneiss usually occurs as enclaves in younger gneissic granites (Fig. 2D). Occasionally, older gneisses are found to be intruded by Cretaceous fine-grained granites (Fig. 2E). 3.2. Five gneissic granite samples (PD30-1, PD26-3, PD34-7, PD25-6, and PD52-3) In general, gneissic granites also show some degrees of migmatization, with low-angle (<15°) shearing bands (Fig. 2C). Foliation and stretching lineation defined by mica and amphibole in gneissic granites are essentially identical to those found in paragneisses. The stretching lineation in the gneissic granites is usually consistent with the fold axis in the paragneiss enclaves (plunge <20° to the NE or SW). Sample PD30-1 was collected from a roadside outcrop on Highway S201 near Jiaotou village, Xingchen town, Dongshan County (Fig. 1; Table 1). The other samples came from Baikeng village in Gangwei town (PD26-3), Guleishan in Gulei peninsula (PD34-7), Gangwei town (PD25-6), and the southern part of Nan’ao Island (PD52-3). The coarse-grained gneissic granites consist of 40–55% plagioclase feldspar, 5–35% K-feldspar, 25–30% quartz, 0–8% hornblende, 1–12% biotite, 0–10% muscovite, 0–2% garnet, and accessory opaque minerals, apatite, titanite, zircon, allanite and monazite (Fig. 2D).

J. Cui et al. / Journal of Asian Earth Sciences 62 (2013) 237–252

239

Fig. 1. Simplified geological map of the coastal area of SE China. A–B shows a generalized profile across the Changle-Nan’ao tectonic zone. The sampling localities of the studied rocks are indicated.

3.3. Five granitoid samples (PD14-1, PD14-2, PD05-1, PD32-3, and PD51-1) Granitoids are commonly undeformed with igneous textures varying from equigranular, fine grained to porphyritic. Typically, the five granitic rocks are composed of 15–50% plagioclase feldspar, 20–50% K-feldspar, 25–30% quartz, 1–8% biotite, 0–5% hornblende, 0–5% muscovite, 0–3% garnet, and accessory apatite, titanite, zircon, allanite and monazite (Fig. 2E). Sample PD14-1 was collected from a garnet-bearing leucogranite dyke (Fig. 2F). The decimeter-scale dyke crops out in a quarry at Liucuo town, Hui’an County. Sample PD14-2 represents the host fine-grained garnet-bearing pluton in the same quarry at Liucuo town. Sample PD05-1 was taken from a microgranite dyke that intruded into the deformed Late Mesozoic sediments at Qiulu town, Putian city. Note that two granite samples (PD51-1 and PD32-3) were collected

from two weakly deformed plutons in the Nan’ao Island and Duxun town, Zhangpu County. The sample locations and field characteristics and dating results are summarized in Table 1. 4. Analytical procedures Zircon U–Pb isotopic compositions were analyzed using SHRIMPII instrument at Curtin University of Technology, Western Australia. Internal structures of zircons were revealed by CL imaging. The SHRIMP instrumental conditions and data acquisition procedures are the same as described by Williams (1998). The measured 206 Pb/238U ratios were corrected using a reference zircon TEMORA (417 Ma; Black et al., 2003). Ages and concordia diagrams were produced using software SQUID 1.03 (Ludwig, 2001) and ISOPLOT 3.00 (Ludwig, 2003). Correction for common Pb was made by the measured 204Pb.

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Fig. 2. Photographs showing the field occurrences of all rock types in the SE part of the Changle-Nan’ao zone. (A) T3–J metasedimentary rocks in the SE margin of Dongshan Island. (B) Migmatitic gneiss (PD23-2) from Shenhu. (C) Gneissic granites with penetrative migmatization (PD30-1) from Jiaotou village. (D) The early Cretaceous gneissic granite (PD25-6) at Gangwei. (E) The early Cretaceous fine-grained granitoids (PD32-3) at Duxun. (F) Garnet-bearing leucogranite dyke (PD14-1) from Liucuo.

The LA-ICPMS analyses were conducted on an Elan 6100 DRC ICPMS equipped with 193 nm laser, which is housed at the Department of Geology, Northwest University in Xi’an, China. Zircon 91500 was used as a standard and the standard silicate glass NIST was used to optimize the running conditions. The spot size was about 30 lm. No common Pb correction was applied, since the signal intensity of 204Pb was much lower than the other Pb isotopes and there is a large isobaric interference from Hg. Raw data were processed using the GLITTER program and 206Pb/238Pb ratios were used to compute the age. The detailed analytical technique is described by Yuan et al. (2004). The analytical data is listed in Tables 2 and 3. Errors on individual analyses by SHRIMP and LA-ICPMS are quoted at the 1r level. Errors on pooled analyses and age computation are quoted at 2r or 95% confidence level. In this study, one gneissic granite sample (PD34-7) was analyzed using both SHRIMP and LA-ICPMS methods to examine the reliability of the age data produced. The results show that the SHRIMP age of 137 ± 1 Ma and the LA-ICPMS age of 139 ± 1 Ma are totally comparable (Table 1). 5. Geochronological results 5.1. Paragneisses and migmatitic gneisses Zircon crystals from sample PD02-2 are mostly yellow and translucent to opaque, and show prismatic to irregular shape. Their

grain sizes range from 50 to 220 lm. The CL images show oscillatory-zoned core, thin planar/sector-zoned mantle and homogeneous rim in most zircon grains (Fig. 3a and b). The result of 30 spot analyses on 30 zircon grains show that most oscillatory-zoned cores have moderate abundances of U (114–1094 ppm) and Th (66–1216 ppm), and Th/U ratios of 0.19–2.04 except for spot 10.1 (0.13) and spot 26.1 (0.05). Excluding six spots (1.1, 5.1, 8.1, 12.1, 19.1, and 26.1), were removed, 20 concordant or near-concordant data-points yielded several groups of 206Pb/238U ages: 764– 730 Ma (n = 4), 637 Ma (n = 1), 461–410 Ma (n = 4), 347–313 Ma (n = 2), 285 Ma (n = 1), 251–207 Ma (n = 4), 202–196 Ma (n = 4) (Fig. 4a, Table 2). In addition, four analyses yielded 207Pb/206Pb ages from 1839 Ma to 1772 Ma, and the three near-concordant grains yielded a weighted mean 207Pb/206Pb age of 1827 ± 32 Ma (MSWD = 0.74). The internal textures revealed by CL imaging and the Th/U ratios indicate that they are of magmatic origin. Most zircon grains of sample PD23-2 are euhedral and 120– 320 lm long with length/width ratios of 1.5–3. They show simple zoning patterns characterized by a bright oscillatory-zoned core and a grey oscillatory-zoned rim (Fig. 3c and d). Thirteen spot analyses on 13 zircon grains were carried out on this sample. Twelve of the 13 oscillatory-zoned domains have medium concentrations of U (192–1187 ppm) and Th (103–636 ppm) and Th/U ratios of 0.27–0.92 (Table 2). Seven of the 13 concordant analyses on the grey oscillatory-zoned rims are tightly clustered with a weighted mean 206Pb/238U age = 140 ± 1 Ma, which is interpreted as the migmatization age for the migmatitic gneiss sample PD23-2 (Figs. 2B

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J. Cui et al. / Journal of Asian Earth Sciences 62 (2013) 237–252 Table 1 Sampling localities, field characteristics, lithology, and age results of the dated rocks from the coastal Changle-Nan’ao zone. Order

Sample

Location

Coordinates

1

PD02-2

Nanwuli

25°29.750

2

PD23-2

Shenhu

3

PD30-1

4

PD26-3

5

6

PD34-7

PD34-7

Lithology

Field characteristics

Ages(Ma)

Methods

Geological significances of ages

119°43.910

Paragneiss

An outcrop of >40  30  4 m3 (L  W  H)

1827 ± 23 764–730 637 ± 6 461–410 347–313 285 ± 2 251–207 202–196 <196

LA-ICPMS LA-ICPMS LA-ICPMS LA-ICPMS LA-ICPMS LA-ICPMS LA-ICPMS LA-ICPMS LA-ICPMS

Age Age Age Age Age Age Age Age Age

24°36.780

118°40.750

Migmatitic gneiss

160–145 140 ± 1

LA-ICPMS LA-ICPMS

Age of protolith Age of migmatization

Jiaotou

23°43.230

117°24.490

Gneissic granite

Enclaves with penetrative migmatization Gneissic granite with penetrative migmatization

160–150

SHRIMP

146–135

SHRIMP

Age of magmatic intrusion Age of migmatization

Baikeng

24°18.700

118°06.990

Gneissic granite

141 ± 1

SHRIMP

0

0

Gneissic granite

Strongly deformed zone Strongly deformed zone

165–150

LA-ICPMS

139 ± 1

LA-ICPMS

137 ± 1

SHRIMP

125 ± 2

SHRIMP

147 ± 1

LA-ICPMS

160–155

LA-ICPMS

136 ± 1

LA-ICPMS

133–123

LA-ICPMS

140 ± 2

SHRIMP

132 ± 1

SHRIMP

124 ± 1

SHRIMP

126–124

SHRIMP

120 ± 1

SHRIMP

117 ± 1

SHRIMP

Undeformed leucogranite dyke of several decimeters wide

145 ± 3

LA-ICPMS

135 ± 1 130–120

LA-ICPMS LA-ICPMS

A dike of 50–100 cm wide cutting Late Mesozoic strata

138 ± 3

LA-ICPMS

117 ± 2

LA-ICPMS

Guleishan

Guleishan

23°46.38

23°46.380

117°36.45

117°36.450

Gneissic granite

7

PD25-6

Gangwei

24°20.580

118°05.030

Gneissic granite

8

PD52-3

Nanao Island

23°24.060

117°07.070

Gneissic granite

9

10 11

12

13

PD14-2

PD51-1 PD32-3

PD14-1

PD05-1

Liucuo

Nanao Island Duxun

Liucuo

Qiulu

25°10.720

118°58.480

23°24.830

117°08.580

0

0

23°56.83

25°10.72

0

25°31.000

117°38.40

118°58.48

0

119°05.710

Fine-grained granite

Weakly deformed granite Weakly deformed granite

Garnet-bearing Leucogranite

Microgranite

and 4b). One analysis produced a younger age (120 Ma) probably due to late Pb loss. Five older oscillatory-zoned cores (161 Ma, 152 Ma, 149 Ma, 145 Ma, and 145 Ma) suggest that the protoliths of the migmatitic gneisses are likely to be late Jurassic magmatic rocks. 5.2. Gneissic granites Three gneissic granite samples (PD30-1, PD26-3, and PD34-7) were dated using the SHRIMP technique. Zircons from sample PD30-1 are mostly euhedral, up to 90–200 lm long, and have length to width ratios of 1–3. Most grains are clear and prismatic with magmatic oscillatory zonings (Fig. 5a–d). They have U and Th concentrations of 145–1239 ppm (except for 6.1, 7.2) and

Strongly deformed zone

Strongly deformed zone Strongly deformed zone

Undeformed domain

Weakly deformed domain Weakly deformed domain

of of of of of of of of of

magmatic event magmatic event magmatic event magmatic event magmatic event magmatic event magmatic event magmatic event metamorphism

Age of magmatic intrusion Age of inherited magmatic zircon Age of magmatic intrusion Age of magmatic intrusion Age of later thermal event Age of magmatic intrusion Age of inherited magmatic zircon Age of magmatic intrusion Age of later thermal events Age of inherited magmatic zircon Age of magmatic intrusion Age of magmatic intrusion Age of inherited magmatic zircon Age of magmatic intrusion Age of later thermal event Age of inherited magmatic zircon Age of migmatization Age of later thermal events Age of inherited magmatic zircon Age of magmatic intrusion

61–808 ppm (except for 6.1), respectively, and Th/U ratios between 0.29 and 1.01 (Table 3). 15 of 25 SHRIMP analyses on 21 zircons form a coherent group with a weighted mean 206Pb/238U age of 146 ± 1 Ma (Fig. 6a). The rest of zircon grains, also of magmatic origin, yielded 206 Pb/238Pb ages of ca. 160–150 Ma (n = 5) and ca. 140–135 Ma (n = 5). Zircons from sample PD26-3 are mostly euhedral, about 80– 200 lm long, and have length/width ratios of 1–3. Besides, they exhibit prismatic faces and well-developed growth zoning (Fig. 5e–h). Most zircon grains have moderate Th (51–294 ppm) and U (102–478 ppm) concentrations. Their Th/U ratios vary from 0.46 to 0.89 (Table 3). 11 of 12 analyses on 12 zircons from the gneissic granite samples yielded a concordant age of 141 ± 1 Ma (Fig. 6b).

Spot

Th (ppm)

U

Th/U

Sample PD23-2 (Compound gneiss) Oscillatory-zoned core 1.1 364 931 2.1 568 1170 3.1 636 1187 5.1 341 845 10.1 301 699 Oscillatory-zoned rim 4.1 268 528 6.1 283 616 7.1 169 299 8.1 545 593 9.1 31 1036 11.1 103 192 12.1 152 210 13.1 194 724 Sample PD25-6 (Gneissic granite) Oscillatory-zoned core 1.1 212 441 2.1 240 397 3.1 309 494

Pb/

235

U

Age (Ma) ±r (%)

206

238

Pb/

U

±r (%)

207

206

Pb/

Pb

±r (%)

207

Pb/235U

±r

206

Pb/238U

±r

207

Pb/206Pb

±r

0.76 0.89 1.65 0.28 0.19 0.24 0.36 0.61 0.42 0.13 0.41 0.90 1.29 0.87 0.57 2.04 1.11 1.79 0.53 0.53 0.67 0.38 0.91 0.94 0.38 0.05 0.40 0.29 0.84 0.36

4.0348 0.5446 0.2716 0.2939 4.1559 1.1500 4.7288 7.5783 0.4006 4.5770 4.5996 8.9174 4.7281 0.2228 0.2169 0.8963 0.3239 1.0844 7.9666 0.4987 0.2524 0.2164 0.2315 1.1115 1.1889 0.3087 0.5792 0.5699 0.3532 0.2284

0.0971 0.0127 0.0050 0.0112 0.0767 0.0315 0.0707 0.1186 0.0071 0.0737 0.0644 0.2584 0.0895 0.0083 0.0060 0.0279 0.0069 0.0325 0.1071 0.0132 0.0064 0.0073 0.0055 0.0269 0.0583 0.0087 0.0183 0.0249 0.0099 0.0072

0.2647 0.0677 0.0371 0.0397 0.2708 0.1258 0.3159 0.3770 0.0553 0.2964 0.2964 0.3975 0.3111 0.0317 0.0313 0.1039 0.0453 0.1199 0.3731 0.0657 0.0345 0.0309 0.0326 0.1254 0.1254 0.0407 0.0741 0.0703 0.0497 0.0319

0.0027 0.0006 0.0003 0.0004 0.0023 0.0026 0.0024 0.0041 0.0003 0.0022 0.0017 0.0090 0.0023 0.0003 0.0003 0.0011 0.0003 0.0011 0.0024 0.0005 0.0002 0.0003 0.0003 0.0009 0.0038 0.0005 0.0008 0.0013 0.0004 0.0003

0.1111 0.0584 0.0528 0.0533 0.1111 0.0663 0.1083 0.1453 0.0524 0.1116 0.1123 0.1609 0.1103 0.0510 0.0502 0.0626 0.0518 0.0655 0.1539 0.0548 0.0528 0.0505 0.0514 0.0640 0.0692 0.0545 0.0570 0.0580 0.0514 0.0521

0.0028 0.0014 0.0009 0.0018 0.0020 0.0013 0.0015 0.0018 0.0009 0.0017 0.0016 0.0027 0.0021 0.0019 0.0014 0.0019 0.0011 0.0020 0.0021 0.0014 0.0013 0.0016 0.0012 0.0015 0.0029 0.0012 0.0019 0.0021 0.0015 0.0017

1641 441 244 262 1665 777 1772 2182 342 1745 1749 2330 1772 204 199 650 285 746 2227 411 229 199 211 759 795 273 464 458 307 209

20 8 4 9 15 15 13 14 5 13 12 26 16 7 5 15 5 16 12 9 5 6 5 13 27 7 12 16 7 6

1514 422 235 251 1545 764 1769 2062 347 1673 1674 2157 1746 201 199 637 285 730 2044 410 218 196 207 761 762 257 461 438 313 202

14 3 2 3 12 15 12 19 2 11 8 42 11 2 2 6 2 6 11 3 2 2 2 5 22 3 5 8 2 2

1818 543 320 343 1817 817 1772 2291 302 1826 1839 2465 1806 239 211 694 276 791 2390 467 320 217 257 743 906 391 500 528 257 287

45 47 42 76 33 41 25 26 41 28 25 29 35 85 97 69 47 66 23 59 57 81 56 47 85 52 72 78 65 76

0.39 0.49 0.54 0.40 0.43

0.1681 0.1587 0.1597 0.1549 0.1537

0.0044 0.0038 0.0037 0.0046 0.0041

0.0253 0.0239 0.0234 0.0228 0.0227

0.0002 0.0002 0.0002 0.0002 0.0002

0.0483 0.0479 0.0492 0.0493 0.0493

0.0013 0.0011 0.0011 0.0015 0.0014

158 150 150 146 145

4 3 3 4 4

161 152 149 145 145

1 1 1 1 1

122 95 167 167 167

58 56 52 70 69

0.51 0.46 0.57 0.92 0.03 0.54 0.73 0.27

0.1414 0.1555 0.1441 0.1506 0.1198 0.1562 0.1525 0.1529

0.0051 0.0056 0.0063 0.0054 0.0037 0.0083 0.0077 0.0046

0.0219 0.0219 0.0221 0.0221 0.0188 0.0221 0.0220 0.0220

0.0002 0.0002 0.0003 0.0002 0.0001 0.0003 0.0003 0.0002

0.0469 0.0515 0.0475 0.0495 0.0462 0.0527 0.0513 0.0506

0.0017 0.0019 0.0021 0.0018 0.0014 0.0030 0.0028 0.0016

134 147 137 142 115 147 144 144

5 5 6 5 3 7 7 4

140 140 141 141 120 141 140 140

1 1 2 1 1 2 2 1

43 265 76 172 9 317 254 233

85 85 113 83 74 127 126 72

0.48 0.60 0.63

0.1601 0.1582 0.1590

0.0023 0.0013 0.0013

0.0229 0.0231 0.0232

0.0002 0.0001 0.0002

0.0507 0.0497 0.0498

0.0007 0.0003 0.0003

151 149 150

2 1 1

146 147 148

1 1 1

228 189 187

33 17 19

J. Cui et al. / Journal of Asian Earth Sciences 62 (2013) 237–252

Sample PD02-2 (Paragneiss) Oscillatory-zoned core 1.1 43.1 56.8 2.1 410 463 3.1 4214 2551 4.1 157 558 5.1 41.1 221 6.1 86.9 361 7.1 95.6 265 8.1 369 604 9.1 596 1405 10.1 54.8 435 11.1 140 341 12.1 86.4 95.6 13.1 167 129 14.1 331 381 15.1 418 727 16.1 248 122 17.1 1216 1094 18.1 204 114 19.1 525 989 20.1 227 427 21.1 501 750 22.1 191 496 23.1 920 1011 24.1 176 187 25.1 24.2 64.3 26.1 48.4 891 27.1 66.3 165 28.1 49.3 167 29.1 329 393 30.1 185 517

Isotopic ratios 207

(ppm)

242

Table 2 LA-ICPMS Th–U–Pb analyses of zircons for metamorphic and magmatic rocks from the Changle-Nan’ao zone.

4.1 5.1 6.1 7.1 8.1 9.1 10.1 11.1 12.1 13.1 14.1 15.1 16.1 17.1 18.1 19.1

86 215 129 175 167 195 192 351 411 103 478 257 269 104 42 135

0.45 0.46 0.81 0.52 0.39 0.53 0.34 0.53 0.40 0.36 0.37 0.44 0.45 0.51 0.39 0.36

0.1581 0.1564 0.1577 0.1647 0.1579 0.1588 0.1562 0.1567 0.1587 0.1588 0.1588 0.1591 0.1610 0.1630 0.1710 0.1589

0.0021 0.0015 0.0024 0.0016 0.0016 0.0017 0.0014 0.0014 0.0016 0.0020 0.0012 0.0015 0.0016 0.0023 0.0034 0.0019

0.0232 0.0232 0.0231 0.0232 0.0232 0.0232 0.0231 0.0231 0.0232 0.0231 0.0231 0.0232 0.0231 0.0231 0.0231 0.0231

0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0001 0.0002 0.0002 0.0001 0.0001 0.0002 0.0002 0.0002 0.0002

0.0496 0.0489 0.0496 0.0518 0.0495 0.0498 0.0491 0.0492 0.0497 0.0499 0.0498 0.0498 0.0506 0.0513 0.0538 0.0499

0.0005 0.0003 0.0006 0.0005 0.0003 0.0004 0.0003 0.0003 0.0002 0.0006 0.0002 0.0004 0.0004 0.0006 0.0010 0.0005

149 148 149 155 149 150 147 148 150 150 150 150 152 153 160 150

2 1 2 1 1 1 1 1 1 2 1 1 1 2 3 2

148 148 147 148 148 148 147 147 148 147 147 148 147 147 147 147

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

176 143 176 276 169 183 154 167 189 187 183 183 233 257 361 191

31 19 34 22 17 23 15 17 11 28 11 21 17 28 73 22

0.66 0.32 0.72 0.73 0.75 0.87 0.59 0.51 0.96 0.70 0.56 0.59 0.84 0.52 0.76

0.1432 0.1314 0.1457 0.1418 0.1651 0.1347 0.1412 0.1703 0.1342 0.1447 0.1364 0.1451 0.1375 0.1617 0.1306

0.0041 0.0087 0.0072 0.0080 0.0059 0.0060 0.0035 0.0050 0.0033 0.0064 0.0066 0.0065 0.0119 0.0049 0.0026

0.0209 0.0201 0.0215 0.0213 0.0244 0.0194 0.0212 0.0249 0.0193 0.0214 0.0212 0.0218 0.0207 0.0244 0.0200

0.0002 0.0003 0.0003 0.0003 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0003 0.0003 0.0004 0.0002 0.0001

0.0498 0.0474 0.0492 0.0484 0.0490 0.0504 0.0482 0.0495 0.0506 0.0490 0.0468 0.0484 0.0482 0.0480 0.0474

0.0015 0.0032 0.0025 0.0028 0.0018 0.0023 0.0013 0.0016 0.0013 0.0022 0.0023 0.0023 0.0042 0.0015 0.0010

136 125 138 135 155 128 134 160 128 137 130 138 131 152 125

4 8 6 7 5 5 3 4 3 6 6 6 11 4 2

133 128 137 136 156 124 136 159 123 137 135 139 132 155 127

1 2 2 2 2 1 1 1 1 2 2 2 2 1 1

187 68 158 118 148 213 109 173 221 150 36 118 108 101 70

69 155 115 132 86 104 61 72 60 103 116 107 196 73 51

0.82 0.39 0.64 0.59 0.55 0.42 0.50 0.65 0.97 0.49 0.61 0.65 0.56 0.59 0.55 0.65 0.58 0.59 0.66 0.64 0.62 0.48

0.1419 0.1446 0.1416 0.1278 0.1421 0.1371 0.1391 0.1448 0.1354 0.1366 0.1452 0.1396 0.1426 0.1440 0.1400 0.1507 0.1366 0.1449 0.1405 0.1352 0.1438 0.1550

0.0112 0.0114 0.0063 0.0067 0.0101 0.0066 0.0061 0.0052 0.0086 0.0166 0.0041 0.0103 0.0095 0.0124 0.0087 0.0080 0.0212 0.0116 0.0104 0.0056 0.0147 0.0075

0.0210 0.0216 0.0211 0.0191 0.0211 0.0205 0.0208 0.0216 0.0201 0.0204 0.0210 0.0207 0.0212 0.0214 0.0210 0.0219 0.0205 0.0211 0.0211 0.0199 0.0214 0.0228

0.0005 0.0005 0.0004 0.0004 0.0005 0.0004 0.0004 0.0004 0.0004 0.0006 0.0004 0.0005 0.0005 0.0005 0.0004 0.0004 0.0008 0.0005 0.0005 0.0004 0.0006 0.0004

0.0491 0.0485 0.0489 0.0486 0.0489 0.0485 0.0484 0.0487 0.0489 0.0485 0.0502 0.0488 0.0488 0.0489 0.0485 0.0499 0.0484 0.0498 0.0484 0.0492 0.0488 0.0495

0.0042 0.0041 0.0025 0.0029 0.0038 0.0027 0.0025 0.0022 0.0034 0.0061 0.0019 0.0039 0.0035 0.0045 0.0033 0.0030 0.0077 0.0043 0.0038 0.0024 0.0052 0.0028

135 137 135 122 135 131 132 137 129 130 138 133 135 137 133 143 130 137 134 129 136 146

10 10 6 6 9 6 5 5 8 15 4 9 8 11 8 7 19 10 9 5 13 7

134 138 134 122 134 131 133 138 128 130 134 132 135 136 134 140 131 135 135 127 137 145

3 3 3 2 3 2 2 2 3 4 2 3 3 3 3 3 5 3 3 2 4 3

153 125 141 126 145 123 120 133 143 125 205 140 137 142 122 191 118 186 116 159 136 171

187 188 117 133 171 125 117 101 155 273 86 177 162 201 153 133 339 187 177 112 234 125

granite) 278 174 136 126 155 159 537 262 517 139 121 222 310 226 623

Sample PD14-1 (Leucogranite dyke) Oscillatory-zoned grain 1.1 154 187 2.1 36 92 3.1 103 159 4.1 100 170 5.1 47 85 6.1 102 242 7.1 169 339 8.1 191 293 9.1 160 166 10.1 52 105 11.1 439 725 12.1 62 94 13.1 53 96 14.1 44 75 15.1 87 158 16.1 103 160 17.1 38 65 18.1 52 88 19.1 74 112 20.1 153 238 21.1 54 87 22.1 138 285

243

(continued on next page)

J. Cui et al. / Journal of Asian Earth Sciences 62 (2013) 237–252

Sample PD52-3 (Gneissic Oscillatory-zoned grain 1.1 184 2.1 56 3.1 98 4.1 91 5.1 116 6.1 138 7.1 319 8.1 133 9.1 494 10.1 97 11.1 68 12.1 130 13.1 260 14.1 116 15.1 474

193 467 160 336 432 368 556 664 1029 286 1299 583 604 202 107 374

160 135 76 132 114 106 119 63 54 115 211 124 139 117 122 28 141 111 220 112 121 118 118 138 112 112 124 116 122 121 122 118 116 112 121 116 114 113 8 9 8 10 8 9 7 9 10 12 9 11 8 13 8 14 10 9 9

3 3 3 3 3 3 2 3 3 3 3 3 3 3 2 4 3 3 3

Pb/206Pb 207

±r 206

±r

Pb/238U

168 184 170 204 159 199 154 193 226 241 176 221 167 263 176 289 214 191 191

J. Cui et al. / Journal of Asian Earth Sciences 62 (2013) 237–252

±r

244

Sample PD34-7 is a two-mica granite with gneissic texture. 19 analyses form a coherent group with a weighted mean 206 Pb/238U age of 137 ± 1 Ma (Fig. 6c), which is interpreted as the emplacement age. The CL images and SHRIMP results are presented in Figs. 5i–l and 6c and the analytical data are listed in Table 3. Two other gneissic granite samples (PD25-6 and PD52-3) were analyzed using the LA-ICPMS technique. Most zircon grains are of magmatic origin, as evidenced from the oscillatory/zoning (Fig. 3e–h) and Th/U ratios of 0.32–0.96 (Table 2). A few grains also have inherited cores and/or very thin rims (<5 lm). The analyses yielded ages from 147–136 Ma (Fig. 4c and d), which are interpreted as the period of magmatic emplacement. A few slightly older inherited magmatic zircon grains were found in three samples (PD30-1, PD34-7, and PD52-3). These zircons gave ages of 165 Ma, 160 Ma, 155 Ma, and 150 Ma. Their CL images are show in Figs. 3 and 5 and the age data are given in Tables 1–3 and further illustrated in Figs. 4 and 6.

114 122 116 118 137 112 113 121 114 122 125 122 119 116 112 116 117 114 118 0.0037 0.0041 0.0036 0.0045 0.0034 0.0043 0.0033 0.0041 0.0048 0.0053 0.0040 0.0049 0.0037 0.0059 0.0038 0.0061 0.0048 0.0042 0.0044 0.0493 0.0487 0.0475 0.0487 0.0483 0.0481 0.0484 0.0473 0.0471 0.0483 0.0503 0.0485 0.0488 0.0484 0.0485 0.0466 0.0489 0.0482 0.0506 0.0004 0.0004 0.0004 0.0005 0.0005 0.0004 0.0004 0.0005 0.0005 0.0005 0.0004 0.0005 0.0004 0.0006 0.0004 0.0006 0.0005 0.0004 0.0004 0.0175 0.0190 0.0185 0.0184 0.0217 0.0176 0.0176 0.0193 0.0182 0.0191 0.0189 0.0191 0.0185 0.0181 0.0174 0.0189 0.0182 0.0179 0.0176 0.0083 0.0099 0.0084 0.0108 0.0094 0.0099 0.0074 0.0101 0.0114 0.0134 0.0098 0.0122 0.0086 0.0140 0.0085 0.0154 0.0113 0.0096 0.0101

207

Sample PD05-1 (Microgranite dyke) Oscillatory-zoned grain 1.1 132 148 2.1 111 119 3.1 105 115 4.1 83 82 5.1 87 167 6.1 70 85 7.1 109 134 8.1 84 98 9.1 56 75 10.1 133 125 11.1 149 132 12.1 94 106 13.1 116 113 14.1 123 102 15.1 201 182 16.1 58 72 17.1 142 127 18.1 114 125 19.1 54 74

(ppm)

0.89 0.93 0.91 1.01 0.52 0.82 0.82 0.86 0.74 1.06 1.13 0.89 1.03 1.21 1.10 0.80 1.12 0.91 0.74

0.1188 0.1275 0.1215 0.1237 0.1445 0.1167 0.1173 0.1261 0.1184 0.1271 0.1312 0.1278 0.1247 0.1207 0.1165 0.1213 0.1225 0.1191 0.1228

Age (Ma) U Th

(ppm)

Spot

Table 2 (continued)

Th/U

Pb/235U

±r (%)

206

Pb/238U

±r (%)

207

Pb/206Pb

±r (%) Isotopic ratios

207

Pb/235U

5.3. Granitoids Sample PD14-2 came from a fine-grained garnet-bearing granite pluton without deformation. Most zircon grains are euhedral, transparent, 150–300 lm long, and have length/width ratios of 2 to 4 (Fig. 5m–p). Concentrations of U range from 63 to 704 ppm, and Th from 35 to 329 ppm. Th/U ratios vary from 0.33 to 0.95 (Table 3). Among the 21 spot analyses on 19 zircons, 19 data-points form a coherent group with a weighted mean 206Pb/238U age of 132 ± 1 Ma (Fig. 6d), which is interpreted as the emplacement age of the granite. Two weakly deformed granite samples were analyzed: (1) most zircon grains from sample PD51-1 are subhedral to euhedral, 100–200 lm long with length/width ratios of 1–3. They show a simple zoning pattern with an osillatery-zoned core and a grey planary rim (Fig. 5q–t). The measured U and Th concentrations range from 72 to 816 ppm and from 70 to 453 ppm, respectively. The Th/U ratios vary between 0.2 and 1.31 (Table 3). The isotopic analyses yielded a weighted mean 206Pb/238U age of 124 ± 1 Ma (Fig. 6e). (2) Most zircons grains from sample PD32-3 are euhedral, prismatic, 100–250 lm long, and have length/width ratios of 1.5–3. They show magmatic oscillatory zonings in CL image (Fig. 5u–x). Seventeen analyses on 17 zircons yielded quite variable U (112–1152 ppm) and Th (79–1319 ppm) concentrations and Th/U ratios (0.65–1.53). The bulk of the analyses (n = 12) is concordant or nearly so, yielding a weighted mean age of 120 ± 1 Ma (Fig. 6f). In addition, a leucogranite dyke sample (PD14-1) was analyzed using the LA-ICPMS technique. Zircons from sample PD14-1, a garnet-bearing leucogranite, are mostly euhedral, 130–300 lm long, and have length/width ratios of 1.5–3. Most grains are transparent and have low to medium concentrations of Th (36– 439 ppm) and U (65–725 ppm). Th/U ratios vary between 0.39 and 0.97, but most of them cluster about 0.5–0.65. For 16 of 22 analyses, 206Pb/238U ratios agree internally to within analytical precision (Fig. 3i and j). The mean 206Pb/238U age of 135 ± 1 Ma is interpreted as crystallization age of the granite (Fig. 4e, Tables 1 and 2). To further constrain the time-span of the Cretaceous plutonism and deformation, one undeformed granitic dyke (PD05-1) was dated. The dyke is intrusive to Late Mesozoic sedimentary rocks. It was dated at 117 Ma (Fig. 4f). The dating results also indicate that the younger (undeformed or weakly deformed) granites often contain some inherited zircons from the older deformed rocks (Fig. 4e and f). The oscillatory-zoned zircon domains of the four magmatic rocks (PD14-2, PD51-1, PD32-3, and PD05-1) from the coastal zone show four ages of 132–117 Ma.

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J. Cui et al. / Journal of Asian Earth Sciences 62 (2013) 237–252 Table 3 SHRIMP Th–U–Pb analyses of zircons for Gneissic granite and granitoids from the Changle-Nanao zone. Spot

U (ppm)

Th (ppm)

Th/ U

f206

Isotopic ratios

(%)

207

0.00 0.00 0.12 0.00 0.18 0.02 0.00 0.00 0.00 0.09 0.04 0.23 0.26 0.00 0.00 4.98 0.09 0.43 1.19 0.83 0.00 0.00 0.00 0.00 0.00

0.45 0.24 0.27 0.73 0.95 1.26 0.63 0.18 0.13 0.15 1.33

±r (%)

0.1769 0.1610 0.1607 0.1715 0.1602 0.1653 0.1631 0.1798 0.1695 0.1550 0.1596 0.1830 0.1523 0.1705 0.1810 0.2040 0.1495 0.1483 0.1380 0.1484 0.1801 0.1707 0.1687 0.1863 0.1950

0.0050 0.0042 0.0056 0.0053 0.0069 0.0053 0.0075 0.0031 0.0034 0.0033 0.0024 0.0183 0.0082 0.0043 0.0065 0.0571 0.0040 0.0096 0.0128 0.0061 0.0090 0.0031 0.0040 0.0084 0.0064

0.1410 0.1509 0.1544 0.1362 0.1357 0.1320 0.1350 0.1524 0.1509 0.1540 0.1180

Sample PD30-1 (Gneissic Oscillatory-zoned grain 1.1 420 286 1.2 472 197 2.1 673 364 2.2 454 239 3.1 717 268 4.1 881 566 5.1 1016 435 6.1 2236 1210 6.2 1152 359 7.1 1196 520 7.2 2817 783 8.1 594 288 9.1 609 252 10.1 521 196 11.1 577 291 12.1 145 61 13.1 747 553 14.1 465 251 15.1 294 162 16.1 325 205 17.1 322 316 18.1 958 808 19.1 1239 585 20.1 238 81 21.1 197 70

granite)

Sample PD26-3 (Gneissic Oscillatory-zoned core 1.1 179 138 3.1 411 196 4.1 288 134 5.1 208 162 6.1 208 172 7.1 102 51 8.1 312 161 9.1 217 187 10.1 478 294 11.1 242 108 12.1 179 110

granite)

Sample PD34-7 (Gneissic Oscillatory-zoned core 1.1 109 103 2.1 131 112 2.2 225 233 3.1 305 140 4.1 226 197 5.1 244 135 6.1 183 60 7.1 465 260 8.1 492 295 9.1 277 158 10.1 462 182 11.1 152 139 12.1 307 149 13.1 755 381 14.1 280 267 15.1 266 278 16.1 550 715 17.1 1960 1024 17.2 358 245 18.2 538 278

two-mica granite)

0.70 0.43 0.56 0.54 0.39 0.66 0.44 0.56 0.32 0.45 0.29 0.50 0.43 0.39 0.52 0.44 0.77 0.56 0.57 0.65 1.01 0.87 0.49 0.35 0.37

0.80 0.49 0.48 0.81 0.85 0.52 0.53 0.89 0.64 0.46 0.63

0.97 0.89 1.07 0.47 0.90 0.57 0.34 0.58 0.62 0.59 0.41 0.94 0.50 0.52 0.98 1.08 1.34 0.54 0.71 0.53

Ages (Ma) 206

Pb/

235

U

238

±r (%)

207

0.0232 0.0227 0.0227 0.0229 0.0228 0.0229 0.0227 0.0252 0.0240 0.0224 0.0232 0.0220 0.0225 0.0232 0.0228 0.0233 0.0214 0.0221 0.0215 0.0215 0.0229 0.0231 0.0242 0.0238 0.0225

0.0003 0.0002 0.0003 0.0003 0.0003 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0003 0.0002 0.0003 0.0003 0.0006 0.0002 0.0003 0.0003 0.0003 0.0003 0.0002 0.0002 0.0003 0.0003

0.0155 0.0041 0.0085 0.0083 0.0079 0.0124 0.0122 0.0085 0.0047 0.0131 0.0142

0.0214 0.0220 0.0215 0.0224 0.0221 0.0224 0.0221 0.0222 0.0224 0.0224 0.0221

Pb/

U

206

±r (%)

206

0.0552 0.0514 0.0512 0.0544 0.0509 0.0525 0.0521 0.0517 0.0512 0.0502 0.0500 0.0605 0.0492 0.0532 0.0575 0.0640 0.0507 0.0487 0.0466 0.0501 0.0571 0.0535 0.0507 0.0569 0.0628

0.0014 0.0014 0.0014 0.0014 0.0014 0.0014 0.0014 0.0014 0.0014 0.0014 0.0014 0.0014 0.0014 0.0014 0.0014 0.0014 0.0014 0.0014 0.0014 0.0014 0.0014 0.0014 0.0014 0.0014 0.0014

0.0004 0.0004 0.0003 0.0004 0.0004 0.0004 0.0004 0.0004 0.0004 0.0004 0.0004

0.0476 0.0498 0.0521 0.0440 0.0445 0.0428 0.0444 0.0498 0.0489 0.0500 0.0387

Pb/

Pb

Pb/238U

±r

208

Pb/232Th

±r

207

Pb/206Pb

148 145 145 146 146 146 145 161 153 143 148 140 143 148 145 148 136 141 137 137 146 147 154 151 144

2 2 2 2 2 2 2 2 2 1 1 2 2 2 2 4 1 2 2 2 2 2 2 2 2

155 148 145 155 144 145 147 159 157 145 150 157 139 159 157 198 136 135 138 134 154 144 161 173 194

4 4 4 4 7 3 7 3 4 3 3 6 7 4 5 50 3 7 9 4 5 2 4 9 8

421 261 252 386 235 306 289 271 248 204 194 620 157 338 512 727 226 134 28 201 494 350 226 487 700

57 54 76 66 96 69 100 31 40 42 27 220 120 50 76 600 58 150 220 92 110 34 51 95 63

0.0048 0.0048 0.0048 0.0048 0.0048 0.0048 0.0048 0.0048 0.0048 0.0048 0.0048

137 140 137 143 141 143 141 142 143 143 141

3 2 2 3 2 3 2 2 2 2 3

132 134 146 133 133 115 132 141 142 144 123

8 4 6 5 5 7 9 5 4 11 9

81 188 288 (110) (84) (178) (89) 185 145 196 (436)

240 51 120 140 140 230 220 130 64 190 320

±r

0.00 1.48 0.48 0.52 0.30 0.00 0.00 0.00 0.91 0.00 2.30 0.00 0.00 0.04 0.00 0.00 0.08 0.01 0.11 0.43

0.2230 0.1300 0.1710 0.1424 0.1570 0.1705 0.1990 0.1633 0.1200 0.1452 0.1560 0.1822 0.1426 0.1391 0.1640 0.1469 0.1425 0.1494 0.1482 0.1342

0.0468 0.0182 0.0123 0.0063 0.0220 0.0066 0.0199 0.0049 0.0108 0.0058 0.0172 0.0084 0.0051 0.0050 0.0131 0.0059 0.0074 0.0033 0.0068 0.0063

0.0217 0.0215 0.0214 0.0212 0.0219 0.0213 0.0225 0.0214 0.0206 0.0209 0.0196 0.0219 0.0215 0.0213 0.0210 0.0213 0.0214 0.0216 0.0216 0.0211

0.0006 0.0005 0.0004 0.0004 0.0004 0.0004 0.0005 0.0003 0.0004 0.0004 0.0004 0.0004 0.0004 0.0003 0.0004 0.0004 0.0003 0.0003 0.0004 0.0003

0.0750 0.0438 0.0580 0.0487 0.0521 0.0580 0.0640 0.0553 0.0421 0.0503 0.0579 0.0604 0.0481 0.0473 0.0567 0.0499 0.0482 0.0502 0.0497 0.0461

0.0158 0.0158 0.0158 0.0158 0.0158 0.0158 0.0158 0.0158 0.0158 0.0158 0.0158 0.0158 0.0158 0.0158 0.0158 0.0158 0.0158 0.0158 0.0158 0.0158

138 137 137 135 140 136 144 137 132 134 125 140 137 136 134 136 137 138 138 135

4 3 3 2 3 2 3 2 2 2 2 3 2 2 2 2 2 2 2 2

167 125 140 132 140 143 196 148 117 132 162 148 139 132 140 130 134 133 136 126

21 9 6 5 10 6 24 5 7 5 17 6 5 4 6 4 3 3 5 5

1062 (123) 529 131 290 529 742 423 (221) 208 527 619 104 67 479 191 111 203 181 2

420 330 150 95 320 75 210 56 220 82 250 89 76 78 170 84 120 37 100 110

Sample PD14-2 (Fine-grained granite) Oscillatory-zoned grain 1.1 121 73 0.62 1.37 2.1 77 44 0.58 3.81 3.1 630 329 0.54 0.08 4.1 452 250 0.57 0.71 5.1 403 226 0.58 0.00 6.1 194 89 0.47 0.00 6.2 82 46 0.57 2.15

0.1420 0.1020 0.1518 0.1288 0.1568 0.1582 0.1190

0.0213 0.0459 0.0058 0.0057 0.0064 0.0065 0.0167

0.0203 0.0206 0.0220 0.0214 0.0212 0.0210 0.0201

0.0004 0.0007 0.0004 0.0004 0.0003 0.0004 0.0005

0.0507 0.0360 0.0501 0.0436 0.0536 0.0547 0.0431

0.0076 0.0076 0.0076 0.0076 0.0076 0.0076 0.0076

129 131 140 137 135 134 128

3 5 2 2 2 2 3

128 109 135 122 141 145 122

14 30 7 4 5 6 11

226 (640) 198 (131) 354 398 (163)

350 1200 80 100 85 83 340

(continued on next page)

246

J. Cui et al. / Journal of Asian Earth Sciences 62 (2013) 237–252

Table 3 (continued) Spot

Th/ U

f206

Isotopic ratios

(%)

207

±r (%)

206

0.51 0.75 0.55 0.75 0.94 0.33 0.84 0.92 0.72 0.95 0.54 0.89 0.78 0.39

0.00 1.28 0.00 1.21 0.00 0.18 0.00 1.08 0.35 0.00 2.96 1.10 0.00 25.87

0.1850 0.1380 0.2000 0.1370 0.1690 0.1362 0.1870 0.1340 0.1340 0.1628 0.0770 0.1740 0.1608 0.1500

0.0222 0.0128 0.0300 0.0219 0.0220 0.0060 0.0580 0.0228 0.0133 0.0080 0.0200 0.0103 0.0071 0.1185

0.0860 0.0780 0.1308 0.1448 0.1380 0.1730 0.1550 0.1440 0.1511 0.1483 0.1250 0.1580 0.1698 0.1473

Sample PD32-3 (Weakly deformed granite) Oscillatory-zoned grain 1.1 1152 1319 1.18 0.38 0.1292 2.1 112 79 0.73 0.00 0.2260 3.1 357 226 0.65 0.31 0.1310 4.1 867 1174 1.40 0.08 0.1355 5.1 221 293 1.37 0.00 0.1717 6.1 835 983 1.22 0.15 0.1370 7.1 221 295 1.38 0.69 0.1291 8.1 166 206 1.28 0.48 0.1370 9.1 241 283 1.21 0.22 0.1362 10.1 116 133 1.19 3.50 0.0760 11.1 237 351 1.53 1.50 0.1145 12.1 140 155 1.14 0.00 0.1670 13.1 558 499 0.92 0.00 0.1357 14.1 173 123 0.73 0.00 0.1551 15.1 289 339 1.21 0.00 0.1590 16.1 711 850 1.24 0.59 0.1159 17.1 327 412 1.30 0.61 0.1220

7.1 8.1 8.2 9.1 10.1 11.1 12.1 13.1 14.1 15.1 16.1 17.1 18.1 19.1

U

Th

(ppm)

(ppm)

150 190 65 63 198 704 96 145 229 144 155 150 218 254

74 138 35 45 180 225 78 130 161 132 81 130 165 96

Sample PD51-1 (Weakly Oscillatory-zoned core 1.1 72 70 2.1 93 98 3.1 431 250 4.1 786 152 5.1 816 239 6.1 203 196 7.1 368 419 8.1 468 216 9.1 623 288 10.1 267 228 11.1 753 340 13.1 266 178 14.1 310 394 15.1 449 453

Pb/235U

Ages (Ma) Pb/238U

±r (%)

207

0.0210 0.0211 0.0218 0.0205 0.0205 0.0206 0.0216 0.0208 0.0205 0.0202 0.0199 0.0209 0.0204 0.0204

0.0004 0.0004 0.0006 0.0005 0.0004 0.0003 0.0007 0.0005 0.0004 0.0004 0.0004 0.0004 0.0004 0.0011

0.0344 0.0413 0.0097 0.0061 0.0086 0.0152 0.0095 0.0128 0.0091 0.0061 0.0074 0.0111 0.0044 0.0062

0.0192 0.0192 0.0194 0.0198 0.0190 0.0203 0.0196 0.0196 0.0188 0.0195 0.0193 0.0194 0.0199 0.0198

0.0067 0.0362 0.0118 0.0033 0.0053 0.0030 0.0065 0.0164 0.0079 0.0342 0.0071 0.0127 0.0038 0.0057 0.0132 0.0071 0.0122

0.0197 0.0197 0.0189 0.0194 0.0189 0.0196 0.0189 0.0185 0.0191 0.0185 0.0187 0.0191 0.0182 0.0189 0.0187 0.0182 0.0182

Pb/206Pb

±r (%)

206

0.0639 0.0475 0.0670 0.0487 0.0598 0.0479 0.0630 0.0466 0.0473 0.0584 0.0282 0.0601 0.0571 0.0530

0.0076 0.0076 0.0076 0.0076 0.0076 0.0076 0.0076 0.0076 0.0076 0.0076 0.0076 0.0076 0.0076 0.0076

0.0004 0.0005 0.0002 0.0002 0.0002 0.0003 0.0002 0.0003 0.0002 0.0003 0.0002 0.0002 0.0002 0.0003

0.0320 0.0290 0.0489 0.0529 0.0526 0.0616 0.0575 0.0533 0.0583 0.0552 0.0471 0.0591 0.0618 0.0539

0.0002 0.0007 0.0002 0.0002 0.0002 0.0002 0.0004 0.0003 0.0004 0.0004 0.0002 0.0003 0.0002 0.0002 0.0002 0.0003 0.0002

0.0476 0.0830 0.0503 0.0506 0.0658 0.0508 0.0496 0.0536 0.0518 0.0300 0.0443 0.0632 0.0540 0.0596 0.0617 0.0461 0.0485

Pb/238U

±r

208

134 135 139 131 131 132 138 133 131 129 127 134 130 130

3 2 4 3 3 2 5 3 2 3 3 3 3 7

0.0128 0.0128 0.0128 0.0128 0.0128 0.0128 0.0128 0.0128 0.0128 0.0128 0.0128 0.0128 0.0128 0.0128

125 126 124 126 121 128 124 124 119 123 123 122 125 126

0.0024 0.0024 0.0024 0.0024 0.0024 0.0024 0.0024 0.0024 0.0024 0.0024 0.0024 0.0024 0.0024 0.0024 0.0024 0.0024 0.0024

126 126 121 124 121 125 121 118 122 118 120 122 117 121 120 116 116

Pb/232Th

±r

207

Pb/206Pb

±r

158 124 176 136 137 128 157 124 132 142 88 138 135 131

17 8 23 13 10 7 27 10 8 8 14 6 5 130

740 76 830 132 597 96 701 28 65 547 (1349) 608 497 320

240 220 320 360 270 97 650 400 230 97 830 120 86 1800

2 2 1 1 1 2 1 2 1 2 1 1 1 2

102 106 123 137 129 142 130 131 134 136 119 139 126 131

13 15 6 11 9 7 4 10 8 4 6 7 3 3

(930) (1220) 143 326 310 662 510 340 541 420 55 571 666 366

1200 1600 170 94 140 190 130 200 130 84 140 150 51 91

1 5 1 1 1 1 2 2 2 3 1 2 1 2 1 2 1

123 166 124 121 129 124 118 122 124 104 111 131 120 138 127 110 120

3 21 7 2 3 2 4 6 4 11 3 5 2 7 5 4 4

80 1275 209 222 801 230 175 354 275 (1180) (92) 716 370 587 663 4 121

120 300 210 52 60 47 110 260 120 1400 150 160 60 76 180 140 240

deformed granite) 1.02 1.09 0.60 0.20 0.30 1.00 1.18 0.48 0.48 0.88 0.47 0.69 1.31 1.04

3.68 3.55 0.34 0.00 0.14 0.00 0.00 0.00 0.00 0.00 0.41 0.00 0.00 0.00

f206 (%) represents the proportion of common

206

Pb in total

206

Pb measured. Common Pb was corrected using the measured

204

Pb. All uncertainties are 1r.

6. Discussion

6.1. Extension/rifting in early Jurassic (ca. 200–170 Ma)

A paragneiss sample (PD02-2) from the Changle-Nan’ao zone contains magmatic zircons with U–Pb ages of 1823 Ma, 764–730 Ma, 637 Ma, 461–410 Ma, 347–313 Ma, 285 Ma, 251–207 Ma and 202– 196 Ma. The Neoproterozoic to Early Jurassic igneous events have been well-documented for the entire SE China. In fact, such a lengthy evolution seems to support that the magmatism in the coastal zone started in early Neo-proterozoic time (Grenville orogey). The abundant Triassic magmatic zircons indicate that the clastic deposits in the T3–J sedimentary rocks could have come from an Indosinian orogen (Fig. 7). The predominant Jurassic to Cretaceous rocks and structures (>90%) suggest that Late Mesozoic geological evolution is important for the formation of the coastal zone. As a result, the new zircon U–Pb ages from the paragneiss and other rock types actually provide strong arguments for an extended geological process, especially for the Late Mesozoic evolution in the coastal zone.

The presence of Late Triassic to Early Jurassic magmatic zircons (ca. 202–196 Ma) in the paragneiss (sample PD02-2) from the Pingtan Island indicates that the protoliths of the metamorphic rocks are T3–J1 sedimentary rocks in the coastal zone. The rock and mineral assemblages of the metasedimentary rocks show that the T3–J1 sediments probably formed in pull-apart basin(s). The available age data suggest that the Jurassic extension/riftingrelated basins and igneous rocks (including alkaline basalts, bimodal volcanic rocks and I- and A-type granites) were formed in the period of ca. 200–170 Ma in SE China (Zhu et al., 2010; He et al., 2010). The idea of early Jurassic extension (ca. 200–170 Ma) seems to be supported by the geochemical and high-precision age data of igneous rocks, as well as structural and sedimentation analyses (Li and Li, 2007; Zhang et al., 2009; Zhu et al., 2010; He et al., 2010).

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Fig. 3. Cathodoluminescence (CL) images of zircons from metamorphic rocks and granitoids from the Changle-Nan’ao zone. (a and b) Oscillatory-zoned zircons from paragneiss sample PD02-2 showing a grey luminescent core and a thin slight bright luminescent rim, for ex., 14.1 of (a) PD02-2. (c and d) Oscillatory-zoned zircons from migmatitic gneiss sample PD23-2 showing a slight bright luminescent core and a grey luminescent rim, for ex., 1.1 of (c) PD23-2. (e and f) Oscillatory-zoned zircons from gneissic granite sample PD25-6 showing a slight bright luminescent core and a grey luminescent rim, for ex., 10.1 of (e) PD25-6. (g and h) Oscillatory-zoned zircons from gneissic granite sample PD52-3, some of which show a slight bright luminescent core and a thin grey luminescent rim, for ex., 11.1 of (g) PD52-3. (i and j) Oscillatory-zoned zircons from garnet-bearing leucosome sample PD14-1, for ex., 8.1 of (i) PD14-1. (k and l) Oscillatory-zoned zircons from micro-granite dyke sample PD05-1, for ex., 15.1 of (l) PD05-1. Ages are given at 1r (see Table 2). Scale bars are 50 lm.

6.2. A late Jurassic to early Cretaceous orogeny (ca. 165–135 Ma) In the coastal Changle-Nan’ao zone, the Dakeng Formation (T3) and Lishan Formation (J1) are unconformably overlain by the Nanyuan Formation (K1) (BGMR, 1985; Tong and Tobisch, 1996; Xing et al., 2008; He and Xu, in press) or occur as enclaves in the gneissic granites of 147–136 Ma. Some characteristic minerals in the metasedimentary rocks (PD02-2), such as amphibolite, garnet and sillimanite, show that the T3–J1 continental sediments have been subjected to the amphibolite facies metamorphism. The oscillatory zoning of zircons readily suggests that the ages of 147–136 Ma obtained for the gneissic granites (PD301, PD26-3, PD34-7, PD25-6, and PD52-3) can be interpreted as time of early Cretaceous magmatism. However, many inherited zircons with late Jurassic ages (ca. 165–150 Ma) have also been identified in the early Cretaceous gneissic granites (e.g. PD30-1, PD34-7, PD52-3). This suggests that some Jurassic granitoids had been reworked during the early Cretaceous magma generation. In fact, this is supported by the Jurassic age data of the volcanic activities (ca. 165–150 Ma) in other coastal areas of SE China (e.g. Davis et al., 1997; Xing et al., 2008; Mei et al., 2011; Sewell et al., 2012). In summary, the late Jurassic magmatic episode (165–150 Ma), as earlier identified in the hinterland of SE China (Li, 2000; Hsieh et al., 2008), has also affected the coastal zone.

We like to underline that geological significance of the new zircon U–Pb ages for the gneissic granites is not always clear cut. Sample PD30-1 serves as an example. The age of 146 Ma (n = 15) obtained for the gneissic granites from the Dongshan Island may be interpreted in two ways: (1) It represents the emplacement time of the gneissic granites. The older zircon grains of ca. 160– 150 Ma (n = 5) could be inherited from Jurassic igneous rocks (protolith?), while the younger zircon grains of ca. 140–135 Ma (n = 5) were formed during the early Cretaceous magma generation. (2) The gneissic granites were emplaced at ca. 160–150 Ma, and all the younger ages (146 Ma and 140–135 Ma) were due to isotopic reset and Pb-loss in the early Cretaceous thermal events. The facts that the penetrative migmatization in sample PD30-1 cannot be observed in other gneissic granite samples (PD26-3, PD34-7, PD25-6, and PD52-3) and the little deformed granitoids (132–117 Ma) indicate that the second interpretation is more probable. Considering the age information from the early Jurassic extension (ca. 200–170 Ma), the emplacement of late Jurassic to early Cretaceous gneissic granites (ca. 165–150 Ma and 147–136 Ma) and the little deformed fine-grained granitoids (132–117 Ma), the deformation and metamorphic events for the T3–J1 sedimentary rocks (Fig. 2A) and migmatitic gneisses (Fig. 2B) can be bracketed between 170 Ma and 132 Ma. Consequently, a Late Mesozoic orogeny during ca. 165–135 Ma is established in the coastal zone of SE China

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Fig. 4. LA-ICPMS zircon U–Pb concordia diagrams for metamorphic rocks and granitoids from the coastal Changle-Nan’ao zone. (a) Detrital zircons from sample PD02-2; (b) the age shown is the average of 7 data points (sample PD23-2), excluding 6 data (the dotted circle); (c) the age shown is the average of 19 data points (sample PD25-6); (d) the age shown is the average of 6 data points (sample PD52-3), excluding 9 data (the dotted circle); The younger ages (n = 6) could be due to late Pb loss. The older ages (n = 3) are interpreted as inheritance; (e) the age shown is the average of 16 data points (sample PD14-1), excluding 6 data (the dotted circle). The older age (n = 1) is interpreted as inheritance. The younger ages (n = 5) could be due to late Pb loss; (f) the age shown is the average 18 data points (sample PD05-1), excluding 1 data that is interpreted as xenocryst (the dotted circle). The wide age range is possibly due to a combination of factors including inheritance, magmatic crystallization, and metamorphic overgrowths.

(Fig. 8). Note that the time of this orogeny coincided with that of migmatization in the coastal zone (PD23-2, PD30-1 and PD14-1). Geochemical and isotopic data indicate that most magmatic rocks from the coastal zone are I-type granites, but are probably derived by remelting of Proterozoic rocks with little oceanic plate component (our unpublished data). Typical arc-related andesite and diorite are scarce in the coastal zone, hence the generation of the Late Mesozoic igneous rocks could not be directly linked to the andesite-type magmatism and volcanism. Considering the coastal orogeny in Late Mesozoic, we suggest here that the ca. 165–135 Ma igneous rocks were formed in syn-orogenic settings. Using geochemical and zircon U–Pb age data, Li et al. (2007) proposed that the late Jurassic Nankunshan A1-type granite (ca.

160 Ma) in Guangdong was emplaced in an extensional tectonic setting resulting from the mantle upwelling. However, the tectonic scenario is opposite to most other geological arguments for the Late Mesozoic tectonic evolution (Jahn et al., 1976; BGMR, 1985; Isozaki, 1997; Chen et al., 2004; Chen et al., 2008a, 2008b; Xing et al., 2008; Zhang et al., 2009). In fact, most zircon grains from the Nankunshan granite are dark and opaque due to high U concentrations; only a few zircons with low U contents gave two 206 Pb/238Pb age groups of ca. 200 Ma (n = 2) and 179–149 Ma (n = 12) (Li et al., 2007). Moreover, Late Mesozoic Gaojiping Group (ca. 165–130 Ma) and Fogang batholith (6000 km2) were intruded by the Nankunshan granite (Liu et al., 2005; Li et al., 2007; Hsieh et al., 2008; Mei et al., 2011). The Gaojiping and Fogang granitoids

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Fig. 5. Cathodoluminescence (CL) images of zircons from gneissic granites and fine-grained granitoids from the Changle-Nan’ao zone. (a–d) Oscillatory zircon grains from sample PD30-1. (e–h) Oscillatory-zoned zircons from the gneissic granite sample PD26-3, some of which have a very thin and discontinued dark luminescent overgrowth rim. (i–l) Oscillatory-zoned zircons from the gneissic two-mica granite sample PD34-7, a few of which have a very thin and discontinued dark luminescent overgrowth rim. (m–p) Oscillatory-zoned zircons from fine-grained granite sample PD14-2. (q–t) Oscillatory-zoned zircons from the weakly deformed granite sample PD51-1, most of which have a very thin and discontinued dark luminescent overgrowth rim. (u–x) Oscillatory-zoned zircons from the weakly deformed granite sample PD32-3, a few of which have a very thin and discontinued dark luminescent overgrowth rim. Ages are given at 1r (see Table 3). Scale bars are 20 lm.

are known to include S-, I-, and A-types based on mineralogical, whole-rock geochemical and Sr–Nd isotopic arguments (Li et al., 2007 and references therein; Hsieh et al., 2008). The emplacement time of the Nankunshan pluton was determined at ca. 160–110 Ma by the Rb–Sr isochron method (Liu et al., 2003; Li et al., 2007). Therefore, all the zircon grains with ages of ca. 200 Ma and 179– 149 Ma from the Nankunshan pluton (sample 2KG1.3) could be inherited zircons. If so, the late Mesozoic magma series in this part

of SE China probably erupted and intruded in different tectonic regimes and the A-type granite in Nankunshan might be emplaced in late- to post-orogenic stages (ca. 140–117 Ma?). 6.3. Mechanism for the Late Mesozoic magmatism Though controversial, most authors proposed that East Asia had an andean-type continental margin along the southeastern coast of

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Fig. 6. SHRIMP zircon U–Pb concordia diagrams for the gneissic granites and fine-grained granitoids from the coastal Changle-Nan’ao zone. (a) The age shown is the average of 15 data points (sample PD30-1), excluding 10 data (the dotted circle); (b) the age shown is the average of 11 data points (sample PD26-3); (c) the age shown is the average of 19 data points (sample PD34-7), excluding 1 data (the dotted circle); (d) the age shown is the average of 19 data points (sample PD14-2), excluding 2 data (the dotted circle); (e) the age shown is the average of 12 data points (sample PD51-1), excluding 2 data (the dotted circle); (f) the age shown is the average 12 data points (sample PD323), excluding 5 data (the dotted circle). The wide age range is possibly due to a combination of factors including inheritance, magmatic crystallization, and metamorphic overgrowths.

the continent in Late Mesozoic (Jahn, 1974; Jahn et al., 1976, 1990; Isozaki, 1997; Lapierre et al., 1997; Li and Li, 2007; Chen et al., 2008a; Zhang et al., 2009). The identification of the ca. 165– 135 Ma orogeny in the coastal Changle-Nan’ao zone has contributed significantly to our understanding of the mechanism for the Late Mesozoic magmatism in the continent. Forces from the oceanic plate (for example, oceanic plateau(s) related collision or low-angle subduction) can be quickly transmitted into interiors of the continent, where they are subsequently deflected around fold belts and focused toward cratons, causing severe intraplate deformation and orogeny, such as the Early Yanshan orogeny in

eastern China and Daebo orogeny in Korean peninsula (Dong et al., 2008; Zhang et al., 2009; Lim and Cho, 2011). Consequently, the periodic Late Mesozoic magmatism, resulted from crustal thickening and thinning, in the continent probably occurred in respond to the interaction between the continental margin of Eurasia and the paleo-Pacific plate. Based on the new geochronological data presented in this paper, we use the ca. 165–135 Ma orogeny model to explain the Late Mesozoic evolution of the exceptionally-wide magmatic belt in the continent (Fig. 8). A late Jurassic crustal contraction/thickening could cause the folding and metamorphism in the T3–J1

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251

et al., 2009) indicate crustal thinning/extension in late- to postorogenic tectonic settings. 7. Conclusions The present zircon U–Pb geochronological study leads to the following conclusions:

Fig. 7. Age probability diagram for the inherited magmatic zircons from the paragneiss (PD02-2) in the coastal Changle-Nan’ao zone.

Fig. 8. Age pattern of magmatic and metamorphic rocks from the coastal ChangleNan’ao zone, illustrating that a change of tectonic setting from the Late Mesozoic orogeny to post-orogenic extension occurred at ca. 135 Ma in the coastal areas of SE China.

sediments, the magmatic pulse in ca. 165–150 Ma, and the regional unconformity underneath the Lower Cretaceous igneous rocks (ca. 145–117 Ma). Decompression during the late-orogenic exhumation might be responsible for the 147–135 Ma magmatism, migmatization and shearing deformation in the coastal zone. The orogenic processes in ca. 165–135 Ma could result in large scale lithospheric delamination, leading to a mantle avalanche, asthenosphere upwelling, and the post-orogenic extension related crustal melting/thinning/cooling in 132–117 Ma (Chen et al., 2002, 2008a; Wong et al., 2009; He and Xu, in press). Note that the magmatic pulse in 132–117 Ma was followed by a middle Cretaceous crustal contraction/thickening process (ca. 115– 100 Ma?), which can be used to explain the folding and metamorphism in the Lower Cretaceous volcanic rocks (ca. 145–117 Ma), the magmatic lull at ca. 115–110 Ma, the magmatic complex of ca. 110–100 Ma, and the regional unconformity underneath the Upper Cretaceous volcanic rocks (ca. 100–80 Ma) (Lapierre et al., 1997; Chen et al., 2008a,b; Hsieh et al., 2008; He and Xu, in press). The magmatic pulse in ca. 100–90 Ma, accompanied by emplacement of A-type granites, and the bimodal igneous rocks of ca. 90–80 Ma (Lapierre et al., 1997; Chen et al., 2008a, 2008b; Wong

(1) The Late Mesozoic magmatic rocks from the coastal Changle-Nan’ao tectonic zone, SE China, were emplaced during 147–117 Ma. The intrusive rocks of 147–136 Ma are mostly deformed and/or migmatized, while the fine-grained granitoids of 132–117 Ma show little/no sign of deformation and migmatization. The new age data provide better timing constraints on Late Mesozoic magmatism and tectonic processes. (2) The presence of the youngest detrital magmatic zircons of 202–196 Ma in the amphibolite facies metasediments (PD02-2) indicates that the protoliths of the metasediments are T3–J1 sedimentary rocks. Abundant inherited magmatic zircons of ca. 165–150 Ma in early Cretaceous migmatized gneissic granites (PD30-1) and migmatitic gneisses (PD232) suggest that magmatic protoliths of these rocks were emplaced in late Jurassic. The ages for the migmatized/migmatitic rocks (PD30-1 and PD23-2) and leucogranite dykes (PD14-1) show that migmatization was active during 146– 135 Ma. All the new zircon age data reported herein support a late Jurassic to early Cretaceous orogeny (ca. 165–135 Ma) in the coastal zone. Consequently, the magmatic rocks of 147–117 Ma probably represent the syn-kinematic magmatism (147–136 Ma) and the post-kinematic plutonism (132–117 Ma). The tectonic mode switch from syn-orogenic compression to post-orogenic extension occurred at ca. 136–132 Ma. (3) U–Pb analyses of zircon from paragneiss reveal multiple episodes of magmatism from Paleo-proterozoic to early Jurassic in the coastal zone. Incidentally, the early Neo-proterozoic to early Jurassic magmatic events had been discovered in other parts of SE China.

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