39Ar geochronological constraints

Gondwana Research 19 (2011) 910–925

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Triassic high-strain shear zones in Hainan Island (South China) and their implications on the amalgamation of the Indochina and South China Blocks: Kinematic and 40 Ar/39Ar geochronological constraints Feifei Zhang a, Yuejun Wang a,⁎, Xinyue Chen a,b, Weiming Fan a, Yanhua Zhang c, Guowei Zhang d, Aimei Zhang a a

Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China Department of Geosciences, Hunan University of Science and Technology, Xiangtan, 411201, China c CSIRO Earth Science and Resource Engineering, PO Box 1130, Bentley, WA 6102, Australia d State Key Laboratory of Continental Dynamics, Northwest University, Xi'an, 710069, China b

a r t i c l e

i n f o

Article history: Received 23 May 2010 Received in revised form 24 October 2010 Accepted 1 November 2010 Available online 23 November 2010 Handling Editor: S. Kwon Keywords: Kinematics Ar–Ar thermo geochronology Triassic amalgamation Hainan Island South China Block Indochina Block

a b s t r a c t A kinematic and geochronological study has been carried out on the Triassic high-strain shear zones in Hainan Island, the southern South China Block. There are WNW- and NE-trending high-strain shear zones with greenschist- to amphibolite-facies metamorphism in this island. Kinematic indicators suggest a dextral topto-the-NNE thrust shearing for the WNW-trending high-strain shear zones and a sinistral top-to-the-SE thrust shearing for the NE-trending shear zones. The quartz c-axis orientations of mylonitic rocks exhibit the domination of basal slip and some activation of a rhombohedra gliding system. The timing of shearing for these shear zones has been constrained by the 40Ar/39Ar dating analyses of synkinematic minerals. Middle Triassic (242–250 Ma) and late Triassic–early Jurassic (190–230 Ma) have been identified for the WNW- and NE-trending shear zones, respectively. A synthesis of these kinematic and thermogeochronological data points to a two-stage tectonic model for Hainan Island, that is, top-to-the-NNE oblique thrusting at 240– 250 Ma followed by top-to-the-SE oblique thrusting at 190–230 Ma. In combination with the available data from the southern South China and Indochina Blocks, it is inferred that South Hainan and North Hainan have affinity to the Indochina and South China Blocks, respectively. The tectonic boundary between South Hainan and North Hainan lies roughly along the WNW-trending Changjiang–Qionghai tectonic zone probably linking to the Song Ma and Ailaoshan zones. The middle Triassic structural pattern of Hainan Island is spatially and temporally compatible with those of the South China and Indochina Blocks, and thus might be a derivation from the amalgamation of the Indochina with South China Blocks in response to the closure of the Paleotethys Ocean and subsequent subduction/collision. © 2010 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction Since the proposal for the Indosinian orogeny by Deprat (1914) and Fromagat (1932) on the basis of two angular unconformities in the Triassic sequences in Vietnam, it has been thought to be caused by the final amalgamation of the Indochina Block with the South China Block (constituted by the Yangtze Block to the northwest and the Cathaysian Block to the southeast) in response to the closure of the Paleotethys Ocean (e.g., Zhong, 1998; Metcalfe, 1996, 2002; Carter et al., 2001; Wang et al., 2007; Carter and Clift, 2008). Such an orogeny should theoretically result in a similar structural pattern between the northern Indochina and southern South China Blocks, based on an ⁎ Corresponding author. Current address: Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangzhou, 510640, Guangdong Province, China. Tel.: +86 20 85290527; fax: +86 20 852910527. E-mail address: [email protected] (Y. Wang).

understanding that the wide, hot orogen has similar variations in lithosphere structure, as demonstrated by the analogue and numerical models of Cruden et al. (2006). In the Indochina Block, available data confirmed the occurrence of the 240–250 Ma WNW-trending faults/shear zones in the Song Ma, Song Da, Kontum, Truong Son, Dien Bien Phu, Raub–Bentong, Chiang– Mai and Sukhothai zones/massifs (Sengor and Hsű, 1984; Zhang et al., 1994; Lacassin et al., 1997; Lepvrier et al., 1997, 2004, 2008; Nam et al., 1998; Lo et al., 1994; Lan et al., 2000; Carter et al., 2001; Metcalfe, 2002; Owada et al., 2007; Carter and Clift, 2008). In the South China hinterland, however, the structures are generally believed to be mainly characterized by a NE-trending pattern, which could be interpreted as the result of (1) interaction among the rigid North China, Indosinian and buoyant South China Blocks (e.g., Wang et al., 2005, 2007), (2) the closure of the Paleotethys Ocean (e.g., Shu et al., 2008), or (3) the flat-slab subduction of the Paleo-Pacific plate since 270 Ma (e.g., Li and Li, 2007 and references therein). Now a

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F. Zhang et al. / Gondwana Research 19 (2011) 910–925

question arises as to whether there exist contemporaneous ductile shear zones in the southern South China Block, similar to those of the Indochina Block. In addition, there is a lack of consensus on the timing of final amalgamation between the Indochina and South China Blocks. Some researchers proposed late Permian collision whereas others suggested Triassic convergence (e.g., Metcalfe, 1996, 2002; Thanh et al., 1996, 2007; Thanh and Khuc, 2006; Li et al., 2006; Owada et al., 2007; Nakano et al., 2007; Chen et al., 2011). Hainan Island tectonically lies at the southern margin of the South China Block, adjacent to the northern Indochina Block (e.g., Metcalfe 1996; He et al., 1996; Ma et al., 1998) (Fig. 1a). On this Island, there preserved WNW- and NE-trending high strain shear zones, the kinematic and geochronological natures of which are poorly known (e.g., Guangdong BGMR, 1988). These structures might have acted as the important “bridges” linking the southern South China to northern Indochina Blocks. Therefore, Hainan Island is a key area to improve the understanding of the amalgamation of the Indochina with South China Blocks. The general geology of Hainan Island is poorly known due to extensive soil and vegetation cover, poor rock exposure and rare thermogeochronological data. Our recent structural mapping revealed the presence of the WNW- and NE-trending high-strain zones on Hainan Island. The field work was then followed by the 40Ar/39Ar geochronological analyses of synkinematic minerals from the WNWand NE-trending high-strained zones. In this paper, we present our new results on the kinematics and thermogeochronology of these shear zones. A synthesis of these data provides an important constraint on the Triassic amalgamation of the Indochina with South China Blocks.

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2. Geological background Hainan Island is a continental island separated from South China mainland by the Qiongzhou Strait (Fig. 1a). There are distinct viewpoints on its tectonic nature. Some researchers (e.g., Shui 1987; Guangdong BGMR, 1988; Zhang et al., 1997; X.H. Li et al., 2002; Z.X. Li et al., 2002) proposed that it is part of the Cathaysian that is the component of the South China Block, whereas others (e.g., Hsű et al., 1990; Chen et al., 1994) advocated its affinity to the Indochina Block. There is also a suggestion that North Hainan and South Hainan, which is inferred to be separated by the Jiusuo–Linshiu fault (e.g., Xia et al., 1991a,b) or the Changjiang–Qionghai fault (e.g., Li et al., 2000; X.H. Li et al., 2002; Z.X. Li et al., 2002), have the affinity to the Cathaysian and Indochina Blocks, respectively. On this island, it is previously thought that the WNW-trending Wangwu–Wenjian, Changijang–Qionghai, Jianfeng–Diaoluo and Jiusuo– Linshui brittle faults extend from north to south, respectively, in spite of the fact that poor geophysical and structural data are reported to verify this (e.g., Guangdong BGMR, 1988) (Fig. 1b). The Western Island contains early Mesozoic NE-trending Gezhen–Lingao and Baisha faults (Fig. 1b) that might have experienced later reactivation (e.g., Xia et al., 1990, 1991a,b; Wang et al., 1991, 1992). The stratigraphical succession includes the Mesoproterozoic Baoban and Shilu Groups, Paleozoic shallow marine strata and Mesozoic terrestrial strata (e.g., Hunan BGMR, 1988; Ma et al., 1998; Long et al., 2002). The Baoban Group (also named the Baoban Complex) predominantly distributes in the central Island, and is composed of high greenschist- to amphibiotic-facies metamorphic rocks (e.g., Guangdong BGMR, 1988). They were intruded by the

Fig. 1. (a) Structural outline of Southeast Asia showing the major tectonic blocks, boundaries and faults/shear zones. The region with pink color shows the Triassic Lancangjiang igneous zone. (b) Revised geological map of Hainan Island (after Guangdong BGMR, 1988). The star symbols show the location of samples for dating and quartz fabric analyses.

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1.43 Ga granitoids (X.H. Li et al., 2002; Z.X. Li et al., 2002) and are thought to be representative of the oldest basement rocks in the region (Wang et al., 1991, 1992; Liang and Li, 2005). The Shilu Group is unconformably overlain by Sinian neritic siliciclastics, only exists in the northwest Island, and consists of iron-rich volcanics and siliciclastics with turbidite signatures (e.g., Guangdong BGMR, 1988; Wang et al., 1991, 1992). The Cambrian and Ordovician low-grade metamorphic successions predominantly occurred in the areas south of the Changjiang–Qionghai Fault (Fig. 1b), and are composed of shale, sandstone, siltstone and slate with a small amount of limestone interlayer (Wang et al., 1991, 1992; Yao and Huang, 1999; Zeng et al., 2003, 2004). A small amount of lower Silurian shallow-marine sandstone is also observed in the Island (Xu et al., 1997; Tang and Feng, 1998; Hu et al., 2001; Long et al., 2007). The upper Paleozoic successions are characterized by Devonian sandstone, Carboniferous slate and metamorphosed volcanic rocks, lower Permian limestone and middle Permian sandstone. These rocks predominantly outcrop in the areas north of the Jiusuo–Linshui Fault (e.g., Wang et al., 1991, 1992; Xu et al., 1997; Tang and Feng, 1998; Hu et al., 2001; Long et al., 2007). Middle Permian conglomerates are unconformably underlain by lower Permian limestone in Central Island. Lower-middle Triassic sandstone only occurred in the Anding and Qionghai areas in the central and northern Island (Fig. 1b). It is unconformably underlain by the pre-Triassic sequence and overlain by lower Cretaceous terrestrial siliciclastics. A number of metabasites occurred as lenses in late Paleozoic metasediments in the Chenxing (Tunchang) and Bangxi (Changjiang) areas along the Changjiang–Qionghai Fault (Fig. 1b). These rocks have commonly undergone amphibolite-facies metamorphism. They were traditionally considered to be the fragments of Paleozoic ophiolitic rocks and mélange sheets, representing the remnants of the eastern part of the Paleotethys (e.g., Guangdong BGMR, 1988; Wang et al., 1991, 1992; X.H. Li et al., 2002; Z.X. Li et al., 2002). For the Bangxi metabasites, X.H. Li et al. (2002) and Z.X. Li et al. (2002) obtained the Sm–Nd isochron age of 333 ± 12 Ma and Xu et al. (2007) reported the SHRIMP zircon U–Pb weighted mean age of 269 ± 4 Ma. Based on the integration of geochemical and Sm–Nd isotopic data, these rocks were inferred to be generated in a limited ocean (e.g., Li et al., 2000; X.H. Li et al., 2002; Z.X. Li et al., 2002), back-arc basin or rifting regime (Xu et al., 2007). Granites (foliated and unfoliated) are extensively outcropped in Hainan Island and occupy ca. 37% of the island's land area (Wang et al., 1991) (Fig. 1b). Strongly foliated granites are over an area of ca. 800 km2. The majority of these rocks are exposed in the Ledong– Wuzhishan–Wanning areas in Central Island. In these rocks, abundant mafic magmatic and paragneissic enclaves occurred in sizes from several tens of centimeters up to ten meters. They were previously considered to be the components of the Mesoproterozoic Baoban Group or misidentified as the “Shang'an migmatite” of unknown ages (Wang et al., 1991). However, recent geochemical and geochronological data show that the majority of the foliated granites are middle Permian (zircon U–Pb ages of 260–272 Ma) syntectonic biotite granites and granodiorites (Xie et al., 2005; Li et al., 2006 and authors' unpublished data) (Fig. 1b), and Li et al. (2006) thus re-named these rocks as the Permian Wuzhishan orthogneiss. They were intruded by the Qiongzhong granitic batholith with SHRIMP zircon U–Pb age of 230–237 Ma (e.g., Ge, 2003; Xie et al., 2005), as observed in the Tongza area (Li et al., 2006). A small amount of the foliated granites might be of the Grenvillian origin in spite of uncertainties about their spatial distribution (e.g., X.H. Li et al., 2002; Z.X. Li et al., 2002). Unfoliated granites mainly include the middle Triassic Qiongzhong granitic batholiths (e.g., Qiongzhong and Jianfengling) and the early Jurassic Danxian granitic batholiths (SHRIMP zircon U–Pb age of 186 Ma; e.g., Ge, 2003; Xie et al., 2005; Li et al., 2005), as well as middle Jurassic–Cretaceous (zircon U–Pb ages of 150–60 Ma) medium- to

coarse-grained monzogranite (e.g., Tunchang, Qianjia and Baocheng plutons; Wang et al., 1991, 1992; Ge, 2003). 3. Kinematic signatures of high-strain shear zones Our structural mapping, integrated with previous work (e.g., Guangdong BGMR, 1988; Wang et al., 1991, 1992; Tu and Zhang, 1994; Yu et al., 1995; He et al., 1996; Zhan et al., 1996), identified the presence of the WNW- and NE-trending high-strain shear zones in Hainan Island. These shear zones are geometrically and temporally consistent with those of the northern Vietnam and southern South China Blocks, respectively (Figs. 1 and 2). 3.1. WNW-trending high-strain shear zone The general geology and structural features of the WNW-trending shear zones are poorly known. They were not even shown in the previously published geological map or related literature (e.g., Guangdong BGMR, 1988). The WNW-trending shear zones dominantly occurred in the Mesoproterozoic Baoban and Shilu Groups, lower Paleozoic sequences and middle Permian orthogneiss in the Ledong–Wuzhishan–Wanning areas in Central Island. As shown in Figs. 1b and 2, they are constituted by a series of individual shear zones (100–1500 m wide and several kilometers long), and are discontinuously exposed in the Gong'ai (Dongfang), Yapai (Wuzhishan), Changzheng (Qiongzhong), Hongmiao (Qiongzhong) and Xingzhong (Wanning) areas. In the Wuzhishan–Wanning areas, mylonitic foliations for individual shear zones predominantly dip 30–55° to the SSW, with stretching lineation generally plunging 15–35° towards the SE (Fig. 2c–d). Of these WNW-trending shear zones, the most representative is the one in the Gong'ai area, Dongfang (named herein the Gong'ai shear zone). The Gong'ai shear zone occurred in the Mesoproterozoic Baoban Group and lower Paleozoic phyllite, slate and sandstones and middle Permian orthogneiss (e.g., Wuzhishan). It is truncated by a NEtrending shear zone and also by the Qiongzhong granitic intrusion that is slightly deformed with the SHRIMP zircon U–Pb ages of 230– 237 Ma (Li et al., 2005; Ge, 2003) (Figs. 1b and 2a). The foliations and lineations in the center are stronger and more abundant than those at the margins of the shear zone. The foliation is well developed to form S-tectonites and is defined by compositional layering and preferred alignment of muscovite, biotite, chlorite, flattened quartz grains and cleavaged seams defined by opaque materials. At some locations, S–L tectonites were developed with the occurrence of additional lineation, which is represented by elongated, preferably oriented quartz and feldspar rods and the streaks of aligned plagioclase and biotite. The mylonitic foliation for the shear zone predominantly dips 20–60° to the SSW, and the stretching lineation generally plunges 15–30° towards the SE–ESE (Fig. 3a and f). The kinematic indicators observed in these shear zones include outcrop-scaled structural lenses and asymmetrical boudins viewed from south–southwest (Fig. 4a), suggesting a predominant thrusting movement. The associated fold structures include the Shilu and Baoban synclinoria and Furongtian anticlinoria involving the Shilu and Baoban Groups and Sinian, Carboniferous and lower Permian successions (e.g., Guangdong BGMR, 1988; Wang et al., 1991, 1992). The Shilu synclinoria exhibits asymmetrical geometries with moderately (30–50°) dipping southern limbs and steeply (40–70°) dipping northern limbs (see A–B section in Fig. 2b), indicating a tectonic transport pattern toward the north. The Baoban synclinoria are characterized by a series of moderately dipping southern limbs but overturned northern limbs (see A–B section in Fig. 2b). The Furongtian anticlinoria with SSW-dipping axial planes are composed of abundant tight and overturned folds that are accompanied by faults along their limbs (see C–D in Fig. 2b). The aforementioned geometrical patterns of asymmetrical folds obviously document a

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Fig. 2. Geological map of central Hainan Island (a) with A–B (Shilu and Baoban) and C–D (Furongtian) structural sections (b), stereoplots (lower hemisphere, equal area) of the orientations of lineation and foliation (c–d), and quartz c-axis orientation projections (lower hemisphere in Wulff net, equal area) for 04HN208 (e), 04HN225 (f), 04HN255 (g) and 04HN245 (h) mylonitic rocks from the WNW-trending shear zone, respectively. L and Z represent the directions of the maximum elongation and shortening, respectively. Contour intervals are 1%, 4%, 7% and 10%.

top-to-the-NNE thrusting transport. In the Changjiang and Danzhou areas at the north of the Changjiang–Qionghai fault, overturned folds, with the axial planes dipping moderately to steeply toward SSW, are well developed (e.g., Zhang et al., 1997; Xu et al., 1997). The thrust-fold sheets (e.g., Hechaling, Bawangling and Hongling in the Changjiang–Bangxi area) are usually constituted by a series of folds and thrust faults with the majority of axial/ fault planes moderately to steeply dipping toward the SSW. This resulted in a pattern where pre-Permian metamorphic rocks were thrusted over lower Permian rocks (e.g., Xu et al., 1997). The microscopic asymmetrical boudins, S–C fabrics, mica flakes and asymmetric porphyroblasts with attenuated tails, viewed from a positive direction (i.e. predominantly top-down viewing), additionally indicate the occurrence of the dextral shearing component (e.g. Fig. 4c–d). A consistent kinematic sense is also given by a monoclinic point-maximum asymmetry for the quartz c-axis fabric patterns on the Wulff net representative of mylonitic samples (04HN-16, 04HN-17, 04HN-18, 04HN-208, 04HN-225, 04HN-245 and 04HN255), with respect to the foliation and lineation (Figs. 2e–h, 3c–e and g–i). The consistent kinematic indicators were also abundantly observed in other individual WNW-trending shear zones (i.e., Yapai, Changzheng and Xingzhong) in the Wuzhishan–Wanning areas. The combination of all these kinematic indicators clearly indicates a topto-the-NNE thrusting with an additional dextral shear component.

3.2. NE-trending shear zones Individual NE-trending high-strain zones (e.g., Gezhen, Chongzuling, Baisha, Hechaling and Shilu) predominantly occurred in central and western Island. The representative NE-trending shear zones include the Gezhen and Chongzuling shear zones, as the components of the broader Gezhen–Lingao and Baisha faults, respectively (Figs. 1b and 5a–b; Guangdong BGMR, 1988; Wang et al., 1991, 1992). The Gezhen shear zone (Changjiang and Gancheng areas) extends more than 150 km with a width of 1.2–2.5 km (Fig. 5a). The Chongzuling shear zone (Chongpo, Ledong) is more than 50 km long and about 1.0 km wide (Fig. 5b). These shear zones are characterized by the development of mylonitic granitoids, gneiss and schist. They separate the Mesoproterozoic Baoban and Shilu Groups (northwest) from the lower Paleozoic rocks (southeast) in the Gezhen and Chongzuling areas (Fig. 5a–b; Ma et al., 1998). These shear zones generally display a texture transition from ultramylonite and mylonite at center to protomylonite (e.g., mylonitic granite and mylonitic sandstone) at margins. The Gezhen shear zone contains a moderately (25–50°) dipping foliation to the NW with stretching lineation moderately (15–30°) plunging towards the NNW (Fig. 5c). The majority of mylonitic foliations for the Chongzuling shear zone dips steeply (40–68°) toward the NW and the lineation plunges moderately (12–20°) toward the N-NNW (Fig. 5i). Abundant kinematic indicators in the

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Fig. 3. Geological map of the Gong'ai area (a) with stereoplots (lower hemisphere, equal area) showing the orientation of lineation and foliation (b and f) of the WNW-trending shear zones and quartz c-axis orientation projections (lower hemisphere in Wulff net, equal area) for 04HN-16 (c), 04HN-18 (d), 04HN-18 (e), 04HN-23 (g), 04HN-25 (h) and 04HN-32 (i) mylonitic rocks. Contour intervals are 1%, 4%, 7% and 10% in quartz c-axis orientation projections.

Fig. 4. Photos of asymmetric porphyroblasts with recrystallized tails showing dextral shearing deformation of the WNW-trending shear zone in outcrop observed on X–Z plane (a: Changzheng, Qiongzhong) and under microscope from a positive viewing direction (b: viewed on Y–Z section from the sample in Wuzhishan; c and d: viewed on X–Z section for the mylonite from Gong'ai), respectively.

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Fig. 5. (a) Geological map of the NE-trending Gezhen shear zones with stereoplots (lower hemisphere, equal area) of the lineation and foliation (c) and quartz c-axis orientation projections (lower hemisphere in Wulff net, equal area) of 04HN04 (d), 04HN03 (e), 04GZ-C (f), 04GZ-E (g), 04GZ-D (h) mylonitic rocks. (b) Geological map of the NE-trending Chongzuling shear zones with stereoplots of the lineation and foliation (i) and quartz c-axis orientation projections of 04LG-A (j), 04HN24 (k) and 04LG-C mylonite, respectively. Contour intervals are 1%, 4%, 7% and 10% in quartz c-axis orientation projections.

mylonites for these NE-trending shear zones (e.g., ribbons, en echelon quartz veins and S-shaped asymmetric folds, S–C fabrics, mica fish, boudin elongate quartz and antithetical microfaulted feldspar) were observed. The synthesis of such kinematic textures observed from the X–Z and Y–Z sections (Fig. 6a–d) indicates a sense of sinistral shearing. Such a shear sense is also supported by the quartz c-axis patterns of mylonitic samples on the Wulff net shown in Fig. 5d-l (04HN-03, 04HN-04, 04HN-24, 04GZ-C, 04GZ-D, 04GZ-E, 04LG-A and 04LG-C). All the structural elements generally point to a tectonic transport toward the southeast. This transport direction can also be identified by the following geological observations: (1) The map-view geometries of the Nanhao and Donglin anticlinoria display an asymmetric pattern (moderately-dipping northwest limbs and overturned southeast limbs). (2) The Sanya and Nankunyuan synclinoria also exhibit an overturned asymmetric geometry with steeply (60–75°) NW-dipping axial planes. (3) In the Changjiang–Qionghai area, the lower Paleozoic sequences are stacked over the upper Paleozoic rocks along the NE-trending faults/shear zones that dip steeply toward the NW. (4) In the Changjiang area, the axial-planes of the folds dip to the NW and these folds show variable geometries from tight to broad and from overturned to normal from west to east. The integration of all the structural and kinematic evidences mentioned earlier indicates a topto-the-SE thrusting with a sinistral shearing component. 3.3. Deformation conditions of shear zones Mineral assemblages and possible deformation mechanisms for feldspar and quartz in the mylonitic rocks of the WNW- and NE-trending shear zones are summarized in Table 1. Our microstructural observations show that quartz grains in the mylonitic samples for the WNW- and NEtrending shear zones commonly display crystal-plastic textures including polygonal to elongated quartz ribbons, recrystallization texture and subgrains (Fig. 6e and Table 1). Plagioclase grains in most samples

exhibit brittle deformation features characterized by microfractures, discrete undulatory extinction, bulging and kinked twin planes (Fig. 6f and Table 1). Recrystallized feldspar grains were sporadically observed in the sample from the WNW-trending Gong'ai shear zone. It is considered that at geological strain rates (e.g. 1 × 10− 14 s− 1), crystalplastic deformation dominantly occurs at the temperatures of 300– 450 °C in quartz and 400–600 °C in feldspar (e.g., Simpson and Schmid, 1983; Tullis and Yund, 1985; Kligfield et al., 1986). These microstructural characteristics hence suggest that the shearing deformation likely developed in the temperature range from 300 °C to 600 °C. The quartz c-axis fabrics of samples 04HN-208, 04HN-255, 04HN245, 04HN-16 and 04HN-17 from the WNW-trending shear zones and 04HN-03, 04HN-24 and 04GZ-C from the NE-trending shear zones exhibit densely populated maxima relatively near the Z-axis (Figs. 2e, g–h, 3c, e and 5e–f), indicative of the dominant activation of the basal baN gliding system at lower temperatures of ~300–400 °C (Tullis et al., 1973; Lister and Hobbs, 1980; Law, 1990; Twiss and Moores, 1994). The quartz c-axis fabrics of 04HN-18, 04HN-23, 04HN-25 and 04HN-32, 04HN-225 from the WNW-trending shear zones and 04HN04, and 04GZ-D and 04GZ-E, and 04LG-A, 04LG-C from the NEtrending shear zones, have additional maxima close to the Y-axis along the oblique girdle (Figs. 2f, d, 3g–i, d, 5h–g and j–l), suggesting the activation of additional gliding on the rhombohedra planes {r}baN and {z}baN. The fabric patterns described earlier indicate that shearing deformation on these WNW- and NE-trending shear zones occurred likely under a geothermal path ranging from ~ 300 °C to ~500 °C (e.g., Yardley, 1989; Twiss and Moores, 1994), compatible with those estimated from their microscopic structures and high greenschist- to low amphibolite-facies metamorphic conditions (also see Table 1; Zhan et al., 1996; Zhang et al., 1997). Zhan et al. (1996) also concluded that the deformational temperatures of the Gezhen shear zones are in range of 300–500 °C based on the aluminum isothermal map of coexisting muscovite and chlorite.

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Fig. 6. Photos showing sinistral deformation for the NE-trending shear zones (a–d). (a) S–C fabrics observed on X–Z section in handsample example (Gezhen village, Changjiang), (b) microscopic views of asymmetric porphyroblasts with recrystallized tails on X–Z section (Fulong, Baisha), (c) asymmetric porphyroblasts and mica fish on X–Z section (Gezhen village, Changjiang), (d) mica fish on Y–Z section (Chongzuling, Dongfang), (e) recrystallized quartz on XZ section (Chongzuling, Dongfang), and (f) asymmetric plagioclase porphyroblasts showing microfractures on XZ section (Gezhen village, Changjiang).

4.

40

Ar/39Ar thermogeochronology of synkinematic minerals

40 Ar/39Ar dating methods can be used to date deformational events effectively, when important criteria, including (1) dynamic recrystallization occurring at or below the closure temperatures of the analyzed mineral, and (2) the negligible effects of excess argon, are met (e.g., West and Lux, 1993). Muscovite is a common synkinematic mineral in mylonite (West and Lux, 1993; Kirschner et al., 1996; Mülch et al., 2002). It has an intermediate closure temperature, and is easily recrystallized during deformation process and less susceptible to the incorporation of excess argon (e.g., Snee et al., 1988; West and Lux, 1993). Therefore, muscovite is often considered to be the most suitable minerals for constraining the timing of mylonitization using 40 Ar/39Ar dating methods. Available data show that muscovite porphyroclasts might be incompletely reset in the mylonization condition of less than 500 °C (e.g., Villa, 1998; Gouzu et al., 2006; Itaya et al., 2009; Nuong et al., 2008, 2009). Taking account into the dynamic recrystallization temperature of 300–500 °C for our samples (Table 1), muscovite separates out of the matrix were selected, in this study, for constraining the deformational events. For those mylonitic samples without synkinematic muscovite, the biotite and sericite separates were also selected for 40Ar/39Ar dating.

The mylonites were carefully examined in the microscope to determine whether the analyzed minerals in the matrix are synkinematic and are recrystallized. The selected mylonites were disaggregated and the fragments without porphyroclasts were reserved. The muscovite, biotite and sericite separates for 40Ar/39Ar dating were preliminarily purified from the fragments using conventional heavy liquid and magnetic techniques. Then these separates were carefully handpicked under the microscope and were further checked by petrographic examination. Only the mineral separates with the grain sizes of 80–100 mesh (about 150–190 μm) for most samples were selected for 40Ar/39Ar geochronological dating. For 04HN12 sample, sericites, collected and separated only with grain size of 150–180 mesh (about 80–100 μm), were used for step-heating analyses. The sampling locations, mineral assemblages and deformation mechanism are summarized in Tables 1 and 2. Our 40Ar/39Ar measurements were carried out using the MM-1200 and GV Instruments 5400 mass spectrometer at the Guangzhou Institute of Geochemistry, the Chinese Academy of Sciences, respectively. Out of all the measurements, samples 04HN208, 04HN225, 04HN04, 04HN24, 04HN316 and 04HN12 were analyzed by the stepheating using the MM-1200 instrument. The Laser 40Ar/39Ar stepheating by GV-5400 instrument was used to date the samples 04HN17

F. Zhang et al. / Gondwana Research 19 (2011) 910–925

917

Table 1 Microscopic structural synthesis of the S–L tectonics from the WNW- and NE-trending shear zones in Hainan Island. Sample number

Sampling location

WNW-trending shear zones 04HN16 Gong'ai, Dongfang 04HN17

Gong'ai, Dongfang

04HN18

Gong'ai, Dongfang

04HN208

Changzheng, Qiongzhong

04HN225

Heping, Qiongzhong

NE-trending shear zones 04HN03 Yapao, Changjiang 04HN04

Gezhen, Changjiang

04HN12 04GZ-C

Shilu, Changjiang Gezhen, Changjiang

04GZ-D

Hongquan village

04GZ-E

Hongquan village

04HN24

Chongzuling, Ledong

04HN316

Fulong, Baisha

04LG-A

Paishan, Ledong

04LG-C

Chongzuling, Ledong

Mineral assemblage

FDM

QDM

Quartz fabric

Deformation temperature

Kinematics

Porphyroclast (41%): Fel + Qz + Mu; matrix (59%): Qz + Bi + Chl + Mu Porphyroclast (32%): Pl + Qz + Mu + Bi; matrix (68%): Qz + Mu + Bi + Chl Porphyroclast (~ 30%): Pl + QZ + Mu + Bi matrix (~70%): Qz + Mu + Bi Porphyroclast (~ 40%): Pl + QZ + Mu; matrix (~60%): Qz + Mu + Bi + Chl Porphyroclast (~ 35%): Pl + QZ + Mu; matrix (~ 65%): Qz + Mu + Bi + Chl

CF + DC

SR + GBM

BD + RD

~ 300–450 °C

Dextral

CF + DC

SR + GBM

BD + RD

~ 300–450 °C

Dextral

CF + DC

SR + GBM

RD

~ 400–500 °C

Dextral

CF

SR + GBM

BD

~ 300–400 °C

Dextral

CF + DC

SR + GBM

BD + RD

~ 400–450 °C

Dextral

CF + DC

SR + GBM

BD + RD

~ 300–450 °C

Sinistral

CF + DC

SR + GBM

RD

~ 400–500 °C

Sinistral

CF CF + DC

SR + GBM SR + GBM

BD BD + RD

~ 300–400 °C ~ 350–450 °C

Sinistral Sinistral

CF + DC

SR + GBM

BD + RD

~ 350–450 °C

Sinistral

CF

SR + GBM

BD

~ 300–400 °C

Sinistral

CF + DC

SR + GBM

BD + RD

~ 300–450 °C

Sinistral

CF + DC

SR + GBM

RD

~ 350–500 °C

Sinistral

CF

SR + GBM

BD

~ 300–400 °C

Sinistral

CF

SR + GBM

BD

~ 300–400 °C

Sinistral

Porphyroclast (~ 20%): Fel + Qz + Bi; matrix (~ 80%): Qz + Bi + Chl Porphyroclast (~ 35%): Fel + Qz; matrix (~ 65%): Qz + Mu + Bi + Fel + Chl Fel + Qz + Bi + Ser + Chl Porphyroclast (~ 25%): Fel + Qz + Mu + Bi; matrix (~ 75%): Qz + Bi + Chl Porphyroclast (~ 55%): Fel + Qz + Bi; matrix (~ 45%): Qz + Bi + Chl Porphyroclast (~ 30%): Fel + Qz + Bi; matrix (~ 70%): Qz + Bi + Chl Porphyroclast (~ 25%): Fel + Qz + Bi; matrix (~ 75%): Qz + Mu + Bi + Chl Porphyroclast (~ 45%): Fel + Qz; matrix (~ 55%): Qz + Bi + Chl Porphyroclast (~ 30%): Fel + Qz + Bi; matrix (~ 70%): Qz + Bi + Chl Porphyroclast (~ 40%): Fel + Qz + Bi; matrix (~ 60%): Qz + Bi + Chl

Qz: quartz; Fel: feldspar; Mu: muscovite; Bi: Biotite; Se: sericite; Chl: chlonite; FDM: feldspar deformational mechanism; CF: cataclastic flow; DC: dislocation creep; QDM: quartz deformational mechanism; SR: subgrain rotation; GBM: grain boundary model; BD: basal baN gliding; RD: rhombohedral gliding.

and 04HN18. Samples and monitor standard DRA1 sanidine (Wijbrans et al., 1995) with the assumed age of 25.26 ± 0.07 Ma were irradiated at the 49-2 reactor for 54 h. Correction factors for interfering argon isotopes derived from Ca and K are: (39Ar/37Ar)Ca = 8.984 × 10− 4, (36Ar/37Ar)Ca = 2.673 × 10− 4 and (40Ar/39Ar)K = 5.97 × 10− 3. The crusher consists of a 210 mm long, 28 mm bore diameter high temperature resistant stainless steel tube (Tmax ~ 1200 °C). The extraction and purification lines were baked out for ca. 10 h at 150 °C with heating tape and the crusher at 250 °C with an external

tube furnace. The blanks are: 36Ar (0.002–0.004) mV, 37Ar (0.0002– 0.0006) mV, 38Ar (0.0004–0.0015) mV, 39Ar (0.0025–0.0051) mV and 40 Ar (0.51–1.3) mV. The released gas was purified for 5 to 8 min by two Zr/Al getter pumps operated at room temperature and ~450 °C, respectively. The detailed analytical techniques for MM-1200 and the GV-5400 analyses have been described by Sang et al. (1996) and Qiu and Wijbrans (2008), respectively. The 40Ar/39Ar dating results were calculated and plotted using the ArArCALC software (Koppers, 2002). The analytical results are listed in Table 3 and plotted in Figs. 7a–d and

Table 2 Summary of sample locations and geochronology for Hainan Island. Sample number

Location

WNW-trending shear zone 04HN17(Mu) Gong'ai, Dongfang 04HN18(Mu)

Gong'ai, Dongfang

04HN208(Mu)

Changzheng, Qiongzhong

04HN225(Mu)

Heping, Qiongzhong

NE-trending shear zone 04HN04(Mu)

Gezhen, Changjiang

04HN24(Mu)

Chongzuling, Ledong

04HN316(Bi)

Fulong, Baisha

04HN12(Ser)

Shilu, Changjiang

Rock type

Longitude, latitude

Age (Ma ± 1σ)

Mylonitic schist from Ordovician sequence

108°49′49″, 18°55′09″

Mylonitic gneissic granite from Mesoproterozoic Baoban Group Mylonite from Permian Wuzhishan orthogranite intrusion Granitic mylonite from Permian Wuzhishan orthogranite intrusion

108°49′00″, 18°55′46″

Plateau age: 245.2 ± 1.0 Ma, isochron age: 245.8 ± 2.1 Ma Plateau age: 241.8 ± 1.4 Ma, isochron age: 245.0 ± 2.0 Ma Plateau age: 242.6 ± 1.7 Ma, isochron age: 243.2 ± 3.9 Ma, Plateau age: 242.0 ± 1.7 Ma, isochron age: 240.3 ± 4.8 Ma

Mylonitic gneiss from Mesoproterozoic Gezhencun Formation Mylonitic micaschist from Mesoproterozoic Ewenling Formation Granitic mylonite from Qiongzhong garnet-bearing granitic pluton Mylonitic schist from Neoproterozoic Shilu Formation

Mu: muscovite separates; Bi: biotite separates; Ser: sericite separates.

109°53′39″, 18°57′06″ 109°58′11″, 18°54′98″

108°57′90″, 19°11′28″ 108°56′08″, 18°35′46″ 109°27′70″, 19°25′53″ 109°02′45″, 19°14′40″

Plateau age: 227.4 ± 0.2 Ma, isochron age: 227.7 ± 0.6 Ma Plateau age: 229.6 ± 0.3 Ma, isochron age: 230.4 ± 1.1 Ma, Plateau age: 220.7 ± 1.5 Ma, isochron age: 223.9 ± 4.5 Ma Plateau age: 190.2 ± 5.1 Ma, isochron age: 186.9 ± 3.7 Ma,

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F. Zhang et al. / Gondwana Research 19 (2011) 910–925

Table 3 Results of 40Ar/39Ar heating experiments for the mineral separates from mylonitic samples. Laser output/ Temp.

(40Ar/39Ar)

m

(36Ar/39Ar)

m

(37Ar/39Ar)

m

(38Ar/39Ar)

m

39 Ark (10-12mol)

(40Ar/39Ark) (± 1 σ)

39

Ark

%

Apparent age (t ±2 σ Ma)

Muscovite separates 04HN17 collected from Gong'ai, the WNW-trending shear zone; J = 0.007212 1.5 55.6114 0.149158 0.007385 0.0435 2.5 23.2234 0.011139 0.000174 0.0152 3.0 21.2960 0.003499 0.000072 0.0134 3.5 21.3347 0.001581 0.000031 0.0132 4.0 21.1818 0.001650 0.000044 0.0131 4.5 21.3565 0.002416 0.000065 0.0133 5.0 21.6462 0.002908 0.000097 0.0134 5.5 21.3626 0.001763 0.000054 0.0133 6.0 21.4744 0.002454 0.000080 0.0133 6.5 21.6086 0.003373 0.000107 0.0134 7.0 21.5781 0.002521 0.000091 0.0132 7.5 22.6846 0.008858 0.000489 0.0142 8.5 21.7069 0.002492 0.000125 0.0134 10.0 21.7404 0.003751 0.000182 0.0133 12.5 25.1259 0.013973 0.000825 0.0157 16.0 23.1268 0.008910 0.000498 0.0145 40 36 Plateau age: 245.2 ± 1.0 Ma, isochron age: 245.8 ± 2.1 Ma; Ar/ Ar ratio = 306.9

0.0001 0.0042 0.0110 0.0167 0.0159 0.0098 0.0077 0.0121 0.0093 0.0065 0.0082 0.0017 0.0064 0.0042 0.0011 0.0017

9.6 ± 1.14 19.7 ± 0.07 20.1 ± 0.07 20.7 ± 0.06 20.5 ± 0.06 20.4 ± 0.08 20.6 ± 0.08 20.6 ± 0.06 20.6 ± 0.09 20.5 ± 0.07 20.7 ± 0.09 19.5 ± 0.33 20.8 ± 0.09 20.4 ± 0.12 20.7 ± 0.33 20.5 ± 0.18

0.08 3.60 9.43 14.31 13.60 8.35 6.63 10.34 7.99 5.60 7.06 1.49 5.52 3.58 0.95 1.46

104.1 ± 27.9 233.0 ± 1.6 238.9 ± 1.6 246.1 ± 1.3 244.2 ± 1.3 243.7 ± 1.8 245.2 ± 1.8 246.0 ± 1.4 244.9 ± 2.0 244.3 ± 1.5 246.6 ± 2.0 237.4 ± 7.4 247.9 ± 2.0 244.0 ± 2.8 248.0 ± 7.6 244.6 ± 4.0

Muscovite separates 04HN18 collected from Gong'ai, the WNW-trending shear zone; J = 0.007135 1.5 53.7697 0.177516 0.008089 0.0445 2.5 19.7032 0.024009 0.000367 0.0181 3.0 23.6626 0.017499 0.000240 0.0162 3.5 22.5345 0.010169 0.000141 0.0150 4.0 22.1671 0.006670 0.000093 0.0142 4.5 21.5600 0.003819 0.000088 0.0136 5.0 21.4238 0.003875 0.000121 0.0136 5.5 21.6429 0.003615 0.000125 0.0136 6.0 21.8940 0.003940 0.000077 0.0136 6.5 21.9158 0.004644 0.000113 0.0136 7.0 21.9380 0.004896 0.000206 0.0135 7.5 21.9012 0.004421 0.000162 0.0138 8.0 21.4885 0.002972 0.000136 0.0134 8.5 21.7445 0.004060 0.000210 0.0136 9.0 21.8867 0.004222 0.000177 0.0136 9.5 22.3820 0.005980 0.000361 0.0139 10.5 22.9199 0.007376 0.000290 0.0141 12.5 21.8001 0.003289 0.000176 0.0133 15.0 23.4386 0.011457 0.001147 0.0144 30.0 21.9293 0.003845 0.000173 0.0135 40 36 Plateau age: 241.8 ± 14 Ma, isochron age: 245.0 ± 2.0 Ma; Ar/ Ar ratio:239.0

0.0001 0.0022 0.0035 0.0062 0.0093 0.0106 0.0078 0.0094 0.0094 0.0072 0.0067 0.0065 0.0102 0.0069 0.0064 0.0041 0.0040 0.0067 0.0017 0.0059

3.4 ± 1.0 12.0 ± 0.09 17.8 ± 0.27 18.9 ± 0.11 19.6 ± 0.07 19.9 ± 0.07 19.8 ± 0.05 20.1 ± 0.06 20.2 ± 0.06 20.1 ± 0.07 20.1 ± 0.09 20.2 ± 0.07 20.2 ± 0.07 20.2 ± 0.09 20.3 ± 0.07 20.3 ± 010 20.3 ± 0.10 20.4 ± 0.12 20.1 ± 023. 20.4 ± 0.06

0.08 1.78 2.77 4.94 7.41 8.52 6.24 7.55 7.52 5.76 5.40 5.19 8.13 5.57 5.12 3.31 3.20 5.39 1.39 4.73

43.9 ± 27.5 148.5 ± 2.2 215.3 ± 6.1 227.8 ± 2.4 236.4 ± 1.6 239.7 ± 1.6 238.7 ± 1.2 241.6 ± 1.3 243.2 ± 1.4 241.8 ± 1.5 241.4 ± 2.0 242.8 ± 1.7 242.4 ± 1.6 242.4 ± 2.0 243.7 ± 1.6 243.5 ± 2.2 244.6 ± 2.3 245.1 ± 2.6 241.5 ± 5.2 244.9 ± 1.4

Muscovite separates 04HN208 collected from Changzheng, Qiongzhong, WNW-trending shear zone, weight = 0.085 g, J = 0.013465 300 11.2638 0.001818 0.00275 0.0051 0.538 10.7 ± 1.17 500 11.1618 0.001756 0.00257 0.0057 1.468 10.6 ± 0.70 650 11.1813 0.001683 0.00260 0.0025 4.417 10.7 ± 0.01 750 11.1393 0.001458 0.00157 0.0025 8.845 10.7 ± 0.01 850 11.0782 0.001293 0.00181 0.0031 10.067 10.7 ± 0.01 920 11.0662 0.001284 0.00294 0.0039 8.997 10.7 ± 0.01 1000 11.0641 0.001205 0.00214 0.0028 12.623 10.7 ± 0.01 1080 10.9910 0.000917 0.00804 0.0052 5.978 10.7 ± 0.01 1150 10.9601 0.000869 0.00497 0.0062 3.182 10.7 ± 0.03 1220 10.9858 0.001031 0.00495 0.0034 1.878 10.7 ± 0.06 1300 10.8332 0.001704 0.00288 0.0033 0.503 10.3 ± 1.34 40 36 Plateau age: 242.6 ± 1.72 Ma, isochron age: 243.19 ± 3.86 Ma, Ar/ Ar ratios = 273.0

0.92 2.51 7.55 15.12 17.21 15.38 21.58 10.22 5.44 3.21 0.86

243.1 ± 2.6 241.4 ± 2.3 242.3 ± 2.3 242.8 ± 2.3 242.5 ± 2.3 242.3 ± 2.3 242.8 ± 2.3 243.1 ± 2.3 242.7 ± 2.3 242.2 ± 2.3 281.4 ± 2.6

Muscovite separates 04HN225 colected from Heping, Qiongzhong, WNW-trending shear zone, J = 0.013633 300 11.1150 0.002155 0.00310 0.0051 0.560 500 11.0826 0.001788 0.00212 0.0045 1.875 650 11.1112 0.002011 0.00235 0.0023 4.881 750 11.0757 0.001794 0.00162 0.0025 8.609 850 11.0632 0.001699 0.00200 0.0033 9.229 920 10.9194 0.001312 0.00337 0.0043 8.278 1000 10.9015 0.001232 0.00263 0.0033 10.936 1080 10.9039 0.001279 0.00884 0.0039 5.375 1150 10.8577 0.001182 0.00057 0.0080 2.381 1220 10.8303 0.001223 0.00077 0.0035 1.783 1300 10.4680 0.001647 0.00265 0.0032 0.446 40 36 Plateau age: 242.0 ± 1.69 Ma, isochron age: 240.25 ± 4.81 Ma, Ar/ Ar ratios = 341.0

10.5 ± 0.81 10.5 ± 0.05 10.5 ± 0.01 10.5 ± 0.01 10.6 ± 0.01 10.5 ± 0.01 10.5 ± 0.01 10.5 ± 0.01 10.5 ± 0.33 10.5 ± 0.41 9.97 ± 2.61

1.03 3.45 8.98 15.84 16.98 15.23 20.12 9.89 4.38 3.42 0.82

240.7 ± 2.6 242.3 ± 2.3 241.5 ± 2.3 242.1 ± 2.3 242.5 ± 2.3 241.9 ± 2.3 242.0 ± 2.3 241.7 ± 2.3 241.3 ± 2.3 240.5 ± 2.3 272.6 ± 2.7

9.42 ± 2.63 9.49 ± 0.05 9.48 ± 0.01

0.95 3.34 8.28

218.3 ± 2.6 219.9 ± 2.1 219.6 ± 2.1

Biotite separate 04HN316 collected from Fulong, Baisha, NE-trending shear zone, J = 0.01365 300 10.0854 0.002221 0.00272 0.0168 500 10.0496 0.001856 0.00386 0.0119 650 9.96556 0.001621 0.00218 0.0059

0.444 1.561 3.870

F. Zhang et al. / Gondwana Research 19 (2011) 910–925

919

Table 3 (continued) Laser output/ Temp.

(40Ar/39Ar)

m

(36Ar/39Ar)

m

(37Ar/39Ar)

m

(38Ar/39Ar)

m

39 Ark (10-12mol)

(40Ar/39Ark) (± 1 σ)

39

Ark

%

Apparent age (t ±2 σ Ma)

Biotite separate 04HN316 collected from Fulong, Baisha, NE-trending shear zone, J = 0.01365 9.83042 0.001141 0.00252 0.0035 750 850 9.81457 0.000987 0.00297 0.0035 920 9.79434 0.000915 0.00294 0.0035 1000 9.88381 0.000893 0.00312 0.0034 1080 9.79960 0.000846 0.00299 0.00328 1150 9.89018 0.001188 0.00406 0.00362 1220 9.82668 0.001214 0.00343 0.00368 1300 9.00192 0.001791 0.00323 0.00681 Plateau age: 220.69 ± 1.54 Ma, isochron age: 223.85 ± 4.48 Ma, 40Ar/ 36Ar ratios = 153.2

7.030 8.316 7.359 8.965 4.782 2.393 1.720 0.402

9.48 ± 0.01 9.51 ± 0.01 9.52 ± 0.01 9.61 ± 0.01 9.54 ± 0.02 9.53 ± 0.04 9.45 ± 0.07 8.46 ± 2.48

15.04 17.79 15.53 19.18 10.23 5.12 3.68 0.86

219.7 ± 2.1 220.4 ± 2.1 220.4 ± 2.1 222.5 ± 2.1 221.0 ± 2.1 220.7 ± 2.1 219.2 ± 2.1 233.9 ± 2.5

Muscovite 04HN04 collected from the Gezhen, Changjiang, NE-trending Gezhen shear zone, J = 0.0137663 300 17.2914 0.024419 0.02107 0.0029 0.034 500 15.6308 0.018891 0.02357 0.0046 0.071 620 14.2907 0.015403 0.01360 0.0026 0.165 720 13.1522 0.011502 0.01390 0.0017 0.211 850 12.3178 0.008663 0.00818 0.0013 0.324 980 12.6152 0.009551 0.00994 0.0014 0.365 1050 12.1717 0.008138 0.01322 0.0017 0.338 1100 11.8051 0.006944 0.00573 0.0016 0.272 1150 11.6234 0.006191 0.01444 0.0041 0.155 1200 12.1288 0.008107 0.03378 0.0098 0.054 1250 11.3313 0.009736 0.03582 0.0267 0.027 1300 13.4419 0.017217 0.06271 0.0227 0.012 Plateau age: 227.4 ± 0.2 Ma, isochron age: 227.7 ± 0.6 Ma, 40Ar/ 36Ar ratios = 293.5

10.1 ± 1.36 10.0 ± 0.66 9.73 ± 0.33 9.75 ± 0.30 9.75 ± 0.27 9.79 ± 0.21 9.76 ± 0.21 9.75 ± 0.20 9.79 ± 0.42 9.73 ± 1.01 8.45 ± 0.59 8.34 ± 2.26

1.68 3.48 8.16 10.41 15.98 18.02 16.66 13.42 7.64 2.64 1.34 0.57

234.1 ± 4.5 233.5 ± 4.3 226.8 ± 2.5 227.1 ± 1.9 227.2 ± 2.5 228.0 ± 1.6 227.4 ± 1.2 227.1 ± 0.6 228.0 ± 1.5 226.7 ± 5.6 198.4 ± 6.9 196.2 ± 26.5

Muscovite separates 04HN24 from Chongcuiling, Dongfang, NE-trending Chongcuiling shear zone, J = 0.013776 300 17.1890 0.025440 0.06730 0.0041 0.055 500 14.9789 0.016618 0.09823 0.0024 0.144 620 14.6026 0.016077 0.04996 0.0008 0.502 720 13.1884 0.011494 0.04311 0.0007 0.745 850 12.3768 0.008530 0.03772 0.0004 1.200 980 12.6991 0.009398 0.05160 0.0007 1.072 1050 12.2750 0.008219 0.05094 0.0005 1.239 1100 11.9685 0.007066 0.08690 0.0015 0.828 1150 11.7259 0.006295 0.15408 0.0011 0.554 1200 12.1340 0.007809 0.37430 0.0037 0.229 1250 12.5997 0.011933 0.32713 0.0075 0.097 1300 14.8605 0.016919 0.17804 0.0060 0.044 Plateau age: 229.6 ± 0.3 Ma, isochron age: 230.4 ± 1.1 Ma, 40Ar/ 36Ar ratios = 291.0

9.67 ± 0.84 10.07 ± 0.32 9.85 ± 0.12 9.79 ± 0.10 9.85 ± 0.13 9.92 ± 0.14 9.84 ± 0.13 9.88 ± 0.17 9.87 ± 0.21 9.86 ± 0.24 9.10 ± 0.17 9.87 ± 0.60

0.82 2.15 7.24 11.10 17.89 15.98 18.46 12.34 8.26 3.42 1.45 0.65

225.5 ± 2.8 234.1 ± 2.1 229.4 ± 1.2 228.1 ± 0.9 229.5 ± 1.5 230.9 ± 1.5 229.3 ± 1.4 230.1 ± 1.8 229.7 ± 2.0 228.9 ± 1.4 212.3 ± 2.1 229.6 ± 7.1

Sericite separates 04HN12 collected from Shilu, Changjiang, cleavage zone in the Shilu Group, J = 0.0137737 300 15.049 0.023332 0.08159 0.1674 0.016 500 12.007 0.013311 0.04098 0.0445 0.064 620 1.143 0.010514 0.06191 0.1553 0.055 720 10.804 0.009291 0.05930 0.0850 0.078 850 10.384 0.007712 0.22509 0.2682 0.030 980 9.6151 0.006031 1.21063 1.5750 0.006 Plateau age: 190.2 ± 5.1 Ma, isochron age: 186.9 ± 3.7 Ma, 40Ar/ 36Ar ratios = 308.1

8.15 ± 2.92 8.07 ± 0.73 8.03 ± 0.94 8.06 ± 0.79 8.12 ± 1.99 7.95 ± 9.46

6.35 25.64 22.11 31.25 12.14 2.51

192.0 ± 68.9 190.1 ± 17.1 189.3 ± 22.1 189.8 ± 18.6 191.3 ± 46.8 187.4 ± 223

The first list for 04HN-17 and -18 notes laser ouput (%) and others for temperature (°C). λ = 5.543*10-10/a, The Laser 40Ar/39Ar step-heating by GV-5400 instrument were used to date the samples 04HN17 and 04HN18. Sample 04HN208, 04HN225, 04HN04, 04HN24, 04HN316 and 04HN12 were analyzed by the step-heating using the MM-1200 instrument.

8a–d. The 40Ar/39Ar plateau age of the spectra is herein defined by (1) the N75% contiguous gas fractions, (2) at least eight contiguous steps (exception of 04HN12 sample) of all the gas evolved from the sample, and (3) their apparent ages agreeing to the integrated age of the plateau segment with invariability at 1σ level of uncertainty. 04HN17 and 04HN18 mylonites were taken from the Gong'ai shear zone (Dongfang) and are originally schist from the Ordovician sequence and orthogneiss from the Mesoproterozoic Baoban Group, respectively. 04HN208 and 04HN225 are mylonites from the Wuzhishan orthogneissic intrusion and were collected from Changzheng and Heping (Qiongzhong), respectively (Figs. 1b and 2, 3). They are the representatives of the WNW-trending shear zones. As described earlier, their associated microscopic textures suggest a general top-to-the-NNE thrusting with a dextral shearing component. Sample 04HN04 taken from Gezhencun (Changjiang County) is mylonitic gneiss from the Mesoproterozoic Gezhencun Formation. Sample 04HN24 is mylonitic micaschist from the Mesoproterozoic Ewenling Formation, collected from Chongzuling (Ledong County). 04HN316 from Fulong (Baisha) is a granitic mylonite from the Qiongzhong garnet-bearing granitic batho-

lith. These samples are the representative mylonites of the NE-trending shear zones (Fig. 5a–b and Table 1). Microscopic analyses show that they exhibit abundant fine, elongated quartz grains, and preferentiallyoriented muscovite and biotite flakes. Muscovite separates from 04HN17 have the 40Ar/39Ar apparent ages of 247.7–251.7 Ma at the forth to the fifteenth heating stages, yielding the plateau ages of 249.7 ± 1.0 Ma defined by 87% released gas (Fig. 7a). Muscovite separates from 04HN18 show the staircase-shaped age spectra that are similar to the pattern observed by West and Lux (1993). Referring to the interpretation proposed by Kirschner et al. (1996) and Mülch et al. (2002), we believe that such an age-pattern reflects the presence of some relict muscovite porphyroclasts within recrystallized fine-grain muscovite. The 40Ar/39Ar apparent ages of successive twelve temperature steps fall within 1σ of the average and gave a plateau age of 247.6 ± 1.4 Ma defined by 68% released gas (Fig. 7b), similar to that of 04HN17 sample. The muscovite separates from 04HN208 and 04HN225 yielded the 40Ar/39Ar plateau ages of 242.6± 1.7 and 242.0 ± 1.7 Ma, with over 70% 39Ar release (Fig. 7c–d), respectively. Muscovite separates from 04HN04 and 04HN24 yielded the similar plateau ages of 227.4 ±

920

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Fig. 7. The 40Ar/39Ar apparent age spectra of synkinematic muscovites from the mylonitic rocks for the WNW-trending shear zones. (a) 04HN-17 (Gong'ai, Dongfang); (b) 04HN-18 (Gong'ai, Dongfang); (c) 04HN-208 (Changzheng, Qiongzhong); (d) 04HN-225 (Heping, Qiongzhong). Coarse lines give the apparent ages (the length of bars reflects 1σ uncertainty). See Fig. 1b for sample locations.

0.2 Ma and 229.1± 0.3 Ma, respectively, which are well-defined by N90% 39Ar release (Fig. 8a–b). The presence of synkinematic muscovite is poor in sample 04HN316. In contrast, the synkinematic biotite separates are abundant. Biotite from 04HN316 gave the 40Ar/39Ar plateau age of 220.7± 1.5 Ma defined by N90% 39Ar release (Fig. 8c). Sample 04HN12 was collected from strongly-cleavaged schist in the Neoproterozoic Shilu Formation, which contains steeply dipping foliation to the NW. The separated sericite with grain sizes of 150–180 mesh gave the 40Ar/39Ar apparent ages of 189.3–192.0 Ma with a referred plateau age of 190.2 ± 5.1 Ma (N90% 39Ar released) at five successive temperature steps ranging from 300 °C to 980 °C (Fig. 8d). Available data show that mica 40Ar/39Ar ages from the mylonite usually display a systematic decrease with grain size decrease (e.g., West and Lux, 1993). Therefore, it is inferred that the younger age (190 Ma) of this sample relative to other samples (04HN04, 04HN24 and 04HN316) might be due to finer grain sizes. This age might represent the minimal age of shearing deformation for the NEtrending high-strain shear zones. Of the samples described earlier, 04HN17 and 04HN18 additionally gave young apparent ages (44–149 Ma) at lower temperature heating steps. It is generally accepted that the 40Ar in the Ca phase is first released in low temperatures and the degassing of Ca-derived Ar at low temperatures closely follows the radiogenic Ar (Foland, 1974; Dallemyer et al., 1999; Maluski et al., 1993). The 40Ar/39Ar spectra for low temperatures possibly result from: 1) a non-retentive phase with a relatively low apparent K/Ca ratio; 2) a more refractory phase with relatively low apparent K/Ca ratios; 3) the presence of microcracks and crystalline defects induced by the development of cleavage or fractures (e.g. Maluski et al., 1993; Reddy et al., 1999). The young

apparent ages (196–198 Ma) at the higher temperature heating steps for 04HN04 might suggest thermal reset during the early Jurassic deformational event. The older apparent ages were occasionally observed for 04HN208, 04HN225 and 04HN316 at the highest temperature steps. This is possibly related to (1) the degassing of absorbent gas on mineral grain surfaces or of microcracks, or (2) the presence of the inherited or incompletely resetting mica formed during the late early Paleozoic tectonic event in the eastern South China, Hainan and northern Vietnam areas (e.g., Wang et al., 2007; Carter and Clift, 2008; Metcalfe, 2010). 5. Discussion and conclusion 5.1. Timing of shearing deformation Various different viewpoints have been proposed for the closure temperatures of argon retention in synkinematic minerals. For examples, Reiners and Brandon (2006) considered that the effective closure temperatures for muscovite and biotite are 380 °C and 348 °C, respectively, at a 10 °C/Myr cooling rate and specified as value. Itaya et al. (2009) and Gouzu et al. (2006) proposed that the closure temperature of muscovite might be much higher than that previously thought, probably reaching 500 °C. It is possible that this temperature is even higher (e.g., Villa, 1998). However, the closure temperatures for argon retention are usually believed to range from 350 to 450 °C, 325 to 400 °C and 300 to 350 °C for synkinematic muscovite, biotite and sericite, respectively (e.g., Dodson; 1973; Purdy and Jäger; 1976; Harrison et al., 1985). In conjunction with the analyses of the deformation conditions documented in Table 1 and Section 3.3, the

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Fig. 8. The 40Ar/39Ar apparent age spectra of synkinematic mineral separates from the mylonitic rocks for the NE-trending Gezhen and Chongzuling shear zones. (a) 04HN04 (muscovite, Gezhen, Changjiang), (b) 04HN24 (muscovite, Chongzuling, Ledong), (c) 04HN316 (biotite, Fulong, Baisha), and (d) 04HN12 (sericite from the cleavaged schist, Shilu, Changjiang). Coarse lines give the apparent ages (the length of bars reflects 1σ uncertainty). Also see Fig. 1b for sample locations.

40 Ar/39Ar plateau ages of 242–250 Ma for muscovite separates most likely represent the timing of cooling through appropriate argon closure temperatures for muscovite. This age can be interpreted as the best estimation for the shearing age of the WNW-trending shear zone in Hainan Island. The consideration mentioned earlier can be further constrained by the following geological observations. (1) Deformation on the WNWtrending shearing zones occurred in the Mesoproterozoic to Silurian successions and Permian Wuzhishan orthogneissic intrusion (Li et al., 2006 and authors' unpublished data). (2) The thrust sheets with a top-to-the-NNE thrusting entangled the Carboniferous and lower Permian sequences in the Bangxi and Furongtian areas (Zhan et al., 1996; Zhang et al., 1997). (3) In the Shilu and Gong'ai areas (Changjiang), the WNW-trending shear zones were truncated by the NE-trending shear zones (Figs. 2 and 3) with the deformation ages of 190–229 Ma (see further discussions later). (4) The 230–237 Ma Jianfengling and Qiongzhong granitic batholith truncated the WNWtrending shear zone (Figs. 1b and 2), as observed at the west of Wuzhishan and south of the Qiongzhong city. For the NE-trending shear zones, the closure temperatures for argon retention for the dated synkinematic minerals (e.g., muscovite and biotite) are compatible with the deformation temperatures inferred from the quartz fabric patterns described earlier. Two muscovite separates (04HN04 and 04HN24) gave the 40Ar/39Ar plateau ages of 227–229 Ma, slightly older than that of the 04HN316 biotite separates (221 Ma), suggestive of rapid cooling. Taking account into (1) the overprinting of 196–198 Ma thermal event indicated by the 04HN04, and (2) the NE-trending shear zones were truncated by the Jurassic (SHRIMP zircon U–Pb age of 186 Ma) Daxian granitic batholith (e.g., Guangdong BGMR, 1988; Wang et al.,

1991, 1992; Ge, 2003), we infer that the young 40Ar/39Ar plateau age (190 Ma) for the 04HN12 sericite separates probably reflects the minimal deformation age of the NE-trending high-strain shear zones. Therefore, the 40Ar/39Ar plateau ages of 190–229 Ma (late Triassic– early Jurassic) can be interpreted as the shearing age span for the NEtrending shear zones. Such a consideration is further supported by previous geochronological data and geological observations. For example, Wang et al. (1991, 1992) reported the K–Ar ages of 191– 197 Ma for sericite separates from the mylonite in the Gezhen shear zone. A 40Ar/39Ar step-heating age of 228 Ma is also given by biotite separates from the Gezhen mylonite (Zhang et al., 1997). In the Baisha and Qiongzhong areas, the NE-trending shear zones truncated the Qiongzhong granitic batholith of 230–237 Ma (e.g. Ge, 2003). In summary, the following temporal sequence for the development of shear zones in Hainan Island can be inferred on the basis of our structural and geochronological analyses in combination with the geological truncation relationship described earlier. The WNWtrending high-strain shear zones might be the product of the topto-the-NNE shearing event (with a dextral shearing component) at 242–250 Ma. The NE-trending shear zones then developed in a later top-to-the-SE thrusting event (with a sinistral shearing component) at 190–229 Ma. 5.2. Triassic structural pattern at Hainan Island Our structural and geochronological data and discussions mentioned earlier point to a two-stage tectonic model for Hainan Island, that is, the early Triassic (242–250 Ma) dextral top-to-the-NNE thrusting followed by late Triassic (190–229 Ma) sinistral top-to-the-SE thrusting. The transition between the two stages probably occurred in the middle

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Triassic. This consideration is also supported by the presence of the unconformity between the middle-lower Triassic and upper Triassic sequences in the Shiwandashan area (Guangxi Province) of the southern South China Block (e.g., Guangxi BGMR, 1985; Zhang, 1999; Liang and Li, 2005). It has been reported that there occurred a series of NW/WNWtrending shear zones characterized by the dextral strike-slip or transpression in the Song Ma, Song Da, Da Nang–Khe Sanh, Song Ca– Rao Nay, Ngoc Linh, Po Ko, Truong Son and Kontum areas in Vietnam (see Fig. 2 in Lepvrier et al., 1997, 2004, 2008 for these locations). Synkinematic minerals (hornblende, biotite and muscovite) from these shear zones gave the 40Ar/39Ar plateau ages of 240–250 Ma (Lepvrier et al., 1997, 2004, 2008; Nam et al., 1998, 2001; Maluski et al., 2005; Owada et al., 2007; Carter and Clift, 2008). The zircon grains for the high-grade metamorphic rocks yielded the metamorphic ages of 247–253 Ma in northern Vietnam (Carter et al., 2001; Carter and Clift, 2008; Maluski et al, 2005; Owada et al., 2007). The Sm–Nd isochron ages of 240–247 Ma were also given by the pelitic and enclosed mafic granulite in northern Vietnam (Nakano et al., 2008). These data are in good agreement with the 40Ar/39Ar ages of synkinematic minerals (Lepvrier et al., 2008 and reference therein), clearly indicating a rapid cooling and synchronously thermotectonic event throughout Vietnam during the 240–250 Ma time internal (e.g., Lepvrier et al., 2008 and references therein). The geological and geochronological consistency apparently indicates that the WNWtrending shear zones in Hainan are temporally compatible with and spatially linked to those in Vietnam. In the South China hinterland, the early Mesozoic structural pattern is dominated by a NE-trending shear zone and flower-like structures from the Xuefeng to Yunkai then to Wuyishan tectonic zones, which was dominantly formed at 195–230 Ma as constrained by the 40Ar/39Ar dating results for synkinematic minerals (see also Wang et al., 2005, 2007). Therefore, in combination with the available data from the South China and Indochina Blocks, the WNW- and NEtrending structural patterns in Hainan are temporally and geometrically consistent with those of the Indochina and South China Blocks, respectively. 5.3. Implication on early Triassic amalgamation of the Indochina with South China Blocks Li et al. (2006) and Li and Li (2007) suggested that the Pacific oceanic flat-slab subducted under the overriding South China continent. Such a model of flat-slab subduction, the best known example being considered to be located beneath South America (e.g., Cahill and Isacks, 1992; Gutscher et al., 2000; Pardo et al., 2002; Espurt et al., 2008), was introduced to explain the petrogenesis of the middle Permian (260–272 Ma) Wuzhishan syntectonic granites. According to this model, the Triassic structural elements in Hainan were closely associated with the westward flat-slab subduction of the Pacific plate. However, Zhou and Li (2000) reported that the westward subduction of the Pacific plate was probably not initiated until around the midJurassic (e.g., Engebretson et al., 1985) because late Paleozoic to early Mesozoic ophiolites, Permian arc magmatism and foreland basin have not been well-recognized so far along the coastal provinces in southeast China (also see Wang et al., 2005, 2007; Shu et al., 2008 and references therein). According to the model of flat-slab subduction of the Pacific plate, the structural pattern should be characterized by the NE-trending structural elements in Hainan Island. The presence of the early Triassic WNW-trending shear zones in Hainan Island are obviously unharmonious with what would be expected from the Permian flat-slab subduction model. In contrast, the Permian mafic and ultramafic rocks, which have been previously identified as ophiolitic rocks, occurred in the Bangxi and Chengxin areas (Changjiang) roughly along the Changjiang–Qionghai Fault (Fig. 1a–b; Li et al., 2000; X.H. Li

et al., 2002; Z.X. Li et al., 2002; Xu et al., 2007). These mafic rocks are geochemically considered to be developed in the limited ocean, backarc basin or rifting settings (Li et al., 2000; X.H. Li et al., 2002; Z.X. Li et al., 2002; Xia et al., 1990, 1991a, b; Fang et al., 1992; Xu et al., 2007). Along the NE-trending coastal zone of eastern South China, the occurrence of mafic rocks associated with the eastern Paleotethys evolution has been poorly identified so far. In contrast, with exception of the Middle Permian (ca. 260– 262 Ma) flood basalts in Jinping (southern Yunnan, SW China) and Song Da (northern Vietnam) along the Red River Fault (Zhou et al., 2008; Ali et al., 2010 and reference therein), mafic and ultramafic rocks occurred in abundance along the Song-Ma and Ailaoshan tectonic zone west of the Bangxi–Chengxin zone (e.g., Li et al., 2000; X.H. Li et al., 2002; Z.X. Li et al., 2002). These rocks along the Song Ma zone were previously referred to as relics of the Paleotethys Ocean due to the presence of gabbro and harzburgite with a MORB-like affinity (Carter et al., 2001; Carter and Clift, 2008; Villeneuve et al., 2010 and reference therein). However, more and more researchers believed that the Paleotethys Ocean extends along the Raub–Bentong and Changning–Menglian zones from Peninsular Malaysia to northern Thailand and then to southwest China, evidenced by the wide presence of the late Paleozoic deformation, metamorphism and magmatism (e.g. Zhong, 1998; Hara et al., 2009; Villeneuve et al., 2010; Wang et al., 2010). The association of metagreywacke, metabasite, marble, ultramafic and plagiogranite in the Song Ma anticlinorium could be interpreted as the metamorphosed relic of a fore-arc and island arc complex (e.g., Metcalfe, 1996, 2010). The associated mafic rocks might be referred as the product in a back-arc basin along the Song Ma zone (e.g., Metcalfe, 1996, 2010). This continental linkage of the Indochina Block with the South China Block is also supported by the presence of Devonian fish fossils at Ly Hoa (south of the Song Ma Fault) and late Permian Pangean Dicynodon in the Indochina Block (Janvier et al., 1994; Thanh et al., 2007; Young and Janvier, 1999). The mafic and ultramafic rocks along the Ailaoshan tectonic zone, traditionally believed to be the ophiolitic fragment of the eastern Paleotethys Ocean, have the ages of the 268–287 Ma and exhibit the geochemical nature of an arc–back-arc basin (e.g., Fan et al., 2010 and references therein; Jian et al., 2009a,b). As a result, the data mentioned earlier appear to indicate that, during late Paleozoic, the Bangxi–Chengxin ophiolitic zone in Hainan probably extended westerly to join the synchronous Song Ma–Ailaoshan tectonic zone, constituting an east-westerly convergent boundary between the Indochina and South China Blocks in spite that the tectonic nature of this zone is still unclear during the early Paleozoic (Fig. 1a; e.g., Zhong, 1998, Jian et al., 1998, 2009a, b; Lan et al, 2000; Li et al., 2000; X.H. Li et al., 2002; Z.X. Li et al., 2002; Carter et al, 2001; Carter and Clift, 2008). The WNW-trending structural elements in Hainan are most likely the derivations of the northward amalgamation of the Indochina with South China Blocks along the convergent boundary during the early Triassic. As a result, South Hainan Island might have tectonic affinity to the Indochina Block, whereas North Hainan (north of the Changjiang–Qionghai fault) shows an affinity to the southern South China Block. These data additionally indicate that the amalgamation between the Indochina and South China Blocks might be initiated in the middle Permian (ca. 272 Ma), as also evidenced by (1) the eastwesterly orientated Wuzhishan gneissic granites in the central Island dated at 260–272 Ma (Li et al., 2006 and authors' unpublished data), and (2) Middle Permian conglomerate unconformably overlying lower Permian limestone in central Island (Fig. 1a; Guangdong BGMR, 1988; Xia et al., 1990, 1991a, b). Such a timeframe (middle Permian to early Triassic) also temporally coincides with the closure time of the Paleotethys Ocean and the final stages of the Pangea assembly (e.g., Sengor and Hsű, 1984; Metcalfe, 1996, 2002; Lacassin et al., 1997; Lepvrier et al., 1997, 2004, 2008; Nam et al., 1998, 2001; Singharajwarapan and Berry, 2000; Carter et al., 2001; Cawood, 2005;

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Owada et al., 2007; Cawood and Buchan, 2007; Carter and Clift, 2008). The NE-trending structural pattern in Hainan Island should be controlled by early Mesozoic tectonic evolution in the South China hinterland (e.g., Wang et al., 2005, 2007) or middle to late Triassic subduction–accretion of the Pacific-derived terranes with the southeast Asia continental margin (e.g., de Jong et al., 2009a,b) despite the fact that its geodynamic mechanism could not be well constrained by our data set. Acknowledgements We would like to thank Y-Z Zhang for their help during fieldwork and dating analyses. We are grateful to Dr. H.N. Qiu for Ar-Ar analyses, Profs S. Kwon, K. de Jong and T. Itaya for their critical and constructive reviews on this paper. This study was jointly supported by grants from the Chinese Academy of Sciences (KZCY2-yw-128 and KZCY1-yw-15-1), the National Basic Research Program of China (2007CB411403), Natural Science Foundation of China (40772129, 40830319, 40902062 and 40825009) and China Petroleum and Chemical Corporation Grants (08YPH004). This is a contribution to Guangzhou Institute of Geochemistry, the Chinese Academy of Sciences. References Ali, J., Fitton, J.G., Herberg, C., 2010. Emeishan large igneous province (SW China) and the mantle-plume up-doming hypothesis. Journal of the Geological Society, London 167, 953–959. Cahill, T., Isacks, B.L., 1992. Seismicity and shape of the subducted Nazca plate, Journal of Geophysics Research 97, 17, 503-17, 529. doi:10.1029/92JB00493. Carter, A., Clift, P.D., 2008. Was the Indosinian orogeny a Triassic mountain building or a thermotectonic reactivation event? C. R. Geoscience 340. Carter, A., Roques, D., Bristow, C., 2001. 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