Neoproterozoic granitoids in South China: crustal melting above a mantle plume at ca. 825 Ma?

Precambrian Research 122 (2003) 45–83

Neoproterozoic granitoids in South China: crustal melting above a mantle plume at ca. 825 Ma? Xian-Hua Li a,b,∗ , Zheng-Xiang Li b , Wenchun Ge a , Hanwen Zhou a,c , Wuxian Li a , Ying Liu a , Michael T.D. Wingate b b

a Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, P.O. Box 1131, Guangzhou 510640, China Tectonics Special Research Centre, School of Earth and Geographical Sciences, The University of Western Australia, Crawley, WA 6009, Australia c Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China

Received 1 January 2002; received in revised form 3 April 2002; accepted 15 April 2002

Abstract Neoproterozoic pre-rifting granitoids are widespread in the Yangtze craton, South China. Mineralogical, petrographic and geochemical characteristics indicate that there are two types of peraluminous, S-type granitoids—muscovite-bearing leucogranites (MPG) and cordierite-bearing granodiorites (CPG), and two types of I-type granitoids—K-rich calc-alkaline granitoids (KCG) and tonalite–trondhjemite–granodiorite (TTG). Sm–Nd isotopic data suggest that all were generated by partial melting of various crustal rocks without appreciable involvement of new mantle-derived magmas, i.e. pelitic and psammitic sources for MPG and CPG, and tonalitic to granodioritic and amphibolite sources for KCG and TTG, respectively. Despite pronounced geochemical and isotopic heterogeneity of these granitoids, and their vast areal extent, new SHRIMP U–Pb zircon ages, together with previous U–Pb data, indicate that they formed within short time interval of ∼5 Ma at ca. 825–820 Ma. The granitoids were essentially coeval with ∼825 Ma mafic/ultramafic intrusions in South China. The granitoids exhibit no temporal or spatial zonation, hence no simple genetic relationship can be established between the different granitoid types and geodynamic environments such as subduction, continental collision, and post-collisional relaxation, as suggested previously. Instead, the coeval intrusion of such mixed types of granitoids and mafic/ultramafic rocks over an area of >1000 km × 700 km suggests that these rocks are more likely to have resulted from extensive crustal anatexis caused by conductive heating above a mantle plume beneath South China at ∼825 Ma. This interpretation has significant implications to better understanding of geodynamic environments in which the granitoids formed, i.e. granitoids can form not only in various environments during different stages of the Wilson cycle but also above mantle plumes. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Neoproterozoic; Granitoids; Yangtze Craton; South China; Mantle plume; Ion microprobe U–Pb geochronology

1. Introduction Granitoid rocks are amongst the most common rocks in the continental crust, and play an important ∗

Corresponding author. Fax: +86-20-85290130. E-mail address: [email protected] (X.-H. Li).

role in crustal evolution. Granitoids are generated typically during periods of heat and/or mass transfer from the mantle to the crust, and are thus linked commonly to major tectonic events. Various petrogenetic classification schemes have been proposed (see review of Barbarin, 1999, and references therein). Amongst these classification schemes and many case

0301-9268/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 9 2 6 8 ( 0 2 ) 0 0 2 0 7 - 3

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studies of granitoids, particular attention has focussed on relationships between granitoid types, their origins, and the geodynamic environments in which they formed. It is now generally accepted that granitoids can form in various environments during different stages of the Wilson cycle, such as thinning and rifting of continental lithosphere, divergence of oceanic lithosphere, convergence between oceanic and/or continental lithosphere, and orogenic collapse. However, possible relationships between granitoid formation and mantle plume activity has rarely been discussed, although plumes constitute an important process of heat and mass transfer from mantle to crust (e.g. Hill et al., 1992). Widespread Neoproterozoic granitoids in South China have been interpreted traditionally to be syn-orogenic (e.g. Xing et al., 1989) and late- to post-orogenic (e.g. Li, 1999) during the early Neoproterozoic Jinningian orogeny. Li et al. (1999) proposed the existence of a ∼825 Ma mantle plume beneath South China, based on (1) the identification in South China of 828 ± 7 Ma mafic to ultramafic intrusions that are coeval with the plume-related Gairdner dyke swarm in Australia (Zhao et al., 1994; Wingate et al., 1998), (2) the presence of contemporaneous granitoids accompanied by continental scale unroofing, and (3) the subsequent continental rifting. The 828 Ma mafic and ultramafic intrusions are associated closely in time and space with the 819–826 Ma granitoids in northern Guangxi (Li, 1999). Thus, Li et al. (1999) considered that these 819–826 Ma granitoids were products of melting above a mantle plume. In this study, we report new SHRIMP U–Pb zircon ages and systematic geochemical and Nd isotopic analyses for the major Neoproterozoic granitoids that commonly are overlain unconformably by Neoproterozoic rift strata over a much of South China, and explore possible links between the origin of the granitoids, the proposed mantle plume and continental rifting.

2. General geology and sampling South China consists of two major tectonic blocks, the Yangtze craton to the northwest, and the Cathaysia block to the southeast (Fig. 1). The oldest rock unit in the Yangtze craton is the Kongling Complex that crops

out in the northern interior of the craton, consisting of the Archean to Paleoproterozoic high-grade metamorphic TTG (tonalite, trondhjemite and granodiorite) gneisses, metasedimentary rocks and amphibolites (Ling et al., 1998; Gao et al., 1999; Qiu et al., 2000). Mesoproterozoic metamorphic rocks are widespread around the craton, and were intruded by Neoproterozoic granitoids and overlain unconformably by Neoproterozoic rift successions. The Yangtze and Cathaysia blocks are generally considered to have collided in the early Neoproterozoic. The age of the collision is bracketted between the formation of NE Jiangxi Ophiolite at ∼1000 Ma (Chen et al., 1991; Li et al., 1994) and granitoid magmatism at ∼820 Ma (e.g. Li, 1999). Li et al. (2002a) reported evidence of Grenville-aged metamorphism, and sediments likely derived from Cathaysia, in possible late Mesoproterozoic foreland basins in southern Yangtze, although they suggested that compressional tectonism in South China probably continued for some time after 1.0 Ga. Neoproterozoic granitoids in the Yangtze craton (Fig. 1) have been grouped into an older Jinningian suite, that intruded Mesoproterozoic metamorphic basement and is overlain by Neoproterozoic rift successions, and a younger Chengjiangian suite, that formed during Neoproterozoic rifting and commonly intruded the lower rift successions and is overlain unconformably by late Neoproterozoic (<750 Ma) platform strata. This paper focuses on the early pre-rifting granitoids from Yunnan, Guangxi, Jiangxi, Anhui and Huibei Provinces, whereas the younger syn-rifting magmatism is the subject of a separate paper by Li et al. (2003, this issue). 2.1. Yunnan Province Neoproterozoic pre-rifting granitoids in Yunnan Province are represented by the Eshan Pluton (Fig. 1a), which crops out over ∼200 km2 to the south of Eshan County, central Yunnan Province. The pluton intruded Mesoproterozoic metamorphic rocks of the Kunyang Group, and is overlain unconformably by the lower Sinian System sandstones (Chengjiang Formation) and glacial deposits (Nantuo Formation). K-feldspar porphyritic granite is the dominant rock type, although quartz diorite and granodiorite are exposed over ∼7 km2 in the northern part of the pluton. Quartz

Fig. 1. Simplified map showing the distribution of the Neoproterozoic granitoids in SE China, modified after Yunnan (1990), Guangxi (1985), Jiangxi (1984), Anhui (1987) and Hubei (1990). U–Pb zircon ages are from Ma et al. (1989) for the Huangling Pluton; Roger and Calassou (1997) for the Gezong Pluton; Li (1999) for the Bendong, Sanfang and Yuanbaoshan Plutons in Northern Guangxi; Sinclair (2001) for the Tongde Gabbro; Zhang et al. (2001) for Hannan Complex; and this study. (a) Geological map of Eshan K-rich granitoids in Yunnan; (b) geological map of muscovite-bearing leucogranites and biotite-rich granodiorites in northern Guangxi; (c) geological map of Jiuling cordierite-bearing granitoids in northern Jiangxi; (d) geological map of cordierite-bearing granitoids in southern Anhui; (e) geological map of Huangling tonalites, trondhjemites and granodiorites (TTG) in Hubei.

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Fig. 1. (Continued ).

X.-H. Li et al. / Precambrian Research 122 (2003) 45–83

49

Fig. 1. (Continued ).

diorite and granodiorite generally display transitional or cross-cutting relationships with the surrounding K-rich granitoids, and diorites occur as enclaves within the K-rich granitoids (Yunnan, 1990; Ma, 1991). These observations suggest that the two rocks are essentially coeval, although the diorites appear to

have formed slightly earlier. Ma (1991) reported a five-point Rb–Sr isochron age of 836 ± 36 Ma (2σ, recalculated using ISOPLOT of Ludwig, 1998), and a U–Pb upper intercept age of 828 ± 43 Ma, based on relatively discordant zircon and monazite data, which was interpreted as the time of granite formation.

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The K-rich granitoids contain 20–30% quartz, 35–50% K-feldspar, 15–20% plagioclase (An10–30 ), and 7–15% biotite. Hornblende is absent. Accessory minerals include zircon, apatite, titanite, ilmenite, magnetite, and rutile. The quartz diorite and granodiorite contain 10–20% quartz, 5–20% K-feldspar, 50–60% plagioclase (An25–45 ), 10–13% biotite, and 7–15% hornblende. Sample 98KD154 is a medium-grained K-feldspar porphyritic granite collected from the northern part of the pluton for U–Pb zircon analysis (Fig. 1a). 2.2. Guangxi Province Neoproterozoic granitoids outcrop over ∼1500 km2 in northern Guangxi Province (Fig. 1b). They intruded the tightly folded Mesoproterozoic Sibao Group, and are overlain unconformably by Neoproterozoic rift strata of the Danzhou Group. Coarse-grained muscovite leucogranite is the dominant rock type, and is exposed over ∼1400 km2 , including the Sanfang batholith (∼1000 km2 ), the Yuanbaoshan batholith (∼300 km2 ) and several small plutons and stocks. Biotite granodiorite (∼85 km2 ) makes up the remainder of the granitoids in the region, including the Bendong Pluton (∼40 km2 ) and a few smaller plutons and stocks. Previous high precision SHRIMP and conventional zircon analyses yield concordant U–Pb ages of 819 ± 9, 826 ± 10 and 824 ± 4 Ma for the Bendong, Sanfang and Yuanbaoshan Plutons, respectively (Li, 1999), indicating that these granitoids are essentially coeval at ca. 820–825 Ma. The Sanfang leucogranite contains 26–35% quartz, 35–44% K-feldspar, 26–30% sodic plagioclase (An7–13 ), 2–6% biotite and 1–5% muscovite, and minor tourmaline, epidote, zircon, monazite, and ilmenite. Tourmaline enclaves ranging up to 20 cm occur ubiquitously in the marginal to transitional phases of the batholith. The Yuanbaoshan leucogranite is similar petrographically to the Sanfang body, containing 22–40% quartz, 27–45% K-feldspar, 12–36% sodic plagioclase (An5–20 ), 1–7% biotite, 1–3% muscovite and trace amounts of tourmaline, zircon, epidote, ilmenite, and monazite. The Bendong biotite granodiorite contains 18–38% quartz, 4–20% K-feldspar, 30–46% plagioclase (An31–45 ), and 10–17% biotite. Secondary muscovite occurs as rims on biotite. Hornblende is absent. Accessory min-

erals include zircon, apatite, epidote, rutile, ilmenite, and magnetite. Chromite crystals were detected in some mafic samples (Zhao et al., 1987). Biotite-rich enclaves are common. 2.3. Jiangxi Province The Jiuling batholith in northern Jiangxi Province is the largest Neoproterozoic granitoid intrusion in South China, with an exposed area of ∼2500 km2 (Fig. 1c). The batholith intruded Mesoproterozoic Shuangqiaoshan Group metasediments and is overlain unconformably by the Neoproterozoic Dongmen Formation, which has a basal granitoid conglomerate. Biotite-rich, cordierite-bearing granodiorite is the dominant rock type, making up ∼99% of the batholith. A late phase of two-mica leucogranite makes up the remainder of the batholith (Jiangxi, 1984). There are few isotopic results available for the Jiuling granitoid. A biotite 40 Ar–39 Ar age of 936 ± 15 Ma was reported by Hu et al. (1985). However, apparent ages ranging from 464 to 939 Ma with increasing heating temperature, and the mean age of 936 ± 15 Ma was obtained from the last three high temperature (900–1200 ◦ C) steps. The reliability of this 40 Ar–39 Ar age is thus questionable. Cordierite-bearing granodiorite contains 25–32% quartz, 10–40% K-feldspar, 20–44% plagioclase (An10–42 ), 6–13% biotite, 1–5% cordierite, and variable amounts of secondary muscovite. Accessory minerals include zircon, garnet, apatite, ilmenite, monazite, and rutile (Jiangxi, 1984). Two coarse- to medium-grained cordierite-bearing biotite granodiorites samples 2KJL26-1 and 2KJL14-8 were collected from northern and southwestern parts of the Jiuling Batholith, respectively, for U–Pb zircon analysis (Fig. 1c). 2.4. Anhui Province Neoproterozoic granitoids in southern Anhui have a total outcrop area of ∼200 km2 , and include three main intrusions: the Xucun (133 km2 ), Xiuning (32 km2 ) and Shexian Plutons (32 km2 ) (Fig. 1d). These granitoids intruded Mesoproterozoic Shangxi Group metasediments, and are overlain unconformably by Neoproterozoic sediments of the Xiuning Formation. Zhou and Wang (1988) reported a six-point Rb–Sr

X.-H. Li et al. / Precambrian Research 122 (2003) 45–83

isochron age of 964 ± 184 Ma for the Xiuning Pluton, but with a very low initial 87 Sr/86 Sr ratio of 0.704. Xing et al. (1989) obtained a five-point Rb–Sr isochron age of 765 ± 126 Ma for the Shexian Pluton, with an initial 87 Sr/86 Sr ratio of 0.711. The reported 87 Rb/86 Sr ratios range from 1.2 to 2.5, and the Rb–Sr isochrons are poorly defined. Xing et al. (1989) also reported a mean 207 Pb/206 Pb age of 926 Ma for the Shexian Pluton, based on two highly discordant U–Pb zircon analyses, and biotite K–Ar ages of 744 and 913 Ma for the Xucun Pluton. However, none of these results are regarded as reliable ages of crystallisation. All three plutons consist of biotite-rich, cordieritebearing granodiorite with very similar mineralogy: 24–28% quartz, 18–24% K-feldspar, 30–38% plagioclase (An22–36 ), 8–15% biotite, 3–7% cordierite, and minor secondary muscovite (Zhou and Wang, 1988; Xing et al., 1989). Accessory minerals include zircon, garnet, apatite, ilmenite, monazite, and xenotime. Cordierite is euhedral to sub-euhedral, indicating a magmatic origin (Zhou and Wang, 1988). Sample Qnt, a medium-grained cordierite-bearing biotite granodiorite, was collected from the central part of the Xucun Pluton for U–Pb zircon analysis (Fig. 1d). 2.5. Hubei Province The Huangling granitoid complex, northwest of Yichang City in Hubei Province, is the only Neoproterozoic granitoid in the interior of the Yangtze craton (Fig. 1e). The Huangling Complex, with an exposed area of ∼970 km2 , intruded the Archean–Paleoproterozoic Kongling Complex and is overlain unconformably by the Liantuo Formation, dated at 748 ± 12 Ma (Ma et al., 1989). Four magmatic suites have been identified within the Huangling Complex, based on petrographic features and geological relationships: the Sandouping quartz diorite–tonalite suite, the Huanglingmiao trondhjemite–granodiorite suite, the Dalaoling monzodiorite–monzogranite suite, and the Xiaofeng mafic–felsic composite dyke swarms (Wang et al., 1996). These four suites can be grouped into two series based on mineralogy and petrochemical features: (1) Na-rich TTG (the Sandouping and Huanglingmiao suites), which is the dominant rock type with an outcrop area of ∼890 km2 and (2) minor K-rich calc-alkaline intrusions (the Dalaoling Suite and the Xiaofeng dykes).

51

Ma et al. (1989) reported a SHRIMP U–Pb zircon age of 819 ± 7 Ma for the Huanglingmiao trondhjemite–granodiorite suite. Zircon U–Pb and whole-rock Rb–Sr isotopic dating was conducted on other granitoid suites as part of geological investigations during construction of the Yangtze Gorge Dam. An upper intercept age of 831±30 Ma by conventional U–Pb zircon analysis and a five-point Rb–Sr isochron age of 834 ± 48 Ma were obtained for the Sandouping quartz diorite–tonalite suite (Li et al., 1994). These two ages are indistinguishable within error from the SHRIMP U–Pb zircon age of 819 ± 7 Ma for the Huanglingmiao Suite. Therefore, the dominant TTG rocks of the Huangling Complex were emplaced at ∼820 Ma. Imprecise Rb–Sr isochron ages of 787 ± 57 and 754 ± 253 Ma were reported for the Dalaoling Suite and the Xiaofeng Dykes, respectively (Ma and Du, 1994). 2.6. Summary Previous isotopic results yield loose age constraints of ca. 960–800 Ma for the pre-rifting granitoids. Precise U–Pb zircon ages for the granitoids in Hubei and northern Guangxi indicate a narrow interval between ca. 825–820 Ma for granitoid formation. To expand the reliable age database and to explore possible origins of the Neoproterozoic granitoids in South China, we conducted SHRIMP U–Pb zircon analyses and systematic geochemical and Nd isotopic analyses.

3. Analytical methods Samples for U–Pb analysis were processed by conventional magnetic and density techniques to concentrate non-magnetic, heavy fractions. A representative selection of zircons was extracted from each concentrate by hand-picking under a binocular microscope. Zircons, together with a zircon U–Pb standard, were cast in an epoxy mount, which was then polished to section the crystals for analysis. Zircons were documented with transmitted and reflected light micrographs and cathodoluminescence images, and the mount was vacuum-coated with a ∼500 nm layer of high-purity gold. Measurements of U, Th, and Pb (Tables 1–3) were conducted using the Perth Consortium SHRIMP II ion microprobe at Curtin University

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Table 1 Ion microprobe analytical data for zircons from the Xucun grantoid sample Qnt (29◦ 57 37 N, 118◦ 20 21 E) Grain 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 21b

238 U

232 Th

(ppm)

(ppm)

167 231 349 339 183 295 228 308 125 167 984 318 316 269 171 166 194 195 274 216 235 52

24 34 160 97 43 35 59 41 15 28 376 66 175 27 16 19 23 44 25 6 31 26

Th/U 0.14 0.15 0.46 0.29 0.23 0.12 0.26 0.13 0.12 0.17 0.38 0.21 0.55 0.10 0.09 0.11 0.12 0.22 0.09 0.03 0.13 0.50

f206 (%) 0.017 0.227 0.064 0.050 0.079 0.015 0.012 0.085 0.242 0.047 0.005 0.051 0.017 0.017 0.017 0.148 0.313 0.017 0.130 0.202 0.155 0.017

207 Pb∗ /206 Pb∗

206 Pb∗ /238 U

207 Pb∗ /206 Pb∗

(±1σ)

(±1σ)

(Ma) (±1σ)

(Ma) (±1σ)

752 759 844 812 817 882 877 818 831 804 829 811 844 846 897 827 691 837 803 789 808 925

816 826 851 845 831 820 830 821 808 829 864 824 841 808 812 839 822 821 834 831 831 873

0.06432 0.06453 0.06721 0.06618 0.06634 0.06843 0.06827 0.06638 0.06679 0.06594 0.06671 0.06614 0.06720 0.06727 0.06893 0.06665 0.06250 0.06699 0.06591 0.06545 0.06604 0.06989

0.00156 0.00134 0.00137 0.00097 0.00213 0.00138 0.00123 0.00178 0.00180 0.00193 0.00050 0.00096 0.00075 0.00078 0.00099 0.00147 0.00137 0.00105 0.00177 0.00245 0.00179 0.00248

0.1349 0.1367 0.1412 0.1400 0.1375 0.1356 0.1374 0.1358 0.1336 0.1373 0.1433 0.1364 0.1394 0.1335 0.1342 0.1390 0.1360 0.1358 0.1382 0.1375 0.1376 0.1451

0.0019 0.0022 0.0019 0.0014 0.0018 0.0023 0.0021 0.0020 0.0019 0.0033 0.0016 0.0016 0.0027 0.0015 0.0021 0.0020 0.0020 0.0015 0.0021 0.0049 0.0015 0.0035

age

51 43 42 30 66 41 37 55 55 60 16 30 23 24 29 45 46 32 55 77 56 71

206 Pb∗ /238 U

age

11 13 11 8 10 13 12 12 11 19 9 9 15 9 12 11 11 9 12 28 9 20

Table 2 Ion microprobe analytical data for zircons from the Jiuling granitoid samples 2KJL26-1 (29◦ 05 15 N, 114◦ 58 33 E) and 2KJL14-8 (28◦ 29 08 N, 114◦ 32 27 E) 238 U (ppm)

207 Pb∗ /206 Pb∗

232 Th (ppm)

Th/U

Sample 2KJL26-1 1 315 2 623 3 346 4 236 5 259 6 340 7 361 8 250 9 332 10 362 11 271 12 451 13 399 14 322 15 358

73 286 54 67 30 102 160 52 36 92 33 90 63 60 62

0.23 0.46 0.16 0.28 0.12 0.30 0.44 0.21 0.11 0.25 0.12 0.20 0.16 0.19 0.17

0.017 0.014 0.015 0.017 0.132 0.075 0.017 0.158 0.042 0.002 0.034 0.016 0.034 0.018 0.044

0.06638 0.06605 0.06653 0.06709 0.06459 0.06602 0.06606 0.06539 0.06558 0.06634 0.06642 0.08206 0.06617 0.06612 0.06622

Sample 2KJL14-8 1 115 2 272 3 281 4 403

80 127 29 61

0.70 0.47 0.10 0.15

0.228 0.105 0.048 0.017

0.06748 0.06735 0.06653 0.06607

Grain

f206 (%)

206 Pb∗ /238 U (±1σ)

207 Pb∗ /206 Pb∗

0.00075 0.00146 0.00095 0.00054 0.00129 0.00080 0.00045 0.00094 0.00132 0.00144 0.00095 0.00073 0.00052 0.00070 0.00340

0.1341 0.1355 0.1371 0.1340 0.1348 0.1365 0.1377 0.1360 0.1364 0.1346 0.1346 0.2215 0.1343 0.1350 0.1372

0.0018 0.0022 0.0018 0.0014 0.0017 0.0016 0.0032 0.0027 0.0014 0.0041 0.0019 0.0037 0.0026 0.0022 0.0049

819 808 823 841 761 807 808 787 793 818 820 1247 812 810 813

24 46 30 17 42 25 14 30 42 45 30 17 16 22 104

811 819 828 810 816 825 832 822 824 814 814 1290 812 816 829

10 12 10 8 10 9 18 15 8 23 11 19 15 13 28

0.00123 0.00095 0.00163 0.00047

0.1491 0.1481 0.1349 0.1335

0.0029 0.0024 0.0017 0.0014

853 849 823 809

38 29 50 15

896 891 816 808

16 14 10 8

(±1σ)

age

(Ma) (±1σ)

206 Pb∗ /238 U age (Ma) (±1σ)

X.-H. Li et al. / Precambrian Research 122 (2003) 45–83

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Table 3 Ion microprobe analytical data for zircons from the Eshan grantoid sample 98KD154 (102◦ 18 07 N, 24◦ 05 25 E) Grain 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

238 U

232 Th (ppm)

Th/U

(ppm) 111 405 199 610 439 749 696 184 202 174 331 286 250 323 290 266 290

76 128 105 371 245 214 135 100 99 143 153 157 118 155 138 109 137

0.68 0.32 0.53 0.61 0.56 0.29 0.19 0.55 0.49 0.82 0.46 0.55 0.47 0.48 0.48 0.41 0.47

f206 (%)

207 Pb∗ /206 Pb∗

(±1σ) 0.273 0.101 0.110 0.232 0.025 0.064 0.259 0.184 0.028 0.072 0.017 0.151 0.017 0.057 0.017 0.017 0.017

0.06404 0.06647 0.06545 0.06387 0.06654 0.06632 0.07195 0.06505 0.06774 0.06619 0.06781 0.06575 0.06681 0.06680 0.06636 0.06678 0.06724

0.00177 0.00211 0.00118 0.00064 0.00061 0.00109 0.00149 0.00113 0.00119 0.00090 0.00068 0.00085 0.00072 0.00105 0.00054 0.00058 0.00065

of Technology. Decay constants employed are those recommended by Steiger and Jäger (1977). U–Th–Pb ratios and absolute abundances were determined relative to the CZ3 standard zircon (206 Pb/238 U = 0.09143 corresponding to 564 Ma, 550 ppm 238 U; Pidgeon et al., 1994; Nelson, 1997), analyses of which were interspersed with those of unknown grains, using operating and data processing procedures similar to those described by Nelson (1997). Measured compositions were corrected for common Pb using non-radiogenic 204 Pb. Corrections are sufficiently small to be insensitive to the choice of common Pb composition, and an average crustal composition (Cumming and Richards, 1975) appropriate to the age of the mineral was assumed. Uncertainties on individual analyses in data tables are reported at a 1σ level; mean ages for pooled 206 Pb/238 U analyses are quoted with 95% confidence interval, except where noted otherwise, including uncertainty in calibration against the CZ3 zircon standard. Major element oxides were determined using a Philips PW 1400 X-ray fluorescence spectrometer (XRF) at the Department of Geology and Geophysics, the University of Western Australia, a Rigaku RIX 2000 XRF at the Department of Geology, National Taiwan University, and a Varian Vista PRO ICP-AES at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. XRF analytical procedures

206 Pb∗ /238 U (±1σ)

207 Pb∗ /206 Pb∗

(Ma) (±1σ)

206 Pb∗ /238 U age (Ma) (±1σ)

0.1372 0.1343 0.1354 0.1170 0.1374 0.1322 0.1006 0.1374 0.1341 0.1363 0.1367 0.1370 0.1336 0.1349 0.1354 0.1338 0.1348

743 821 789 737 823 816 985 776 861 812 863 799 832 832 818 831 845

829 813 818 713 830 800 618 830 811 824 826 827 808 816 818 810 815

0.0024 0.0046 0.0017 0.0041 0.0017 0.0020 0.0024 0.0015 0.0012 0.0023 0.0023 0.0012 0.0035 0.0020 0.0013 0.0020 0.0013

57 65 37 21 19 34 42 36 36 28 21 27 22 32 17 18 20

age

14 26 10 24 10 11 14 9 7 13 13 7 20 11 8 11 8

were similar to those of Norrish and Hutton (1969) and Lee et al. (1997), and analytical precision is generally better than 1–5%. Analytical procedures for major element analysis using ICP-AES are similar to those described by Ramsey et al. (1995), with sample powders being fused with lithium metaborate at ∼1100 ◦ C. The ICP-AES analytical precision is better than 1–2%. Trace elements were determined using a PerkinElmer Sciex ELAN 6000 ICP-MS at the Guangzhou Institute of Geochemistry. Procedures for ICP-MS trace element analysis were similar to those described by Li (1997). About 50 mg sample powders were dissolved in high-pressure Teflon bombs using a HF + HNO3 mixture. An internal standard solution containing the single element Rh was used to monitor signal drift during counting. The USGS standards BCR-1 and G-2 were chosen for calibrating element concentrations of measured samples. Analytical precision for most elements was better than 2%. Major and trace element data are presented in Table 4. Nd isotopic compositions were determined using a Micromass Isoprobe multi-collector (MC-ICPMS) at the Guangzhou Institute of Geochemistry and a Finnigan MAT-261 mass spectrometer at the China University of Geosciences (Wuhan). The Isoprobe MC-ICPMS was operated in static mode, and yielded 143 Nd/144 Nd = 0.512125 ± 11 on 14 runs during

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Table 4 Chemical compositions for the Neoproterozoic granitoids from South China Sample

Northern Guangxi leucogranite 97GX-1

96G25

96G27

96G29

96G47

98GX9-1

YBS2-5

98GX6-1

98GX6-5

Major elements (%) SiO2 75.68 TiO2 0.08 13.16 Al2 O3 Fe2 O3 1.89 MnO 0.02 MgO 0.33 CaO 0.48 Na2 O 2.43 K2 O 5.32 0.17 P 2 O5 A/CNK 1.24

77.70 0.07 11.19 1.88 0.03 0.28 0.44 3.20 4.00 0.15 1.14

74.61 0.22 12.64 2.86 0.07 0.88 0.46 2.64 5.06 0.16 1.19

74.92 0.03 12.74 1.78 0.06 0.13 0.39 3.10 5.25 0.12 1.11

76.60 0.15 12.35 2.30 0.05 0.34 0.80 3.16 3.56 0.12 1.18

75.94 0.03 12.65 1.90 0.05 0.39 0.35 3.01 5.10 0.10 1.14

75.89 0.11 12.90 1.93 0.08 0.20 0.50 2.56 4.97 0.14 1.23

76.33 0.06 12.40 2.03 0.04 0.39 0.45 3.12 4.70 0.11 1.12

76.61 0.07 12.27 2.13 0.06 0.30 0.50 3.04 4.50 0.15 1.14

Trace elements (ppm) V 2.62 Cr 2 Co 0.75 Ni 1 Ga 18.0 Rb 453 Sr 9.27 Y 21.6 Zr 53.9 Nb 13.4 Cs 16.2 Ba 17.5 La 6.41 Ce 16.0 Pr 2.02 Nd 6.68 Sm 2.17 Eu 0.06 Gd 2.35 Tb 0.57 Dy 3.87 Ho 0.72 Er 1.97 Tm 0.31 Yb 1.93 Lu 0.25 Hf 2.27 Ta 3.46 Th 12.4 U 3.40

2.6 3 1.15 4 14.8 435 11.4 23.4 70.9 7.19 10.8 30.5 4.00 9.61 1.38 4.55 1.53 0.07 1.68 0.41 2.95 0.59 1.64 0.30 1.86 0.26 3.51 1.40 10.2 3.92

16.8 13 4.26 9 18.2 294 42.6 41.1 102 9.13 10.9 154 16.8 39.8 5.6 20.1 4.84 0.41 4.87 0.98 6.51 1.36 3.88 0.67 3.95 0.55 3.48 0.99 13.2 2.13

0.15 3 1.04 2 16.7 428 10.5 22.1 38.9 4.57 14.2 11.2 3.29 7.85 1.21 4.26 1.58 0.04 1.59 0.42 3.00 0.60 1.77 0.36 2.47 0.35 1.79 0.91 7.33 3.64

11.5 9 2.92 7 20.5 233 69.4 33.8 103 9.32 5.74 203 13.9 32.0 4.39 15.0 3.90 0.29 3.68 0.74 4.52 0.82 2.08 0.32 1.74 0.22 3.57 1.21 16.1 5.25

5.23 4 0.60 1 14.9 417 11.7 19.7 59.6 7.36 16.3 26.7 4.60 11.2 2.14 4.96 1.87 0.21 1.82 0.52 3.28 0.66 2.04 0.41 2.61 0.40 2.80 1.36 9.41 5.06

6.91 7 1.49 2 15.4 388 23.5 27.1 43.3 7.65 19.9 59.3 11.0 24.5 3.01 10.7 2.78 0.17 2.95 0.65 4.26 0.85 2.50 0.41 2.63 0.35 1.52 1.22 9.83 10.7

14.9 7 1.17 1 16.5 442 6.54 26.5 53.1 10.0 30.4 16.0 6.93 16.9 2.08 7.60 2.32 0.09 2.68 0.64 4.39 0.82 2.23 0.38 2.59 0.38 2.18 2.74 9.80 14.5

10.8 6 0.87 1 16.6 485 11.0 28.0 52.8 10.0 63.7 23.9 7.94 19.7 2.41 7.72 2.66 0.09 3.01 0.67 4.62 0.85 2.32 0.41 2.71 0.41 2.44 2.34 11.9 15.6

X.-H. Li et al. / Precambrian Research 122 (2003) 45–83

55

Table 4 (Continued ) Northern Guangxi leucogranite 98GX24-2

Northern Guangxi granodiorite

98GX24-4

98GX26-3

98GX34-2

98GX34-4

98GX34-5

96G15

92LPC24

92LPC26

Major elements (%) SiO2 75.56 0.03 TiO2 Al2 O3 12.67 Fe2 O3 2.13 MnO 0.02 MgO 0.27 CaO 0.57 Na2 O 3.02 5.14 K2 O P 2 O5 0.10 A/CNK 1.10

75.57 0.04 12.35 2.43 0.03 0.40 0.35 2.89 5.07 0.16 1.14

76.05 0.05 12.45 1.87 0.03 0.41 0.52 2.82 5.50 0.11 1.08

75.55 0.05 12.32 2.69 0.04 0.24 0.37 3.10 4.97 0.20 1.10

77.72 0.03 11.54 2.58 0.04 0.37 0.47 3.06 3.51 0.16 1.19

74.95 0.05 12.54 2.75 0.04 0.32 0.56 3.26 4.86 0.17 1.08

62.08 0.46 12.93 6.42 0.12 7.63 2.74 2.38 3.20 0.13 1.05

68.37 0.37 15.56 3.73 0.13 1.47 1.96 3.04 3.28 0.19 1.28

68.00 0.37 15.50 3.70 0.13 1.53 2.40 2.51 3.81 0.20 1.23

Trace elements (ppm) V 9.39 Cr 6 Co 0.87 Ni 0.9 Ga 16.8 Rb 374 Sr 15.5 Y 27.8 Zr 56.0 Nb 11.5 Cs 8.73 Ba 30.4 La 6.71 Ce 16.9 Pr 2.08 Nd 7.21 Sm 2.37 Eu 0.06 Gd 2.73 Tb 0.65 Dy 4.58 Ho 0.87 Er 2.47 Tm 0.45 Yb 3.01 Lu 0.46 Hf 2.69 Ta 2.84 Th 12.2 U 3.88

0.10 5 0.36 2 16.5 448 8.33 16.2 56.4 9.28 5.36 30.4 5.80 15.4 1.87 6.42 1.80 0.04 1.73 0.37 2.73 0.55 1.74 0.3 2.17 0.3 2.58 2.15 8.75 3.81

17.1 5 0.75 1 15.3 387 7.71 34.4 51.1 9.54 7.75 10.5 7.90 19.8 2.46 8.22 2.79 0.04 3.16 0.75 5.42 1.07 3.16 0.58 3.93 0.61 2.10 1.75 12.6 2.52

3.90 3 0.45 2 21 778 3.22 15.4 76.2 14.1 25.5 8.20 4.80 14.6 1.76 6.30 2.03 0.01 1.88 0.42 2.68 0.48 1.36 0.22 1.45 0.19 3.88 2.92 10.8 22.1

7.59 6 0.74 0.6 18.7 514 4.19 13.8 9.95 17.8 31.3 8.46 2.01 5.68 0.74 2.45 1.15 0.01 1.41 0.38 2.55 0.42 1.05 0.19 1.25 0.16 0.69 3.59 9.18 29.0

6.55 9 0.79 0.6 18.9 643 4.20 24.9 50.7 15.7 37.0 11.2 5.88 15.7 1.98 6.91 2.58 0.02 2.69 0.64 4.24 0.77 2.07 0.37 2.41 0.33 2.49 3.21 13.9 24.9

74.0 383 27.0 125 11.9 80.1 131 14.3 120 5.69 7.32 219 18.3 38.8 4.95 17.8 3.37 0.77 2.84 0.50 2.88 0.59 1.72 0.25 1.65 0.25 3.35 0.45 6.07 1.27

43.6 60 7.79 19 20.7 217 183 26.8 153 10.8 11.6 475 21.8 48.5 6.34 22.7 4.85 0.99 4.27 0.70 4.09 0.80 2.19 0.36 2.20 0.31 4.70 1.07 9.65 2.22

42.9 67 9.33 25 20.2 176 202 28.3 131 9.23 8.12 491 22.4 50.0 6.51 23.4 5.03 0.93 4.25 0.71 4.06 0.79 2.10 0.35 2.06 0.29 4.05 0.83 10.7 1.69

56

X.-H. Li et al. / Precambrian Research 122 (2003) 45–83

Table 4 (Continued ) Northern Guangxi granodiorite 98GX14-1

98GX15

69.94 0.36 13.81 4.47 0.08 1.87 2.23 2.75 4.24 0.12 1.05

68.74 0.34 14.17 4.21 0.08 2.35 2.54 2.52 3.50 0.13 1.13

Trace elements (ppm) V 56 Cr 54 Co 9.29 Ni 22 Ga 19.2 Rb 157 Sr 104 Y 28.9 Zr 141 Nb 10.1 Cs 4.72 Ba 499 La 38.6 Ce 87.3 Pr 9.58 Nd 37.1 Sm 6.95 Eu 0.91 Gd 6.28 Tb 0.90 Dy 5.26 Ho 1.03 Er 3.03 Tm 0.41 Yb 2.74 Lu 0.40 Hf 4.44 Ta 1.00 Th 17.1 U 3.28

52.6 73 10.0 24 19.1 133 110 29.7 121 9.65 6.16 493 36.5 76.6 9.07 35.5 6.89 1.08 6.54 0.92 5.47 1.07 3.06 0.42 2.75 0.40 3.87 0.88 14.4 2.76

SiO2 TiO2 Al2 O3 Fe2 O3 MnO MgO CaO Na2 O K2 O P 2 O5 A/CNK

98GX16

98GX17

Majorelements(%) 68.82 64.05 0.32 0.47 14.18 13.92 5.15 6.91 0.11 0.10 1.81 3.75 2.38 3.83 2.07 2.24 3.47 2.88 0.19 0.15 1.23 1.01 46.0 61 8.31 19 20.0 321 166 31.8 135 11.5 8.77 692 39.7 92.5 8.53 34.8 6.63 1.29 6.35 0.98 5.69 1.10 3.18 0.50 2.85 0.43 4.09 1.04 14.4 2.58

114 164 17.4 54.6 17.6 125 159 29.5 145 11.1 11.7 452 36.5 73.7 8.71 32.5 6.25 1.10 5.33 0.84 4.99 0.96 2.61 0.42 2.67 0.43 4.05 0.90 13.5 2.50

97GX18

97GX19

98GX39-2

98GX27-1

67.91 0.46 15.36 4.10 0.08 1.63 1.97 3.53 4.37 0.14 1.09

70.41 0.39 14.97 3.94 0.08 1.92 2.01 2.63 3.22 0.12 1.31

64.62 0.39 14.08 6.56 0.10 3.63 4.71 1.85 2.65 0.14 0.97

66.06 0.52 14.57 5.49 0.07 3.32 2.89 2.39 3.38 0.18 1.13

29.8 56 6.51 22 13.9 127 111 24.4 159 11.6 2.83 523 39.1 73.3 8.28 31.9 6.76 1.19 5.44 0.85 4.89 0.98 2.56 0.43 2.80 0.42 5.88 1.24 16.2 2.49

30.5 50 7.36 22.4 14.1 134 177 28.0 134 12.0 3.59 494 36.1 67.8 7.95 30.7 6.90 1.10 5.66 0.95 5.57 1.12 3.04 0.46 3.25 0.48 5.11 1.27 17.9 3.82

111 165 17.3 61.6 16.6 130 143 26.7 132 10.4 8.44 411 29.9 61.8 7.25 27.2 5.40 1.00 4.98 0.77 4.67 0.90 2.44 0.39 2.46 0.40 3.78 0.88 13.5 2.37

89.5 112 14 37.1 20.7 157 126 29.2 193 11.3 9.52 659 37.1 80.8 9.11 35.6 6.32 1.07 6.26 0.86 5.30 1.06 3.1 0.44 2.93 0.43 5.62 0.96 15.0 2.29

X.-H. Li et al. / Precambrian Research 122 (2003) 45–83

57

Table 4 (Continued ) Northern Guangxi granodiorite

Yunan granodiorite

98GX27-2 Major elements (%) SiO2 65.68 TiO2 0.48 Al2 O3 14.49 6.39 Fe2 O3 MnO 0.11 MgO 3.08 CaO 3.39 Na2 O 2.26 3.17 K2 O P2 O5 0.14 A/CNK 1.09 Trace elements V Cr Co Ni Ga Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Th U

(ppm) 84.0 106 12.4 37 20.1 137 117 27.9 149 10.5 7.60 488 36.4 80.7 8.86 33.8 6.03 1.08 5.83 0.81 4.92 0.99 2.97 0.41 2.77 0.40 4.60 0.88 13.3 2.76

98GX29-1

98GX30-1

98KD152

98KD153

98KD154

98KD155

98KD156

98KD157

64.48 0.58 14.56 6.12 0.12 3.54 3.05 2.40 3.25 0.19 1.12

65.19 0.53 14.63 6.69 0.11 3.20 2.74 2.19 2.96 0.16 1.24

66.98 0.49 15.25 4.2 0.08 1.38 1.56 3.25 3.86 0.17 1.23

67.61 0.53 15.12 3.85 0.06 0.98 2.51 2.9 4.4 0.19 1.07

67.34 0.49 15.22 4.23 0.06 1.06 2.78 2.86 3.85 0.17 1.09

71.06 0.34 14.21 2.74 0.05 0.62 1.24 3.02 4.86 0.13 1.14

71.26 0.34 14.23 2.66 0.05 0.52 0.81 3.11 4.86 0.12 1.20

70.56 0.34 14.34 2.62 0.05 0.58 1.06 3.03 5.29 0.13 1.13

96.2 126 13.6 38.5 17.6 137 154 30.3 141 10.8 11.6 493 32.8 67.7 7.99 29.6 6.02 1.07 5.33 0.83 4.97 0.99 2.71 0.43 2.70 0.44 4.05 0.91 13.6 2.56

94.4 113 15.8 42 19.3 154 192 27.6 173 10.3 14.7 370 32.4 66.0 8.00 29.0 5.73 1.19 5.09 0.80 4.78 0.97 2.77 0.42 2.74 0.42 4.59 0.74 13.2 2.54

46.9 16 7.10 14 19.4 153 433 23.9 173 13.0 6.53 866 44.3 85.9 8.62 30.8 5.86 1.27 4.81 0.77 4.54 0.89 2.54 0.40 2.49 0.38 5.33 1.64 17.8 5.58

44.4 14 6.12 13 19.2 170 563 24.9 216 20.1 2.08 1280 87.9 144 15.3 52.2 8.45 1.62 6.28 0.88 4.83 0.91 2.55 0.40 2.49 0.38 6.55 1.79 35.6 6.79

51.3 17 8.02 14 18.6 184 457 22.2 164 12.0 8.00 1066 54.1 91.4 10.0 35.1 6.42 1.32 5.30 0.75 4.21 0.82 2.24 0.36 2.16 0.34 4.84 1.29 18.3 4.66

25.8 13 4.03 14 18.6 230 324 26.5 176 24.3 4.12 922 62.4 104 11.0 38.9 6.70 1.09 5.34 0.80 4.66 0.94 2.68 0.44 2.70 0.45 5.66 3.03 36.3 10.9

26.35 11 43.6 4 18.0 208 266 25.7 184 23 3.19 739 63.3 114 12.5 42.1 6.68 1.08 5.05 0.83 4.43 0.91 2.67 0.43 2.86 0.45 5.49 2.78 32.4 1.57

24.3 9 3.21 13 18.8 241 353 26.2 184 22.7 3.73 1082 74.2 124 13.2 43.8 7.45 1.22 5.55 0.89 5.00 0.97 2.72 0.43 2.70 0.40 5.69 2.09 45.4 8.08

58

X.-H. Li et al. / Precambrian Research 122 (2003) 45–83

Table 4 (Continued ) Yunan K-rich granitoid 98KD158

Southern Anhui granodiorite

98KD81-32

98KD81-35

LG546

99SC62-2

99SC62-3

99SC62-5

99SC62-6

AW178

Major elements (%) SiO2 70.76 TiO2 0.34 Al2 O3 14.22 2.8 Fe2 O3 MnO 0.04 MgO 0.75 CaO 0.97 Na2 O 2.99 4.87 K2 O P 2 O5 0.13 A/CNK 1.19

67.28 0.48 15.28 4.44 0.11 1.52 2.67 3.2 3.65 0.16 1.09

60.2 0.86 16.37 6.51 0.14 2.68 3.85 3.4 3.38 0.31 1.01

68.94 0.46 14.56 4.21 0.14 1.00 1.36 3.01 3.67 0.19 1.28

67.91 0.52 16.14 4.08 0.07 1.13 0.89 3.29 4.77 0.21 1.32

68.78 0.44 16.21 3.74 0.07 1.03 1.61 3.59 4.91 0.21 1.15

67.70 0.47 15.93 4.02 0.08 1.11 0.92 3.32 5.16 0.19 1.25

70.96 0.46 14.52 3.56 0.07 0.92 1.07 3.24 4.52 0.19 1.19

65.65 0.75 15.68 6.50 0.10 1.61 1.54 2.54 3.61 0.18 1.44

Trace elements (ppm) V 26.5 Cr 11 Co 28.9 Ni 4 Ga 17.7 Rb 210 Sr 278 Y 22.9 Zr 180 Nb 21.1 Cs 3.61 Ba 749 La 56.0 Ce 99.8 Pr 10.9 Nd 37.0 Sm 5.86 Eu 1.01 Gd 4.63 Tb 0.77 Dy 4.27 Ho 0.82 Er 2.47 Tm 0.38 Yb 2.49 Lu 0.37 Hf 5.43 Ta 2.08 Th 31.2 U 2.27

57.7 12 8.42 10 19.8 175 418 21.4 188 11.2 8.72 950 56.4 101 11.3 36.9 6.22 1.34 4.77 0.73 3.88 0.73 2.07 0.3 1.94 0.29 4.83 1.11 15.5 3.52

122 34 13.6 13 23.5 150 963 25.7 343 15.2 3.8 2641 104 166 19.5 63.4 9.43 2.15 7.08 0.93 4.83 0.90 2.49 0.34 2.14 0.32 7.41 0.82 21.5 3.15

41.9 34 8.65 18 19.7 125 214 26 172 7.68 5.73 601 23.0 52.9 7.20 27.3 5.86 0.99 5.29 0.83 4.51 0.84 2.21 0.35 2.05 0.29 5.15 0.55 9.75 2.10

51.7 34 7.64 14 18.0 145 202 29.7 151 8.76 6.85 1090 25.9 56.6 7.27 27.3 5.99 1.26 5.61 0.93 5.32 1.01 2.75 0.42 2.57 0.40 4.46 0.63 9.41 2.82

57.5 39 7.62 15 18.8 150 187 33.5 161 10.2 7.41 1103 31.8 69.3 8.93 33.2 7.12 1.22 6.42 1.08 6.06 1.14 3.06 0.45 2.81 0.42 4.78 0.87 12.3 2.51

51.5 37 7.68 15 18.5 149 187 31.5 148 9.37 7.75 1044 28.0 62.0 7.96 29.8 6.62 1.30 6.08 1.01 5.75 1.08 2.99 0.45 2.88 0.42 4.84 0.65 10.8 1.64

49.8 32 7.90 14 17.2 137 154 30.5 126 9.72 6.13 850 24.3 53.9 6.96 25.7 5.78 1.11 5.47 0.94 5.42 1.05 2.93 0.46 2.88 0.43 3.77 0.83 9.56 3.08

80.9 47 13.2 24 21.2 135 223 30.0 211 10.3 6.73 720 26.6 60.4 8.07 30.4 6.10 1.09 5.38 0.81 4.45 0.86 2.38 0.38 2.29 0.34 6.04 0.63 11.7 1.35

X.-H. Li et al. / Precambrian Research 122 (2003) 45–83

59

Table 4 (Continued ) Southern Anhui granodiorite AW213

LG552

LG502

Major elements (%) SiO2 67.83 TiO2 0.44 Al2 O3 15.83 4.53 Fe2 O3 MnO 0.07 MgO 1.13 CaO 1.44 Na2 O 2.90 3.65 K2 O P 2 O5 0.19 A/CNK 1.40

65.90 0.65 15.28 5.68 0.12 1.50 1.96 2.82 3.64 0.18 1.26

65.62 0.69 15.39 6.20 0.14 1.58 1.86 2.59 3.78 0.18 1.31

69.8 48 12.3 26 20.9 141 204 31.5 179 9.58 6.56 686 28 62.8 8.28 31.1 6.42 1.1 5.96 0.93 5.29 1.04 2.83 0.46 2.77 0.44 5.26 0.68 11.3 2.02

95.9 54 12.0 20 26.8 65.4 93 31.1 224 11.3 9.64 500 25.9 59.5 6.47 24.2 5.81 0.98 5.61 0.95 5.65 1.20 3.45 0.53 3.41 0.52 6.37 0.67 9.17 2.14

Trace elements V Cr Co Ni Ga Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Th U

(ppm) 41.5 26 8.24 22 21.2 141 202 24.8 159 7.36 11.6 691 26.5 60.1 8.03 30.2 6.34 1.16 5.67 0.86 4.73 0.87 2.25 0.35 2.05 0.28 4.67 0.53 9.82 1.94

Northern Jiangxi granodiorite Qnt

LG511

2KJL12-1

2KJL12-2

2KJL14-2

2KJL14-8

65.88 0.72 15.38 5.14 0.09 1.64 1.74 3.82 3.51 0.16 1.16

68.50 0.59 14.33 5.91 0.15 1.21 1.13 2.22 3.97 0.14 1.43

69.53 0.50 14.75 4.53 0.08 1.32 1.03 2.28 4.04 0.13 1.48

71.23 0.46 13.74 4.54 0.08 1.07 1.23 2.44 3.53 0.09 1.36

67.48 0.73 14.97 5.92 0.11 1.79 1.71 2.82 2.89 0.10 1.38

68.03 0.68 15.01 5.49 0.10 1.73 1.63 2.72 3.45 0.13 1.34

85.14 41 16.6 25 20.2 144 189 43.9 223 11.4 9.94 624 37.3 78.1 9.64 35.8 7.69 1.25 7.72 1.26 7.68 1.57 4.48 0.67 4.27 0.63 5.81 0.88 13.3 2.44

68.1 56 12.3 21 19.2 212 47.8 38.7 180 8.77 7.48 374 26.5 60.4 8.16 30.4 6.38 0.85 6.11 1.02 6.14 1.24 3.47 0.57 3.31 0.46 5.57 0.65 14.5 2.08

58.2 45 9.46 15 16.7 191 72.7 36.8 163 9.79 14 403 26.9 58.2 7.4 26.5 5.72 0.91 6.05 1.03 6.27 1.26 3.38 0.54 3.46 0.53 4.97 0.96 12.6 2.01

62.8 49 9.54 15 17.0 193 72.0 38.6 148 9.72 12.4 360 27.4 60.4 7.64 27.5 6.02 0.83 6.24 1.06 6.45 1.3 3.47 0.57 3.65 0.55 4.59 0.83 13.5 1.88

55.56 46 9.86 15 18.2 154 79.84 41.3 158 8.96 12.8 347 26.7 58.4 7.43 27 5.89 1.05 5.87 1.07 6.63 1.33 3.75 0.58 3.63 0.54 4.59 0.81 11.7 2.41

79.9 63 13.87 21 18.5 165 83.7 52.2 221 11 13.6 313 30.7 66.7 8.38 30.4 6.63 1.03 7.41 1.27 8.37 1.77 4.89 0.82 5.41 0.83 6.51 1.06 14.2 6.79

60

X.-H. Li et al. / Precambrian Research 122 (2003) 45–83

Table 4 (Continued ) Northern Jiangxi granodiorite 2KJL14-9

2KJL14-12

2KJL15-1

2KJL15-4

2KJL16-1

2KJL23-1

2KJL23-6

2KJL23-8

2KJL25-3

Major elements (%) SiO2 66.84 TiO2 0.63 Al2 O3 15.52 5.22 Fe2 O3 MnO 0.08 MgO 1.59 CaO 1.89 Na2 O 2.64 3.91 K2 O P 2 O5 0.12 A/CNK 1.29

67.82 0.73 14.94 6.05 0.10 1.94 1.73 2.63 3.10 0.12 1.38

68.96 0.43 16.00 4.14 0.08 1.36 2.66 3.07 2.65 0.11 1.25

69.44 0.47 15.38 4.15 0.08 1.22 2.49 3.18 2.34 0.10 1.25

67.23 0.45 15.94 3.76 0.08 1.24 3.48 3.79 1.92 0.09 1.09

66.08 0.68 15.89 5.43 0.10 1.77 2.94 3.07 2.68 0.13 1.20

68.05 0.53 16.17 4.90 0.09 1.55 2.45 2.84 3.35 0.16 1.27

66.36 0.57 16.07 4.89 0.09 2.13 3.27 3.11 2.34 0.10 1.18

65.22 0.69 15.99 6.17 0.13 2.16 2.39 2.53 3.21 0.13 1.33

Trace elements (ppm) V 78.4 Cr 64 Co 12.71 Ni 20 Ga 17.5 Rb 178 Sr 81.2 Y 31.8 Zr 208 Nb 10.2 Cs 10.8 Ba 471 La 28.9 Ce 61.7 Pr 7.68 Nd 27.7 Sm 5.88 Eu 1.18 Gd 5.98 Tb 0.97 Dy 5.66 Ho 1.1 Er 2.93 Tm 0.48 Yb 3.18 Lu 0.49 Hf 6.24 Ta 0.87 Th 12.3 U 1.95

94.37 71 15.42 25 20.5 180 90.5 27.7 220 10.8 17.5 267.3 33.9 71.9 8.86 32.6 6.77 1.01 6.20 0.95 5.19 0.97 2.67 0.39 2.63 0.40 5.73 1.00 15.7 3.56

66.7 39 9.17 13 17.2 117 126 15.8 153 8.13 6.87 349 22.2 46.9 5.76 20.4 4.08 0.91 3.66 0.53 2.87 0.54 1.41 0.23 1.56 0.25 4.27 0.79 8.98 2.65

66.87 44 9.57 16 18.79 119 144 20.3 153 7.47 9.24 303 23.5 48.0 5.79 20.9 4.26 0.96 3.74 0.63 3.55 0.70 1.98 0.29 1.91 0.29 3.84 0.64 9.00 1.82

73.6 19 8.19 6 17.8 83.6 169 16.2 140 6.53 4.66 310 16.9 35.3 4.34 15.5 3.25 0.86 3.06 0.47 2.78 0.55 1.46 0.24 1.64 0.26 4.07 0.70 6.39 1.39

83.9 48 12.4 22 18.7 121 116 27.4 145 10.7 9.59 334 27.6 58.2 7.22 27.1 5.34 1.16 4.91 0.82 4.64 0.91 2.46 0.38 2.42 0.39 3.95 0.89 10.5 2.00

71.2 46 10.3 17 17.0 139 108 28.6 144 9.84 13.1 388 24.8 53.7 6.73 24.1 5.24 1.03 5.25 0.85 5.04 0.95 2.47 0.40 2.56 0.39 4.28 1.06 10.0 3.16

88.4 51 13.3 26 18.0 114 160 34.1 146 7.75 10.9 329 23.3 49.5 6.14 22.2 4.72 1.02 5.00 0.84 5.32 1.12 3.05 0.50 3.14 0.49 4.09 0.69 9.90 1.34

101 66 15.4 25 19.9 164 98.9 64.6 208 10.6 15.2 357 30.5 66.6 8.47 30.7 6.67 0.98 7.35 1.34 9.48 2.22 6.55 1.14 7.50 1.18 5.99 0.96 13.6 3.16

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Table 4 (Continued ) Northern Jiangxi granodiorite 2KJL25-4 Major elements (%) SiO2 67.50 TiO2 0.61 Al2 O3 15.42 5.10 Fe2 O3 MnO 0.09 MgO 1.68 CaO 2.58 Na2 O 2.87 2.97 K2 O P2 O5 0.12 A/CNK 1.22 Trace elements V Cr Co Ni Ga Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Th U

(ppm) 84.3 52 12.7 22 18.1 139 94.3 34.1 235 9.43 11.8 343 27.4 59.8 7.54 27.3 5.83 1.05 5.90 0.95 5.76 1.13 3.08 0.51 3.33 0.51 6.80 0.86 12.0 2.97

Hubei TTG

2KJL25-7

2KJL26-1

2KJL26-2

2KJL26-13

99SC8

99SC10

99SC11

H12-14

66.36 0.64 16.00 5.15 0.08 1.83 2.17 2.89 2.94 0.13 1.35

67.60 0.58 15.29 4.87 0.08 1.63 2.05 2.55 3.51 0.15 1.30

68.16 0.57 15.09 4.82 0.08 1.66 1.98 2.50 3.47 0.14 1.32

66.76 0.68 15.15 5.15 0.08 1.79 1.63 2.98 3.91 0.13 1.25

61.17 0.65 17.29 6.03 0.11 2.78 5.73 4.02 1.48 0.20 0.93

70.73 0.31 15.73 2.52 0.06 0.66 3.22 4.66 1.51 0.10 1.04

72.02 0.19 14.82 2.21 0.05 0.39 2.63 4.13 2.29 0.07 1.05

72.79 0.22 13.93 2.70 0.05 0.50 2.55 4.06 1.93 0.06 1.04

91.09 48 13.3 24 20.1 153 123 31.8 212 8.88 13.8 312 31.7 66.3 8.38 31.0 6.59 1.23 6.15 0.97 5.61 1.10 3.09 0.45 2.89 0.44 5.50 0.71 13.2 1.73

83 58 12.0 25 18.5 167 89.6 37.2 171 10.1 22.2 384 32.4 70.7 8.84 32.1 6.93 1.18 7.00 1.10 6.44 1.25 3.33 0.53 3.37 0.51 5.05 0.93 14.6 2.8

73.9 61 11.9 24 18.1 164 86.9 34.2 174 9.92 20.6 346 29.1 63.3 7.93 28.8 6.16 1.11 6.25 1.00 5.87 1.15 3.02 0.48 3.09 0.48 5.05 0.92 13.6 4.35

88.3 60 12.8 26 18.2 182 94.0 28.9 232 11.4 15.2 450 31.9 68.8 8.64 31.4 6.63 1.06 6.35 0.96 5.28 0.98 2.56 0.41 2.63 0.42 6.79 0.95 14.0 1.55

95.0 26 16.3 21 18.7 37.6 465 22.5 119 8.28 1.41 560 90.7 181 18.5 60.1 7.05 1.54 4.60 0.73 3.60 0.71 1.94 0.30 1.99 0.31 2.80 0.52 24.7 1.17

20.0 5 3.28 3 17.0 26.8 376 12.5 124 7.62 0.74 630 33.1 55.2 6.88 24.3 3.89 0.90 2.86 0.43 2.16 0.40 1.06 0.17 1.15 0.18 3.19 0.65 8.34 0.52

10.9 4 1.88 2 15.2 17.8 297 6.58 112 5.6 0.39 1153 26.0 47.3 5.03 16.8 2.33 0.70 1.49 0.23 1.14 0.22 0.57 0.09 0.61 0.10 2.96 0.34 5.04 0.45

11.3 6 3.04 2 16.1 57.4 294 8.18 91 8.59 1.71 766 31.4 57.7 6.14 21.8 3.29 0.87 1.91 0.33 1.54 0.28 0.76 0.13 0.79 0.12 2.54 1.00 8.72 1.27

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Table 4 (Continued ) Hubei TTG

Major elements (%) SiO2 TiO2 Al2 O3 Fe2 O3 MnO MgO CaO Na2 O K2 O P2 O5 A/CNK Trace elements (ppm) V Cr Co Ni Ga Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Th U

H12-15

H12-16

65.03 0.39 16.71 4.52 0.10 1.85 4.45 4.43 1.68 0.08 0.97

74.44 0.13 13.65 1.88 0.06 0.38 1.76 4.01 3.29 0.04 1.02

53.8 22 9.33 13 16.1 34.2 445 9.35 48.3 7.97 1.19 608 29.1 49.2 4.99 17.4 2.56 0.79 1.70 0.32 1.71 0.33 0.88 0.16 0.89 0.14 1.07 0.87 4.23 0.45

8.26 7 2.50 1 14.7 63.1 334 7.49 82.6 9.30 0.64 910 20.9 37.5 3.81 13.6 2.35 0.54 1.53 0.28 1.36 0.25 0.66 0.13 0.73 0.11 2.44 1.17 4.56 0.99

this study for the Shin Etou JNdi-1 standard. Measured 143 Nd/144 Nd ratios were normalised to 146 Nd/144 Nd = 0.7219. The Finnigan MAT-261 mass spectrometer in Wuhan was operated in static multi-collector mode. The 143 Nd/144 Nd ratio of the

La Jolla standard measured during this study was 0.511856 ± 6. Sm–Nd isotopic data are listed in Table 5; the 143 Nd/144 Nd ratios are adjusted relative to the La Jolla standard of 0.511860 (corresponding to the Shin Etou JNdi-1 standard of 0.512115, Tanaka et al., 2000).

4. SHRIMP U–Pb geochronology 4.1. Xucun Pluton, Anhui Province (sample Qnt) Zircons are mostly euhedral, range up to ∼200 ␮m in length, and have length to width ratios up to 3:1. Most are relatively transparent and colourless, although a few are dark brown and turbid. Rounded zircon cores can be observed within a few idiomorphic grains. Euhedral concentric zoning is common in most crystals. Twenty-two analyses of 21 zircons were obtained in sets of seven scans during a single analytical session (Table 1). 238 U concentrations range from 50 to 350 ppm, with one analysis at 985 ppm. Thorium ranges from 6 to 380 ppm, with a median of 33 ppm, and Th/U ratios vary between 0.03 and 0.55. Common Pb is low; the proportion of common 206 Pb in total measured 206 Pb (f206 in data tables) is <0.3%, with a median of 0.05%. Most results are within error of concordia (Fig. 2). Analysis #21b, of a zircon core, is significantly older than the main group. The 207 Pb/206 Pb ratios for 19 of the remaining 21 analyses agree to within analytical precision, and their weighted mean yields an age of 828.4 ± 7.8 Ma (1σ, MSWD = 0.6). Corresponding 238 U/206 Pb ratios are dispersed beyond analytical precision. Mixture modelling (Sambridge and Compston, 1994) identifies a main age component at 822 ± 2.7 (1σ, n = 17, MSWD = 0.7) and an older component made up of four analyses ranging in age from 841 to 863 Ma, with a mean of 851 Ma. Compared to the main group of analyses, the five analyses in the older group have high U concentrations and Th/U ratios (Fig. 3), the latter similar to that measured in the zircon core (#21b). Post-analysis microscopic examination of the four older analysis sites did not reveal the presence of cores; their older ages and distinctly higher Th/U ratios suggest that these zircons are inherited. The best estimate of the crystallisation age of sample Qnt is given by the mean 206 Pb/238 U age for

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Table 5 Sm–Nd isotopic data for the Neoproterozoic granitoids in South China Sm (ppm)

Nd (ppm)

147 Sm/144 Nd

143 Nd/144 Nd

TDM (Ga)

T2DM (Ga)

εNd (T)

fSm/Nd

Northern Guangxi 96G25 96G27 96G29 96G47 YBS2-5 97GX-1 98GX6-1 98GX6-5 98GX9-1 98GX24-2 98GX26-3 98GX34-4 98GX34-5

leucogranite 1.53 4.84 1.58 3.96 2.78 2.17 2.32 2.66 1.87 2.37 2.79 1.15 2.58

0.2031 0.1456 0.2236 0.1591 0.1571 0.1967 0.1845 0.2083 0.2279 0.1983 0.2051 0.2850 0.2253

0.512355 0.511960 0.512493 0.512050 0.512155 0.512176 0.512154 0.512393 0.512374 0.512402 0.512454 0.512866 0.512435

± ± ± ± ± ± ± ± ± ± ± ± ±

20 11 13 14 13 15 5 6 6 7 7 8 7

11.06 2.65 −10.50 3.05 2.66 8.51 5.13 20.01 −8.59 7.26 11.94 −0.61 −9.74

1.99 2.12 1.94 2.10 1.91 2.22 2.15 1.97 2.17 1.87 1.85 1.88 2.05

−6.20 −7.88 −5.66 −7.54 −5.27 −9.03 −8.18 −6.01 −8.44 −4.78 −4.48 −4.82 −6.97

0.033 −0.260 0.137 −0.191 −0.202 0.000 −0.062 0.059 0.159 0.008 0.043 0.449 0.145

Northern Guangxi 92LPC24 92LPC26 96G15 97GX18 97GX19 98GX16 98GX17 98GX39-2 98GX29-1 98GX30-1

biotite granodiorite and tonalite 4.85 22.7 0.1292 5.03 23.4 0.1299 3.37 17.8 0.1142 6.76 31.9 0.1284 6.90 30.7 0.1361 6.63 34.8 0.1153 6.25 32.5 0.1161 5.40 27.2 0.1199 6.02 29.6 0.1230 5.73 29.0 0.1194

0.512021 0.512034 0.511966 0.511997 0.512031 0.511938 0.511933 0.511955 0.512042 0.512019

±6 ±5 ± 12 ±5 ±5 ±9 ±9 ±4 ±5 ±5

2.03 2.02 1.81 2.05 2.19 1.87 1.89 1.94 1.86 1.82

1.89 1.87 1.85 1.92 1.93 1.90 1.92 1.91 1.80 1.81

−4.96 −4.79 −4.46 −5.35 −5.50 −5.13 −5.31 −5.28 −3.90 −3.98

−0.343 −0.339 −0.419 −0.347 −0.308 −0.414 −0.410 −0.390 −0.375 −0.393

Southern Anhui and northern Jiangxi cordierite-bearing granodiorite AW178 6.10 30.4 0.1213 0.512148 AW213 6.34 30.2 0.1269 0.512179 LG511 6.38 30.4 0.1269 0.512136 LG538 4.92 23.6 0.1260 0.512203 LG552 6.42 31.1 0.1248 0.512212 Qnt 7.69 35.8 0.1297 0.512223 99SC62-2 5.99 27.3 0.1326 0.512259 99SC62-3 7.12 33.2 0.1296 0.512267 99SC62-5 6.62 29.8 0.1343 0.512261 99SC62-6 5.78 25.7 0.1360 0.512283 2KJL12-1 5.72 26.5 0.1305 0.512123 2KJL12-2 6.02 27.5 0.1323 0.512143 2KJL23-8 4.72 22.2 0.1285 0.512211 2JKL25-3 6.67 30.7 0.1313 0.512216 2KJL25-4 5.83 27.3 0.1291 0.512245 2KJL26-2 6.16 28.8 0.1293 0.512192

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

9 13 8 14 6 12 10 12 11 9 8 9 10 8 13 10

1.65 1.70 1.78 1.64 1.60 1.68 1.67 1.60 1.70 1.70 1.88 1.88 1.68 1.72 1.63 1.73

1.62 1.62 1.69 1.57 1.55 1.57 1.54 1.50 1.55 1.53 1.74 1.72 1.58 1.60 1.53 1.62

−1.65 −1.64 −2.47 −1.07 −0.77 −1.07 −0.67 −0.20 −0.81 −0.55 −3.11 −2.91 −1.18 −1.38 −0.58 −1.63

−0.383 −0.355 −0.355 −0.359 −0.366 −0.341 −0.326 −0.341 −0.317 −0.309 −0.337 −0.327 −0.347 −0.332 −0.344 −0.343

Eshan (Yunnan) K-rich granitoids 98KD81-35 9.43 98KD152 5.86 98KD154 6.42 98KD155 6.70 98KD157 7.45

± ± ± ± ±

12 6 8 9 8

1.74 2.13 2.09 1.86 1.93

2.01 2.17 2.19 2.01 2.11

−6.50 −8.43 −8.71 −6.52 −7.69

−0.543 −0.416 −0.438 −0.470 −0.478

4.55 20.1 4.26 15.0 10.7 6.68 7.60 7.72 4.96 7.21 8.22 2.45 6.91

63.4 30.8 35.1 38.9 43.8

0.0899 0.1149 0.1105 0.1042 0.1027

0.511731 0.511767 0.511729 0.511807 0.511739

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Table 5 (Continued ) Sm (ppm) Huangling (Hubei) TTG 99SC8 7.05 99SC10 3.89 99SC11 2.33 H12-14 3.29 H12-15 2.56 H12-16 2.35

Nd (ppm)

147 Sm/144 Nd

143 Nd/144 Nd

60.1 24.3 16.8 21.8 17.4 13.6

0.0709 0.0967 0.0839 0.0912 0.0889 0.1045

0.511500 0.511266 0.511071 0.511419 0.511567 0.511157

± ± ± ± ± ±

6 10 8 13 15 14

TDM (Ga) 1.76 2.44 2.43 2.15 1.93 2.76

T2DM (Ga)

εNd (T)

fSm/Nd

2.22 2.80 3.00 2.51 2.26 3.04

−9.03 −16.30 −18.77 −12.74 −9.61 −19.25

−0.639 −0.508 −0.574 −0.536 −0.548 −0.469

T: crystallisation age of granitoids.

the main group of 17 analyses at 823 ± 8 Ma (95% confidence interval, including uncertainty arising from calibration against the CZ3 standard). 4.2. Jiuling Pluton, Jiangxi Province (samples 2KJL26-1 & 2KJL14-8) Zircons in both samples are sub- to euhedral, range up to 100–150 ␮m in length, and have length to width ratios about 2:1. Most are relatively transparent and colourless. Euhedral concentric zoning is common in most crystals; no inherited zircon cores were observed. Fifteen analyses of 15 zircons from sample 2KJL26-1

were obtained in sets of seven scans during a single analytical session (Table 2). 238 U concentrations are moderate, ranging from 240 to 620 ppm. Thorium ranges from 30 to 290 ppm, and Th/U ratios vary between 0.11 and 0.46. Values for f206 range up to 0.16%, with a median of 0.02%. The results form a single, essentially concordant group with a mean 206 Pb/238 U age of 819 ± 9 Ma (95% confidence interval), with one exception of analysis #12 that, at ∼1250 Ma, is significantly older than the main group and is interpreted to be a xenocryst (Fig. 4). The 207 Pb/206 Pb ratios for the remaining 14 analyses agree to within analytical precision, yielding

Fig. 2. U–Pb concordia diagram showing analytical data for zircons from granodiorite sample Qnt, from the Xucun Pluton in southern Anhui Province. Data not included in the calculated age for the sample are shown with solid symbols.

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Most are relatively transparent and colourless. Euhedral concentric zoning is common in most crystals; inherited zircon cores were not observed. Seventeen analyses were conducted of 17 zircons in sets of seven scans (Fig. 5 and Table 3). Concentrations of 238 U range from 110 to 750 ppm, and 232 Th ranges from 75 to 370 ppm. Th/U ratios vary between 0.19 and 0.82. Values for f206 range up to 0.3%, with a median of 0.06%. For 15 of 17 analyses, ratios of 207 Pb/206 Pb and 238 U/206 Pb agree internally to within analytical precision and yield ages of 825.6 ± 6.6 Ma (1σ, MSWD = 0.8) and 819.2 ± 2.5 Ma (1σ, MSWD = 0.8). Of the two rejected analyses, one (#7, not shown in Fig. 5) is strongly discordant, yields a 207 Pb/206 Pb age of 985 Ma, and is interpreted as a xenocyst that has lost radiogenic Pb. The other (#4) yields a 206 Pb/238 U age of 713 Ma, indicating loss of radiogenic Pb from the analysed site. The best estimate of the age of crystallisation of sample 98KD154, based on the mean 206 Pb/238 U ratio, is 819 ± 8 Ma (95% confidence interval).

5. Geochemical characteristics Fig. 3. Variation of 206 Pb/238 U age with (a) uranium concentration and (b) Th/U ratio in zircons from sample Qnt. Analyses not included in the calculated age for the sample are labelled and shown with solid symbols.

a weighted mean age of 813.4±6.6 Ma (1σ, MSWD = 0.4). Corresponding 206 Pb/238 U ratios are also in agreement and yield a mean age of 818.8 ± 3.0 (1σ, n = 14, MSWD = 0.4). Thus, the best estimate of the crystallisation age of sample 2KJL26-1 is 819 ± 9 Ma. Four zircons were analysed from sample 2KJL14-8 (Table 2 and Fig. 4). Two yielded concordant ages of 808 and 816 Ma, consistent with the results for sample 2KJL26-1. Two others produced slightly reversely discordant results, with 206 Pb/238 U ages of 890 and 896 Ma, and are interpreted to be xenocrysts. 4.3. Eshan Pluton, Yunnan Province (sample 98KD154) Zircons are sub- to euhedral, range up to ∼200 ␮m in length, and have length to width ratios about 2:1.

5.1. Muscovite-bearing leucogranites The northern Guangxi leucogranites are highly siliceous (SiO2 = 74–78%), and plot in the granite field in the TAS diagram (Fig. 6). They are exclusively peraluminous with A/CNK values of 1.08–1.24 (Table 4 and Fig. 7), and high in K2 O (mostly 4.5–5.4%, with a few highly fractionated samples having slightly lower K2 O of 3.5–4.0%; Fig. 8), K2 O/Na2 O (1.2–2.5), Rb (200–800 ppm), and Rb/Sr (3–300), but low in Al2 O3 (11–13%), TiO2 + FeOt + MgO (<3%), Sr (2–100 ppm), Ba (3–200 ppm), Zr (20–100 ppm), and total REE (13–110 ppm). In addition, they are enriched in Ta relative to Nb, and thus have very low Nb/Ta ratios ranging from 3.7 to 9.2 (mostly 4–6), charcateristics similar to those of peraluminous leucogranites (Harris et al., 1986). The leucogranites are characterised by low REE abundance and flat (V-type) REE patterns with pronounced Eu negative anomaly (Fig. 9a), similar to many leucogranites worldwide (e.g. Mittlefehldt and Miller, 1983; Bernard-Griffiths et al., 1985; Inger and Harris, 1993; Williamson et al., 1996). Most

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Fig. 4. U–Pb concordia diagram showing analytical data for zircons from granodiorite samples 2KJL26-1 (open symbols) and 2KJL18-4 (solid symbols), both from the Jiuling Pluton in northern Jiangxi Province. Analysis #12, of a ∼1250 Ma xenocrystic zircon, is not shown.

Fig. 5. U–Pb concordia diagram showing analytical data for zircons from K-feldspar porpyritic granite sample 98KD154, from the Eshan Pluton in Yunnan Province. Analysis #7, of a highly discordant xenocrystic zircon, is not shown.

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Fig. 6. Total alkalis vs. SiO2 diagram (Middlemost, 1994) for classification of the granitoids. Data from Guangxi (1985), Zhao et al. (1987), Mao et al. (1988), Xing et al. (1988), Zhou and Wang (1988), Yunnan (1990), Ma and Du (1994), and this study.

leucogranite samples, particularly those having very low REE abundance (such as 98GX34-4, 96G29, and 96G25), show clearly the “tetrad effect” (Masuda and Ikeuchi, 1979). On the ocean ridge granite (ORG)normalised trace element diagram (Pearce et al., 1984), they show a general increase in normalised

abundance from Yb to Rb, with the exception of Ba which has a pronounced negative anomaly (Fig. 10a). Rb is highly enriched. Patterns for Hf to Yb are generally flat and lower than the ORG. Normalised (Ce/Nb)N ratios are mostly ≥1, and thus no significant negative Nb–Ta anomalies are observed.

Fig. 7. Plot of molar Al/(Ca + Na + K) vs. Al/(Na + K) showing the strong peraluminous to weakly metaluminous compositions of Neoproterozoic granitoids in South China.

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Fig. 8. Chemical variation diagrams of Neoproterozoic granitoids in South China. See text for explanation. The calc-alkaline and high-K fields in the K2 O vs. SiO2 plot are after Peccerillo and Taylor (1976).

5.2. Biotite granodiorites The biotite granodiorites in northern Guangxi and cordierite-bearing biotite granodiorites in northern Jiangxi and southern Anhui have a wide range of

SiO2 (60–71%) and plot mainly in the granodiorite field (Fig. 6). The Guangxi samples have lower SiO2 (60–63%), plotting in the diorite field. All samples but one (98GX39-2) are peraluminous with a wide range of A/CNK of 1.0–1.5. The Jiangxi and Anhui

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Fig. 9. Chondrite-normalised REE diagrams for Neoproterozoic granitoids in South China. Normalisation values are from Sun and McDonough (1989).

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Fig. 10. ORG-normalised spidergrams for Neoproterozoic granitoids in South China, after Pearce (1983).

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cordierite-bearing granodiorites are strongly peraluminous with A/CNK > 1.1 (Table 4 and Fig. 7), resembling typical S-type granites in the Lachlan Fold Belt of SE Australia (Chappell and White, 1992). Some Guangxi biotite granodiorite samples are metaluminous to weakly peraluminous (A/CNK = 0.97–1.1), possibly indicative of mixed I- and S-type compositions. The biotite granodiorites are high in Al2 O3 (14–16%) and low in MgO (<3%) over a SiO2 range of 65–72% (Fig. 8). In contrast, the Guangxi biotite granodiorites with SiO2 < 65% have low Al2 O3 (12–14%) but high MgO (3–10%) which decrease and increase, respectively, with decreasing SiO2 (Fig. 6). These samples also have highly variable Ni (37–125 ppm) and Cr (66–383 ppm). The curved trends on MgO and Cr versus SiO2 plots (Fig. 8) are indicative of a cumulate origin for these more mafic, high-Mg samples. Insufficient separation between cumulate mafic minerals and liquids will produce such curved trends, whereas restite-unmixing or two-component mixing will result in a linear trend (Chappell and White, 1992). Overall, the biotite granodiorites are characterised by relatively high TiO2 + FeOt + MgO (4–16%), variable K2 O (2–5%) and K2 O/Na2 O (0.7–1.8), constant and moderate Rb (100–200 ppm), Sr (100–200 ppm) and Rb/Sr (1–3). Furthermore, they have generally similar REE patterns showing variable LREE-enrichment and moderate Eu anomalies (Fig. 9b–d). Some Jiangxi cordierite-bearing granodiorites show elevated HREE patterns due to the presence of garnet. On the ORG-normalised spidergram, the biotite granodiorites are characterised by strong enrichment in K, Rb, Ba and Th and pronounced negative anomalies of Nb–Ta and Zr–Hf relative to neighbouring elements (Fig. 10b–d). These trace element patterns generally resemble those of “collision granites” (Pearce et al., 1984).

(A/CNK = 1.05–1.35, Fig. 7), but show a clear genetic link with the metaluminous amphibole-bearing quartz diorites and granodiorites in chemical variation diagrams (Fig. 8), suggesting that the strongly peraluminous high-SiO2 samples belong to the fractionated felsic I-type. This is also supported by the negative correlation between P2 O5 and SiO2 (Fig. 8) due to fractionation of apatite in the absence of Y-bearing accessory minerals in I-type granites (Chappell and White, 1992).

5.3. K-rich granitoids

6. Sm–Nd isotopes

A salient feature of the Eshan (Yunnan) K-rich porphyritic granitoids is their high K2 O + Na2 O contents (6.7–8.3%) (Fig. 6), high Sr (350–560 ppm), Ba (870–1280 ppm) and LREE (LaN = 200–400), and variably higher Nb (11–24 ppm), Th (18–45 ppm) and U (3.5–11 ppm). They are exclusively peraluminous

Sm–Nd isotopic data, expressed as εNd (T) values (T = crystallisation age) and Nd model ages (Table 5), provide useful information on the origin and average crustal residence age of the protolith of granitoid magma. For granitoid rocks with 147 Sm/144 Nd ratios, or fSm/Nd values [fSm/Nd = (147 Sm/144 Nd)sample /

5.4. TTG suite The Huangling (Hubei) TTG samples have a wide range in SiO2 , from 57 to 72%, and consequently straddle the fields of diorite, granodiorite, and granite (Fig. 6). They are metaluminous to weakly peraluminous (A/CNK = 0.89–1.09, Fig. 7) with a negative P2 O5 to SiO2 correlation (Fig. 8) typical of I-type granites (Chappell and White, 1992). With high Al2 O3 (14–18%), Na2 O (3.6–4.8%), Sr (300–460 ppm) and Sr/Y (21–45), and low K2 O (0.9–2.9%), K2 O/Na2 O (0.2–0.7), Rb (18–38 ppm), and Zr (89–124 ppm), the TTG samples show geochemical affinities with Archean TTG and/or modern adakites (e.g. Martin, 1999). All TTG samples show strongly fractionated REE patterns (Fig. 9f) with variable LREE enrichment (LaN = 100–400), significant HREE depletion (YbN = 4–12), and high La/YbN ratios of 20–30. Eu anomalies vary from negative (Eu/Eu∗ = 0.8) to slightly positive (Eu/Eu∗ = 1.1). On the ORG-normalised spidergram, TTG samples show variable enrichment in K, Rb, Ba, Th, Ce, and Sm relative to Nb, Ta, Zr, Hf, Y, and Yb, and display clear negative anomalies of Nb–Ta and Zr–Hf, with their patterns having steeper slopes relative to other granitoids (Fig. 10f).

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(147 Sm/144 Nd)CHUR − 1, (147 Sm/144 Nd)CHUR = 0.1967], close to that of the average continental crust (147 Sm/144 Nd = 0.118, corresponding to fSm/Nd = fcc = −0.40; Jahn and Condie, 1995), a single-stage Nd model age (TDM ) relative to a depleted mantle source with linear Nd isotopic growth gives a reasonable estimate of the average crustal residence age of the protolith. The single-stage Nd model age is defined as: TDM =

1 λSm

 (143 Nd/144 Nd)sample − 0.51315 +1 , ln 147 ( Sm/144 Nd)sample − 0.2137


where λSm = decay constant of 147 Sm = 6.54×10−12 per year. However, for granitoid rocks with 147 Sm/144 Nd ratios (or fSm/Nd values) more than 10% different from that of the average continental crust (such as the Guangxi leucogranites and the Huangling TTG rocks in this study), the single-stage Nd model ages are affected significantly by Sm/Nd fractionation. To minimise this effect, a two-stage Nd model age (T2DM ) is calculated using the same formulation as (Keto and Jacobsen, 1987): T2DM = TDM − (TDM − t)

fcc − fs , fcc − fDM

where fcc , fDM , and fs are the fSm/Nd values for average continental crust (fcc = −0.4), the depleted mantle (fDM = 0.08592), and the sample, respectively, and t is the formation age of the granitoids. In this study, we use T2DM model ages for all rocks to make all Nd model ages comparable, although we recognise that the assumption that all granitoid protoliths had a uniform Sm/Nd ratio may not be true.

indicating that the leucogranites were derived mainly from a crustal source having Paleoproterozoic residence ages. 6.2. Biotite granodiorites Ten biotite granodiorite samples from northern Guangxi have εNd (T) values between −5.5 and −3.9 (Table 5 and Fig. 11) and T2DM model ages between 1.80 and 1.93 Ga. Their εNd (T) values and T2DM model ages are relatively constant, in contrast to the strong increases in MgO and Cr with decreasing SiO2 (Fig. 8). Sixteen samples of cordierite-bearing biotite granodiorite from northern Jiangxi and southern Anhui exhibit relatively constant 147 Sm/144 Nd (0.121–0.136) and 143 Nd/144 Nd ratios (0.51212–0.51228). Consequently, they gave tightly-grouped εNd (T) values between −0.2 and −3.1 and Nd model ages of 1.5–1.7 Ga (Table 5 and Fig. 11), indicating that these rocks were likely generated by melting of a relatively homogeneous crustal source having Mesoproterozoic residence ages. 6.3. K-rich granitoids Five Eshan (Yunnan) K-rich granitoid samples have tightly-grouped 147 Sm/144 Nd (0.090–0.115) and 143 Nd/144 Nd (0.51173–0.51181) ratios, yielding relatively constant εNd (T) values (–6.5 to −8.7) and Nd model ages of 2.0–2.2 Ga (Table 5 and Fig. 11). Sample 98KD81-35, a quartz diorite with SiO2 = 60%, has a similar Nd isotopic composition to other high-silica K-rich granitoids. Thus, the Eshan K-rich granitoids and quartz diorites were dominantly derived from crustal sources with Paleoproterozoic residence ages. 6.4. TTG suite

6.1. Muscovite-bearing leucogranites Thirteen leucogranite samples from northern Guangxi were analysed for Nd isotopes. Due to significant fractionation between Sm and Nd, 147 Sm/144 Nd ratios range between 0.15 and 0.29, and measured 143 Nd/144 Nd ratios vary between 0.51196 and 0.51287. Calculated εNd (T) values range from −9.0 to −4.5 (Table 5 and Fig. 11). T2DM model ages for the Guangxi leucogranites cluster around 1.85–2.22 Ga,

Six TTG samples have strongly fractionated ratios (0.071–0.105) and variably low 143 Nd/144 Nd ratios (0.51107–0.51157), corresponding to εNd (T) values between −9.0 and −19.5 and Paleoproterozoic to Archean Nd model ages of 2.2–3.0 Ga (Table 5 and Fig. 11). Thus, these TTG rocks were likely derived mainly from a dominantly crustal source with Paleoproterozoic to Archean residence ages. 147 Sm/144 Nd

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Fig. 11. Histogram of εNd (820 Ma) values for Neoproterozoic granitoids in South China.

7. Granitoid origin and petrogenesis 7.1. Guangxi leucogranite and associated biotite granodiorite Field relations, petrographic features, whole-rock major and trace element geochemistry, and Sm–Nd isotope compositions, are inconsistent with production of the leucogranites and granodiorites from a common parent magma by crystal fractionation processes.

Instead, two types of metasedimentary rocks, with distinct mineralogical and chemical compositions, are suggested as protoliths for these two suites of granitoids. Based on the diagnostically low CaO/Na2 O ratio of <0.3 and high Rb/Sr (3.4–330) and Rb/Ba (1.1–170) ratios, the leucogranites are considered to be derived from a clay-rich, plagioclase-poor pelitic source (Fig. 12, Sylvester, 1998). In contrast, a clay-poor, plagioclase-rich psammitic source is suggested for the biotite granodiorites, based on their

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Fig. 12. Rb/Sr vs. CaO/Na2 O and Rb/Ba ratios for the Neoproterozoic peraluminous granitoids. Symbols as in Fig. 6.

high CaO/Na2 O ratio of >0.3 and low Rb/Sr (0.6–1.9) and Rb/Ba (0.2–0.5) ratios. Variable amounts of mafic to ultramafic components might have been involved in the high-Mg samples, because they have abnormally high MgO, Cr, and Ni contents that are difficult to be explained solely by insufficient separation of cumulate mafic minerals. Two types of mafic to ultramafic rocks occur in northern Guangxi Province: komatiitic basalts and peridotites interlayered within the Sibao metasediments (Zhou et al., 2000) and the ∼825 Ma mafic/ultramafic intrusions (Li et al., 1999). Komatiitic basalts and peridotites have variably high MgO contents between 5 and 32% (Li, 1996; Zhou et al.,

2000) and negative εNd values ≈ −2 at 820 Ma (Li, 1996). There are ∼825 Ma mafic dykes/sills exposed around the Sangfang and the Bengdong Plutons, and ultramafic intrusives (dunite and olivine pyroxenite) of possibly similar age occur around the Yuanbaoshan Pluton (Fig. 1b). The former have relatively low MgO contents of 5–10% and negative εNd values of −0.5 to −7, whereas the latter have higher MgO contents of 20–36% and positive εNd values of +3 to +5 (Ge et al., 2001). Two-component mixing model calculation demonstrates that the komatiitic basalts/peridotites and/or the coeval (∼825 Ma) dunite/olivine pyroxenite could have been involved in production of the granodioritic magma, resulting

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Fig. 13. Logarithmic plots of Rb and Ba vs. Sr concentration for Neoproterozoic peraluminous granitoids. Arrows indicate the influence of fractionation of plagioclase (Pl), K-feldspar (Kf), hornblende (Hb) and biotite (Bt) on the composition of the residual liquids. Symbols as in Fig. 6.

in high MgO, Cr, and Ni contents without significant influence on εNd (T) values. However, involvement of the coeval (∼825 Ma) dyke/sills is unlikely because they cannot influence significantly the MgO contents of these samples. The leucogranites exhibit εNd (T) values of −9.0 to −4.5 and T2DM model ages of 1.85–2.22 Ga, resembling closely those of the regional Mesoproterozoic Sibao metasedimentary rocks (εNd values = −6 to −8 at 820 Ma, Nd model ages = 2.0–2.1 Ga; Li and McCulloch, 1996). Although the biotite granodiorites seem to have slightly higher εNd (T) values and younger Nd model ages relative to the

leucogranites (due possibly to variable involvement of mafic/ultramafic components), εNd (T) values and Nd model ages for the high-silica biotite granodiorites overlap with those of the leucogranites and Sibao metasedimentary rocks. The metasedimentary sources of leucogranites (pelite-dominated) and biotite granodiorites (psammite-dominated), therefore, have Sm–Nd isotopic compositions similar to those of the Sibao metasedimentary rocks. The petrogenesis of muscovite-bearing leucogranites worldwide is controversial (e.g. Williamson et al., 1996). Most major and trace element variations in the leucogranites can be explained by either fractionation,

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or by variable degrees of partial melting. For example, covariation between Rb, Sr, and Ba in the Guangxi leucogranites (Fig. 13) can be explained either by fractional crystallisation of feldspars, or by low degrees of partial melting with feldspars being present in the residuum. The general decrease in these rocks of LREE and Eu/Eu∗ with increasing SiO2 and Rb/Sr, can also be explained by either of these genetic processes. Although controversial, there is a tendency to consider partial melting as the dominant process in the production of muscovite-bearing leucogranite (i.e. Inger and Harris, 1993; Williamson et al., 1996; Pressley and Brown, 1999). This is mainly because of apparent difficulties with the crystal fractionation model: (1) feldspar fractionation in felsic magmas is impeded by the lack of density contrast between the solid phase and the melt and by the high viscosity of granitic liquid (e.g. Martin and Nokes, 1989; Inger and Harris, 1993); (2) the crystal fractionation model invokes large amounts of genetically related, less evolved granite at depth, which is not supported by geophysical and field observations (Williamson et al., 1996). Accordingly, partial melting of a pelitic source is inferred for the formation of the Guangxi leucogranites. Their high K2 O contents of ca. 5% support a model of muscovite dehydration equilibrium eutectic melting (Miller, 1985; Pressley and Brown, 1999). However, a few highly evolved leucogranite samples with extremely high Rb/Sr ratios (>100) could have resulted from feldspar fractionation (Halliday et al., 1991). Note that these high Rb/Sr samples also have very low REE abundances, pronounced negative Eu anomalies, and tetrad REE patterns (Fig. 9a). The tetrad REE patterns in high-silica granites is attributed commonly to the presence of an aqueous-like system and to intense interaction of residual melts with aqueous hydrothermal fluids (Bau, 1996; Irber, 1999; Jahn et al., 2001), which might play an important role in enhancing feldspar fractionation. 7.2. Cordierite-bearing granitoids in Jiangxi and Anhui These rocks are characterised by strong peraluminous nature, high CaO/Na2 O (>0.3), and low Rb/Sr (0.5–2.7) and Rb/Ba (0.1–0.7) ratios (Fig. 12). In addition, they have homogeneous εNd (T) values between −0.2 and −3.1. Therefore, a clay-poor,

plagioclase-rich, psammitic source is inferred for these rocks (Sylvester, 1998). Sm–Nd isotopic compositions are comparable with those of the regional Mesoproterozoic metasedimentary rocks, i.e. the Shuangqiaoshan Group, which have εNd (T) values of between −0.4 and −2.5 at 820 Ma and Nd model ages of 1.5–1.7 Ga (Chen and Jahn, 1998). In comparison with the metasedimentary source for the Guangxi granitoids, the psammitic sources for the cordierite-bearing granitoids are chemically immature with younger, early Mesoproterozoic residence ages. The cordierite-bearing granitoids in Jiangxi and Anhui are chemically and mineralogically similar to S-type granitoids in the Lachlan Fold Belt of SE Australia. Chemical variations within and between related granitoid units in the Lachlan Fold Belt have been interpreted as the result of variable degrees of separation of restitic materials from the melt, i.e. the “restite model” (White and Chappell, 1977; Chappell et al., 1987). However, others have argued against the restite model and its general applicability (e.g. Wall et al., 1987; Clemens, 1989; Collins, 1998). Unlike the cordierite in the Lachlan Fold Belt S-type granitoids (White and Chappell, 1977; Chappell et al., 1987) and the biotite in the East Greenland S-type granitoids (Kalsbeek et al., 2001), both of which were interpreted to be of restitic origin, the biotite and cordierite in the Jiangxi and Anhui granitoids are of magmatic origin, based on petrographic observations (Zhou and Wang, 1988). The restite model thus does not seem to be applicable. Moreover, on logarithmic bivariate plots of Rb versus Sr and Ba versus Sr (Fig. 13), the Jiangxi granodiorite samples show covariation consistent with an evolution controlled by plagioclase, rather than by biotite. Considering the strong peraluminous nature and low Rb/Sr ratios of 1 to 2 for the Jiangxi and Anhui granodiorites, we suggest that they were generated by vapour-present melting (Harris et al., 1995; Harris and Inger, 1992) of a clay-poor, plagioclase-rich psammitic source (Sylvester, 1998) similar to the Shuangqiaoshan metasedimentary rocks. 7.3. TTG suite Major and trace element data indicate that the Huangling TTG are typical I-type granitoids, geochemically similar to those of Archean TTG (Martin,

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1994) and modern adakitic rocks (Martin, 1999). Modern adakites are produced exclusively by melting of subducted oceanic crust, where abnormally high geothermal gradients are expected along the Benioff plane (e.g. Defant and Drummond, 1990; Atherton and Petford, 1993). Archean TTG rocks were generated by partial melting of either a young and hot subducting slab (e.g. Martin, 1994, 1999), or a tholeiitic source related to underplating and/or a subjacent mantle plume (Arndt and Goldstein, 1989; Kröner, 1991; Kröner and Layer, 1992). Although it is debatable whether Archean TTG series are analogous to modern adakites (Martin, 1999; Smithies, 2000), these Na-rich calc-alkaline magmas are generated exclusively by partial melting of hydrated basaltic crust (either oceanic crust or underplated mafic lower crust) at pressures high enough to stabilise garnet ± amphibole (e.g. Drummond and Defant, 1990; Rapp et al., 1991; Martin, 1999). A basaltic crustal source is thus reasonably assumed for the Huangling TTG rocks. Moreover, Sm–Nd isotopes suggest that the TTG were derived mainly from a Paleoproterozoic or Archean crustal source with little involvement of Neoproterozoic juvenile material. Amongst the various rock types of the regionally-exposed Archean–Paleoproterozoic Kongling Complex, the amphibolites are the most likely source for the Huangling TTG suite in view of its tholeiitic composition and comparable Sm–Nd isotopic compositions (εNd values ≈ −9 to −13 at 820 Ma; Gao et al., 1999). The metaluminous to weakly peraluminous nature of the Huangling TTG suite precludes its formation under low pressure and H2 O-saturated conditions, because experimental melts produced at low pressure (<8 kbar) are strongly peraluminous, and an increase in H2 O pressure results in formation of even more peraluminous melts than at lower pressures (Beard and Lofgren, 1989, 1991). Thus the TTG are likely to have been produced by dehydration melting of a basaltic source (Beard and Lofgren, 1991). Low HREE abundance (<10 × chondritic values, with the exception of sample 99SC-8) and fractionated REE patterns (chondrite-normalised (La/Yb)N = 20–33) indicate the possible presence of garnet in the residuum after melting. However, fractionation of HREEs in the TTG rocks is insignificant, as shown by their relatively flat HREE patterns, with (Ho/Yb)N = 1.04–1.10 and Y/Yb = 10–11. These values are comparable with

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chondritic ratios, but contrast with Archean TTG and modern adakites that show significant fractionation of HREEs (steep HREE patterns) with a pronounced decrease from Gd towards Lu and Y/Yb > 14 (e.g. Drummond et al., 1996; Martin, 1999). Such flat HREE patterns suggest that amphibole was likely the main residual mineral during partial melting of the Huangling TTG magmas. Moreover, Sr enrichment relative to LREE is not observed in the Huangling TTG rocks, because these samples do not have a positive Sr anomaly relative to Ce and Nd (primitive-mantle normalised SrN /(CeN /2 + NdN /2) = 0.3–1.0). This implies that plagioclase could also have been a residual phase during their genesis (Martin, 1999). Based on experimental results on high-pressure mafic granulites (Rushmer, 1993) and metabasalts (Rapp and Watson, 1995), the residual mineral assemblage of amphibole + plagioclase + pyroxene ± garnet suggests that the melting of the Huangling TTG took place at condition of P = 8–12 kbar and T = 800–1050 ◦ C. 7.4. K-rich granitoids Major and trace element characteristics indicate that the Eshan (Yunnan) K-rich granitoids are I-type, and therefore that they were likely derived from an igneous source. In view of their consistent negative εNd (T) values (−6.5 to −8.7) and Paleoproterozoic Nd model ages (2.0–2.2 Ga), significant involvement of mantle-derived juvenile material in the granites is unlikely. The high-K nature of the granites suggests that their source contained a substantial concentration of K-bearing silicates such as biotite and K-feldspar. The felsic to intermediate composition of the K-rich granitoids is consistent with derivation from an ancient source of tonalitic to granodioritic composition (Singh and Johannes, 1996). Melting experiments of Patiño Douce (1997) demonstrate that shallow (≤4 kbar) dehydration melting of hornblende-bearing tonalite and granodiorite at >900 ◦ C generates metaluminous A-type granites, whereas deep (≥8 kbar) dehydration melting generates hornblende-absent peraluminous granites. The Eshan K-rich granitoids were therefore most likely generated by dehydration melting of a tonalite–granodiorite source at deep crustal levels with P ≥ 8 kbar and T > 900 ◦ C.

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8. Tectonic implications Genetic links between granitoid types and the geodynamic environments in which they formed has been the focus of many studies since Chappell and White (1974) introduced the concept of I- and S-type granites. Barbarin (1996, 1999) proposed a detailed and practical classification scheme, in which six types of granitoid were defined: two types of peraluminous granitoids (MPG and CPG) of purely or essentially crustal origin, two types of calc-alkaline granitoids (KCG and ACG) of mixed origin, and two types of “tholeiitic” or alkaline granitoids (ATG or RTG, and PAG) of mainly mantle-derived origin. Based on mineralogical, petrographic and geochemical characteristics, the Neoproterozoic granitoids of the Yangtze craton in South China fall into four types in Barbarin’s classification: (1) MPG (muscovite-bearing peraluminous granitoids), represented by the muscovite-bearing leucogranites in northern Guangxi; (2) CPG (cordierite-bearing peraluminous granitoids), represented by the northern Jiangxi and southern Anhui cordierite-bearing granodiorites (the northern Guangxi biotite granodiorites are also similar to the CPG type, though they are lacking in cordierite); (3) KCG (high K–low Ca calc-alkaline granitoids), represented by the K-rich porphyritic granitoids; and (4) the Huangling TTG, which show geochemical affinities with adakitic rocks. According to Barbarin (1999), peraluminous MPG and CPG are generally formed in collisional orogens and are associated with the climax of collisional orogenesis, succh as the Massif Central and Brittany, France and High Himalaya leucogranites. KCG also form in collisional belts and are associated with MPG and CPG, but they occur typically during post-collisional uplift and orogenic relaxation. Adakites are considered to be associated with ACG and ATG in subduction environments. Therefore, the Neoproterozoic granitoids in South China could have formed in subductional (the Huangling TTG), collisional (the peraluminous granitoids in Guangxi, Jiangxi and Anhui), or post-collisional (the Yunnan K-rich granitoids) environments. However, several lines of evidence argue against such an interpretation: (1) All four granitoid types were emplaced during a narrow time interval at 825–820 Ma,

(2) No spatial or temporal pattern is apparent in the distribution of the granitoids in South China. Moreover, the Huangling TTG suite was emplaced in the interior of the Yangtze craton and thus represents “within-plate” magmatism, (3) None of the granitoids is significantly deformed or metamorphosed, (4) There was no contemporaneous metamorphism in the region, whereas older, Grenville-aged (1.3–1.0 Ga) metamorphism has been reported (Li et al., 2002a). The 825–820 Ma granitoids are thus unrelated to the Grenvillian orogenesis in South China. As discussed above, the geochemical characteristics of these granitoids point to their derivation from diverse sources. For instance, the Huangling TTG plot in the “volcanic arc granitoid” field on the Y + Nb versus Rb diagram (Fig. 14a), but straddle the boundary between “within-plate” and “late- to post-collisional” granitoids on the Rb–Hf–Ta ternary diagram (Fig. 14b). The peraluminous granodiorites and K-rich granitoids plot exclusively within the “post-collision granitoid” field on the Y + Nb versus Rb plot (Fig. 14a), but on the Rb–Hf–Ta ternary diagram, the former fall into the “volcanic arc granitoid” field, and the latter mostly into the field of “lateto post-collisional” granitoids (Fig. 14b). Therefore, these geochemical discrimination diagrams may not be useful for diagnosing the tectonic settings of the studied Neoporterozoic granitoids in South China. The diagrams could instead be diagnostic of the tectonic settings in which the protoliths were formed, or have become non-diagnostic owing to modification of geochemical compositions by melting of mixed sources. An alternative explanation for the widespread crustal melting across South China at 825–820 Ma is the presence of a mantle plume. Crustal melting above a ∼825 Ma mantle plume, as proposed by Li et al. (1999), is considered here to be a plausible mechanism for the generation of coeval, geochemically diverse granitoid suites. The 828 ± 7 Ma mafic to ultramafic intrusions in South China are not only coeval with, but also geochemically similar to, the plume-related Gairdner Dyke Swarm in Australia (Zhao et al., 1994; Wingate et al., 1998; Ge et al., 2001). The plume model is further supported by the identification of ca.

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Fig. 14. Geochemical tectonic discrimination diagram for Neoproterozoic granitoids: (a) Rb/(Y + Nb) plot after Pearce et al. (1984) and Pearce (1996); (b) Rb–Hf–Ta ternary diagram after Harris et al. (1986). Abbreviations: VAG, volcanic arc granites; ORG, ocean ridge granites; WPG, within plate granites; syn-COLG and post-COLG, syn- and post-collision granites; L/P-COLG, late- and post-collision granite. Symbols as in Fig. 6.

820 Ma plume-related Tiechuanshan tholeiites along the northwestern margin of the Yangtze craton (Ling et al., 2003, this issue), and ca. 800 Ma bimodal volcanic rocks within a rift basin (Li et al., 2002). The plume activity resulted in extensive basaltic magma intrusion and extrusion in South China, as demonstrated by mafic intrusions such as the 819 Ma Tongde Gabbro in South Sichuan (Sinclair, 2001), ca. 820 Ma mafic intrusions within the Hannan Complex (Zhang

et al., 2001), and 828 Ma mafic dykes, sills, and stocks in northern Guangxi that are slightly older than the granitoid plutons. There are also mafic and ultramafic intrusions associated with the Huangling TTG in Hubei (Fig. 1e), although none are dated precisely. These intrusions appear to be coeval with the Huangling TTG based on field relations. Extensive underplating and intrusion of hot, mantle-derived basaltic magma related to plume activity could have provided

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the heat source for widespread crustal melting in South China, as is predicted by numerical modelling (Huppert and Sparks, 1988; Bergantz, 1989).

9. Conclusions New SHRIMP U–Pb zircon age determinations, together with previous zircon data, indicate that the pre-rifting Neoproterozoic granitoids in South China were emplaced synchronously at ca. 825–820 Ma. Four granitoid types are identified, based on mineralogical, petrographic and geochemical characteristics: two types of peraluminous (S-type) granitoids (muscovite-bearing leucogranites and cordieritebearing granodiorites), and two types of I-type granitoids (calc-alkaline K-rich granitoids and TTG). Sm–Nd isotopic data indicate that all four granitoid types were generated by partial melting of local basement rocks with little involvement of new mantle-derived magmas. The narrow time interval of magmatism and the mixture of compositions are inconsistent with an orogenic environment (including subduction, continental collision, post-collisional uplift, or orogenic collapse). Instead, we suggest that the 825–820 Ma granitoids were formed by extensive crustal anatexis resulting from underplating and intrusion of basaltic magma caused by a mantle plume beneath South China at ∼825 Ma.

Acknowledgements We appreciate the assistance of C.-Y. Lee and R. Chang in major element analysis by XRF, G.Q. Hu and W. Zeng in major element analysis by ICP-AES, X.L. Tu for trace element analysis by ICP-MS, and X.R. Liang and W.L. Ling for Nd isotope analysis by MC-ICPMS and TIMS. L.G. Zhi, J. Wang, and S. Zhang provided field support. XHL thanks the Australian Academy of Science for providing a 2-month visiting fellowship to the University of Western Australia under the agreement of scientific cooperation between the Chinese and Australian Academies of Sciences. U–Pb analyses were conducted using the SHRIMP II ion microprobe in Perth, Australia, which is operated by a consortium consisting of Curtin University of Technology, the Geological Survey of

Western Australia, and The University of Western Australia, with the support of the Australian Research Council. This work was supported by NFSC (grants 49725309 and 40032010-B) and Chinese Academy of Sciences (grant KZCX2-101). This is Tectonics Special Research Centre publication no. 193, and a contribution to IGCP 440.

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