Geochronology and geochemistry of volcanic rocks from the Jingtan Formation in the eastern Jiangnan orogen, South China: Constraints on petrogenesis and tectonic implications

Accepted Manuscript Geochronology and geochemistry of volcanic rocks from the Jingtan Formation in the eastern Jiangnan orogen, South China: Constraints on petrogenesis and tectonic implications Longming Li, Shoufa Lin, Guangfu Xing, Yang Jiang, Xiaoping Xia PII: DOI: Reference:

S0301-9268(16)30236-4 http://dx.doi.org/10.1016/j.precamres.2017.02.012 PRECAM 4677

To appear in:

Precambrian Research

Received Date: Revised Date: Accepted Date:

30 June 2016 8 February 2017 20 February 2017

Please cite this article as: L. Li, S. Lin, G. Xing, Y. Jiang, X. Xia, Geochronology and geochemistry of volcanic rocks from the Jingtan Formation in the eastern Jiangnan orogen, South China: Constraints on petrogenesis and tectonic implications, Precambrian Research (2017), doi: http://dx.doi.org/10.1016/j.precamres.2017.02.012

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Geochronology and geochemistry of volcanic rocks from the Jingtan Formation in the eastern Jiangnan orogen, South China: Constraints on petrogenesis and tectonic implications Longming Lia, *, Shoufa Linb, a, Guangfu Xingc, Yang Jiangc, Xiaoping Xiad

a

School of Resources and Environment, Hefei University of Technology, Hefei 230026,

PR China b

Department of Earth and Environmental Sciences, University of Waterloo, 200

University Avenue West, Waterloo, Ontario N2L 3G1, Canada c

d

Nanjing Institute of Geology and Mineral Resources, Nanjing 210016, PR China

Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou

510460, PR China

Abstract An integrated study of zircon U-Pb geochronology and geochemistry, together with Nd-Hf isotopes, have been carried out on the rhyodacite and rhyolitic tuff of the Jingtan Formation in the eastern part of the Jiangnan orogen. SIMS zircon U-Pb dating of two samples yielded weighted mean 206Pb/238U ages of 784 ± 6 Ma and 788 ± 6 Ma, respectively. Geochemically, they are peraluminous (A/CNK mostly around 1.2) and are characterized by enrichments in Rb, Th, REEs and HFSEs (e.g. Zr and Y) but depletions in Ba, Sr, P, Eu and Ti. The volcanic rocks show a clear A-type granite geochemical signature with high total alkalis (K2O + Na2O = 5.3-7.64 wt.%), FeOt/MgO ratios and low CaO, MgO and TiO2 contents. They have negative whole-rock εNd(t) (-4.2 to -1) and positive zircon εHf(t) (+1.26 to +11.6) values, illustrating decoupled Nd-Hf isotopes which may be genetically related to their 1

petrogenesis process. The positive εHf(t) values and juvenile THfDM1 (0.89-1.3Ga) of zircons indicate that the volcanic rocks may have been derived from the partial melting of the Neoproterozoic to late Mesoproterozoic crustal materials. Combined with the occurrence of significant volumes of contemporary bimodal volcanic rocks in eastern section of the Jiangnan orogen, it is inferred that the Jingtan felsic volcanic rocks formed during post-collisional extension shortly after the final amalgamation of Yangtze and Cathaysia blocks.

Key words: Jiangnan orogen, Jingtan Formation, Post-collisional extension, South Anhui Province, Volcanic rocks

1. Introduction The Jiangnan orogen is commonly considered to be the collisional belt between the Yangtze and Cathaysia blocks which were amalgamated to form the South China Block (SCB) (Charvet et al., 1996; Shu and Charvet, 1996; Wang et al., 2004, 2006, 2008; Zheng et al., 2007, 2008; Li et al., 2007, 2008, 2009b; Charvet, 2013; Zhao, 2015). Generally, the evolution processes of an orogenic belt, from plate subduction to continental collision and post-collisional / post-orogenic extension, may be recorded in sedimentation, magmatism, deformation and metamorphism (e.g. Shu et al., 1995; Li et al., 2002, 2016a). Since there are not enough diagnostic features in the regional metamorphism and deformation in the Jiangnan orogen, magmatism becomes an important factor in studying the evolution of the orogen (Wang et al., 2012b). Neoproterozoic magmatic rocks are widespread in the orogenic belt (Li et al., 2003a, b; Wang et al., 2004, 2006, 2008; Wu et al., 2006b). These igneous rocks can provide pivotal information about the tectonic evolution of the South China Block. 2

They have thus received great attention in recent years (e.g., Zheng et al., 2008; Li et al., 2008, 2013a,b, 2016a,b; Wang et al., 2012b, 2013b, 2014; Zhang et al., 2012a, 2013a,b). However, interpretation of geochemical data of these igneous rocks are not unequivocal (i.e. plume, plate-rift or arc-related setting), which has led to contrasting views on the timing of each stage of the orogenic evolution processes (e.g. Li et al., 2003a; Zheng et al., 2008; Zhao et al, 2011; Wang et al.,2012b; Zhang et al.,2013c). Moreover, due to a lack of high grade metamorphic rocks which are a common product of collisional orogeny, it is difficult to constrain the final collision of the orogen (Li et al., 2016a). As a result, evolution history of the Jiangnan orogen is still poorly constrained. Some researchers believe that the southeastern margin of the Yangtze Block was a passive continental margin during late Mesoproterozoic (Li et al., 2013b), plate subduction started in early Neoproterozoic (ca. 930-890 Ma) (Ye et al., 2007; Li et al., 2009b) and arc-continental collision occurred at ca. 880 Ma (Li et al., 2008) or at ca. 860 Ma (Zheng et al., 2008), whereas others suggest that subduction continued to ca. 830 Ma or even later and the final arc-continental collision happened at ca. 820-810 Ma (Zhao et al., 2011; Zhang et al., 2012a,2013a,b; Li et al., 2013a, 2016a; Wang et al., 2013c, 2014; Zhao, 2015). Neoproterozoic igneous rocks, particularly those located in the eastern part of the Jiangnan orogen, have been one of the important targets in this controversy. Previous geochronological and geochemical investigations show that there are two phases of magmatism in the eastern part of the Jiangnan orogen, with age peaks of ca. 820 Ma and ca. 780 Ma, respectively (Li et al., 2003; Wu et al., 2006b; Zheng et al., 2008). The first phase of magmatism has been studied extensively (e.g. Li et al., 2003, 2013a, 2016a; Wang et al., 2006; Wu et al., 2006b), whereas data from the second phase is limited (e.g. Zheng et al., 2008). The two phases of magmatism may have formed during different stages of the evolution of the orogen. Therefore, potential difference 3

in geodynamic setting for the two phases of magmatism may shed light on the evolution of the Jiangnan orogen. In this paper, we present results of an integrated study of zircon U-Pb and Lu-Hf isotopes, whole-rock geochemistry and Nd isotopes for the volcanic rocks of the second phase from the Jingtan Group in south Anhui Province. Our results offer new perspectives on the geodynamic setting of the igneous rocks in the eastern part of the Jiangnan Orogen, contributing to a better understanding of Neoproterozoic tectonic evolution of the South China Block.

2. Geological background The South China Block, one of the major crustal blocks in Eastern Asia, is traditionally considered to be made up of the Yangtze Block to the northwest and the Cathaysia Block to the southeast (Fig.1a), which were amalgamated along the Jiangnan orogen (Zheng et al., 2007; Li et al., 2009b). The Yangtze Block has a Archean-Paleoproterozoic continental nucleus in the Kongling area, and is surrounded by late Meso- to Neoproterozoic strata and magmatic rocks (Qiu et al., 2000; Zhang et al., 2006; Gao et al., 2011b; Li et al., 2014a; Wu et al., 2014). Exposed basement in the Cathaysia Block is dominantly late Neoproterozoic with minor Paleoproterozoic outcrops (Li et al., 2011; Yu et al., 2009a, 2012; Xia et al., 2012; Chen et al., 2016). The Jiangnan orogen comprises mainly low-grade Proterozoic metasedimentary and igneous rocks. The metasedimentary strata are structurally divided into folded basement sequences and overlying cover sequences separated by an angular unconformity (Wang and Li, 2003). The folded sequences include the Sibao Group in Guangxi Province, the Fanjingshan Group in Guizhou Province, the Lengjiaxi Group in Hunan Province, the Shuangqiaoshan Group in Jiangxi Province and the Xikou Group in Anhui Province, which have similar ages and show similar depositional, 4

deformational, metamorphic and structural characteristics (BGMRGX, 1985; BGMRHN, 1988; BGMRJX, 1997; BGMRAH, 2008). They may have been deposited in an arc-back-arc basin setting corresponding to subduction between the Yangtze and Cathaysia blocks (Wang et al., 2012a; Yin et al., 2013; Li et al., 2013, 2016a, b) and subsequently experienced deformation at low-greenschist facies conditions. These folded basement sequences are intruded by voluminous ca. 835-800 Ma granitoids (Li et al., 2003; Wang et al., 2006; Wu et al., 2006b). There are also two Precambrian ophiolite complexes occurred in the eastern part of the Jiangnan orogen, i.e. the Northeastern Jiangxi Ophiolite and the Fuchuan Ophiolite in Southern Anhui Province (e.g. Zhang et al., 2012, 2013c, 2015; Wang et al., 2015). The overlying

volcano-sedimentary sequences

are

unmetamorphosed

or

weakly

metamorphosed. In the eastern part of the Jiangnan orogen, the Shangshu, Puling and Jingtan formations are parts of the overlying volcano-sedimentary sequences (Table 1). The Shangshu Formation records at least two stages of volcanic eruption, with basalts, basaltic andesites, andesites, dacites, rhyolites and tuffs (Shu et al., 1995, Wang et al., 2012). This formation was intruded by the ca.780 Ma Shi’ershan granites (Li et al, 2003). The Puling Formation is composed of andesites and tuffs, and lesser tuffaceous sandstones (Ma et al., 2001). The Jingtan Formation, which is considered to be equivalent to the Puling Formation (Li et al., 2014b), mainly consists of andesites, dacites, rhyolites and pyroclastic rocks (Wu et al., 2007). These formations were suggested to have formed in an extensional setting (Wu et al., 2007; Wang et al., 2012). They are conformably overlain by the Xiuning Formation (Ma et al., 2001; Li et al., 2008; Wang et al., 2012), which consists mainly of sandstones and siltstones that formed during the late stage of rifting and was overlain by the Nantuo glacial 5

deposits (Li et al., 2008). Eighteen volcanic rocks of Jingtan Formation including rhyodacite and rhyolitic tuff were sampled from Jiezhuying, Changgai and Jingtan area of Southern Anhui Province (Fig. 1b). Field observations show that these felsic volcanic rocks exhibit eruptive contact relationship. They display porphyritic texture and flow structure (Fig. 2a) with a fine-grained matrix. They contain phenocrystic K-feldspar, hornblende, biotite, and intergrown magnetite. Many of the feldspar crystals contain abundant inclusions and embayments of glass whose composition is indistinguishable from that of the matrix. In some thin sections, the orientation of the quartz microlites around larger phenocrysts gives a flow banding appearance (Fig. 2b).

3. Analytical methods Representative samples were selected for zircon U-Pb dating. Zircon concentrates were separated using standard density and magnetic separation techniques. Zircon grains were embedded in a polished epoxy mount and imaged by backscattered electron (BSE) and cathode luminescence (CL). Both BSE and CL images were used to select areas for analysis, preferentially targeting primary, uniform crystal domains and avoiding cracks and alteration. Secondary ion mass spectrometry (SIMS) zircon U-Pb analyses were conducted using a CAMECA IMS1280-HR system at the SKLaBIG GIG CAS. Analytical procedure is similar to that described by Li et al. (2009c). Each measurement consisted of seven cycles, and the total analytical time per measurement was ~12 minutes.Calibration of Pb/U ratios is relative to the standard zircon Plesovice (337.13 Ma) (Slama et al., 2008) ,which was analyzed once every four unknowns, based on an observed linear relationship between ln (206Pb/238U) and ln (238U16O2/238U) 6

(Whitehouse et al., 1997). A long-term uncertainty of 1.5% (1 RSD) for

206

Pb/238U

measurements of the standard zircons was propagated to the unknowns (Li et al., 2010), despite that the measured

206

Pb/238U error in a specific session is generally

around 1% (1 RSD) or less. U and Th concentrations of unknowns were also calibrated relative to the standard zircon Plesovice, with Th and U concentrations of 78 and 755ppm, respectively (Slama et al., 2008). An average of present-day crustal composition (Stacey & Kramers, 1975) is used for the common Pb. A secondary standard zircon Qinghu (Li at al 2013) were analyzed as unknown to monitor the reliability of the whole procedure. 5 analytical spots conducted during the course of this study yield a Concordia age of 159.2Ma, identical to its recommended value. The U-Pb analytical results are given in Table 2. Uncertainties on single analyses are reported at the 1σ level; mean ages for pooled U-Pb analyses are quoted with a 95% confidence interval. Data reduction was carried out using the Isoplot/Ex 3 software (Ludwig, 2003). For elemental and isotopic analyses, samples were ground in an agate mill to ~200 mesh. Major element contents of whole rocks were determined using a Rigaku RIX 2000 X-ray fluorescence spectrometer (XRF) at the Guangzhou Institute of Geochemistry (GIS), Chinese Academy of Sciences (GAS), on fused glass beads. Based on the measured values of rock standards (BHVO-1 and AGV-1), the analytical uncertainties are estimated to be better than 3% for all the major elements. Details of procedures were described by Li et al. (2006). Sample solution for trace elements was prepared in the Department of Earth Sciences, University of Hong Kong. The procedures were similar to those described by Li et al. (2009a). Trace elements contents were measured using Perkin-Elmer Sciex ELAN 6000 inductively coupled plasma-mass spectrometer (ICP-MS) at the GIG, CAS. A set of international 7

standards including BHVO-1, G-2, GSR-3 and AGV-1 was used to estimate the accuracy and precision of the analyses. Detailed sample analytical procedure was described by Wei et al. (2002). The results are presented in Table 3. Sr-Nd isotopic analyses were carried out at the GIG, CAS. Detailed descriptions of the techniques are given in Wei et al. (2002) and Liang et al. (2003). Isotopic compositions were determined using a Micro Mass Isoprobe Multi-collector Mass Spectrometer (MC-ICPMS). The mass fractionation corrections for Sr and Nd isotopic ratios are based on 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219, respectively. 87

Rb/86Sr and

147

Sm/144Nd ratios were calculated using the Rb, Sr, Sm and Nd

abundances measured by ICP-MS. The measured

87

Sr/86Sr ratio of the (NIST) SRM

987 standard and 143Nd/144Nd ratio of the La Jolla standard are 0.710265 ± 12 (2σ) and 0.511862 ± 10 (2σ), respectively. The results are presented in Table 4. Zircon Hf isotopic measurements were performed on a Neptune Plus multi-collection inductively coupled plasma mass spectrometry at the GIG, CAS. Details of Hf isotopic analytical method followed Wu et al. (2006a). Normalizing factor used to correct the mass fractionation of Hf during the measurements is 0.7325 for 179Hf/177Hf. The reference value of 176Hf/177Hf of standard Penglai Zircon is 0.282906 ± 0.000010 (2s) (Li et al., 2010). Analyses of standard Penglai zircon over the measurement period provided176Hf/177Hf = 0.282926 ± 0.000016 (2s) (n = 6). Initial 176Hf/177Hf values were calculated based on Lu decay constant of 1.865E−11 (Scherer et al., 2001). The model ages were computed under the assumption that the 176Lu/177Hf of average crust is 0.015, and the

176

Hf/177Hf and 176Lu/177Hf ratios of

chondrite and depleted mantle at the present are, respectively, 0.282772 and 0.0332, and 0.28325 and 0.0384 (BlichertToft and Albarede, 1997). The analytical results of Hf isotopic compositions are given in Table 5. 8

4. Results 4.1 SIMS zircon U-Pb geochronology Sample 12WN-12-8 (N29054′49.7″, E118044′15.2″) is a dacite. Zircons from the sample range from 80 to 150 µm in length, with length/width ratios nearly 2:1. They are generally prismatic with clear oscillatory zoning (Fig. 3a). Some grains have clear rounded xenocrystic or inherited cores. A total of twenty zircons were analyzed. Their Th concentrations range from 39 to 397 ppm, U from 50 to 575 ppm, and Th/U ratios from 0.13 to 0.98. Fifteen of the analyses form a single, Concordant group with a weighted mean

206

Pb/238U concordia age of 784 ± 6 Ma (MSWD = 0.4), which is

interpreted as the crystallization age of the dacite. The remaining five analyses give distinctly older ages of ca. 820 Ma (Fig. 4a), and are likely to be xenocrysts. Zircons from sample 12WN-16-1 (N29040′48.6″, E118028′34.4″), a rhyolitic tuff, are mostly euhedral, translucent and light in color. Most grains are from 40 to 100 µm long, with length/width ratios of 1:1 to 2:1. Magmatic oscillatory zoning is obvious in all crystals (Fig. 3b). A total of sixteen analyses were conducted. U concentrations range from 124 to 329 ppm with Th/U ratios of 0.58-0.73. One analysis (#13) is strongly discordant. The remaining fifteen data are concordant or nearly concordant and give a weighted average 206Pb/238U age of 788 ± 6 Ma (MSWD = 0.69) (Fig. 4b), interpreted as the crystallization age of the rhyolitic tuff. 4.2 Major and trace elements Totals for major oxides of the Jingtan volcanic rocks are recalculated to 100% for presentation (volatile free). The felsic rocks have SiO2 contents ranging from 65.4 to 79.6 wt. % and are characterized by variable total alkalis (K2O + Na2O = 5.3-7.64 9

wt. %) and Al2O3 content (12-15 wt. %). On the SiO2-Zr/TiO2 diagram (Fig. 5a), they classify as rhyolites and rhyodacites. They have A/CNK ratios of 0.94-1.74, and thus are slightly peraluminous (Fig. 5b). There is a negative correlation between some major elements (e.g. Al2O3, CaO, MgO and FeOt) and SiO2 in Harker diagram (Fig. 6). The felsic volcanic rocks all have similar REE patterns showing LREE-enrichment with (La/Yb)N value of 4.12-14.8, and strong negative Eu anomalies (&Eu = 0.16-0.82) (Fig. 7a). In the primitive mantle-normalized spidergram (Fig. 7b), they have pronounced depletion in Nb, Sr, P and Ti, similar to many Neoproterozoic granitoids along the Jiangnan orogen (Li et al., 2003; Wu et al., 2006b). 4.3 Whole-rock Nd isotopes The felsic rocks have

147

Sm/144Nd ratios of 0.1296-0.1606 and

143

Nd/144Nd

varying from 0.512192 to 0.512241. They have negative calculated εNd(t) of -4.2 to -1.0 at their ages of magma emplacement, with corresponding single-stage Nd model ages (TNd DM1) of 1726-1773 Ma. 4.4 Zircon Hf isotopes The zircons from the two samples that were dated by U-Pb were also analyzed for their Lu-Hf isotopes. Initial

176

Hf/177Hf ratios and εHf(t) values were calculated at

t=784 Ma and 788 Ma, respectively, depending on their U-Pb ages. Twenty-two spots were analyzed for zircon Lu-Hf isotopes in dacite 12WN-12-8. Except for one negative εHf(t) values of -4.69 (spots #7 in Table 4), all other analyses show positive ε Hf(t) values ranging from +1.26 to +11.6. Their Corresponding Hf single-stage model ages (THf DM1) are 0.89-1.3 Ga. The negative ε Hf(t) spot has Hf model age of 1.55 Ga. Twenty-two spots were also analyzed for zircon Lu-Hf isotopes in rhyolite 10

12WN-16-1. Except for one negative εHf(t) values of -0.79 (spots #12 in Table 4), all other spots show positive εHf(t) values ranging from +2.85 to +10.3. Their Corresponding THfDM1 ages are 0.95-1.24 Ga. The negative εHf(t) spot has Hf model age of 1.42 Ga.

5. Discussions 5.1. Origin of the felsic volcanic rocks The genetic type of the felsic volcanic rocks in this study has been uncertain. Although its status as an A-type granite may be debated (e.g. Wu et al., 2007), it has most of the features associated with the definition of A-type granites by Loiselle and Wones (1979) and Frost et al. (2001). They show high SiO2 content with high FeOt/MgO ratio (Fig. 8a) and they are high in total alkalis (5.3-7.64 wt. %). In general, these volcanic rocks are variably enriched in REE and Zr, and their sum of contents of Zr, Nb, Ce and Y ranges from 173 to 603ppm (mostly > 350ppm), typical of A-type granites (e.g., Collins et al., 1982; Whalen et al.,1987). In a 10,000*Ga/Al versus Zr + Ce + Nb + Y diagram, they plot in the A-type granite field (Fig. 8b). Although highly evolved I-type granite can also possess higher values of Ga/Al ratios and HFSE contents (Eby, 1992; King et al., 2001), the Zr saturation thermometer (up to 900 0C, Fig. 8c) indicate that they have much higher initial temperatures than I-type granite (Clemens et al., 1986; King et al., 2001; Bonin, 2007). Different from peralkaline A-type granites, the Jingtan volcanic rocks can be strongly peraluminous with A/CNK ratios of 1.03–1.74 (mostly around 1.2), indicating they are affinitive to peraluminous A-type granite (Turner et al., 1992; King et al., 1997; Brewer et al., 2004). Petrogenetic models for A-type magma are not unanimous. It essentially falls into 11

two categories, including extensive fractional crystallization from mantle-derived basaltic magmas (Turner et al., 1992; Frost et al., 1999), and partial melting of crustal protoliths, either a granulitic residue depleted by a prior extraction of granitic melt (Collins et al., 1982; Clemens et al., 1986), or a tonalite to granodioritic metaigneous source (Anderson, 1983; Creaser et al., 1991; Skjerlie and Johnston, 1993; Patiño Douce, 1997). However, melting of previously dehydrated metasedimentary source rocks can also form A-type granite (Anderson and Thomas, 1985; Whalen et al., 1987), which may be the most suitable model to explain the peraluminous affinity of the Jingtan felsic rocks. High δ18O values of zircons from the Jingtan felsic rocks also point to a supra crustal origin (e.g. Wu et al., 2007; Zheng et al., 2008). Since the Hf isotope composition of zircons is more resistant to later hydrothermal alteration, it can be more reliable in tracing the sources of the felsic volcanic rocks. The Jingtan volcanic rocks have depleted initial

176

Hf/177Hf ratios (Fig. 9), with a mean εHf(t)

value of +7.1 ± 0.8, and an average THfDM1 of 1.1 ± 0.05 Ga. The positive ε Hf(t) values for the zircons indicate the source region of the felsic volcanic rocks is mainly composed of juvenile crust material, while their THfDM1 ages further suggest that the extraction of new crust from the depleted mantle was during late Mesoproterozoic period. Many Neoproterozoic granitic rocks in the eastern Jiangnan orogen also have late Mesoproterozoic zircon THfDM1 ages at ca.1.2 Ga (Wu et al., 2006b; Zheng et al., 2008; Wang et al., 2014). A compilation of these THfDM1 ages (ca. 1.1-1.2 Ga) may define a major episode in the growth of juvenile crust, which is supported by the occurrence of mafic volcanism with age of ca. 1150 Ma in the eastern part of the Jiangnan orogen (e.g. Li et al., 2013b). Hence, it may be reasonable to interpret the felsic rocks as products of partial melting of late Mesoproterozoic juvenile crustal material. In addition, some zircons have relatively young THfDM1 of ca. 0.9 Ga, thus 12

the products of partial melting of early Neoproterozoic arc materials (e.g Shuangxiwu arc) may also have been incorporated into the source region of the felsic rocks. Element variations indicate that plagioclase and K-feldspar fractionation is also required after partial melting of the juvenile crust (Fig. 6). This is further evidenced by the pronounced depletion in Sr, Ba and Eu in Fig. 7a-b, together with an increase of Rb/Sr ratios with decreasing Sr. In addition, negative P and Ti anomalies become significant (Fig. 7b), which implies that fractionation of apatite and Fe-Ti oxides also occurred in magma evolution. 5.2. Decoupling of Nd-Hf isotopes Lu-Hf and Sm-Nd isotopic systems behave analogously during most magmatic processes, and thus Hf-Nd isotopic compositions are positively correlated (Vervoort et al., 1999). However, the volcanic rocks of Jingtan Formation have positive εHf(t) (+1.26 to +11.6) and negative whole-rock εNd(t) (-4.2 to -1) values, illustrating a Hf-Nd isotope decoupling (Fig. 10). Zircon is a highly refractory accessory mineral, it can preserve its primary Hf isotope composition during chemical weathering or magma crystallization. In contrast, whole rock Sm-Nd isotopic system can be disturbed by fluid-rock interaction in late hydrothermal events (e.g. Pronost et al., 2008; Luais et al., 2009), or high grade metamorphism (Liu et al., 2015). Since the volcanic rocks of Jingtan Formation did not experience obvious metamorphism and no significant fluid-rock interaction texture was observed, it excludes the possibility that their whole rock Nd isotope compositions may have been affected by the secondary disturbances. Alternatively, the Hf-Nd isotopic decoupling was genetically related to the source region of the felsic volcanic rocks which have experienced chemical weathering before partial melting (c.f. Wu et al., 2006b). In this scenario, the zircon preserves its primary Hf isotope composition during chemical weathering and 13

magma crystallization, whereas the whole-rock Sm-Nd system was readily equilibrated with the new melt and hence gave the lower εNd(t) values. However, this is not consistent with their A-type granite affinity. Thus, it suggests that the decoupling between Nd-Hf isotopes could be genetically related to their sources (i.e., the juvenile crust rocks). Hf-Nd isotope ratios are decoupled in many Phanerozoic arc lavas due to their different mobility along the slab-mantle wedge interface (e.g. Pearce et la., 1999; Tollstup and Gill, 2005; Hoffmann et al., 2011). Models explain Hf-Nd isotope decoupling of mafic arc rocks, including metasomatism prior to partial melting of peridotites in subduction zone (Wittig et al., 2007) or melt-peridotite interaction in the magma source (Bizimis et al., 2003). Frisby et al. (2016) proposed that serpentinization caused by seawater would lower the Sm/Nd relative to Lu/Hf ratio of the abysal peridotite, generating less radiogenic Nd isotopes for a given Hf isotopes over time. Silicate melt is expected to produce enrichments in most incompatible elements including Lu and Hf, so it is likely to be responsible for resetting of Lu-Hf and Sm-Nd isotope system (c.f. Yu et al., 2009b). Addition of oceanic sediments can also shift the composition of the Earth’s mantle towards more radiogenic Hf values through time (Garçon et al., 2013). Since the mantle beneath the Jiangnan Orogen has been modified by the flux of the slab-derived fluid and input of the recycled sediment-derived melt (e.g. Zhang et al., 2013b), the juvenile mafic rocks probably contain decoupled Hf-Nd isotopic signature. Collectively, we propose that the felsic volcanic rocks probably retain the decoupled Hf-Nd isotopic feature of juvenile crust rocks which derived from partial melting of mantle source metasomatized by fluid/melt in subduction zone. 5.3. Evolution process of the Jiangnan orogen 14

In the Jiangnan orogen, ca.850-750 Ma is a prominent period of extensive magmatism and two age peaks have been identified for these igneous rocks: ca.820 Ma and ca.780 Ma (e.g., Li et al., 2003; Wu et al., 2006b; Zheng et al., 2007; Wang et al., 2012). Previously, two different models have been proposed for the geodynamic system of the two episodes of igneous rocks, particularly granitoid rocks, including a plume rift model (Li et al., 2003) and a plate-rift model (Zheng et al., 2007). In the plume-rift model, the first phase (ca.830-795 Ma) and second phase (ca.780-745 Ma) of granitoid rocks formed during the initial and peak stage of the rifting caused by super plume, respectively. In the plate-rift model, two episodes of granitic magmatism took place during the pre-rift and syn-rift stage in response to orogenic collapse related to arc-continental collision and Rodinia supercontinent rifting, respectively. A key prerequisite is that the basement rocks (the Xikou Group and its equivalents) were folded before 880 Ma for the plume-rift model (Li, et al., 2008a) or before ca.860 Ma for plate-rift model (Zheng et al., 2008). However, the basement sequences formed in a short period of 850-810 Ma, according to recent geochronological data of the sedimentary rocks and intercalated volcanic rocks (Gao et al., 2008, 2010, 2011a, 2014; Zhang et al., 2012b, 2013b; Li et al., 2013a; 2016a; Wang et al., 2013a; Yin et al., 2013; Xu et al., 2014). Moreover, most of the volcanic rocks at various locations of the Jiangnan orogen erupted in a back-arc basin setting (Zhang et al., 2012a, 2013a, b; Li et al., 2013a, 2016a). Thus, Li et al. (2016a) proposed that the basement sequences of the entire Jiangnan orogen formed in a back-arc basin setting during ca. 850-810 Ma, implying that the collision between the Yangtze and Cathaysia blocks occurred at some time after the formation of these sequences. Zhang et al.(2012a) further suggest that the back-arc basin was rapidly closed following arc-continental collision at about ca. 800Ma along the Jiangnan orogen. Considering the final amalgamation of the Yangtze and Cathaysia 15

blocks was also marked by the angular unconformity between the basement sequences and the overlying vocano-sedimentary strata (Li et al., 2002; Wang et al., 2007), it suggests that the first phase igneous rocks intruding the basement sequences may have formed during the amalgamation of the Yangtze and Cathaysia blocks, whereas the second one intruding the overlying volcano-sedimentary strata should have formed shortly after the collision of the two blocks. It is generally accepted that the occurrence of A-type granite is commonly associated with an extensional environment, either post-orogenic or anorogenic setting (Whalen et al., 1987; Sylvester, 1989). According to the geochemical subdivision of A-type granites by Eby (1992), the felsic volcanic rocks of Jingtan Formation belong to A2-subtype rocks (Fig. 12a). These samples plot within the post-collisional granite field on the (Y+Nb) versus Rb diagram (Fig. 12b), supporting their formation shortly after the collision of the Yangtze and Cathaysia blocks. The orogenic cycle comprises a period of collision accommodated by crustal thickening. Partial melting of juvenile curst due to the orogenic collapse resulted in the formation of the Jingtan volcanic rocks. The post-collisional extensional setting is further supported by the occurrence of widespread Neoproterozoic (ca. 800-760 Ma) bimodal igneous rocks and ca. 790 Ma A-type granite in the eastern part of the Jiangnan orogen (Li et al., 2008b; Shu et al., 2012; Wang et al., 2012b). However, alkaline mafic rocks have not been found in this area, probably suggesting that the extension in the eastern part of Jiangnan orogen was not intense enough (Wang et al., 2012b). On the basis of the above discussion, the igneous rocks have recorded the evolution of the Jiangnan orogeny as follows. The southeastern margin (present coordinate) of the Yangtze Block remains a passive continental margin until the late Mesoproterozoic (Li et al., 2013b). The subduction of the Cathaysia Block initiated in early Neoproterozoic (ca. 930-890 Ma) (Ye et al., 2007; Li et al., 2009b; Zhao et al., 16

2015). Back-arc basins developed during ca. 850-810 Ma (Zhang et al., 2012a, b, 2013a; Li et al., 2013a, 2016a, b) and the final arc-continental collision happened at ca. 800 Ma (Zhang et al., 2012a). The following post-collisional collapse and post-orogenic extension occurred shortly after the final amalgamation of the two blocks at ca. 800-760 Ma (Wang et al., 2012b and this study).

6. Conclusions New SIMS U-Pb zircon dating indicates that the volcanic rocks of Jingtan Formation in the eastern Jiangnan orogen formed at ca. 784-788 Ma. Geochemical and Nd-Hf isotopic data suggest that the felsic volcanic rocks derived from partial melting of the Neoproterozoic to late Mesoproterozoic juvenile crustal material. The Jingtan volcanic rocks belong to A2-type granitic rocks that formed in a post-collisional extension setting. They were broadly synchronous with widespread bimodal magamatism (ca. 790-760 Ma) in the eastern part of the Jiangnan orogen, suggesting the tectonic collapse of the collision-thickened orogen happened soon after the amalgamation between Yangtze and Cathaysia blocks.

Acknowledgements The authors gratefully acknowledge C.L. Zhang and the anonymous reviewer for helpful comments and constructive suggestions. This work was financially supported by China NSFC grant (41573023, 41472166) and Natural Sciences and Engineering Research Council of Canada. The Research Funding for Young Huangshan Scholar of HFUT to Longming Li is gratefully acknowledged. J. Wong is thanked for solution preparation. We also appreciate Y. Liu for major element, X.L. Tu for trace element, X.R. Liang for Sr-Nd isotopic analyses and L. Zhang for Hf isotopic analyses. 17

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32

Figure captions Fig.1. (a) Simplified geological map of China showing the location of the Jiangnan Orogen. (b) Geological map of south Anhui area in the eastern part of the Jiangnan Orogen, South China, showing the Xikou Group and Jingtan Formation.

Fig.2. Field (a) and microscope (b) photographs for the rhyolitic tuff of the Jingtan Formation in the eastern part of the Jiangnan Orogen.

Fig.3. CL images of analyzed zircons from (a) dacite (12WN-12-8) and (b) rhyolitic tuff (12WN-16-1) of the Jingtan Formation.

Fig.4. Zircon SIMS U–Pb Concordia diagram for the dated dacite (12WN-12-8) and rhyolitic tuff (12WN-16-1) from the Jingtan Formation in the eastern part of Jiangnan orogen.

Fig.5. (a) Zr/TiO2 vs. SiO2 (wt.%) diagram (Winchester and Floyd, 1977), and (b) ANK (Al2O3+ Na2O + K2O) vs. A/CNK (Al2O3/(CaO + Na2O + K2O)) (molecular) diagram (Maniar and Piccoli, 1989) for the Jingtan volcanic rocks.

Fig.6. Relationship diagram between major elements and SiO2 contents for felsic volcanic rocks of the Jingtan Formation in the eastern part of the Jiangnan orogen.

Fig.7. (a) Chondrite normalized REE distributions (b) Primitive mantle normalized spidergram of the Jingtan felsic volcanic rocks. Chondrite and primitive mantlenormalized values are from Sun and McDonough (1989). 33

Fig.8. (a) Plots of the Jingtan felsic volcanic rocks in (a) (Zr + Ce + Nb + Y) vs.10,000 × Ga/Al (modified after Whalen et al., 1987) showing an A-type granite affinity; (b) (Na + K + 2Ca)/(Al*Si) (cation ratio) vs. Zr diagram (Watson and Harrison, 1983).

Fig.9. Zircon εHf(t) versus whole-rock εNd(t) for the Neoproterozoic igneous rocks in the eastern part of the Jiangnan orogen. εHf(t) and εNd(t) are calculated based on the U–Pb ages for different plutons. Two equations of Nd–Hf isotopes and the terrestrial array are from Vervoort and Blichert-Toft (1999).

Fig.10. Initial

176

Hf/177Hf ratio versus U-Pb age diagram for magmatic zircons from

the felsic volcanic rocks of the Jingtan Formation.

Fig.11. Plots of the Jingtan felsic volcanic rocks in (a) Nb–Y–Ga and Nb–Y–Ce diagrams (Eby, 1992) and (b) Rb vs (Y+Nb) diagram after Pearce et al.(1984) showing their

tectonic

setting.

Solid

circles represent rhyolitic

tuffs of

Shuangqiaoshan Group from Li et al. (2016a); Solid squares represent rhyolites of Tieshajie Group from Li et al. (2013b). Abbreviations: VAG, volcanic arc granites; ORG, ocean ridge granites; WPG, within plate granites; syn-COLG and post-COLG, syn- and post-collision granites.

34

Table 1. Stratigraphy of the Neoproterozoic strata in the eastern Jiangnan orogen modified from Zhao and Cawood (2012).

Table 2. SIMS U-Pb ages for dacite (12WN-12-8) and rhyolite (12WN-16-1) from the Jingtan Formation in the eastern part of the Jiangnan orogen, South China. Grain

U

Th/U

207

12WN-12-8 dacite 1 575 0.52 2 174 0.67 285 0.38 3 4 193 0.75 50 0.78 5 6 234 0.87 7 348 0.13 8 127 0.78 406 0.98 9 10 268 0.26 358 0.88 11 12 271 0.98 13 350 0.44 14 150 0.38 220 0.91 15 16 346 0.45 248 0.42 17 18 179 0.39 19 249 0.33 20 248 0.39 12WN-16-1 rhyolitic tuff 165 0.66 1

Pb/235U

Ratios (common-Pb corrected) 206 1σ Pb/238U 1σ

Err*

207

Ages (common- Pb corrected, Ma) Pb/206Pb ±1σ 207Pb/235U ±1σ 206Pb/238U ±1σ %Disc

1.17478 1.18684 1.25375 1.16797 1.22293 1.26633 1.16736 1.16065 1.16170 1.18390 1.23392 1.20557 1.24522 1.18071 1.27475 1.17005 1.18065 1.18434 1.16834 1.18531

1.5606 1.8998 1.9155 1.6262 2.9659 3.3570 1.7321 1.8959 1.6622 1.5766 2.1892 1.7692 1.6831 1.9166 1.6414 1.8241 3.3489 1.8739 2.1355 1.7716

0.12887 0.12748 0.13666 0.12817 0.12967 0.13677 0.12891 0.12773 0.12926 0.12913 0.13258 0.13255 0.13495 0.12906 0.13868 0.12887 0.13027 0.12983 0.12898 0.12978

1.5004 1.6987 1.7679 1.5035 1.6317 2.2758 1.5286 1.7484 1.6070 1.5019 2.1238 1.5055 1.6159 1.7800 1.5578 1.7517 1.5444 1.5617 1.9822 1.6598

0.9614 0.8941 0.9229 0.9245 0.5502 0.6779 0.8825 0.9222 0.9668 0.9526 0.9701 0.8509 0.9601 0.9287 0.9491 0.9603 0.4612 0.8334 0.9282 0.9369

810 854 823 809 881 842 796 803 780 822 853 805 835 818 827 802 798 811 797 814

9 18 15 13 50 51 17 15 9 10 11 19 10 15 11 11 61 22 17 13

789 794 825 786 811 831 785 782 783 793 816 803 821 792 835 787 792 793 786 794

9 11 11 9 17 19 10 10 9 9 12 10 10 11 9 10 19 10 12 10

781 773 826 777 786 826 782 775 784 783 803 802 816 782 837 781 789 787 782 787

11 12 14 11 12 18 11 13 12 11 16 11 12 13 12 13 11 12 15 12

-3.7 -10.0 0.3 -4.2 -11.4 -2.0 -1.9 -3.7 0.5 -5.1 -6.3 -0.4 -2.5 -4.6 1.3 -2.7 -1.1 -3.2 -2.0 -3.6

1.13127

1.6950

0.12863

1.5351

0.9056

735

15

768

9

780

11

7

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

124 277 228 199 227 261 243 148 166 319 147 350 274 220 329

0.59 0.60 0.58 0.99 0.63 0.68 0.69 0.65 0.65 0.63 0.70 0.71 0.73 0.59 0.68

1.13359 1.11160 1.18370 1.11309 1.15205 1.20027 1.14667 1.18544 1.14955 1.16014 1.18045 0.57756 1.16386 1.13855 1.19255

2.6457 2.1648 2.2162 6.0530 1.6858 1.6838 3.1498 1.9381 1.7209 1.7617 2.0762 25.4897 1.6416 1.6656 1.5600

0.12617 0.12923 0.12982 0.12681 0.13092 0.13257 0.13142 0.13245 0.13072 0.12921 0.13050 0.10495 0.13029 0.12847 0.13186

2.4544 1.5002 1.6178 1.7286 1.5584 1.5022 1.5012 1.6580 1.5206 1.6494 1.7100 1.6646 1.5427 1.5080 1.5038

0.9277 0.6930 0.7300 0.2856 0.9244 0.8921 0.4766 0.8555 0.8836 0.9362 0.8236 0.0653 0.9397 0.9054 0.9640

780 687 810 730 736 796 718 772 734 778 794 -356 767 751 793

21 33 31 118 14 16 58 21 17 13 25 555 12 15 9

769 759 793 760 778 801 776 794 777 782 792 463 784 772 797

14 12 12 33 9 9 17 11 9 10 11 99 9 9 9

766 783 787 770 793 803 796 802 792 783 791 643 790 779 798

18 11 12 13 12 11 11 13 11 12 13 10 11 11 11

-2 15 -3 6 8 1 12 4 8 1 0 295 3 4 1

Table 3. Major and trace element compositions of volcanic rocks of the Jingtan Formation, eastern part of the Jiangnan orogen, South China. Sample

SiO2 TiO2

12WN

12WN

12WN

12WN

12WN

12WN

12WN

12WN

12WN

12WN

12WN

12WN

12WN

2WN

12WN

12WN

12WN

12WN

12-1

12-2

12-3

12-4

12-5

12-6

12-7

12-8

13-1

13-2

13-3

14-1

15-1

16-1

17-1

18-1

18-3

19-1

67.7 0.58

66.1 0.56

63.9 1.03

70.5 0.52

74.5 0.25

70.2 0.47

68.3 0.54

68.0 0.58

65.0 0.80

65.8 0.79

63.7 0.77

73.0 0.18

78.0 0.18

76.4 0.18

72.6 0.31

74.8 0.22

68.3 0.49

66.9 0.73

Al2O3 Fe2O3T MnO MgO CaO Na2O K2O P2 O5 LOI Total V Cr Co Ni Ga Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd

14.5 4.72 0.08 1.14 2.23 3.26 3.53 0.14 1.51 99.44 51.0 16.1 7.06 9.44 17.4 112 153 42.9 250 10.1 4.95 635 36.5 77.8 9.45 36.0 7.78 1.47 4.47

14.2 7.39 0.11 0.86 1.47 4.12 2.70 0.20 1.70 99.45 12.8 6.91 7.39 4 16.0 88.5 61.1 59.5 303 12.3 2.97 616 39.6 86.4 10.9 41.7 9.63 2.58 8.97

14.6 7.19 0.10 2.15 2.22 4.48 1.85 0.19 1.76 99.46 121 40.4 16.4 22.6 17.6 44.8 106 41.3 242 10.2 2.10 420 29.2 62.4 7.93 30.7 7.09 1.51 7.98

13.1 4.67 0.08 0.58 1.47 3.93 3.24 0.18 1.24 99.57 22.6 4.80 4.33 3.38 19.8 116 91.6 63.6 416 13.5 7.24 721 51.4 109.4 14.2 55.3 12.2 2.02 10.2

11.7 2.71 0.07 0.35 1.34 3.37 3.33 0.11 1.78 99.48 7.63 4.02 2.03 2.58 14.1 110 54.2 57.9 234 12.3 8.24 854 48.7 106 13.6 52.2 11.8 1.59 12.0

14.0 3.59 0.04 0.82 1.41 2.92 3.47 0.16 2.44 99.49 38.6 18.2 6.50 11.2 18.5 160 107 25.5 220 10.3 11.7 677 43.0 93.7 11.6 42.4 9.15 1.63 8.44

14.8 3.98 0.05 0.94 1.96 2.57 3.93 0.16 2.14 99.47 48.2 24.5 7.26 13.9 20.8 179 142 30.7 254 11.3 13.3 827 48.4 104 12.9 48.2 10.0 1.79 9.26

15.1 4.16 0.06 0.91 1.88 3.18 3.41 0.16 2.10 99.49 47.4 20.8 7.42 11.1 20.2 162 146 36.5 281 11.8 12.0 745 47.7 105 13.1 49.3 10.5 1.87 9.92

14.5 7.76 0.11 0.84 3.35 3.15 2.03 0.23 1.70 99.46 9.48 0.41 1.14 0.50 20.0 122 49.3 39 157 7.08 3.46 213 40.0 92.8 11.5 41.7 8.23 0.40 7.33

14.1 6.95 0.12 0.67 2.93 2.99 2.92 0.23 1.89 99.46 34.4 10.2 10.1 5.99 20.2 148 91.6 48.1 283 11.0 14.7 526 39.4 86.2 10.5 40.3 8.70 1.78 8.97

14.0 8.01 0.13 0.67 3.18 4.08 2.18 0.24 2.48 99.49 30.2 8.62 10.0 5.39 18.9 91.3 116 55.0 278 11 125 431 37.5 82.9 10.4 40.9 9.24 2.14 10.2

13.3 1.57 0.04 0.55 1.42 4.28 3.18 0.16 1.85 99.47 17.9 6.16 2.27 3.9 13.6 126 106 23.1 104 9.18 8.38 533 16.9 37.5 4.86 18.6 4.91 0.68 5.34

11.5 2.17 0.01 0.05 0.30 1.96 3.88 0.02 1.34 99.44 4.20 1.88 0.43 1.39 18 165 33.3 57.8 281 14.4 8.97 763 45.5 101 12.7 49.0 11.0 1.61 11.1

13.0 1.90 0.01 0.11 0.19 0.67 5.56 0.02 1.47 99.54 7.29 7.45 0.42 3.77 18.5 221 45.7 58.6 287 14.5 12.3 1389 53.1 115 14.5 55.8 12.7 1.86 13.5

12.9 3.36 0.10 0.45 1.52 3.27 3.41 0.19 1.28 99.43 13.9 20.8 2.83 8.78 17.0 147 89.5 60.9 311 13.6 8.44 648 60.0 135 16.7 64.5 13.4 1.99 13.3

13.0 2.06 0.02 0.56 0.39 4.88 2.55 0.17 0.82 99.43 9.38 8.27 1.41 3.27 16.6 64.9 87.4 43.2 220 13.8 4.42 811 51.8 120 14.6 54.5 11.9 1.59 11.3

12.8 6.96 0.11 0.47 2.16 2.86 2.69 0.17 2.50 99.49 10.9 535 8.66 242 15.7 134 82.1 53.2 298 11.5 48 380 42.4 93.0 11.5 44.8 10.0 2.02 10.5

14.5 5.72 0.11 1.82 1.34 3.85 1.56 0.15 2.79 99.51 77.0 67.9 14.0 30.9 16.1 58.9 196 26.7 189 8.08 10.4 322 28.8 59.9 7.31 27.5 5.69 1.47 5.35

Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U FeOt /MgO (La/Yb)N &Eu 104*Ga/Al

0.69 4.10 0.81 2.11 0.29 1.78 0.27 2.71 1.27 2.21 2.25 0.5 4.13 5.72 0.59 2.26

1.48 9.81 2.02 5.62 0.80 5.11 0.80 5.78 0.75 4.75 3.76 1.81 8.62 4.28 0.63 2.13

1.28 8.35 1.74 4.95 0.73 4.57 0.72 7.41 0.90 22.0 13.61 2.90 3.35 4.56 0.64 2.27

1.70 11.3 2.42 6.86 1.02 6.64 1.02 8.67 1.09 10.17 14.46 3.05 8.08 5.30 0.49 2.84

1.92 12.5 2.54 7.04 1.04 6.60 1.02 8.01 1.15 10.6 16.5 3.15 7.76 5.29 0.41 2.27

1.20 6.24 1.04 2.48 0.34 2.08 0.30 7.17 0.98 28.8 15.6 2.66 4.36 14.8 0.57 2.49

1.32 7.26 1.27 3.12 0.43 2.62 0.39 7.65 1.01 24.3 16.9 2.66 4.25 13.2 0.57 2.64

1.45 8.36 1.47 3.84 0.52 3.21 0.47 8.65 1.07 24.8 18.0 2.88 4.58 10.6 0.56 2.53

1.25 8.02 1.67 4.72 0.75 4.89 0.77 6.15 0.67 13.9 8.9 2.17 9.29 5.87 0.16 2.60

1.47 9.76 2.06 5.88 0.89 5.76 0.91 8.56 1.03 16.9 12.6 2.64 10.4 4.91 0.62 2.70

1.67 11.2 2.35 6.69 0.98 6.34 1.00 8.33 1.03 22.2 12.3 2.54 12 4.24 0.68 2.54

0.91 5.47 0.98 2.46 0.36 2.22 0.33 4.36 1.36 17.0 7.92 3.28 2.88 5.48 0.41 1.94

1.99 13.1 2.71 7.88 1.21 7.92 1.23 10.3 1.43 31.3 15.6 2.90 41.8 4.12 0.45 2.96

2.25 14.6 2.96 8.23 1.24 7.96 1.21 10.8 1.46 27.4 15.4 3.25 17.4 4.78 0.44 2.68

2.17 13.7 2.84 7.84 1.12 6.80 1.04 10.7 1.35 21.2 19.5 3.38 7.53 6.33 0.46 2.49

1.82 10.8 2.05 5.52 0.77 4.77 0.72 8.55 1.47 8.18 18.1 2.80 3.64 7.79 0.42 2.41

1.71 11.2 2.38 6.58 0.99 6.42 1.00 8.92 1.13 32.6 13.1 2.64 14.9 4.74 0.61 2.32

0.87 5.59 1.15 3.21 0.48 3.14 0.49 5.63 0.71 13.7 7.01 1.65 3.15 6.56 0.82 2.09

Table 4. Whole rock Nd isotope compositions of volcanic rocks of the Jingtan Formation, eastern part of the Jiangnan orogen, South China. Sample Sm(ppm) Nd (ppm) Dacites 12WN-12-3 7.09 30.7 12WN-12-6 9.15 42.4 12WN-12-8 10.5 49.3 12WN-13-2 8.7 40.3 Rhyolites 12WN-14-1 4.91 18.6 12WN-16-1 12.7 55.8

147

Sm/144Nd

143

Nd/144Nd

2σ

εNd(t) TDM1(Ma)

0.140560 0.131344 0.129627 0.131392

0.512298 0.512216 0.512192 0.512208

0.000008 0.000008 0.000006 0.000008

-1.0 -1.7 -2.0 -1.9

1776 1729 1737 1744

0.160665 0.138523

0.512241 0.512289

0.000016 0.000008

-4.2 -1.0

2608 1746

Table 5. Zircon Lu-Hf isotopic data for felsic volcanic rocks of the Jingtan Formation in eastern part of the Jiangnan Orogen, South China. Analysis

Age(Ma)

12WN-12-8 Dacite 1 784 2 784 3 784 4 784 5 784 6 784 7 784 8 784 9 784 10 784 11 784 12 784 13 784

176

Hf/177Hf

0.282450 0.282603 0.282590 0.282546 0.282533 0.282428 0.282176 0.282469 0.282642 0.282425 0.282339 0.282444 0.282533

2σ 0.000016 0.000013 0.000015 0.000012 0.000014 0.000013 0.000013 0.000012 0.000015 0.000019 0.000012 0.000012 0.000013

176

Yb/177Hf

0.037544 0.060412 0.110284 0.128162 0.103023 0.123125 0.093788 0.069285 0.129021 0.055735 0.073528 0.061778 0.137572

2σ 0.002396 0.000191 0.004173 0.000750 0.003269 0.000825 0.000471 0.000335 0.004078 0.002626 0.000424 0.001020 0.001679

176

Lu/177Hf

0.000657 0.001161 0.001920 0.002406 0.001855 0.002383 0.001716 0.001207 0.002184 0.000978 0.001377 0.001145 0.002650

2σ 0.000047 0.000004 0.000071 0.000015 0.000059 0.000014 0.000010 0.000007 0.000061 0.000043 0.000008 0.000023 0.000015

176

Hf/177Hf initial 0.282440 0.282586 0.282562 0.282511 0.282506 0.282393 0.282151 0.282451 0.282610 0.282411 0.282319 0.282427 0.282494

εHf(t)

TDM1 (Ga)

5.57 10.72 9.88 8.07 7.89 3.89 -4.69 5.96 11.58 4.53 1.26 5.09 7.47

1.12 0.92 0.96 1.04 1.04 1.21 1.55 1.11 0.89 1.17 1.30 1.15 1.06

14 784 15 784 16 784 17 784 18 784 19 784 20 784 21 784 22 784 12WN-16-1 Rhyolitic tuff 1 788 2 788 3 788 4 788 5 788 6 788 7 788 8 788 9 788 10 788 11 788 12 788 13 788 14 788 15 788 16 788 18 788 19 788 20 788 21 788 22 788

0.282463 0.282458 0.282472 0.282526 0.282550 0.282496 0.282412 0.282474 0.282483

0.000018 0.000013 0.000012 0.000012 0.000012 0.000015 0.000025 0.000012 0.000012

0.282588 0.282514 0.282581 0.282549 0.282512 0.282524 0.282563 0.282565 0.282503 0.282412 0.282386 0.282302 0.282610 0.282455 0.282560 0.282589 0.282454 0.282607 0.282580 0.282516 0.282477

0.000014 0.000027 0.000016 0.000016 0.000015 0.000015 0.000015 0.000019 0.000013 0.000017 0.000018 0.000027 0.000020 0.000018 0.000016 0.000010 0.000024 0.000016 0.000015 0.000014 0.000020

0.077687 0.059382 0.072028 0.064546 0.126782 0.088115 0.148018 0.069763 0.084705 0.098761 0.182396 0.120047 0.093597 0.082402 0.079983 0.089632 0.095440 0.069155 0.072885 0.070844 0.124012 0.155540 0.089805 0.101344 0.085992 0.121801 0.115222 0.084996 0.090681 0.103765

0.002794 0.001074 0.001143 0.000790 0.003436 0.002963 0.001122 0.001365 0.000210 0.001939 0.000803 0.000349 0.001240 0.001071 0.000900 0.000608 0.000788 0.000442 0.001536 0.000591 0.003541 0.002279 0.000435 0.001756 0.001008 0.002143 0.000797 0.000595 0.000399 0.000679

0.001371 0.001046 0.001387 0.001252 0.002371 0.001633 0.003046 0.001370 0.001587 0.001921 0.004076 0.002578 0.001865 0.001611 0.001596 0.001760 0.002027 0.001407 0.001491 0.001619 0.002980 0.003420 0.001922 0.001999 0.001761 0.002815 0.002425 0.001640 0.001886 0.002094

0.000046 0.000016 0.000020 0.000011 0.000067 0.000053 0.000030 0.000030 0.000012 0.000035 0.000056 0.000029 0.000029 0.000018 0.000012 0.000021 0.000017 0.000019 0.000052 0.000026 0.000121 0.000055 0.000016 0.000036 0.000028 0.000089 0.000016 0.000016 0.000017 0.000030

0.282443 0.282443 0.282451 0.282508 0.282516 0.282472 0.282368 0.282454 0.282460

5.65 5.66 5.96 7.97 8.23 6.71 2.99 6.05 6.27

1.13 1.12 1.12 1.03 1.03 1.09 1.26 1.11 1.11

0.282560 0.282455 0.282544 0.282522 0.282488 0.282501 0.282537 0.282536 0.282482 0.282390 0.282362 0.282259 0.282559 0.282427 0.282531 0.282563 0.282412 0.282572 0.282556 0.282488 0.282446

9.90 6.14 9.31 8.55 7.35 7.79 9.08 9.03 7.14 3.89 2.89 -0.79 9.86 5.17 8.86 9.99 4.66 10.3 9.76 7.36 5.85

0.96 1.14 0.99 1.02 1.07 1.05 1.00 1.00 1.07 1.20 1.24 1.42 0.97 1.16 1.01 0.96 1.19 0.95 0.97 1.07 1.13

Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Fig. 6

Fig. 7

Fig. 8

Fig. 9

Fig. 10

Fig. 11

Highlights 1. Volcanic rocks of Jingtan Formation in the Jiangnan orogen were formed at ca.780Ma 2. The decoupled Nd-Hf isotopes of the rock maybe genetically related to their source 3. The volcanic rocks were derived from partial melting of juvenile crustal materials 4. They formed during post-collision stage after the formation of the Jiangnan orogen