1. Introduction
The genesis of mafic rocks generated within orogenic belts remains the focus of considerable debate (Foden et al. Reference Foden, Song, Turner, Elburg, Smith, Van Der Steldt and Van Penglis2002; Vilà et al. Reference Vilà, Pin, Enrique and Liesa2005). Post-orogenic mafic rocks are generally thought to be formed in extensional environments, such as post-orogenic extension (e.g. Teixeira et al. Reference Teixeira, Pinese, Iacumin, Girardi, Piccirillo, Echeveste, Ribot, Fernandez, Renne and Heaman2002; Zhou, M. F. et al. Reference Zhou, Lesher, Yang, Lia and Sun2004), continental rifts (e.g. Coish & Sinton, Reference Coish and Sinton1992; Pedersen et al. Reference Pedersen, Craig, Upton, Rämö, Jepsen and Kalsbeek2002), etc. Distinguishing mafic magmatism formed in post-orogenic extension from that formed in other geodynamic settings is crucial for studying the evolution of orogenic belts.
The Proterozoic Jiangnan orogen is located between the Yangtze and Cathysia blocks (Fig. 1a). Igneous rocks in the orogen are dominated by Neoproterozoic S-type granitoids (Zhou, Wang & Qiu, Reference Zhou, Wang and Qiu2005; Wang et al. Reference Wang, Zhou, Qiu, Zhang, Liu and Zhang2006). Li et al. (Reference Li, Li, Kinny, Wang, Zhang and Zhou2003) proposed that Neoproterozoic felsic and mafic magmatism in the Jiangnan orogen was related to the c. 840–740 Ma superplume activities that led to the break-up of the Rodinia supercontinent. However, compelling geological and petrological evidence for the hypothesis is lacking (Jiang, Sohl & Blick, Reference Jiang, Sohl and Blick2003; Wang et al. Reference Wang, Zhou, Qiu and Gao2004a; Zhou, J. C. et al. Reference Zhou, Wang, Qiu and Gao2004). On the contrary, several lines of evidence seem to support the existence of a normal orogenic cycle for the Jiangnan orogen: (1) the c. 1000–950 Ma ophiolite suites (Chen et al. Reference Chen, Foland, Xing, Xu and Zhou1991; Zhou & Zhu, Reference Zhou and Zhu1993; Li et al. Reference Li, Zhou, Zhao, Fanning and Compston1994) and c. 910–875 Ma arc volcanic rocks (Cheng, Reference Cheng1993; Wang, Reference Wang2000) in the eastern part of the Jiangnan orogen might record the early subduction of oceanic crust; (2) high-pressure blueschists with a K–Ar age of 866±14 Ma probably indicate the collisional peak (Shu et al. Reference Shu, Zhou, Shi and Yin1994); and (3) it is proposed that the c. 835–800 Ma strongly peraluminous granitoids and mafic–ultramafic rocks in the western part of this orogenic belt would represent the post-collisional magmatism (Zhou, Wang & Qiu, Reference Zhou, Wang and Qiu2005; Wang et al. Reference Wang, Zhou, Qiu and Gao2004b, Reference Wang, Zhou, Qiu, Zhang, Liu and Zhang2006). Thus, the 1000–800 Ma igneous rocks in the Jiangnan orogen may have resulted from the collision and the orogenesis between the Yangtze and Cathysia blocks. However, the geodynamic setting during c. 800–740 Ma for the Jiangnan orogen is not clear. Recently, OIB (ocean island basalt)-like mafic rocks with ages of c. 760–750 Ma have been reported in the western Jiangnan orogen (Wang et al. Reference Wang, Zhou, Qiu and Gao2004b; Zhou, Wang & Qiu, Reference Zhou, Wang and Qiu2005). However, was the occurrence of these mafic rocks related to the alleged superplume activities? Also, it is still controversial whether South China was part of the Rodinia supercontinent during the period 800–740 Ma. In this work, we present new geochronological, element geochemical and Nd–Hf isotopic data for the mafic rocks in western Hunan. Their ages and petrogenesis will shed some light on the understanding of tectonic evolution of South China and some Neoproterozoic geological events.
Geological sketch maps of western Hunan, South China (modified after HRGST, 1965; Zheng et al. Reference Zheng, Jia, Liu and Cao2001; Ma et al. Reference Ma, Qiao, Min, Fan and Ding2002 and Wang et al. Reference Wang, Zhou, Qiu and Gao2004b). (a) South China; (b) Hunan Provinces, with the dashed rectangle indicating the western Hunan area; (c) Qianyang; (d) Guzhang.
2. Geological setting
The ENE-striking Jiangnan orogen is about 1500 km long and is situated on the southeastern margin of the Yangtze Block (Fig. 1a). The western part of the orogenic belt encompasses parts of northern Guangxi, western and northern Hunan, as well as northeastern Guizhou provinces (Fig. 1b). Two pre-Sinian sedimentary sequences, the Lengjiaxi and Banxi groups, occur in the area (Fig. 1b). They underwent low greenschist-facies metamorphism and are separated by an unconformity that is believed to mark the Jinningian orogenesis in the area (BGMRHN, 1988). Both sequences have been described in detail by Wang et al. (Reference Wang, Zhou, Qiu and Gao2004b). The Neoproterozoic Banxi Group constitutes the majority of the Precambrian sedimentary sequences in western Hunan. It is weakly deformed and is considered to be younger than c. 820 Ma (Wang & Li, Reference Wang and Li2003). The Banxi Group is subdivided into two formations. The lower Madiyi Formation comprises purple sandy conglomerates, arkose, pelitic dolomite and turbidite formed in a continental–neritic environment, whereas the upper Wuqiangxi Formation contains arkose, carbonaceous slate and marl deposited in a littoral–neritic environment. The two formations exhibit characteristics of an extensional setting (Wang & Li, Reference Wang and Li2003). The Banxi Group is conformably overlain by the Lower Sinian System, which represents the deposits in a rift basin (Liu, Hao & Li, Reference Liu, Hao and Li1999). The Lower Sinian System, with age ranging from c. 750 Ma to 680 Ma, comprises the Chang'an, Xiangmeng and Nantuo formations from bottom to top in the area (Wang et al. Reference Wang, Zhou, Qiu and Gao2004b; Wang & Li, Reference Wang and Li2003). In particular, it should be noted that the Chang'an and Nantuo formations in South China are considered to represent the Neoproterozoic Sturtian and Marinoan glacial deposits, respectively.
Neoproterozoic mafic magmatic rocks in western Hunan are mostly distributed in the Guzhang, Qianyang and Tongdao areas, constituting an approximately 300 km long NNE-trending rock belt (Fig. 1b), but their total outcrop area is less than 10 km2 (BGMRHN, 1988). The NNE-trending belt may be extended southward to the Longsheng area of northern Guangxi (Fig. 1b). The mafic–ultramafic rocks in western Hunan occur predominantly as dykes, veins and sills. The intrusions are generally NNE-striking with individual widths of 10–200 m (BGMRHN, 1988). Most intrude the Wuqiangxi Formation and show conformable bedding planes with the country rocks (Fig. 1c, d; BGMRHN, 1988). Some diabases intersect the bedding of the country rocks. The contact zone between the mafic intrusions and the country rocks shows weak metamorphism, with contact metamorphic aureoles. There are a few basalts in the Wuqiangxi Formation near Qianyang city (Zheng et al. Reference Zheng, Jia, Liu and Cao2001). Mafic rocks in the Guzhang area are conformably overlain by the sandstones of the Lower Sinian System (Chang'an Formation; see Fig. 1d), suggesting their ages should be older than the lower age limit of the Sinian strata. According to their relationships with the overlying sedimentary sequences and their petrological features, the mafic–ultramafic rocks in western Hunan have been regarded as the products of a single episode of magmatism (BGMRHN, 1988; Zheng et al. Reference Zheng, Jia, Liu and Cao2001). However, their ages remain unknown. Zheng et al. (Reference Zheng, Jia, Liu and Cao2001) obtained Sm–Nd whole-rock isochron ages of 868±30 Ma and 855±6 Ma for the basalts and pyroxenite-diabases, respectively, but both ages are questionable for their limited variation in Sm/Nd ratios. Recently, the ages of these mafic rocks from western Hunan have been estimated at 760–750 Ma based on new petrological and stratigraphic evidence in the adjacent areas (Wang et al. Reference Wang, Zhou, Qiu and Gao2004b). Therefore, more precise geochronological studies are needed.
3. Petrography and mineralogy
Sampling was designed to cover the representative rock exposures in the southern (Tongdao area), middle (Qianyang area) and northern (Guzhang area) parts of western Hunan. Exposed rock types in the study area include altered pyroxenites, gabbros, gabbro-diabases, diabases and basalts. Gabbros and diabases are the dominant rock types (>90%) (BGMRHN, 1988). The mafic–ultramafic rocks from western Hunan generally experienced varying alteration and were replaced by tremolite, chlorite, clinozoisite, serpentine and calcite due to the low greenschist-facies metamorphism. Some relatively fresh clinopyroxene and plagioclase phenocrysts are observed in some mafic samples, but contents of the preserved clinopyroxenes and plagioclases vary in different samples. The accessory minerals within the mafic rocks are ilmenite and titanite.
Clinopyroxenes are subhedral to euhedral homogeneous augites with TiO2 contents ranging from 0.40 wt% to 1.96 wt% (Table 1). The augites from Qianyang have higher TiO2 and normative Wo than those from Guzhang, suggesting that the host basalts of the former have relatively high alkalis (Qiu & Zeng, Reference Qiu and Zeng1987). Plagioclases are subhedral to euhedral, with normative Ab as high as 92.33 to 99.21. Albitization of the plagioclases is probably due to the regional low greenschist-facies metamorphism in the area.
Representative mineral compositions of clinopyroxenes (Py) and plagioclases (Pl) in the mafic rocks from western Hunan
Mg no. = 100*Mg2+/(Mg2++Fe2+).
4. Ion microprobe U–Pb zircon dating
4.a. Analytical techniques
Zircons for U–Pb analysis were separated from two samples (QY-2 and 05TD-3–3) by conventional magnetic and density techniques to concentrate non-magnetic, heavy fractions. About 3.0 kg from each sample was crushed. The zircons, together with several grains of TEMORA, were mounted in epoxy and polished down to half-section. Transmitted and reflected light micrographs and cathodoluminescence (CL) images were used to guide the U–Th–Pb isotope analyses, and the mount was vacuum-coated with a layer of high-purity gold. The CL study was carried out on a FEI-XL30SFEG electron microscope at the Department of Electronics, Beijing University. The zircon grains generally are 100–150 μm in length and 30–50 μm in width. CL images show typical rhythm zoning, indicating that the zircons are of magmatic origin. Zircon U–Pb isotopes were analysed by the Sensitive High-Resolution Ion MicroProbe (SHRIMP-II), the Beijing SHRIMP Centre, Chinese Academy of Geological Sciences. Detailed analysing processes are similar to Compston et al. (Reference Compston, Williams, Kirschvink, Zhang and Ma1992). A 3.2 nA primary beam, with about 30 μm diameter, was used for ion production. Five scans through the mass stations were made for each age determination. Both Pb/U and Pb/Th ratios and absolute Pb, Th and U abundances of the standard Sri Lanka zircon SL13 (206Pb/238U=0.0928 corresponding to 572 Ma, 238 ppm 238U: Williams, Buick & Cartwright, Reference Williams, Buick and Cartwright1996) and TEM with an age of 417 Ma (Black et al. Reference Black, Kamo, Williams, Mundil, Davis, Korsch and Foudoulis2003) have been used to monitor the analyses of the zircon under study. 204Pb was applied for the common lead correction and data processing was carried out using the SQUID 1.0 (Ludwig, Reference Ludwig1999) and PRAWN (Williams, Buick & Cartwright, Reference Williams, Buick and Cartwright1996) programs. Uncertainties on individual analyses are quoted at the 1σ level, whereas those on pooled alignment analyses are quoted at the 95% confidence level. Isotopic data are listed in Table 2.
Ion microprobe U–Th–Pb data for zircons from mafic rocks of Qianyang and Guzhang, western Hunan
PbC and Pb* indicate the common and radiogenic lead proportions.
4.b. Results
QY-2 is a diabase sampled from the Qianyang area. Fourteen spot analyses were performed on 14 zircon grains of the sample. The measured U and Th concentrations of these zircon grains range from 623 ppm to 1542 ppm and 2101 ppm to 2941 ppm, respectively. Th/U ratios are higher, varying between 2.55 and 6.34. Most of the analyses are concordant and yield a weighted mean 206Pb–238U age of 747±18 Ma (Fig. 2a; 2σ, 95% conf., n=14, MSWD=0.14). This age agrees with the weighted mean 207Pb–206Pb age of 776±41 Ma of the sample (2σ, 95% conf., n=12, MSWD=1.7) within analytical precision. For Neoproterozoic and younger rocks, the analysed zircon 206Pb–238U ages generally have higher precision than the 207Pb–206Pb ages (Black et al. Reference Black, Kamo, Williams, Mundil, Davis, Korsch and Foudoulis2003). The 206Pb–238U age of 747±18 Ma is thus interpreted as the crystallization age of the Qianyang mafic rocks.
SHRIMP U–Pb zircon concordia plots and recalculated weighted mean 206Pb/238U ages for the Neoproterozoic mafic rocks from western Hunan. (a) QY-2, from Qianyang; (b) 05TD-3–3, from Tongdao.
Sample 05TD-3–3 is from the Tongdao area, in the southern end of western Hunan. Sixteen analyses were obtained from 16 zircon grains from the sample. U and Th concentrations (179–2483 ppm and 388–4683 ppm, respectively) of the zircons have relatively wider ranges than those of QY-2. Th/U ratios are relatively high, ranging from 1.95 to 4.58. Common Pb is low, with a proportion generally less than 0.40%, except for one analysis at 0.64%. If all 16 analyses are treated as a single group they yield a weighted mean 206Pb–238U age of 756±12 Ma (2σ, 95% conf., n=16, MSWD=1.6). However, as shown in Figure 2b, some analyses are slightly discordant, which may result in a relatively young mean 206Pb–238U age. The 207Pb/206Pb ratios seem to be stable, giving a weighted average 207Pb–206Pb age of 772±11 Ma (2σ, 95% conf., n=16, MSWD=0.9). Considering the mean 207Pb–206Pb age has a relatively low MSWD value, we think 772±11 Ma could represent the best estimate of the crystallization age of the Tongdao ultramafic rocks in these circumstances. This age is consistent with that of the Qianyang mafic rocks within analytical error.
5. Geochemistry
5.a. Analytical techniques
All samples were prepared by crushing in an agate shatterbox. Major elements were analysed using a VF-320 X-ray fluorescence spectrometer (XRF) at the Centre of Modern Analysis, Nanjing University (NJU), with an analytical precision less than 1%, following the procedures described by Franzini, Leoni & Saitta (Reference Franzini, Leoni and Saitta1972). Rare earth elements (REE) and trace elements were determined by ICP-MS (Finnigan MAT Element 2) machines housed at the State Key Laboratory for Mineral Deposits Research, NJU. The analytical precision for most elements is better than 5%. International standards were used to monitor the quality of analyses throughout the analytical processes for ICP-MS. Precisely weighed 50 mg sample powders were dissolved in Teflon bombs in HF+HNO3. An internal standard solution containing the single element Rh was used to monitor signal drift during counting. Analytical procedures are similar those in Zhou, J. C. et al. (Reference Zhou, Wang, Qiu and Gao2004).
Sm–Nd isotopic compositions were determined at the Isotope Laboratory, Institute of Geology & Geophysics, Chinese Academy of Sciences (CAS), using a MAT-262 mass spectrograph. The analytical details have been given in Shen, Zhang & Liu (Reference Shen, Zhang and Liu1997). Analysis of the standard BCR-1 gives 143Nd/144Nd=0.512656±13. The ϵNd(t) values were calculated based on the Nd isotopic compositions of 143Nd/144Nd (CHUR)=0.512638 and 147Sm/144Nd (CHUR)=0.1967.
Zircon Lu–Hf analyses reported here were carried out in situ using a Geolas CQ 193 nm ArF excimer laser ablation system at the Institute of Geology & Geophysics, CAS. The laser ablation system is attached to a Neptune MC-ICP-MS which has a double focusing multi-collector ICP-MS and the capability of high mass resolution measurements in a multiple collector mode. Both He and Ar carrier gases were used to transport the ablated sample from the laser-ablation cell via a mixing chamber to the ICP-MS torch. The analytical techniques are similar to those described in detail by Xu et al. (Reference Xu, Wu, Xie and Yang2004). Most analyses were carried out using a beam with an approximately 32 μm diameter and a 4 Hz repetition rate. A new TIMS determined value of 0.5887 for 176Yb/172Yb was applied for correction (Vervoort et al. Reference Vervoort, Patchett, Söderlund and Baker2004). During the analytical process, we applied the mean βYb value in the same spot to the interference correction of 176Yb on 176Hf in order to get precise data for the individual analyses. Zircon 91500 was used as the reference standard, with a recommended 176Hf/177Hf ratio of 0.282302±8 (Goolaerts et al. Reference Goolaerts, Mattielli, de Jong, Weis and Scoates2004). The decay constant for 176Lu of 1.865×10−11 a−1 proposed by Scherer, Münker, & Mezger (Reference Scherer, Münker and Mezger2001) was adopted in this work. ϵHf values were calculated according to the chondritic values of Blichert-Toft, Chauvel & Albarede (Reference Blichert-Toft, Chauvel and Albarede1997).
5.b. Classification
Most of the rocks selected for this study are basalts and basaltic trachyandesites (SiO2=46.28–57.47 wt%; Table 3; Fig. 3a, b). On the alkali–SiO2 diagram (Fig. 3a), the samples from Qianyang and Guzhang are classified as basalts, trachybasalts and trachyandesites. Because the elements Ti, Zr, Nb and Y are not as susceptible to change during alteration as Na and K, a Zr/TiO2–Nb/Y diagram (Winchester & Floyd, Reference Winchester and Floyd1977) may be more reliable in distinguishing rock types than alkali–SiO2 schemes. On such a diagram, the mafic rocks from Guzhang plot in the alkali basalt fields (Fig. 3b), while those from Qianyang are in the alkali basalts and trachyandesites areas. Both diagrams indicate that the samples from Qianyang and Guzhang are alkaline, whereas the samples from Tongdao plot in the sub-alkaline basalt area in both diagrams, similar to the mafic–ultramafic rocks from Longsheng, northern Guangxi (Fig. 3a, b). The rocks from Tongdao have high MgO (24.88–26.85 wt%), Ni (955–1237 ppm), Cr (1867–1975 ppm), Co (154–160 ppm) contents and low Na2O, K2O and P2O5 concentrations, indicating that they are ultramafic cumulates rich in normative enstatite and olivine.
Major and trace element analyses for the Neoproterozoic mafic rocks from western Hunan, South China
aData sources: Ge et al. (Reference Ge, Li, Li, Zhou, Wang and Li2000) and Zhou J. C. et al. (Reference Zhou, Wang, Qiu and Gao2004); bdata from Wang et al. (Reference Wang, Zhou, Qiu and Gao2004b), trace elements were re-analysed by ICP-MS; cFe2O3 as total iron; dLOI, loss on ignition; eMg no. = 100*Mg2+/(Mg2++Fe2+), Fe2+ is calculated from total Fe; fMajor oxides were recalculated to 100% on a volatile-free basis; gTD – Tongdao, LS – Longsheng, QY – Qianyang.
Rock classification diagrams for the Neoproterozoic mafic rocks from western Hunan. (a) TAS diagram (after Maitre et al. Reference Maitre, Bateman, Dudek, Keller, Lemeyre, Bas, Sabine, Schmid, Sorensen, Streckeisen, Wooley and Zanettin1989); (b) Zr/TiO2–Nb/Y diagram (after Winchester & Floyd, Reference Winchester and Floyd1977). The shaded areas outline the ranges of compositions for the mafic–ultramafic rocks from the Longsheng area of northern Guangxi with data sources from Zhou, J. C. et al. (Reference Zhou, Wang, Qiu and Gao2004) and Ge et al. (Reference Ge, Li, Li, Zhou, Wang and Li2000).
5.c. Post-crystallization alteration
The mafic–ultramafic rocks collected for this study might undergo post-crystallization alteration and metamorphism in varying degrees, as indicated by their high LOI values (Table 3). This may disturb the abundance of individual elements. In this work, we put an emphasis on some elements, such as MgO, TiO2, Al2O3, Ni, Cr, Y, high field strength elements (HFSEs) (Nb, Th, Ta, Zr and Hf) and REE (except Eu), which are relatively immobile during hydrothermal processes or low-grade metamorphism. To assess the mobility of individual elements during post-crystallization processes, we have plotted the less mobile MgO, Th, Y and La and the mobile Na2O, K2O, Sr and Ba elements against Zr (Fig. 4) which is relatively immobile (Winchester & Floyd, Reference Winchester and Floyd1977; Macdonald et al. Reference Macdonald, Millward, Beddoe-Stephens and Laybourn-Parry1988). Plots of Na2O, K2O, Sr and Ba v. Zr for the rocks from Guzhang (Fig. 4) show a large scatter confirming their mobile nature, though some weak overall trends are observed. Plots of Th, Y and La v. Zr show correlations, suggesting that they might be immobile during post-crystallization alteration. Although a roughly negative correlation between MgO and Zr has been shown, there is still a mild scatter of data for the less mobile element. This is likely due to alteration effects. Both the normalized REE and trace element patterns of the mafic rocks from western Hunan (Fig. 5) are generally regular, implying that most of the incompatible trace elements might be immobile during post-crystallization alteration and could be used to trace the magma sources.
Major, trace elements and incompatible element ratios v. Zr for the Neoproterozoic mafic–ultramafic rocks from western Hunan. FC – fractional crystallization.
Normalized REE and trace element patterns for the Neoproterozoic mafic rocks from western Hunan. Chondrite, primitive mantle and OIB data are from Sun & McDonough (Reference Sun, Mcdonough, Saunders and Norry1989).
5.d. Nature of parental magma
As shown in Table 3, mafic rocks from Qianyang and Guzhang show a range of compositions from fairly primitive (Mg no. = 65, Ni = 462 ppm, Cr = 661 ppm) to highly differentiated (Mg no. = 25, Ni = 2.9 ppm, Cr = 2.68 ppm). It is therefore important in later discussion of mantle source characters to choose element ratios that are not significantly affected by crystal fractionation processes. These rocks have high TiO2, moderate Al2O3 contents and low Al2O3/TiO2 ratios. Total alkalis are relatively high (3.62–8.36 wt%). LREE (light rare earth elements) are highly enriched relative to HREE (heavy rare earth elements) (Fig. 5; (La/Yb)N=9.26–25.29, (Gd/Yb)N=1.75–3.85). The samples from Qianyang, with higher TiO2, P2O5 and Zr contents and Nb/Y ratios (Table 3; Fig. 3b), are more alkaline than those from Guzhang. The Qianyang mafic rocks also have higher LREE contents (average La concentrations of 46.13 ppm for Qianyang as opposed to 26.04 ppm for Guzhang samples) and fractionated REE patterns ((La/Yb)N values of 11.83–25.29 v. 9.26–12.49). There is an overall similarity in the trace element patterns for the mafic rocks from Qianyang and Guzhang, and most of them are enriched in incompatible elements (Fig. 5). The HFSE, such as Nb, Zr, Hf and Ti, are generally not depleted. Although the rocks show a slight Nb depletion relative to U and La, they do not show extreme Nb depletion like the island arc basalts (Kelemen et al. Reference Kelemen, Johnson, Kinzler and Irving1990). Zr and Hf are even enriched in some samples. The variations in Rb and Ba might result from the post-crystallization alteration. Overall, the primitive-mantle normalized patterns of the mafic rocks from Qianyang and Guzhang are regular and similar to those of typical ocean island basalts (OIB). Incompatible trace element ratios, such as Zr/Nb (6.15–12.15) and La/Nb (0.96–1.56, average of 1.26), are also similar to EM-1 type OIB (Weaver, Reference Weaver1991). Their low Rb/Sr and Sm/Nd ratios (0.02–0.13 and 0.19–0.22, respectively) as well as lower 143Nd/144Nd ratios (between 0.51228 and 0.51237, Table 4) and initial (87Sr/86Sr) ratios (Wang et al. Reference Wang, Zhou, Qiu and Gao2004b), are similar to those of the EM-1 type OIB in Walvis Ridge (Saunders & Tarney, Reference Saunders and Tarney1988). The εNd(t) values of these alkaline mafic rocks range from −0.47 to 2.69 (Table 4), suggesting that they might be derived from a weakly depleted mantle source. Moreover, it is evident that most εNd(t) values of the mafic intrusive rocks from Qianyang and Guzhang are near chondritic (−0.47 to 0.93) except for one basalt sample (QY-8). This suggests the involvement of some enriched components in their sources.
The Tongdao ultramafic rocks as well as the mafic–ultramafic rocks from Longsheng of northern Guangxi show clearly different geochemical characteristics. They generally have low TiO2, Na2O, K2O and P2O5 and are enriched in MgO, MnO, Ni and Cr (Table 3). The enrichment of LREE for the Tongdao ultramafic rocks is relatively low, with average (La/Yb)N of 3.94 (Fig. 5), similar to those of the mafic–ultramafic rocks from Longsheng. The REE patterns of these rocks show moderate negative Eu anomalies (Eu/Eu*=0.57–0.80), implying the fractionation of plagioclase. The evident Sr troughs in the primitive-normalized patterns also support this conclusion (Fig. 5). There are moderate to weak negative anomalies in Nb and Ti, but Zr and Hf are not depleted. Compared to the mafic rocks from Qianyang and Guzhang, the Tongdao ultramafic rocks have high 147Sm/144Nd and 143Nd/144Nd values with a εNd(t) value of −2.91 (Table 4), implying the enriched signatures in their source.
Sm–Nd isotopic data for the Neoproterozoic mafic rocks from western Hunan, South China
aData from Wang et al. (Reference Wang, Zhou, Qiu and Gao2004b)
5.e. Zircon Hf isotopes
Since zircon preserves a high-quality record of near-initial Hf-isotope ratios, it can be utilized as a geochemical tracer of a host rock's origin (Woodhead et al. Reference Woodhead, Hergt, Shelley, Eggins and Kemp2004). The dated zircons of this work were further analysed for micro-area Lu–Hf isotopes and the results are listed in Table 5 (supplementary material, available online at the Geological Magazine website http://journals.cambridge.org/geo). Initial 176Hf/177Hf ratios were calculated using their determined U–Pb zircon ages. We determined the Lu–Hf isotopic compositions of 18 and 17 spots for samples QY-2 and 05TD-3–3, respectively. As shown in Figure 6, the weighted average initial 176Hf/177Hf ratios of the zircons from QY-2 are basically similar to those from 05TD-3–3. The zircon ϵHf(t) values of QY-2 vary from −1.96 to +8.50, while the ϵHf(t) values of 05TD-3–3 range from −0.86 to +5.65. Although the 176Hf/177Hf ratios of the two samples display multi-peak distributions (Fig. 6), they are generally restricted to a range of 0.2823–0.2825. Therefore, we think the weighted mean values of the Hf isotopes and the corresponding ϵHf(t) values (Fig. 6; 1.0±1.1 for QY-2 and 1.91±0.95 for 05TD-3–3, respectively) could approximately represent the initial Hf isotopic compositions of the two samples. The average ϵHf(t) value of QY-2 (1.0±1.1), consistent with its ϵNd(t) value (0.85, Table 4), is near chondritic. It indicates that the Nd–Hf isotopes of mafic rocks from Qianyang and Guzhang are coupled with each other. In the ultramafic rocks from Tongdao, however, the ϵNd(t) value of 05TD-3–3 (−2.91, Table 4) is evidently decoupled from its ϵHf(t) value (1.91±0.95).
Hf isotopes and the recalculated weighted average ϵHf(t) values for the Neoproterozoic mafic rocks from western Hunan. QY-2, from Qianyang; 05TD-3–3, from Tongdao. The shaded area is used for comparison.
6. Magma genesis
6.a. Crustal contamination
Mantle-derived magmas may be affected by crustal contamination during their ascent, and/or temporary residence in magma chambers within continental crust. Some samples (QY-3, 4, 5, 7, 8) have normative nepheline (Table 3), which does not support the significant high-SiO2 crustal component assimilation. In addition, there are no positive correlations between La/Sm v. Zr/Nb, La/Sm and La/Nb v. 143Nd/144Nd, and negative correlation of Ti/Zr v. La/Nb for the mafic rocks from Qianyang and Guzhang, implying that the crustal contamination was not an important factor for the generation of these Neoproterozoic mafic rocks.
6.b. Fractional crystallization
The mafic rocks from Qianyang and Guzhang have Mg no. ranging from 65 to 25, suggesting that fractional crystallization is an important factor in the evolution of the mafic magmas in the area. Moreover, the Zr/Nb–Zr plot (Fig. 4) implies that the slight chemical differences between samples would reflect variations in the degree of fractional crystallization rather than partial melting. The effects of fractional crystallization for the mafic rocks from western Hunan are indicated by the compositional variations of both major and trace elements with respect to Mg no. as fractionation index (Fig. 7). CaO, CaO/Al2O3 and CaO/TiO2 increase with the increased Mg no. (Fig. 7), which indicates the crystallization of clinopyroxene (Class et al. Reference Class, Altherr, Volker, Eberz and McCulloch1994). It should be pointed out that two types of correlations with Mg no. have been shown by some elements and element ratios (Fig. 7). Under Mg no. ≥45 (predominantly including the ultramafic and mafic rocks), negative correlations are revealed by TiO2, Al2O3, Na2O+K2O and V versus Mg no., whereas positive correlations exist between Al2O3/TiO2 and Mg no., implying the fractional crystallizations of olivine and clinopyroxene. Under Mg no. <45, however, there are positive correlations of TiO2 and V versus Mg no., relatively constant Na2O+K2O and Al2O3 versus Mg no., and negative correlation of Al2O3/TiO2 versus Mg no., probably indicating the fractionation of Fe–Ti oxides. Incompatible trace elements (e.g. Zr, Nb and Y) show negative correlations with Mg no., while the compatible trace elements (e.g. Ni and Cr) have positive correlations with Mg no., supporting the fractionation of olivine and pyroxene. Furthermore, a Eu anomaly is lacking in most mafic samples (Fig. 5), indicating that plagioclase fractionation was not important in the mafic magma evolution.
Major and trace elements v. Mg no. plot for the Neoproterozoic mafic rocks from western Hunan.
6.c. Magma source
Although the mafic rocks from Tongdao, Qianyang and Guzhang occur in the same magmatic belt, the elemental geochemical and Sm–Nd isotopic features discussed above suggest that these contemporaneous rocks may have been derived from rather different mantle sources.
6.c.1. Tongdao
As shown in Figure 1b, the Neoproterozoic mafic rock belt in western Hunan extends southward to the Longsheng area of northern Guangxi Province. The mafic rocks from Longsheng yielded a single zircon U–Pb age of 761±8 Ma (Ge et al. Reference Ge, Li, Li and Zhou2001), which is indistinguishable from that of the Tongdao rocks (772±11 Ma). It is therefore necessary to bring together different studies of the mafic–ultramafic rocks from both Tongdao and Longsheng in order to discuss their petrogenesis. The mafic–ultramafic rocks from Longsheng have been believed to be derived from lithospheric mantle (Ge et al. Reference Ge, Li, Li, Zhou, Wang and Li2000; Zhou, J. C. et al. Reference Zhou, Wang, Qiu and Gao2004). The ultramafic rocks from Tongdao show obvious geochemical affinities with those from Longsheng, implying a petrogenetic correlation between them. In tectonic discriminant diagrams (Fig. 8), the ultramafic rocks from Tongdao all plot in the volcanic arc basalt area, similar to those from Longsheng.
Tectonic discriminative diagrams for the Neoproterozoic mafic rocks from western Hunan. (a) 2Nb–Zr/4–Y plot (after Meschede, Reference Meschede1986); (b) Ti–Zr plot (after Pearce, Reference Pearce and Thorpe1982). WPA – within-plate alkaline basalts; WPB – within-plate basalts; WPT – within-plate tholeiites; VAB – volcanic arc basalts; MORB – middle ocean ridge basalts; N-MORB – normal MORB; P-MORB – plume MORB. The shaded areas outline the ranges of compositions for the mafic–ultramafic rocks from the Longsheng area with data sources from Zhou, J. C. et al. (Reference Zhou, Wang, Qiu and Gao2004) and Ge et al. (Reference Ge, Li, Li, Zhou, Wang and Li2000).
The decoupled Nd–Hf isotopic compositions of the Tongdao ultramafic rocks are likely to reflect the influence of an early subduction process in their mantle source. In the subduction zone, the addition of subducted components would increase Nd more than Hf concentrations in the mantle wedge. This is because Nd is more soluble than Hf in slab-derived fluids and melts that have typically high Nd/Hf ratios (Polat & Münker, Reference Polat and Münker2004). In the Baotan area of northern Guangxi, about 140 km southwest of the Longsheng area, the occurrence of the Mesoproterozoic and c. 820 Ma mafic–ultramafic rocks is believed to be related to the early subduction and post-collisional processes in the area (Zhou, J. C. et al. Reference Zhou, Wang, Qiu and Gao2004; Wang et al. Reference Wang, Zhou, Qiu, Zhang, Liu and Zhang2006). The c. 760 Ma mafic magmatism from Tongdao in western Hunan and Longsheng in northern Guangxi clearly took place following the orogenic process. Their mantle source might have been metasomatized by the early slab-derived components in the area. Overall, the decoupled Nd–Hf isotopic characteristics of the Tongdao ultramafic rocks provide another line of important evidence for the origination of these rocks from the partial melting of early metasomatized lithospheric mantle.
6.c.2. Qianyang and Guzhang
The mafic rocks from Qianyang and Guzhang are characterized by their high alkali and TiO2 contents and OIB-like trace element patterns. They consistently plot in the within-plate basalt area in tectonic discriminant diagrams (Fig. 8). The origin of the within-plate mafic rocks is still the subject of debate. They can be generated by the melting of the continental lithospheric mantle, the asthenospheric mantle or both (McDonough, Reference McDonough1990; McKenzie & O'Nions, Reference McKenzie and O'Nions1995). Most authors believe that they might be derived from a relatively enriched mantle source (e.g. Halliday et al. Reference Halliday, Lee, Tommasini, Davies, Paslick, Fitton and James1995; Lassiter et al. Reference Lassiter, Blichert-Toft, Hauri and Barsczus2003). During continental extension under very high stretching factors (β=3.5–6), lithosphere which has been previously enriched by CO2–H2O-rich metasomatic fluids derived from the asthenosphere could be partially melted to generate the OIB-like magmas (McKenzie & Bickle, Reference McKenzie and Bickle1988; Arndt & Christensen, Reference Arndt and Christensen1992). However, there is no evidence for significantly stretched lithosphere in the western part of the Proterozoic Jiangnan orogen. Therefore, a lithospheric mantle source for the alkaline mafic magmas from Qianyang and Guzhang of western Hunan is less likely. These alkaline mafic rocks are geochemically similar to the OIB and comparable to the alkali basalts from the Basin and Range Province of the western U.S.A. (Bradshaw, Hawkesworth & Gallagher, Reference Bradshaw, Hawkesworth and Gallagher1993). This indicates that an asthenospheric mantle source might be suitable for them.
In addition, it has been reported that some geochemical criteria could be used to distinguish the asthenospheric and lithospheric mantle-derived basalts. Thompson & Morrison (Reference Thompson and Morrison1988) showed that basaltic rocks with La/Ta ratios of 10–12 might be derived from the asthenospheric mantle, whereas basalts having La/Ta>30 were derived from the lithospheric mantle or had suffered crustal contamination. Fitton et al. (Reference Fitton, James, Kempton, Ormerod, Leeman, Cox and Menzies1988) and Saunders et al. (Reference Saunders, Storey, Kent, Norry, Storey, Alabaster and Pankhurst1992) also used La/Nb ratios to distinguish the basalts derived from different mantle sources. They found that the basalts derived from uncontaminated asthenospheric mantle have La/Nb ratios of <1.5, while those from a lithospheric mantle have a La/Nb>1.5. In the Neoproterozoic alkaline mafic rocks from Qianyang and Guzhang, La/Ta ratios range from 13.01 to 22.96 (average 17.11) and La/Nb ratios vary from 0.96 to 1.56 (average 1.26). It suggests that their parental magmas have been derived from the asthenospheric mantle.
Moreover, the εNd(t) values of the intrusive rocks from Qianyang and Guzhang are near chondritic (between −0.43 and 0.97; Table 4) except for a basalt sample (QY-8), as pointed out above. Similarly, the mean εHf(t) value of zircon grains in QY-2 is also near chondritic (1.0±1.1). The coupled Nd–Hf isotopic compositions of these rocks might provide some useful information for their petrogenesis. During the earlier extension stage immediately following the orogeny, some older enriched lithospheric mantle components might be incorporated into the depleted asthenospheric mantle. The mixing between the incorporated enriched lithospheric mantle and the depleted mantle would cause the mantle source to have near-chondritic Sm–Nd and Lu–Hf isotopic signatures. Therefore, essentially near-chondritic εNd(t) and εHf(t) values for the mafic intrusive rocks from Qianyang and Guzhang likely reflect the interactions between the depleted asthenospheric and enriched lithospheric mantle along the Jiangnan orogen during Neoproterozoic times.
7. Tectono-magmatic evolution model
In recent years, much discussion has focused on the Precambrian tectonic evolution of South China (e.g. Zhao & Cawood, Reference Zhao and Cawood1999; Li et al. Reference Li, Li, Kinny and Wang1999, Reference Li, Li, Kinny, Wang, Zhang and Zhou2003; Zhou et al. Reference Zhou, Yan, Kennedy, Li and Ding2002; Yang et al. Reference Yang, Sun, Yang and Pei2004). It is generally accepted that the Jiangnan orogen resulted from the collision between the Yangtze and Cathysia blocks (Guo, Shi & Ma, Reference Guo, Shi and Ma1980; Chen et al. Reference Chen, Foland, Xing, Xu and Zhou1991; Zhou & Zhu, Reference Zhou and Zhu1993; Zhao & Cawood, Reference Zhao and Cawood1999; Wu, Zheng & Wu, Reference Wu, Zheng and Wu2005). Corresponding petrological records include the 1.0–0.96 Ga ophiolites, the c. 910–875 Ma arc volcanic rocks, the high-pressure blueschists of 870–850 Ma and the post-collisional S-type granitic rocks emplaced at 835–800 Ma. There was no magmatism during 790–770 Ma in the western part of the Jiangnan orogen, which might suggest a transition of tectonic regime in the area. The occurrence of c. 760 Ma mafic–ultramafic rocks in western Hunan as well as those in Longsheng of northern Guangxi may be the products of post-orogenic magmatism.
There are two possible explanations for the c. 760 Ma mafic magmatism of the western Jiangnan orogen. The first explanation involves a (super-)mantle plume event (Li et al. Reference Li, Li, Kinny and Wang1999, Reference Li, Li, Kinny, Wang, Zhang and Zhou2003), whereas the second one can be viewed in a model of post-orogenic extension that follows continental collision.
The mantle plume model encounters significant difficulties in explaining the origin of the Neoproterozoic mafic rocks in the western Jiangnan orogen. First, a considerable lack of outcrop of large-scale mafic lavas and dykes seems to conflict with the (super-)plume model. There is only about 100 km2 of outcropping area for the c. 760 Ma mafic rocks in the western part of the Jiangnan orogen (including western Hunan and northern Guangxi: BGMRHN, 1988; BGMRGX, 1985). Second, the aeromagnetic anomaly resulting from mafic rocks in the area is very weak (Rao, Wang & Cao, Reference Rao, Wang and Cao1993), which suggests that the amount of mafic rocks exposed and buried in the area is limited. Third, high-magnesium and high-alkali mafic–ultramafic rocks are absent in the area.
Post-orogenic extension might be considered as an alternative model for the geodynamic setting of the c. 760 Ma magmatism in the western part of the Jiangnan orogen. Bonin et al. (Reference Bonin, Sekkal, Bussy and Ferrag1998) have given definitions for the different episodes of a whole orogenic Wilson cycle, and proposed that the post-orogenic episode would be likely to occur after the orogenic processes and during the within-plate stage. Delamination of the subcontinental lithosphere (Wu et al. Reference Wu, Wilde, Zhang and Sun2004), upwelling of new asthenospheric mantle, thinning of overlying lithospheric mantle (Zhou, M. F. et al. Reference Zhou, Lesher, Yang, Lia and Sun2004) and collapse of the orogenic belt (England & Houseman, Reference England and Houseman1989) would lead to the extension in the post-orogenic stage. The NNE distribution style of the c. 760 Ma mafic magmatism in the western part of the Jiangnan orogen (Fig. 1a) is consistent with the strike of the pre-existing subduction zone in the area. It is believed that the pre-existing metasomatized lithospheric mantle would be easily melted during the post-orogenic extension. Therefore, for the petrogenesis (see discussion below) of the c. 760 Ma mafic rocks in the western Jiangnan orogen, the post-orogenic extension model would also be preferred.
It is well known that the coexistence of nearly coeval mafic magmas derived from different sources like this is common in orogenic belts worldwide (Dunphy, Ludden, & Francis, Reference Dunphy, Ludden and Francis1995; Foden et al. Reference Foden, Song, Turner, Elburg, Smith, Van Der Steldt and Van Penglis2002; Cvetković et al. Reference Cvetković, Prelević, Downes, Jovanović, Vasellid and Pécskay2004). It has been noted that the mafic rocks from Qianyang and Guzhang derived from asthenospheric mantle have only an area of less than 10 km2 (BGMRHN, 1988), whereas the mafic–ultramafic rocks in Tongdao and Longsheng areas derived from lithospheric mantle have an area of c. 90 km2 (BGMRGX, 1985). This spatial evolution of the c. 760 Ma mafic–ultramafic magmatism in the western Jiangnan orogen resembles that in the Basin and Range Province, in which the mafic magmatism evolved from the sub-alkaline and calc-alkaline series to the OIB-like basaltic rocks, and the magmatic sources accordingly evolved from lithosphere to asthenospheric mantle (Fitton, James & Leeman, Reference Fitton, James and Leeman1991; Kempton et al. Reference Kempton, Fitton, Hawkesworth and Ormerod1991). In particular, the early lithosphere-derived magmas are voluminous, whereas the late asthenosphere-derived OIB-like magmas are relatively less so (<5%: Bradshaw, Hawkesworth & Gallagher, Reference Bradshaw, Hawkesworth and Gallagher1993; Daley & DePaolo, Reference Daley and DePaolo1992). The extension of lithosphere accompanied with the upwelling of asthenosphere may be the key factor for the coeval magmas with different sources. According to the model of McKenzie & Bickle (Reference McKenzie and Bickle1988), only the lithosphere could be partially melted during the early stage of the extension. However, the contribution of asthenosphere to the generation of mafic magmatism would become ever greater along with the increase in extension of the lithosphere. The geochemical and isotopic characteristics of the c. 830–810 Ma arc-like mafic–ultramafic rocks in the Baotan area of northern Guangxi suggest that the lithospheric mantle at the time had been metasomatized by the slab-derived fluids (Zhou, J. C. et al. Reference Zhou, Wang, Qiu and Gao2004). The upwelling of asthenospheric mantle would immediately follow the break-off and detachment of the subducted oceanic slab (Davies & Blanckenburg, Reference Davies and Blanckenburg1995) during the post-orogenic stage, which would at first lead to the extension of overlying metasomatized lithospheric mantle. According to the studies on the sedimentary rocks of the Banxi Group, it has been shown that the Banxi Group was deposited in a littoral–neritic deposition environment at continental margins (Wu, Dai & He, Reference Wu, Dai and He2001). The depth of the ancient sea might turn deeper from the north to the south (that is, from Guzhang in western Hunan to Longsheng in northern Guangxi) (Tang, Huang & Guo, Reference Tang, Huang and Guo1997), which may indicate that the extension in the south would take place earlier than that in the north. The extension and heating accompanied by the upwelling of the asthenosphere would lead to the partial melting of the metasomatized lithospheric mantle to generate the sub-alkaline mafic rocks in the Tongdao and the Longsheng areas. However, the extension in the Qianyang and Guzhang areas probably took place later and deeper. The small degrees of decompression melting of the deeper asthenospheric mantle would result in the generation of minor alkaline mafic magmas in the area.
The post-orogenic extensional setting revealed by the c. 760 Ma mafic rocks of western Hunan supports a normal evolution process of the Jiangnan orogen. It was reported that the Sinian sedimentary sequences conformably overlying these mafic rocks show some features of rifting basin (Liu, Hao & Li, Reference Liu, Hao and Li1999). From this point of view, the formation of the post-orogenic mafic rocks may suggest that the initial rifting in South China might take place after c. 760 Ma. This regional rifting that occurred in South China might be a part of the global rifting event of the Rodinia supercontinent. Palaeomagnetic studies on the mafic–ultramafic rocks of western Hunan and comparisons with the published palaeomagnetic data from other areas will shed light on the studies of Rodinia reconstruction in the study area.
8. Conclusions
(1) The Neoproterozoic mafic rocks in western Hunan of South China, the western part of the Jiangnan orogen, yielded ion microprobe U–Pb zircon ages of 747±18 Ma (Qianyang) and 772±11 Ma (Tongdao). They are geochemically divided into two subtypes. The ultramafic rocks from Tongdao are depleted in Nb and Ti and show a decoupled Nd–Hf isotopic signature. Their geochemical features are similar to those of the c. 761 Ma sub-alkaline rocks in the Longsheng area of northern Guangxi. However, the alkaline mafic rocks from Qianyang and Guzhang are geochemically similar to OIB and have near-chondritic Nd and Hf isotopic characteristics.
(2) The petrogenesis of two subtypes of the c. 760 Ma mafic rocks in western Hunan is different. The ultramafic rocks from Tongdao as well as the mafic–ultramafic rocks from Longsheng in northern Guangxi might be derived from the partial melting of the lithospheric mantle that had been metasomatized during the early oceanic subduction in the area, whereas the mafic rocks from Qianyang and Guzhang might be derived from asthenospheric mantle.
(3) The c. 760 Ma mafic magmatism in western Hunan may have been formed in a post-orogenic extensional setting, representing the transition of the tectonic regime from orogenic to anorogenic rifting. The break-off and detachment of the subducted oceanic slab and the accompanied upwelling of the deep asthenospheric mantle might lead to post-orogenic lithospheric extension. The extension might have taken place earlier in the Tongdao and Longsheng areas to generate the relatively great amount of sub-alkaline rocks, whereas the less alkaline mafic rocks in Qianyang and Guzhang might have been generated in the relatively later stage of the extension.
Acknowledgements
This research was financially supported by the National Natural Science Foundation of China (grant nos 40572039 and 40221301), Postgraduate Training Project of Nanjing University and Jiangsu province, China. The manuscript benefited from the comments of the referees, Prof. M. F. Zhou and an anonymous reviewer, and the editors. We also thank Senior Engineers J. Z. Huang, X. S. Tang and J. J. Zheng for their earnest direction and assistance on the field trip, Prof. F. Y. Wu and Dr L. W. Xie for La-MC-ICP-MS zircon Hf isotopic analyses, Prof. F. K. Chen and Dr Z. Y. Chu for Sm-Nd isotopic analysis, Prof. D. Y. Liu for U–Pb zircon analyses, J. F. Gao for trace element analysis, and M. Q. Zhang for major element analysis.
