Melting of enriched Archean subcontinental lithospheric mantle: Evidence from the ca. 1760 Ma volcanic rocks of the Xiong’er Group, southern margin of the North China Craton

Precambrian Research 182 (2010) 204–216

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Melting of enriched Archean subcontinental lithospheric mantle: Evidence from the ca. 1760 Ma volcanic rocks of the Xiong’er Group, southern margin of the North China Craton Xiao-Lei Wang a,b,∗ , Shao-Yong Jiang a , Bao-Zhang Dai a a b

State Key Laboratory for Mineral Deposits Research, Department of Earth Sciences, Nanjing University, Nanjing 210093, PR China CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, PR China

a r t i c l e

i n f o

Article history: Received 14 December 2009 Received in revised form 22 July 2010 Accepted 13 August 2010

Keywords: Petrogenesis Volcanic rocks Lithospheric mantle Extension North China Craton

a b s t r a c t The Xiong’er Group is an important geologic unit in the southern margin of the North China Craton. It is dominated by the volcanic rocks, dated at 1763 ± 15 Ma, that have SiO2 contents ranging from 52.10 wt% to 73.51 wt%. These volcanic rocks are sub-alkaline and can be classified into three subgroups: basaltic andesites, andesites and rhyolites. They unexceptionally show enrichment of light rare earth elements (LREE) and share similar trace element patterns. Depletions in Nb, Ta, Sr, P and Ti relative to the adjacent elements are evident for all the samples. The volcanic rocks are evolved with low MgO contents (0.29–5.88 wt%) and accordingly low Mg# values of 11–53. The Nd isotopes are enriched and show a weak variation with εNd (t) = −7.12 to −9.63. Zircon Hf isotopes are also enriched with εHf (t) = −12.02 ± 0.45. The volcanic rocks of the Xiong’er Group are interpreted to represent fractional crystallization of a common mantle source. The volcanic rocks might have been generated by high-degree partial melting of a lithospheric mantle that was originally modified by the oceanic subduction in the Late Archean. This brings a correlation with the subduction-modified lithospheric mantle in an extensional setting during breakup of the Columbia supercontinent in the late Paleoproterozoic, rather than in an arc setting. The elevated SiO2 contents and evolved radiogenic isotope features indicate the possible incorporation into their source of lower crustal materials that have similar Nd isotopic characteristics to the subcontinental lithospheric mantle. The existence of extensive Xiong’er volcanic rocks (60,000 km2 ) indicates an early large-scale subduction-related metasomatism in the area and probably suggest a flat subduction model for the plate-margin magmatism in the Late Archean. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Juvenile arc magmatism is unequivocally characterized by depleted isotopic compositions and negative anomalies in the high field strength elements (i.e. Nb, Ta) relative to large ion lithophile (LIL) elements (e.g. La and K), representing the growth of juvenile crust. However, igneous rocks with arc-like geochemical features are not definitely generated in island-arc settings, but could have resulted from several other processes, such as crustal contamination, magma mixing, reworking of old arc materials or melting of old, metasomatized mantle. Especially for the basaltic and basaltic andesitic rocks that occur in the previously developed arc-continent collisional belts, they generally have arc-like geochemical signatures but with unradiogenic Nd isotopic ratios,

∗ Corresponding author at: State Key Laboratory for Mineral Deposits Research, Department of Earth Sciences, Nanjing University, 22 Habkou Road, Jiangsu Province, Nanjing 210093, PR China. Tel.: +86 25 83686336; fax: +86 25 83686016. E-mail address: [email protected] (X.-L. Wang). 0301-9268/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2010.08.007

which may led to the debate when assessing the petrogenesis and tectonic settings of the rocks. Many of the rocks are enriched in K2 O, and are suggested to have formed due to low-degree melting of subcontinental lithospheric mantle that was metasomatized by aqueous fluid (Arnaud et al., 1992; Turner et al., 1993, 1996; Miller et al., 1999; Ding et al., 2003), although the possibilities of contributions from mafic granulitic or eclogitic lower-crustal source (Cooper et al., 2002) and hot, metasomatized asthenosphere (Guo et al., 2006) have been suggested. The metasomatism of subcontinental lithospheric mantle may take place due to the incorporation of crustally derived melts and/or aqueous fluids driven out from the down-going slab during the subduction of oceanic crust. Alternatively, the old sub-arc mantle that had accreted to the continent may potentially be another source for the type of rocks if, following the early metasomatism, no melt was immediately extracted (Zhang et al., 2009). For example, partial melting of two different sources have been proposed for the petrogenesis of Tibetan potassic rocks: (1) the enriched lithospheric mantle that has been isolated from the convecting asthenosphere for a long period of time (Turner et al., 1996); and (2) Cenozoic mantle lithosphere

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Fig. 1. Geological sketch map of the volcanic rocks of the Xiong’er Group, southern margin of the North China Craton. (a) Outline of the geological units of North China Craton (modified after Zhao et al., 2009); (b) Geological map of the Xiong’er Group in Southern Henan Province (modified after BGMRHP, 1989), LL-Luonan-Luanchuan Fault; (c) comparisons of the different rock types of the volcanic rocks of the Xiong’er Group with the frequency diagram of SiO2 contents quoted from Zhao et al. (2002), and the cake-shape diagram based on the cropping thickness in representative section (after BGMRHP, 1989). The thickness of the basaltic andesites may be more than what the cake-shape figure shows, because it is difficult to distinguish the basaltic andesites from the andesites in the field due to the similar color and textures.

that was metasomatized by subduction of ancient continental crust (Ding et al., 2003). Distinguishing the timing and mechanism for metasomatism of the mantle source is critical for understanding the petrogenesis and tectonic settings of the type of mafic rocks (Zhao and Zheng, 2009), especially for the geological evolution of old continental margins that lack good geophysical constraints. The volcanic rocks of the Xiong’er Group occur in the southern margin of North China Craton (Fig. 1a), occupying a large eruption area of over 60,000 km2 (Zhao et al., 2009). In the last three decades, there has been considerable debate over the petrogenesis and tectonic settings of the Xiong’er volcanic rocks (see Zhao et al., 2009, and references therein). Some authors suggest that the rocks were formed in a rift (e.g. Sun et al., 1981; Zhang, 1989; Yang, 1990; Zhai et al., 2000; Zhao et al., 2002) or in a mantle plume (e.g. Peng et al., 2008) environment, while others prefer a continental arc setting (e.g. Hu and Lin, 1988; Jia, 1987; He et al., 2008; Zhao et al., 2009). However, key issues remain over two aspects: (1) what are the roles of the continental lithospheric mantle and crust in the petrogenesis of these volcanic rocks, and (2) when did the proposed subduction-related metasomatism of their mantle source take place? Based on available published and our new geochemical data, this paper presents a new synthesis of petrology, geochronology, geochemistry and isotopes of the volcanic rocks comprising

the Xiong’er Group. From this it is proposed that ancient (Archean) lithospheric mantle was melted in a post-orogenic extensional setting to generate the volcanics. 2. Geological background The mainland of China is composed of two blocks, the North China and South China, which are separated by the Qinling and Dabie-Sulu orogenic belts. The Qinling Orogen experienced a complex divergent and convergent continental history onto the northern margin of the Yangtze Block (Meng and Zhang, 2000; Ratschbacher et al., 2003; Lu et al., 2004). The North China Craton can be subdivided into the Eastern and Western Blocks that were amalgamated along the ∼1850 Ma Trans-North China Orogen (Fig. 1a; Zhao et al., 2001). The Xiong’er Group is a distinct geological unit on the southern margin of the North China Craton occurring in a roughly triangular area bounded to the south by the Luonan-Luanchuan Fault and the Qinling Orogen (Fig. 1a and b). The Xiong’er Group is dominantly composed of lavas interlayered with minor pyroclastic rocks and sedimentary rocks (<5%) and covers an area of more than 60,000 km2 . The Xiong’er Group is stratigraphically constrained in that it unconformably overlies Archean to Paleoproterozoic basement rocks that include

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dated 2800–2500 Ma tonalitic–trondhjemitic–granodioritic (TTG) gneisses and Paleoproterozoic amphibolite- and granulite-facies metamorphosed complex of Taihua Group. The Group in turn is unconformably overlain by terrigenous sandstones, limestones and calc-silicate rocks of the Meso- to Neoproterozoic Ruyang Group (1126 Ma to ca. 900 Ma; Gao et al., 2009) and the Guandaokou Group. The Guandaokou Group is overlain by the ca. 830 Ma (our unpublished data) volcanic rocks of the Luanchuan Group. The Xiong’er Group volcanic rocks are dominantly composed of basaltic andesites and andesites with subordinate rhyolites, dacites, minor silicic tuff and mafic to felsic sub-volcanic rocks (e.g. Zhao et al., 2002; Fig. 1c). The basaltic andesites and andesites are generally extremely fine-grained to glassy in the groundmass with few phenocrysts of plagioclase, augite, and hornblende. Plagioclase is the dominant phenocryst phase in the rocks and the margins of some crystals are embayed due to melt resorption, suggesting that they are the early crystallizing mineral phase. The plagioclases phenocrysts are generally replaced by albite due to post-magmatic alteration, suggesting the mobility of Na2 O in the rocks. Evidence of similar low-temperature alteration is common in the volcanic rocks, with the occurrence of mineral assemblages including chlorite, actinolite, epidote and carbonate. Amygdales filled by quartz and chlorite are very common in the basic to intermediate volcanic rocks. The apparent occurrence of abundant sodium-rich plagioclase phenocrysts led some authors (e.g. Xia et al., 1990) to conclude that the Xiong’er volcanic rocks represented a spilitekeratophyre rock association. The subordinate acid rocks of the Xiong’er Group are dominated by mauve-coloured rhyolites and rhyolitic porphyries characterized by K-feldspar and quartz as the major minerals and with minor biotite. Based on whole-rock Rb–Sr isochrons and single zircon U–Pb dating methods, the volcanic rocks of the Xiong’er Group have been previously dated within the period of 1850–1400 Ma (see Zhao and Jin, 1999, and references therein). However, the Rb–Sr isotopic system with Rb–Sr isochron ages ranging from 1650 Ma to1400 Ma may have been affected by the post-magmatic alteration and again in later tectonic activities, and the single zircon U–Pb dating with results concentrated at ca. 1850–1730 Ma lacks a detailed analysis of the internal structure of zircons. Moreover, it is difficult to imagine that a single magmatic event could last over 400 Myr. In the last five years, more precise dating analyses have been carried out on the volcanic rocks. Using SHRIMP and LA-ICP-MS in situ zircon U–Pb dating techniques, more modern results suggest a shorter duration of 1780–1750 Ma for the generation of most of the volcanic rocks of the Xiong’er Group (e.g. Zhao et al., 2004; He et al., 2009). Minor felsic volcanic rocks in the area yield a SHRIMP zircon U–Pb age of 1450 ± 31 Ma (He et al., 2009), possibly representing a later magmatic activity in the southern margin of North China Craton.

3. Analytical methods We selected representative sample of fresh rhyolitic porphyry (07XE-13, N33◦ 54 54.1 , E112◦ 27 33.09 ) for LA-ICP-MS zircon U–Pb dating. Sampling locations are shown in Fig. 1. Zircons were separated using conventional heavy liquid and magnetic techniques, mounted in epoxy resin and polished down to expose the grain centers. Cathodo-luminescence (CL) images (Fig. 2) were acquired with a Mono CL3+ (Gatan, U.S.A.) attached to a scanning electron microscope (Quanta 400 FEG) at the State Key Laboratory of Continental Dynamics, Northwest University, Xi’an. U–Pb zircon dating was carried out at the State Key Laboratory for Mineral Deposits Research, Nanjing University (NJU), using an Agilent 7500a ICP-MS attached to a New Wave 213 nm laser ablation system. He carrier gas was used to transport the ablated sample

Fig. 2. Representative cathodo-luminescence (CL) images for the zircons from the sample 07XE-13. The dating spots are denoted by the open circles with the spot number and analyzed 206 Pb/238 U ages. The broken circles represent the analytical areas of Hf isotopes with the numbers indicating the εHf (t) values.

from the standard laser-ablation cell, and then mixed with the Ar gas via a mixing chamber before entering into the ICP-MS torch. All of the spot analyses were carried out using a beam with a 24 ␮m diameter and a repetition rate of 5 Hz and 60% energy, which produce an energy density of 0.121 J/cm2 . U–Pb fractionation was corrected using zircon standard GEMOC GJ-1 (207 Pb/206 Pb age of 608.5 ± 1.5 Ma; Jackson et al., 2004) and analytical accuracy was monitored using zircon standards Mud Tank (intercept age of 732 ± 5 Ma; Black and Gulson, 1978). Zircon samples were analyzed in runs of ca. 15 analyses which included 5 zircon standards and 10 sample points. U–Pb ages were calculated from the raw signal data using the on-line software package GLITTER (ver. 4.4) (www.mq.edu.au/GEMOC). Because 204 Pb could not be measured due to low signal and interference from 204 Hg in the gas supply, common lead correction was carried out using the EXCEL program ComPbCorr#3 15G (Andersen, 2002). However, no analysis has been corrected effectively because of the concordance between different U–Th–Pb isotopic ratios and the very low common lead. The Th and U concentrations of the zircons were calculated according to the comparison of relative signal intensity between the standard zircon GJ-1 in our laboratory (Th = 8 ppm, U = 330 ppm) and zircon samples using the EXCEL program Data Templatev2b from GEMOC. Major elements were analyzed using an ICP-AES (JY38S) at the State Key Laboratory for Mineral Deposits Research, NJU, following the procedures described by Liu et al. (1999), with an analytical precision generally less than 2% (RSD). Rare earth and other trace elements were analyzed using ICP-MS (Finnigan MAT-Element 2) techniques at the State Key Laboratory for Mineral Deposits Research, NJU. Analytical precision for most elements by ICP-MS is better than 5% with analytical procedures similar to Wang et al. (2004).

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Sr–Nd isotopes were analyzed using the ID-TIMS (Finnigan MAT Triton TI) at the State Key Laboratory for Mineral Deposits Research, NJU. Chemical separation procedures are similar to Pu et al. (2005), with relative standard deviation (RSD) lower than 5 × 10−6 . Mass fractionation was corrected according to 86 Sr/88 Sr = 0.1194. The long-term 87 Sr/86 Sr analyzing values of the international standard NIST SRM987 is 0.710252 ± 0.000016 (2, n = 65), consistent with the results (0.710252 ± 0.000013) of Weis et al. (2006). Mass fractionation of Nd isotopes was corrected with 146 Nd/144 Nd = 0.7219. The long-term 143 Nd/144 Nd analyzing values of the international standard JNdi-1 is 0.512121 ± 0.000016 (2, n = 67), identical to the value (0.512115 ± 0.000007) of Tanaka et al. (2000). The εNd (t) values were calculated based on the Nd isotopic compositions of 143 Nd/144 Nd (CHUR) = 0.512638 and 147 Sm/144 Nd (CHUR) = 0.1967. Zircon Lu–Hf analyses were carried out in situ using a New Wave 213 nm solid laser ablation system attached to a New Plasma MCICP-MS at the Institute of Geochemistry (Guiyang), CAS. He gas was used to transport the ablated sample from the laser-ablation cell, and then to the ICP-MS torch after mixing with the Ar gas. The analytical techniques are similar to those described in detail by Griffin et al. (2004) and Tang et al. (2008). Most analyses were carried out using a beam with a ca. 40 ␮m diameter and a 10 Hz repetition rate. A new TIMS determined value of 0.5887 for 176 Yb/172 Yb was applied for correction (Vervoort et al., 2004). Zircon 91500 was used as the reference standard, with a recommended 176 Hf/177 Hf ratio of 0.282302 ± 8 (Goolaerts et al., 2004). The decay constant for 176 Lu of 1.865 × 10−11 year−1 proposed by Scherer et al. (2001) was adopted in this work. εHf values were calculated according to the chondritic values of Blichert-Toft et al. (1997). In case of any influence from sampling limitations, we collected all of the available published geochemical data when discussing the petrogenesis of the volcanic rocks of the Xiong’er Group. These data are plotted with our results for comparison. 4. Results 4.1. U–Pb dating Zircons from the sample 07XE-13 are light pink to transparent, euhedral, with lengths of 50–150 ␮m and widths of 40–100 ␮m and are mainly stubby prisms with a broad oscillatory zoning (Fig. 2). Some zircon grains are embayed and some have a corerim structure. Twenty spots were analyzed from 20 zircons, and the analytical results are listed in Table 1. Th contents range from 19 ppm to 966 ppm, and U ranges from 20 ppm to 1118 ppm in the analyses, generating Th/U ratios ranging from 0.44 to 2.09. Three analyses are variably discordant with relatively old 207 Pb/206 Pb ages (ca. 2334–2116 Ma). One analysis spot with old 207 Pb/206 Pb ages (ca. 2225 Ma) is concordant. The other 21 analyses are concordant and yield a weighted average 206 Pb/238 U age of 1763 ± 15 Ma (95% confidence, MSWD = 0.22; Fig. 3), and a 207 Pb/206 Pb age of 1772 ± 26 Ma (95% confidence, MSWD = 0.025). Considering the mean 206 Pb/238 U age has a lower error, the age of 1763 ± 15 Ma is interpreted as the crystallization age of the rhyolitic porphyry. The age is consistent with the results (ca. 1780–1750 Ma) of Zhao et al. (2004) and He et al. (2009). 4.2. Major and trace element geochemistry The Xiong’er volcanic rocks have highly variable concentrations of SiO2 , ranging from 52.10 wt% to 73.51 wt%. In the (Na2 O + K2 O)–SiO2 diagram (Fig. 4a), they can be distinguished as basaltic andesites, basaltic trachy-andesites, andesites, dacites and rhyolites. Considering the intense alteration visible in thin section and the relatively high loss on ignition values (>3 wt%) in some

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Fig. 3. LA-ICP-MS zircon U–Pb zircon concordia plots and recalculated weighted mean 206 Pb/238 U ages for the rhyolitic porphyry 07XE-13.

basaltic and andesitic samples, some major elements (i.e. Na2 O) may have been mobile after the eruption of the rocks. Nb, Y, Zr and TiO2 are usually considered as being immobile during alteration. In the Zr/TiO2 –Nb/Y diagram (Fig. 4b), the rocks plot in the areas of basalts, andesites and dacites that belong to sub-alkaline basalts. According to the SiO2 contents, the volcanic rocks could be subdivided into four subgroups. Group 1 comprises rare volcanic rocks with SiO2 lower than 52 wt% found in the available published data (Zhao et al., 2002; He et al., 2008, 2010) but not in this work. Group 2 has SiO2 contents ranging from 52 wt% to 57 wt%, and is basaltic andesites. Group 3 comprises andesites with SiO2 values of 57–65 wt%. Group 4 comprises rhyolitic rocks with SiO2 contents higher than 65 wt%. Most of the samples have relatively high K2 O contents and belong to the high-K calc-alkaline or shoshonitic series in the K2 O–SiO2 diagram (Fig. 4c), apart from three samples that plot amongst low-K tholeiitic series. In case of the alteration and the variation of Na2 O contents, the AFM diagram may be questionable. However, the FeO* and MgO contents are relatively immobile during the alteration process or low-grade metamorphism. In the FeO*/MgO–SiO2 diagram (Fig. 4d), the rocks from the Xiong’er Group plot in the tholeiitic series suggesting that they should be tholeiitic rather than calc-alkaline series. The basaltic andesites of the Xiong’er Group have moderate MgO contents (Table 2; 3.06–5.88 wt%), producing evolved Mg# values of 37–53. The andesites have relatively lower MgO contents (1.00–2.85 wt%) and Mg# (24–35), while the rhyolitic rocks have the lowest MgO contents of 0.29–0.41 wt%. The TiO2 contents of the andesites and basaltic andesites range from 0.95 wt% to 1.70 wt%. Bearing in mind the significant degree of alteration, only those immobile major element oxides (MgO, Al2 O3 and TiO2 ), compatible trace elements (Ni, Cr), HFSE (Nb, Th, Ta, Zr and Hf) and REE (except Eu) will be further used to draw inferences on the petrogenesis of the volcanic rocks and their tectonic environment. The Ba and K2 O contents have roughly negatively correlations with the Mg# values and positively correlations with the Zr and SiO2 contents (not shown here), suggesting they could also been used when discussing the petrogenesis of the Xiong’er volcanic rocks. Although the compositions of the Xiong’er volcanic rocks vary greatly, their REE contents and chondrite-normalized patterns are consistent with each other (Fig. 5a, c and e). The light rare earth elements (LREE) are enriched relative to the heavy REE, resulting in high (La/Yb)N of 8.10–10.93. Negative Eu anomalies occur in some samples interpreted to reflect either plagioclase fractionation at

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Fig. 4. Rock classification diagrams for the Neoproterozoic mafic rocks from western Hunan. (a) TAS diagram (after Le Maitre et al., 1989); (b) Zr/TiO2 –Nb/Y diagram (after Winchester and Floyd, 1977); (c) K2 O–SiO2 diagram (after Le Maitre et al., 1989); (d) FeO*/MgO–SiO2 diagram (after Miyashiro, 1974).

the magmatic stage, or possibly, a greater mobility of Eu2+ relative to the other trivalent lanthanides during alteration processes, or both. The Eu negative anomalies are weak (Eu/Eu* = 0.66–0.86) for the basaltic andesites and andesites but moderate for the rhyolites (Eu/Eu* = 0.50–0.53). The HREE patterns for all the samples are nearly flat ((Dy/Yb)N = 0.99–1.30). Similarly, the trace elements also show identical primitive-mantle normalized patterns for all the rock types (Fig. 5b, d and f), displaying evident positive Ba and K2 O anomalies and impoverishment in Nb, Ta, Sr, P and Ti relative to the neighboring other trace elements of similar degree of incompatibility. The negative anomalies of Th and U are greater in the basaltic andesites and andesites than in those in the rhyolites. The basaltic andesites and andesites can be distinguished from ocean island basalts (OIB) by their high contents of large ion lithophile elements (LILEs) and remarkable Nb and Ta negative anomalies. These characters of the Xiong’er volcanic rocks are also different from the typical oceanic island-arc basalts and continental island-arc basalts, though the negative Nb and Ta troughs are similar (Fig. 5b and d). It is interesting to note that the primitive-mantle normalized patterns of the basaltic andesites and andesites are similar to the average upper continental crust (Fig. 5b and d), probably indicating the recycling of crust materials in the petrogenesis of the volcanic rocks of the Xiong’er Group.

4.3. Whole-rock Sr–Nd isotopes A total of eight samples were analyzed for Sr–Nd isotope analyses (Table 3). Apart from one sample (07XE-04, Rb/Sr = 0.50), the basaltic andesites and andesites (Rb/Sr = 0.11–0.16) have consistent initial 87 Sr/86 Sr ratios of 0.706–0.709. The rhyolitic rocks have extremely low initial 87 Sr/86 Sr ratios lower than 0.700, which may be generated from their high Rb/Sr ratios (1.09–1.26). The volcanic rocks of the Xiong’er Group show identical Nd isotopes with a narrow initial 143 Nd/144 Nd ratios (0.50987–0.51000). The εNd (t) values range from −7.12 to −9.63, indicating an enriched magmatic source for the volcanic rocks. The two-stage Nd model ages are Late Archean between 2750 Ma and 2920 Ma. 4.4. Zircon Lu–Hf isotopes A total of 20 zircons from the dated sample 07XE-13 were further analyzed for the Lu–Hf isotopes with results listed in Table 4. Initial 176 Hf/177 Hf ratios were calculated using their determined average U–Pb zircon ages. Apart from three spots, which have relatively old age (206 Pb/207 U ages of 2329–2116 Ma), the other spots show consistent 176 Hf/177 Hf isotopic ratios, which result in a normal distribution curve for the εHf (t) values with a mean value of

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Fig. 5. Normalized REE and trace element patterns for the volcanic rocks of the Xiong’er Group. Chondrite, primitive-mantle and OIB data are from Sun and McDonough (1989). The average upper crust data is from Rudnick and Gao (2004) and the average oceanic and continental-arc basalts data are from Kelemen and Hanghøj (2004).

−12.02 ± 0.45 at t = 1760 Ma (MSWD = 2.3, n = 17; Fig. 6). This value is roughly consistent with the whole-rock εNd (t) value (−9.05) of the sample. Correspondingly, the two-stage Hf model ages for the zircons range from 3000 Ma to 3313 Ma, slightly older than the Nd model ages. 5. Discussion 5.1. Crustal contamination Considering the very large area over which the Xiong’er volcanic rocks crop out, their crust-like trace element patterns and their enriched Nd isotopic signatures, it is necessary to evaluate the influence of crustal contamination during the magma ascent, and/or temporary residence in magma chambers within continental crust.

Because the upper crust and moderate-degree partial melts of the lower crust are enriched in LREE but depleted in high field strength elements (HFSE), crustal contamination will lead to a large variation of La/Nb ratios and a positive correlation between La/Nb and La/Sm. Moreover, the εNd (t) values can shift toward much lower when the continental crust increases in thickness and in age, and plausible crustal contaminations of a mantle-derived magma would result in much higher values of SiO2 , Zr and lower MgO than those observed. The basaltic andesites and andesites of the Xiong’er Group have no significant correlation between La/Nb and La/Sm ratios (Fig. 7a), εNd (t) values with MgO (Fig. 7b), Nb/Nb* (Fig. 7c) and 1/Nd (Fig. 7d) and SiO2 (Fig. 7e), only showing roughly positive or negative correlations. This probably suggests that the crustal contamination may not be an important factor in their petrogenesis. The basaltic andesites and andesites have relatively lower SiO2 contents but similar Nd

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Fig. 6. Frequency distribution of the εHf (t) values for the spots of the sample 07XE13.

contents compared with the rhyolitic rocks with high SiO2 contents, also precluding significant crustal contamination. In addition, there is no correlation between the La/Sm and Zr/Nb ratios (Fig. 7f), which seems to indicate that the crustal contamination may be minor. Conversely, it may be caused by the constant Zr/Nb ratios that were mainly controlled by the fractional crystallization. Overall, the isotopic and trace elemental signatures of the basaltic andesites and andesites demonstrate an origin from an enriched mantle reservoir. Crustal contamination, if any, was minor and limited to the earliest lavas. 5.2. Crystallization or partial melting? The Xiong’er volcanic rocks have compositions ranging from basaltic to rhyolitic, which can be produced by fractional crystallization of a basic magma chamber or by partial melting of different magma sources at different levels. If the latter, the volcanic rocks would have different evolution trends and different elemental and isotopic signatures. However, several lines of evidence suggest that the fractional crystallization may play an important role in the petrogenesis of the rocks. (1) The ratios of some incompatible elements (i.e. Zr/Nb and La/Sm) which have similar geochemical behavior are constant during fractional crystallization, while the element concentrations will change. The Xiong’er volcanic rocks show an obvious fractional crystallization tendency in the Zr/Nb–Zr diagram (Fig. 8a). Two samples of Zhao et al. (2002) have relatively higher Zr/Nb ratios, which might be generated from the inaccurate analytical Nb concentrations. Similarly, the La/Sm–La diagram (Fig. 8b) also defines a fractional crystallization trend for the volcanic rocks, although a weak partial melting tendency is indicated. (2) Variations in major element abundances (e.g. MgO = 0.29–5.88 wt% and Mg# = 11–53) reflect fractional crystallization of clinopyroxene. (3) The negative correlation (Fig. 7e) between SiO2 and εNd (t) values also indicate the existence of assimilation-fractional crystallization (AFC) processes. (4) The effects of fractional crystallization for the Xiong’er volcanic rocks are indicated by the compositional variations of both major and trace elements with respect to Mg# as fractionation index (Fig. 9). The positive correlations between SiO2 , MnO, Al2 O3 , TiO2 and Mg# suggest normal magmatic evolution trend for the volcanic

rocks of the Xiong’er Group. The jumbled distribution in the Na2 O vs. Mg# diagram suggests that Na was mobile in the rocks. Since Ni, Cr and partly Co are strongly compatible in mantle mineral phases, such as olivine and pyroxene, on the other hand, La, Ba and K are strongly incompatible in these two minerals, both positive correlations of Mg# vs. Cr and Co, and the negative correlations of Mg# vs. La and K2 O, would indicate that fractional crystallization of olivine and pyroxene should play an important role in the petrogenesis of the volcanic rocks. However, the SiO2 contents suggest that the olivine may be the early mineral phase that controlled the fractional crystallization during the early stage of magmatic evolution, while the pyroxene may be the dominant controlling factor for the fractional crystallization of the volcanic rocks. The positive correlation between CaO/TiO2 and Mg# also indicate the fractional crystallization of clinopyroxene. The main fractional crystallization phases may have changed in the volcanic rocks when Mg# exceeds 30. When Mg# is lower than 30, there are positive correlations between it with TiO2 and P2 O5 , indicating the fractional crystallization of apatite. A negative correlation between Al2 O3 /TiO2 ratios and Mg# is evident when Mg# < 30, whereas the correlation turn to be positive when Mg# > 30, which may suggest the fractional crystallization of plagioclase and clinopyroxene, respectively. Furthermore, an obvious Eu anomaly in all the Xiong’er volcanic rocks (Fig. 5a, c and e) indicates that plagioclase fractionation was an important process in their magmatic evolution. 5.3. Magma source: enriched Archean lithospheric mantle The above discussion suggests that the varied compositions of the volcanic rocks from the Xiong’er Group was generated by the fractional crystallization and magmatic evolution accompanied with minor crustal contamination. The andesites and rhyolites may have resulted from the fractional crystallization of a precursor basaltic melt. The sub-parallel trace and REE element distribution patterns (Fig. 5) for the rocks of different types suggest a common mantle source for them. The Zr/Nb ratios could not be affected by the fractional crystallization and partial melting during the petrogenesis of the rocks, and may reflect the characteristics of the magma source. The Xiong’er volcanic rocks have constant Zr/Nb ratios ranging from 23.50 to 28.63, close to the mean value for the source of typical N-MORB (Zr/Nb = 30; Weaver, 1991), suggesting that the local mantle had a degree of depletion broadly similar to that of the sub-oceanic mantle. However, the volcanic rocks are characterized by enrichments in the LIL elements and negative Nb and Ti anomalies relative to the neighboring REE, suggesting the melting of a subduction-modified mantle. The homogeneous unradiogenic Nd isotopic characteristics with εNd (t) of −7.12 to −9.63 indicate the partial melting of an isotopically heterogeneous and enriched mantle reservoir. We suggest that the isotope characteristics of these rocks were inherited from their mantle source region. There are four possibilities for achieving the enriched isotopic features: (1) crustal contamination; (2) a mantle wedge affected by fluids or melts from dehydration of the subducting oceanic plate; (3) a mantle metasomatized by volatile fluxes from a mantle plume; and (4) the long-term evolution of an older, enriched lithospheric mantle. The first explanation needs significant crustal contamination. However, it is precluded by the trace element and Nd isotopic features as discussed above. Metasomatism by melt and/or fluids can happen in subduction zones where the melts of the subducted sediments and the fluids released during the melting may have enriched Nd isotopic features. However, the enrichment of Nd isotopes may not be significant because depleted sub-arc mantle is the dominant source for the rocks. In addition, interaction and mixing of the mantle wedge and the slab components would have produced a series of mixtures with different Sm/Nd ratios, which are

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Fig. 7. Crustal contamination discriminative diagrams for the volcanic rocks of the Xiong’er Group.

correlated with the proportions of the slab component and the presence of different metasomatic mineral assemblages. The metasomatism can take place at oceanic and continental arcs. However, the Xiong’er volcanic rocks has constant Sm/Nd ratios of 0.17–0.19. The inherited old zircons from the rocks (this work and Zhao et al., 2002) and the minor crustal contamination preclude the oceanic arc environments. In addition, the primitive-normalized trace element patterns of the volcanic rocks are obviously different from the typical oceanic island basalts in their evident negative Nb anomalies. Continental arc magmatism could not generate so unradiogenic Nd

isotopic signatures at relatively constant SiO2 contents. Metasomatism of the lithospheric mantle by volatile fluxes from mantle plumes has been proposed to be also a common phenomenon, and probably with effects similar to subduction-derived metasomatism (e.g., Baker et al., 1998). Separation of carbonatitic melts and hydrous fluids from mantle plumes may affect the composition of lithospheric mantle. However, the homogeneous geochemical features and the absence of alkaline mafic rocks of the Xiong’er volcanic rocks suggest the mantle plume might be impossible. More geological evidence and isotopic data are necessary to confirm this

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be precluded due to the high SiO2 contents if the local lower crust was composed of immature, isotopically juvenile materials at Late Archean. 5.4. Tectonic setting

Fig. 8. Fractional crystallization discriminative diagrams for the volcanic rocks of the Xiong’er Group. (a) Zr/Nb–Zr diagram; (b) La/Sm–La diagram.

idea. However, the enriched isotopic and elemental characteristics of the Xiong’er volcanic rocks can be well explained by the partial melting of a long-term evolved old mantle source. The two-stage whole-rock Nd and zircon Hf model ages of the Xiong’er volcanic rocks are both Late Archean. Considering the minor crustal contamination, an enriched lithospheric mantle with a Late Archean age may be a possible magma source for the volcanic rocks. The models invoking partial melting of Late Archean metasomatized mantle and possibly subsequent mixing between mantleand crust-derived melts include: (1) Origin of the volcanic rock suite by fractional crystallization of the upper mantle-derived mafic magmas. (2) Formation of the mafic and intermediate volcanics by fractional crystallization of the mantle-derived magmas, and origin of the rhyolites by crustal anatexis during underplating of the continental crust by mafic magmas. (3) Generation of the large volume of andesitic magmas by mixing of mantle-derived mafic magmas with crust-derived felsic magmas. The homogeneous Nd isotopic characteristics of the Xiong’er volcanic rocks and especially the overlapping between the andesites and rhyolites as well as the limited cropping area of the rhyolites preclude the last two models. It thus can be deduced that all of the volcanic rocks of the Xiong’er Group may be products of fractional crystallization of a common mantle-derived mafic magmas. The incorporation of some lower crustal materials could not

Several petrogenetic models have been proposed to explain the genesis of the Xiong’er volcanic rocks. The first one is an “arc” model in which it was argued that the volcanic rocks formed in a continental arc setting (Jia, 1987; Hu and Lin, 1988), representing the out-boarding of the Columbia supercontinent (Zhao et al., 2009). A second model can be summarised as variations of a “rift” model which suggests that the eruption of the volcanic rocks was related to decompression melting of the mantle under rifting (Sun et al., 1981; Zhang, 1989; Yang, 1990; Zhai et al., 2000; Zhao et al., 2002) or a mantle plume (Peng et al., 2008), rather than plate subduction. The negative Nb anomalies of the rocks and the presence of large amounts of andesitic material have been regarded as evidence for the “arc” model (Jia, 1987; Hu and Lin, 1988; He et al., 2008; Zhao et al., 2009). However, the timing of the primary subduction process that led to these negative Nb anomalies is unclear. It has been proposed that such anomalies can be generated by the melting of metasomatized lithospheric mantle after several hundred million years, such as in the case of the Scourie dike swarm of Scoland (Tarney and Weaver, 1987), the ca. 1080 Ma low-Ti tholeiitic basalts of the Stuart and Kulgera of central Australia (Zhao and McCulloch, 1993), and the ca. 1830 Ma ultrapotassic rocks of the Christopher Island Formation of the western Churchill Province (Peterson et al., 1994; Cousens et al., 2001). The negative Nb anomalies of the Xiong’er volcanic rocks suggest a metasomatized magma source. The anomalies could not be obtained by crustal contamination of a mantle-derived OIB-like magma because evidence of only the minor crustal contamination is present, suggesting that the Nb anomalies were inherited from the magma source. However, the trace element concentrations and normalized patterns are not similar to the typical oceanic- and continental-arc basalts (Fig. 5b, d and f), which suggests that the Xiong’er volcanic rocks with negative Nb anomalies might be the products of the reworking of early, rather than juvenile, arc materials. Moreover, the andesites are not always formed in the subduction zones or active continental margins. It can also be resulted from the fractional crystallization of basaltic magmas and have no special diagnostic implications for the tectonic settings (e.g. Condie, 1989; Curtis et al., 1999; Hollings and Kerrich, 1999; Iacumin et al., 2001). As discussed above, the volcanic rocks of the Xiong’er Group have obvious trends of fractional crystallization of a common mafic source. It is argued that the presence of the andesitic rocks has no special tectonic significance. Some authors believed that the Xiong’er volcanic rocks have been formed in a “rift” setting (i.e. Zhao et al., 2002 and references therein). However, the “rift” model did not indicate when and how the geochemical enrichment for the Xiong’er volcanic rocks happened. Moreover, volcanic rocks associated with intracontinental rifts generally display a typically bimodal mafic–felsic suites with alkaline and tholeiitic basalts and ryholitic rock associations, or alkaline rock such as carbonatites, nephelinites, and alkali-basalts. However, the volcanic rocks are dominated by the basaltic andesites and andesites, with minor rhyolites (Zhao et al., 2002), and the mafic samples of the Xiong’er Group are subalkaline, rather than alkaline. Their trace element patterns indicate enriched lithospheric mantle and rule out asthenospheric mantle source that produce typical rifting-related mafic rocks (i.e. OIB, CFB). Since the intermediate and acid rocks are the products of fractional crystallization of more mafic rocks, the rock association of the Xiong’er volcanic rocks could not be regarded as typical bimodal magmatism. Furthermore, bimodal magmatic associations cannot be used as unequivocal evidence of anorogenic rifting, a point well

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Fig. 9. Major and trace elements vs. Mg# plot for the volcanic rocks of the Xiong’er Group.

illustrated by the strongly bimodal magmatic assemblage in arc lavas of North Island, New Zealand (Hamilton, 1988). The evident depletion of Nb and Ta suggest the volcanic rocks have arc-like feature of trace elements. They are plotted in the volcanic arc and N-MORB areas in the Ti/Y–Nb/Y diagram (Fig. 10a), precluding the intraplate magmatism for them. In the Ti–Zr diagram (Fig. 10b), the volcanic rocks are plotted in the evolved area and define an evolved tendency from basaltic andesites to

andesites and then to the rhyolites. The observed geochemical and isotopic signatures are best explained in terms of partial melting of an isotopically evolved, subduction-modified subcontinental lithospheric mantle that was isolated for several hundred million years prior to partial melting. The precursor subduction and related arc magmatism in the area may have happened in Late Archean as indicated by the whole-rock Nd and zircon Hf isotopes. The extensive eruption of the Xiong’er volcanic rocks could not be regarded

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Fig. 10. Tectonic discriminative diagrams for the volcanic rocks of the Xiong’er Group. (a) Ti/Y–Nb/Y plot (after Pearce, 1982); (b) Ti–Zr plot (after Pearce, 1982 and Condie, 1989). Symbols as in Fig. 8.

as a juvenile crustal growth event due to their enriched Nd and Hf isotopic features and relatively high trace element concentrations compared to the typical oceanic arc basalts. Subduction zone and mantle plume are the two major regimes where juvenile crust grow significantly (Condie, 1998; Zheng et al., 2008). The enriched, evolved Xiong’er volcanic rocks may have been generated by the reworking of the early arc materials during the breakup of the Columbia supercontinent. Extensional collapse of collisional orogens may be responsible for partial melting of the older, enriched subcontinental lithospheric mantle (Zhao and Zheng, 2009). The distinct geochemical features such as low MgO and high Zr concentrations suggest that partial melting probably occurred at a depth above the garnet stability field. In addition, the melting temperature may have been depressed if the lithospheric mantle was hydrous due to previous subduction-related metasomatism. However, the Xiong’er volcanic rocks have an extensive area, indicating great heat inputs. At 1760 Ma, accompanying the breakup of the Columbia supercontinent, crustal extension easily took place at mobile zones or thickened orogens. Stretching and thinning of such hydrous lithosphere and the collapse of orogens after the closure of oceanic basin would allow hot asthenospheric mantle material to flow beneath the lithosphere and would result in decompression melting. The potassic chemical features indicated the high-degree of partial melting for them. The higher abundances of LREE and LILE, as well as the lower εNd (t) values, are interpreted to reflect a greater contribution of recycled continental materials. The mafic rocks of the Xiong’er Group have evaluated SiO2 contents with the lowest SiO2 content being 52.10 wt% of this work, suggest the possibility of the incorporation of some lower crust materials in the magma source of the Xiong’er volcanic rocks. 5.5. Implication for the subduction style of Archean The mechanism of subduction at plate margins in the Precambrian remains controversial (Condie, 1989; Zhao and McCulloch, 1993; Stern, 2005). Our study indicates that the Xiong’er volcanic rocks were generated from the partial melting of lithospheric mantle modified by subduction processes closely associated with major periods of crust formation at Late Archean. This provides further support for the view that subduction is the dominant process responsible for continental growth and evolution since Late Archean. Similar processes of Late Archean subduction-modified partial melted in absolutely young periods have been reported in the western Churchill Province and other areas (Peterson et al., 1994; Cousens et al., 2001 and references therein).

As discussed above, the Xiong’er volcanic rocks that have enriched and homogeneous Sr–Nd isotopic features have been formed by partial melting of the late Archean subduction-modified lithospheric mantle. This indicates the subduction event at late Archean. The Xiong’er volcanic rocks cover a triangular area of ca. 60,000 km2 , rather than a linear profile like the Andean-type subduction, suggesting that the early subduction-related metasomatism may take place within a large area. The homogeneous metasomatism over a large area rules out the high- or low-angle subduction that will generate a linear distribution of volcanic rocks. However, the flat subduction is an alternative explanation for the genesis of extensive metasomatism and the subsequent or later eruption of arc-like volcanic rocks. Similarly, flat subduction at late Archean has been proposed by Cousens et al. (2001) based on the studies of Paleoproterozoic ultra-potassic rocks (>240,000 km2 ) of the western Churchill Province. Therefore, we suggest that the flat subduction may be dominant in the Precambrian crustal evolution. 6. Conclusions (1) The volcanic rocks of the Xiong’er Group give a LA-ICP-MS zircon U–Pb age of 1763 ± 15 Ma. They have varied geochemical compositions ranging from mafic to rhyolitic. They have evolved geochemical features (low MgO and Mg# ), enriched, and homogeneous Nd isotopic characteristics and arc-like trace elemental signatures (Nb and Ta depletions). (2) The volcanic rocks of the Xiong’er Group may be generated from the high-degree partial melting of Late Archean subductionmodified lithospheric mantle in an extensional setting during the breakup of the Columbia supercontinent. The elevated SiO2 contents and evolved geochemical features of the Xiong’er volcanic rocks indicate the possible incorporation into their source of lower crust materials. (3) The extensive early subduction-related metasomatized magma source for the Xiong’er volcanic rocks suggests a flat subduction model for the plate-margin magmatism in the Late Archean. Acknowledgements This research was financially supported by Major State Basic Research Development Program of China (973 Program) (Grant No. 2006CB403506) and the State Key Laboratory of Mineral Deposit Research, Nanjing University (Grant No. 2008-III-01). We thank L.W. Qiu and T. Yang (NJU) for major and trace element analyses, respectively. The first author appreciates the comments and corrections of Prof. Y.-F. Zheng (USTC) on the first draft and discussions with Prof. G.C. Zhao (HKU) and Dr. L.H. Chen (NJU). Thoughtful and

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