Melting of Enriched Archean Subcontinental Lithospheric Mantle: Evidence from the Ca

Precambrian Research 182 (2010) 204–216

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Precambrian Research

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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 State Key Laboratory for Mineral Deposits Research, Department of Earth Sciences, Nanjing University, Nanjing 210093, PR China b 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 article info abstract

Article history: The Xiong’er Group is an important geologic unit in the southern margin of the North China Craton. It is Received 14 December 2009 dominated by the volcanic rocks, dated at 1763 ± 15 Ma, that have SiO2 contents ranging from 52.10 wt% Received in revised form 22 July 2010 to 73.51 wt%. These volcanic rocks are sub-alkaline and can be classified into three subgroups: basaltic Accepted 13 August 2010 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 adja- cent elements are evident for all the samples. The volcanic rocks are evolved with low MgO contents Keywords: (0.29–5.88 wt%) and accordingly low Mg# values of 11–53. The Nd isotopes are enriched and show a weak Petrogenesis ε − − ε − ± Volcanic rocks variation with Nd(t)= 7.12 to 9.63. Zircon Hf isotopes are also enriched with Hf(t)= 12.02 0.45. Lithospheric mantle The volcanic rocks of the Xiong’er Group are interpreted to represent fractional crystallization of a com- Extension mon mantle source. The volcanic rocks might have been generated by high-degree partial melting of North China Craton 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 which may led to the debate when assessing the petrogenesis and tectonic settings of the rocks. Many of the rocks are enriched in Juvenile arc magmatism is unequivocally characterized by K2O, and are suggested to have formed due to low-degree melting depleted isotopic compositions and negative anomalies in the of subcontinental lithospheric mantle that was metasomatized by high field strength elements (i.e. Nb, Ta) relative to large ion aqueous fluid (Arnaud et al., 1992; Turner et al., 1993, 1996; Miller lithophile (LIL) elements (e.g. La and K), representing the growth et al., 1999; Ding et al., 2003), although the possibilities of con- of juvenile crust. However, igneous rocks with arc-like geochemi- tributions from mafic granulitic or eclogitic lower-crustal source cal features are not definitely generated in island-arc settings, but (Cooper et al., 2002) and hot, metasomatized asthenosphere (Guo could have resulted from several other processes, such as crustal et al., 2006) have been suggested. The metasomatism of subconti- contamination, magma mixing, reworking of old arc materials or nental lithospheric mantle may take place due to the incorporation melting of old, metasomatized mantle. Especially for the basaltic of crustally derived melts and/or aqueous fluids driven out from and basaltic andesitic rocks that occur in the previously devel- the down-going slab during the subduction of oceanic crust. Alter- oped arc-continent collisional belts, they generally have arc-like natively, the old sub-arc mantle that had accreted to the continent geochemical signatures but with unradiogenic Nd isotopic ratios, may potentially be another source for the type of rocks if, follow- ing the early metasomatism, no melt was immediately extracted (Zhang et al., 2009). For example, partial melting of two differ-

∗ ent sources have been proposed for the petrogenesis of Tibetan Corresponding author at: State Key Laboratory for Mineral Deposits Research, potassic rocks: (1) the enriched lithospheric mantle that has been Department of Earth Sciences, Nanjing University, 22 Habkou Road, Jiangsu Province, Nanjing 210093, PR China. Tel.: +86 25 83686336; fax: +86 25 83686016. isolated from the convecting asthenosphere for a long period of E-mail address: [email protected] (X.-L. Wang). time (Turner et al., 1996); and (2) Cenozoic mantle lithosphere

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 the Xiong’er Group. From this it is proposed that ancient (Archean) (Ding et al., 2003). Distinguishing the timing and mechanism for lithospheric mantle was melted in a post-orogenic extensional set- metasomatism of the mantle source is critical for understanding ting to generate the volcanics. the petrogenesis and tectonic settings of the type of mafic rocks (Zhao and Zheng, 2009), especially for the geological evolution of 2. Geological background old continental margins that lack good geophysical constraints. The volcanic rocks of the Xiong’er Group occur in the southern The mainland of China is composed of two blocks, the North margin of North China Craton (Fig. 1a), occupying a large erup- China and South China, which are separated by the Qinling and tion area of over 60,000 km2 (Zhao et al., 2009). In the last three Dabie-Sulu orogenic belts. The Qinling Orogen experienced a decades, there has been considerable debate over the petrogene- complex divergent and convergent continental history onto the sis and tectonic settings of the Xiong’er volcanic rocks (see Zhao et northern margin of the Yangtze Block (Meng and Zhang, 2000; al., 2009, and references therein). Some authors suggest that the Ratschbacher et al., 2003; Lu et al., 2004). The North China Cra- rocks were formed in a rift (e.g. Sun et al., 1981; Zhang, 1989; Yang, ton can be subdivided into the Eastern and Western Blocks that 1990; Zhai et al., 2000; Zhao et al., 2002) or in a mantle plume were amalgamated along the ∼1850 Ma Trans-North China Oro- (e.g. Peng et al., 2008) environment, while others prefer a conti- gen (Fig. 1a; Zhao et al., 2001). The Xiong’er Group is a distinct nental arc setting (e.g. Hu and Lin, 1988; Jia, 1987; He et al., 2008; geological unit on the southern margin of the North China Craton Zhao et al., 2009). However, key issues remain over two aspects: (1) occurring in a roughly triangular area bounded to the south by the what are the roles of the continental lithospheric mantle and crust Luonan-Luanchuan Fault and the Qinling Orogen (Fig. 1a and b). in the petrogenesis of these volcanic rocks, and (2) when did the The Xiong’er Group is dominantly composed of lavas interlay- proposed subduction-related metasomatism of their mantle source ered with minor pyroclastic rocks and sedimentary rocks (<5%) take place? Based on available published and our new geochemical and covers an area of more than 60,000 km2. The Xiong’er Group data, this paper presents a new synthesis of petrology, geochronol- is stratigraphically constrained in that it unconformably over- ogy, geochemistry and isotopes of the volcanic rocks comprising lies Archean to Paleoproterozoic basement rocks that include 206 X.-L. Wang et al. / Precambrian Research 182 (2010) 204–216 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 Na2O 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 spilite- keratophyre 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. Fig. 2. Representative cathodo-luminescence (CL) images for the zircons from the Based on whole-rock Rb–Sr isochrons and single zircon U–Pb sample 07XE-13. The dating spots are denoted by the open circles with the spot dating methods, the volcanic rocks of the Xiong’er Group have been number and analyzed 206Pb/238U ages. The broken circles represent the analytical ε previously dated within the period of 1850–1400 Ma (see Zhao and areas of Hf isotopes with the numbers indicating the Hf(t) values. 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 from the standard laser-ablation cell, and then mixed with the Ar in later tectonic activities, and the single zircon U–Pb dating with gas via a mixing chamber before entering into the ICP-MS torch. results concentrated at ca. 1850–1730 Ma lacks a detailed analysis All of the spot analyses were carried out using a beam with a of the internal structure of zircons. Moreover, it is difficult to imag- 24 ␮m diameter and a repetition rate of 5 Hz and 60% energy, ine that a single magmatic event could last over 400 Myr. In the which produce an energy density of 0.121 J/cm2. U–Pb fractiona- last five years, more precise dating analyses have been carried out tion was corrected using zircon standard GEMOC GJ-1 (207Pb/206Pb on the volcanic rocks. Using SHRIMP and LA-ICP-MS in situ zircon age of 608.5 ± 1.5 Ma; Jackson et al., 2004) and analytical accuracy U–Pb dating techniques, more modern results suggest a shorter was monitored using zircon standards Mud Tank (intercept age duration of 1780–1750 Ma for the generation of most of the vol- of 732 ± 5Ma;Black and Gulson, 1978). Zircon samples were ana- canic rocks of the Xiong’er Group (e.g. Zhao et al., 2004; He et al., lyzed in runs of ca. 15 analyses which included 5 zircon standards 2009). Minor felsic volcanic rocks in the area yield a SHRIMP zircon and 10 sample points. U–Pb ages were calculated from the raw U–Pb age of 1450 ± 31 Ma (He et al., 2009), possibly representing signal data using the on-line software package GLITTER (ver. 4.4) a later magmatic activity in the southern margin of North China (www.mq.edu.au/GEMOC). Because 204Pb could not be measured Craton. due to low signal and interference from 204Hg in the gas supply, common lead correction was carried out using the EXCEL program ComPbCorr#3 15G (Andersen, 2002). However, no analysis has 3. Analytical methods been corrected effectively because of the concordance between different U–Th–Pb isotopic ratios and the very low common lead. The We selected representative sample of fresh rhyolitic porphyry Th and U concentrations of the zircons were calculated according (07XE-13, N33◦5454.1, E112◦2733.09) for LA-ICP-MS zircon to the comparison of relative signal intensity between the standard U–Pb dating. Sampling locations are shown in Fig. 1. Zircons were zircon GJ-1 in our laboratory (Th = 8 ppm, U = 330 ppm) and zircon separated using conventional heavy liquid and magnetic tech- samples using the EXCEL program Data Templatev2b from GEMOC. niques, mounted in epoxy resin and polished down to expose Major elements were analyzed using an ICP-AES (JY38S) at the the grain centers. Cathodo-luminescence (CL) images (Fig. 2) were State Key Laboratory for Mineral Deposits Research, NJU, follow- acquired with a Mono CL3+ (Gatan, U.S.A.) attached to a scanning ing the procedures described by Liu et al. (1999), with an analytical electron microscope (Quanta 400 FEG) at the State Key Labora- precision generally less than 2% (RSD). Rare earth and other trace tory of Continental Dynamics, Northwest University, Xi’an. U–Pb elements were analyzed using ICP-MS (Finnigan MAT-Element zircon dating was carried out at the State Key Laboratory for Min- 2) techniques at the State Key Laboratory for Mineral Deposits eral Deposits Research, Nanjing University (NJU), using an Agilent Research, NJU. Analytical precision for most elements by ICP-MS 7500a ICP-MS attached to a New Wave 213 nm laser ablation sys- is better than 5% with analytical procedures similar to Wang et al. tem. He carrier gas was used to transport the ablated sample (2004). X.-L. Wang et al. / Precambrian Research 182 (2010) 204–216 207

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 86Sr/88Sr = 0.1194. The long-term 87Sr/86Sr 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 146Nd/144Nd = 0.7219. The long-term 143Nd/144Nd analyzing values of the international standard JNdi-1 is 0.512121 ± 0.000016 (2, n = 67), identical to ± ε the value (0.512115 0.000007) of Tanaka et al. (2000). The Nd(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 were carried out in situ using a New Wave 213 nm solid laser ablation system attached to a New Plasma MC- ICP-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 Fig. 3. LA-ICP-MS zircon U–Pb zircon concordia plots and recalculated weighted 206 238 et al. (2004) and Tang et al. (2008). Most analyses were carried mean Pb/ U ages for the rhyolitic porphyry 07XE-13. out using a beam with a ca. 40 ␮m diameter and a 10 Hz repeti- 176 172 tion rate. A new TIMS determined value of 0.5887 for Yb/ Yb basaltic and andesitic samples, some major elements (i.e. Na2O) was applied for correction (Vervoort et al., 2004). Zircon 91500 was may have been mobile after the eruption of the rocks. Nb, Y, Zr 176 177 used as the reference standard, with a recommended Hf/ Hf and TiO2 are usually considered as being immobile during alter- ratio of 0.282302 ± 8(Goolaerts et al., 2004). The decay constant ation. In the Zr/TiO2–Nb/Y diagram (Fig. 4b), the rocks plot in the for 176Lu of 1.865 × 10−11 year−1 proposed by Scherer et al. (2001) areas of basalts, andesites and dacites that belong to sub-alkaline ε was adopted in this work. Hf values were calculated according to basalts. According to the SiO2 contents, the volcanic rocks could be the chondritic values of Blichert-Toft et al. (1997). subdivided into four subgroups. Group 1 comprises rare volcanic In case of any influence from sampling limitations, we collected rocks with SiO2 lower than 52 wt% found in the available published all of the available published geochemical data when discussing the data (Zhao et al., 2002; He et al., 2008, 2010) but not in this work. petrogenesis of the volcanic rocks of the Xiong’er Group. These data Group 2 has SiO2 contents ranging from 52 wt% to 57 wt%, and is are plotted with our results for comparison. 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 4. Results K2O contents and belong to the high-K calc-alkaline or shoshonitic series in the K O–SiO diagram (Fig. 4c), apart from three samples 4.1. U–Pb dating 2 2 that plot amongst low-K tholeiitic series. In case of the alteration and the variation of Na O contents, the AFM diagram may be Zircons from the sample 07XE-13 are light pink to transparent, 2 questionable. However, the FeO* and MgO contents are relatively euhedral, with lengths of 50–150 ␮m and widths of 40–100 ␮m immobile during the alteration process or low-grade metamor- and are mainly stubby prisms with a broad oscillatory zoning phism. In the FeO*/MgO–SiO diagram (Fig. 4d), the rocks from (Fig. 2). Some zircon grains are embayed and some have a core- 2 the Xiong’er Group plot in the tholeiitic series suggesting that they rim structure. Twenty spots were analyzed from 20 zircons, and should be tholeiitic rather than calc-alkaline series. The basaltic the analytical results are listed in Table 1. Th contents range from andesites of the Xiong’er Group have moderate MgO contents 19 ppm to 966 ppm, and U ranges from 20 ppm to 1118 ppm in the (Table 2; 3.06–5.88 wt%), producing evolved Mg# values of 37–53. analyses, generating Th/U ratios ranging from 0.44 to 2.09. Three The andesites have relatively lower MgO contents (1.00–2.85 wt%) analyses are variably discordant with relatively old 207Pb/206Pb and Mg# (24–35), while the rhyolitic rocks have the lowest MgO ages (ca. 2334–2116 Ma). One analysis spot with old 207Pb/206Pb contents of 0.29–0.41 wt%. The TiO contents of the andesites and ages (ca. 2225 Ma) is concordant. The other 21 analyses are concor- 2 basaltic andesites range from 0.95 wt% to 1.70 wt%. dant and yield a weighted average 206Pb/238U age of 1763 ± 15 Ma Bearing in mind the significant degree of alteration, only those (95% confidence, MSWD = 0.22; Fig. 3), and a 207Pb/206Pb age of immobile major element oxides (MgO, Al O and TiO ), compat- 1772 ± 26 Ma (95% confidence, MSWD = 0.025). Considering the 2 3 2 ible trace elements (Ni, Cr), HFSE (Nb, Th, Ta, Zr and Hf) and REE mean 206Pb/238U age has a lower error, the age of 1763 ± 15 Ma (except Eu) will be further used to draw inferences on the pet- is interpreted as the crystallization age of the rhyolitic porphyry. rogenesis of the volcanic rocks and their tectonic environment. The age is consistent with the results (ca. 1780–1750 Ma) of Zhao The Ba and K O contents have roughly negatively correlations et al. (2004) and He et al. (2009). 2 with the Mg# values and positively correlations with the Zr and SiO2 contents (not shown here), suggesting they could also been 4.2. Major and trace element geochemistry used when discussing the petrogenesis of the Xiong’er volcanic rocks. The Xiong’er volcanic rocks have highly variable concen- Although the compositions of the Xiong’er volcanic rocks vary trations of SiO2, ranging from 52.10 wt% to 73.51 wt%. In the greatly, their REE contents and chondrite-normalized patterns are (Na2O+K2O)–SiO2 diagram (Fig. 4a), they can be distinguished as consistent with each other (Fig. 5a, c and e). The light rare earth basaltic andesites, basaltic trachy-andesites, andesites, dacites and elements (LREE) are enriched relative to the heavy REE, resulting in rhyolites. Considering the intense alteration visible in thin section high (La/Yb)N of 8.10–10.93. Negative Eu anomalies occur in some and the relatively high loss on ignition values (>3 wt%) in some samples interpreted to reflect either plagioclase fractionation at 208 X.-L. Wang et al. / Precambrian Research 182 (2010) 204–216

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) K2O–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+ rela- 4.3. Whole-rock Sr–Nd isotopes tive to the other trivalent lanthanides during alteration processes, or both. The Eu negative anomalies are weak (Eu/Eu* = 0.66–0.86) A total of eight samples were analyzed for Sr–Nd isotope anal- for the basaltic andesites and andesites but moderate for the rhy- yses (Table 3). Apart from one sample (07XE-04, Rb/Sr = 0.50), the olites (Eu/Eu* = 0.50–0.53). The HREE patterns for all the samples basaltic andesites and andesites (Rb/Sr = 0.11–0.16) have consis- 87 86 are nearly flat ((Dy/Yb)N = 0.99–1.30). Similarly, the trace elements tent initial Sr/ Sr ratios of 0.706–0.709. The rhyolitic rocks have also show identical primitive-mantle normalized patterns for all extremely low initial 87Sr/86Sr ratios lower than 0.700, which may the rock types (Fig. 5b, d and f), displaying evident positive Ba and be generated from their high Rb/Sr ratios (1.09–1.26). The vol- K2O anomalies and impoverishment in Nb, Ta, Sr, P and Ti relative to canic rocks of the Xiong’er Group show identical Nd isotopes with 143 144 ε the neighboring other trace elements of similar degree of incom- a narrow initial Nd/ Nd ratios (0.50987–0.51000). The Nd(t) patibility. The negative anomalies of Th and U are greater in the values range from −7.12 to −9.63, indicating an enriched magmatic basaltic andesites and andesites than in those in the rhyolites. The source for the volcanic rocks. The two-stage Nd model ages are Late basaltic andesites and andesites can be distinguished from ocean Archean between 2750 Ma and 2920 Ma. island basalts (OIB) by their high contents of large ion lithophile elements (LILEs) and remarkable Nb and Ta negative anomalies. These 4.4. Zircon Lu–Hf isotopes characters of the Xiong’er volcanic rocks are also different from the typical oceanic island-arc basalts and continental island-arc basalts, A total of 20 zircons from the dated sample 07XE-13 were fur- though the negative Nb and Ta troughs are similar (Fig. 5b and ther analyzed for the Lu–Hf isotopes with results listed in Table 4. d). It is interesting to note that the primitive-mantle normalized Initial 176Hf/177Hf ratios were calculated using their determined patterns of the basaltic andesites and andesites are similar to the average U–Pb zircon ages. Apart from three spots, which have rel- average upper continental crust (Fig. 5b and d), probably indicating atively old age (206Pb/207U ages of 2329–2116 Ma), the other spots the recycling of crust materials in the petrogenesis of the volcanic show consistent 176Hf/177Hf isotopic ratios, which result in a nor- ε rocks of the Xiong’er Group. mal distribution curve for the Hf(t) values with a mean value of X.-L. Wang et al. / Precambrian Research 182 (2010) 204–216 209

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 Because the upper crust and moderate-degree partial melts of ε − is roughly consistent with the whole-rock Nd(t) value ( 9.05) of the lower crust are enriched in LREE but depleted in high field the sample. Correspondingly, the two-stage Hf model ages for the strength elements (HFSE), crustal contamination will lead to a large zircons range from 3000 Ma to 3313 Ma, slightly older than the Nd variation of La/Nb ratios and a positive correlation between La/Nb ε model ages. and La/Sm. Moreover, the Nd(t) values can shift toward much lower when the continental crust increases in thickness and in age, 5. Discussion and plausible crustal contaminations of a mantle-derived magma would result in much higher values of SiO2, Zr and lower MgO than 5.1. Crustal contamination those observed. The basaltic andesites and andesites of the Xiong’er Group have no significant correlation between La/Nb and La/Sm ε Considering the very large area over which the Xiong’er volcanic ratios (Fig. 7a), Nd(t) values with MgO (Fig. 7b), Nb/Nb* (Fig. 7c) rocks crop out, their crust-like trace element patterns and their and 1/Nd (Fig. 7d) and SiO2 (Fig. 7e), only showing roughly positive enriched Nd isotopic signatures, it is necessary to evaluate the influ- or negative correlations. ence of crustal contamination during the magma ascent, and/or This probably suggests that the crustal contamination may not temporary residence in magma chambers within continental be an important factor in their petrogenesis. The basaltic andesites crust. and andesites have relatively lower SiO2 contents but similar Nd 210 X.-L. Wang et al. / Precambrian Research 182 (2010) 204–216

rocks of the Xiong’er Group. The jumbled distribution in the Na2O 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 K2O, 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 TiO and P O5, indicating the fractional crystalliza- Fig. 6. Frequency distribution of the εHf(t) values for the spots of the sample 07XE- 2 2 13. tion of apatite. A negative correlation between Al2O3/TiO2 ratios and Mg# is evident when Mg# < 30, whereas the correlation turn to be positive when Mg# > 30, which may suggest the fractional contents compared with the rhyolitic rocks with high SiO contents, 2 crystallization of plagioclase and clinopyroxene, respectively. Fur- also precluding significant crustal contamination. In addition, there thermore, an obvious Eu anomaly in all the Xiong’er volcanic rocks is no correlation between the La/Sm and Zr/Nb ratios (Fig. 7f), which (Fig. 5a, c and e) indicates that plagioclase fractionation was an seems to indicate that the crustal contamination may be minor. important process in their magmatic evolution. Conversely, it may be caused by the constant Zr/Nb ratios that were mainly controlled by the fractional crystallization. Overall, the iso- 5.3. Magma source: enriched Archean lithospheric mantle topic and trace elemental signatures of the basaltic andesites and andesites demonstrate an origin from an enriched mantle reservoir. The above discussion suggests that the varied compositions Crustal contamination, if any, was minor and limited to the earliest of the volcanic rocks from the Xiong’er Group was generated by lavas. the fractional crystallization and magmatic evolution accompa- nied with minor crustal contamination. The andesites and rhyolites 5.2. Crystallization or partial melting? may have resulted from the fractional crystallization of a precursor basaltic melt. The sub-parallel trace and REE element distribution The Xiong’er volcanic rocks have compositions ranging from patterns (Fig. 5) for the rocks of different types suggest a common basaltic to rhyolitic, which can be produced by fractional crystal- mantle source for them. The Zr/Nb ratios could not be affected by lization of a basic magma chamber or by partial melting of different the fractional crystallization and partial melting during the pet- magma sources at different levels. If the latter, the volcanic rocks rogenesis of the rocks, and may reflect the characteristics of the would have different evolution trends and different elemental and magma source. The Xiong’er volcanic rocks have constant Zr/Nb isotopic signatures. However, several lines of evidence suggest that ratios ranging from 23.50 to 28.63, close to the mean value for the the fractional crystallization may play an important role in the pet- source of typical N-MORB (Zr/Nb = 30; Weaver, 1991), suggesting rogenesis of the rocks. that the local mantle had a degree of depletion broadly similar to that of the sub-oceanic mantle. However, the volcanic rocks are (1) The ratios of some incompatible elements (i.e. Zr/Nb and La/Sm) characterized by enrichments in the LIL elements and negative Nb which have similar geochemical behavior are constant during and Ti anomalies relative to the neighboring REE, suggesting the fractional crystallization, while the element concentrations will melting of a subduction-modified mantle. The homogeneous unra- change. The Xiong’er volcanic rocks show an obvious fractional ε − − diogenic Nd isotopic characteristics with Nd(t)of 7.12 to 9.63 crystallization tendency in the Zr/Nb–Zr diagram (Fig. 8a). Two indicate the partial melting of an isotopically heterogeneous and samples of Zhao et al. (2002) have relatively higher Zr/Nb ratios, enriched mantle reservoir. We suggest that the isotope charac- which might be generated from the inaccurate analytical Nb teristics of these rocks were inherited from their mantle source concentrations. Similarly, the La/Sm–La diagram (Fig. 8b) also region. defines a fractional crystallization trend for the volcanic rocks, There are four possibilities for achieving the enriched isotopic although a weak partial melting tendency is indicated. features: (1) crustal contamination; (2) a mantle wedge affected by (2) Variations in major element abundances (e.g. fluids or melts from dehydration of the subducting oceanic plate; MgO = 0.29–5.88 wt% and Mg# = 11–53) reflect fractional (3) a mantle metasomatized by volatile fluxes from a mantle plume; crystallization of clinopyroxene. and (4) the long-term evolution of an older, enriched lithospheric ε (3) The negative correlation (Fig. 7e) between SiO2 and Nd(t) mantle. The first explanation needs significant crustal contamina- values also indicate the existence of assimilation-fractional tion. However, it is precluded by the trace element and Nd isotopic crystallization (AFC) processes. features as discussed above. Metasomatism by melt and/or fluids (4) The effects of fractional crystallization for the Xiong’er volcanic can happen in subduction zones where the melts of the subducted rocks are indicated by the compositional variations of both sediments and the fluids released during the melting may have major and trace elements with respect to Mg# as fractionation enriched Nd isotopic features. However, the enrichment of Nd iso- index (Fig. 9). topes may not be significant because depleted sub-arc mantle is the dominant source for the rocks. In addition, interaction and mixing The positive correlations between SiO2, MnO, Al2O3, TiO2 and of the mantle wedge and the slab components would have pro- Mg# suggest normal magmatic evolution trend for the volcanic duced a series of mixtures with different Sm/Nd ratios, which are X.-L. Wang et al. / Precambrian Research 182 (2010) 204–216 211

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 isotopic signatures at relatively constant SiO2 contents. Metaso- presence of different metasomatic mineral assemblages. The meta- matism of the lithospheric mantle by volatile fluxes from mantle somatism can take place at oceanic and continental arcs. However, plumes has been proposed to be also a common phenomenon, the Xiong’er volcanic rocks has constant Sm/Nd ratios of 0.17–0.19. and probably with effects similar to subduction-derived metaso- The inherited old zircons from the rocks (this work and Zhao et al., matism (e.g., Baker et al., 1998). Separation of carbonatitic melts 2002) and the minor crustal contamination preclude the oceanic arc and hydrous fluids from mantle plumes may affect the composition environments. In addition, the primitive-normalized trace element of lithospheric mantle. However, the homogeneous geochemical patterns of the volcanic rocks are obviously different from the typ- features and the absence of alkaline mafic rocks of the Xiong’er vol- ical oceanic island basalts in their evident negative Nb anomalies. canic rocks suggest the mantle plume might be impossible. More Continental arc magmatism could not generate so unradiogenic Nd geological evidence and isotopic data are necessary to confirm this 212 X.-L. Wang et al. / Precambrian Research 182 (2010) 204–216

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

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 sim- Fig. 8. Fractional crystallization discriminative diagrams for the volcanic rocks of ilar to the typical oceanic- and continental-arc basalts (Fig. 5b, d and the Xiong’er Group. (a) Zr/Nb–Zr diagram; (b) La/Sm–La diagram. f), which suggests that the Xiong’er volcanic rocks with negative Nb anomalies might be the products of the reworking of early, rather idea. However, the enriched isotopic and elemental characteristics than juvenile, arc materials. Moreover, the andesites are not always of the Xiong’er volcanic rocks can be well explained by the partial formed in the subduction zones or active continental margins. It can melting of a long-term evolved old mantle source. The two-stage also be resulted from the fractional crystallization of basaltic mag- whole-rock Nd and zircon Hf model ages of the Xiong’er volcanic mas and have no special diagnostic implications for the tectonic rocks are both Late Archean. Considering the minor crustal con- settings (e.g. Condie, 1989; Curtis et al., 1999; Hollings and Kerrich, tamination, an enriched lithospheric mantle with a Late Archean 1999; Iacumin et al., 2001). As discussed above, the volcanic rocks age may be a possible magma source for the volcanic rocks. of the Xiong’er Group have obvious trends of fractional crystalliza- The models invoking partial melting of Late Archean metaso- tion of a common mafic source. It is argued that the presence of the matized mantle and possibly subsequent mixing between mantle- andesitic rocks has no special tectonic significance. and crust-derived melts include: Some authors believed that the Xiong’er volcanic rocks have been formed in a “rift” setting (i.e. Zhao et al., 2002 and refer- (1) Origin of the volcanic rock suite by fractional crystallization of ences therein). However, the “rift” model did not indicate when the upper mantle-derived mafic magmas. and how the geochemical enrichment for the Xiong’er volcanic (2) Formation of the mafic and intermediate volcanics by fractional rocks happened. Moreover, volcanic rocks associated with intra- crystallization of the mantle-derived magmas, and origin of the continental rifts generally display a typically bimodal mafic–felsic rhyolites by crustal anatexis during underplating of the conti- suites with alkaline and tholeiitic basalts and ryholitic rock asso- nental crust by mafic magmas. ciations, or alkaline rock such as carbonatites, nephelinites, and (3) Generation of the large volume of andesitic magmas by mix- alkali-basalts. However, the volcanic rocks are dominated by the ing of mantle-derived mafic magmas with crust-derived felsic basaltic andesites and andesites, with minor rhyolites (Zhao et magmas. al., 2002), and the mafic samples of the Xiong’er Group are sub- alkaline, rather than alkaline. Their trace element patterns indicate The homogeneous Nd isotopic characteristics of the Xiong’er enriched lithospheric mantle and rule out asthenospheric man- volcanic rocks and especially the overlapping between the tle source that produce typical rifting-related mafic rocks (i.e. OIB, andesites and rhyolites as well as the limited cropping area of the CFB). Since the intermediate and acid rocks are the products of frac- rhyolites preclude the last two models. It thus can be deduced that tional crystallization of more mafic rocks, the rock association of the all of the volcanic rocks of the Xiong’er Group may be products of Xiong’er volcanic rocks could not be regarded as typical bimodal fractional crystallization of a common mantle-derived mafic mag- magmatism. Furthermore, bimodal magmatic associations cannot mas. The incorporation of some lower crustal materials could not be used as unequivocal evidence of anorogenic rifting, a point well X.-L. Wang et al. / Precambrian Research 182 (2010) 204–216 213

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 andesites and then to the rhyolites. The observed geochemical and lavas of North Island, New Zealand (Hamilton, 1988). isotopic signatures are best explained in terms of partial melting of The evident depletion of Nb and Ta suggest the volcanic rocks an isotopically evolved, subduction-modiﬁed subcontinental litho- have arc-like feature of trace elements. They are plotted in the vol- spheric mantle that was isolated for several hundred million years canic arc and N-MORB areas in the Ti/Y–Nb/Y diagram (Fig. 10a), prior to partial melting. The precursor subduction and related arc precluding the intraplate magmatism for them. In the Ti–Zr dia- magmatism in the area may have happened in Late Archean as gram (Fig. 10b), the volcanic rocks are plotted in the evolved indicated by the whole-rock Nd and zircon Hf isotopes. The exten- area and deﬁne an evolved tendency from basaltic andesites to sive eruption of the Xiong’er volcanic rocks could not be regarded 214 X.-L. Wang et al. / Precambrian Research 182 (2010) 204–216

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 As discussed above, the Xiong’er volcanic rocks that have Hf isotopic features and relatively high trace element concentra- enriched and homogeneous Sr–Nd isotopic features have been tions compared to the typical oceanic arc basalts. Subduction zone formed by partial melting of the late Archean subduction-modified and mantle plume are the two major regimes where juvenile crust lithospheric mantle. This indicates the subduction event at late grow significantly (Condie, 1998; Zheng et al., 2008). The enriched, Archean. The Xiong’er volcanic rocks cover a triangular area of evolved Xiong’er volcanic rocks may have been generated by the ca. 60,000 km2, rather than a linear profile like the Andean-type reworking of the early arc materials during the breakup of the subduction, suggesting that the early subduction-related meta- Columbia supercontinent. somatism may take place within a large area. The homogeneous Extensional collapse of collisional orogens may be responsible metasomatism over a large area rules out the high- or low-angle for partial melting of the older, enriched subcontinental litho- subduction that will generate a linear distribution of volcanic rocks. spheric mantle (Zhao and Zheng, 2009). The distinct geochemical However, the flat subduction is an alternative explanation for the features such as low MgO and high Zr concentrations suggest genesis of extensive metasomatism and the subsequent or later that partial melting probably occurred at a depth above the gar- eruption of arc-like volcanic rocks. Similarly, flat subduction at late net stability field. In addition, the melting temperature may have Archean has been proposed by Cousens et al. (2001) based on the been depressed if the lithospheric mantle was hydrous due to pre- studies of Paleoproterozoic ultra-potassic rocks (>240,000 km2)of vious subduction-related metasomatism. However, the Xiong’er the western Churchill Province. Therefore, we suggest that the flat volcanic rocks have an extensive area, indicating great heat inputs. subduction may be dominant in the Precambrian crustal evolution. At 1760 Ma, accompanying the breakup of the Columbia supercontinent, crustal extension easily took place at mobile zones or 6. Conclusions thickened orogens. Stretching and thinning of such hydrous lithosphere and the collapse of orogens after the closure of oceanic (1) The volcanic rocks of the Xiong’er Group give a LA-ICP-MS basin would allow hot asthenospheric mantle material to flow zircon U–Pb age of 1763 ± 15 Ma. They have varied geochem- beneath the lithosphere and would result in decompression melt- ical compositions ranging from mafic to rhyolitic. They have ing. The potassic chemical features indicated the high-degree of evolved geochemical features (low MgO and Mg#), enriched, partial melting for them. The higher abundances of LREE and LILE, and homogeneous Nd isotopic characteristics and arc-like trace ε as well as the lower Nd(t) values, are interpreted to reflect a greater elemental signatures (Nb and Ta depletions). contribution of recycled continental materials. The mafic rocks of (2) The volcanic rocks of the Xiong’er Group may be generated from the Xiong’er Group have evaluated SiO2 contents with the lowest the high-degree partial melting of Late Archean subduction- SiO2 content being 52.10 wt% of this work, suggest the possibility modified lithospheric mantle in an extensional setting during of the incorporation of some lower crust materials in the magma the breakup of the Columbia supercontinent. The elevated SiO2 source of the Xiong’er volcanic rocks. contents and evolved geochemical features of the Xiong’er volcanic rocks indicate the possible incorporation into their source 5.5. Implication for the subduction style of Archean of lower crust materials. (3) The extensive early subduction-related metasomatized magma The mechanism of subduction at plate margins in the Precam- source for the Xiong’er volcanic rocks suggests a flat subduction brian remains controversial (Condie, 1989; Zhao and McCulloch, model for the plate-margin magmatism in the Late Archean. 1993; Stern, 2005). Our study indicates that the Xiong’er volcanic rocks were generated from the partial melting of lithospheric Acknowledgements mantle modified by subduction processes closely associated with major periods of crust formation at Late Archean. This This research was financially supported by Major State Basic provides further support for the view that subduction is the Research Development Program of China (973 Program) (Grant No. dominant process responsible for continental growth and evo- 2006CB403506) and the State Key Laboratory of Mineral Deposit lution since Late Archean. Similar processes of Late Archean Research, Nanjing University (Grant No. 2008-III-01). We thank subduction-modified partial melted in absolutely young periods L.W. Qiu and T. Yang (NJU) for major and trace element analyses, have been reported in the western Churchill Province and other respectively. The first author appreciates the comments and correc- areas (Peterson et al., 1994; Cousens et al., 2001 and references tions of Prof. Y.-F. Zheng (USTC) on the first draft and discussions therein). with Prof. G.C. Zhao (HKU) and Dr. L.H. Chen (NJU). Thoughtful and X.-L. Wang et al. / Precambrian Research 182 (2010) 204–216 215 constructive reviews of this manuscript by Prof. C.R.L. Friend and Jackson, S.E., Pearson, N.J., Griffin, W.L., Belousova, E.A., 2004. The application of laser an anonymous reviewer are gratefully acknowledged. ablation-inductively coupled plasma-mass spectrometry to in situ U–Pb zircon geochronology. Chem. Geol. 211, 47–69. Jia, C.-Z., 1987. 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