The geochemical characteristics of metabasites with a pseudo-pillow structure from Ganghe, Dabie Orogen, China

Journal of Asian Earth Sciences 63 (2013) 151–161

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The geochemical characteristics of metabasites with a pseudo-pillow structure from Ganghe, Dabie Orogen, China Zhi Xie a,⇑, Bin Wang a,b, Jiangfeng Chen a, Hui Qian a a b

CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China Gold Geological Institute of CAPF, Langfang, Hebei 065000, China

a r t i c l e

i n f o

Article history: Available online 23 May 2012 Keywords: Ganghe metabasites Lens Sr–Nd–Pb isotopes Crustal contamination Dabie Orogen

a b s t r a c t Several metabasite lenses in Ganghe, Central Dabie, that were previously described as pillow lavas are studied by elemental, Sr–Nd–Pb isotopic, and mineral oxygen isotopic analysis as well as zircon SHRIMP U–Pb dating. Zircon U–Pb geochronology results indicate that the protolith emplacement age of these metabasites is approximately 717 ± 38 Ma, consistent with the age of the volcanoclastic rocks in the same unit, and that they experienced the Triassic HP eclogite-facies retrograde metamorphism at 221 ± 2 Ma during exhumation after subduction to mantle depth and peak ultra-high pressure metamorphism. The low d18O values of 5.5‰ to 2.0‰ indicate that the protoliths underwent high temperature meteoric-hydrothermal alteration before subduction but had no seawater interaction. These metabasites had similar formation processes, water–rock interactions and metamorphisms as other eclogite-facies rocks cropped out in the Central Dabie terrain. They showed negative abnormalities in Nb, Sr, and Ti content and positive abnormalities in Ba, Th, and Pb content; they also showed LREE enrichment. The insusceptible Sm–Nd isotopes during metamorphism yielded eNd (t) = 12 to 10 and TDM = 2.2–2.8 Ga for samples from lenses #1 to #3 and 7 to 6 and 2.1–2.2 Ga for lens #4; the samples also showed low radiogenic Pb isotope compositions of (206Pb/204Pb)i = 15.34–16.50, (207Pb/204Pb)i = 15.23–15.32, and (208Pb/204Pb)i = 35.93–37.04. The data suggest that the protolith sources of the metabasites were contaminated to variable degrees by old crustal materials during formation. Unlike the Maowu layered intrusions, which were contaminated by upper crust, the magmas of the metabasites were contaminated by lower crust in the magma chamber and during eruption. It can be concluded that the protoliths of these metabasites were derived from old crustal-contaminated mantle sources and initially emplaced in the crust at the Neoproterozoic and that they were altered by meteoric water at high temperatures. In this respect, they might be similar to the Neoproterozoic mafic intrusions in the North Huaiyang terrain. However, the studied metabasites experienced the Permo-Triassic subduction and metamorphism, whereas the North Huaiyang Neoproterozoic mafic intrusions did not. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction It is widely accepted that the Yangtze Block that was subducted northward under the North China craton during the Permo-Triassic (Liu et al., 2008; Cheng et al., 2011) and the Dabie-Sulu Orogen represents a collisional boundary. A previous tectonic model suggested that oceanic crust existed between the two plates before the subduction (Faure et al., 1999). Jahn et al. (1999) also proposed that oceanic crust could be subducted to mantle depth and converted to eclogite-facies rock. Pillow basalts, which are among the most abundant volcanic rocks occurring throughout the geological record, are common on modern ocean floors, in Phanerozoic ophiolites, and in Archean greenstone belts (Polat et al., 2003). ⇑ Corresponding author. E-mail address: [email protected] (Z. Xie). 1367-9120/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jseaes.2012.05.010

Pillow basalts may undergo a series of physical and chemical changes during seafloor hydrothermal alternation after volcanic eruption. The axiolitic metabasite lenses from Ganghe, Dabie Orogen, have been described as pillow lavas by Oberhänsli et al. (2000, 2002) and Schmid et al. (2003); their petrologic and tectonic characteristics have also been described. The lenses, along with other rocks such as volcanoclastic rock, pelite, and chert, forms the Ganghe complex that overlies the gneissic basement of the Yangtze Block (Schmid et al., 2003). It was suggested that the complex was compatible with formation in a rift setting along a passive continental margin (Schmid et al., 2003). Ames et al. (1996) offered a similar interpretation, suggesting that the rocks of the Dabie Orogen represent a metamorphosed Proterozoic rift sequence. Thus, the pillow lavas in the Ganghe area had been considered to be part of an ocean-continent transfer belt that existed before

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the Permo-Triassic subduction, survived and was exhumed to the crustal level with eclogite-facies rocks after ultra-high pressure metamorphism (UHPM), while most of the oceanic crust was eventually detached from the continental lithosphere (e.g., Jahn et al., 1999). Subduction of continental margin rocks such as pillow lavas may have direct implications for crustal recycling and subsequent crust-mantle interactions. However, Zheng et al. (2003) suggested that the protoliths of most eclogites in Dabie Orogen were continental basaltic rocks. These metabasites, therefore, may offer the opportunity to verify the existence of oceanic crust relic after the subduction. Because Oberhänsli et al. (2002) and Schmid et al. (2003) already conducted detailed petrologic research on these lenses, this paper attempts to: (1) present new geochemical information on the lenses to identify their material source; (2) compare them to other metabasites with similar histories in the Dabie Orogen; and (3) discuss the progress of crustal-mantle interactions in the north margin of the Yangtze Block. 2. Geologic setting and sample description The general geology of the Dabie Orogen has been described in numerous publications (e.g., Hacker et al., 1996; Liou et al., 1996; Faure et al., 1999; Zheng et al., 2003; Zhao et al., 2011). The Dabie Orogen is composed of four major petro-tectonic units; from north to south, these are: (1) the North Huaiyang belt (NHY), (2) the North Dabie terrain (NDT) with HT/HP granulite-facies metamorphism, (3) the Central Dabie terrain (CDT) with ultra-high pressure metamorphism (UHPM), and (4) the South Dabie terrain (SDT, formerly the Susong Group) with dominant LT/HP blueschist-facies metamorphism (Fig. 1). These units are bounded in the southwest by the Xiangfan-Guangji Fault, in the east by the Tan-Lu Fault and in the north by the Hefei Basin. The most abundant rocks within the four units are gneisses, and all four units contain Cretaceous igneous rocks. UHPM rocks occur widely in the Central Dabie and the Sulu terrains. The Sulu unit was offset at least 500 km northward to the Shandong Peninsula along the Tan-Lu Fault. In both regions, numerous coesite- and microdiamond-bearing eclogites crop out within orthogneiss, peridotite, and marble (e.g., Okay et al., 1989; Wang et al., 1989; Xu et al., 1992; Zheng et al., 2011). These

observations imply the subduction of supracrustal materials to mantle depths of more than 120 km, then brief recrystallization under UHP conditions, followed by rapid exhumation (Zheng et al., 2003). Geochemical and isotopic studies (Jahn, 1998; Zhang and Liou, 1998) suggested that the protoliths of most eclogites in the Dabie-Sulu terrain were continental basalts and gabbros that exchanged H, O and C isotopes with ancient meteoric water (Zheng et al., 1998, 1999, 2000, 2003; Gao et al., 2011). Eclogite lenses within marble could have been derived from marls or basaltic rocks, which cut the carbonate sediments, and eventually contaminated by calcareous sediments. Permo-Triassic ages ranging from 255 to 210 Ma have been obtained by zircon U–Pb, mineral Sm–Nd, Ar–Ar, and other isotopic methods for analyzing eclogites and gneisses (Ames et al., 1993, 1996; Li et al., 1993, 1994, 1999, 2000; Okay et al., 1993; Hacker and Wang, 1995; Chavagnac and Jahn, 1996; Rowley et al., 1997; Webb et al., 1999; Hacker et al., 1998, 2000; Ayers et al., 2002; Liu et al., 2008; Zheng, 2008; Cheng et al., 2011). However, the exact timing of the UHPM is still controversial. Based on the zircon U– Pb ages of 210–225 Ma (Ames et al., 1993, 1996; Rowley et al., 1997) and the mineral Sm–Nd isochron ages of 210–226 Ma (Li et al., 1993, 1994, 2000; Chavagnac and Jahn, 1996), some authors considered that the UHP event occurred during the late Triassic. Others have advocated a peak UHPM during the early to middle Triassic based on a few Sm–Nd mineral isochron ages of approximately 245 Ma for eclogites (Li et al., 1993, 2000; Okay et al., 1993), a SHRIMP U–Pb age of approximately 240 Ma on overgrowths of zircon grains in granitic gneisses (Hacker et al., 1998), and zircon and monazite U–Th–Pb ages of 230–237 Ma (Ayers et al., 2002). Zheng (2008) concluded that the UHP metamorphism in the diamond stability field took place at approximately 238–235 Ma and lasted approximately 10 Ma; in this scenario, high-pressure eclogite-facies recrystallization was carried out in 225–215 Ma during exhumation, followed by the amphibolite-facies retrograde metamorphism in 215–205 Ma. The Ganghe complex occurs in the southwest part of Yuexi county in the CDT. The main rock assemblages in this area are a sequence of metasandstones, coarse- to fine-grained metavolcanoclastic rocks, metacherts, meta-pillow lavas (metabasites), ash beds, metapelites and marls; these rocks represent a metamorphosed cover sequence of the Yangtze Block (Oberhänsli et al.,

Fig. 1. (a) Sketch map of the Dabie Orogen. (b) Simplified geological map of the Ganghe area, the Dabie Orogen and the sampling location of the lenses (modified from Oberhänsli et al. (2000)).

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2002; Schmid et al., 2003). This complex was earlier interpreted as low-grade metamorphic rock (Tang et al., 1995; Dong et al., 1997), however, detailed work has revealed that the protoliths of metabasites and bandage volcanic rocks were formed at approximately 700–800 Ma (Schmid et al., 2001) and underwent UHPM in the Triassic (Oberhänsli et al., 2002). The metamorphic history of the rocks is similar to those of other famous eclogite bodies such as the Bixiling (Chavagnac and Jahn, 1996), Shuanghe (Li et al., 1993), and others (e.g., Ames et al., 1993, 1996; Jahn, 1998; Hacker et al., 1998). The existence of lava breccia close to metabasites might be due to failure to record the UHPM or its overprinting by a later event (Schmid, 2001; Oberhänsli et al., 2002). Further details of the petrology of the Ganghe complex can be found in Oberhänsli et al. (2002) and Schmid et al. (2003). The zircon U–Pb age of volcanoclastic breccia bordering the metabasites is 761 ± 33 Ma, similar to previously reported ages (Dong et al., 2002), which were interpreted as depositional ages (Schmid et al., 2003). Since it is still a doubt about the existence of oceanic relic, the studied metabasites will be described as lenses though they show pillow like structures. The samples were collected from several lenses outcropped near the Ganghe School (30°410 5100 N, 116°140 5100 E), Yuexi county (Fig. 1a and b). The lenses were previously described in detail by Oberhänsli et al. (2002). They are elongated with a long axis of approximately 3 m and a short axis of approximately 1 m. Some lenses contain eclogite, garnet-amphibolite, or a mica layer. The main mineral assemblage is garnet, ±omphacite, ±amphibole, and white mica, and rutile, zircon, glaucophane as accessory mineral. Petrologic work has indicated that the lenses experienced metamorphism at ca. 800 °C and 2.7–2.9 GPa (Oberhänsli et al., 2002), similar to the other UHPM rocks in the CDT. Nine samples were collected from four different lenses, which are designated #1 to #4, as shown in Table 1. Samples DB466 and DB473 were from two different lenses, #1 and #3. Samples DB468–DB472 were from different parts of lens #2, which crops out between lenses #1 and #3. Of that, DB469 was from the core, and the other four samples were from each end of the two axes (Fig. 2). The three lenses are all banded by thin muscovite schist layers. Samples DB474 and DB475 were from lens #4, approximately 50 m from lenses #1, #2 and #3; both samples contained abundant zoisite phenocrysts that were not found in the other seven samples. Bulk chemical, Sr–Nd–Pb isotopic composition and mineral oxygen isotope analyses have been conducted for these samples. Zircons were also separated from DB468 and DB474 and dated using the SHRIMP method.

Fig. 2. The photograph of sample locations of DB466 from lens #1 and DB468– DB472 from Lens #2.

3. Analytical procedures The nine rock samples were crushed in a steel-jawed crusher and powdered in an agate mill to a <200 mesh size. Mineral fragments were also separated after crushing, and a shaking bed was used to concentrate heavy minerals. This was followed by magnetic separation and hand picking. Analyses of major and trace elements were carried out at the Analytical Institute in the Hubei Bureau of Geology and Mineral Resources. The analytical methods have been described by Ma et al. (2000). For the major elements, SiO2 content was determined by gravimetry, TiO2 and P2O5 content by spectrophotometry, Al2O3, Fe2O3, and FeO content by volumetry, and MnO, MgO, CaO, Na2O and K2O content by atomic absorption spectrometry. The analytical uncertainty was usually <5%. For REE and Nb, Ta, Zr, Hf, Th, and Ba, the samples were digested by alkaline fusion and analyzed using the ICP-AES method. The analytical uncertainty was better than 5% for REE and <10% (2r) for other trace elements. The results of analyses of international standard reference samples were reported by Gao et al. (1991). Sr–Nd–Pb isotopic analyses were conducted at the CAS Key Laboratory of Crust-Mantle Materials and Environments (CASKLCMME), University of Science and Technology of China (USTC). Rb, Sr and REE were purified by Bio-rad AG 50  8 cation exchange resin, Sm and Nd were purified by Ln Spec ion exchange resin,

Table 1 Zircon SHRIMP U–Pb dating of the metabasites from Ganghe, Dabie Orogen. Spots no.

U (ppm)

Th (ppm)

Th/U

204

Pb/206Pb  103

207

Pb/235U

±1r (%)

206

Pb/238U

±1r (%)

207

Pb/206Pb

±1r (%)

Appearance age (Ma) Pb/238U

DB468-1 DB468-2 DB468-3 DB474-1 DB474-2 DB474-3 DB474-5 DB474-6 DB474-7 DB474-8 DB474-9 DB474-10 DB474-11 DB474-12 DB474-13 DB474-14 DB474-15 DB474-16

216 491 472 242 158 264 372 151 792 1829 820 1813 1274 672 188 231 217 111

137 13 9 164 74 158 127 3 26 26 6.6 9.7 12 340 138 168 190 89

0.64 0.03 0.02 0.67 0.47 0.60 0.34 0.02 0.03 0.01 0.01 0.01 0.01 0.52 0.76 0.75 0.90 0.82

0.52 0.38 0.53 0.45 1.3 0.27 0.29 0.38 3.2 0.48 0.65 0.27 0.00 0.49 1.6 1.7 1.3 0.33

0.490 0.274 0.210 0.983 0.585 0.889 0.638 0.267 0.470 0.245 0.263 0.254 0.230 0.873 0.819 0.982 1.0391 1.1706

6 5 12 5 9 5 4 6 31 4 5 3 4 4 9 7 6 5

0.0623 0.0372 0.0344 0.1081 0.0769 0.0994 0.0755 0.0350 0.0590 0.0351 0.0364 0.0347 0.0300 0.0994 0.1114 0.1143 0.1219 0.1216

3.7 3.7 3.8 3.7 3.8 3.7 3.7 3.9 4.7 1.3 1.2 1.5 1.3 1.3 1.9 1.5 1.5 1.3

0.0647 0.05890 0.0520 0.07242 0.0739 0.0687 0.06547 0.0608 0.1032 0.0505 0.0525 0.0529 0.0500 0.0637 0.0533 0.0623 0.0618 0.0698

2.0 1.7 2.4 1.4 2.1 1.5 1.3 4.2 7.9 3.2 4.7 2.3 3.4 3.4 8.5 6.9 5.3 4.5

206

±2r

390 236 218 662 477 611 469 222 370 223 230 220 211 611 681 698 741 740

14 8 8 23 17 22 17 9 17 3 3 3 3 8 12 10 10 9

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and Pb was purified by Bio-rad AG 1  8 anion exchange resin. Isotope data were collected using a Finnigan MAT-262 multi collector mass spectrometer. Sr and Nd isotopic ratios were corrected for mass fractionation relative to 86Sr/88Sr = 0.1194, 149 Sm/152Sm = 0.51686 and 146Nd/144Nd = 0.72190. The standard La Jolla yielded 143Nd/144Nd = 0.511860 ± 4 (2r), and NBS-987 yielded 87Sr/86Sr = 0.70250 ± 4 (2r). Fractionations of Pb isotopes were corrected according to the standard NBS-981 data. Rb–Sr and Sm–Nd isochron calculations were made using the regression programs of ISOPLOT (Ludwig, 1994). Input errors used in age computations were: 147Sm/144Nd = 0.2%, 143Nd/144Nd = 0.005%, 87 Rb/86Sr = 2% and 87Sr/86Sr = 0.01%. The analytical precision of isotope ratio measurements was given as ±2 standard errors (2r), and the quoted errors in ages and initial isotopic ratios represented ±2 standard deviations. Oxygen isotope analysis of mineral separates was also carried out at CAS-KLCMME, USTC. Approximately 1.5–2.0 mg of mineral fragments were ablated by the laser fluorination technique using a 25-W CO2 laser MIR-10, and reacted with BrF5 in a vacuum; obtained O2 was directly transferred to Delta+ for measurement of O isotope ratios (Zhao et al., 2007; Zheng et al., 2002). The 18O/16O ratios were reported in d18O notation relative to the VSMOW standard. The reproducibility of the analysis was better than ±0.1. During the laser fluorination, two international standards were used: (1) d18O = 5.8‰ for garnet UWG-2 (Valley et al., 1995) and (2) d18O = 11.11‰ for quartz GBW04409 (Zheng et al., 1998). Mineral-pair O isotopic temperatures were also calculated using the fractionation parameters of Zheng (Zheng, 1991, 1993a,b), assuming preservation of isotope equilibration. Temperature errors were estimated to be in the range of 30–50 °C in terms of the analytical uncertainties. Judgment and interpretation of O isotope equilibrium or disequilibrium between coexisting minerals were based on the measured fractionation values and the resultant sequence of O isotope temperatures in combination with rates of O diffusion in the concerned minerals and the corresponding sequence of closure temperatures (e.g., Zhao et al., 2007). Zircons were concentrated from samples DB468 and DB474 by a magnetic separator and then by hand-picking for SHRIMP analysis. Mounts were made, photographs were taken and the analysis was performed in the Beijing Ion-probe Central and Microprobe Lab in the Chinese Academy of Geological Sciences, Beijing, and in the SHRIMP Lab, Geological Survey of Canada. Zircon grains were mounted in epoxy with the zircon standards TEMORA (age 417 Ma) and/or 6266 and polished. After photographs were taken under transmitted and reflected light, cathodoluminescence (CL) pictures were taken to determine internal structure. The measurement was performed using SHRIMP II. The procedures have been described in detail by Compston et al. (1992), Williams (1998) and Davis et al. (2006). The standard zircons SL13, TEMORA and/or 6266 were measured, the former for calibration of U and Th concentrations and the latter two for age. The diameter of the beam was 20–25 lm. The results were corrected for common Pb using 204Pb, and data were calculated using SQUID and ISOPLOT (Ludwig, 1994).

Fig. 3. CL photographs and dating results of the zircon of metabasite samples DB468 and DB474 from Ganghe. The 206Pb/238U ages are marked beside the analyzed spots with concordant ages; the analyzed numbers are also marked beside the spots with discordant ages.

majority consists of euhedrally zoned zircon mantled by structureless, bright overgrowth (Fig. 3). In many grains, the zoning is intact without truncations that might be attributable to metamorphic corrosion. Most grains contain a distinct core over which the zoned zircon itself has grown. Some grains have preserved earlier oscillatory zoning (Fig. 3, DB468-A), indicating that the cores may be overgrown crystal fragments. The U–Th–Pb results obtained by SHRIMP for the zircons are listed in Table 1 and are plotted in a Wetherill concordia diagram (Fig. 4a) and in a weighted-average 206Pb/238U age diagram (Fig. 4b and c). All of the spots are scattered along the discordia, giving an upper-intercept age of 725 ± 120 Ma and a lower-intercept age of 224 ± 130 Ma, with large errors. Because the zircon grains are small in size, the discordia ages might result from a mixture of older core and younger overgrowth margins. However, a few spots are located very close to the upper and lower-intercept, and the spots close to upper-intercept give a weighted-average 206 Pb/238U age of 717 ± 38 Ma and MSWD = 7.7 (Fig. 4b), the spots close to the lower-intercept can be separated to three groups with 206 Pb/238U ages of 231 ± 25 Ma and MSWD = 2.0 include two spots, 221 ± 2 Ma and MSWD = 0.91 include four spots, and 211 Ma of one spot (Fig. 4c), respectively.

4. Results and discussion 4.1. Protolith ages and UHP metamorphism of the metabasites Zircon U–Pb dating is a powerful tool for geochronology study, especially for metamorphic rocks, because of its ability to record multi-stage event ages. In this study, the zircons separated from the two samples DB468 and DB474 are of low yield, approximately 50 lm in size, anhedral, and yellowish in color. The grains have the rounded shape common in zircon from high-grade rocks (Williams and Claesson, 1987). CL images of the grains show that the great

Fig. 4. Zircon SHRIMP dating results for the Ganghe metabasites, Dabie Orogen. The results of DB468 from lens #2 and DB474 from lens #4 are displayed together. (a) The discordant ages of all the spots. (b) The weighted-average 206Pb/238U age of the zircon cores, which are plotted in the upper-intercept, is approximately 717 ± 38 Ma, and the MSWD = 7.7. (c) The weighted-average 206Pb/238U age of the zircon margins, which are plotted in the low-intercept, can be separated to three groups with age of 231 ± 25 Ma, MSWD = 2.0; 221 ± 2 Ma, MSWD = 0.91; and 211 Ma.

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Z. Xie et al. / Journal of Asian Earth Sciences 63 (2013) 151–161 Table 2 Element compositions of the metabasites from Ganghe, Dabie Orogen. Sample no.

DB466

DB468

DB469

DB470

DB471

DB472

DB473

DB474

DB475

Lens no. SiO2 Al2O3 Fe2O3 FeO MgO CaO Na2O K2O MnO TiO2 P2O5 Lost Total Mg#

1 46.46 15.84 4.24 8.05 6.79 8.45 3.54 1.48 0.34 1.65 0.57 1.40 98.81 53

2 46.5 15.7 4.48 8.45 5.45 10.59 3.3 0.9 0.34 1.69 0.63 0.76 98.79 46

2 47.06 16.22 2.41 9.15 6.8 9.19 3.44 1.62 0.34 1.65 0.54 0.28 98.70 54

2 45.46 16.56 4.11 8.55 6.43 6.39 3.42 2.99 0.35 1.7 0.62 2.20 98.78 51

2 46.91 16.13 4.08 7.65 6.74 8.94 3.06 1.57 0.34 1.65 0.58 1.22 98.87 54

2 46.34 16.5 4.48 7.4 6.37 8.65 3.5 1.42 0.34 1.63 0.67 1.66 98.96 52

3 48.27 16.44 2.74 8.9 6.53 8.78 3.65 0.5 0.27 1.68 0.59 0.48 98.83 53

4 49.59 16.6 3.76 8.42 5.92 9.24 2.6 0.37 0.19 1.3 0.29 0.56 98.84 50

4 49.05 16.46 4.85 7.35 5.43 9.67 2.32 0.85 0.23 1.35 0.31 1.02 98.89 48

Trace elements (ppm) La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y Rb Ba Th Nb Pb Sr Zr Hf U Ga (La/Yb(La/Lu)n Zr/Hf Th/U Nb/U

25.18 51.68 7.32 30.24 6.39 2.34 6.30 1.04 5.54 1.13 2.92 0.46 2.60 0.39 25.77 30.9 895 1.3 7.4 6.7 416 176 4.6 0.6 17.1 6.5 38.3 2.2 12.3

22.86 46.61 6.68 26.12 5.83 2.19 6.06 0.98 5.96 1.20 3.13 0.48 2.81 0.42 29.50 18.8 931 1 6.1 5.7 575 198 4.5 0.6 20.5 5.5 44.0 1.7 10.2

19.21 40.01 5.74 24.68 5.72 2.14 5.66 0.94 5.15 0.98 2.61 0.40 2.43 0.36 24.05 41.5 1051 1 5.4 4.2 337 178 4.4 0.6 15.9 5.3 40.5 1.7 9.0

30.29 64.65 8.59 36.05 7.87 2.74 7.87 1.30 7.25 1.34 3.47 0.50 3.11 0.46 33.74 73.6 943 1.8 8.0 4.9 252 192 4.8 0.6 23.2 6.6 40.0 3.0 13.3

30.38 60.49 8.39 34.16 7.10 2.55 6.50 1.04 5.60 1.07 2.88 0.41 2.60 0.39 26.53 38.0 936 1.6 6.5 6.1 465 179 4.3 0.6 18.2 7.9 41.6 2.7 10.8

39.04 76.96 9.94 41.56 8.27 3.09 7.61 1.21 6.21 1.15 3.06 0.47 2.73 0.41 27.77 25.0 565 1.3 6.8 5.7 298 180 4.0 0.7 21.4 9.6 45.0 1.9 9.7

39.52 82.99 10.71 42.76 8.56 2.94 7.06 1.07 5.81 1.13 3.05 0.44 2.84 0.42 27.14 14.7 340 1.7 7.1 3.0 130 191 4.5 0.6 17 9.4 42.4 2.8 11.8

16.09 33.92 4.69 19.40 4.45 1.77 4.64 0.72 4.08 0.80 2.19 0.34 2.17 0.33 18.70 25.6 498 1.0 4.5 4.1 682 105 2.6 0.8 16.3 5.0 40.4 1.3 5.6

15.53 32.47 4.41 18.88 4.24 1.76 4.93 0.78 4.55 0.96 2.66 0.41 2.66 0.41 23.44 15.0 814 1.3 5.7 15.5 824 108 2.7 0.6 27.1 3.9 40.0 2.2 9.5

The upper-intercept spots with Neoproterozoic ages have U and Th concentrations in ranges of 111–242 ppm and 89–164 ppm, respectively, and the Th/U ratios of these spots range from 0.67 to 0.90. The lower-intercept spots with the Triassic ages have U and Th contents in ranges of 151–1829 ppm and 3–28 ppm, respectively, and their Th/U ratios range from 0.006 to 0.026. The discordia spots have U concentrations in range of 158–792 ppm and higher Th concentrations of 26–340 ppm; their scattered Th/ U ratios range from 0.03 to 0.90, implying an old-young domains mixture. Although the errors in the data described above are large, it can still be suggested that the upper-intercept age represents the time of protolith formation. This age is consistent with the protolith ages of most orthogneisses and UHP rocks in the Dabie Orogen (e.g., Ames et al., 1996; Rowley et al., 1997; Zheng et al., 2003, 2004) and is also consistent with the single zircon age determined from volcanoclastic rocks (761 ± 33 Ma) near the Ganghe Bridge (Schmid et al., 2003), which belong to the same complex. The high Th/U ratios suggest that metabasite protoliths should be magma formation. Though the weighted-average 206Pb/238U ages of the lowerintercept spots (Fig. 4c) can be divided to three groups, the two spots with older weighted-average ages give larger errors, which

are the same as those of the four spots with age of 221 ± 2 Ma, the youngest ages of the only one spot might be result of analytical uncertainty. In that case, the more precise age of 221 ± 2 Ma with MSWD = 0.91 is discussed in this study. The age is younger than the age of 238–235 Ma representing UHP metamorphism in the diamond stability field, but the same as the time of HP eclogite-facies recrystallization during exhumation (225–215 Ma; Zheng, 2008; Zheng et al., 2005). The lower Th/U ratios may suggest that the overgrowth was not of magmatic origin but resulted from fluid precipitation (Hoskin and Black, 2000; Rubatto and Hermann, 2003). However, the absent of ages in range of 238–225 Ma do not suggest that the metabasites did not experience UHP metamorphism. Metamorphic fluid should be a critical medium for the transfer of Zr and Si and also a required element for zircon growth. Because the pressure of UHPM is generally higher than 25 kbar, the fluid mobility would be very limited, especially in the mantle-derived rocks with lower water content, and metamorphic overgrowth would be difficult (Zheng et al., 2004, 2005); thus, the metamorphic event would not be recorded in zircon (Corfu et al., 2003). During exhumation, fluid should be released due to the pressure decrease; this would accelerate the retrogression and formation of new minerals as well as zircon overgrowth (Zheng et al., 2003). In that case, the age recorded by zircon should be younger

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than the time of UHPM. Overall, it can be concluded that the metabasites in question experienced UHPM and subsequent exhumation as well as HP eclogite-facies retrogression. 4.2. Geochemical characteristics The results of chemical analyses are presented in Table 2. The SiO2 contents of the four different lenses range from 45% to 50%, and the Mg# values are calculated as molecular proportion of 100  MgO/(MgO + FeO), and values range from 46 to 54. The samples are low in MgO (5.4–6.8%) but much higher in ‘‘fertile elements’’ such as Al, Ca, Fe, Ti and Na, and the trace elements Zr and Sr are also relatively enriched compared to real pillow lava (Frei and Jensen, 2003). The scattered K2O values and higher Na2O content of the samples may result from thermal alterations in posteruption or later metamorphic events because these elements are more mobile than the trace elements (Polat et al., 2003). All of the samples are plotted in a total alkali vs. silica (TAS) diagram (Fig. 5), along with Bixiling eclogites and peridotites (Chavagnac and Jahn, 1996), Maowu eclogites (Jahn et al., 2003) and Neoproterozoic mafic intrusions from NHY (Xie et al., 2002). The samples from lenses #1 to #3 fall in the field of alkaline basalt, as well as in the field of the Neoproterozoic mafic intrusions from NHY, stretching across the alkaline and sub-alkaline areas. The other two samples, DB474 and DB475 from lens #4, have obviously lower amounts of K2O and Na2O and slightly higher amounts of SiO2 than the other samples, which are located in the sub-alkaline basalt field. All studied samples show obvious differences with the samples from Bixiling and Maowu, implying their different sources. The trace element data of the nine samples are given in Table 2. The REE distribution patterns of the samples are presented in Fig. 6a, and a primitive mantle (PM)–normalized spider diagram is shown in Fig. 6b. The seven samples from lenses #1, #2, and #3 have slightly higher RREE concentrations and (La/Lu)n ratios of 5.3–9.6 than the two samples from lens #4, which have (La/ Lu)n ratios from 3.9 to 5.0. However, all of the samples show rather similar patterns (Fig. 6a) that differ from the typical normal-type mid-ocean ridge basalt (N-MORB) pattern by enrichment LREE. This suggests that they were not derived through differentiation from N-MORB parental magma. The lower Th content of these metabasites (1.0–1.8 ppm) would limit the amount of monazite to <0.01%, which is too low to sufficiently enrich LREE abundances to the levels shown in Fig. 6a. The enrichment is most likely due to metasomatism similar to that which occurred in the eclogite from Maowu (Jahn et al., 2003). Consistent slightly positive Eu anomalies indicate an original plagioclase cumulates. Significant negative Nb, Sr, and Ti anomalies and positive Ba, K, and Pb anomalies are found in all samples (Fig. 6b). The negative

Nb anomaly is most characteristic of subduction zone volcanic rocks or crustal contamination in the pre-metamorphic magmatic evolution. Because the lenses might be formed in an extensional environment, as other continental basaltic rocks (Zheng et al., 2003), the continental signature of mantle-derived magmas may come from crustal contamination. The La/Nb ratios of the metabasites fall within the ranges of 2.7–5.7, and Ba/Nb ratios are in ranges of 48–195, which are presented in arc volcanics area (Fig. 7). These values are substantially different from those of most intraplate volcanic rocks

Fig. 5. Total alkali vs. silica (TAS) classification diagram for the Ganghe metabasites, Dabie Orogen. The Bixiling eclogites (Chavagnac and Jahn, 1996), Maowu eclogites (Jahn et al., 2003) and the region of Neoproterozoic mafic intrusions from NHY (Xie et al., 2002) are also shown.

Fig. 7. Diagram of Ba/Nb vs. La/Nb, showing that the metabasites are characterized by high Ba/Nb and La/Nb ratios in the field of arc volcanics (The data for these fields is from Jahn et al. (1999)).

Fig. 6. Diagrams of trace elements of the metabasite from Ganghe, Dabie Orogen. (a) Chondrite-normalized REE patterns, which show LREE enrichment; those from lenses #1, #2, and #3 show Eu-positive abnormalities, but those from lens #4 do not. Chondrite values (Boynton, 1984) used for normalization are as follows: La = 0.31, Ce = 0.808, Pr = 0.122, Nd = 0.60, Sm = 0.195, Eu = 0.0735, Gd = 0.259, Tb = 0.0474, Dy = 0.322, Ho = 0.0718, Er = 0.21, Tm = 0.0324, Yb = 0.209, Lu = 0.0332. (b) Primitive mantle (PM)-normalized spider diagrams. Negative Nb anomalies are clearly shown in the rocks. The PM values (in ppm) used are from McDonough et al. (1991): Rb = 0.635, Ba = 6.99, K = 249, Nb = 0.713, La = 0.687, Ce = 1.775, Sr = 21.1, P = 96, Nd = 0.354, Zr = 11.2, Sm = 0.444, Eu = 0.168, Ti = 1300, Gd = 0.596, Dy = 0.737, Y = 4.55, Er = 0.48, Yb = 0.498, and Lu = 0.077.

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Z. Xie et al. / Journal of Asian Earth Sciences 63 (2013) 151–161 Table 3 Sr–Nd–Pb isotopic compositions of the metabasites from Ganghe, Dabie Orogen. Sample no.

DB466

DB468

DB469

DB470

DB473

DB474

DB475

Lens no. Rb (ppm) Sr (ppm) 87 Rb/86Sr 87 Sr/86Sr 2r ISr (220)

1 30.92 415.6 0.215 0.708210 16 0.707537

2 18.82 574.8 0.095 0.708698 17 0.708402

2 41.48 337.4 0.356 0.709019 16 0.707906

2 73.61 251.6 0.847 0.710864 16 0.708214

3 14.68 129.8 0.327 0.708244 16 0.707221

4 7.08 682.3 0.030 0.705392 13 0.705298

4 25.55 824.0 0.090 0.706022 16 0.705741

Sm (ppm) Nd (ppm) 147 Sm/144Nd 143 Nd/144Nd 2r fSm/Nd eNd (0) eNd (220) eNd (t) TDM

6.95 34.10 0.1232 0.511712 15 0.37 18 16 11 2.4

6.48 30.48 0.1285 0.511693 12 0.35 18 17 12 2.6

7.27 32.53 0.1351 0.511718 13 0.31 18 16 12 2.8

8.51 41.17 0.1249 0.511704 12 0.36 18 16 12 2.5

7.90 42.55 0.1122 0.511703 10 0.43 18 16 10 2.2

4.19 19.32 0.1310 0.512032 17 0.33 12 10 6 2.1

4.18 18.77 0.1345 0.512002 16 0.32 12 11 7 2.2

206

16.737 3 15.276 3 37.082 7 16.058 15.232 36.617

16.772 3 15.318 3 37.218 6 15.972 15.266 36.796

16.799 3 15.325 3 37.230 6 15.712 15.255 36.657

16.864 2 15.305 2 37.294 5 15.931 15.245 36.408

16.886 3 15.329 2 37.295 6 15.361 15.231 35.928

16.818 3 15.340 3 37.067 6 15.336 15.244 36.481

16.794 2 15.337 2 37.237 5 16.499 15.318 37.035

Pb/204Pb

2r 207

204

Pb/

Pb

2r 208

Pb/204Pb 2r (206Pb/204Pb)i (207Pb/204Pb)i (207Pb/204Pb)i

including N-MORB, OIB, alkali basalt and kimberlites, which have La/Nb ratios of 0.5–2.5 and much smaller Ba/Nb ratios of 1–20 (Jahn et al., 1999). The data also suggest involvement of continental rocks in the pre-metamorphic magma genesis. 4.3. Whole-rock Rb–Sr and Sm–Nd isochrones The whole-rock (WR) Rb–Sr, Sm–Nd and Pb isotopic data of the studied samples are given in Table 3 and are plotted in both 87 Rb/86Sr–87Sr/86Sr and 147Sm/144Nd–143Nd/144Nd diagrams (Fig. 8a and b). All of the samples have highly variable Rb concentrations (7.1–31 ppm) and 87Rb/86Sr ratios (0.030–0.85) as well as widely ranged initial 87Sr/86Sr ratios of 0.702–0.708. The scattered data and generally high 87Sr/86Sr ratios for the mantle-derived rocks suggest that the rocks have undergone variable degrees of crustal contamination during magmatic differentiation. Apart from DB474 and DB475 from lens #4, the samples have extraordinarily high 87Rb/86Sr ratios for mantle-derived mafic rocks, further suggesting crustal contamination. Samples DB474 and DB475 exhibited very low 87Rb/86Sr ratios that are similar to those of Maowu eclogite (Jahn et al., 2003); this may result from Rb depletion during UHP metamorphism. The WR Rb–Sr and Sm–Nd isotopic data are too scattered to form a meaningful isochron (Fig. 8a and b), only reference lines, which may indicate the post-eruption alteration and/or retrograde metamorphism, can be obtained. DB474 and DB475 from lens #4 yield an Rb–Sr isochron with an age of 740 Ma with large errors and do not give a reliable Sm–Nd isochron with higher 143Nd/144Nd ratios. Other samples yield a reference Rb–Sr isochron and Sm–Nd isochron indicating ages of 240 Ma and 237 Ma, respectively. The reference ages are comparable, within error limits, with Sm–Nd and zircon U–Pb ages obtained from other eclogites, such as the Maowu eclogite and the Bixiling eclogite, which have Sm–Nd mineral isochron ages of 221–236 Ma (Jahn et al., 2003) and 210– 220 Ma (Chavagnac and Jahn, 1996), respectively. The reference ages are also in agreement with the U–Pb SHRIMP ages of the zircons in the studied samples.

4.4. Mineral O isotopes, WR Sr–Nd–Pb isotope compositions and the magma-forming process Metamorphic rocks usually have similar O isotopic compositions to crustal igneous and sediment rocks, these are typically in the range of d18O = 6–18‰ depending on the protolith and the degree of water–rock interaction during metamorphism (e.g., Zhao et al., 2007). Since the extremely negative d18O values of about 10‰ were observed in the eclogite from Qinglongshan, Sulu region (Yui et al., 1995; Zheng et al., 1996), a large number of O isotopic data reported on metamorphic rocks in the Dabie-Sulu Orogen have been used to successfully characterize the protolith and fluid regime during continental subduction and subsequent exhumation (Zheng et al., 2003 and references therein). The distinct low d18O values (in the range of 2‰ to 11‰) of the metamorphic rocks indicate both intensive and extensive O isotope exchange of meteoric water with the protolith during formation (Zhao et al., 2007 and references therein). In this study, minerals were separated from samples DB468 and DB474 for oxygen isotope measurement, and the results are presented in Table 4. Both samples exhibit relatively low d18O values of 5.7‰ to 3.4‰ for garnet, 10‰ to 6.7‰ for rutile, 3.87‰ for omphacite and 2.80‰ for amphibole. Because the main mineral assemblage of sample DB467 is garnet and omphacite and that of sample DB474 is garnet and amphibole, the whole-rock d18O values are estimated to be 5.5‰ to 4.0‰ and 3.5‰ to 2.0‰, respectively. The low d18O values are in the same range as those of eclogites from the Dabie-Sulu Orogen and were probably acquired by meteoric-hydrothermal alteration of their protolith at high temperature. The calculated d18O quartz-mineral equilibrium temperatures of sample DB474 are also listed in Table 4. These are consistent with the equilibrium data obtained from Chinese Continental Scientific Drilling cores (Zhao et al., 2007). Therefore, it is inferred that the minerals achieved re-equilibrium during the HP metamorphism and were not disturbed by later thermal events. For sample DB468, the difference between garnet and omphacite is greater

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Fig. 8. Diagrams of Rb–Sr and Sm–Nd isochrones of the metabasites from Ganghe, Dabie Orogen. (a) Rb–Sr isotopic compositions. The two spots from lens #4 form a reference line with age 740 Ma with a large error. The other spots also form a reference line indicating an age of approximately 240 Ma with a large error. (b) Sm– Nd isotopic compositions. The two data points from lens #4 do not form a reasonable line; the other data points form a reference line indicating an age of approximately 237 Ma with a large error.

Table 4 The mineral oxygen isotopes and calculated mineral pair equilibrium temperatures of the metabasites from Ganghe, Dabie Orogen. Mineral

d18O

DB468 Omphacite Garnet Rutile

3.87 5.68 10.1

DB474 Quartz Muscovite Amphibole Epidote Glaucophane Garnet Rutile

0.50 2.38 2.80 3.02 3.14 3.40 6.69

Pair

D18O

T (°C)

Qtz-Mu Qtz-Am Qtz-Ep Qtz-Gl Qtz-Grt Qtz-Rt

2.88 3.30 3.52 3.64 3.90 7.19

541 637 480 487 624 462

than 1‰, which may suggest that they did not arrive at reequilibrium. The WR Sr–Nd–Pb isotope compositions of the studied samples are significantly different from those of the Bixiling and Maowu eclogites (Figs. 9a, b and 10a, b). The samples can be divided into two groups, lenses #1 to #3 and lens #4. Samples from the former group have higher initial 87Sr/86Sr ratios (within the range of 0.7072–0.7087) than those of Bixiling, but similar to those of Maowu, and lower eNd (220 Ma) values of 17 to 16 than both; the TDM values are within the range of 2.2–2.8 Ga. The Sr–Nd isotope

Fig. 9. Diagrams of the Sr–Nd isotope compositions of the metabasites from Ganghe, Dabie Orogen. Data of the Bixiling eclogites (Chavagnac and Jahn, 1996), Maowu eclogites (Jahn et al., 2003), and North Huaiyang Neoproterozoic mafic intrusions (our unpublished data) are also plotted. (a) eNd vs. initial 87Sr/86Sr ratios are calculated to 220 Ma and plotted for the metabasites from the Ganghe unit, however, this does not represent the real eruption time of the protoliths. Because of the mobility of Rb and Sr during alteration and metamorphism, Sr isotopic characterization of the protolith is difficult (Jahn et al., 2003). The extremely negative eNd values suggest lower crustal contamination of the protoliths. The lenses can be separated into two groups; the group containing lens #4 is much closer to the mantle source. (b) Diagram of ages-eNd (t) of the metabasites. The studied samples are located between the crustal evolution lines of 1.4 and 2.4 Ga, lower than those of the Bixiling and Maowu eclogites but in a similar region as the Dabie orthogneisses and late Mesozoic granitic intrusions as well as the NHY Neoproterozoic mafic intrusions. The data indicate that the sources of the metabasite protoliths were contaminated by old crustal materials differentiated during the early Paleoproterozoic to the Archean.

compositions of the studied samples are similar to those of the NHY Neoproterozoic mafic intrusions (our unpublished data). The two samples from lens #4 are closest to the Bixiling eclogite (Jahn et al., 2003), suggesting a more mantle-like composition. Previous work has suggested that the Bixiling intrusions have undergone a small degree of lower crustal contamination (Chavagnac and Jahn, 1996) and that the Maowu intrusion recorded variable degrees of upper crust contamination (Jahn et al., 2003). For the studied metabasites, such negative eNd (220 Ma) values suggest that the protolith source have undergone variable degrees of crustal contamination during magma differentiation. This is consistent with the hypothesis derived from the trace element diagrams presented earlier and with the Sr isotopic compositions of the samples. On the other hand, the different Sr–Nd isotope compositions may indicate distinctive source materials of the lenses or different degrees of contamination. In Fig. 9b, the studied metabasites are plotted under the 1.4 Ga crustal evolution line, suggesting the contribution of old crustal

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159

lavas are generated by a process analogous to the budding of slowmoving pahoehoe flows, predominately at slow spreading centers (Kennish and Lutz, 1998). Following their eruption, pillow lavas may undergo seafloor hydrothermal alteration; thus, their primary chemical compositions and d18O values may be substantially changed. Modern pillow lavas normally yield positive eNd (t) data (Polat et al., 2003). Although the studied metabasites have pillow structures, they are unlike real pillows derived from mantle, as mentioned above. The negative d18O values and low eNd (t) data both suggest that they should be considered as continental basalts but not oceanic crust. The similar element and Nd isotope compositions and also protolith forming time imply that the studied metabasites may be derived from the similar mantle source as the NHY Neoproterozoic mafic intrusion, occurred as continental basaltic rocks after formation. 5. Conclusions The element and Sr–Nd–Pb isotope data and the mineral O isotope compositions of the samples suggest that the metabasites studied here are significantly different from the Bixiling and Maowu eclogites. Several additional conclusions can be drawn from the obtained data.

Fig. 10. Diagrams of the initial Pb isotope compositions of metabasites from Ganghe, Dabie Orogen. Data for the North Huaiyang Neoproterozoic mafic intrusions (our unpublished data) and for metamorphic and igneous rocks from the Dabie-Sulu Orogen (Huang et al., 2007; Li et al., 2009) are also plotted. (a) Diagram of 206Pb/204Pb–207Pb/204Pb. All the data points are located to the left of the GEOCHRON line, indicating Pb isotope compositions lower than those of the metamorphic and igneous rock from the Dabie-Sulu Orogen; however, the samples display 206Pb/204Pb ratios similar to those of the NHY Neoproterozoic mafic intrusions. (b) Diagram of 206Pb/204Pb–208Pb/204Pb. The metabasites have Pb isotope compositions that are lower than those of metamorphic and igneous rocks from the Dabie-Sulu Orogen but similar to those of the NHY Neoproterozoic mafic intrusions.

materials. However, the absence of relic zircons with old ages might imply that contamination occurred in the source but not took place during the magma ascending. Furthermore, the low eNd values suggest that the crustal materials were derived from mantle before the early Paleoproterozoic. Zhao et al. (2011) suggested that crustal architecture could be separated into three layers according to the zircon Hf compositions of granitic gneisses. The CDT is in the upper layer, which shows predominantly young Hf model ages of the late Mesoproterozoic; the NDT lies in the middle layer and is associated with Meso-Paleoproterozoic ages, and the lower layer is associated in the model ages from the Paleoproterozoic to the Paleoarchean, which was the source of Dabie granitic rocks. In the adjacent area, Xing et al. (1994) indicated the existence of the Paleoproterozoic basement at the north margin of Yangtze Block, with a TDM of 2.7–2.6 Ga. Thus, it can be concluded that the protolith magma sources of the studied metabasites were contaminated to different degrees by old crustal materials derived from mantle during the early Paleoproterozoic to the Archean, but not by upper crustal materials such as those of Maowu; this is also shown by their much lower radiogenic Pb compositions (Fig. 10a and b). As presented by Schmid et al. (2003), the metabasites occurring in Ganghe, Dabie Orogen, were formed along the passive continental margin of the Yangtze Block. Although these metabasites experienced UHPM, their primary pillow structures were well preserved but deformed (Oberhänsli et al., 2002). Normally, pillow

(1) The protoliths of the metabasites were formed during the Neoproterozoic and experienced high-temperature meteoric-hydrothermal interaction. (2) The source of protolith magma was contaminated by old crust materials derived during the early Paleoproterozoic to the Archean. The studied samples have a source similar to that of the Neoproterozoic mafic intrusions that occur in NHY, which were not subducted. (3) The mafic intrusions were subducted to mantle depth with other UHPM rocks during the Triassic and subsequently exhumed; they showed a metamorphic history similar to that of other UHPM rocks occurring in the CDT. (4) Although these metabasites share pillow-like structures, they are not real pillow lavas but are continental basaltic rocks; this can be inferred from their negative d18O and extra-low eNd (t) values.

Acknowledgements We would like to express our respect to Professor B.-M. Jahn for his excellent work in the geochemical field over the past 50 years. This research was supported by the Chinese Academy of Sciences (CAS) (KZCX2-YW-Q08-3), the Natural Science Foundation of China (40921002), and the CAS (KZCX2-EW-QN508). Thanks are due to Drs. W.J. Davis, Y. Amelin, Y.-S. Wan and Z.-Q. Yang for their kind help in SHRIMP dating. We are also grateful to Profs. Y.-F. Zheng and Z.-F. Zhao for performing mineral oxygen isotope measurements and for helpful discussions, and Prof. X.C. Liu and another anonymous reviewer for their constructive comments. References Ames, L., Tilton, G.R., Zhou, G.Z., 1993. Timing of collision of the Sino-Korean and Yangtze cratons: U–Pb zircon dating of coesite-bearing eclogites. Geology 21, 339–342. Ames, L., Zhou, G., Xiong, B., 1996. Geochronology and geochemistry of ultrahighpressure metamorphism with implications for collision of Sino-Korea Cratons, Central China. Tectonics 15, 472–489. Ayers, J.C., Dunkle, S., Gao, S., Miller, C.F., 2002. Constraints on timing of peak and retrograde metamorphism in Dabie Shan Ultrahigh-Pressure Metamorphic Belt, east central China, using U–Th–Pb dating of zircon and monazite. Chemical Geology 186, 315–331. Boynton, W.V., 1984. Geochemistry of the rare earth elements: meteorite studies. In: Henderson, P. (Ed.), Rare Earth Element Chemistry. Elsevier, pp. 63–114.

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