Protolith age and exhumation history of metagranites from the Dabie UHP metamorphic belt in east-central China: A multi-chronological study

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1 Geochemical Journal, Vol. 38, pp. 345 to 362, 2004 Protolith age and exhumation history of metagranites from the Dabie UHP metamorphic belt in east-central China: A multi-chronological study RU-CHENG WANG, 1 * SHI-JIN XU, 1 ZHONG FANG, 1 YUCH-NING SHIEH, 2 HUI-MIN LI, 3 DA-MING LI, 4 JING-LIN WAN 4 and WEI-PING WU 5 1 Department of Earth Sciences, and State Key Laboratory for Mineral Deposits Research, Nanjing University, Nanjing , China 2 Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907, U.S.A. 3 Tianjin Institute of Geology and Mineral Resources, Tianjin , China 4 Institute of Geology, China Seismological Bureau, Beijing , China 5 Anhui Institute of Geology, Hefei , China (Received May 16, 2003; Accepted February 8, 2004) Metagranites (granitic orthogneisses) constitute one of the important lithologic units in the Dabie ultrahigh pressure metamorphic belt. They often display direct contact with paragneisses and eclogites. TIMS zircon U-Pb dating on the metagranites reveal that their protolith was crystallized as granitoids in Neoproterozoic ages of 755 to 611 Ma and thus correspond to rift magmatism associated with breakup of the Rodinia supercontinent in the northern margin of the Yangtze Block. Zircon U-Pb age and Ar-Ar ages of amphibole and biotite indicate that the granitoids experienced eclogite-facies metamorphism in Early Mesozoic with rapid exhumation at cooling rates of 8.7 to 10.2 C/Ma from 225 ± 6 to 181 ± 3 Ma in response to amphibolite-facies retrogression. Fission track ages of sphene, zircon and apatite indicate that the metagranites experienced slow exhumation with cooling rates of 0.88 to 1.25 C/Ma in Late Mesozoic. It is possible that in the first period of exhumation, the metagranites were uplifted from the mantle depth to lower crustal level at ca. 225 Ma, then rapidly reached middle to upper crustal levels between 204 Ma and 180 Ma. After the intensive magmatism in Early Cretaceous, the Dabie terrane was uplifted at the slow rate in response to unroofing. Either the rapid exhumation in the Early Mesozoic or the slow uplift in the Late Mesozoic in the Dabie terrane were contemporaneous with two rapid subsidence and sedimentation of the Hefei Basin in Early Jurassic and Paleogene, respectively. This suggests that the orogeny and basin-forming events were closely related to each other rather than independently separated. Keywords: metagranite, geochronology, cooling history, eclogite-facies, amphibolite-facies INTRODUCTION The Dabie Mountains belong to the east-central part of the Qinling-Dabie-Sulu orogen between the Yangtze Block and the North China Block. This region has become well-known since 1980s because of the discovery of coesite and micro-diamond in eclogites (Okay et al., 1989; Wang et al., 1989; Xu et al., 1992), thus being identified as one of the largest ultrahigh pressure (UHP) metamorphic belts in the world. The eclogite-facies metamorphic event has been dated to occur at Triassic with an age range of 245 to 225 Ma (Ames et al., 1993, 1996; Li, S.- G. et al., 1993, 1994, 1999, 2000; Chavagnac and Jahn, 1996; Rowley et al., 1997; Hacker et al., 1998; Ayers et al., 2002). However, it is still controversial with respect *Corresponding author ( rcwang@nju.edu.cn) Copyright 2004 by The Geochemical Society of Japan. to the exact timing of UHP metamorphic event at either Early-middle Triassic or Late Triassic. In the Dabie UHP metamorphic belt, granitic orthogneisses are widespread (Zhai et al., 1995; Cong, 1996), which is termed as metagranites in this paper following the nomenclature of Xu et al. (1998). It was suggested that these metagranites did not experience the UHP metamorphism because no UHP index mineral was found until However, this suggestion was questioned by Carswell et al. (2000), who deduced from mineralogy and mineral chemistry that the metagranites were metamorphosed at P-T conditions of 36 to 18 kbar and 750 to 690 C. Furthermore, coesite inclusions were observed in zircons by Liu, F-L. et al. (2001) and Liu, J-B. et al. (2001), demonstrating that the metagranites did experience the UHP metamorphism. With respect to ages of metagranite protolith and UHP event in the Dabie terrane, a number of zircon U-Pb dating was carried out by the TIMS technique. The results show discordia upper and lower intercept ages of 800 to 345

2 700 Ma and 245 to 215 Ma, respectively (Ames et al., 1993, 1996; Rowley et al., 1997; Hacker et al., 1998; Chen et al., 2000, 2003; Chavagnac et al., 2001; Zheng et al., 2003a, 2003b, 2004). These data indicate that the metagranites were probably crystallized in the Neoproterozoic and subject to UHP metamorphism in the Triassic. In this study, we present new data from zircon U-Pb ages, mineral 40 Ar/ 39 Ar ages and fission track ages for the metagranites from the Dabie UHP metamorphic belt. The results not only confirm the previous dating on both protolith and metamorphic ages, but also provide further constraints on the exhumation history of metagranites within the framework of the geodynamic evolution of a collisional orogenic belt. GEOLOGICAL BACKGROUND The Qinling-Dabie-Sulu orogenic belt, stretching in E-W direction over 1000 km, is located in east-central China between the Yangtze Block and the North China Block (Fig. 1). This orogenic belt is characterized by the occurrence of UHP metamorphic rocks including eclogite, kyanite schist, jadeite quartzite, coesite-bearing marble, coesite-bearing gneisses (Wang et al., 1995; Cong, 1996; Liou et al., 1996; Xu et al., 1999). The UHP metamorphic belt in E-W trend is separated into two blocks by the Tan-Lu fault, i.e., the Dabie terrane in west and the Sulu terrane in east (Fig. 1). The Dabie terrane is a major segment consisting of a Table 1. Concentrations of major, trace and rare-earth elements in metagranites from the Dabie terrane 346 R.-C. Wang et al.

3 series of fault-bounded metamorphic units. From north to south, they are: (1) the Beihuaiyang low-grade unit, (2) North Dabie high-t/high-p amphibolite/granulitefacies unit, (3) South Dabie UHP eclogite-facies unit, (4) South Dabie low-t/high-p blueschist-facies unit. Besides coesite and micro-diamond, many other minerals indicative of UHP metamorphic conditions have been identified in South Dabie (Zheng, 2003). Lately, microdiamond has been found in eclogite from North Dabie (Xu et al., 2003), demonstrating both South Dabie and North Dabie underwent the Triassic UHP metamorphism. But the occurrence of granulite-facies rocks in North Dabie but their absence in South Dabie indicate that the two units experienced differential histories of exhumation subsequent to the peak UHP metamorphism (Zheng et al., 2001). Paleoproterozoic to Archean ages were reported to occur in zircons from the North Dabie granulite-facies rocks (Chen et al., 1996; Jian et al., 1999; Wu et al., 2002) and the South Dabie UHP parametamorphic rocks (Chavagnac et al., 2001; Ayers et al., 2002; Chen et al., 2002, 2003). This indicates the presence of very old basement in the northern margin of the Yangtze Block and thus its involvement during continental deep subduction. Metagranites occur in the Dabie terrane with exposure areas from less than 10 to greater 100 km 2. Field observations show that the metagranites are often in direct contact with the other UHP metamorphic rocks. The Table 1. (continued) Metagranites from the Dabie UHP metamorphic belt 347

4 Fig. 1. (A) Sketch map of the geology and tectonics of the Dabie-Sulu orogenic belt; (B) geologic sketch map of metagranites in the Dabie terrane. Roman numerals represent tectonic units: I LT/HP blueschist-facies unit, II UHP unit, III HT/HP granulite-facies unit, IV low-grade metamorphic unit, V Mesozoic sedimentary basin. Bracketed numbers denote the faults: (1) the Xiaotian-Mozitan fault, (2) the Wuhe-Shuihou fault, (3) the Taihu-Mamiao fault, (4) the Haizhou-Siyang faul, (5) the Yantai-Wupeng fault, (6) the Shangchen-Machen fault, (7) the Luoshan-Dawu fault. Dabie terrane is structurally characterized by a series of tight isoclinal folds with moderate S-dipping axial planes. Such a type of structure occurs not only in the UHP paragneisses but also in the marginal zone of metagranites. Within the metagranites, the granoblastic texture is much more common than the gneissic schistosity. All these facts suggest that metagranite protolith experienced a regional metamorphic event contemporaneously with its UHP country rocks. Detailed petrographic descriptions of the samples can be found in Xu et al. (1998) and Sun, H. T. et al. (2002). A brief petrographic description is recalled here. Major minerals in metagranites are plagioclase (An < 5) 25 to 35%, K-feldspar (microcline and orthoclase) 20 to 30%, quartz about 30%. Typically metamorphic minerals are garnet, hornblende (arfvedsonite), biotite, muscovite, epidote, and allanite. Accesory minerals include zircon, sphene, apatite, magnetite etc. Two kinds of paragenetic association of minerals may be established in the different metagranite blocks: (1) plagioclase + K-feldspar + quartz + biotite + Mn-rich garnet + zircon + apatite + allanite + sphene; (2) plagioclase + K-feldspar + quartz + hornblende (arfvedsonite) + Mn-poor garnet + phengitic muscovite + biotite + epidote + zircon + apatite + sphene etc. Two compositional and textural kinds of garnet are commonly found. Mn-rich garnet, typically skeletal, is commonly surrounded by K-feldspar and quartz, and contains to mol.% spessartine. Such a garnet is considered the relict of primary magmatic mineral. In contrast, calcic Mn-poor garnet appears as euhedral discrete grains with limited spessartine content (0.46 to 1.36 mol.%) and is associated with phengite. This type of garnet was certainly metamorphic under conditions similar to those of UHP metamorphic paragneiss associated with the eclogite. Major, trace and rare-earth element contents of the metagranites are presented in Table 1. The samples all have high Si and (Na + K). A/CNK values are 0.85 to Most samples of metagranites are metaluminous. In the R1 R2 diagram, the protolith of metagranites is plotted in the area of late- or post-orogenic granites (Sun, H. T. et al., 2002). Their REE patterns are characterized by enriched LREE, nearly flat HREE and strong negative Eu anomaly, e.g., LREE/HREE ratios: 2.80 to 8.77, mostly 4 to 7; δeu: 0.4 to 0.79 (Fig. 2(a)). Some samples (e.g., G , 09, and 12) also show negative Nb, Sr and Eu anomalies in the trace element spider-diagrams. However, in the samples from Huangzhen (G and -05), only a negative Sr anomaly was observed (Fig. 2(b)). On the other hand, the metagranites are characterized by posi- 348 R.-C. Wang et al.

5 Chondrite normalized Primary-mantle normalized La Pr Eu Tb Ho Tm Lu Ce Nd Sm Gd Dy Er Yb G b G D-025a 98D-035a 98D-035b G (a) (b) 98D-025b G9811-2b G G G G Rb Th K Ce Pr Nd Zr Eu Yb Y Lu U Ba La Nb Sr Hf Sm Gd Er Tm Fig. 2. Chondrite-normalized REE patterns and primitive mantle-normalized trace-elements spider diagram of the metagranites in the Dabie UHP metamorphic belt. Chondrite normalizing values used are from Taylor and McLennan (1985), and primitive mantle normalizing factors are from Hofmann (1988). tive anomalies of Rb, Ba, Nd, Ce and Zr, as well as by negative anomalies of La, Nb, Sr and Eu. The patterns of element geochemistry suggest that the protolith of metagranites were from ancient crustal rocks (Wood et al., 1979). Based on mineral paragenesis Carswell et al. (2000) estimated peak P-T conditions for the metagranites, which range from 690~715 C and 36 kbar to 710~755 C and 18 kbar. Oxygen isotope analysis shows that the metagranites have lower and variable δ 18 O values like associated eclogites (Zheng et al., 1998, 1999, 2004; Fu et al., 1999; Zhang et al., 2003), indicating significant 18 O depletion before plate subduction at Triassic. SAMLES AND ISOTOPIC ANALYSES Sample locations Sample locations are shown in Fig. 1 and relevant regional geology is referred to previous studies in the localities. The samples for this study are described as follows. G B: From Huangzhen in Taihu County, collected at roadside outcrops close to the western side of Hualiangting Reservoir. There is a cold eclogite near the Huangzhen metagranite block (Okay, 1993; Carswell et al., 1997). G : From Shuanghe in Qianshan County, collected at outcrops of a quarry adjacent to the roadside. Some coesite-bearing eclogites are near the metagranite block (Cong et al., 1995). G : From Bixiling in Yuexi County, collected at roadside outcrops near the eastern end of Shuifan Bridge on the Qianshui River. Some xenoliths of coesitebearing eclogites appear in the marginal part of the metagranite block (Zhang et al., 1995; Xiao et al., 2000). G : From Xindian, northern area of Qianshan County, collected at roadside outcrops. A micro-diamond inclusion in garnet grain was found from eclogite near the metagranite block (Xu et al., 1992; Okay, 1993). Analytical methods Zircons were extracted from approximately 20 kg rock for each sample, using standard techniques of density and magnetic separation. The non-magnetic zircon grains were selected for analyses based on size, clarity, color and morphology. U-Pb TIMS isotopic analyses of single-grain zircon were performed at Tianjing Institute of Geology and Mineral Resources. Size fractions used for analysis varied somewhat, depending on U content and grain size of zircons. Separation of U and Pb after zircon digestion and isotope dilution mass-spectrometry using a 205 Pb- 235 U enriched tracer solution were done following the procedures described by Krogh (1973) and modified by Lu and Li (1991). Total blanks for Pb and U are less than 0.05 and ng, respectively. Isotope ratios were measured by single collector peak-jumping in the VG-354 TIMS equipped with a Daly-type detector operating in ioncounting mode. Errors to the atomic and isotopic ratios are quoted at the 2σ confidence level. All ages were calculated using the isotopic abundance ratios and decay constants of Jaffey et al. (1971). Common Pb correction was made by the model Pb compositions at the determined age of the samples. Upper and lower intercept ages were calculated by a regression technique described by Davis (1982). In addition to isotope dilution mass-spectrometry, separated zircon grains from the Shuanghe metagranite (G ) were selected for elemental analysis on a JEOL JXA 8800 electron microprobe at State Key Laboratory for Mineral Deposits Research, Nanjing University. The operating conditions were: accelerating voltage 20 kv, probe current 20 na, measuring time 20 s for peak Metagranites from the Dabie UHP metamorphic belt 349

6 Table 2. Average electron-microprobe results (wt.%) of zircon from the Shuanghe metagranite (G ) in the Dabie terrane n = number of analyses. SiO 2 ZrO 2 HfO 2 UO 2 ThO 2 PbO Y 2 O 3 Zr/Hf Th/U G Metamorphic zircon (n = 4) G Magmatic core (n = 1) Metamorphic rim (n = 4) G Metamorphic zircon (n = 4) G Magmatic core (n = 5) Metamorphic rim (n = 2) and 10 s for background. The following standards were used: fayalite (Si), (Zr, Y)O 2 (Zr, Y), uraninite (U, Th) and metallic Hf. Analytical results, shown in Table 2 and similar to those found by Rowley et al. (1997) in eastern Dabie, reveal two types of zircons. The first type displays inherited magmatic core with high Th/U and Zr/Hf ratios and metamorphic overgrown rim with contrasting Th/U and Zr/Hf ratios. The second one consists exclusively of metamorphic zircons with lower Th/U and Zr/Hf ratios and lacking igneous core (Table 2). 40 Ar/ 39 Ar dating was carried out on relatively coarsegrained (0.3 to 0.15 mm) mineral separates. Four mineral concentrates (muscovite, biotite, amphibole and potassic feldspar) were analyzed for 40 Ar/ 39 Ar using stepwiseheating techniques. All analyses were performed in a MM noble gas mass spectrometer at Institute of Geology, China Seismological Bureau, Beijing, China. The operating procedure can be found in Chen et al. (1992). A plateau age is defined when four or more continuous gas fractions yielded similar apparent K/Ca ratios and each fraction represents >5% of the total 39 Ar and all fractions together constitute >50% of the total 39 Ar. The samples used for fission track (FT) dating include mineral concentrates of apatite, zircon and sphene separated from the metagranites at Shuanghe (G ), and Bixiling (G ), respectively. All FT ages were not corrected for sampling elevations, because they were approximately at the same elevations, i.e., approximately 120 m as measured by GPS. The mineral separates were obtained by heavy-liquid separation and handpicking under binocular microscope. Sample purity was 100%. With the Exo-detector method the spontaneous tracks were counted in the etched mineral whilst the induced tracks were also counted in the same area with low uranium muscovite held against the mineral during irradiation and subsequent etching. The tracks were counted and corrected by International Standard Durango Apatite and SRM612 Glass from NBS, USA. Sample preparation and FT counting of six mineral concentrates were completed at Institute of Geology, China Seismological Bureau, Beijing. Operating procedures were described by Liu et al. (1984) and Wan et al. (2000). FT ages were calculated using a zeta of ± 18 for SRM612 glass from NBS, USA and followed by the procedures described by Hurford and Green (1982). ISOTOPE GEOCHRONOLOGY Zircon U-Pb ages Euhedral and non-magnetic zircon grains were selected for U-Pb isotopic analyses. The crystals were different in size, color, shape and transparency among grains. Both transparent and opaque zircons were found in samples G , -09, and -12, whereas in sample G , only transparent zircons were present. Analytical results are shown in Table 3. The upper and lower intercept ages of zircon from sample G are 726 ± 15 Ma and ± 6.3 Ma, respectively (Table 3, Fig. 3(a)). Note that two zircon grains (No. 1 and No. 2) yield nearly concordant ages ca. 375 Ma and 386 Ma, respectively (Table 3, Fig. 3(a)). The eclectron-microprobe analyses show high Th/U ratios of 1.12 to 6.98 for magmatic cores, but low Th/U ratios of 0.06 to 0.66 for metamorphic rims (Table 2). Two groups of zircons were separated for U-Pb dating from G : five opaque zircons have upper and lower intercept ages of 714 ± 14 Ma and 209 ± 14 Ma, respectively (Fig. 3(b)), whereas seven transparent zircons yield intercept ages of 611 ± 109 Ma and 343 ± 62 Ma, respectively (Fig. 3(c)), suggesting complex zircon crystallization history. However, U-Pb analyses of three fractions (Nos. 6, 8, and 9) yield a weighted mean concordant age of 365 ± 8 Ma. Note that seven transparent zircons in this metagranite display nearly concordant ages, 350 R.-C. Wang et al.

7 Table 3. Zircon U-Pb isotopic data for metagranites in the Dabie terrane Metagranites from the Dabie UHP metamorphic belt 351

8 352 R.-C. Wang et al. Table 3. (continued)

9 ranging from 361 Ma to 552 Ma (Table 3, Fig. 3(c)). Besides two nearly concordant ages of 284 Ma and 382 Ma, the intercept ages of five zircons from the sample G are 720 ± 61 Ma and 197 ± 21 Ma, respectively (Table 3, Fig. 3(d)). Eight transparent zircon grains from Huangzhen metagranite (G B) yield upper and lower intercept ages of 755 ± 34 Ma and 299 ± 33 Ma, respectively (Table 3, Fig. 3(e)). Note that no post- Paleozoic ages have ever been measured in zircons from this sample. Note: [a] Concentrations are known to ±30% for sample weights of about 30 µg, and ±5% for sample >100 µg. [b] Common Pb, assuming all has blank isotopic composition. [c] 206 Pb/ 204 Pb ratios have been corrected by experimental blank (Pb = ng, U = ng) and the 205 Pb/ 235 U spike. [d] 206 Pb, 207 Pb and 208 Pb are radiogenic Pb only; and the number in brackets is uncertainty in 2σ. For example: (8) means ± (2σ). Mineral 40 Ar/ 39 Ar ages Amphibole (G H) was identified as arfvedsonite by electron-microprobe analysis. The Na-rich metamorphic amphibole may represent the product of retrograde metamorphism. The arfvedsonite (G H), biotite (G B) and muscovite (G M) yielded well-defined intermediate and high temperature plateaus, corresponding to ages of ± 3.5 Ma, ± 2.5 Ma and ± 2.7 Ma, respectively. The plateau ages were obtained from Ar representing 64.4%, 90.7% and 89.5% of total Ar evolved in stepwise-heating (Table 4, Figs. 4(a), (b), (c)). Arfvedsonite (G H) gave an older apparent age of Ma in the second lowest temperature step; it could indicate Ar release from minor inclusions in the sample. 40 Ar/ 39 Ar ages of arfvedsonite and biotite were Ma and Ma, respectively. Such a difference in age is probably attributed to their distinctive Ar closure temperatures (hornblende: 500 C; biotite: 300 C) (Harrison et al., 1979, 1985; Yamada and Harayama, 1999). K-feldspar (G ) yielded a complex age spectra of >327 Ma, 206 Ma and 102 Ma (Table 4, Fig. 4(d)). Fission track ages FT dating results (e.g., central ages) of sphene, zircon and apatite are listed in Table 5. The FT ages decrease regularly from the oldest age shown by sphene through intermediate age shown by zircon to the youngest age shown by apatite (Table 5). The age patterns are consistent with FT closure temperatures of sphene (250 C), zircon (220 C) and apatite (110 C) as determined by Wagner et al. (1977), Gleadow and Brooks (1979), Harrison et al. (1979, 1985), Yamada and Harayama (1999), and Wan et al. (2000). The tracks in apatites from G Ap and G Ap have their mean length distribution of ± 2.61 µm and ± 2.25 µm, respectively. The frequency distribution of FT lengths in the apatite samples follows a similarly normal single-peak distribution pattern. The results suggest that the metagranites were not disturbed by any other geological heating event, which were more covered than the exhumation in the Cenozoic (Gleadow et al., 1986). Metagranites from the Dabie UHP metamorphic belt 353

10 206 Pb/ 238 U (a) Shuanghe metagranite (G ) Intercepts at 726+/-15 and /-6.3 Ma (MSWD=1.3) Pb/ 235 U Pb/ 238 U (b) Bixiling metagranite (G , 5 opaque zircon grains) Intercepts at 714+/-14Ma and 209+/-14Ma (MSWD=1.9) 207 Pb/ 235 U 206 Pb/ 238 U (c) Bixiling metagranite (G , 7 transparent zircon grains) /-8Ma Intercepts at 611+/-109 and 343+/-62 Ma (MSWD=1.5) Pb/ 235 U Pb/ 238 U (d) Xindian metagranite (G ) Intercepts at 720+/-61 and 197+/-21 Ma (MSWD=0.45) 207 Pb/ 235 U (e) Huangzhen metagranite (G B) Pb/ 238 U Intercepts at 755+/-34 and 299+/-33Ma (MSWD=0.61) Pb/ 235 U Fig. 3. U-Pb concordia plot for zircons from the metagranites in the Dabie UHP metamorphic belt. DISCUSSIONS Neoproterozoic protolith The zircon U-Pb discordia upper-intercept ages for four metagranite samples range from 755 ± 34 to 611 ± 109 Ma (Table 3, Figs. 3(a) (d)). Some transparent and short-prismatic zircons are plotted on the upper part of their U-Pb discordia; in fact they lie very close to the upper intercepts (Table 3, Figs. 3(a) (d)). These zircons have lower U contents than the non-transparent ones. The Neoproterozoic ages were also obtained by previous studies (Zheng et al., 2003b, and references therein), and thus interpreted as crystallization timing of metagranite protolith which was formed as a result of the Neoproterozoic rift-magmatism. Although Xie et al. (2001) reported a zircon upper-intercept age of 576 ± 49 Ma, it can be the minimum age of metagranite protolith. The all available zircon U-Pb ages for the Dabie metagranites indicate a strong granitic magmatic activity in the period from 800 to 700 Ma in the northern margin of the Yangze Block (Zheng et al., 2004). In fact, Neoproterozoic granitoids have been found 354 R.-C. Wang et al.

11 Table 4. Step-heating data of 40 Ar/ 39 Ar dating for metagranites in the Dabie terrane Notice: Before irradiation, mineral fractions were packed in Sn-foil and irradiated at the fast neutron bombardment reactor in Beijing Institute of Academic Energy, Chinese Academy of Sciences. Using the double-vacuum resistance furnace to melt mineral samples, the furnace has regular temperature calibration by means of an optical pyrometer correlated to precise furnace power output. The temperature of furnace is at 10 C for each step with 30-minute step-duration. Interfering reactions on K, Ca and Cl were determined by irradiation of pure salts together with the samples; variations in Cl/K ratios were monitored through the induced 38 Ar Cl / 39 Ar K ratios. 40 Ar* = radiogenic 40 Ar. 38 Ar Cl = 38 Ar transformed from Cl. 39 Ar K = 39 Ar transformed from 39 K. Metagranites from the Dabie UHP metamorphic belt 355

12 Apparent Age (Ma) (a) Arfvedsonite, Shuanghe (G H) Plateau age:203.9 Ma % 39 Ar cumulative Apparent Age (Ma) (b) Biotite, Shuanghe (G B) Plateau age:180.8 Ma % 39 Ar cumulative Apparent Age (Ma) (c) Muscovite, Bixiling (G M) Plateau:196.4 Ma Apparent Age (Ma) (d) K-feldspar, Xindian (G K) % 39 Ar cumulative % 39 Ar cumulative Fig Ar/ 39 Ar age spectra of minerals from metagranites in the Dabie UHP metamorphic belt. Table 5. Mineral fission-track data from the Shuanghe and Bixiling metagranites in the Dabie terrane Sample No. Mineral Counted grain U (ppm) ρ s /n (10 6 /cm 2 ) ρ i /n (10 6 /cm 2 ) ρ s /ρ i Age ± 1σ (Ma) Shuanghe G Ap Apatite ± 1.9 G Zr Zircon ± 3.8 G Sp Sphene ± 3.2 Bixiling G Ap Apatite ± 2.8 G Zr Zircon ± 1.7 G Sp Sphene ± 4.2 around the Yangtze Block, namely, in its southeastern, western and southern margins (see summary by Li, Z-X. et al. (2003) and Zheng et al. (2003b)). The numerous Neoproterozoic granitoid magmatism in South China was traditionally interpreted as a product of orogenesis, marking the cratonisation of the Yangtze Block (e.g., Li and McCulloch, 1996; Li, 1999). Recently, some researchers interpreted their occurrence as a result of the Neoproterozoic rift-type magmatism, crustal unroofing or melting above mantle superplume (Li, Z-X. et al., 1999, 2003; Deng et al., 2001; Li, X-H. et al., 2002, 2003). On the basis of a combined study of zircon oxygen isotope and U-Pb dating for the metagranites, Zheng et al. (2003a, 2003b, 2004) demonstrate that the protolith of metagranites in the Dabie terrane were produced from rifttype magmatism associated with breakup of the Rodinia supercontinent; meteoric water of cold climate in response to the snowball Earth event took part in water-rock interaction during the Neoproterozoic to result in low δ 18 O magmas due to coeval superwelling beneath rifting continental margins. Early Mesozoic metamorphism Apart from metamorphic zircons in the metagranites, some grains show secondary overgrowth, e.g., an inherited igneous core with lower U content and higher Th/U ratio, and a metamorphic overgrowth rim with contrasting U-Th characters (Table 2). Both overgrown rims and metamorphic grains are usually plotted on the lower portion of the U-Pb discordia with lower intercept ages of 225 ± 6 Ma (G ), 209 ± 14 Ma (G ) and 197 ± 21 Ma (G ) (Table 3, Figs. 3(a), (b), (d)). 356 R.-C. Wang et al.

13 Interestingly, these ages fall within, but somewhat later than, the Dabie UHP metamorphic event from 245 to 225 Ma (Ames et al., 1993, 1996; Li, S.-G. et al., 1993, 1994, 1999, 2000; Chavagnac and Jahn, 1996; Rowley et al., 1997; Hacker et al., 1998; Chavagnac et al., 2001; Ayers et al., 2002; Zheng et al., 2003b). Amphibolite-facies retrogression may be a major reason to zircon overgrow and associated resetting of U-Pb system to Early Jurassic ages (Li et al., 2000; Zheng et al., 2003c). It is worthy to note that zircon U-Pb lower intercept age of 225 ± 6 Ma from the metagranite at Shuanghe (G ) is consistent with Sm-Nd mineral isochron ages of Ma from coesite-bearing eclogite and of Ma from UHP paragneiss at Shuanghe (Li et al., 2000). A few rutile inclusions can be observed in the zircon, and rutile concentrates were separated from the Shuanghe metagranite. Carswell et al. (2000), You et al. (2000), Liu, F-L. et al. (2001) and Liu, J-B. et al. (2001) also observed that the above-mentioned mineral assemblages in metagranites (i.e., granitic orthogneisses in their notation) from eastern Dabie were similar to those in the eclogite-bearing paragneisses despite some distinct differences in mineral chemistry. Therefore, the metagranites would experience a similar episode of UHP metamorphism to the associated eclogite and paragneiss. Possible interpretation of Paleozoic ages Did the Neoproterozoic granites experience a single UHP metamorphic event in the Early Mesozoic or did they go through polymetamorphism? It is evident from available geochronological data on different radiometric systems that the UHP metamorphism in the Dabie terrane took place in the Early Mesozoic. However, a few chronological data seem to suggest a Paleozoic metamorphic event in this region. Among all interesting data on zircon U-Pb dating, some SHRIMP 206 Pb/ 238 U ages are worthy to mention. These include: (1) 424 ± 5 Ma for core and 301 ± 13 Ma for rim, respectively, in zircons from the Xiongdian eclogite, northwestern Dabie (Jian et al., 2000; Sun, W.-D. et al., 2002); (2) 757 ± 7 to 296 ± 4 Ma for cores and 494 ± 12 to 210 ± 7 Ma for rims, respectively, in zircons from the Bixiling eclogite in eastern Dabie (Cheng et al., 2000). In addition, magmatic events during the Ordovician to Devonian are demonstrated by concordant whole-rock Rb-Sr and Sm-Nd isochron ages of 445 ± 31 Ma and 446 ± 23 Ma, respectively, for metavolcanics from the Dingyuan Formation in northwestern Dabie (Li et al., 2001). Among the zircon U-Pb ages presented in this paper, Paleozoic ages were also obtained in the metagranites of Neoproterozoic protolith: (1) the lower-intercept age of 299 ± 33 Ma for the zircon in the Huangzhen metagranite (G B) (Table 3, Fig. 3(e)); (2) two concordant ages of 375 Ma and 386 Ma for transparent zircons in the Shuanghe metagranite (G ) (Table 3, Fig. 3(a)). We also obtained the discordia intercept ages of 726 ± 15 Ma and ± 6.3 Ma for this sample (Table 3, Fig. 3(a)); (3) in the Xindian metagranite (G ), zircons yield two concordant ages of 284 ± 5 Ma and 382 ± 12 Ma in addition to the upper and lower intercept ages of 720 ± 61 Ma and 197 ± 21 Ma, respectively (Table 3, Fig. 3(d)); (4) a discordia constructed from the analyses of seven transparent zircons from the Bixiling metagranite (G ) yield the upper and lower intercept ages of 611 ± 109 Ma and 343 ± 62 Ma, respectively (Table 3, Fig. 3(c)). However, the five opaque zircons yield a discordia with upper and lower intercept ages at 714 ± 14 Ma and 209 ± 14 Ma, respectively (Table 3, Fig. 3(b)). Although the above-mentioned U-Pb ages are somewhat variable, the Dabie metagranites, like the eclogites, still give the information concerning the possible occurrence of Paleozoic metamorphic events. However, it is unclear whether the Paleozoic ages reflect geological events that took place during the Paleozoic in the Dabie terrane. On the other hand, it is possible that differential U-Pb resetting occurred in the metagranite protolith during the Triassic UHP metamorphism because the protolith experienced the meteoric-hydrothermal alteration in the Neoproterozoic (Zheng et al., 2003a, b). As a consequence, the zircons in the altered granitoids may have spectacular behaviors of U/Pb differentiation by both Pb loss and U gain from those zircons in altered igneous rocks during the UHP metamorphism (Zheng et al., 2004), resulting in the apparently concordant ages in the Paleozoic. EXHUMATION HISTORY OF DABIE METAMORPHIC BELT Early Mesozoic period A detailed study of mineral Sm-Nd and Rb-Sr isochron dating was accomplished by Li et al. (2000) for UHP eclogites and gneisses at Shuanghe in eastern Dabie. The results show that the Sm-Nd isochrons give consistent Triassic ages of 213 to 238 Ma for eclogite-facies metamorphism, but the Rb-Sr isochrons give Jurassic ages of 171 to 174 Ma for the same samples. Oxygen isotope geothermometry of Zheng et al. (2003c) for the Sm-Nd and Rb-Sr isochron minerals yields two sets of temperatures of 600 to 720 C and 420 to 550 C, respectively, corresponding to cessation of isotopic exchange by diffusion at about 225 ± 5 Ma during eclogite-facies recrystallization and that at about 175 ± 5 Ma during amphibolite-facies retrogression. Although the closure temperatures of O diffusion may not simply correspond to those of Sr or Nd diffusion (Zheng et al., 2002, 2003c), O, Sr and Nd isotopic resetting is evident by two episodes of metamorphism in the Late Triassic and at the beginning of Middle Jurassic, respectively. Accordingly, Li et al. (2000) and Zheng et al. (2003c) reconstructed Metagranites from the Dabie UHP metamorphic belt 357

14 Age (Ma) Temperature ( o C) eclogite-facies metamorphism Amphibolite-facies retrogression Fig. 5. Cooling paths of metagranite G at Shuanghe in the Dabie UHP metamorphic belt. Dash-dot line with asterisk denotes a two-stage cooling history of eclogite and gneiss at Shuanghe subsequent to eclogite- and amphibolite-facies metamorphism, respectively (data for age and closure temperature are after Li et al. (2000) and Zheng et al. (2003a)). the cooling history of two stages for the Shuanghe UHP metamorphic rocks as follows. A path of slow cooling from ~215 Ma at ~600 C to ~180 Ma at ~550 C (~1.4 C/ Ma) is thus bracketed by the first path of rapid cooling from ~225 Ma at ~750 C to ~215 Ma at ~600 C (~15 C/ Ma) and the second path of rapid cooling from ~180 Ma at ~550 C to ~170 Ma at ~400 C (~15 C/Ma). Assuming that the peak metamorphism of the Shuanghe metagranite (G ) occurred at 225 ± 6 Ma as determined by the zircon U-Pb lower-intercept age. The 40 Ar/ 39 Ar plateau age of arfvedsonite (G H) yielded an older retrograde metamorphic age of 204 ± 4 Ma, and that of biotite (G B) yielded a younger age of 181 ± 3 Ma. It has been proposed that the U-Pb closure temperatures of zircon dating range from >850 C to 650 C (Cliff, 1985). In this study, an average closure temperature of 710 C was assumed. It coincides with the peak metamorphic temperature at Shuanghe estimated from the petrological data of paragenetic minerals in the metagranite (Carswell et al., 2000). The Ar closure temperatures for hornblende and biotite were estimated at 500 C and 300 C, respectively (Harrison et al., 1979, 1985; Yamada and Harayama, 1999). Therefore, the calculated earlier cooling rate of the metagranite (G ) is 10.2 C/Ma in the early stage (from 225 to 204 Ma), and the cooling rate is 8.7 C/Ma (from 204 to 181 Ma) in the later stage (Fig. 5). The two cooling rates are similar to each other and thus may point to a single-stage cooling subsequent to the eclogite-facies metamorphism of the metagranite. This is significantly different from the previous studies of cooling history for the UHP eclogites that suggested not only high rates of about 40 C/Ma for initial cooling (Chavagnac and Jahn, 1996; Li et al., 2000) but also two-stage cooling history with the first period of rapid cooling following by the second period of slow cooling (Li et al., 2000). It appears that relatively less rapid rates of initial cooling are obtained for the granitic orthogneisses (metagranites) subsequent to the peak metamorphism. The zircon U-Pb lower intercept age of 209 Ma and the muscovite 40 Ar/ 39 Ar plateau age of Ma were determined from the Bixiling metagranites (G ). Assuming the Ar closure temperature of muscovite at 375 C (Wagner et al., 1977), a cooling rate of 26.6 C/ Ma can be estimated for the metagranite. However, the amphibolite-facies retrogression may start to take place at about 215 Ma with a protracted period of slow cooling till about 180 Ma (Li et al., 2000; Zheng et al., 2003c). In this regard, a rapid cooling at 26.6 C/Ma for the UHP metamorphic rocks in the period from 209 Ma to Ma is incompatible with the known history of exhumation (Chavagnac and Jahn, 1996; Li et al., 2000). On the other hand, the differences in the cooling rate that was estimated by different workers may be due in part to the difference in assuming closure temperatures for the minerals and the ages obtained from the minerals. An alternative explanation would involve diverse paths of exhumation between subducted and exhumed slabs in the orogenic belt, which remains to resolve. Late Mesozoic period We have obtained the 40 Ar/ 39 Ar ages of 102 Ma, 206 Ma and >327 Ma for K-feldspar from the Xindian metagranite (G ) by stepwise heating (Table 4). In addition, there exists a set of secondary apparent ages (113.5, and Ma) between two main age spectra of 102 Ma and 198 Ma. The volume of 39 Ar released from the three heating steps was less than 5% of the total volume of gas (Table 4, Fig. 4(d)). These data seem to suggest the Xindian metagranite was first metamorphosed at Triassic, and then reset at Early Cretaceous. Chen et al. (1995) and Zhou et al. (2003) reported cooling ages of 120 to 123 Ma for the Tiantangzhai granite in northwestern Dabie. In the present study, however, there is no data to suggest that a rapid tectonic uplift-cooling event ever occurred in this period. Since some practically concordant results from FT and (U + Th)/He dating for the Tiantangzhai granite within the limits of analytical uncertainty were obtained, a possible explanation may be that the cooling ages of 120 Ma were most likely a response to intensive magma-emplacement and relevant 358 R.-C. Wang et al.

15 cooling event in shallow levels (Sang et al., 2000) rather than a simple cooling event responsible for tectonouplift. The occurrence of voluminous Early Cretaceous volcanics and granites in the Dabie terrane indicates that crustal extension may have taken place at that time. Because of intensive magmatism at Early Cretaceous throughout the Dabie terrane (e.g., Hacker et al., 1998; Jahn et al., 1999), it is difficult to estimate a starting point for cooling of the metagranites at 120 to 110 Ma. The present study obtained that the FT ages of sphene, zircon and apatite from the metagranite at Shuanghe (G ) are 93.8 Ma, 53.9 Ma and 50.0 Ma, respectively (Table 5). The FT closure temperatures of these minerals are usually assumed to be 250 C, 200 C and 110 C, respectively (Gleadow and Brooks, 1979; Harrison et al., 1979; Zaun and Wagner, 1985; Tagami et al., 1996). Thus cooling rates are calculated to be 1.25 C/Ma and 23.1 C/ Ma, respectively. The very slow cooling rates indicate that the rocks remained isothermally from about 94 to 54 Ma (Fig. 5). Similarly, the FT ages of sphene, zircon and apatite in sample G from Bixiling are Ma, 57 Ma and 53.6 Ma, respectively (Table 5). Correspondingly, cooling rates estimated are 0.88 C/Ma and 26.5 C/Ma, respectively. It appears that very slow cooling at 0.88 to 1.25 C/Ma occurred during the Late Mesozoic, but much fast cooling at 23.1 to 26.5 C/Ma took place during Early Cenozoic. Therefore, the FT dates indicate that the Dabie slab experienced significantly different rates of exhumation in the transition from Late Mesozoic to Early Cenozoic. Block-fault differential uplift in Cenozoic Pairs of FT central ages determined from zircons and apatites from the metagranites (G and -09) are 53.9 ± 3.8 Ma vs ± 1.9 Ma and 57.0 ± 1.7 Ma vs ± 2.8 Ma, respectively. Based on the difference in FT closure temperatures between zircon and apatite discussed above, their cooling rates were 23.1 C/Ma and 26.5 C/Ma for the two rocks, respectively (Fig. 5). Frequency distribution of FT lengths as measured from two apatites (G Ap and -09Ap) shows normal singlepeak style distribution. Those patterns mean that results obtained for the FT dating of apatites are reliable. Thus the uplifted metagranites did not experience significant tectonism during uplift-cooling event since the Early Cenozoic. We believe that the main structural configurations of the Dabie UHP orogenic belt were developed during the rapid exhumation in the Early Cenozoic. Relationship to sedimentation in the Hefei Basin The tectonic-thermal events of the Dabie UHP metamorphic belt may have triggered other geological events in adjacent areas. The Hefei backland basin in the southern margin of the North China Block was closely bound to the south by the Dabie orogenic belt. The basin subsidence was structurally coupled with the exhumation of the Dabie orogenic belt (Liu et al., 1992; Han, 1996; Wang and Cong, 1997). For instance, in Early Jurassic, the basin received coarse-clastic sediments of Triassic eclogites and metagranites (Li, R. W. et al., 2003), with average thickness of about 4.5 km; it reaches a maximum thickness of 6 to 7 km in Shucheng trough (Han, 1996). The sedimentary period coincides with the first rapid exhumation of the Dabie UHP rocks. Li, Z-X. et al. (1999, 2001) also suggested that the sources of detrital sedimentary rocks were transported from the Dabie terrane. During the approximately isothermal uplift stage of the Dabie orogenic belt in the Middle Mesozoic, there was only 1.1 km thickness of sedimentary rocks deposited in the southern part of the basin. In the Early Cenozoic, however, thick-bedded detrital sedimentary rocks became predominant deposits in the basin; its thickness is over 1.8 km in the south and over 2 km in the northern part of the basin (Han, 1996). The sedimentary period of the thick-bedded clastic rocks coincides with the slow uplift period of the Dabie orogenic belt in the Early Cenozoic. In summary, the subsidence and sedimentation in the Hefei backland basin and the final exhumation of the Dabie orogenic belt were spatially and temporally related to each other; they were not isolated as independent geologic events. Acknowledgments Prof. S. T. Xu provided field guidance and assistance, and valuable discussions. We thank Profs. B. L. Cong, S.-G. Li, Y.-F. Zheng and Z.-Q. Zhong for suggestions and discussions. Comments by Dr. B.-m. Jahn and two anonymous reviewers greatly helped improvement of the manuscript. This work was supported from the Natural Science Foundation of China (Grants and ). REFERENCES Ames, L., Zhou, G.-Z. and Xiong, B.-C. (1996) Geochronology and isotopic character of ultrahigh-pressure metamorphism with implications for the collision of the Sino-Korean and Yangtze cratons, central China. Tectonics 15, Ames, S., Tilton, G. R. and Zhou, G. (1993) Timing of collision of the Sino-Korean and Yangtze Craton: U-Pb zircon dating of coesite-bearing eclogites. Geology 21, Ayers, J. C., Dunkle, S., Gao, S. and Miller, C. E. (2002) Constraints on timing of peak and retrograde metamorphism in the Dabie Shan ultrahigh-pressure metamorphic belt, eastcentral China, using U-Th-Pb dating of zircon and monazite. Chem. Geol. 186, Carswell, D. A., O Brien, P. J., Wilson, R. N. and Zhai, M. (1997) Thermobarometry of phengite-bearing eclogites in the Dabie Mountains of central China. J. Metamor. Geol. 15, Carswell, D. A., Wilson, R. N. and Zhai, M. (2000) Metamorphic evolution, mineral chemistry and thermobarometry of Metagranites from the Dabie UHP metamorphic belt 359

16 schists and orthogneisses hosting ultrahigh pressure eclogites in the Dabieshan of Central China. Lithos 52, Chavagnac, V. and Jahn, B. M. (1996) Coesite-bearing eclogites from the Bixiling complex, Dabie Mountains, China: Sm- Nd ages, geochemical characteristics and tectonic implications. Chem. Geol. 133, Chavagnac, V., Jahn, B.-m., Villa, I. M., Whitehouse, M. J. and Liu, D.-Y. (2001) Multichronometric evidence for an in situ origin of the ultrahigh-pressure metamorphic terrane of Dabieshan, China. J. Geol. 109, Chen, D. G., Clark, I. C., Zi, X., Zhou, T., Chen, H. and Xia, Q. (2000) Zircon U-Pb ages for gneiss from Qianshan, Anhui. Chinese Sci. Bull. 45(8), Chen, D.-G., Deloule, E., Xia, Q.-K., Wu, Y.-B. and Cheng, H. (2002) Metamorphic zircon from Shuanghe ultra-high pressure eclogite, Dabieshan: ion microprobe and internal micro-structure study. Acta Petrol. Sin. 18(3), (in Chinese with English abstract). Chen, D.-G., Deloule, E., Cheng, H., Xia, Q.-K. and Wu Y.-B. (2003) Preliminary study of micro-scale zircon oxygen isotopes for Dabie-Sulu metamorphic rocks: Ion probe in site analyses. Chinese Sci. Bull. 48, Chen, J.-F., Xie, Z., Liu, S.-S., Li, X.-M. and Foland, K. A. (1995) Cooling age of Dabie orogen, China, determined by 40 Ar- 39 Ar and fission track techniques. Sci. China (B) 38, Chen, N.-S., You, Z.-D., Suo, S.-T., Yang, Y. and Li, H.-M. (1996) Zircon U-Pb ages of intermediate granulite and deformed granites in the Dabie Mountains, central China. Chinese Sci. Bull. 41, Chen, W. J., Harrison, T. M. and Lovera, O. M. (1992) Thermochronology of the Ailoshan Red River shear zone: A case study of multipe diffusion domain model. Seismol. Geol. 14(2), (in Chinese with English abstract). Cheng, Y., Liu, D., Williams, I. S., Jian, P., Zhuang, Y. and Gao, T. (2000) SHRIMP U-Pb dating of zircons of a darkcoloured eclogite and a garnet-bearing gneissic-granitic rock from Bixiling, eastern Dabie Area: Isotope chronological evidence of Neoproterozoic HP-UHP metamorphism. Acta Geol. Sin. 74, 721. Cliff, R. A. (1985) Isotopic dating of metamorphic belts. J. Geol. Soc. Lond. 142, Cong, B.-L. (1996) Ultrahigh-Pressure Metamorphic Rocks in the Dabieshan-Sulu Region of China. Science Press, Beijing, 224 pp. Cong, B.-L., Zhai, M.-G., Carswell, D. A., Wilson, R. N., Wang, Q.-C., Zhao, Z.-Y. and Windley, B. F. (1995) Petrogenesis of ultrahigh-pressure rocks and their country rocks at Shuanghe in the Dabie Mountains, Central China. Eur. J. Mineral. 7, Davis, D. W. (1982) Optium linear regression and error estimation applied to U-Pb data. Can. J. Earth Sci. 19, Deng, S. X., Wang, J. H. and Zhu, B. Q. (2001) PTt path of metamorphism for the Julin Group and its geodynamical implications in Yuanmou, Yunnan. Sci. China (D) 44(7), Fu, B., Zheng, Y.-F., Wang, Z.-R., Xiao, Y.-L., Gong, B. and Li, S.-G. (1999) Oxygen and hydrogen isotope geochemistry of gneisses associated with ultrahigh pressure eclogites at Shuanghe in the Dabie Mountains. Contrib. Mineral. Petrol. 134, Gleadow, A. J. W. and Brooks, C. K. (1979) Fission track dating, thermal histories and tectonics of igneous intrusions in East Greenland. Contrib. Mineral. Petrol. 71(1), Gleadow, A. J. W., Duddy, I. R., Green, P. F. and Lovering, J. F. (1986) Confined fission track lengths in apatite: a diagnostic tool for thermal history analysis. Contrib. Mineral. Petrol. 94, Hacker, B. R., Ratshbacher, L., Webb, L., Ireland, T., Walker, D. and Dong, S. (1998) U/Pb zircon ages constrain the architecture of the ultrahigh-pressure Qinling-Dabie Orogen, China. Earth Planet. Sci. Lett. 161, Han, S. (1996) Reviews of Mesozoic-Cenozoic Sedimentary Basins, Northern Anhui Province. Geol. Press. House, Beijing, 141 pp. (in Chinese). Harrison, T. M., Armstrong, R. L., Naeser, C. W. and Harakal, J. E. (1979) Geochronology and thermal history of the coast plutonic complex, near Prince Rupert, B.C. Can. J. Earth Sci. 16, Harrison, T. M., Duncan, I. and McDougall, I. (1985) Diffusion of 40 Ar in biotite: Temperature, pressure and compositional effects. Geochim. Cosmochim. Acta 49, Hofmann, A. W. (1988) Chemical differentiation of the Earth, the relationship between mantle, continental crust and oceanic crust. Earth Planet. Sci. Lett. 90, Hurford, A. J. and Green, P. F. (1982) A user s guide to fission track dating calibration. Earth Planet. Sci. Lett. 59, Jaffey, A. H., Flynn, K. F., Glendenin, L. E., Bentely, W. C. and Essling, A. M. (1971) Precision measurement of half-lives and specific activities of 235 U and 238 U. Phys. Rev., Sect. C: Nucl. Phys. 4, Jahn, B.-m., Wu, F.-Y., Lo, C.-H. and Tsai, C.-H. (1999) Crustmantle interaction induced by deep subduction of the continental crust: geochemical and Sr-Nd isotopic evidence from post-collisional mafic-ultramafic intrusions of the northern Dabie complex, central China. Chem. Geol. 157, Jian, P., Yang, W.-R. and Zhang, Z.-C. (1999) 207 Pb/ 206 Pb zircon dating of the Huangtuling hypersthene-garnet-biotite gneiss from the Dabie Mountains, Luotian County, Hubei Province, China: New evidence for Early Precambrian evolution. Acta Geol. Sin. 73, Jian, P., Liu, D., Yang, W. and Williams, I. S. (2000) Petrographical study of zircons and SHRIMP dating of the Caledonian Xiongdian eclogites, northwestern Dabie Mountains. Acta Geol. Sin. 74, Krogh, T. E. (1973) A low contamination method for hydrothermal decomposition of zircon and extraction of U and Pb for isotope age determinations. Geochim. Cosmochim. Acta 37, Li, R. W., Sang, H. Q., Zhang, R.-H., Chu, Z. Y., Li, S. Y., Jin, F. Q. and Jiang, M. S. (2003) Geochronology of source materials from high-pressure and ultrahigh-pressure metamorphic rocks in Jurassic sedimentary rocks of Hefei Ba- 360 R.-C. Wang et al.