Dados Geocronologicos Craton Da China

J. metamorphic Geol., 2009, 27, 125–138

doi:10.1111/j.1525-1314.2008.00810.x

Geochronological and petrological constraints on Palaeoproterozoic granulite facies metamorphism in southeastern margin of the North China Craton Y.-C. LIU,1,2 A.-D. WANG,1 F. ROLFO,3,4 C. GROPPO,3 X.-F. GU1 AND B. SONG2 1 CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China ([email protected]) 2 Beijing SHRIMP Center, Chinese Academy of Geological Sciences, Beijing 100037, China 3 Department of Mineralogical and Petrological Sciences, University of Torino, Via Valperga Caluso 35, 1-10125 Torino, Italy 4 C.N.R.–I.G.G., Section of Torino, Via Valperga Caluso 35, 1-10125 Torino, Italy

ABSTRACT

In the southeastern margin of the North China Craton, high-pressure (HP) granulite facies meta-basic rocks exposed as bands or lenses in the Precambrian metamorphic basement (e.g. Bengbu) and as xenoliths in Mesozoic intrusions (e.g. Jiagou) are characterized by the assemblage garnet + clinopyroxene + plagioclase + quartz + rutile ± Ti-rich hornblende. Cathodoluminescence imaging and mineral inclusions reveal that most zircon from the three dated samples displays distinct core-mantle-rim structures. The cores show typical igneous zircon characteristics and give ages of 2.5–2.4 Ga, thus dating the protolith of the metabasites. The mantles formed at granulite facies conditions as evidenced by inclusions of the HP granulite mineral assemblage garnet + clinopyroxene + rutile + plagioclase + quartz ± hornblende and Ti-rich biotite and yield ages of 1839 ± 31, 1811 ± 19 and 1800 ± 15 Ma. An inclusion-free rim yields an age of 176 ± 2 Ma with the lower Th ⁄ U ratio of 0.02. The geochronological and preliminary petrological data of this study suggest that the lower crust beneath the southeastern margin of the North China Craton formed at 2.5–2.4 Ga and underwent HP granulite facies metamorphism at c. 1.8 Ga. This HT-HP metamorphic event may be ascribed to largescale crustal heating and thickening related to mantle-derived magma underplating at the base of the lower crust, as evidenced by widespread extension, rifting and related mafic magma emplacement in the North China Craton during this period. The age of 176 ± 2 Ma most likely records the late amphibolite facies retrogression occurring during exhumation. Key words: HP granulite-facies metamorphism; lower crust; North China Craton; Palaeoproterozoic; U–Pb dating.

INTRODUCTION

High-pressure (HP) granulites are generally considered to represent high-grade metabasites, characterized by a main mineral assemblage of clinopyroxene + plagioclase + garnet + quartz (Yardley, 1989; OÕBrien & Ro¨tzler, 2003; Pattison, 2003), with subordinate minerals such as amphibole and kyanite depending upon the activity of H2O and bulk composition, respectively (Indares, 2003). They are distinguished from eclogites by the presence of plagioclase and ⁄ or jadeite-poor clinopyroxene and from medium-pressure granulites by the lack of orthopyroxene in the main assemblage, although orthopyroxene may occur in HP granulites as symplectites coronas formed during post-peak decompression (Zhao et al., 2001a). HP granulites are exposed in a number of continental collision belts ranging in age from Palaeoproterozoic (e.g. Hengshan complex, China; Zhao et al., 2001a) to Cenozoic (e.g. Himalayas; Liu & Zhong, 1997). Documented meta 2009 Blackwell Publishing Ltd

morphic pressure conditions may exceed 1.4 GPa at temperatures above 800 C (OÕBrien & Ro¨tzler, 2003), implying that lower levels of thickened crust (or crust undergoing subduction) can be subjected to particularly high temperatures. In addition, HP granulites are occasionally associated with medium-T eclogites, as for instance in the Variscan belt (Carswell & OÕBrien, 1993). Documenting HP granulites in a given belt can provide insights into the evolution of the lower crust involved in continental collision and related processes (e.g. Zhao et al., 2000; Indares, 2003; Shervais et al., 2003), whereas direct petrological and geochronological observations of HP granulite facies metamorphism are crucial for understanding the relationships between the metamorphism and lower crustal evolution. However, it is difficult to constrain the exact timing of HP granulite facies metamorphism. The difficulty arises mainly because of later multiple episodes of metamorphism and related processes, resulting in resetting or disequilibrium of isotopes (especially 125

126 Y.-C. LIU ET AL.

Sm–Nd and Rb–Sr) between minerals (e.g. Li et al., 2000; Liu et al., 2005), and thus constraining the geodynamic evolution and tectonic setting in which the rocks formed. Zircon is a resistant mineral and has very low rates of Pb diffusion (Cherniak & Watson, 2003). It is often multistage and commonly displays complex growth and overgrowth features of both magmatic and metamorphic origin in high-grade metamorphic rocks involved in complex processes (e.g. Rubatto et al., 2001; Ayers et al., 2002; Mo¨ller et al., 2002; Liu et al., 2004, 2007a,b). Thus, in situ U–Pb dating of zircon is a powerful method to obtain reliable U–Pb ages in polymetamorphic rocks. However, zircon in high-grade metamorphic rocks displays a wide diversity and complexity of textures that reflect variations in the physico-chemical conditions and the duration of each metamorphic event, and are caused by modifications of pre-existing structures and ⁄ or by growth of new zircon (Corfu et al., 2003). But, inclusions of metamorphic minerals in zircon can provide a direct link between dating and metamorphism, and the complicated internal structures, including irregular boundaries and various core, mantle and rim domains, of zoned zircon grains can be revealed by cathodoluminescence (CL) images (e.g. Gebauer et al., 1997; Hermann et al., 2001; Liu et al., 2004), recorded complicated magmatic and metamorphic histories of the rocks. Many studies focused on petrology, geochemistry and geochronology of the North China Craton (NCC) have been carried out and major advances in understanding its formation and evolution have been achieved in the past decade, leading to a new tectonic subdivision of the craton into Eastern and Western Blocks, separated by the Trans-North China Orogen or Central Orogenic Zone (fig. 1; Zhao et al., 2000, 2001b, 2005). There is now a broad consensus that the assembly of the NCC was finally completed by the amalgamation of the Eastern and Western Blocks along the Trans-North China Orogen at c. 1.85 Ga (e.g. Zhao et al., 2000, 2005; Guo et al., 2002, 2005; Wilde et al., 2002; Kro¨ner et al., 2005; Wilde & Zhao, 2005; Hou et al., 2006). Following this amalgamation, the craton underwent a number of extensional and rifting events in its interior or along the margins during the period 1.85–1.6 Ga, forming aulacogens and marginal rift basins with the emplacement of mafic dyke swarms, anorthosite–gabbro–mangerite–rapakivi granite suite and A-type granites, and the eruption of superhigh-potassium volcanic rocks (e.g. Zhai et al., 2000; Zhai & Liu, 2003; Peng et al., 2005, 2008; Hou et al., 2006, 2008a; b; Lu et al., 2008). In addition, late Palaeoproterozoic HP granulite-facies metamorphism was reported mainly from the Central Orogenic Zone (e.g. Zhai et al., 1992, 2000; Zhao et al., 2000, 2001a, 2005; Guo et al., 2002, 2005; Zhai & Liu, 2003; Kro¨ner et al., 2005; and references therein), and only a few occurrences from Jiaodong in the eastern part of the Eastern Block (e.g. Li et al., 1997; Zhou et al., 2004)

and Xinyang in the southwestern part of the Eastern Block (e.g. Zheng et al., 2003). However, Palaeoproterozoic HP granulite-facies metamorphism has not been previously reported in the Xuzhou–Suzhou– Benbu area, southeastern margin of the NCC. Furthermore, the tectonic setting of the Palaeoproterozoic HP granulite-facies metamorphism in the NCC is still debated, with two models proposed. The first involves a collisional environment created by the assembly process between the Eastern and Western Blocks of the NCC (e.g. Zhao et al., 2000, 2005; Guo et al., 2002, 2005; Wilde et al., 2002), and the second a plumedriven upwelling (e.g. Zhai et al., 2000; Zhai & Liu, 2003; Zheng et al., 2003; Peng et al., 2005). One of the important reasons for the debate is due to the lack of direct petrological and geochronological observations or evidence concerning the HP granulite-facies metamorphism, especially in the southeastern margin of the NCC or southern part of the Eastern Blocks of the NCC. To date, only petrological evidence on HP granulite-facies metamorphism and ambiguous (late) Palaeoproterozoic ages have been separately reported, especially in the studied area. Recently, Xu et al. (2002, 2006) found eclogite xenoliths in the Xuzhou-Suzhou region and regarded them to be formed by thickening of the Archean NCC mafic lower crust at c. 220 Ma. Except for these published data, to our knowledge, no other eclogite samples have been found in the area. Furthermore, only a garnet + whole rock Sm–Nd isochron of 219 ± 5 Ma and a 206 ± 15 Ma 206 Pb ⁄ 238U age of a zircon grain were reported from the eclogite. Thus, more reliable data on the supposed Triassic eclogite and related xenoliths are needed to better constrain this event. In order to clarify: (1) precise age and tectonic setting of HP granulite facies metamorphism and (2) whether or not there was a Triassic eclogite facies event, we present a preliminary study of metamorphic petrology and SHRIMP U–Pb ages and mineral inclusions of zircon from metabasic rocks occurring in the Precambrian metamorphic basement and from xenoliths occurring in Mesozoic intrusions from the southeastern margin of the NCC. The results provide for the first time unambiguous evidence of the late Paleoproterozoic HP granulite-facies metamorphism in the southeastern margin of the NCC, but do not support a Triassic eclogitic event in the exposed Precambrian metamorphic basement in this area. Therefore, these shed new light on the formation and evolution of the lower crust in the NCC. GEOLOGICAL SETTING

The NCC is the largest and oldest known cratonic block in China, preserving crustal remnants as old as 3.8 Ga (Liu et al., 1992), and is bounded by faults and younger orogenic belts (Fig. 1). The early Palaeozoic Qilianshan orogen and the late Palaeozoic Tianshan– Inner Mongolia–Daxinganling orogen bound the  2009 Blackwell Publishing Ltd

CONSTRAINTS ON PALAEOPROTEROZOIC GRANULITE FACIES METAMORPHISM IN NORTH CHINA CRATON 127

Fig. 1. Schematic geological map of the Qinling–Dabie–Sulu collision zone and adjacent parts of the North China Craton (modified after Xu et al., 2006), with inset showing the major tectonic divisions of China and the location of the study area. YZ and SC denote the Yangtze Craton and South China Orogen, respectively. Also shown are the subdivisions of the North China Craton (Zhao et al., 2000, 2001b), where WB, TNCO and EB denote the Western Block, Trans-North China Orogen and Eastern Block, respectively.

craton to the west and the north, respectively, whereas in the south, the Mesozoic Qinling–Dabie–Sulu highto ultrahigh-pressure belt separates the NCC from the Yangtze Craton. Based on age, lithological assemblages, tectonic evolution and P–T–t paths, the NCC can be divided into the Eastern Block, the Western Block and the Trans-North China Orogen or Central Orogenic Zone in between (e.g. Zhao et al., 2000, 2001b; Kusky & Li, 2003; Zhai & Liu, 2003). The Bengbu and Xuzhou–Suzhou areas are located in the Eastern Block along the southeastern margin of the NCC, about 100 km west of the Tan-Lu fault zone on the southwestern termination of the Su–Lu orogen and about 300 km north of the Dabie orogen (Fig. 1). The deformed Neoproterozoic to late Palaeozoic cover and late Archean to Palaeoproterozoic metamorphic basement in the region were intruded by several small Mesozoic intrusions (e.g. Liguo, Banjing and Jiagou; fig. 1) mainly composed of dioritic to monzodioritic porphyry. The Precambrian metamorphic basement in the studied area is exposed at the surface in Bengbu, while no exposed basement occurs in the XuzhouSuzhou area, where abundant deep-seated enclaves or xenoliths occur in the Mesozoic intrusions (Xu et al., 2002, 2006). In order to better understand the formation and evolution of deep crust (especially lower crust) in the southeastern margin of the NCC, the samples analyzed in this study were collected from the exposed Precambrian metamorphic basement at Fengyang (07FY01) near Bengbu and xenoliths in the Mesozoic intrusions at Jiagou (07JG12 & 07JG14) near Suzhou (Figs 1 & 2).  2009 Blackwell Publishing Ltd

PETROGRAPHY AND METAMORPHIC EVOLUTION OF THE STUDIED SAMPLES

All the analyzed samples have basic compositions with similar paragenesis and different mineral modes. Amphibolite facies assemblages predominate, although granulite facies relics are widespread and chloritization sometimes locally occurs in the samples (as detailed below). Representative mineral compositions are reported in Table 1. Mineral abbreviations in figures and tables are after Kretz (1983). Sample 07FY01 is from the Precambrian basement in which garnet amphibolite occurs as tectonic blocks or bands within impure marble (Fig. 2a), probably suggesting their different evolutionary histories because of their different protolith precursors, i.e. igneous and sedimentary origin, respectively (as detailed below), and tectonic relationship between them. The sample is composed mainly of garnet, plagioclase and hornblende with minor clinopyroxene and titanite, and rare rutile (Fig. 3a–c). Garnet occurs as millimetre-sized fresh crystals and locally includes plagioclase and rutile. Garnet is homogeneous in composition and it is an almandine-pyrope-grossular solid solution with low amounts of Mn (XMg = 0.23; XCa = 0.19; XMn = 0.02). Plagioclase occurs as inclusions in garnet (An = 49 mol.%), in symplectite associated with green hornblende (An = 22 mol.%) and in the matrix (An = 47–51 mol.%). Ti-rich hornblende, typically showing a dark brown colour, has TiO2 contents up to 3.82 wt.% (Table 1) and occurs as inclusions in plagioclase (Fig. 3b) or in the matrix. Green hornblende has virtually no Ti (Table 1)

128 Y.-C. LIU ET AL.

(a)

(b)

Fig. 2. Photographs showing the field occurrence of garnet amphibolite tectonically contacted with impure marble in the Precambrian metamorphic basement at Fengyang near Bengbu (a) and a xenolith of garnet amphibolite in the Mesozoic dioritic porphyry at Jiagou (b).

and occurs in the matrix and in symplectite with plagioclase (Fig. 3c). Clinopyroxene relics in the matrix are diopside, with XMg = 0.63. Sample 07JG12 consists of garnet, plagioclase, hornblende, rutile and quartz with minor clinopyroxene. Garnet crystals, up to a few mm in size, are homogeneous in composition and are almandinepyrope-grossular solid solutions (XMg = 0.28; XCa = 0.19; XMn = 0.02). Plagioclase occurs as inclusions in garnet (An = 42 mol.%) (Fig. 3d), in symplectites together with clinopyroxene and ⁄ or hornblende (An = 23–32 mol.%) and in the matrix (An = 37 mol.%) (Fig. 3d,e). Most rutile is replaced by ilmenite and clinopyroxene is replaced by symplectite structures consisting of hornblende and plagioclase (Fig. 3d,e). Sample 07JG14 consists of garnet, plagioclase, hornblende, clinopyroxene, quartz, rutile, titanite and minor chlorite (Fig. 3f–i). Clinopyroxene is diopside and occurs as inclusions in garnet and titanite together

with rutile and quartz (Fig. 3g,i) or in symplectites associated with plagioclase (Fig. 3f). Diopside is locally replaced by chlorite (Fig. 3f). Diopside included in garnet contains higher amounts of Na (up to 0.09 p.f.u.) compared with diopside in symplectites (Table 1). Locally, clinopyroxene including rutile and hornblende needles is rimmed by retrograde hornblende (Fig. 3g). Garnet is typically characterized by oriented needle-like rutile exsolutions (Fig. 3g–i) and is similar in composition to garnet from sample 07JG12, with an increase in XCa towards the rim from 0.18 to 0.23. Plagioclase mainly occurs in the matrix or in symplectites (Fig. 3f). Some discrete rutile grains are partially replaced by titanite (Fig. 3h). Amphibole in the different samples is classified following Leake et al. (1997). The brown hornblende is pargasite to ferro-pargasite whereas the green hornblende is magnesio-hastingsite or edenite, and contains higher and lower amounts of TiO2, respectively (Fig. 3; Table 1). This suggests that the two amphiboles formed under HP granulite and amphibolite facies, respectively, because it has been demonstrated that Ti increases with metamorphic grade (Raase, 1974; Pattison, 2003). This difference is also supported by textural evidence with the green hornblende occurring in retrograde symplectites and brown hornblende as inclusions. Sample 07FY01 contains relatively more Ti-rich hornblende with respect to samples 07JG12 and 07JG14, probably as a result of different bulk compositions. As a whole, all investigated samples show a consistent peak assemblage of garnet, clinopyroxene, hornblende, plagioclase and quartz with accessory rutile, which is indicative of HP (c.1.1 GPa) granulite facies metamorphism (Rogers, 1977). In addition, these rocks are characterized by the absence of aluminous phases such as kyanite and sillimanite, suggesting that the protoliths were of igneous rather than sedimentary origin (Dessai et al., 2004). Based on microstructural observations and mineral relationships as mentioned above, at least three generations of mineral assemblages can be discerned: (i) garnet + plagioclase + clinopyroxene + quartz + rutile ± Ti-rich hornblende; (ii) plagioclase + green hornblende + ilmenite + titanite; (iii) chlorite + calcite + magnetite. These assemblages are representative of HP granulite, amphibolite and greenschist facies metamorphism, respectively. Therefore, it seems very likely that in all samples the peak metamorphic conditions were at HP granulite facies. Preliminary thermobarometric data and mineral assemblage described above suggest P–T conditions of 667–856 C and 1.0–1.2 GPa for the HP granulite facies metamorphic stage (Table 2). Because almost all granulites show evidence for resetting of Fe–Mg ratio because of slow cooling (Frost & Chacko, 1989), these conditions should be considered as minimum temperatures for this metamorphic stage (Davis et al., 2003), especially for sample 07FY01, which might have experienced a slow exhumation (see below). In addition, samples  2009 Blackwell Publishing Ltd

CONSTRAINTS ON PALAEOPROTEROZOIC GRANULITE FACIES METAMORPHISM IN NORTH CHINA CRATON 129

Table 1. Electron microprobe analyses of representative minerals from the metabasites in southeastern margin of the North China Craton. Mineral No. Site

Garnet

Pyroxene

Amphibole

Plagioclase

07FY01 07FY01 m

07JG12 m

07JG14 m

07JG14 i

07FY01 m

07JG14 Sy

i

Sy

07JG12 m

07JG14 Sy

07FY01 m

07JG12 i

07JG14 Sy

SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2 O

38.3 0.00 21.86 1.31 25.57 1.03 6.02 6.60 0.00 0.00

39.02 0.00 21.34 1.54 24.07 0.76 7.32 6.85 0.00 0.00

39.02 0.00 21.37 0.67 26.21 0.00 6.77 6.54 0.00 0.00

53.85 0.00 2.74 1.00 6.12 0.00 13.75 21.87 1.24 0.00

51.98 0.00 2.31 1.50 12.10 0.00 12.07 20.54 0.48 0.00

52.65 0.00 0.85 1.74 6.97 0.00 14.21 23.93 0.00 0.00

38.32 3.82 13.70 0.00 18.17 0.01 7.47 11.51 1.24 2.77

40.59 0.01 17.35 0.00 19.36 0.01 6.17 11.48 1.75 0.73

45.11 0.01 11.73 0.00 15.64 0.01 11.30 10.55 1.70 0.01

45.65 0.01 9.00 3.19 11.20 0.01 13.94 11.64 1.38 0.82

55.81 0.00 27.56 0.00 0.00 0.00 0.00 10.51 6.05 0.00

58.71 0.00 26.84 0.00 0.00 0.00 0.00 8.45 6.42 0.00

68.07 0.00 19.36 0.00 0.00 0.00 0.00 0.91 11.52 0.00

Total O Si AlIV AlVI Fe3+ Ti Fe2+ Mg Mn Ca Na K

100.69 12 2.96 0.04 1.96 0.08 0.00 1.65 0.69 0.07 0.55 0.00 0.00

100.9 12 2.99 0.01 1.92 0.09 0.00 1.54 0.84 0.05 0.56 0.00 0.00

100.58 12 3.01 0.00 1.94 0.04 0.00 1.69 0.78 0.00 0.54 0.00 0.00

100.57 6 1.97 0.03 0.09 0.03 0.01 0.19 0.75 0.00 0.86 0.09 0.00

100.98 6 1.95 0.05 0.05 0.04 0.00 0.38 0.67 0.00 0.82 0.03 0.00

100.35 6 1.96 0.04 0.00 0.05 0.00 0.22 0.79 0.00 0.95 0.00 0.00

97.01 23 5.99 2.01 0.51 0.00 0.45 2.37 1.74 0.00 1.93 0.38 0.55

97.45 23 6.16 1.83 1.27 0.00 0.00 2.46 1.40 0.00 1.87 0.51 0.14

96.06 23 6.75 1.25 0.82 0.00 0.00 1.96 2.52 0.00 1.69 0.49 0.00

96.84 23 6.76 1.24 0.33 0.36 0.00 1.39 3.08 0.00 1.85 0.40 0.15

99.93 8 2.51 1.46

100.42 8 2.62 1.41

99.86 8 2.98 1.00

0.00 0.00 0.02 0.00 0.00 0.51 0.53 0.00

0.00 0.00 0.00 0.00 0.00 0.40 0.56 0.00

0.00 0.00 0.00 0.00 0.00 0.04 0.98 0.00

Note: m, matrix; i, inclusion in garnet; Sy, symplectite. Garnet ⁄ pyroxene stoichiometries and the amount of Fe3+ and Fe2+ were estimated on the base of eight ⁄ four cations and the chargebalance constraint; Fe3+ content in amphibole was calculated as Si + Al + Ti + Mg + Fe + Mn = 13 for O = 23.

07FY01 and 07JG12 give similar temperatures (801– 856 C) but sample 07JG14 defines a lower temperature of 667 C at 1.0 GPa during HP granulite facies stage (Table 2), probably pointing to their various degrees of re-equilibration or intracrystalline diffusion in minerals during later metamorphic or tectonothermal events. The detailed estimation of the P–T conditions for the different metamorphic stages is beyond the aim of this work, and will be discussed in a separate paper. ANALYTICAL METHODS

Zircon was separated from approximately 1–5 kg of each sample by crushing and sieving, followed by magnetic and heavy liquid separation and hand-picking under binoculars. Approximately, 200 zircon grains from each of the samples 07FY01 and 07JG12 and 16 grains from sample 07JG14 were mounted in epoxy, together with a zircon U–Pb standard TEM (417 Ma) (Black et al., 2003). The mount was then polished until all zircon grains were approximately cut in half. The internal zoning patterns of the crystals were observed by CL imaging at Beijing SHRIMP Center, Chinese Academy of Geological Sciences (CAGS). Zircon was dated using a SHRIMP II at the Beijing SHRIMP Center. Uncertainties in ages are quoted at the 95% confidence level (2r). A spot size of about 30 lm was used. Common Pb corrections were made using measured 204Pb. The SHRIMP analyses followed  2009 Blackwell Publishing Ltd

the procedures described by Williams (1998). Both optical photomicrographs and CL images were taken as a guide to select the U–Pb dating spots. Five scans through the mass stations were made for each age determination. Standards used were SL13, with an age of 572 Ma and U content of 238 ppm, and TEM, with an age of 417 Ma (Williams, 1998; Black et al., 2003). The U–Pb isotope data were treated following Compston et al. (1992) with the ISOPLOT program of Ludwig (2001). Measurement of standard zircon TEMORA 1 yielded a weighted 206Pb ⁄ 238 U age of 417 ± 2 Ma (MSWD = 2.3, n = 30), which is in good agreement with the recommended isotope dilution-thermal ionization mass spectrometry (ID-TIMS) age of 416.75 ± 0.24 Ma (Black et al., 2003). Mineral inclusions in zircon were identified using Raman spectroscopy and ⁄ or an electron microprobe analyzer (EMPA). Micro-Raman spectra were acquired at the Department of Mineralogical and Petrological Sciences (University of Torino, Italy) using an integrated micro ⁄ macro Raman system Horiba Jobin Yvon HR800; a polarized solid state Nd 80 mW laser operating at 532.11 nM was used as the excitation source. Correct calibration of the instrument was verified by checking the position of the Si band at ±520.7 cm)1. The spectra were acquired using a 100· objective, resulting in a laser beam size at the sample of the order of 2 lm. Furthermore, minerals were analyzed with a Cambridge Stereoscan 360 SEM equipped with an EDS Energy 200 and a Pentafet detector (Oxford

130 Y.-C. LIU ET AL.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

Fig. 3. Photomicrographs of metabasites from Bengbu and Jiagou in southeastern margin of the North China Craton. (a) Rutile (rimmed by titanite) + garnet + plagioclase + hornblende, sample 07FY01; (b) Ti-rich dark brown hornblende in plagioclase and green hornblende in the matrix, sample 07FY01; (c) Diopside and symplectite of hornblende + plagioclase, sample 07FY01; (d) Plagioclase inclusion in garnet, sample 07JG12; (e) Ilmenite after rutile and symplectite of diopside + plagioclase + quartz + hornblende, sample 07JG12; (f) Back scattered electron (BSE) image showing diopside + plagioclase symplectite; diopside partially replaced by chlorite, sample 07JG14; (g) Diopside (partially replaced by hornblende at its rim) + rutile inclusions in garnet, sample 07JG14; (h) Rutile inclusion in titanite; (i) Diopside + quartz inclusions in garnet, sample 07JG14.

Instruments) at the Department of Mineralogical and Petrological Sciences, University of Torino, Italy (operating conditions: 50 s counting time; 15 kV accelerating voltage; 2 lm spot size) and the Institute of Mineral Resources, CAGS, China (operating conditions: 300 s counting time for Zr; 20 kV accelerating voltage; 100 nA beam current; 5 lm spot size). The representative CL images for the studied samples are presented in Fig. 4. The compositions of representative mineral inclusions in zircon are reported

in Table 3. The U–Pb data for zircon dating are listed in Tables 4–6. RESULTS CL imaging and mineral inclusions in zircon

In situ SHRIMP-dating, combined with CL information and mineral inclusions for the selection of specific locations in the crystal, is one of the most powerful  2009 Blackwell Publishing Ltd

CONSTRAINTS ON PALAEOPROTEROZOIC GRANULITE FACIES METAMORPHISM IN NORTH CHINA CRATON 131

Table 2. Preliminary P-T estimates of samples 07JG14, 07JG12 and 07FY01 from southeastern margin of the NCC. Metamorphic stage

Sample

HP granulite facies 07JG14 07JG12 07FY01

Temperature (C)

Garnet-clinopyroxene K RG 660 637 667 845 833 856 789 778 801 Zr in rutile W Z 780 906 843

07FY01 07JG14

625 ± 108***

Pressure (GPa)

EG 705

1.0*

890 836

1.1–1.2** 0.92 ± 0.21***

K, Krogh (1988); RG, Raheim & Green (1974); EG, Ellis & Green (1979); Z, Zack et al. (2004); W, Watson et al. (2006). *Reference pressure to calculate temperature. **The HP granulite-facies pressures are inferred from the Al content in amphibole (Schmidt, 1992), though these values must be taken with caution, they are also supported by the inferred pressure ( 1.1 GPa) from the mineral assemblage in the text. *** Calculated using Thermocalc, Ôall reaction between end-membersÕ option. The following end-members have been considered: Prp, Grs, Alm, Di, Hd, Cats (Ca-Tschermaks pyroxene), An, Qtz. Five reactions involving these end-members have been calculated. A stable intersection between these reactions occurs at 625 ± 108 C and 0.92 ± 0.21GPa.

techniques to unravel complex histories of individual zircon grains occurring in polyphase metamorphic rocks (e.g. Vavra et al., 1996; Gebauer et al., 1997; Hermann et al., 2001; Liu et al., 2004, 2007a,b). Although mineral inclusions in zircon, especially from lower crustal rocks or deep-seated xenoliths, are rare, they can be occasionally found and provide a direct link between zircon formation and metamorphism (e.g. Gebauer et al., 1997; Hermann et al., 2001; Liu et al., 2007a). Sample 07FY01 contains comparatively abundant, clear, coarse-grained (commonly 150–250 lm) and rounded zircon. CL images of polished grains reveal a homogeneous zircon population characterized by nearly spherical to multifaceted morphology and internal sector- to fir-tree zoning, indicative of granulite facies metamorphic origin (e.g. Vavra et al., 1999; Pidgeon et al., 2000; Rubatto et al., 2001; Schmitz & Bowring, 2003). Such zircon does not show any igneous cores but sometimes has very thin rims that are dark in CL images (Fig. 4a–c); they contain inclusions of rutile, apatite and clinopyroxene (Fig. 4a–c; Table 3), the latter with a composition similar to Cpx in the hosting rock, further suggesting HP granulite facies origin. On the basis of CL images and mineral inclusions, core-mantle-rim domains have been clearly recognized in zircon from samples 07JG12 and 07JG14 (Fig. 3d–l). Most of the cores exhibit oscillatory growth zoning, which is typical of igneous zircon (e.g. Hanchar & Rudnick, 1995; Gebauer et al., 1997; Corfu et al., 2003). However, some cores are truncated, embayed or irregularly shaped (Fig. 4d,e), suggesting that they were partially or completely resorbed, probably in the presence of a fluid (e.g. Ayers et al., 2002; Corfu et al.,  2009 Blackwell Publishing Ltd

2003). The mantle domains contain garnet, clinopyroxene, plagioclase, hornblende, quartz and Ti-rich biotite (TiO2 up to 4.79 wt.%) inclusions (Fig. 4d,f,h; Table 3), suggesting a typical Opx-free HP granulite facies assemblage (e.g. Indares, 2003; Pattison, 2003). The rims are usually thin and do not contain mineral inclusions. Most zircon from the three studied samples shows a mantle and a thin rim, and preserves relic igneous cores. The occurrence of garnet, clinopyroxene, plagioclase, hornblende, quartz and rutile inclusions in zircon mantle domains suggests that they formed at HP granulite facies conditions. SHRIMP zircon U–Pb dating

Nineteen analyses on 16 zircon grains from sample 07FY01 yield a monomodal age distribution with a weighted mean 206Pb ⁄ 238U age of 1839 ± 31 Ma with MSWD of 1.2 (Table 4; Fig. 5a). The high Th ⁄ U ratios (0.1–0.4) of the metamorphic zircon mantles is likely attributed to their granulite facies metamorphic origin, because it is well documented that granulitic zircon has high Th ⁄ U ratios (e.g. Vavra et al., 1999; Pidgeon et al., 2000; Liu et al., 2007a), in contrast to UHP metamorphic zircon. A granulitic origin for such zircon is also supported by the rutile + clinopyroxene inclusion assemblage and by the sector- to fir-tree zoning documented by CL imaging, together with their spherical shapes (e.g. Vavra et al., 1999; Pidgeon et al., 2000) (Fig. 4a–c). Therefore, the age of 1839 ± 31 Ma represents the best estimate for the HP granulite facies metamorphic event in sample 07FY01. Eighteen U–Pb spot analyses were made on 15 zircon grains from 07JG12 (Table 5; Fig. 5b). The 18 analyses of both igneous cores (7 spots) and granulite facies metamorphic mantle domains (11 spots) define a discordia line with an upper intercept age of 2416 ± 160 Ma and a lower intercept age of 1834 ± 80 Ma (MSWD = 11.3), corresponding to Neoarchean primary crystallization and late Palaeoproterozoic metamorphism, respectively (Fig. 5b). The 11 analyses of the zircon mantle domains record 206 Pb ⁄ 238U concordant ages ranging from 1783 to 1837 Ma with a weighted mean age of 1800 ± 15 Ma (MSWD = 2.3), consistent with the lower intercept age of 1834 ± 80 Ma within error. In addition, granulite facies mineral inclusions such as garnet, clinopyroxene and plagioclase (Fig. 4d,g,h) were found in mantle domains. Therefore, the age of 1800 ± 15 Ma should record the timing of granulite facies metamorphism. The rims of zircon grains from sample 07JG12 are too thin (general < 10 lm) (e.g. Fig. 4d–g) to be analyzed by SHRIMP II. Although zircon grains collected from sample 07JG14 are not numerous, three different textural populations of zircon can be easily discerned by means of integrated CL imaging, i.e. core, mantle and rim, each with record discrete ages (Figs 4 & 5c). A total of

132 Y.-C. LIU ET AL.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

(j)

(k)

(l)

Fig. 4. Cathodoluminescene (CL) images (a–g, i–l) and back scattered electron (BSE) images (h) for zircon from sample 07FY01 (a–c), 07JG12 (d–h) and 07JG14 (i–l). Zircon (h) is an enlargement part of (g). The open circles are spot analysis with available 206 Pb ⁄ 238U ages.

14 U–Pb spot analyses were made on eight zircon grains from 07JG14 (Table 6; Fig. 5c). The 14 analyses of the magmatic zircon cores and metamorphic mantle and rim domains define a discordia line with an upper intercept age of 2324 ± 190 Ma and a lower intercept age of 1753 ± 180 Ma (MSWD = 3.3): the former is consistent with a concordant age of 2563 ± 31 Ma given by the igneous core within error, corresponding to Neoarchean primary crystallization; the latter is

consistent with Palaeoproterozoic metamorphism (Fig. 5c). Three analyses of the zircon mantle domains record 206Pb ⁄ 238U concordant ages with a weighted mean age of 1811 ± 19 Ma (MSWD = 2.1), which is in agreement with the lower intercept age of 1753 ± 180 Ma within error. A concordant age of 387 ± 13 Ma defined by a zircon mantle domain, with higher Th ⁄ U ratio of 0.82, may represent a tectonothermal event or a late Pb loss event. Most of  2009 Blackwell Publishing Ltd

CONSTRAINTS ON PALAEOPROTEROZOIC GRANULITE FACIES METAMORPHISM IN NORTH CHINA CRATON 133

Table 3. Electron microprobe analyses of representative minerals in zircon from the garnet amphibolites in southeastern margin of the North China Craton. Mineral No. Site

Garnet 07JG12 Zir-M

Diopside

Hornblende 07JG12 Zir-M

07JG12 Zir-M

07FY01 Zir-M

Plagioclase 07JG12

Rutile 07FY01 Zir-M

SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2 O

38.22 0.00 20.61 1.04 27.30 1.52 4.72 6.60 0.00 0.00

52.05 0.00 2.40 2.56 5.74 0.00 14.00 22.20 0.67 0.00

52.08 0.00 2.57 0.00 11.09 0.00 11.93 22.32 0.00 0.00

43.34 0.01 13.36 0.00 16.76 0.01 10.45 9.57 2.02 0.35

0.03 99.01 0.03 0.174* 0.19** 0.11

Total O Si AlIV AlVI Fe3+ Ti Fe2+ Mg Mn Ca Na K

100.01 12 3.01 0.00 1.91 0.06 0.00 1.80 0.55 0.10 0.56 0.00 0.00

99.62 6 1.94 0.06 0.04 0.07 0.00 0.18 0.78 0.00 0.88 0.05 0.00

99.99 6 1.96 0.04 0.08 0.00 0.00 0.35 0.67 0.00 0.80 0.00 0.00

95.87 23 6.57 1.43 0.96 0.00 0.00 2.12 2.36 0.00 1.55 0.59 0.07

99.54

Biotite 07JG12

Zir-M

Zir-M

Zir-M

58.86 0.00 26.17 0.00 0.00 0.00 0.00 8.00 6.55 0.42

60.68 0.00 24.83 0.00 0.00 0.00 0.00 6.31 7.68 0.62

33.85 4.79 14.16 12.91 8.81 0.00 12.19 1.80 0.00 6.52

100.00 8 2.64 1.38

100.12 8 2.70 1.30

95.03 11 2.55 1.26

0.00 0.00 0.00 0.00 0.00 0.38 0.57 0.02

0.00 0.00 0.00 0.00 0.00 0.30 0.66 0.03

0.73 0.27 0.55 1.37 0.00 0.14 0.00 0.63

Note: Zir–M represents the mantle domain of zircon. Garnet ⁄ pyroxene stoichiometries and the amount of Fe3+ and Fe2+ were estimated based on eight ⁄ four cations and the charge-balance constraint; the ferric iron content in amphibole was calculated as Si + Al + Ti + Mg + Fe + Mn = 13 for O = 23. *Denotes the content of ZrO2; ** represents all Fe.

Table 4. SHRIMP zircon U–Pb data for garnet amphibolite (sample 07FY01). Spot

Domain

Inclusion

07FY01-1.1 07FY01-2.1 07FY01-3.1 07FY01-4.1 07FY01-5.1 07FY01-6.1 07FY01-7.1 07FY01-8.1 07FY01-9.1 07FY01-10.1 07FY01-11.1 07FY01-12.1 07FY01-6.2 07FY01-13.1 07FY01-14.1 07FY01-15.1 07FY01-16.1 07FY01-14.2 07FY01-16.2

me, me, me, me, me, me, me, me, me, me, me, me, me, me, me, me, me, me, me,

No No No No No No No No No Cpx, Ap Ap No No No No No No No No

m m m m m m m m m m m m m m m m m m m

206

Pbc (%)

U (ppm)

Th (ppm)

Th ⁄ U

4.84 1.10 0.58 0.82 1.22 1.11 3.33 1.25 1.88 0.82 3.60 1.37 2.22 2.89 2.26 1.09 1.11 1.76 3.61

9 35 72 49 48 75 13 24 31 60 12 21 25 18 26 126 31 54 11

1 7 17 11 12 28 2 7 5 17 1 6 3 5 4 51 2 11 1

0.11 0.20 0.24 0.22 0.25 0.37 0.15 0.29 0.16 0.28 0.08 0.28 0.12 0.28 0.15 0.40 0.06 0.20 0.09

206

Pb* (ppm) 2.83 10.4 21.0 14.0 13.6 22.0 3.61 6.70 8.78 17.1 3.32 5.91 7.04 5.07 7.35 36.7 8.84 16.1 3.22

207

Pb* ⁄ 206Pb*

0.125 0.1176 0.1158 0.1147 0.1101 0.1100 0.117 0.1039 0.1059 0.1078 0.122 0.125 0.1072 0.1050 0.1215 0.1109 0.1159 0.1058 0.109

Pbc and Pb* indicate the common and radiogenic portions, respectively. Pb*, corrected for common

the rim domains were too thin to be analysed; only one rim has been dated, giving a 206Pb ⁄ 238U concordant age of 176 ± 2 Ma with a lower Th ⁄ U ratio of 0.02, probably suggesting a later metamorphic overprint. In summary, zircon from the three dated samples exhibits clear core-mantle-rim patterns evidenced by CL images and mineral inclusions distribution. All mantle domains of zircon from the three analysed samples define identical 206Pb ⁄ 238U concordant ages  2009 Blackwell Publishing Ltd

204

±%

207

Pb* ⁄ 235U

11 3.0 2.1 2.5 4.5 2.8 11 5.4 4.6 3.0 8.7 9.7 5.9 8.5 6.0 2.2 3.3 3.8 13

Pb using measured

5.69 5.46 5.42 5.25 4.93 5.16 5.20 4.68 4.67 4.89 5.41 5.58 4.83 4.70 5.49 5.14 5.31 4.95 5.00 204

±%

12 3.5 2.7 3.0 4.8 3.5 12 5.9 5.0 3.4 9.2 10 6.2 8.9 6.5 2.6 3.8 4.2 13

206

Pb* ⁄ 238U

0.331 0.3366 0.3396 0.3320 0.3249 0.3403 0.323 0.3269 0.3198 0.3289 0.3218 0.3237 0.3268 0.3246 0.3277 0.3362 0.3321 0.3398 0.332

±%

206

Pb ⁄ 238U Age (Ma)

3.3 1.9 1.7 1.7 1.9 2.2 3.3 2.4 2.0 1.6 3.1 2.6 2.2 2.6 2.6 1.4 1.8 1.8 3.4

1845 1870 1885 1848 1814 1888 1803 1823 1789 1833 1798 1808 1823 1812 1827 1868 1849 1886 1849

±53 ±32 ±27 ±28 ±30 ±35 ±52 ±37 ±30 ±26 ±48 ±40 ±34 ±42 ±41 ±23 ±30 ±30 ±55

Pb; all errors are 1r; me, metamorphic zircon; m, mantle.

within analytical uncertainty, i.e. 1839 ± 31, 1800 ± 15 and 1811 ± 19 Ma, respectively. The ages represent c. 1.8 Ga HP granulite facies metamorphism, based on robust mineral inclusion assemblage constraints (garnet + clinopyroxene + rutile + plagioclase + quartz ± hornblende). In addition, zircon cores preserved in samples 07JG12 and 07JG14 record a 2.5–2.4 Ga age, which is consistent with the dominant age of the NCC Archean basement (e.g. Zhai et al., 2000; Zhao et al., 2000, 2001b; Kusky & Li,

134 Y.-C. LIU ET AL.

Table 5. SHRIMP zircon U–Pb data for garnet amphibolite (sample 07JG12). Spot

Domain

07JG12-1.1 07JG12-2.1 07JG12-3.1 07JG12-4.1 07JG12-5.1 07JG12-6.1 07JG12-7.1 07JG12-8.1 07JG12-9.1 07JG12-10.1 07JG12-11.1 07JG12-12.1 07JG12-13.1 07JG12-14.1 07JG12-15.1 07JG12-4.2 07JG12-2.2 07JG12-13.2

me, m ma, c me, m ma, c me, m me, m ma, c me, m ma, c me, m me, m me, m me, m me, m me, m me, m me, m ma, c

Inclusion

Qtz No No No No Grt, Pl, Bt No No No No No Qtz No No Cpx No Pl No

206

Pbc (%)

U (ppm)

Th (ppm)

Th ⁄ U

1.90 0.38 0.90 0.48 1.64 1.23 0.44 1.76 0.60 1.26 0.42 2.16 2.07 0.27 0.12 1.63 3.04 0.27

15 156 91 116 35 86 155 33 145 66 232 45 54 229 794 31 21 132

20 85 62 41 33 70 88 33 67 61 155 40 49 142 479 32 26 76

1.33 0.54 0.68 0.35 0.94 0.81 0.57 1.00 0.46 0.92 0.67 0.89 0.91 0.62 0.60 1.03 1.24 0.57

206

Pb* (ppm)

207

3.40 57.3 25.3 44.4 9.81 24.3 49.7 9.05 55.1 18.6 65.9 12.8 15.9 62.8 218 9.53 6.14 47.0

Pb* ⁄ 206Pb*

0.1076 0.1533 0.1060 0.1524 0.1067 0.1075 0.1284 0.1020 0.1468 0.1066 0.1098 0.1001 0.1031 0.1151 0.1071 0.1225 0.1006 0.1524

Pbc and Pb* indicate the common and radiogenic portions, respectively. Pb*, corrected for common zircon; c, core; m, mantle.

204

±%

207

Pb* ⁄ 235U

7.7 0.82 2.0 1.1 5.1 2.7 1.1 7.0 1.1 3.0 1.2 5.1 4.8 0.99 0.53 4.2 8.0 1.3

Pb using measured

3.79 9.02 4.70 9.31 4.70 4.82 6.559 4.43 8.90 4.77 4.991 4.50 4.74 5.054 4.716 5.87 4.52 8.64 204

±%

206

7.9 1.1 2.2 1.4 5.3 2.9 1.4 7.2 1.4 3.4 1.4 5.4 5.3 1.3 0.69 4.7 8.3 1.7

Pb* ⁄ 238U

±%

206

Pb ⁄ 238U Age (Ma)

0.2555 0.4268 0.3212 0.4429 0.3192 0.3255 0.3706 0.3151 0.4399 0.3241 0.3297 0.3259 0.3333 0.3186 0.3192 0.3474 0.3256 0.4114

2.1 0.76 0.90 0.85 1.6 1.1 0.76 1.8 0.82 1.4 0.68 1.6 2.3 0.89 0.44 1.9 2.2 1.1

1467 2291 1796 2363 1786 1817 2032 1766 2350 1810 1837 1819 1854 1783 1786 1922 1817 2221

±28 ±15 ±14 ±17 ±25 ±17 ±13 ±28 ±16 ±22 ±11 ±25 ±37 ±14 ±7 ±32 ±35 ±20

Pb; all errors are 1r; me, metamorphic zircon; ma, magmatic

Table 6. SHRIMP zircon U–Pb data for garnet amphibolite (sample 07JG14). Spot

Domain

Inclusion

206

Pbc (%)

U (ppm)

Th (ppm)

Th ⁄ U

07JG14-3.1 07JG14-4.1 07JG14-5.1 07JG14-6.1 07JG14-3.2 07JG14-7.1 07JG14-8.1 07JG14-9.1 07JG14-5.2 07JG14-10.1 07JG14-9.2

me, m me, m ma, c me, m me, m ma, c me, m me, m me, m me, m me, r

No No No No No No No No No No No

0.25 0.21 0.81 0.14 0.60 0.42 0.13 0.07 12.5 0.04 1.46

367 1119 81 438 189 198 472 1364 102 649 721

166 289 55 75 66 266 150 81 84 358 17

0.45 0.26 0.68 0.17 0.35 1.34 0.32 0.06 0.82 0.55 0.02

206

Pb* (ppm)

103 308 34.1 124 49.8 77.1 139 364 6.22 168 17.4

207

Pb* ⁄ 206Pb*

0.1140 0.1111 0.1634 0.1069 0.1024 0.1475 0.1153 0.1192 0.073 0.1033 0.0590

Pbc and Pb* indicate the common and radiogenic portions, respectively. Pb*, corrected for common zircon; c, core; m, mantle; c, rim.

2003; Zhai & Liu, 2003; Gao et al., 2004; Guo et al., 2005; Kro¨ner et al., 2005; Hou et al., 2006 and references therein) and thus represents the protolith age. The age of 176 ± 2 Ma recorded in the inclusion-free zircon rim with lower Th ⁄ U ratio (0.02, sample 07JG14) could be a record of the Jurassic amphibolite facies metamorphic event. However, no evidence was obtained of a Triassic eclogite facies event as documented in the eclogite xenoliths from the same area, which gave a metamorphic age of c. 220 Ma (Xu et al., 2002, 2006). Furthermore, most importantly, the obtained geochronological data combined with the preliminary petrological observations suggest that the studied samples preserved the same geochronological records and display a similar metamorphic evolutional process, although different samples have different occurrences, especially, sample 07FY01 comes from the exposed Precambrian metamorphic basement and occurs as tectonic lens within the marble (Fig. 2), probably indicating both different evolutionary histories mentioned above.

204

±%

2.4 2.3 1.4 0.76 1.5 0.92 7.1 0.84 38 1.6 5.6

Pb using measured

207

Pb* ⁄ 235U

±%

5.130 4.890 11.00 4.839 4.309 9.170 5.450 5.107 0.620 4.285 0.226

2.6 2.5 2.0 1.4 2.0 1.4 7.2 1.7 38 2.0 5.8

204

206

Pb* ⁄ 238U

±%

206

Pb ⁄ 238U Age (Ma)

0.3267 0.3194 0.4881 0.3283 0.3053 0.4511 0.3429 0.3107 0.0618 0.3008 0.02775

1.0 0.97 1.4 1.1 1.3 1.1 1.0 1.5 3.6 1.1 1.3

1822 1787 2563 1830 1717 2400 1901 1744 387 1695 176

±16 ±15 ±31 ±18 ±19 ±22 ±17 ±23 ±13 ±17 ±2

Pb; all errors are 1r; me, metamorphic zircon; ma, magmatic

DISCUSSION AND CONCLUSIONS

In China, the c. 1900–1800 Ma tectono-metamorphic event is generally named as the Lu¨liang or Zhongtiao Movement. This event was followed at 1800–1650 Ma by the formation of volcano-sedimentary rift successions, anorogenic magmatism with anorthosite– rapakivi intrusions, and the emplacement of mafic dyke swarms (cf. Zhai & Liu, 2003 for a detailed discussion). Some researchers (e.g. Zhao et al., 2000, 2001a,b) consider the 1900–1800 Ma metamorphism and the 1800–1650 Ma rifting as representing, respectively, a continental collision event and a subsequent extensional event within the craton. Other authors suggest that metamorphism and rifting are possibly related to a single tectonic event caused by PalaeoMesoproterozoic mantle upwelling and supercontinent break-up (Zhai et al., 2000; Zhai & Liu, 2003). In addition, evidence for a late Palaeoproterozoic tectonothermal event is common in the Neoarchean rocks throughout the entire North China Craton, and not confined to the Central Orogenic Belt (e.g. Li et al.,  2009 Blackwell Publishing Ltd

CONSTRAINTS ON PALAEOPROTEROZOIC GRANULITE FACIES METAMORPHISM IN NORTH CHINA CRATON 135

Fig. 5. Zircon SHRIMP U–Pb dating for garnet amphibolites from Bengbu (sample 07FY01) (a) and Jiagou (samples 07JG12 & 07JG14) (b and c).

1997; Zhou et al., 2004; Hou et al., 2006), thus suggesting that the metamorphism might not be closely related to the building of the Central Orogenic Belt (Zhai & Liu, 2003; cf. Hou et al., 2008b for review). Late Palaeoproterozoic (1.84–1.77 Ga) mafic dyke swarms are widespread in the NCC, occurring in the  2009 Blackwell Publishing Ltd

Western Block and Central Orogenic Zone (Shanxi Province) as well as in the Eastern Block (Shandong Province) (e.g. Zhai et al., 2000; Hou et al., 2008a,b; Peng et al., 2008; and references therein). According to Hou et al. (2006, 2008a), these mafic dyke swarms are related to the rifting of the aulacogens. In a broad view, the mafic dyke swarms and rifting in the whole NCC could represent an important extensional event in response to the breakup of the Late Palaeoproterozoic Columbia supercontinent (e.g. Zhai & Liu, 2003; Hou et al., 2006). Thus, the c. 1800 Ma event is most likely related to mantle upwelling and crustal uplift, as previously suggested by Zhai et al. (2000). Mantle upwelling provided a large-scale heat source and resulted in crustal thickening by accretion of basaltic magma for the widespread development of HP granulite facies metamorphism in the lower crust. Coeval extension and rifting most likely triggered magmatic underplating and lower crustal metamorphism (e.g., Davis, 1997; Zheng et al., 2003; Liu et al., 2007a; Zhai et al., 2007). Of course, such a model remains to be further verified by other studies such as trace element and isotope geochemistry. The geochronological data combined with the preliminary petrology provided in this paper support a scenario in which the lower crust in the southeastern margin of the NCC formed at 2.5–2.4 Ga and underwent c. 1.8 Ga HP granulite facies metamorphism. The studied metamorphic rocks could have experienced amphibolite-facies retrograde overprinting at 176 ± 2 Ma. However, more data are required in order to: (i) better constrain this age with additional methods such as Sm–Nd and Rb–Sr dating, and (ii) better understand the metamorphic evolution of the studied samples. Although Xu et al. (2002) reported rare Triassic eclogite xenoliths in the Xuzhou–Suzhou area (e.g. Jiagou and Liguo) (Fig. 1), no eclogite-facies assemblages were found in the investigated samples. The lack of eclogite-facies assemblages may be explained in two ways: (i) the samples studied in this paper did not experience eclogite-facies metamorphism in the Triassic; (ii) the eclogite-facies paragenesis, consisting of garnet, omphacite and probably minor phengite, was mainly lost during the late amphibolite-facies overprinting, or the rocks experienced eclogite facies metamorphism but did not result in the overgrowth of zircon because of the absence of fluids in the Triassic. Obviously, the second possibility seems to be ruled out, because this interpretation is inconsistent with multiple lines of evidence. The investigated rocks contain granulite facies assemblages, and the granulite facies metamorphism is dated at c. 1.8 Ga; if eclogite facies metamorphism occurred at c. 220 Ma (Xu et al., 2002, 2006), that most likely would have erased the granulite facies assemblage. Also, no Triassic ages and eclogitefacies assemblages are recorded in the zircon grains of the samples. Thus, it seems unlikely that any zircon that formed at c. 220 Ma under eclogite facies

136 Y.-C. LIU ET AL.

conditions would be preferentially resorbed ⁄ recrystallized during late amphibolite facies overprinting, leaving the Palaeoproterozoic mantle zircon domains. That is to say, the samples from the basement and xenoliths did not undergo the Triassic eclogite facies metamorphism. In this context, the observation that some mafic HP rocks in the southeastern margin of the NCC experienced Triassic eclogite-facies metamorphism while rocks in this study did not, may be because the deepest portions of the mafic lower crust experienced eclogite facies conditions and most of them delaminated in the Triassic and foundered into the mantle (e.g. Gao et al., 2004). Therefore, it is most likely that mafic lower-crustal rocks of eclogite facies sank and were recycled into the mantle, resulting in the formation of Mesozoic high-Mg adakitic rocks (e.g. Gao et al., 2004; Xu et al., 2006), and only a few were preserved and exposed at the surface in the area. However, a possible clue in favour of the second hypothesis is found in garnet. Generally speaking, garnet can preserve cores and rims formed during different orogenic cycles, thus providing a powerful tool in the integrated study of polymetamorphic orogenic belts, as for instance the Alps (Rolfo et al., 2004) and the Himalaya (Argles et al., 1999). Thin rims with higher XCa and XFe around garnet have been described in a phengite-amphibole eclogite from Dora-Maira Massif, Western Alps (Groppo et al., 2007a) and interpreted as the product of discontinuous reactions involving the destabilization of eclogitic minerals (omphacite, garnet and phengite) in favour of a new retrograde assemblage (amphibole, Ca-rich garnet rim and Jd-poor omphacite rim). Ca-rich rims of the large garnet crystals in sample 07JG14, occasionally crowded with small quartz and rutile inclusions, may be interpreted in a similar way. Eclogitic omphacite is not preserved, probably being replaced by a fine-grained symplectite of clinopyroxene (diopside) + plagioclase (Fig. 3f). Clinopyroxene + plagioclase symplectitic textures have been reported in the literature from a number of retrogressed eclogites and HP granulites, and there is a general consensus that they indicate the replacement of omphacite by plagioclase and clinopyroxene during decompression (e.g. Smelov & Beryozkin, 1993; Mo¨ller, 1998; Lombardo & Rolfo, 2000; Zhao et al., 2000, 2001a; OÕBrien & Ro¨tzler, 2003; Groppo et al., 2007b). The occurrence of symplectite structures in samples 07JG12 and 07JG14 does not rule out the possibility that some of the xenoliths underwent eclogite-facies metamorphism in the late Triassic, as suggested by Xu et al. (2002). In this regard, some xenoliths probably experienced eclogite facies metamorphism but did not result in the overgrowth of zircon because of the absence of fluids in the Triassic, because the dissolution and overgrowth of zircon hinges on the availability of fluids during HP metamorphism (e.g. Rubatto et al., 1999; Liermann et al., 2002; Liu et al., 2007a). However, no similar retrograded textures were observed in sample 07FY01,

probably indicating that the metamorphic rocks from the Precambrian basement in the Bengbu region had a different exhumation history compared with that of the xenoliths in the Xuzhou-Suzhou region. For example, it is most likely that the xenoliths were brought to the surface rapidly, and that rapid decompression caused the formation and preservation of the symplectites, whereas the basement sample must have been exhumed more slowly. Anyway, the observation that some mafic HP rocks in the southeastern margin of the NCC experienced Triassic eclogite-facies metamorphism, as previously identified by Xu et al. (2002, 2006), requires further study. Worth of note is that the metamorphic basement and xenoliths in Bengbu and Xuzhou-Suzhou areas are dominated by hornblende-bearing assemblages, which differ from most other intracratonic lower-crustal xenoliths that are dominated by ÔdryÕ pyroxene-garnet assemblages (Weber et al., 2002). This may be ascribed to their occurrence near the southeastern margin of the NCC. ACKNOWLEDGEMENTS

This research was financially supported by the National Natural Science Foundation of China (40634023, 90814008 and 40572035), the National Basic Research Program of China (2009CB825002), the Chinese Academy of Sciences (kzcx2-yw-131) and the University of Torino (Project ÔScambi CulturaliÕ ROFRSCUL06 and ROFRSC07). Prof. S.-G. Li is thanked for his suggestions and constructive discussions on earlier versions of this manuscript. Critical reviews and many useful suggestions by D. Robinson and two anonymous reviewers have helped improve the final version of the manuscript. We thank D.-Y. Liu and H. Tao for their help in SHRIMP U–Pb dating on zircon, and Z.-Y. Chen for electron microprobe analysis. REFERENCES Argles, T. W., Prince, C. I., Foster, G. L. & Vance, D., 1999. New garnets for old? Cautionary tales from young mountain belts. Earth and Planetary Science Letters, 172, 301–309. Ayers, J. C., Dunkle, S., Gao, S. & Miller, C. F., 2002. Constraints on timing of peak and retrograde metamorphism in the Dabie Shan ultrahigh-pressure metamorphic belt, east– central China, using U–Th–Pb dating of zircon and monazite. Chemical Geology, 186, 315–331. Black, L. P., Kamo, S. L., Allen, C. M. et al., 2003. TEMORA 1: A new zircon standard for Phanerozoic U-Pb geochronology. Chemical Geology, 200, 155–170. Carswell, D. A. & OÕBrien, P. J., 1993. Thermobarometry and geotectonic significance of high-pressure granulites: examples from the Moldanubian zone of the Bohemian massif in lower Austria. Journal of Petrology, 34, 427–459. Cherniak, D. J. & Watson, E. B., 2003. Diffusion in zircon. Reviews in Mineralogy and Geochemistry, 53, 113–143. Compston, W., Williams, I. S., Kirschvink, J. L., Zhang, Z. & Ma, G., 1992. Zircon U–Pb ages for the early Cambrian  2009 Blackwell Publishing Ltd

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