Petrological and geochronological constraints on the formation, subduction and exhumation of the continental crust in the southern Sulu orogen, eastern-central China

Tectonophysics 475 (2009) 291–307

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Tectonophysics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t e c t o

Petrological and geochronological constraints on the formation, subduction and exhumation of the continental crust in the southern Sulu orogen, eastern-central China Ze-ming Zhang a,⁎, Kun Shen b, Jin-li Wang a, Hai-liang Dong c a b c

Institute of Geology, Chinese Academy of Geological Sciences, No. 26 Baiwanzhuang Road, Beijing 100037, China Shandong Institute and Laboratory of Geological Sciences, Jinan, 250013, China Department of Geology, Miami University, Oxford, Ohio 45056, USA

a r t i c l e

i n f o

Article history: Received 23 December 2007 Received in revised form 12 February 2009 Accepted 24 February 2009 Available online 12 March 2009 Keywords: Bimodal magmatism Neoproterozoic continental rift Ultrahigh-pressure metamorphism Exhumation rate Sulu orogen Scientific Drilling

a b s t r a c t Detailed petrological, geochemical and geochronological studies were carried out for the core samples from the Chinese Continental Scientific Drilling Main Hole (CCSD-MH) with a final depth of 5158 m. This borehole has penetrated into an ultrahigh-pressure (UHP) metamorphic slice consisting mainly of eclogites, gneisses, garnet–pyroxenites and garnet–peridotites. Geochemical characteristics indicate that their protoliths are igneous rocks, and occur in a continental rifting tectonic setting. Quartz-, rutile- and ilmenite-rich eclogites from 0 to 710 m occur as alternating layers; the eclogites, together with interlayers of peridotites and gneisses form a layered ultramafic–mafic–acidic intrusion, which was formed by extensive fractional crystallization of basaltic magma in continental environments. The granitic gneisses from 1190 to 1505 m and 3460 to 5118 m show affinity to within-plate granite, whereas the granitic gneisses from 710 to 1190 m and 1505 to 3460 m exhibit characteristics of volcanic-arc granite. Zircon U–Pb dating demonstrates that the magmatic zircon cores, which have relatively high Th/U ratios (mostly > 0.4), from both eclogites and gneisses, yield the same age at c. 788.8 Ma, suggesting that the protoliths of UHP rocks were formed by bimodal magmatism in Neoproterozoic rifting tectonic zones along the northern margin of the Yangtze Plate, in response to the breakup of the supercontinent Rodinia. U–Pb dating of metamorphic zircons with coesite and other eclogite-facies mineral inclusions and with relatively low Th/U ratios (mostly < 0.14) gives similar Triassic ages, which define two main zircon-forming events at 221.1 Ma and 216.7 Ma. We suggest that the older weighted mean age represents the peak-UHP metamorphic event at a pressure of 5.0 GPa (corresponds to ∼ 165 km depth), whereas the younger mean age reflects the UHP/HP retrograde event at a pressure 2.8 GPa (∼ 92 km depth). Therefore, a maximum rate of vertical movement during early exhumation of the UHP rocks from the Sulu orogen would be 17 mm/year, which is quite similar to initial exhumation rates (16 to 35 mm/year) of many UHP terranes in the world. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The Dabie-Sulu orogen was formed by subduction and collision between North China and the Yangtze plates. It is probably one of the most intensively investigated collisional orogens in the world due to widespread occurrences of coesite-bearing ultra-high pressure (UHP) metamorphic rocks (Liou et al., 1995; Cong and Wang, 1996; You et al., 1996; Wallis et al., 1999). In order to unravel the subsurface structure and to further constrain the mechanism of formation and exhumation of the continental crust, we studied rock core samples from the Chinese Continental Scientific Drilling (CCSD) project which was undertaken from 1997 to 2005. The drill site of the CCSD is located near Maobei village (N34°25′, E118°40′), about 17 km to the south⁎ Corresponding author. Tel.: +86 10 68999735; fax: +86 10 68994781. E-mail addresses: [email protected], [email protected] (Z. Zhang). 0040-1951/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2009.02.042

west of Donghai in the southern segment of the Sulu UHP belt (Fig. 1). The CCSD Main Hole (CCSD-MH) was completed in March, 2005 with a final depth of 5158 m. The core samples from the depth interval of 100 to 2000 m of the CCSD-MH have been previously investigated (e.g. Liu et al., 2004a,b; Xu, 2004; Zhang et al., 2004; Liu et al., 2005a,b; Su et al., 2005; Zhao et al., 2005; Zhang et al., 2006a,b,c,d; Zong et al., 2006; Zhao et al., 2006; Xu et al., 2006; Sun et al., 2007; Chen et al., 2007). These results demonstrate that the drill hole has penetrated an ultrahigh-pressure (UHP) metamorphic rock slice consisting of eclogite, jadeitite– quartzite, garnet–peridotite and minor garnet–pyroxenite, garnet– phengite–quartz schist and kyanite–quartzite. However, some of them have been partly or completely transformed into amphibolite-facies metamorphic rocks. The UHP metamorphic evidence preserved in most gneisses is only documented by the presence of coesite and other eclogite-facies mineral inclusions within zircons. Zircon U–Pb dating

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Fig. 1. Simplified geological map of the Donghai area of the southern Sulu, showing the locations of the CCSD-MH and some typical eclogite and/or peridotite bodies.

suggests that the UHP metamorphism occurred in the Triassic with an age ranging from 240 to 220 Ma, whereas the protolithic age and tectonic environment of the UHP rocks remain debatable. The constructed geologic profile of the borehole provides detailed and critical information with regard to tectonic deformation of the root zone of the Sulu orogen (Xu, 2004). However, in these previous studies only local core samples from some segments of the borehole were used. In this paper, we report petrological, geochemical and geochronological characteristics of eclogites and other rocks nearly of the entire rock sequence between 100 and 5158 m of the CCSD-MH. The main aims were to reveal protoliths and formation tectonic setting of UHP rocks and their protolithic and metamorphic ages, to constrain subduction and exhumation process of the metamorphic supercrustal rocks, and to establish petrological and geochemical profiles of the deeply subducted continental crust. Mineral abbreviations used in this paper are after Kretz (1983) except: Amp = amphibole, Coe = coesite, and Phn = phengite. 2. Analytical methods Chemical compositions of the UHP core samples were analyzed at the National Geological Analysis Center of China, Beijing. Oxides of major elements of the bulk rock samples were determined by X-ray fluorescence (XRF) (Rigaku-3080). The analytical uncertainty is <0.5%. Trace elements Zr, Nb, V, Cr, Sr, Ba, Cl, Zn, Ni, Rb and Y were analyzed using a different XRF instrument (Rigaku-2100). The analytical uncertainty is <5% for Ba, 2–14% for Cl, and <3% for the other trace elements. Other trace elements, such as Rb, Cs, Pb, Zn, Cu, Co, Ni, U, Th, W, Sn, Mo, Be, Cd, Ga, In, Tl, As, Sb, Bi, Y, Sc, and rare earth elements (REE) were analyzed by ICP-MS (TJA-PQ-ExCell). The analytical uncertainties are 1–5% when the abundance is greater than 1 ppm, and 5–10% when the abundance less than 1 ppm. To test the reliability of the results, repeated analyses of about thirty samples were independently completed at the Geoscience Centre of the University Goettingen in Germany and at the Department of Earth Science of Rice University in the United States of America. Results from the three laboratories were well consistent within analytical errors. Zircon grains used for dating were separated by crushing and sieving approximately 500 g to 1000 g of each rock sample, followed by magnetic and heavy liquid separation. Approximately 150 to 200 zircon grains from each sample were mounted onto 25-mm epoxy discs and polished. Mineral inclusions in zircons were identified using a RENISHAW Raman spectrometer (RM100) at the Key Laboratory for Continental Dynamics, Institute of Geology, Chinese Academy of

Geological Sciences (CAGS). Zircon grains were analyzed for U–Th–Pb isotope compositions by sensitive high-resolution ion microprobe (SHRIMP II) and LA-ICP-MS. SHRIMP analyses were performed at the SHRIMP Laboratory in Beijing, China. The instrumental conditions and measurement procedures are the same as those described by Compston et al. (1992). Spots of approximately 20 µm diameter were analyzed; seven scans through the critical mass range were used for data collection. The 206Pb⁄238U ratios of the samples were calibrated using the empirical relationship of Claoué-Long et al. (1995), and corrected using reference zircon of SL13 from Sri Lanka (206Pb/238U = 0.0928; 417 Ma). Duplicate analyses of the standard during the analytical session were used to determine concentrations of U, Th and Pb and to calibrate the inter-elemental isotopic ratios for the samples. The measured 208Pb/ 206 Pb ratio was used for the correction of common Pb, the isotopic compositions of which were assumed by a single-stage evolution model 238 U/204Pb = 8.8 and 232Th/238U = 3.8 (Compston et al., 1989). Employing a two-stage evolution model for common Pb (Stacey and Kramers, 1975), the corrected ages are consistent within experimental error (Compston et al.,1992). LA-ICP-MS analyses were performed at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences. Laser sampling was performed using a GeoLas 2005 System (193 nm, MicroLas, Lambda Physik GmbH, Germany). The laser-generated aerosol was transported from the ablation cell to the ICP-MS instrument using a 1 m transfer tube with an internal diameter of 3 mm. The standard ablation cell in the GeoLas 2005 system is a closed design cell and consists basically of a cylindrical volume of approximately 40 cm3 with an inlet nozzle (i.d. <0.5 mm) and a wide outlet (i.e. 1.5 mm). An in-house sample mount was placed in the cell, which reduces significantly the effective cell volume to ∼14 cm3. Helium is advantageous as a carrier gas (Günther and Heinrich, 1999) and was thus used here. Argon was used as the make-up gas and was mixed with the carrier gas via a T-connector before entering the ICP to maintain stable and optimum excitation conditions. An Agilent 7500a ICP-MS instrument was used to acquire ion-signal intensities. The carrier and make-up gas flows were optimized by ablating NIST 610 to obtain maximum signal intensity for 208Pb, while keeping the ThO/Th ratio below 1%. The ion-signal intensity ratio measured for U and Th (NIST 610) (U/Th≈ 1) was used as an indicator of complete vaporization (Günther and Hattendorf, 2005). 3. Petrography For the first 100 m in the CCSD-MH only rock cuttings were collected. The section from 100 to 5148 m has a core recovery of ∼85%.

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Eclogites and gneisses are the principal lithological types (Fig. 2). Eclogites have a total cumulated thickness of about 1600 m, and mainly occur at depth intervals of 100 to 1100 m, 1600 to 2050 m and 2500 to 3500 m. Gneisses mainly occur at depths between 1100 to 1600 m and below 2050 m. Other rock types include garnet–peridotite with a thickness of ∼ 70 m, rarely, schist and quartzite occurring as thin layers within eclogites and gneisses. According to the spatial distribution, association, and compositional variation of the rocks, the CCSD-MH can be divided into five lithological units (Fig. 2). With increasing depths, they are: unit 1 (from 0 to 710 m) which consists mainly of eclogite, intercalated layers of gneiss and peridotite; unit 2 (from 710 to 1190 m) which is composed mainly of eclogite with abundant interlayers of gneiss; unit 3 (from 1190 to 1505 m) consisting of gneiss with minor eclogite as thin layers; unit 4 (from 1505 to 3460 m) which is composed mainly of gneiss with abundant interlayers of eclogite and retrograded eclogite (amphibolite); and unit 5 (below 3460 m) consisting mainly of gneiss with rare interlayers of eclogite and amphibolite. Eclogites from the CCSD-MH have variable mineral assemblages with different modal abundance. Quartz-rich, rutile-rich, ilmeniterich, phengite-rich and MgO-rich eclogite were recognized (Zhang et al., 2006a,b). Rt- and/or Ilm-rich eclogites that occur in unit 1 have a typical mineral assemblage of Grt + Omp + Rt (or Ilm) ± Ap (Fig. 3A); Qtz-rich eclogite occurs mainly in unit 1 and contains quartz up to

293

20 vol.% with minor rutile (Fig. 3B); Phn-rich eclogite occurs mainly in units 1 and 2, and is characterized by relatively high contents of phengite ± kyanite (Fig. 3C); MgO-rich eclogite occurs as thin layers, lenses or blocks within ultramafic rocks of unit 1, which can be distinguished by absence of minor minerals except for rutile. Inclusions of polycrystalline quartz pseudomorph after coesite are common in the eclogitic garnet and omphacite (Fig. 3B). Coesite, garnet, omphacite and phengite are also found as inclusions in zircons from fresh and retrograde eclogites (Zhang et al., 2006a,b,c), indicating that all eclogites have been subjected to UHP metamorphism. Some eclogites, especially those that occur as thin layers in gneisses from units 3, 4 and 5, have been subjected to amphibolitefacies retrogression to various extents, i.e. omphacite is partly or completely replaced by amphibole + sodic plagioclase symplectite, garnet by amphibole + plagioclase symplectite, and phengite by biotite + plagioclase symplectite or corona (Fig. 3C and D). Ultramafic rocks, including garnet–peridotite and garnet–pyroxenite, occur at the depth interval of 600 to 680 m. They consist of garnet, olivine, clinopyroxene, and orthopyroxene with or without minor Ti-clinohumite and phlogopite. Most ultramafic rocks exhibit variable degrees of hydration alteration as indicated by partial to complete serpentinization. Gneisses occur in all units, and consist of plagioclase, K-feldspar and quartz with minor and variable amounts of biotite, amphibole, muscovite, epidote (or allanite), garnet, apatite, and zircon (Fig. 3E to G). The gneisses

Fig. 2. Lithological and geochemical profiles of the CCSD-MH (oxygen-isotopic data after Zhang et al., 2006d).

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Fig. 3. Microphotographs of UHP rocks from the CCSD-MH. A. Rt-rich eclogite (sample ZN77, 369.0 m) consisting mainly of Grt and Omp, with abundant Ap and Rt; B. Qtz-rich eclogite (ZB75, 511.2 m), the garnet in the viewing center contains inclusion of polycrystalline quartz pseudomorph after coesite, and shows radial fractures around the inclusion; C. Phn-rich eclogite (ZG64, 1074.3 m), omphacite and phengite have thin symplectitic coronas; D. Retrograde eclogite (ZK66, 2710.6 m), omphacite is replaced by the symplectite of Amp + Pl, and phengite is rimmed by Bt+ Pl symplectitic coronas; E. Granitic gneiss (ZJ66, 2698.9 m) consisting of Pl, Kf, Grt, Amp, Bt, Qtz, Aln, Ep and Mag; F. Granitic gneiss consisting of Pl, Kf, Qtz, Aln, Ep and Bt (ZH64, 1425.4 m); G. Retrograde jadeite–quartzite (ZP43, 366.8 m), jadeite has been completely replaced by Amp + Pl symplectite, and phengite by Bt + Pl symplectite; H. Grt–Phn– Qtz schist containing minor Ep and Ttn (ZM77, 365.9 m). Phengite is partly or completely replaced by Bt+ Pl symplectite. All images are taken under plain light.

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295

Table 1 Major element compositions (wt.%) of the representative UHP rocks from the CCSD-MH. Sample ZA73 ZN76 ZN77 ZB75 ZD83 ZE82 ZE83 ZE85 ZG64 ZE87 ZH64 ZL44 ZK65 ZJ66 ZK66 ZO71 ZM75 ZO77

Depth(m)

Rock a

Re. Qtz-rich eclogite Re. Phn-rich eclogite Rt-rich eclogite Qtz-rich eclogite Re. eclogite Ordinary eclogite Ep Bt gneiss Phn Grt gneiss Phn-rich eclogite Phn Grt gneiss Bt Aln gneiss Ep Bt gneiss Re. Phn-rich eclogite Grt Bt Aln gneiss Re. Phn-rich eclogite Grt Bt gneiss Bt gneiss Grt Bt Aln gneiss

249.5 368.0 369.0 511.2 643.8 728.0 812.3 930.2 1074.3 1109.3 1425.4 2566.0 2687.0 2698.9 2710.6 3062.1 3325.8 3949.7

SiO2

TiO2

Al2O3

Fe2O3

FeO

MnO

MgO

CaO

Na2O

K2O

P2O5

H2O+

CO2

Total

60.83 49.43 43.32 47.30 49.11 47.80 56.37 74.36 47.66 75.95 73.80 75.79 60.11 70.72 56.78 73.97 77.58 77.18

1.05 0.98 4.71 2.96 1.78 0.66 1.07 0.31 1.66 0.26 0.28 0.21 0.94 0.51 1.00 0.32 0.15 0.07

16.02 16.08 14.31 13.13 15.39 16.71 15.03 13.42 16.11 12.54 12.60 11.71 15.18 12.44 15.88 12.78 11.72 11.25

3.22 4.58 2.60 4.58 3.03 1.76 3.56 1.03 4.49 0.70 1.13 1.71 1.97 1.99 1.43 1.15 1.32 0.55

5.60 5.66 14.82 12.68 7.81 9.18 3.32 0.94 8.07 0.63 0.97 0.99 5.05 2.71 6.70 1.44 0.83 0.75

0.49 0.22 0.29 0.31 0.19 0.20 0.17 0.03 0.19 0.08 0.03 0.06 0.12 0.17 0.14 0.08 0.05 0.02

0.78 6.16 6.72 4.69 4.95 8.26 4.42 0.52 7.05 0.33 0.29 0.44 4.03 1.12 5.04 0.81 0.09 0.03

3.35 8.90 9.39 9.14 9.25 11.88 6.41 1.19 9.92 1.39 0.64 0.40 5.28 1.76 6.80 1.26 0.22 0.32

6.89 4.25 2.64 3.72 4.11 2.69 2.46 4.80 2.39 3.93 3.19 3.38 2.93 4.16 2.95 4.50 4.21 4.15

0.87 2.07 0.02 0.01 1.92 0.08 3.50 1.97 0.37 2.63 5.51 4.19 2.62 3.14 2.03 3.13 4.56 4.33

0.27 0.17 0.48 0.42 0.32 0.01 0.12 0.05 0.72 0.06 0.06 0.05 0.29 0.08 0.33 0.10 0.01 0.01

0.94 0.98 0.08 0.04 1.30 0.30 1.78 0.80 0.72 0.68 0.70 0.44 1.26 0.48 1.22 0.48 0.28 0.46

0.05 0.09 0.34 0.35 0.25 0.18 1.27 0.13 0.12 0.46 0.52 0.12 0.13 0.28 0.09 0.18 0.09 0.23

100.45 99.63 99.75 99.75 99.57 99.80 99.58 99.53 99.46 99.64 99.72 99.49 99.91 99.56 100.39 100.20 101.11 99.35

a

Re-retrograde.

from units 3 and 5 are characterized by relatively high contents of feldspar and quartz, and relatively low abundances of ferromagnesian minerals, showing only weak foliation or even massive texture. In general, gneisses from the CCSD-MH have amphibolite-facies mineral assemblages. However, symplectite of amphibole+plagioclase after jadeite, and plagioclase+biotite after phengite were recognized in some gneiss samples (Fig. 3G), implying that their UHP counterparts are phengitebearing jadeite–quartzite. As an important indicator for the UHP metamorphism, coesite inclusions are common in gneissic zircons. Other minerals, such as phengite, garnet, jadeite (or omphacite), rutile and apatite also occur as inclusions in gneissic zircons as shown by previous studies (Liu et al., 2004a,b; Zhang et al., 2006a,b,c). These observations strongly suggest that the gneisses have been subjected to early UHP metamorphism before they were overprinted by amphibolitefacies metamorphism, which is consistent with the results obtained from non-mafic rocks from some shallow holes and surface outcrops in the Donghai area (Zhang et al., 2000, 2003; Liu et al., 2001). Schists occur usually as thin layers (<1 m) within gneiss and eclogite. Three types of schists can be recognized according to their mineralogy: garnet–phengite–quartz schist (Fig. 3H), garnet twomica schist, and garnet–epidote–biotite–quartz schist. The schists have mineral assemblages similar to the gneisses but with higher amounts of mica and quartz. Symplectite of biotite + plagioclase after phengite is recognized in some schists (Fig. 3H). Coesite and other

eclogite-facies mineral inclusions are also present as inclusions within zircon of the schists, suggesting an early stage of UHP metamorphism. 4. Petrochemistry 4.1. Major elements A large number of eclogite, gneiss and schist core samples collected at an interval of 2 to 4 m from the CCSD-MH were analyzed for major and trace elements. The whole-rock compositions of representative eclogites from the CCSD-MH have been published elsewhere (e.g., Zhang et al., 2006a,b). Here, chemical compositions of 18 rock samples dated for zircon U–Pb ages are listed in Tables 1–3. Variations of SiO2, TiO2, Nb, REE and oxygen-isotopic compositions with depth are shown in Fig. 2. The eclogites from the borehole cover a relatively wide compositional range from peridotgabbro to diorite, of which gabbros and gabbroic diorites are dominant according to the chemical classification and nomenclature of plutonic rocks (Middlemost, 1994; Zhang et al., 2006a,b). The Rt-rich eclogites are plotted mainly in the fields of gabbro, peridotgabbro and foid-gabbro, and some Ilm-rich eclogites are plotted outside the peridotgabbro field due to their low SiO2 contents (can be as low as 32 wt.%). The Qtz-rich eclogites are of diorite and quartz monzonite composition. The phengite- and MgO-rich eclogites can be classified as gabbro, gabbroic diorite, monzogabbro or monzodiorite. Most gneisses

Table 2 Trace element compositions (ppm) of the representative UHP rocks from the CCSD-MH. Sample

Depth(m)

Sr

Zr

Ba

V

Zn

Cr

Co

Ni

Cu

Ga

Rb

Nb

Ta

Hf

Pb

Th

U

Cl

Sc

Y

ZA73 ZN76 ZN77 ZB75 ZD83 ZE82 ZE83 ZE85 ZG64 ZE87 ZH64 ZL44 ZK65 ZJ66 ZK66 ZO71 ZM75 ZO77

249.5 368.0 369.0 511.2 643.8 728.0 812.3 930.2 1074.3 1109.3 1425.4 2566.0 2687.0 2698.9 2710.6 3062.1 3325.8 3949.7

125.2 127.7 199.0 219.0 465.0 115.0 95.2 289.9 493.4 240.8 53.4 81.5 212.5 144.3 273.4 59.8 51.9 7.7

565.4 69.4 93.4 157.0 163.0 120.0 116.0 266.2 143.2 172.6 345.4 320.3 141.5 573.3 148.8 246.0 126.0 160.0

207.0 247.0 18.8 9.5 997.0 57.4 331.0 1044.0 171.0 839.0 658.0 720.0 992.0 802.0 903.0 352.0 450.0 55.0

8.7 200.8 289.0 457.0 268.0 228.0 167.0 12.6 231.0 28.9 13.2 13.7 133.9 43.4 161.0 31.0 1.9 4.4

158.8 148.5 161.9 168.0 119.0 93.8 109.0 48.4 128.6 41.1 64.4 100.8 73.3 150.4 73.8 77.5 80.3 89.6

0.8 192.4 6.9 9.7 80.5 197.0 430.0 2.6 165.0 4.8 5.5 23.2 77.7 22.9 105.8 15.9 2.4 3.5

1.3 34.1 37.3 44.2 38.8 59.2 26.3 1.6 47.0 2.0 1.5 2.9 26.9 7.6 34.5 4.6 0.2 0.2

0.9 94.9 3.4 6.5 39.3 93.1 53.6 1.2 76.8 2.7 1.5 10.8 33.6 8.7 48.0 7.3 < 0.05 0.9

9.1 54.3 166.7 47.8 57.2 66.3 10.9 20.1 36.1 10.4 5.0 4.8 14.9 58.7 25.4 6.9 4.2 4.2

20.5 17.6 19.4 17.7 18.9 17.5 23.7 17.7 19.4 20.2 23.4 22.5 17.4 22.1 17.6

9.2 59.5 0.5 1.0 60.7 2.2 158.0 33.0 12.0 60.2 128.4 93.3 54.8 71.4 42.5 67.8 109.0 159.0

12.3 4.9 2.5 6.3 7.1 1.7 17.6 9.9 3.9 12.6 17.8 21.5 4.0 16.4 2.8 15.6 13.3 19.2

0.5 0.5 0.2 0.4 0.4 0.1 0.9 0.7 0.4 0.8 1.1 1.4 0.3 0.8 0.2 0.9 1.1 1.6

11.7 2.0 2.3 4.3 5.0 3.3 3.4 6.3 3.3 5.2 12.4 12.6 4.9 19.9 4.5 8.1 5.0 7.4

1.5 4.0 8.3 3.2 6.1 5.9 3.5 5.8 12.2 5.1 15.5 34.8 5.4 7.6 12.9 4.2 23.1 28.6

2.0 0.7 0.5 2.1 3.1 0.1 1.3 13.1 0.8 11.1 13.7 12.1 10.6 7.8 6.2 4.2 10.2 16.1

0.5 0.3 0.1 0.5 0.8 0.1 1.3 1.5 0.4 1.6 1.3 1.9 1.2 1.1 0.8 1.5 0.8 1.9

41.6 25.1 26.0 20.5 71.3 142.0 <20.0 20.4 <15.0 15.8 38.2 20.8 36.3 27.2 226.2 9.1 27.7 53.0

34.6 31.9 55.2 39.5 26.5 42.5 20.9 6.8 35.3 4.4 4.7 4.7 17.5 8.0 24.1 5.0 3.3 2.3

37.6 42.7 31.9 38.2 30.3 24.7 31.8 8.2 29.6 25.5 84.8 49.6 26.3 97.9 25.0 59.1 39.0 90.3

22.8

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Table 3 Rare earth element compositions (ppm) of the representative UHP rocks from the CCSD-MH. Sample

Depth(m)

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

ZA73 ZN76 ZN77 ZB75 ZD83 ZE82 ZE83 ZE85 ZG64 ZE87 ZH64 ZL44 ZK65 ZJ66 ZK66 ZO71 ZM75 ZO77

249.5 368.0 369.0 511.2 643.8 728.0 812.3 930.2 1074.3 1109.3 1425.4 2566.0 2687.0 2698.9 2710.6 3062.1 3325.8 3949.7

14.7 6.7 3.5 17.1 28.4 1.0 7.5 82.4 26.0 59.2 78.5 59.5 53.7 51.5 36.4 25.3 21.8 47.0

37.0 19.0 8.0 41.0 57.9 1.8 24.1 123.0 54.1 100.0 157.0 196.0 98.7 107.0 71.1 56.2 56.7 107.0

5.7 3.0 1.2 5.8 7.3 0.3 3.5 16.5 7.2 10.4 17.1 13.6 10.9 13.1 8.4 7.5 7.5 12.5

28.2 14.7 6.4 26.0 31.2 2.2 15.4 56.3 32.5 34.4 62.3 49.1 40.5 54.2 33.1 30.5 30.2 45.8

7.4 4.8 1.9 6.9 7.4 2.1 4.4 8.7 7.0 5.3 13.0 10.4 6.9 12.1 6.2 7.8 7.8 12.5

7.1 1.8 0.8 2.2 2.1 1.0 1.2 2.0 2.6 0.9 0.8 1.3 1.9 2.8 1.8 0.6 1.1 0.1

8.1 7.0 3.0 7.5 7.4 3.7 5.3 7.0 6.9 4.3 11.0 9.5 5.1 11.9 5.0 8.0 7.0 13.2

1.3 1.4 0.7 1.3 1.2 0.7 1.0 0.7 1.0 0.7 2.2 1.7 0.8 2.2 0.8 1.5 1.2 2.6

7.4 8.7 5.1 8.3 7.1 4.9 6.5 2.5 5.7 4.1 13.5 9.9 4.7 15.3 4.6 9.7 7.7 16.7

1.6 1.7 1.1 1.8 1.5 1.1 1.3 0.4 1.1 0.9 2.9 2.0 0.9 3.6 0.9 2.1 1.5 3.4

4.6 4.3 3.4 5.2 4.1 3.1 3.7 0.9 3.1 2.9 8.6 5.7 2.8 11.7 2.9 6.4 4.3 9.9

0.7 0.6 0.4 0.8 0.6 0.5 0.5 0.1 0.4 0.5 1.2 0.8 0.4 1.9 0.4 1.0 0.6 1.4

4.9 3.4 2.9 4.8 3.7 2.8 3.4 0.6 2.9 3.2 7.9 5.1 2.6 12.9 2.7 6.5 3.6 8.5

0.8 0.5 0.4 0.7 0.6 0.4 0.5 0.1 0.4 0.5 1.2 0.8 0.4 2.1 0.4 1.0 0.5 1.1

lie in the granitic field with the SiO2 content greater than 69 wt.%; whereas some gneisses have the composition of granodiorite, quartz monzonite, diorite and monzonite, indicating a compositional range of intermediate-acidic intrusions (SiO2 = 60–69 wt.%). The eclogites in unit 1 show the largest variation in terms of SiO2 from 32 to 61 wt.%, TiO2 (0.7–6.0 wt.% in general, with the highest up to 8.0 wt.%) and Al2O3 (12–22 wt.%), and are characterized by alternating occurrences of SiO2-rich Qtz eclogite and TiO2-rich Rt- and Ilmeclogites (Fig. 2). Variations of SiO2 vs. MgO, total FeO and TiO2 display distinct negative correlations, implying a fractional crystallization trend (Zhang et al., 2006a). Moreover, these rocks possess low Mg values (mostly <50%), where the Mg value is Mg2+ / (Mg2+ + Fe2+), suggesting that they are not derived from primitive melts. The eclogites from 530 to 600 m are characterized by high TiO2 (mostly >3 wt.%) and total FeO (mostly >20 wt.%) with low SiO2 (<45 wt.%) and alkali (<2 wt.%) contents (Fig. 2). Their chemical compositions are similar to those of Fe–Ti gabbros from layered intrusions (Parsons et al., 1986; Morrison et al., 1986; McBirney, 1989; Wiebe, 1993; Arnason et al., 1997; Brandriss and Bird, 1999), and metamorphosed Fe–Ti gabbros (Zhang et al., 1995; You et al., 1996; Cox et al., 1998; Liou et al., 1998; Cox and Indares, 1999). The eclogites occurring as thin layers, blocks or lenses within ultramafic rocks are characterized by high MgO (mostly >12 wt.%) contents, distinguishing them from all other eclogites, and suggesting a co-genetic origin with the host ultramafic rocks. The unit 2 eclogites have compositions close to the quartz eclogites from unit 1, except its lower total FeO (mostly <10 wt.%) and TiO2 contents (generally around 1 wt.%), showing no fractional crystallization trend. The eclogites from units 3, 4 and 5 have relatively homogeneous compositions with SiO2 content in the range of 47–56 wt.%. In general, all the eclogites show comparable but narrower compositional ranges than the continental plutonic rocks, such as continental intrusions from the East Greenland and North Atlantic Province as indicated by Zhang et al. (2006a). The gneisses that occur as interlayers within the eclogites of unit 1 have intermediate-acidic and acidic compositions with SiO2 = 62– 72 wt.%, and belong to a part of a layered intrusion formed by fractional crystallization of basaltic magma (Zhang et al., 2006b). The gneisses in units 2 and 4 have a relatively large compositional range (SiO2 = 60–80 wt.%, Fig. 2); however, most samples show compositions of granite (SiO2 > 69 wt.%, Na2O + K2O = 5–7 wt.%). The gneisses in unit 3 and 5 have consistent chemical compositions, and are characterized by relatively high SiO2 (mostly >72 wt.%) (Fig. 2) and Na2O + K2O (>6 wt.%) contents, suggesting a granitic protolith. The SiO2 contents of all rocks from the CCSD-MH demonstrate a bimodal variation; the two peaks are centered at c.a. 47 wt.% and

77 wt.%, corresponding to the SiO2 contents of gabbroic and granitic source material, respectively. Therefore, the protoliths of both eclogites and gneisses can be regarded as products of bimodal magmatism that took place in a tectonic zone of continental rifting (also see following section). 4.2. Trace elements A close inspection of the trace element data reveals important compositional differences among various eclogites. The Qtz-rich eclogite from unit 1 possesses relatively high concentrations of Zr (116–726 ppm), Hf (3.4–18 ppm), Nb (3.3–25.5 ppm) and Ta (0.3– 50.6 ppm), which are similar to typical continental basalts (Zhang et al., 2006a). The Rt- and Ap-rich eclogites from unit 1 have relatively high TiO2 (2.4–5.9 wt.%), P2O5 (0.1–4.1 wt.%), but variable Zr (21– 414 ppm), Nb (0.4–14.6 ppm) and Ta (0.1–1.1 ppm) contents. The ilmenite-rich eclogite from 530 to 605 m exhibits a unique trace element characteristics, i.e., very high V (126–1163 ppm) and Co (14– 132 ppm), low Zr (24–85 ppm), Ta (<0.1–0.6 ppm), Nb (0.3–6.9 ppm) and total REE (10.7–334.0 ppm) contents (Fig. 2), suggesting decoupling of Ti and V from Nb and Ta. This characteristic is typical of Fe–Ti gabbros in layered mafic to ultramafic intrusions formed by extensive fractional crystallization of basaltic magma in continental environments (Parsons et al., 1986; Morrison et al., 1986; McBirney, 1989; Wiebe, 1993; Arnason et al., 1997; Brandriss and Bird, 1999). In general, other eclogites from various units are slightly enriched in incompatible elements but have variable Rb, Ba and Th contents, and show distinctly negative Nb, Ta, Sr and Ti anomalies, in contrast to any MORB or OIB (Zhang et al., 2006a). The eclogites have variable REE contents and chondrite-normalized REE patterns. Most eclogites, including the Qtz- and Rt-rich eclogites, are characterized by LREE enrichment and HREE depletion with generally variable positive Eu anomalies (Fig. 4A). In addition, these eclogites have variable trace element compositions, but most of them exhibit negative Nb, Sr and Zr anomalies in the primitive-mantle normalized trace element spider diagram (Fig. 5A); whereas the Ilmrich eclogite has usually LREE-depleted REE patterns with pronounced positive Eu anomalies (Fig. 4B), and positive Sr and Ti, and negative Nb anomalies (Fig. 5B), typical of Fe–Ti gabbro in layered intrusion (Zhang et al., 2006b). As mentioned above, the eclogites from the CCSD-MH, especially those from unit 1, show strongly compositional scatter in both major and trace elements. These distinct compositional variations have been inferred to have resulted from extensive fractional crystallization of basaltic magma formed under continental tectonic setting (Zhang et al., 2006a,b).

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Fig. 4. Chondrite-normalized REE patterns of eclogites and gneisses from the CCSD-MH.

The granitic gneisses from units 3 and 5 have relatively high Rb, Nb, Y and REE contents (Figs. 2, 5Cand F) and consistent REE patterns showing LREE enrichment with pronounced negative Eu anomalies (Fig. 4B and F). It is noted that the rocks with very low Nb contents in units 3 to 5 are actually interlayers of eclogite and/or amphibolite within gneisses. Thus many mafic rocks are too thin in thickness to show up in Fig. 2. The granitic gneisses (SiO2 > 69 wt.%) from units 2 and 4 have relatively high Sr and Ba contents and variable REE contents (Fig. 2), but similar REE patterns showing LREE enrichment and negative Eu anomalies (Fig. 4D and E). Compared with the primitive mantle, all granitic gneisses from this borehole are characterized by strong enrichment of LILE (such as Rb, Th, U, K) and LREE, but pronounced negative anomalies of Sr and HFSE (such as Nb and Ti) (Fig. 5C–F). These patterns of trace element partition are common for the continental crust that is usually assumed to originate from chemical differentiation of rift- or arc-derived magmas (Zhang et al., 2006a). Moreover, the granitic gneisses from units 3 and 5 display generally a compositional character of within-plate granite (WPG) (Fig. 6A); whereas most granitic gneisses from units 2 and 4

have variable but lower Y and Nb concentrations and exhibit mostly a similarity to volcanic-arc granite (VAG) (Fig. 6B). 5. Zircon U–Pb dating 5.1. Zircon morphology and mineral inclusions The zircon U–Pb ages of 18 eclogite and gneiss samples from different depths and units were determined by the SHRIMP and LAICP-MS methods. In general, the zircons can be divided into two types: most type I zircons contain an inherited magmatic core and an UHP metamorphic rim whereas type II zircons have no core–rim structure, and were crystallized during the UHP metamorphism. The type I zircon grains from the eclogites occur usually as inclusions in garnet, omphacite and phengite; they are transparent, colorless and nearly round-shaped. The cathodoluminescence (CL) images reveal that the zircon cores show oscillatory or broad-band magmatic zoning (Fig. 7A and B), typical of zircon from gabbroic intrusives (e.g., Rubatto and Scambelluri, 2003; Zhang et al., 2006b).

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Fig. 5. Primitive-mantle normalized trace element patterns of eclogites and gneisses from the CCSD-MH.

The rims, however, either are unzoned or show patchy or segmented patterns, which are typical of metamorphic overgrowth. The zircon cores contain mineral inclusions of Di, Pl, Bt, Qtz and Ap, characteristic of lower pressure assemblages, whereas the zircon rims have inclusions of Coe, Grt, Omp, Phn and Rt, implying their formation under UHP metamorphic conditions (Zhang et al., 2006a,b). The type I zircons from the gneisses are usually subhedral to euhedral, long prismatic in shape, colorless, and transparent. These zircons show similar core and rim characteristics as those from the eclogites. For example, the cores show oscillatory magmatic zoning and also contain low-pressure mineral inclusions of Pl, Kf, Ep, Qtz and Ap, typical of granitic zircon (Fig. 7C to F); whereas the rims either are unzoned or show unclear patchy patterns, and contain UHP mineral inclusions, such as Coe, Grt, Jd, Phn, and Rt (Fig. 7C). The type II zircons, which are recognized only in some eclogite samples without retrogression, are characterized by multifaceted, colorless and transparent crystals that are relatively small in grain size in comparison with the type I zircons from the eclogites. They are unzoned or only show weakly patchy pattern, and commonly contain eclogite-facies mineral inclusions such as Coe, Grt, Omp, Phn and Rt

(Fig. 7G and H). Therefore, these zircons with typical characteristics of metamorphic origin must have been formed during the UHP metamorphism. Investigations of the zircons in the gneiss and eclogite samples from 100 to 2000 m of the CCSD-MH have demonstrated that the cores of the type I zircons, especially those with abundant mineral and fluid inclusions and wider overgrown rims, commonly exhibit weak magmatic zonation and faint luminescence (Fig. 7D) (Zhang et al., 2006c). These characteristics imply that these zircons have been altered by interactions with metamorphic fluids (Vavra et al., 1996; Gebauer et al., 1997; Zhang et al., 2006b,c). Alternatively, they are interpreted to have been resulted from solid-state recrystallization of protolithic magmatic zircon (Zheng et al., 2006). 5.2. Zircon U–Pb ages The results of zircon U–Pb dating demonstrate that the inherited magmatic cores of the type I zircons from the eclogite and gneiss samples give highly variable 206Pb/238U ages ranging from 805 to 247 Ma, and relatively high Th/U ratios of >0.15 (mostly >0.50)

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Fig. 6. Rb vs. Y + Nb diagram of granitic gneisses from the CCSD-MH (after Pearce et al., 1984). A. Gneisses from units 3 and 5; B. Gneisses from unit 4. ORG: ocean ridge granites; Syn-COLG: syn-collision granites; VAG: volcanic arc granite; WPG: within plate granite.

Figs. 8–11A) that are typical of magmatic zircons (Gebauer et al., 1997; Buick et al., 2006; Liu et al., 2006). For three gneiss samples (ZE87, ZL44 and ZO77), the weighted mean of those older ages (>700 Ma) are 777 ± 41 Ma (Fig. 9D), 767 ± 29 Ma (Fig. 9F) and 776 ± 19 Ma (Fig. 10D), respectively. The older ages (>700 Ma) of the inherited magmatic cores from most other eclogite and gneiss samples, such as 718 Ma (ZN76), 762 Ma (ZB75), 729 Ma (ZB82), 774 Ma (ZK65), 781 Ma (ZK66), 744 Ma (ZE85), 788 Ma (ZH64) and 803 Ma (ZM75), are similar to the weighted mean ages of the former three samples. The type I zircon rims from 15 eclogite and gneiss samples yield similar mean ages in the range from 214.2 ± 2.7 Ma (ZK65; Fig. 8H) to 223 ± 16 Ma (ZH64; Fig. 9E). These zircon rims have relatively low Th/U ratios of 0.004 to 0.40 (mostly <0.15) (Fig. 11B), typical of metamorphic zircon (Williams et al., 1996; Gebauer et al., 1997; Rubatto, 2002; Buick et al., 2006). Twenty spots of the type II metamorphic zircons without magmatic cores from an eclogite sample ZG64 give a weighted mean age of 221.5 ± 3.3 Ma (Fig. 8G), with very low Th/U ratios of ranging from 0.004 to 0.127 (mean = 0.072), typical of metamorphic origin. Available literature data demonstrated that magmatic cores of zircon can be partly altered or reset by later thermal events (e.g. Gebauer et al., 1997; Schaltegger et al., 1999; Zeck and Whitehouse, 2002; Tomaschek et al., 2003; Zheng et al., 2003a; Zhang et al., 2006b, c), and as a result, the isotopic system of the magmatic zircons could be disturbed during UHP metamorphism (Gebauer et al., 1997; Zheng et al., 2003a,b, 2006; Zhang et al., 2006b,c). Therefore, unreasonable ages could be obtained by SHRIMP U–Pb dating on altered magmatic cores of the zircons from UHP rocks of the Alps (Gebauer et al., 1997) as well as Sulu UHP eclogites (Zhang et al., 2006b). Our dating results indicate that the magmatic cores of zircons with narrow overgrowth rims and well-preserved magmatic zoning from samples ZE87, ZL44

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and ZO77 give older but consistent ages with relatively high Th/U ratios (Fig. 7A and F; Appendix A), whereas the zircon cores with a wider overgrowth rim and faint magmatic zoning from samples ZN76, ZE83 and ZO71 give commonly younger and inconsistent ages with lower Th/U ratios (Fig. 7E; Appendix A). Moreover, for gneisses and especially for eclogites, a positive correlation between the ages of magmatic zircon cores and their respective Th/U-ratios was observed (Fig. 11A). These characteristics further indicate that the inherited magmatic zircons have been incompletely reset by the partial Pb and Th loss during the UHP metamorphism (Gebauer et al., 1997). Therefore, we suggest that the calculated mean ages of 767 to 777 Ma for the inherited zircon cores from the three gneiss samples represent the protolith ages of the eclogites and gneisses, whereas those younger ages from the inherited zircon cores might be reset or mixed ages between partly reset protolithic zircon and UHP metamorphic zircon. Thus, the protolith ages of the middle Neoproterozoic are in agreement with the rift magmatism in the periphery of the Yangtze Plate in response to the supercontinental rifting (e.g. Li et al., 2003; Zheng et al., 2006). The ages of 214 to 223 Ma from the overgrowth rims of the type I zircon and from the type II zircon are interpreted as the formation age of metamorphic zircon during the UHP metamorphism because these zircons or the zircon rims contain coesite and other eclogite-facies mineral inclusions and thus were formed under UHP eclogite-facies conditions as mentioned above. All zircon U–Pb ages obtained for the analyzed UHP metamorphic rocks clearly exhibit a double-peak pattern in the age frequency distribution diagram (Fig. 10E). The inherited magmatic cores of the eclogitic and gneissic zircons give the older age peak at 788.8 Ma, whereas the metamorphic zircon and the overgrowth rims of the type I zircon give the younger peak at 216.4 Ma. Based on the reasons described above, the former peak should represent the protolithic ages of the eclogite and gneiss from the CCSD-MH, whereas the latter peak age might reflect the growth time of eclogitic and gneissic zircons during the UHP metamorphism. In addition, as shown in Fig. 10F, the ages of metamorphic zircons also show a multimodal character with a main peak at 216.7 Ma and a smaller peak at 221.1 Ma, suggesting that the zircons were crystallized at different stages of the UHP metamorphism (also see following section). The older age of 221.1 Ma is quite close to a weighted mean of 221.5 Ma given by the type II metamorphic zircons from the eclogite sample ZG 64 (Fig. 8G) and 222 Ma obtained for the eclogitic and gneissic zircons from the CCSD-MH by Chen et al. (2007). 6. Discussions 6.1. Protolith of UHP rocks and tectonic setting during their formation Available literature data have indicated that the protoliths of the UHP eclogites and granitic gneisses from the surface outcrops in the Dabie-Sulu orogenic belt were formed in a Neoproterozoic continental environment (Ames et al., 1993; Li et al., 1993; Rowley et al., 1997; Hacker et al., 1998; Hacker et al., 2000; Rumble et al., 2002; Jahn et al., 2003; Zheng et al., 2003a,b, 2006). Their protoliths had interacted with fluids at shallow crustal depths before subduction, as reflected by negative O-isotopic anomalies with variable magnitudes for some UHP rocks exposed on the surface (e. g. Yui et al., 1995; Zheng et al., 1996; Rumble et al., 2002), and from the shallow drill holes (Zhang et al., 2005a). Recent investigations (Zhang et al., 2006d; Xiao et al., 2006) have shown that most rocks from 100 to 3300 m of the CCSDMH display negative O-isotopic anomalies of different magnitudes (Fig. 2), suggesting that they had undergone hydrothermal alteration with meteoric water with extremely low oxygen isotopic values before subduction (Zhang et al., 2006d). As mentioned above, the UHP rocks from the CCSD-MH show very large compositional variations (Fig. 2), but they generally exhibit geochemical characteristics of igneous rocks. Recent investigation by Zhang et al. (2006b) indicated that the protolith of UHP rocks in unit 1

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Fig. 7. Cathodoluminescence (CL) images and microphotographs of zircon, showing the spot used for dating and their age in Ma. A. Type I zircon in eclogite (ZB75, 511.2 m) has an inherited magmatic core with oscillatory zoning and Pl inclusion, and a thin metamorphic overgrowth rim with weakly patch zoning; B. Type I eclogitic zircons (ZE83, 812.3 m), one has a small magmatic core with weak patchy zonation and a wide metamorphic rim with patch zoning, while another has a magmatic core with broad-band zoning and a thin rim with concentric zoning; C and D. Type I gneissic zircon (ZE87, 1109.3 m) with an uncompleted oscillatory zoned magmatic core with and Qtz, Ms and Ep inclusions, and a wide metamorphic rim with Coe inclusion and weakly concentric zoning; E and F. Type I gneissic zircons with large oscillatory zoned magmatic cores containing Pl and Qtz inclusions, and narrow metamorphic rims with weak patchy zoning (E. ZH64, 1425.4 m and F. ZL44, 2566.0 m); G and H. Type II eclogitic zircon (ZG64, 1074.3 m) shows weakly cloudy zonation and contains Omp, Coe and fluid inclusion (FI). C and G are microphotographs under plane polarized light, others are CL images.

of the CCSD-MH is a layered intrusion consisting of ultramafic–mafic– acidic rocks, which was formed by extensive fractional crystallization of basaltic magma in a Neoproterozoic continental setting. Moreover,

fractional crystallization and possible multiple injections of magma in an evolved basaltic magma chamber are likely the main mechanisms for Ti-enrichment and repetition of Rt-rich eclogitic layers. The

Fig. 8. Zircon U–Pb concordia diagram for 8 eclogite samples. A: ZA73, 249.5 m; B: ZN76, 368.0 m; C: ZN77, 369.0 m; D: ZB75, 511.2 m; E: ZD83, 643.8 m; F: ZE82, 728.0 m; G: ZG64, 1074.3 m; and H: ZK65, 2687.0 m.

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Fig. 9. Zircon U–Pb concordia diagram for 1 eclogite and 5 gneisses. A: ZK66, 2710.6 m; B: ZE83, 812.3 m; C: ZE85, 930.2 m; D: ZE87, 1109.3 m; E: ZH64, 1425.4 m; and F: ZL44, 2566.0 m.

widespread occurrence of thick-layered eclogites with unusually high Ti, P and V contents suggests that these components have potential economic value (Zhang et al., 2006b). Liu et al. (2005a, 2008) demonstrated that parental materials of the eclogites and ultramafic rocks in unit 1 probably represent a complete sequence of fractional crystallization of tholeiitic or picritic magmas that formed in the lower crust in the Neoproterozoic time, and were later metamorphosed under UHP conditions. Zhang et al. (2006a) suggested that the protoliths of the most eclogites and gneisses from the depth interval of 710 to 2050 m of the CCSD-MH are also metamorphosed igneous rocks.

The conclusions mentioned above were strongly supported by the isotopic characteristics of the UHP rocks. Xue and Liu (2007) have demonstrated that the gneisses in unit 3 have low and narrow range of ɛNd(t) values (−8.2 to −13.0), and their Nd model ages are from 2.25 to 2.54 Ga, much older than their protolith ages (700–800 Ma), indicating their protoliths were probably formed by partial melting of Paleoproterozoic crustal rocks in Neoproterozoic. In contrast, the gneisses in units 1, 2 and 4 of the CCSD-MH have a wide range of ɛNd (t) values (+ 1.3 to − 9.4), which are similar to those of the acidic endmember of the Neoproterozoic bimodal volcanic rocks present in the northern margin of the Yangtze Block. Moreover, the gneisses as thin

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Fig. 10. A–D. Zircon U–Pb concordia diagram for 4 gneisses. A: ZJ66, 2698.9 m; B: ZO71, 3062.1 m; C: ZM75, 3325.8 m; D: ZO77, 3949.7 m; E. Histogram of all analyzed zircon U–Pb ages, exhibiting double-peak pattern, the solid line represents the relative probability; and F. Histogram of all metamorphic zircon U–Pb ages, exhibiting double-peak pattern and the solid line represents the relative probability.

layers in the eclogite have near-chondritic ɛNd values (+1.3 to −2.6), similar to or slightly lower than those of the host eclogites, indicating a genetic relationship between the protoliths of the gneisses and the eclogites. Zhao et al. (2005) have shown that the eclogite and gneiss from 734 to 737 m of the CCSD-MH display similar but relatively high ɛNd values of −3.6 to +0.5, whereas the gneiss from 929 to 933 m has a significantly low ɛNd value of − 8.7. These data suggest that the protolith of the eclogite is similar to within-plate basalt, the protolith of gneiss from 734 to 737 m is probably an intermediate rock formed by crustal assimilation of basaltic magma during its emplacement,

whereas the protolith of the gneiss from 929 to 933 m is a granitic rock formed by partial melting of old crustal materials. The Hf isotope data demonstrate that the gneiss from 734 to 737 m has positive ɛHf(t) values of 7.8 to 6.0, with young Hf model age of 1.03 and 1.11 Ga, whereas the gneiss from 929 to 933 m exhibits negative ɛHf(t) values of − 6.9 to − 9.1, with old Hf model ages of 2.11 and 2.25 Ga (Chen et al., 2007). It appears that the protolith of the gneiss from the segment 734–737 m was formed by rapid reworking of juvenile crust, whereas the gneiss from the segment 929–933 m was primarily generated by rift anatexis of the Paleoproterozoic crust. The bimodal

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Fig. 11. Th/U vs. age diagram of the inherited magmatic zircon cores (A) and the metamorphic overgrowth rims and metamorphic zircons (B).

magmatism has been proposed to occur in an arc-continental collision orogen during the supercontinental rifting, in response to the attempted breakup of the supercontinent Rodinia at 780 Ma. This work further demonstrates that the protoliths of the eclogites and gneisses from the CCSD-MH are metamorphosed mafic and acidic (or intermediate-acidic) intrusives, respectively. The main evidence include: (1) most eclogitic zircons have magmatic cores, typical of zircon in mafic intrusion, and most gneissic zircons have magmatic cores with characteristics of granitic zircon, suggesting that they are metamorphosed igneous intrusions, rather than metavolcanic (or even metasedimentary) rocks; (2) all dating results indicate that the eclogites and gneisses have the same protolithic age (Neoproterozoic) (Figs. 8–10); (3) the eclogites have compositional characteristics of continental basalt (or gabbro), and the granitic gneisses have characteristics of withinplate and/or volcanic-arc granites (Fig. 6A and B); (4) the whole-rock compositions of all core samples from the CCSD-MH show clear evidence of a bimodal magmatism. Thus, the conclusion of this paper and previous investigations strongly support the idea that the protoliths of all UHP rocks from the CCSD-MH are the products of Neoproterozoic bimodal magmatism taken place in rifting tectonic zones along the northern margin of the Yangtze Block. Contemporaneous magmatism that occurred in the period from 830 to 740 Ma in other marginal zones of the Yangtze Block is interpreted as a response to breakup of the supercontinent Rodinia (Li et al., 2003; Zheng et al., 2003a,b, 2006; Xu et al., 2006; Liu et al., 2008). The bimodal magmatism along volcanic rift margins may have transported both heat and materials from the mantle into the crust. As a result, it brought about reworking of both the meteoric-hydrothermally altered juvenile crust and the Paleoproterozoic ancient crust (Zheng et al., 2006). 6.2. Continent deep subduction and exhumation As mentioned above, many typical UHP metamorphic rocks, including eclogites and garnet–peridotite, have been collected from

the CCSD-MH. Although the gneisses usually have a matrix mineral assemblage of amphibolite-facies metamorphism, typical symplectitic pseudomorphs after UHP minerals can still be recognized which occur as thin layers in the eclogites (Fig. 3G and H). Moreover, the gneissic zircons commonly contain coesite along with other eclogite-facies mineral inclusions (Fig. 3C). These observations indicate that the gneisses and schists together with the eclogites have been subjected to early UHP metamorphism before amphibolite-facies retrogression, and that their UHP counterparts are jadeite-coesitite and garnet– phengite-coesite schist. P–T estimated are conditions of >816 °C and >4.4 GPa for various types of eclogites from different depths of the CCSD-MH, and ∼ 930 °C and ∼ 5.0 GPa for garnet–peridotite from unit 1 (Zhang et al., 2006a), suggesting that both types of UHP rocks were subducted together into the mantle depth of about 165 km. Similar UHP metamorphic conditions for the eclogites and peridotites from the surface outcrops in the Donghai area have been estimated by previous workers (e.g. Zhang et al., 1995; Yang and Jahn, 2000; Mattinson et al., 2004; Zhang et al., 2005b,c, 2007). In addition, the investigations from shallow holes and surface outcrops have also shown that the eclogites and their country-rock gneisses from the southern Sulu area underwent contemporaneous UHP metamorphism (Zhang et al., 1995, 2000; Liu et al., 2002; Zhang et al., 2003). Therefore, it is concluded that a huge continental rock slab (>50 km long × 100 km wide × 5 km depth) was subducted to the mantle depths of more than 100 km, and then exhumed to the surface. SHRIMP U–Pb ages of metamorphic zircons from the eclogite, garnet–peridotite and gneiss of the Dabie-Sulu UHP belt generally vary between 191 and 266 Ma with weighted means ranging from ∼220 Ma to ∼245 Ma (Ames et al., 1993; Li et al., 1993; Rowley et al., 1997; Hacker et al.,1998; Maruyama et al.,1998; Rumble et al., 2002; Ayers et al., 2002; Jahn et al., 2003; Faure et al., 2003; Yang et al., 2003; Zheng et al., 2003a, b; Liu et al., 2004a,b, 2006; Cosca et al., 2005; Zhang et al., 2005b; Wan et al., 2005; Zhang et al., 2006c; Zhao et al., 2006). Multiple zircon growth during the Triassic metamorphism has been observed in UHP gneisses from the Dabieshan and Sulu belts. Low Th/U overgrowth zones and also whole zircon grains of quartz-feldspathic gneiss from the central Dabieshan yield concordant U–Pb ages of 220-238 Ma, which are formed during UHP metamorphism, whereas thin, euhedral rims yield concordant ages of 214–220 Ma due to retrograde metamorphism (Maruyama et al.,1998). Liu et al. (2004a) studied zircon domains which contain inclusions of the UHP index mineral coesite in the gneiss samples collected from the CCSD-MH and obtained SHRIMP U–Pb ages of 229 ± 4 Ma. Liu et al. (2006) have suggested that the two main zirconforming events in the Sulu-Dabie UHP belt provide chronological constraints on the prograde P–T path from the onset of HP–UHP metamorphism at 242.2 Ma to peak UHP conditions at 227 Ma. However, according to Zhao et al. (2006), the peak UHP metamorphism for the Sulu UHP belt occurred at 240–225 Ma, whereas the HP eclogite-facies recrystallization and amphibolite-facies retrogression occurred at 225– 215 Ma and 215–205 Ma, respectively. Consistent with previous investigations, the present work also shows that the metamorphic zircon U–Pb ages from the eclogite and gneiss of the CCSD-MH are variable, however, in a relatively narrow range of 214 to 228 Ma. Furthermore, this work clearly indicates that the type II zircon and the overgrowth rims of the type I zircon were formed during the UHP metamorphism because they contain coesite and other eclogitefacies mineral inclusions. Combining these current results and data from previous work, we suggest that the age of the peak UHP metamorphism for the UHP rocks from the CCSD-MH should be close to or younger than 228 Ma, whereas the lower limit of the UHP metamorphic age is close to or older than 214 Ma. Considering the weighted mean ages of all metamorphic zircons investigated (Fig. 10F), we propose 221.1 Ma to represent peak-UHP-metamorphic event, whereas the age of 216.7 Ma is related to an early UHP retrograde stage. According to Zhang et al. (2005d), the Donghai eclogites once experienced UHP retrogression at P–T conditions of ∼700 °C and 2.8 GPa during early exhumation. Thus, a

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maximum rate of vertical movement during early exhumation of the UHP rocks would be 17 mm/year, assuming the onset of peak UHP metamorphic pressure of 5.0 GPa (∼165 km depth), and a retrograde UHP metamorphic pressure of 2.8 GPa (∼92 km depth). This is quite similar to the initial exhumation rate of about 20 mm/year for the northern Sulu UHP metamorphic belt (Yoshida et al., 2004), and compatible with the rates derived from others UHP terranes like 31 mm/year for the Dabieshan UHP rocks (Wawrzenitz et al., 2006), 35 mm/year for the Kokchetav Massif, Kasachstan (Hermann, 2003), and 20 to 24 mm/year (Gebauer et al., 1997) and 34 mm/year (Rubatto and Hermann, 2001) for Dora Maira, but is still lower than the exhumation rate of 100 mm/year that have been reported from the Western Gneiss Region, Norway (Carswell et al., 2003). Based on available investigations (Ernst and Liou, 1999; Burov et al., 2000; Gerya and Stoeckhert, 2002; Wawrzenitz et al., 2006), the rapid exhumation of UHP rocks is explained by its large density contrasts to the surrounding mantle that give rise to high buoyancy forces, which in combination with the low viscosity of preserved less dense phases triggers detachment of the crustal rocks soon after their deep subduction and result in high rates for early exhumation from the upper mantle to the lower crust. 7. Conclusions (1) The core samples collected continuously from the CCSD-MH with a depth of 5148 m form a typical UHP rock profile of the southern Sulu orogen, in which eclogite, garnet–peridotite, garnet– pyroxenite and granitic gneiss have been subjected to in-situ UHP metamorphism during deep continental subduction. (2) The protoliths of the UHP rocks were mainly gabbro and granite with the same intrusive age at c. 789 Ma, representing a bimodal magmatism that took place during Neoproterozoic continental rifting, in response to breakup of the Rodinia supercontinent. This demonstrates that the old continental crust has been coherently subducted to the mantle depth, and then exhumed to the surface. (3) Two main metamorphic zircon-forming events suggest that the peak-UHP metamorphism occurred at 221 Ma, whereas the UHP/HP retrogression at 217 Ma. A maximum rate of vertical movement is estimated to be 17 mm/year during the early exhumation of the UHP rocks. Acknowledgments This research was supported by grants from a National 973 Project of the Ministry of Science and Technology of China (2003CB716501), the Natural Science Foundation of China (40399142 and 40472036). Thanks are due to Professors Zhiqin Xu, Jingsui Yang, Zhenmin Jin, and Yongsheng Liu for their assistance at various stages of this work. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.tecto2009.02.042. References Ames, L., Tilton, G.R., Zhou, G.Z., 1993. Timing of collision of the Sino-Korean and Yangtze cratons: U–Pb zircon dating of coesite-bearing eclogites. Geology 21, 339–342. Arnason, J.G., Bird, D.K., Bernstein, S., Rose, N.M., Manning, C.E., 1997. Petrology and geochemistry of the Kruuse Fjord gabbro complex, East Greenland. Geol. Mag. 134, 67–89. 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. Chem. Geol. 186, 315–331.

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