The Grenvillian Songshugou ophiolite in the Qinling Mountains, Central China: Implications for the tectonic evolution of the Qinling orogenic belt

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Journal of Asian Earth Sciences 32 (2008) 325–335 www.elsevier.com/locate/jaes

The Grenvillian Songshugou ophiolite in the Qinling Mountains, Central China: Implications for the tectonic evolution of the Qinling orogenic belt Yun-Peng Dong a,*, Mei-Fu Zhou b, Guo-Wei Zhang a, Ding-Wu Zhou a, Liang Liu a, Qi Zhang c a

The State Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, China b Department of Earth Sciences, University of Hong Kong, Hong Kong, China c Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China

Abstract The Qinling Mountains in Central China mark a gigantic composite orogenic belt with a complex tectonic evolution involving multiple phases of rifting and convergence. This belt separates the North China and South China Blocks and consists of the South and North Qinling terranes separated by the Shangdan suture. The suture is marked by the Grenvillian Songshugou ophiolite along the southern margin of the North Qinling terrane, which is key to understanding the Proterozoic tectonic evolution of the belt. The ophiolite consists of highly metamorphosed ultramafic and mafic rocks. Three groups of meta-basalts are present: group 1 rocks are LREE depleted and have a MORB compositional affinity. Their low Ta/Yb ratios (<0.1) are consistent with high degrees of partial melting of a depleted asthenospheric mantle. Rocks of group 2 have higher TiO2 (1.63–2.08 wt%) and Ta/Yb ratios (>0.12), and display slight enrichment of LREE, suggesting that the original magmas were derived from a depleted mantle source mixed with some enriched material. Samples from group 3 are enriched in LREE and other incompatible elements (Ti, Zr, Ta, Nb), suggesting derivation from an enriched mantle source, possibly a plume. All the basalts have high eNd(t) (+4.2 to +6.9), variable eSr(t) and high 207Pb/204Pb and 208Pb/204Pb ratios for given 206Pb/204Pb ratios. These characteristics are compatible with formation at a mid-ocean ridge system above an anomalous Dupal mantle region. The mafic rocks have a Sm–Nd whole-rock isochron age of 1030 ± 46 Ma. The Songshugou ophiolite was emplaced onto the southern margin of the North Qinling terrane, an active continental margin from the Meso-Proterozoic to Neo-Proterozoic.  2007 Elsevier Ltd. All rights reserved. Keywords: Ophiolite; Geochemistry; Meta-basalt; Proterozoic tectonics; Qinling orogenic belt; Central China

1. Introduction The Qinling orogenic belt, of possibly Grenvillian age, extends from east to west for nearly 2500 km across central China and marks the boundary between the North China and South China Blocks. These two blocks are separated by the Proterozoic Shangdan suture zone which may have significant implication for the assembly of the Rodinian super-continent (e.g., Moores, 1991). However, the exact *

Corresponding author. E-mail address: [email protected] (Y.-P. Dong).

1367-9120/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2007.11.010

timing and mechanism of convergence between these two blocks are not well constrained. Some authors have suggested an Early Paleozoic collision between the North China and South China Blocks (Ren et al., 1991; Kroner et al., 1993; Zhai et al., 1998). Others postulated left-lateral strike-slip faulting along the Shangdan suture at ca. 315 Ma and inferred a pre-Devonian collision between the two blocks (Mattauer et al., 1985; Xu et al., 1988). The geochemistry of fine-grained sediments in the Qinling Mountains was used to argue for a Silurian–Devonian collision (Gao et al., 1995), whereas a Late Triassic

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collision was also proposed (Sengor, 1985; Hsu et al., 1987; Wang et al., 1989), based on the formation of ultrahigh-pressure metamorphic rocks in the easternmost part of the belt at 230 Ma (e.g., Li et al., 1993; Ames et al., 1996). Paleomagnetic data favor a Late Triassic–Middle Jurassic amalgamation of the North China and South China Blocks (Zhao and Coe, 1987; Enkin et al., 1992). The Shangdan suture zone is marked by Grenvillian ophiolites, including the Songshugou ophiolite. Because it represents remnants of a former ocean basin, the age and character of the Songshugou ophiolite are crucial to understanding the Proterozoic tectonic framework of the Qinling orogenic belt and global correlation of Grenvillian belts. This paper reports major and trace element data and Sr, Nd and Pb isotope compositions of mafic rocks from the Songshugou ophiolite. Based on this new dataset, we discuss the petrogenesis of the mafic rocks and their tectonic significance, and attempt

to reconstruct the evolutionary history of this major orogenic belt. 2. Geological setting The Qinling orogenic belt extends from the Qinling Mountains in the west to the Dabie Mountains in the east. It lies between the North China and South China Blocks (Fig. 1), and is bounded on the north by the Lushan fault and on the south by the Mianlue–Bashan–Xiangguang fault (Zhang et al., 2000). The orogenic belt itself is divided into the North and South Qinling terranes by the Shangdan suture zone. The North Qinling terrane was thrust onto the southern margin of the North China Block along the Mesozoic–Cenozoic Lushan intra-continental fault. The Mianlue–Bashan–Xiangguang fault is also a thrust fault, along which the South Qinling terrane was emplaced onto the South China Block in the Late Paleozoic–Middle Triassic (Zhang et al., 1995).

Fig. 1. Simplified geological map of the Songshugou area in the Qinling Range, Central China (after Dong et al., 1996b).

Y.-P. Dong et al. / Journal of Asian Earth Sciences 32 (2008) 325–335

2.1. South Qinling terrane The South Qinling terrane consists of pre-Sinian basement overlain by Sinian and Phanerozoic sedimentary strata. The basement contains several Archean–Paleoproterozoic complexes (e.g., Xiaomoling, Douling, TongbaiDabie, Foping and Yudongzi), all of which contain Mesoto Neo-Proterozoic rift-type volcanic-sedimentary assemblages (Zhang et al., 1995). The sedimentary cover includes Sinian clastic and carbonate rocks, Cambrian–Ordovician limestones and Silurian shales. A few upper Paleozoic– lower Triassic clastic sedimentary rocks are also present in the northern part of the South Qinling terrane (Zhang et al., 2000). The South Qinling terrane is characterized by thinskin, south-vergent thrusts and folds showing imbricated thrust-fold systems. Two detachment surfaces are recognized; one between the Sinian and Cambrian and the other above the Lower Silurian shales (Xu et al., 1988; Zhang et al., 2000). All pre-Mesozoic rocks in the South Qinling terrane underwent low-greenschist facies metamorphism in the Triassic. Early Precambrian metamorphic complexes may have undergone amphibolite facies metamorphism. 2.2. North Qinling terrane The North Qinling terrane consists mainly of Precambrian rocks overlain by sparse Phanerozoic cover rocks. It is characterized by intense Paleozoic-Mesozoic magmatism and thick-skin structures with dominantly north-vergent, imbricated thrusts and folds. The basement underwent amphibolite facies metamorphism at 1.0 Ga and greenschist facies metamorphism at 400 Ma (Liu et al., 1993; Zhang et al., 1994b). This terrane is further divided by the Luonan–Luanchuan fault into a northern and southern belts. The northern belt consists mainly of Archean–Paleoproterozoic basement complexes which are overlain by sedimentary sequences. The northern belt is traditionally thought to be part of the southern margin of the North China Block, which underwent intra-continental orogenic deformation in the Mesozoic–Cenozoic (Xu et al., 1988; Ren et al., 1991; Zhang et al., 1995). In contrast, the southern belt consists dominantly of Paleoproterozoic crystalline basement, and Mesoproterozoic meta-volcanic rocks and ophiolites. The basement is locally overlain by NeoproterozoicLower Paleozoic volcanic-sedimentary assemblages and, in some cases, by Carboniferous–Permian clastic rocks. From north to south the main rock units in this belt are the Kuanping Group, Erlangping Group, Qinling Group, Songshugou ophiolite and Danfeng Group, which are separated from each other by thrust faults or ductile shear zones. The Kuanping Group consists chiefly of greenschists, amphibolites, quartz-mica schists, gneisses and marbles. Protoliths of both the greenschists and amphibolites were

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tholeiitic basalts with N-MORB and T-MORB geochemical characteristics (Zhang and Zhang, 1995). Sm–Nd whole-rock isochron ages of these meta-basalts range from 0.94 to 1.2 Ga (Zhang et al., 1994b). Three tectonic slices make up the Erlangping Group, one of which consists of the Erlangping ophiolite, composed of massive basalts, pillow basalts, sheeted dikes, gabbros, sparse ultramafic rocks and a few of radiolarian cherts. Abundant radiolarians in the chert are Early to Middle Ordovician in age, and the basalt geochemistry suggests formation in a back-arc basin (Sun et al., 1996). The Qinling Group is composed of gneisses, amphibolites and marbles, whose protoliths were clastic rocks and limestones (You et al., 1991) with interlayers of continental tholeiitic lavas (Zhang et al., 1994b). U–Pb isotopic ages of zircon from gneisses range from 2172 to 2267 Ma, whereas the Sm–Nd whole-rock isochron age of the amphibolites (meta-basalts) is 1987 ± 49 Ma (Zhang et al., 1994b). The rocks underwent amphibolite facies metamorphism at 990 ± 0.4 Ma and greenschist facies metamorphism at 425 ± 48 Ma (Chen et al., 1991). Metamorphosed calc-alkaline and tholeiitic basalts of the Danfeng Group formed in an oceanic island-arc setting (Zhang et al., 1994a). A Sm–Nd mineral isochron age of noritic gabbros is 403 ± 17 Ma (Li et al., 1989), consistent with radiolarian ages ranging from Ordovician to Silurian (Cui et al., 1995). 3. Field relationships of the Songshugou ophiolite The Songshugou ophiolite is situated in eastern Shaanxi Province and western Henan Province, where it crops out along the Shangdan suture zone (Fig. 1). It forms a NW– SE trending block about 27 km long and 3 km wide. On the north, it is bounded by the Jieling ductile shear zone, along which the ophiolite has been thrust onto the Qinling Group. To the south, it is separated from the Qinling Group and Fushui gabbroic complex by the Xigou fault. Some lenticular bodies of high-pressure granulite have been recognized along its boundaries (Liu et al., 1996). The Songshugou ophiolite consists mainly of metamorphosed mafic and ultramafic rocks. The ultramafic rocks occur as several hundred knockers enclosed within mafic rocks. The largest knocker is about 18 km long and 2 km wide, and consists chiefly of serpentinized, granular dunite with lesser amounts of coarse-grained dunite and harzburgite. The fabric of the granular dunite indicates that it is an intensely deformed mylonite with relict olivine porphyroclasts (Dong et al., 1996a). Coarse-grained dunite, which makes up about 15% of the largest ultramafic slice, has a typical cumulate texture without plastic deformation (Dong et al., 1996b). Harzburgite occurs as both slightly deformed and undeformed patches, lenses and veins in the dunite mylonite. Detailed structural analyses show that the dunite mylonite has high-temperature deformational fabrics and dislocations, which were probably produced by shearing in a subduction–collision setting (Dong et al., 1996a).

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Mafic rocks in the Songshugou ophiolite are mainly amphibolite, garnet amphibolite and amphibole schists, all of which were metamorphosed at about 1.0 Ga (Zhang et al., 1995). Garnet amphibolites occur as discontinuous lenses, although they were previously considered to form a contact aureole around the biggest ultramafic slice (Huang, 1984). There are also high-pressure mafic granulites (retrograde metamorphosed eclogites) within the amphibolites and some felsic granulites in the regional metamorphic rocks near the Xigou fault. Accordingly, the formation of the garnet-amphibolites and high-pressure metamorphic rocks is considered to have been associated with oceanic subduction and emplacement of the Songshugou ophiolite (Liu et al., 1995). 4. Analytical methods Major oxides were analyzed by X-ray fluorescence (XRF) on fused glass discs at the Institute of Geology, Chinese Academy of Sciences, Beijing. Sc, Co, Hf, Ta, Th, U and rare earth elements (REEs) were analyzed by INAA at the Institute of High Energy Nuclear Physics, Chinese Academy of Sciences, Beijing, whereas the other trace elements were analyzed by inductively coupled plasma mass spectrometry (ICP-MS) at the Chinese Academy of Geological Sciences, Beijing. The analytical precision is generally better than 1% for the major oxides and better than 5% for the trace elements. The isotopic compositions of selected samples were analyzed on a MAT-261 solid-source mass spectrometer in the Chinese Academy of Geological Sciences, Beijing. The analytical procedures are the same as described by Zhang et al. (1994b). Results for international reference samples analyzed in this laboratory are: 143Nd/144Nd = 0.511125 ± 8(2d) for J.M. Nd2O3 and 143Nd/144Nd = 0.512643 ± 12(2d) for BCR-1. All measured 87Sr/86Sr and 143Nd/144Nd ratios were normalized to 88Sr/86Sr = 8.37521 and 146 Nd/144Nd = 0.721900, respectively. Uncertainties for both Rb/Sr and Sm/Nd ratios are better than 0.1%. The decay constants of 87Rb and 147Sm are 1.42 · 10 11 y 1 and 6.54 · 10 12 y 1, respectively. 5. Analytical results 5.1. Major and trace elements Amphibolites and garnet-amphibolites from the Songshugou ophiolite have mafic composition with MgO ranging from 4.82 to 8.63 wt%, TiO2 from 0.91 to 2.54 wt% and Na2O from 0.92 to 2.87 wt% (Table 1). In the SiO2 vs. Na2O + K2O diagram, most samples plot in the field of subalkaline basalt (Fig. 2). The mafic rocks can be divided into three groups on the basis of their chondrite-normalized REE patterns (Fig. 3). Samples of group 1 have relatively flat, LREE-depleted patterns (Fig. 3A), similar to normal mid-ocean ridge

basalt (N-MORB). Their (La/Yb)N ratios range from 0.63 to 0.80 with chondrite-normalized values of (Yb)N ranging from 14 to 20. Rocks of group 2 have somewhat higher total REE and show slight LREE enrichment (Fig. 3B) with (La/Yb)N ratios ranging from 1.4 to 2.3. This pattern is characteristic of enriched mid-oceanic ridge basalt (E-MORB). Samples of group 3 are strongly enriched in LREE (Fig. 3C) with (La/Yb)N ratios ranging from 3.5 to 4.2, similar to ocean island basalts (OIB). However, other trace element features of these three groups of rocks are similar. All of the analysed samples have relatively high Nb and Ta contents (Nb > 5 ppm, Ta > 0.3 ppm). The Nb/La ratio of most samples is roughly 1, whereas the Ti/V ratios range from 15 to 30. MORB-normalized trace element variation diagrams (Pearce, 1983) for the Songshugou meta-basalts show that group 2 samples have flat patterns similar to MORB, especially for the immobile elements (Ta to Yb) (Fig. 4). In the plot of Th/Yb vs Ta/Yb (Fig. 5), group 1 samples again plot in the N-MORB field, whereas group 2 and 3 samples have relatively high Th and Ta contents and plot in the transitional area. When plotted on discrimination diagrams of Hf/3-Th–Ta and 2Nb–Zr/4-Y, group 1 samples again fall in the N-MORB field and those of group 2 and 3 in the E-MORB field (Fig. 6).

5.2. Sr, Nd and Pb isotopic compositions The three groups of rocks all have similar isotopic compositions (Table 2). The eNd(t) values range from +4.2 to +6.9, suggesting a depleted mantle source with some contamination by enriched mantle (Fig. 7). In addition, these samples have relatively high and variable eSr(t) ranging from +11 to +74, similar to ophiolitic samples affected by seawater alteration (Wilson, 1989). The Songshugou meta-basalts have relatively high 207 Pb/204Pb and 208Pb/204Pb ratios, ranging from 15.554 to 15.692 and 37.591 to 38.378, respectively. Their 206 Pb/204Pb ratios range from 18.07 to 18.66. Their calculated initial Pb-isotope ratios are (206Pb/204Pb)i = 16.706– 17.302, (207Pb/204Pb)i = 15.456–15.593 and (208Pb/204Pb)i = 36.112–36.900. All the samples plot above the northern hemisphere reference line (NHRL) in the Pb–Pb isotope diagram (Fig. 8) and show characteristics of an enriched mantle source. Three amphibolite samples (93Lt-1, Lt-014 and Lt-020) and two garnet-amphibolite samples (93Xs-7 and 93Xs-03) with large Sm/Nd fractionation were selected for dating. A reliable Sm–Nd whole rock isochron was obtained with t = 1030 ± 46 Ma, initial 143Nd/144Nd = 0.51161 ± 5(2d) and eNd(t) = +5.7 ± 0.2 (Fig. 9). Depleted mantle model ages (TDM) range from 1271 to 1440 Ma. The linear correlation of analyses in the isochron diagram, small MSWD (1.4) and a narrow range of eNd(t) values (+5.6 to +5.9) suggest that the initial Nd isotope compositions were homogeneous. On the other hand, the large range of

Table 1 Major oxide (wt%) and trace element (ppm) contents of the meta-basaltic rocks from the Songshugou ophiolite Sample

Group 1 LT-016

Total

99.51

Trace elements La 4.83 Ce 13.8 Nd 10.9 Sm 3.81 Eu 1.67 Gd 5.1 Tb 0.955 Ho 1.37 Tm 0.64 Yb 3.98 Lu 0.64 Sc 54.5 Co 65.5 Ta 0.469 Th 2.66 U 0.583 V 375 Cr 248 Ni 65.8 Rb 3.45 Sr 168 Nb 5.17 Hf 3.17 Zr 81.7 Y 30.8 Ba 80.2

T-03

Group 3

93Xs-03

LT-018

XS-011

43.89 0.91 13.55 3.37 12.41 0.35 14.57 7.44 0.92 0.28 0.02 2.35

49.97 1.63 15.74 3.38 8.91 0.21 11.35 5.67 2.05 0.31 0.17 1.13

46.42 1.82 14.7 5.81 7.23 0.22 11.66 6.77 2.32 0.35 0.15 2.22

100.2

100.06

99.8

4.16 9.42 6.55 2.39 0.937 3.59 0.686 1.1 0.486 3.15 0.491 46.2 42.1 0.332 0.266 0.293 363 293 80.6 4.5 139 5.1 1.78 65 33 35.7

4.55 7.25 7.37 2.26 0.813 3.26 0.677 1.29 0.652 4.37 0.724 58.4 74.5 0.318 0.393 0.421 371 347 62.3 9.1 358 5.2 2.63 92.3 33.7 201

8.55 21.4 15.6 4.43 1.77 5.35 1.01 1.47 0.64 3.83 0.597 43.7 59.6 0.583 3.23 0.615 265 339 79.5 4.54 145 9.45 3.61 112 33.2 59.2

47.97 1.21 14.5 2.64 9.13 0.21 11.87 8.38 2.74 0.36 0.14 1.00

ZT-3

93SJ-02

93LT-1

93Xs-2

Xs-027

D-0110

D-0120

T-01

LT-014

49.34 1.8 14.21 5.38 7.01 0.23 10.47 7.03 2.15 0.46 0.17 1.52

48.56 2.08 13.89 3.7 9.66 0.23 10.23 6.38 2.86 0.22 0.21 1.69

48.81 1.87 13.29 3.3 10.53 0.23 10.82 6.31 1.73 0.61 0.21 1.77

48.28 2.03 13.68 4.93 8.57 0.21 11.37 6.72 1.93 0.36 0.21 1.86

48.88 1.93 15.08 1.28 11.85 0.22 10.54 4.82 2.87 0.36 0.21 2.07

45.41 1.83 13.69 3.18 12.89 0.27 12.95 7.06 1.66 0.09 0.16 0.47

45.21 2.03 12.78 2.23 13.17 0.27 13.95 7.48 1.5 0.46 0.18 0.48

40.4 2.54 14.08 3.87 13.19 0.39 11.85 8.2 2.12 0.76 0.33 1.93

41.42 2.27 14.93 3.63 12.16 0.37 12.41 8.28 1.62 0.44 0.25 1.68

99.7

99.77

99.71

99.53

100.05

100.09

99.66

99.74

99.71

99.6

8 19.4 14 4.19 1.41 4.92 0.882 1.17 0.486 3.08 0.456 45.5 64.9 0.625 1.08 0.482 331 112 41.2 6.38 163 9.62 3.64 112 28.5 68.3

7.99 20.1 13.5 3.83 1.45 5.11 0.924 1.24 0.516 2.92 0.401 41.8 58.8 0.492 1.55 0.485 318 130 46.2 7.7 175 8.4 3.33 106 24.7 109

7.91 17.4 13.5 4.21 1.61 5.87 1.01 1.45 0.58 3.33 0.413 41.9 73.5 0.927 1.42 0.56 394 87.3 39.3 4.1 256 13.9 2.57 121 26.5 109

10.5 16.4 15.6 4.63 1.63 5.98 1.13 1.69 0.665 3.71 0.517 43 82.6 0.706 0.976 0.324 341 63.6 31 11.9 139 16 3.23 122 38 117

11.8 23 15.9 4.1 2 5 0.56 2.3 0.56 3.1 0.67 43.1 58.8 0.4 0.94 0.53 333 90.8 58.1 4.6 208 10.4 3.2 111 29.4 111

9.51 22.2 15.4 4.52 1.62 5.23 0.973 1.44 0.603 3.64 0.554 42 57.1 0.872 0.595 0.737 289 239 69.1 4.34 320 14.2 3.27 100 34.2 127

12.5 26.8 15.7 4.61 1.62 6.22 1.19 1.79 0.734 4.13 0.523 50.3 63.4 1.02 3.01 1.4 472 114 73.8 2.1 82 20.4 3.34 109 40 54

7.76 21.1 14.8 4.87 1.75 6.22 1.09 1.47 0.558 3.19 0.486 49.5 55.5 0.507 1.13 1.04 518 141 79.1 37.6 162 10 2.44 72.4 38 55.7

32 64 33.5 7.77 2.39 7.88 1.37 1.87 0.763 4.63 0.664 51.1 59.1 1.63 6.31 0.866 385 452 166 6.7 174 29.8 5.15 212 52.9 245

21.2 49.3 28.8 6.98 2.48 7.54 1.27 1.47 0.72 3.65 0.557 53.7 76.4 1.4 4.67 0.94 353 393 115 7.91 186 20.2 4.48 175 45.4 182

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Major oxides SiO2 44.01 1.64 TiO2 Al2O3 15.46 Fe2O3 2.87 FeO 10.36 MnO 0.24 CaO 12.58 MgO 8.63 Na2O 1.68 K2O 0.32 P2O5 0.16 LOI 1.56

Group 2

LOI, loss on ignition. 329

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Fig. 2. Plots of SiO2 vs. Na2O + K2O for mafic rocks from the Songshugou ophiolite.

Fig. 4. MORB-normalized trace element patterns for metabasalts from the Songshugou ophiolite. Normalization values are from Pearce (1983).

Fig. 3. Chondrite-normalized REE patterns for mafic rocks from the Songshugou ophiolite. 147

Sm/144Nd ratios (0.1539–0.2380) suggests that the isochron age (1030 ± 46 Ma) is reliable. 6. Discussion 6.1. Timing of formation of the Songshugou ophiolite

Fig. 5. Th/Yb versus Ta/Yb diagram for metabasalts from the Songshugou ophiolite. Reference fields are from Pearce (1983).

The TDM is generally interpreted as giving the age of mantle magmatism, whereas Sm–Nd whole-rock isochron age represents the time of crystallization. Assuming that the protoliths of the Songshugou amphibolites were derived directly from a depleted mantle source with initial

eNd = +10, and that the Sm/Nd ratio has not changed since eruption, the TDM should be approximately equal to, or a little greater than, the whole-rock isochron age. The TDM ages of the Songshugou ophiolite samples (1271–

Y.-P. Dong et al. / Journal of Asian Earth Sciences 32 (2008) 325–335

2Nb

Hf/3

Group I

331

Group II Within-plate alkali basalt

N-MORB

Group III

IAT E-MORB CAB

Within-plate tholeiite

E-MORB WPB

Volcanic-arc basalts

Th

Ta

N-MORB

Zr/4

Y

Fig. 6. Discrimination diagrams for metabasalts from the Songshugou ophiolite. A. Hf/3-Th–Ta (Wood, 1980); B. 2Nb–Zr/4-Y (Meschede, 1986).

Table 2 Nd, Sr and Pb isotopic compositions of the meta-basaltic rocks from the Songshugou ophiolite, Qinling Mountains, Central China Sample

Lt-014

Lt-016

Lt-018

93Lt-l

93Xs-7

93Xs-3

Lt-20

Qx-2

Sm (ppm) Nd (ppm) 147 Sm/144Nd 143 Nd/144Nd 2d eNd (0) eNd (t)

7.056 27.73 0.1539 0.512651 0.000010 +0.2 +5.7

3.788 11.10 0.2064 0.512917 0.000008 +5.4 +4.2

4.547 15.47 0.1778 0.512751 0.000029 +2.2 +4.6

4.493 15.89 0.1710 0.512751 0.000012 +2.2 +5.5

8.394 31.39 0.1618 0.512700 0.000019 +1.2 +5.7

2.745 9.415 0.1764 0.512797 0.000010 +3.1 +5.7

1.411 3.586 0.2380 0.513219 0.000019 +11.3 +5.9

19.95 83.23 0.1450 0.512660

Rb (ppm) Sr (ppm) 87 Rb/86Sr 87 Sr/86Sr 2d (87Sr/86Sr)i eSr(t)

8.829 166.91 0.1528 0.70985 0.000030 0.70862 +74

4.974 167.76 0.0856 0.70875 0.000050 0.70753 +58.5

3.065 140.94 0.0625 0.70511 0.000023 0.70421 +11.3

1.987 148.12 0.2142 0.71008 0.000026 0.70702 +51.3

0.671 115.71 0.0167 0.70490 0.000050 0.70466 +17.7

206

18.4359 38.3784 15.5985 17.077 15.500 36.900 16 63

18.6616 38.3583 15.6920 17.302 15.593 36.879 23 23

18.0652 37.5912 15.5541 16.706 15.456 36.112 15 20

18.3090 38.2945 15.6457 16.950 15.547 36.815 22 70

18.2678 38.1981 15.5929 16.909 15.494 36.719 17 65

Pb/204Pb Pb/204Pb 207 Pb/204Pb 206 ( Pb/204Pb)i (207Pb/204Pb)i (208Pb/204Pb)i D207Pb/204Pb D208Pb/204Pb 208

+0.4 +6.9 6.954 44.419 0.4533 0.71095 0.000002 0.70447 +15

Isotopic compositions are analyzed by MAT-262 in the Instiute of Geology, Chinese Academy of Sciences (Beijing). Analyses of QX-2 are from Li et al. (1991). l = 238U/204Pb = 8.8, t = 1030 Ma, (143Nd/144Nd)chur = 0.51264, (147Sm/144Nd)chur = 0.1967, (87Sr/86Sr)chur = 0.7047, (87Rb/86Sr)chur = 0.0847, kNd = 0.00000654 Ma 1, kSr = 0.0000142 Ma 1.

1440 Ma) are somewhat older than the Sm–Nd whole-rock isochron age (1030 ± 46 Ma). A Sm–Nd mineral isochron age of 983 Ma for the garnet-amphibolite was explained as a retrograde metamorphic age (Li et al., 1991). 40Ar/39Ar dating of metamorphic pyroxene megacrysts in ultramafic rocks from the Songshugou ophiolite gives a plateau age of 833.8 ± 2.3 Ma and isochron age of 848.2 ± 4.3 Ma, which were interpreted to be the emplacement age of the ophiolite after subduction and collision (Chen et al., 2002). It is possible that they represent exhumation and cooling age of the rocks. We conclude that the basaltic melts were derived from the depleted mantle and crystallized at about 1030 Ma, which we take as the formation age of the ophiolite. The

results thus suggest a Grenvillian orogeny in the Qinling Mountains. However, the significance of such orogeny in the reconstruction of the Rodinian super-continent is currently unknown.

6.2. Petrogenesis of the mafic rocks of the Songshugou ophiolite The amphibolites and garnet amphibolites from the Songshugou ophiolite are likely metamorphosed basalts representing part of a crustal sequence of the ophiolite. Some trace elements, such as REE and high field strength elements (HFSE) (Nb, Ta, Zr and Hf), and Nd and Pb isotopes are relatively immobile during metamorphism and

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Fig. 7. Plots of eNd(t) (143Nd/144Nd)i versus (87Sr/86Sr)i for mafic rocks from the Songshugou ophiolite. Reference fields are from Zindler and Hart (1986). DM, depleted mantle; BSE, bulk silicate Earth; EMI and EMII, enriched mantle; HIMU, mantle with high U/Pb ratio; PREMA, frequently observed prevalent mantle composition. The mantle array is defined by many oceanic basalts and the bulk earth value for 87Sr/86Sr can be obtained from this trend.

Fig. 8. (206Pb/204Pb)i versus (207Pb/204Pb)i and (208Pb/204Pb)i diagrams for metabasalts from the Songshugou ophiolite. NHRL represents the northern hemisphere reference line (Hart, 1988); the MORB field for the Atlantic and Indian Oceans are from Mertz et al. (1991).

can be used for examining the petrogenesis and tectonic setting of these rocks. The mafic rocks in Songshugou may include N-MORB (group 1), E-MORB (group 2), and OIB (group 3) according to their REE patterns and some trace elemental features (Figs. 4–6). Group 1 samples have low Ta/Yb ratios (<0.1), which suggests that the magma was produced by high degrees of partial melting of a depleted asthenospheric mantle. Group 2 samples have MORB-normalized trace element patterns enriched in Nb and Ta, about 2 times higher than MORB, compatible with an E-MORB compo-

Fig. 9. Sm–Nd isochron of the meta-basalts from the Songshugou ophiolite.

sition (Fig. 4). Group 3 samples with highly enriched LREE are similar to OIB. The isotope compositions provide useful information for elucidating petrogenetic processes in different tectonic settings. Meta-basalts from the Songshugou ophiolite are isotopically similar to Indian Ocean MORB with variable 143 Nd/144Nd and relatively high 87Sr/86Sr ratios and high 207 Pb/204Pb and 208Pb/204Pb ratios relative to 206Pb/204Pb ratios (Xu et al., 2002; Xu and Castillo, 2004). Thus, the ophiolitic basalts are believed to have been derived from a mantle source with a Dupal isotope anomaly produced by exchange between the asthenosphere and a mantle plume (cf., Dupre and Allegre, 1983; Hamelin et al., 1986; Mertz et al., 1991). The Dupal isotope anomaly is not restricted to Indian Ocean crust and mantle, but has also been detected in Archean rocks (Menzies and Kyle, 1990). Some Paleoproterozoic ophiolites have geochemical characteristics similar to Mesozoic–Cenozoic ophiolites (Scott et al., 1990; Vuollo et al., 1992), suggesting plate tectonic processes in the Proterozoic. Thus, available evidence indicates that the meta-basalts in Songshugou are lavas of N-MORB, E-MORB and OIB affinity. These basaltic lavas were derived from a complex mantle source containing both depleted and enriched components. 6.3. Implications for the tectonic evolution of the Northern Qinling orogenic belt The Songshugou ophiolite represents a fragment of oceanic lithosphere that once separated the North Qinling and South Qinling terranes. The Proterozoic oceanic lithosphere is also represented by a N-MORB type ophiolite in the Heihe area, the western part of the Shangdan suture zone. The Heihe ophiolite is dated at 963 ± 10 Ma (Zhang et al., 2000), similar to the Sm–Nd isotope isochron age of 1030 ± 46 Ma for the Songshugou ophiolite. To the north, the Kuanping Group is separated from the Songshugou ophiolite by the Qinling Group and is composed of several thrust slices of greenschist–amphibolite,

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quartz schist-gneiss and marble. These assemblages are believed to be metamorphic products of rocks formed in a back-arc basin environment (Zhang and Zhang, 1995). The protoliths of the quartz schist–gneisses in the Kuanping Group have relatively high SiO2, MgO, Na2O, CaO, and LREE contents but are depleted in Eu, features similar to flysch composed of graywacke, lithic sandstone and arkose (Liu et al., 1993). These clastic protoliths were derived from both the Qinling Group to the south and the Archean rocks of the North China Block to the north (Gao et al., 1990). Based on whole-rock Sm–Nd isochron ages of meta-basalts (986 ± 169 Ma in Shangxian; 1142 ± 18 Ma in Luonan; 1085 ± 37 Ma in Nanzhao) (Zhang et al., 1994b), we infer that the Kuanping backarc basin was formed by extension along the northern border of the North Qinling terrane at about 0.98–1.2 Ga. We believe that this back-arc basin was formed because of northward subduction of the Proterozoic oceanic lithosphere underneath the Qinling Group (Fig. 10). The P–T–t path of the Qinling Group is consistent with an island-arc setting in the North Qinling terrane (Chen et al., 1991; Liu et al., 1993; Zhang et al., 2000), whereas the P–T–t path of the Kuanping Group is interpreted to reflect metamorphism in a back-arc basin during a subduction–collision–uplift process (Liu et al., 1993). This subduction and collision event is consistent with the tectonothermal history of the North Qinling terrane. For example, a variety of collision-related granites, formed between 1.2 and 0.8 Ga, have been identified in the North Qinling terrane. The Laoyu gneissic granite in the Huxian area has ages of 1088 ± 41 Ma (Sm–Nd isochron age) and 883 ± 114 Ma (Rb–Sr isochron age) (Yang et al., 1992). The Shangnan granite in the Shangnan area has a zircon 207 Pb/206Pb age of 889 ± 22 Ma (Pei, 1997). The Dehe granite in the Xixia area has ages of 1156 Ma (Sm–Nd isochron age) and 794 Ma (Rb–Sr isochron age) (You et al.,

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1991). The Niujiaoshan granite in west Henan has an age of 959 ± 4 Ma (zircon U–Pb age) (Wang et al., 1998). Detailed studies have also revealed a phase of upper greenschist to amphibolite facies metamorphism in the Qinling Group and Kuanping Group in the North Qinling terrane at 1000 Ma (Chen et al., 1991, You et al., 1991, Liu et al., 1993, Zhang et al., 1994b). The metamorphic ages of the Paleo-Proterozoic Qinling Group range from 1.2 to 0.9 Ga. For example, Zhang et al. (1994b) reported a Rb–Sr isochron age of 996 ± 76 Ma and a zircon 207 Pb/206Pb age of 891 ± 7 Ma for gneisses from the Shewei area, Xixia County, and a Sm–Nd isochron age of 1169 ± 258 Ma for gneisses and a Rb–Sr isochron age of 1111 ± 189 Ma for amphibolites from the Yongyu area, Danfeng County. Chen et al. (1991) proposed metamorphic ages of 1111 ± 189 Ma (Rb–Sr isochron age) and 850 ± 6 Ma (zircon 207Pb/206Pb age) for gneisses from the Neixiang area. High-pressure (HP) metamorphic rocks, including mafic and felsic granulites, are spatially associated with the Songshugou ophiolite (Fig. 1). The high-pressure mafic granulites are part of the Songshugou ophiolite (Chen et al., 1993; Liu et al., 1995). Both the mafic and felsic HP granulites in the Xigou shear zone are considered to represent retrograded eclogites (Liu et al., 1995) and it is inferred that the ecologite facies metamorphism was associated with subduction along the southern margin of the North Qinling terrane (Zhang et al., 2000). Exhumation of the eclogites and their retrograde metamorphism may have been associated with the collision and obduction of the ophiolite. Based on the isotope compositions of garnet and hornblende in a garnet-amphibolite, Li et al. (1991) reported a Sm–Nd mineral isochron age of 983 Ma, which is considered to be a retrograde metamorphic age roughly equal to the age of the emplacement of the Songshugou ophiolite. Given that the age of the basalt protolith is about

Fig. 10. Tectonic evolution of the Qinling orogenic belt in the Neoproterozoic. SCB, South China block; NQT, North Qinling terrane; NCB, North China block; Qinling Gr., Qinling Group; Kuanping Oph & Gr., Kuanping ophiolite and Group.

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1030 ± 46 Ma, it is reasonable to infer that the HP metamorphism and collision took place at around 983 Ma. All of the available evidence indicates that there was subduction and collision along the southern margin of the North Qinling terrane in the Proterozoic. The emplacement of the ophiolite may have occurred as a result of collision between the South Qinling and North Qinling terranes (Fig. 10). Although Li et al. (1996) argued that the South China Block was separated from the North China Block by the Australia Block in the Neoproterozoic, the present study does not favour such a reconstruction. We suggest that amalgamation between the North China and South China Blocks occurred in the Neoproterozoic. Zhang et al. (1998) suggested that the North Qinling terrane was part of the Yangtze geochemical province and that its collision with the Yangtze Block occurred in the late Mesoproterozoic. Available data confirm that subduction and collision occurred along the southern margin of the North Qinling terrane in late Middle-Proterozoic (Fig. 10). 7. Conclusions The Songshugou ophiolite has a formation age of 1030 Ma and represents a fragment of Proterozoic oceanic lithosphere between the North China and South China Blocks, marking a Grenvillian orogeny in the Qinling Mountains. The protoliths of amphibolite and garnetamphibolite associated with the Songshugou ophiolite were basalts derived from depleted asthenosphere. The Kuanping Group represents a back-arc basin assemblage that developed in the Qinling Group at 0.98 and 1.2 Ga, suggesting that the North Qinling terrane was an active continental margin at this time. Subduction and collision between the North China and South China Blocks produced high-pressure metamorphic rocks at around 983 Ma. Acknowledgments The authors wish to thank Prof. Paul T. Robinson for his many constructive suggestions and Prof. Pat Castillo and Prof. John Shervais for thorough reviews that helped to improve this paper. This research was supported by Grants from the National Natural Science Foundation of China (Grants No. 40472115, 40234041 and 40003003). References Ames, L., Zhou, G., Xiong, B., 1996. Geochronology and isotopic character of ultrahigh pressure metamorphism with implication for collision of the Sino-Korean and Yangtse cratons, central China. Tectonics 15, 472–489. Chen, D.L., Liu, L., Zhou, D.W., Luo, J.H., Sang, H.Q., 2002. Genesis and 40Ar–39Ar dating of clinopyroxene megacrysts in ultramafic terrain from Songshugou, east Qinling Mountain and its geological implication. Acta Petrologica Sinica 18, 355–362 (in Chinese with English Abstract).

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