Geochemistry of the E-MORB type ophiolite and related

Science in China Ser D: Earth Sciences

© 2007 SCIENCE IN CHINA PRESS

Springer-Verlag

www.scichina.com www.springerlink.com Sci China Ser D-Earth Sci | Nov. 2007 | vol. 50 | Supp. II |234-245

Geochemistry of the E-MORB type ophiolite and related volcanic rocks from the Wushan area, West Qinling

DONG YunPeng†, ZHANG GuoWei, YANG Zhao, ZHAO Xia, MA HaiYong & YAO AnPing

State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, China

The mafic-ultramafic assemblages, which thrustthrust into the Wushan-Tangzang boundary fault as some blocks and outcropped in the Yuanyangzhen, Lijiahe, Lubangou and Gaojiahe area, consist mainly of meta-peridotites, gabbros and basalts. The meta-peridotites are characterized by high SiO2 and MgO contents, low ΣREE, as well as their chondrite-normalized rare earth element patterns show some similarities to that of middle oceanic meta-peridotite. The basalts from the Yuanyangzhen, Lijiahe and Lubangou area are characterized by relatively high TiO2 content, low Al2O3 content and Na2O>>K2O. Above all, it is the slight enrichment or flat REE distribution patterns and the unfractionated in HFS elements in the primitive-normalized trace elements distribution patterns that indicate these basalts are similar to that of the typical E-MORB. In comparison, the basalts from the Gaojiahe section are featured by depletion in Nb and Ta contents and enrichment in Th content which show that these were derived from an island-arc setting. From studies of the regional geology, petrology, geochemistry, geochronology and all above evidence, it can be suggested that the mafic-ultramafic rocks from the Wushan area are mainly dismembered E-MORB type ophiolite, which represent the fragments of the lithosphere of the Early-Paleozoic Qinling ocean. It is preferred that these rocks were formed in an initial mid-ocean ridge setting during the beginning stage of the oceanic basin spreading. This ophiolite together with the Gaojiahe island-arc basalts shows that there exists an ophiolitic mélange along the Wushan-Tangzang boundary fault, and marks the suture zone after the closure of the Qinling ocean in early Paleozoic.

E-MORB, geochemistry, Shangdan suture zone, west Qinling, tectonics

Being a composite collisional orogen between the North China Plate and South China Plate, the China central orogenic belt (CCOB) is a preferred area to investigate the formation and evolutionary history of China and Asia continents. Since it is located in the middle part of the CCOB, the west Qinling Mountains not only represent a collisional belt between North China and South China blocks, but also a key tectonic unite that separates the east Qinling and the Qilian Mountains from the Kunlun Mountains. In addition, it is also a link to connect the west plateau with the low topography to the east. Therefore, the west Qinling is a key to understanding the China continental dynamics. After several decades of studies, there still are some debates about the geology. Above all, the connected relationship between the Qin-

ling and the Qilian Mountains is a key issue, especially the tectonic framework and the evolutionary history in Early Paleozoic. More and more studies suggest that the Qinling Orogen is a continental orogen formed by the collision between the North China and the South China block[1―4]. The studies in the east Qinling show that the Shangdan zone is the main suture between the North China block and the South China block, which is marked by amounts of ophiolite and island arc volcanic rocks along this boundary fault[5―8]. However, there still is

Received January 10 2007; accepted May 10, 2007 doi: 10.1007/s11430-007-6004-3 †Corresponding author (email: [email protected]) Supported by the National Natural Science Foundation of China (Grant Nos. 40472115 and 40234041)

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DONG YunPeng et al. Sci China Ser D-Earth Sci | Nov. 2007 | vol. 50 | Supp. II | 234-245 235

some indetermination about the distribution and composition of this suture zone to the west Qinling area. This uncertainty made it difficult to get the truth of the tectonic evolution and its geodynamics, and understand the tectonic and evolutionary relationship between the Qinling and Qilian Orogen. Moreover, in west Qinling, there remains not only some information of the subduction, collisional regime and tectonics between the North China and the South China block from Neoproterozoic to Paleozoic, but also amounts of records about the influences and overprints of the northern branch of Paleotethys. Above all, located on the northeast side of Tibet Plateau, the West Qinling was impacted by the uplifting of Tibet Plateau in Cenozoic. In terms of all above evidences, it seems important to study the tectonic framework and evolutionary history about the West Qinling belt.

Based on regional geological investigations, in this article we report new geochemical data for the metamorphosed mafic-ultramafic rocks outcropped in the Wushan area and use these data to constrain their genesis and tectonic setting. Thenew data also provide new constraints on geodynamic models for the tectonic evolution of the Qinling Orogenic Belt in Early Paleozoic.

1 Geological background

The West Qinling is located in the connectional and transformational area among the Qinling Mountains, the Qilian Mountains and the Kunlun Mountains, and has a complicated composition and deformation (Figure 1). Being formed by the completely convergent between the Ordos Block, the Alxa Block, the Qaidam Block and the Qinling Block, it is a key area to study the transformational relationship between the Qinling Mountains and the Qilian Mountains, as well as to reconstruct the tectonic framework, evolutionary history and geodynamics of the Qinling Ocean and the Qilian Ocean during Paleozoic. Recently, many synthetic researches have approved that the dominant tectonic boundary of the West Qinling Belt is the Wushan-Tangzang Belt excepting the Mesozoic -Cenozoic overprinted structures, which can connect with the Shangdan Suture Zone in East Qinling. The Wushan-Tangzang Belt consists mainly of many tectonic-stratigraphic slices and blocks with different compositions, genesis and ages, which are thrustthrust and separated by east-west trending faults.

The most important blocks are the ophiolite and related island arc volcanic rocks outcropped along the Tangzang, Guanzizhen and Wushan area. According to the synthetic studies of geology, geochemistry and chronology, it is shown that the Wushan-Tangzang Belt represents the west part of the Paleozoic collisional suture zone which socalled as the Shangdan Suture to the East Qinling. It is the Wushan-Tangzang Belt that separates the northern unite from the southern unite with different lithostratigraphic formations. On the north side, there successively outcrops the Qinling Group, the Huluhe Group and the Longshan Group from south to north. The Qinling Group consists mainly of gneisses, amphibolites and a few of marbles. The Huluhe Group is mainly composed by epimetamorphic volcanic and clastic rocks, which similar to that of the Erlangping Group in the East Qinling. The Longshan Group mainly includes some amphibolites (meta-basalt) and gneisses (meta-clastic rocks and meta-acidite), which is the same composition as the Kuanping Group to the East Qinling area. In comparison, it is the Dacaotan Group and the Shujiaba Group that outcrop on the south side of the Wushan- Tangzang Belt from north to south. The Shujiaba Group consists mainly of clastic sedimentary rocks and minor of limestone with abundant of marine fossils, such as corals and brachiopods in Middle Devonian. With the typical sedimentary association, it is suggested that they were deposited in a neritic marine environment within a typical passive continental margin. The Upper Devonian Dacaotan Group is mainly composed of diverse metamorphic greenschists, chloritequartz schists, phyllites, and some unmetamorphosed sandstones, pelitic siltstones and apogrits. On the north side, the Dacaotan Group is thrustthrustby the Wushan- Tangzang ophiolitic mélange, whileas it covers on the different layers of the Shujiaba Group by overlapped or accord unconformity on the side.

2 Geology and analytical method

The Wushan ophiolite is composed mainly of meta- peridotites、gabbros and basalts, and distributed along the Yuanyangzhen section, the Lijiahe section and the Lubangou section. They were imposed by greenschist- facies metamorphism. The ophiolite in the Yuanyangzhen section consists of each unite of a typical ophiolite, such as meta-peridotite, gabbros and basalts, whereas there only exists meta-gabbros and meta-basalts in the

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236 DONG YunPeng et al. Sci China Ser D-Earth Sci | Nov. 2007 | vol. 50 | Supp. II | 234-245

Figure 1 Simplified geological map of the Wushan-Tangzang region. 1, Mesozoic and Cenozoic; 2, Devonian; 3, Huluhe Group/Eelangping Group; 4, ophiolitic mélange and ultramafic rocks; 5, Longshan Group/Kuanping Group; 6, Qinling Group; 7, granite; 8, diorite; 9, fault and hypothetical fault; 10, section position. Loubangou section. Although there only outcrops the meta-peridotites and basalts of the ophiolite along the Lijiahe section, it shows a continuous section with different tectonic blocks. Most of these blocks thrust northward into the chlorite-quartzschists, micaquartzs- chists and marbles of the Liziyuan Group (Figure 2).

The meta-peridotites show blackish-green color and imposed by serpentinization. The meta-gabbros with perfect blastogabbroic texture are composed mainly of plagioclase (35%―40%), pyroxenes (20%―25%), hornblendes (20%―25%) and chlorites (5%―10%), and minor micas and epidotes. The meta-basalts consist mainly of chlorites (25%―35%), hornblendes (10%―

15%), plagioclases (35%―40%) and some other accessory minerals.

The field mapping shows that there not only outcrop

the mafic-ultramafic rocks in the Yuanyangzhen area, Lijiahe area and Lubangou area but also exist some meta-basalts in the Gaojihe area, 20 km far to the west. Although we emphasize the research about the mafic- ultramafic rocks from Lijiahe, Yuanyangzhen and Lubangou sections, in order to compare genesis of the mafic rocks from different area, we also report the geochemical data of the basalts from Gaojiahe area.

27 representative samples with block structure and light alteration were selected for chemical analysis, including 2 samples of meta-peridotite from the Lijiahe section, and 25 samples of basalts which are mainly from the Lijiahe section (15 samples), the Yuanyangzhen section (6 samples), the Lubangou section (2 samples) and the Gaojiahe area (2 samples). The major elements were analyzed by XRF method using RIX-2100, whereas

Figure 2 The Lijiahe section of the Wushan ophiolite mélange. 1, Quaternary; 2, quartzschist; 3, marble; 4, amphibolite; 5, basalt; 6, meta-peridotite; 7, fault breccia; 8, ductile shear zone; 9, fault.



the rare earth and trace elements were determined using ICP-MS (ELAN6100-DRC) in the State Key Laboratory of Continental Dynamics, Northwest University. The data of trace elements were corrected by comparing with standard samples of BHVO-1, AGV-1 and G-2. The precision during the course of the analysis is better than 5%. The data are listed in Table 1.

3 Geochemistry of the major elements

The mantle unit of the Wushan ophiolite is composed mainly by harzburgite and dunite, and both of them experienced intensive serpentinization. The MgO content of two representative samples is 38.34% and 38.40%, whereas the SiO2 content is 40.40% and 40.75%. In view of the high LOI content (11.46% and 11.56%) due to the serpentinization, we conversed the major oxides into dry system which shows obvious high SiO2 content (45.70% and 45.94%) and MgO content (43.29% and 43.37%). Additionally, they are characterized by low TiO2 (0.03% and 0.05%), Al2O3 (1.32% and 1.88%) and CaO (0.05% and 0.14%) contents. All the above characteristics are similar to the major oxides composition of the harzburgite from the Troodos ophiolite and Dingqing ophiolite in the Hengduan Mountains, and also similar to that of the average values of the meta-peridotite from all over the world[9]. In addition, the meta-peridotite is featured by low ΣREE content (2.20―3.08) and the U- shaped Chondrite-normalized REE distribution patterns.

In general, the meta-basalts of the Wushan ophiolite are characterized by relatively high SiO2 and ΣFe2O3

contents, low Al2O3, MgO and P2O5 contents, as well as Na2O>>K2O. Detailed comparison shows that there are some geochemical differences between different sections. The TiO2 contents of the Lijiahe meta-basalts range from 1.16% to 1.92%, while the SiO2, Al2O3 and ΣFe2O3 contents are varied within 47.92%―51.57%, 12.51%―

14.29% and 13.00%―14.87%, respectively. Meanwhile, the MgO contents range from 3.35% to 7.73% showing negative correlation with SiO2 contents. In addition, the Na2O contents (1.37%―4.09%) are higher than the K2O contents (0.16%―0.32%). Compared with the meta- basalts from the Lijiahe section, the meta-basalts from the Yuanyangzhen area have similar compositions of SiO2

(47.43%―50.91%), Al2O3 (13.00%―14.24%), ΣFe2O3

(13.28%―14.92%) and MgO (4.76%―9.22%) contents, except obviously higher TiO2 contents (1.84%―2.79%)

than that of the samples from Lijiahe. In comparison, both samples from the Lubangou section are characterized by low TiO2 contents (1.52% and 1.61%), high MgO (6.79% and 11.36%) and CaO (12.15% and 12.90%) contents.

The basalts from the Gaojiahe area are featured by high SiO2 (49.07% and 53.06%) and Al2O3 (14.14% and 16.76%) contents, low TiO2 (0.93% and 1.36%) and CaO (7.39% and 10.77%) contents, which are very different from that of the samples from Yuanyangzhen, Lijiahe and Loubangou area.

The TAS diagram (Figure 3(a)) shows that the samples from Lijiahe, Yuanyangzhen and Lubangou are all classified as the sub-alkaline series, except the GH-3 sample from Gaojihe being trachyandesite with high Na2O and K2O contents. In view of the mobility of the K and Na elements in metamorphism, deformation and alteration, the Zr/TiO2-Nb/Y diagram (Figure 3(b)) is used to discriminate, which shows that all the samples are attributed to the sub-alkaline series.

4 REE and trace element geochemistry

The basalts from Lijihe have low ΣREE content ranging from 41.39×10−6 to 81.82×10−6, and show flat to slight enrichment in LREE in the chondrite-normalized REE distribution patterns (Figure 4(a)). The (La/Yb)N values range from 1.22 to 2.42, while the (La/Sm)N values vary from 0.96 to 1.50. The REE distribution lines paralleling to each other indicates that all samples were derived from the same magma source. However, it is distinguishable that most samples display slight enrichment in LREE, while the other three samples show slight depletion in LREE. Comparing the chemical compositions of major oxides, REE and trace elements of two kinds of samples, it is suggested that the most samples with slight enrichment in LREE are very similar to a typical E-MORB, and the three samples with slight depletion in LREE possess the transitional geochemistry between N-MORB and E-MORB.

Excepting the YY-14 sample, most samples from Yuanyangzhen have relatively higher ΣREE content (ranging from 63.95×10−6 to 97.64×10−6) than that of the samples from the Lijiahe area. The Chondrite-normalized REE distribution patterns (Figure 4(b)) show slight enrichment in LREE with fractionation between LREE and HREE. The (La/Yb)N and (La/Sm)N values range from 2.20 to 2.42, and from 1.06 to 1.41, respectively.



Table 1 Major (%) and trace elements (10−6) compositions of the ophiolite and related volcanic rocks from the Wushan area Meta-peridotite Basalts from Lijiahe

Sample W-3 W-5

WS-11 WS-12 WS-12R WS-13 WS-14 WS-15 WS-18 WS-22 WS-23 WS-25 WS-26 WS-27

SiO2 40.40 40.75 48.99 48.57 48.46 48.30 47.92 49.69 51.57 50.76 49.78 49.94 49.22 48.22TiO2 0.05 0.03 1.52 1.42 1.43 1.16 1.24 1.68 1.92 1.66 1.64 1.62 1.58 1.63 Al2O3 1.32 1.88 13.68 13.65 13.62 13.89 14.29 12.97 12.51 13.37 12.69 13.31 13.44 13.72ΣFe2O3 8.00 7.48 14.46 13.77 13.71 12.39 13.00 14.87 14.59 13.19 13.56 14.22 13.86 14.55MnO 0.06 0.05 0.20 0.18 0.18 0.17 0.17 0.19 0.18 0.15 0.18 0.16 0.18 0.19 MgO 38.34 38.40 6.92 7.44 7.43 7.73 7.63 6.81 3.75 6.30 6.05 6.54 6.18 6.47 CaO 0.14 0.05 10.49 10.48 10.45 11.02 11.69 8.36 11.87 9.48 9.91 8.51 9.89 10.33Na2O 0.06 0.05 2.69 2.85 2.85 2.77 2.21 3.56 1.37 3.74 4.09 4.03 3.58 2.76 K2O 0.02 0.01 0.24 0.19 0.19 0.22 0.21 0.21 0.16 0.26 0.32 0.23 0.25 0.26 P2O5 0.01 0.01 0.13 0.10 0.10 0.11 0.09 0.13 0.20 0.14 0.14 0.15 0.13 0.15 LOI 11.46 11.56 0.75 1.08 1.18 1.94 1.19 1.30 1.40 1.29 1.60 1.18 1.27 1.45 Total 99.86 100.27 100.07 99.73 99.60 99.70 99.61 99.77 99.52 100.34 99.96 99.89 99.58 99.73

Li 2.51 3.64 6.32 6.30 7.27 7.91 5.91 9.36 6.76 7.15 6.69 8.94 7.25 10.49Be 0.725 0.808 0.550 0.530 0.403 0.375 0.335 0.617 0.668 0.541 0.577 0.511 0.585 0.643Sc 14.6 16.8 49.4 48.3 49.4 48.2 49.4 45.8 41.7 43.4 42.2 46.5 46.0 56.9 V 69.8 80.5 409 403 403 354 363 411 381 358 354 380 366 419 Cr 3364 2472 127 132 128 233 195 71.8 26.9 175 152 72.8 74.8 372 Co 108 107 60.8 62.3 61.1 61.1 63.4 57.8 69.7 54.2 46.1 59.8 58.5 69.1 Ni 2618 2470 83.4 85.1 91.5 109 100 63.0 48.3 83.5 78.7 58.5 61.5 101 Cu 1.77 2.24 149 164 153 134 148 58.1 146 87 154 149 152 163 Zn 54.1 52.1 97.6 97.5 95.5 88.4 81.8 104 101 84.0 87.4 95.6 95.9 113 Ga 1.61 2.46 19.5 19.1 19.0 16.1 17.2 17.5 19.5 21.6 18.4 19.2 19.1 19.3 Ge 1.23 1.10 1.76 1.81 1.70 1.68 1.67 1.63 2.08 2.18 2.58 1.72 1.72 1.68 Rb 0.890 0.886 2.37 2.37 2.04 4.40 2.93 2.16 1.14 3.46 4.28 3.02 3.12 4.14 Sr 9.76 3.12 157 156 157 201 168 95.5 127 186 154 116 127 143 Y 2.46 1.75 32.3 31.8 29.4 24.0 26.9 29.6 37.4 28.3 26.8 29.9 29.1 31.9 Zr 0.745 0.730 86.6 82.4 69.7 50.3 59.4 81.0 108 77.9 74.2 78.2 79.0 94.8 Nb 0.190 0.344 6.48 6.43 4.99 4.07 4.38 6.85 10.1 8.34 7.98 8.74 8.54 9.45 Cs 0.738 0.574 0.115 0.116 0.091 0.161 0.100 0.065 0.041 0.188 0.283 0.258 0.230 0.358Ba 0.886 0.642 83.5 83.5 72.2 125 82.8 94.2 31.4 40.0 54.6 55.8 56.0 72.7 La 0.362 0.321 6.79 6.45 5.15 4.05 4.60 6.97 9.79 8.33 6.89 8.72 8.12 8.56 Ce 0.635 0.932 16.6 15.9 13.2 10.4 11.9 17.4 23.4 20.3 17.3 20.5 19.2 21.1 Pr 0.068 0.116 2.40 2.29 1.99 1.58 1.78 2.49 3.40 2.91 2.50 2.82 2.65 2.83 Nd 0.306 0.508 12.2 12.0 10.6 8.43 9.40 12.8 17.0 15.1 12.8 14.1 13.1 14.7 Sm 0.114 0.170 3.64 3.63 3.27 2.67 2.92 3.71 4.78 4.27 3.66 3.88 3.65 4.23 Eu 0.032 0.036 1.25 1.22 1.18 0.95 1.05 1.37 1.58 1.39 1.32 1.37 1.30 1.45 Gd 0.143 0.219 4.11 4.02 3.78 3.00 3.35 4.11 5.18 4.56 3.96 4.29 3.99 4.85 Tb 0.030 0.044 0.796 0.782 0.728 0.599 0.648 0.789 0.979 0.825 0.729 0.794 0.743 0.903Dy 0.191 0.275 5.10 4.98 4.58 3.73 4.10 4.83 6.15 4.84 4.37 4.88 4.71 5.31 Ho 0.047 0.067 1.08 1.08 1.02 0.819 0.904 1.03 1.32 0.990 0.940 1.05 1.01 1.20 Er 0.112 0.168 2.90 2.90 2.73 2.21 2.40 2.70 3.56 2.53 2.43 2.79 2.67 3.15 Tm 0.018 0.027 0.449 0.437 0.416 0.339 0.368 0.413 0.540 0.369 0.365 0.427 0.400 0.493Yb 0.120 0.168 2.94 2.90 2.71 2.23 2.43 2.69 3.53 2.33 2.36 2.85 2.63 3.32 Lu 0.019 0.027 0.470 0.465 0.430 0.365 0.388 0.430 0.569 0.363 0.364 0.438 0.412 0.521Hf 0.044 0.049 2.37 2.29 1.94 1.44 1.68 2.27 2.94 2.12 1.98 2.19 2.09 2.94 Ta 0.008 0.009 0.440 0.436 0.345 0.277 0.303 0.443 0.730 0.542 0.516 0.571 0.555 0.694Pb 0.839 0.533 1.88 1.83 1.68 2.06 2.10 1.93 3.34 1.42 1.87 1.33 1.57 1.97 Th 0.012 0.023 0.607 0.596 0.443 0.346 0.393 0.715 1.00 0.665 0.634 0.769 0.695 0.867U 0.248 0.240 0.202 0.197 0.154 0.137 0.136 0.239 0.358 0.167 0.157 0.254 0.234 0.266

(To be continued on the next page)



(Continued)

Basalts from Lijiahe Basalts from Yuanyangzhen Basalts from

Lubangou Basalts from

Gaojiahe Sample WS-29 WS-29R WS-32

YY-7 YY-9 YY-10 YY-14 YY-14R YY-15 LB-6 LB-20

GH-3 GH-6 SiO2 48.64 48.58 50.21 47.43 49.25 48.96 50.91 50.88 48.04 48.48 50.85 53.06 49.07 TiO2 1.64 1.62 1.72 1.84 2.24 2.12 2.77 2.79 2.00 1.52 1.61 0.93 1.36 Al2O3 13.12 13.13 12.84 13.05 13.03 13.08 13.03 13.00 14.24 14.51 8.55 14.14 16.76 ΣFe2O3 14.66 14.67 15.54 13.28 13.54 13.64 14.92 14.92 13.52 12.75 10.35 7.32 12.30 MnO 0.18 0.18 0.19 0.19 0.18 0.17 0.22 0.22 0.18 0.19 0.14 0.12 0.21 MgO 6.93 6.93 5.74 7.57 6.13 6.81 4.76 4.77 9.22 6.79 11.36 8.19 5.82 CaO 9.33 9.33 9.01 10.85 9.78 9.96 8.81 8.81 6.79 12.90 12.15 7.39 10.77 Na2O 2.62 2.62 2.71 2.92 3.68 3.59 2.84 2.88 3.15 2.63 1.62 3.59 1.04 K2O 0.21 0.21 0.26 0.48 0.64 0.41 0.35 0.34 0.23 0.11 0.17 2.54 0.69 P2O5 0.13 0.14 0.16 0.17 0.24 0.21 0.48 0.48 0.20 0.12 0.12 0.35 0.47 LOI 2.13 2.19 1.20 1.84 1.29 1.11 0.70 0.69 2.66 2.66 1.90 1.10 Total 99.59 99.60 99.58 99.62 100.00 100.06 99.79 99.78 100.23 100.00 99.58 99.53 99.57

Li 9.49 9.34 7.63 11.4 8.46 9.04 9.44 9.16 24.54 59.6 23.4 21.5 13.5 Be 0.649 0.668 0.671 0.732 1.05 1.06 1.54 1.62 0.882 1.00 0.721 2.20 2.22 Sc 46.0 45.2 45.5 42.5 42.6 45.7 36.7 36.8 44.6 35.9 45.4 22.0 34.3 V 395 398 416 345 347 365 296 304 363 286 304 152 311 Cr 69.6 68.3 106.7 112.2 108 97.3 32.3 36.3 199 237 1718 389 82 Co 55.0 53.1 65.7 54.9 51.8 55.9 61.0 60.1 50.8 46.2 67.6 42.3 63.6 Ni 57.2 57.3 62.8 59.1 52.3 50.5 27.3 27.6 71.9 71.6 283 177 45.9 Cu 132 132 123 15.9 30.4 14.9 15.5 15.3 12.1 54.6 204 35.7 17.9 Zn 95.8 99.7 111 113 98.8 101 90.6 89.3 116 106 90.6 74.8 134 Ga 18.5 18.7 19.1 18.2 18.4 18.9 18.5 18.6 20.1 16.3 13.8 18.3 21.7 Ge 1.58 1.62 1.71 1.57 1.85 2.07 1.99 2.03 1.55 1.96 2.01 1.45 2.52 Rb 2.99 2.95 3.40 11.6 15.7 8.22 5.98 5.98 4.48 7.41 6.03 82.7 22.1 Sr 126 127 153 281 143 195 178 178 175 184 151 564 322 Y 29.6 29.6 34.3 27.4 41.3 37.3 63.9 63.9 28.6 23.7 22.4 21.9 41.2 Zr 81.8 88.2 83.0 83.3 94.3 73.8 199 191 119 74.1 98.6 190 141 Nb 8.81 8.90 10.0 8.76 19.1 13.1 23.6 23.4 9.70 5.36 6.87 8.43 12.7 Cs 0.259 0.261 0.108 0.136 0.259 0.387 0.315 0.307 0.358 0.758 0.373 1.09 0.728 Ba 46.4 46.6 53.7 77.5 104 80.6 55.7 56.4 56.1 18.0 22.8 836 70.9 La 8.06 8.29 9.58 7.70 12.3 10.8 20.2 20.4 6.78 7.53 8.84 58.6 21.0 Ce 19.2 19.5 21.8 19.8 29.5 25.5 49.6 49.7 18.3 18.0 21.2 120 44.4 Pr 2.71 2.74 2.98 2.81 4.14 3.61 6.93 6.95 2.71 2.50 2.98 12.2 5.58 Nd 13.4 13.6 14.6 14.3 20.6 18.0 33.9 34.2 13.9 12.1 15.0 48.1 25.0 Sm 3.78 3.77 4.02 4.00 5.67 5.04 9.14 9.06 4.01 3.31 4.08 8.25 6.16 Eu 1.28 1.29 1.38 1.29 1.73 1.65 2.65 2.66 1.32 1.11 1.22 2.00 1.77 Gd 4.16 4.15 4.44 4.20 5.99 5.29 9.46 9.53 4.23 3.50 4.14 6.98 6.25 Tb 0.779 0.771 0.847 0.758 1.10 1.01 1.74 1.75 0.806 0.649 0.711 0.859 1.09 Dy 4.82 4.85 5.37 4.45 6.57 6.03 10.3 10.3 4.78 3.90 4.04 4.16 6.39 Ho 1.05 1.06 1.20 0.939 1.42 1.29 2.18 2.19 1.00 0.852 0.811 0.735 1.36 Er 2.78 2.79 3.20 2.44 3.75 3.42 5.82 5.87 2.70 2.23 2.01 1.90 3.57 Tm 0.424 0.422 0.493 0.360 0.567 0.495 0.870 0.864 0.411 0.343 0.284 0.253 0.540 Yb 2.84 2.79 3.33 2.37 3.75 3.20 5.73 5.69 2.64 2.24 1.79 1.63 3.57 Lu 0.445 0.443 0.537 0.363 0.583 0.485 0.897 0.890 0.415 0.340 0.272 0.261 0.561 Hf 2.29 2.54 2.30 2.32 2.64 2.16 5.05 4.89 3.16 2.05 2.74 4.74 3.74 Ta 0.579 0.581 0.652 0.568 1.02 0.75 1.49 1.46 0.633 0.368 0.474 0.553 0.905 Pb 5.06 5.02 3.21 4.23 4.33 5.22 2.18 2.15 5.44 17.1 8.42 18.5 17.6 Th 0.724 0.739 0.928 0.305 1.02 0.825 1.75 1.72 0.709 0.823 1.61 17.9 6.73 U 0.237 0.245 0.319 0.203 0.386 0.301 0.529 0.531 0.471 0.448 0.481 3.55 2.37



Figure 3 TAS diagram (a) and Zr/TiO2-Nb/Y diagram (b) for classify the meta-basalts. +, Samples from Lijiahe; □, samples from Yuanyangzhen; △, samples from Lubangou; ▲, samples from Gaojiahe.

Figure 4 Chondrite-normalized REE distribution patterns for the basalts from the Wushan area. (a) Samples from the Lijiahe area; (b) samples from the Yuanyangzhen area; (c) samples from the Lubangou area; (d) samples from the Gaojiahe area. The REE distribution line of each sample is parallel to each other, and also it is parallel to that of a typical E-MORB. Therefore, it is reasonable to infer that the basalts from Yuanyangzhen were derived from an E-MORB type magma source.

Although the basalts from the Lubangou area show similar ΣREE content (58.60×10−6 and 67.34×10−6) to that of the basalts from the Lijiahe and the Yuanyangzhen area, it is obvious that the basalts from Lubangou are more fractionated in LREE and HREE than that of the samples from the other two places. The (La/Yb)N and (La/Sm)N values range from 2.27 to 3.34 , and from 1.36 to 1.43, respectively. The chondrite-normalized REE

distribution patterns (Figure 4(c)) show obvious enrichment in LREE. All the above indicate that the samples from the Lubangou area are similar to E-MORB.

The samples (GH-3 and GH-6) from the Gaojihe area are characterized by high ΣREE content (266×10−6 and 127×10−6) and enrichment in LREE in the chondrite- normalized REE distribution patterns (Figure 4(d)). In detail, there exists geochemical difference between these two samples, for instance, the GH-3 sample displays rather enrichment in LREE ((La/Yb)N=24.3, (La/Sm)N=4.5), while the GH-6 sample is featured by (La/Yb)N =3.98 and (La/Sm)N =2.15. This difference can



be interpreted as being due to varying degree metaso- matism in the magma source.

In view of the trace elements chemistry, it is suggested that both samples from the Lijiahe area and the Yuanyangzhen area are characterized by lower contents of all trace elements, and most trace elements contents are 5 to 20 times that of a primitive mantle. The primitive mantle normalized trace element patterns (Figure 5(a) and (b)) show that the samples from the Lijiahe and the Yuanyangzhen area are both featured by low LILE and unfractionated in HFSE without any enrichment in Th and depletion in Nb and Ta, which are very similar to that of a typical E-MORB. In comparison, the Lijiahe basalts have much lower trace element contents and show a transitional trend between the N-MORB and E-MORB, while the Yuanyangzhen basalts possess the same geochemistry as the typical E-MORB reported by Sun and McDonough[10]. Additionally, though the samples from the Lubangou area show light enrichment in Th and U, there is not any obvious depletion in Nb and Ta compared with Ce value (Figure 5(c)). Especially, the HFSE distribution patterns coincide with that of the typical E-MORB, which are similar to the Yuanyangzhen

and Lijiahe basalts. The geochemistry of the Gaojiahe basalts is apparently

different from that of the basalts from the Lijiahe area, Yuanyangzhen area and Loubangou area. In the primitive mantle normalized trace element patterns (Figure 5(d)), the basalts of Gaojihe area are featured by concentration in LILE and fractionation in HFSE, especially obvious depletion in Nb, Ta and enrichment in Th. All these evidences indicate that the magma source of the Gaojihe basalts was influenced by input of materials from subduction zone.

5 Tectonic setting of the ophiolite

In summary, the geochemistry of REE and other trace elements indicates that the basalts (from Lijiahe, Yuanyangzhen and Lubangou) of the Wushan ophiolite were mainly derived from a slightly enriched mantle source. In general, being chemical stability during alteration and metamorphism, the HFSE are effective indicators for studying the magma source chemistry, petrogenesis and tectonic setting. With the characteristics of low Ta (<0.7×10−6) and Nb (<12×10−6) contents, and

Figure 5 The primitive mantle normalized trace elements patterns. (a) Samples from the Lijiahe area; (b) samples from the Yuanyangzhen area; (c) samples from the Lubangou area; (d) samples from the Gaojiahe area.



Nb/La<1, Hf/Ta>5, La/Ta >15, the typical island-arc basalt (IAB) and some normal mid-oceanic ridge basalt (N-MORB) are very different from the within plate basalt (WPB) and E-MORB[11]. In comparison, the geochemistry of Ta (0.28×10−6―0.69×10−6) and Nb (4.1×10−6―

10×10−6) contents, Nb/La (0.95―1.16), Hf/Ta (3.8―5.6) and La/Ta (12―16) ratios suggests that the Lijiahe basalts have the transitional chemical nature between N-MORB and E-MORB. The basalts from the Yuanyang- zhen area are featured by relatively high Ta (0.57×10−6―

1.49×10−6) and Nb (8.8×10−6―23.6×10−6) contents, and Nb/La (1.14―1.56), Hf/Ta (2.6―5.0) and La/Ta (11―14) ratios, which are similar to that of a typical E-MORB and WPB, and different from IAB and N-MORB.

Generally, in a primitive mantle normalized trace element diagrams, WPB (include OIB) is always characterized by intense fractionation in HFSE, showing a ‘humped’ pattern, while the typical island-arc basalt is

featured by depletion in Nb, Ta and enrichment in Th[12,13]. Most basalts of the Wushan ophiolite show characteristics of not only without any fractionation in HFSE and depletion in Nb and Ta, but also with slight enrichment in LREE. All these suggest that the basalts of the Wushan ophiolite were derived from a typical E-MORB source.

Many immobile elements covariance diagrams for discrimination of a tectonic setting show that most basalts of the Wushan ophiolite formed in an E-MORB setting. For instance, all the basalts from the Yuanyang- zhen, Lijiahe and Loubangou areas drop into the area of volcanic arc and the MORB setting in the Ti/100-Zr-3Y diagram (Figure 6(a)). However, the 2Nb-Zr/4-Y diagram (Figure 6(b)) shows that the basalts from the Yuanyang- zhen area formed in an E-MORB setting, while the basalts from the Lijiahe and Loubangou areas tend to floating into the volcanic arc and N-MORB area. In the Ti/100-Zr-Sr/2 diagram (Figure 6(c)), all the basalts from

Figure 6 Tectonic discrimination diagrams for the basalt from the Wushan area. (a) Ti-Zr-Y diagram[14], A, island arc tholeiites; B, island-arc tholeiites, island-arc calc-alkali basalts and MORB; C, island-arc calc-alkali basalts; D, within-plate basalts; (b) Nb-Zr-Y diagram[15], AI, within-plate alkali basalt; AII, within-plate alkali basalts and tholeiites; B, E-type MORB; C, within-plate tholeiites and volcanic-arc basalts; D, N-type MORB and volcanic-arc basalts; (c) Ti-Zr-Sr diagram[14], OFB, ocean floor basalts; IAB, island-arc basalts; CAB, calc-alkali basalts; (d) Hf-Th-Ta diagram[16], A, N-MORB; B, E-MORB and within-plate basalts; C, within-plate alkali basalt; D, island-arc basalts. Symbols after Figure 3.



the above three places drop into or near the OFB setting and far away from the volcanic arc basalts area. Additionally, further discrimination of the Hf/3-Th-Ta diagram (Figure 6(d)) indicates that the basalts of the Wushan ophiolite from the Yuanyangzhen, Lijiahe and Loubangou areas were formed in a tectonic setting where E-MORB magma can be derived.

All the above studies indicate that the basalts of the Wushan ophiolite mainly belong to E-MORB, except for three samples from the Lijiahe area being featured by flat REE distribution patterns and showing transitional chemistry between E-MORB and N-MORB. The basalts from the Loubangou area are characterized by slight depletion in Nb, Ta and enriched in U, Th, which are attributed to the feeble contamination of continental crust during the initial spreading of the oceanic basin. In view of the N-MORB type ophiolite widely outcropped in Guanzizhen and the ophiolitic mélange in the Yanwan and Tangzang areas along the Wushan-Tangzang fault belts, we deduce that the Wushan ophiolite probably formed in a spreading center of an oceanic basin during its initial expanding.

However, the basalts from the Gaojiahe area are characterized by enrichment in LILE, fractionation in HFSE and apparent depletion in Nb and Ta. These features could show that the magma source had been influenced by inputs of the subduction materials. All the geochemistry and many tectonic discrimination diagrams indicate that the Gaojiahe basalts were formed in an Island-arc setting.

6 Discussion on the tectonic implications

The Qinling Mountains have been traditionally considered as a composite orogenic belt between the South China and North China blocks. After several decades of studies, it is demonstrated that there exist two suture zones named the Shangdan Suture to north and the Mianlue Suture to south along the Qinling Belt. The Mianlue Suture extending from east to west along the southern margin of the Qinling-Dabie Orogenic Belt has already been approved to be a suture after closing of the northern branch ocean of Paleotethys[17―20] (D-T2), and can be connected with the A’nyêmaqên suture zone in the East Kunlun Mountains to the west[8]. The Shangdan suture zone represents a major suture between the South China and the North China block, and is demonstrated by

large amounts of ophiolites, island-arc volcanic and related rocks distributed along the boundary fault between the Northern Qinling terrane and the Southern Qinling terrane. These rocks are a key to studying the characteristics and evolution history of the suture zone and the paleo-ocean. Although there also exists the Meso- Neoproterozoic ophiolite in the Songshugou area on the north side of the Shangdan Zone[21], more and more researches demonstrated that the Shangdan suture is mainly composed mainly of the Early-Paleozoic ophiolites and related volcanic rocks. For instance, the Shangdan suture zone in the East Qinling area is marked by metamorphic basic volcanic rocks outcropped in the Danfeng area. The geochemistry of these rocks shows that their magma origin is related to the partial melting of mantle wedge caused by plate subduction, and formed in an island arc setting[5,22]. There exist Cambrian- Ordovician radiolarian fossils within the radiolarian silicalite interlayers in the Danfeng ophiolitic mélange[23]. To the west, the Shangdan suture is still composed mainly of island-arc volcanic rocks in the Heihe area[24], however, both the E-MORB ophiolites and island-arc volcanic rocks outcrop together until westward to Sifangtai, Yanwan and Tangzang areas.

The formational age of the Yanwan E-MORB type ophiolite is about 518 ± 2.9 Ma (U-Pb dating of zircon for gabbros). Additionally, the U-Pb dating of zircon give an age of 523 ± 26 Ma[25] for the Danfeng volcanic rocks in Fengtai area. Westward to Tianshui area, the N-MORB ophiolite outcropped in Guanzizhen was formed at about 471 ± 1.4 Ma[26]. Extending to the Wushan area, the Shangdan suture is marked by the E-MORB type ophiolite reported in this article, the U-Pb dating of zircon from gabbros gives an age of 457 ± 3 Ma[27], which could be interpreted as the upper limit formational age of the Wushan ophiolite.

Based on the geochemistry and ages of the E-MORB type ophiolite in Wushan, the N-MORB type ophiolite in the Guanzizhen area, and the regional geology and geochronology, we infer that the Wushan-Tangzang boundary fault can be connected with the Shangdan Sutre Zone to the east, which represents the major suture after the closure of the Early-Paleozoic ocean between the North China and the South China block. In addition, it is also indicated that the Paleo-ocean had been evolved into a complete evolutionary process including initial spreading (E-MORB ophiolite), maturated extension (N-MORB ophiolite) and subduction (Island-arc volcanic



rocks). Furthermore, there occur some magma complexes,

which were mainly formed in an active continental margin or island-arc setting, between the Shangdan ophiolitic mélange belt and the Qinling Group. They are mainly represented by the troctolite-gabbroic intrusion in the Sifangtai-Lajimiao area[6] and the island-arc gabbros in the Fushui area[28]. The U-Pb dating of zircons from gabbros gave an age of 514 ± 1.3 Ma which was interpreted as the formational age of the Fushui gabbroic intrusion[29]. Additionally, there also is a large amount of subduction and/or collisional granite intruded into the Qiling Group on the north side of the Shangdan Suture, such as the Huichizi body, the Piaochi body, the Anjiping body, the Zaoyuan body and the Tieyupu body, etc. More and more evidences suggest that the Qinling Group with the intrusive magma complexes formed an island-arc terrane above the subduction zone in Early-Paleozoic. To east Qinling, there occurs the Erlangping Group composed mainly of clastic rocks, carbonate rocks and

metamorphic basalts with pillow-structure. The geochemistry of the basalts suggests that they were formed in a back-arc basin setting[30]. Some Ordovician-Silurian radiolarian fossils were found in the radiolarian silicalite within the Erlangping Group[31]. In comparison, there also exist a suite of a volcanic rocks formed in a back-arc basin setting along the Qingshui-Boyang area, to the north of the Wushan-Tangzang ophiolite mélange. Both volcanic rocks from the Erlangping and the Qingshui- Boyang area indicate that there had existed a back-arc basin on the northern side of the Northern Qinling terrane in Early-Paleozoic.

All the above studies suggest that there are distributed the Wushan-Tangzang-Shangdan ophiolite mélange, the Qinling island-arc terrane and Qingshui-Boyang- Erlangping back-arc basin ophiolitea from south to north long the Northern Qinling Belt. It is further inferred that there had occurred a complete plate tectonic framework with a mid-ocean, trench, island-arc and back-arc basin setting in Northern Qinling in Early-Paleozoic.

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Geochemistry of the E-MORB type ophiolite and related