ARTICLES Chinese Science Bulletin 2006 Vol. 51 No. 8 963—970
DOI: 10.1007/s11434-006-0963-1
Finding of Neoproterozoic (~775 Ma) magmatism recorded in metamorphic complexes from the North Qilian orogen: Evidence from SHRIMP zircon U-Pb dating Chien-Yuan Tseng1, Houng-Yi Yang1, WAN Yusheng2, LIU Dunyi2, Da-Jen Wen3, Tzung-Chi Lin1 & Kuo-An Tung1 1. Department of Earth Sciences, National Cheng Kung University, Tainan 701, China; 2. Beijing SHRIMP Centre, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China; 3. Department of Geosciences, National Taiwan University, Taipei 106, China Correspondence should be addressed to Chien-Yuan Tseng (email:
[email protected])
Abstract SHRIMP U-Pb dating was carried out on zircons from the Niuxinshan gneissic granite and the Leigongshan gneissic tonalite in the North Qilian orogen, NW China. The results yield weighted averaged 206Pb/238U ages of 776±10 Ma and 774±23 Ma respectively. Igneous morphology, oscillatory zonings, and relatively high Th/U ratios for these zircons suggest magmatic origin. These ages are interpreted as timing of magma emplacement and thus representing a Neoproterozoic (~775 Ma) magmatic event in the North Qilian area. It is suggested that this magmatism is probably related to breakup of the supercontinent Rodinia. This finding, together with the similar ages of 750 to 800 Ma reported for neighboring terranes of South-central Qilian and North Qaidam, is of significance to understanding of the Rodinia evolution in West China. Keywords: North Qilian orogen, zircon U-Pb age, Neoproterozoic magmatism, Rodinia breakup.
The Qilian Mountain is considered a part of the Central Orogenic Belt of China[1]. The Central Orogenic Belt of China, extending for a distance of about 4000 km long in the E-W direction from the east coast westward through the mainland China all the way to the westernmost border, consists of the following enorwww.scichina.com
www.springerlink.com
mous mountain ranges: Dabie-Sulu Mountains, Tongbai Mountains, Qinling Mountains, Qilian Mountains, Altun Mountains, and Kunlun Mountains. Despite their different tectonic histories, ultrahigh-pressure metamorphism accompanying deep continental subduction seems to be a common tectonic event for these mountain ranges[1]. Recent studies showed that ultrahigh-pressure metamorphism occurred around 420― ― 500 Ma in its western part (North Qaidam orogen)[1 5] and around 225―240 Ma in its eastern part (Dabie― Sulu orogenic belt)[6 10]. Many U-Pb geochronological studies on zircons grains found that the outer layer of the zircon bearing the ages of ultrahigh-pressure metamorphism often mantles a core of Neoproterozoic ages (ca. 750―800 Ma)[8,9]. This core age is interpreted as protolith ages of the ultrahigh-pressure metaigneous rocks. The formation of the metaigneous protolith is likely related to contemporaneous magmatism that is probably associated with breakup of the supercontinent Rodinia[8,9]. Therefore, pursuing this breakup process and the associated igneous activities should be very helpful to the understanding of the geological histories of the Central Orogenic Belt. The Qilian Mountain lies the northeastern margin of the Tibet Plateau and extends in NWW direction of about 1200 km in the northwestern part of China. It is tectonically divided longitudinally into the North Qilian orogen and the South-central Qilian craton. The North Qilian orogen consists chiefly of the Caledonian ophiolites, arc complexes, circum-Pacific type subduction complexes (LT/HP metamorphic rocks), intermediate-felsic plutonic rocks, mafic-ultramafic associations, and Precambrian metamorphic complexes. So far, there have been no reports on Neoproterozoic igneous activity from the North Qilian orogen related to the Rodinia breakup. But there have been such reports from the following adjacent areas. To the north at the southwestern margin of the Alax craton, Jinchuan ultramafic intrusion was dated to be 827±8 Ma[11]. To the south, the zircon grains in the ultrahigh-pressure metamorphic rocks from the North Qaidam terrane commonly have 750―800 Ma cores[5,12], reflecting that some protoliths of the ultrahigh-pressure metamorphic rocks were probably the product of the Rodinia breakup. Thus, it is worthwhile to pursue whether or not the north Qilian orogen, located between the Alax craton and the North Qaidam ultrahigh-pressure metamorphic belt, also has records of such Neoproterozoic igneous activities. 963
ARTICLES Zircons separated from the rocks of two metamorphic complexes from the North Qilian orogen were dated with SHRIMP. The results yield two U-Pb ages of 776±10 Ma and 774±23 Ma, respectively, representing Neoproterozoic magmatism as reported in the present study. These ages are discussed in relation to the global magmatism (particularly the one in the periphery of the Yangtze craton). In addition, the lithology and ages of the studied rocks from the North Qilian orogen are similar to those reported for low-grade metaigneous intrusives in the northern margin of the Dabie-Sulu orogenic belt. Thus a comparison is made between them, despite about 2000 km apart, with respect to the Neoproterozoic magmatic activity and tectonic emplacement. 1
Geological background
The North Qilian orogen is sandwiched between the North China craton (Alax) and the South-central Qilian craton, and is considered as a typical Caledonian orogenic belt[13]. The volcanic rocks, ophiolites, metasedimentary rocks, subduction complexes, and intermediate-felsic plutonic rocks within the North Qilian orogen have been well described and will not be re― peated here[13 16]. However, one of the most characteristic features of the North Qilian orogen is that numerous pieces of Precambrian metamorphic complex masses of all sizes are embedded in the early Paleozoic
rock formations. Nine such pieces mappable on a 2000000:1 geological map are given by Zuo et al.[17]. They might be autochthonous in-situ residual basement blocks[17] or allochthonous tectonic blocks transported to the present sites by low-angle thrust faults or by gravitational slide[18]. Two rock samples studied in the present work were collected from two such masses of Precambrian metamorphic complexes, one at Niuxinshan and the other at Leigongshan (Fig. 1). The Niuxinshan Precambrian metamorphic complex, well exposed along Zhamashihe and Dongcaohe, consists of migmatites, gneissic granites, amphibole schists, two mica schists, marbles, actinolite schists, biotite schists, and amphibolites. Lin[19] observed that the metamorphism was of greenschist to epidote amphibolite facies. Leigongshan Precambrian metamorphic complex, a much smaller mass with an exposed area of only 0.5 km2, consists of greenschists and gnessic tonalite of greenschist facies, and is accompanied by early Paleozoic metavolcanic, metasedimentary, and intermediate-felsic plutonic rocks[20]. 2 Description of the rock samples and treatment of the zircon grains A gneissic granite (ZMS-5D1) and a gneissic tonalite (RGS-05) were selected respectively from Niuxinshan and Leigongshan metamorphic complexes for the present study. The Niuxinshan gneissic granite consists of
Fig. 1. Map showing the locations of the Niuxinshan and Leigongshan metamorphic complexes in the North Qilian orogen.
964
Chinese Science Bulletin
Vol. 51 No. 8
April 2006
ARTICLES plagioclase (An3―51), K-feldspar, biotite (Xmg = 0.38― 0.43), and quartz with minor amount of muscovite and ilmenite. Well-developed schistosities-gneissosities were formed by parallel arrangement of the biotite, muscovite, and plagioclase. Most mineral grains were strongly sheared and quartz grains often exhibit ribbon textures, suggesting that mylonitization was extensive. The Leigongshan gneissic tonalite consists of plagioclase, hornblende, and quartz with minor amount of biotite and Fe-Ti oxide minerals. It can be seen that igneous textures are still preserved under petrographic microscope, but the handspecimen shows gneissosity. The Niuxinshan gneissic granite display more features of dynamic deformation than the Leigongshan gneissic tonalite, yet they are all surrounded by the greenschist to epidote-amphibolite facies metamorphic country rocks. Obviously, both Niuxinshan and Leigongshan samples are low grade metamorphosed intrusive rocks having suffered different degree of dynamic deformation.
Separation of zircons from the rock samples ZMS-5D1 and RGS-05 was done at the Institute of the Hebei Geological Survey. The sample zircon and standard zircon grains were mounted on the surface of epoxy resin and were polished. To study their internal textures, the polished zircon grains were examined with reflected light under the optical microscope, and their backscattered electron images were also examined with scanning electron microscope (Fig. 2). The ZMS-5D1 zircon grains are mostly prismatic, with their long axes of about 80―200 μm and an average aspect ratio of about 2. Their crystal faces and edges are perfect and clear. Their backscattered electron images show oscillatory zonings parallel to the crystal faces. The RGS-05 zircon grains are generally less than 100 μm in size and are short prisms with an average aspect ratio of less than 2. Their backscattered electron images also exhibit oscillatory zonings. All these textural and morphological features of the ZMS-5D1 and RGS-05 zircon samples are characteristics of magmatic origin.
Fig. 2. (a) The reflected light microphotograph under the optical microscope and the backscattered electron images of zircon grains from the Niuxinshan gneissic granite (ZMS-5D1); (b) backscattered electron images of zircon grains from the Leigongshan gneissic tonalite (RGS-05).
www.scichina.com
www.springerlink.com
965
ARTICLES 3
Method of analysis and result
U-Pb dating of the zircon with SHRIMP was carried out at the Beijing SHRIMP Centre. The analytical processes and conditions follow the standard procedures[21]. Zircon standard SL13 with an age of 572 Ma and a U concentration of 238 μg/g was used to calibrate the U concentration[22]. Zircon standard TEMORA with an age of 417 Ma[22] was used to calibrate U-Th-Pb ratios for every 3―4 analyses. The analytical data were corrected for common Pb content using the measured 204 Pb. SQUID and Isoplot program[23] were used to process the data. Average ages are given at 95% confidence limits. Fifteen points of measurement were obtained from the sample ZMS-5D1, but the point ZMS-5D1-5 was discarded due to its high 204Pb value of 53.8ppb. The rest of the fourteen points yielded a weighted average 206 Pb/238U age of 776±10 Ma (MSWD=1.8) (Table 1, Fig. 3(a)). Their Th/U ratios range from 0.12 to 0.45,
indicating a magmatic origin for granitic rocks. Ten measurements were made on the sample RGS-05. Their Th/U ratios range from 0.44 to 1.01, also indicating a magmatic origin. The weighted average 206Pb/238U age is 774±23 Ma (MSWD=4.0) (Table 1, Fig. 3 (b)). It is interpreted as the age of emplacement of the tonalite. 4
Discussion
The present SHRIMP zircon U-Pb dating provides geochronological records of magmatism at 776±10 Ma in the Niuxinshan gneissic granite and at 774±23 Ma in the Leigongshan gneissic tonalite. These two ages essentially represent the same episode of middle Neoproterozoic magmatism. This is the first report of the middle Neoproterozoic magmatism from the North Qilian orogen. The important question is whether this magmatic event can be correlated with the Rodinia breakup during the middle Neoproterozoic or with the epicontinental rifting of the Alax craton (North China craton) during the late Neoproterozoic. The former has been
Table 1 SHRIMP U-Th-Pb isotope data of zircons from the Niuxinshan and Leigongshan metamorphic complexes of the North Qilian orogenic belt Concentration Radiogenic ratios Age (Ma) (ppm) Spot Th/U 206Pb(%) Pb 206 206 U Th Pb/238U ±% 207Pb/235U ±% 207Pb/206Pb ±% Pb/238U ±1σ 207Pb/206Pb ±1σ rad. Gneissic granite from the Niuxinshan metamorphic complex ZMS-5D1-1 466 57 51.4 0.13 0.19 0.1279 1.7 1.113 2.1 0.06302 1.1 776 ±13 711 ±24 ZMS-5D1-2 243 59 25.9 0.25 0.14 0.1236 2.2 1.104 2.6 0.06475 1.5 751 ±15 767 ±32 ZMS-5D1-3 216 97 23.9 0.46 0.09 0.1286 1.8 1.109 2.6 0.0624 2.0 780 ±13 692 ±41 ZMS-5D1-4 242 79 27.6 0.34 0.04 0.1324 1.8 1.165 2.4 0.0638 1.6 802 ±14 736 ±33 ZMS-5D1-5 492 149 48.3 0.31 2.00 0.1120 1.8 0.973 3.0 0.0626 2.4 684 ±11 709 ±50 ZMS-5D1-6 314 55 34.0 0.18 0.06 0.1259 1.8 1.101 2.0 0.06341 0.96 764 ±13 723 ±20 ZMS-5D1-7 467 76 51.0 0.17 -0.1270 1.7 1.129 2.0 0.06444 0.87 771 ±13 757 ±18 ZMS-5D1-8 242 77 27.1 0.33 0.11 0.1302 1.8 1.143 2.8 0.0637 2.1 789 ±13 731 ±45 ZMS-5D1-9 233 71 26.0 0.32 0.42 0.1296 1.8 1.108 2.9 0.0620 2.3 785 ±14 675 ±48 ZMS-5D1-10 191 59 21.4 0.32 0.08 0.1300 1.8 1.150 2.4 0.0641 1.6 788 ±13 747 ±33 ZMS-5D1-11 327 116 36.2 0.37 0.18 0.1288 1.8 1.164 2.0 0.06553 0.96 781 ±13 792 ±20 ZMS-5D1-12 264 50 30.1 0.20 -0.1328 1.8 1.190 2.2 0.06496 1.3 804 ±13 774 ±28 ZMS-5D1-13 223 78 24.4 0.36 -0.1272 1.8 1.135 2.6 0.0647 1.9 772 ±13 764 ±39 ZMS-5D1-14 254 55 27.5 0.22 0.08 0.1256 1.8 1.103 2.3 0.06370 1.5 763 ±13 733 ±30 ZMS-5D1-15 379 70 39.7 0.19 0.16 0.1219 1.8 1.097 2.1 0.06525 1.2 742 ±12 783 ±25 Gneissic tonalite from the Leigongshan metamorphic complex RGS-05-1 214 139 24.1 0.67 0.42 0.1305 2.2 1.127 3.3 0.0626 2.5 791 ±16 695 ± 53 RGS-05-2 120 74 14.2 0.64 0.29 0.1378 2.1 1.225 3.5 0.0645 2.8 832 ±17 758 ± 59 RGS-05-3 184 110 20.7 0.62 0.38 0.1300 2.1 1.116 3.5 0.0623 2.8 788 ±15 683 ± 61 RGS-05-4 100 49 11.0 0.51 0.68 0.1267 2.2 1.030 4.3 0.0590 3.7 769 ±16 565 ± 81 RGS-05-5 125 77 14.3 0.64 0.86 0.1319 2.3 1.075 4.2 0.0591 3.6 799 ±17 572 ± 77 RGS-05-6 153 88 17.0 0.59 0.60 0.1283 2.1 1.090 3.5 0.0616 2.8 778 ±16 661 ± 60 RGS-05-7 163 110 17.4 0.70 0.58 0.1232 2.1 1.078 3.5 0.0634 2.8 749 ±15 722 ± 59 RGS-05-8 108 46 11.0 0.44 0.91 0.1166 2.4 0.966 5.4 0.0601 4.9 711 ±16 607 ±110 RGS-05-9 95 44 10.2 0.48 0.84 0.1242 2.2 1.011 5.5 0.0591 5.0 755 ±16 570 ±110 RGS-05-10 308 301 33.9 1.01 0.24 0.1281 2.1 1.158 2.4 0.06557 1.3 777 ±15 793 ± 28
966
Chinese Science Bulletin
Vol. 51 No. 8
April 2006
ARTICLES
Fig. 3. (a) Concordia diagram of SHRIMP U-Pb data for zircon from the Niuxinshan gneissic granite; (b) concordia diagram of SHRIMP U-Pb data for zircon from the Leigongshan gneissic tonalite.
well documented in the literature[9,11]. Some of the previous works considered that rifting, followed by the oceanization, of the Alax craton in the Neoproterozoic formed a presumed Paleo-Qilian ocean between the Alax craton and the South-central Qilian craton, and the closure of this presumed ocean formed the North Qilian orogen[15,18]. So far, the known oldest oceanic crust of the North Qilian orogen, the Yushigou ophiolite, was dated to be 550±17 Ma[24]. If these two ages were associated with the epicontinental rifting of the Alax craton, then the time span from the rifting to the appearance of the first oceanic rock was ~220 Ma. It was too long to be reasonable in view of the present-day plate tectonics. Thus, these two magmatic ages, found in the present study, cannot be correlated with the epicontinental rifting of the Alax craton, even if such rifting ever occurred. Furthermore, recent Sm-Nd isotopic studies on www.scichina.com
www.springerlink.com
the metamorphic basement rocks showed that the South-central Qilian craton has a stronger affinity toward the Yangtze craton rather than the North China craton[25,26], implying that the South-central craton might not originally be part of the North China craton. Therefore, these two magmatic ages cannot be related to the epicontinental rifting of the North China craton. Dong (unpublished data) dated the zircon in a granitic gneiss from Hualong area, South-central Qilian craton, with SHRIMP U-Pb method and obtained a magmatic age of 790±12 Ma, very close to the magmatic ages from the present study. In other words, the magmatic event recorded in the Precambrian metamorphic complexes embedded in the North Qilian orogen could be better correlated with the magmatic event recorded in the orthogneisses in the South-central Qilian craton. Thus those masses of Precambrian metamorphic complexes should be deemed to be tectonic blocks transported likely from the South-central Qilian craton by the low-angle thrust faults. Consequently, it should be more appropriate to discuss the significance of the Neoproterozoic magmatic events recorded in the North Qilian orogen in connection with the Yangtze craton. It is well known that the important geological event in the Neoproterozoic time was the assembly and breakup of the supercontinent Rodinia[27,28]. The Rodinia breakup is invoked to be caused by mantle plumes, or superplume, from the interior of the ― earth[29 31]. However, no direct product of the plumes or superplume has been observed in the suggested areas. The South China craton, consisting of the Yangtze and Cathaysia cratons, was considered part of the supercontinent Rodinia[32,33]. Previous studies found two episodes of magmatism at 795-830 Ma and 745―780 Ma, respectively[34]. The former was considered the pre-rift magmatism and the latter the syn-rift magmatism during the Rodinia breakup. The ages reported in the present study, ~775 Ma, were apparently close to the latter. In addition, a single zircon age of 751±14 Ma was reported from the Diaodaban granitic gneiss in the western part of the North Qilian orogen[35]. If the magmatism of this age could also be correlated with the magmatism associated with the Rodinia breakup, then the ages from the eastern part (Leigongshan), middle part (Niuxinshan), and western part (Diaodaban) indicate the occurrence of a period of Neoproterozoic magmatism in the North Qilian orogen, which is closely related with the Rodinia breakup. Therefore, it becomes very important to include the North Qilian orogen in 967
ARTICLES the discussion of the assembly and breakup of the supercontinent Rodinia when making reconstruction of paleo-plate tectonics. From the view of the regional geological tectonics, the North Qaidam orogen was a Caledonian suture belt between the South-central Qilian craton and the Qaidam craton, and is considered as the southern boundary of the Qilian Mountain. The core U-Pb ages of the zircon grains, separated from the ultrahigh-pressure metamorphic rocks from the North Qaidam orogen, were dated with SHRIMP to be around 750―800 Ma[5,12] which were taken to be ages of their protolith. The protolith was further thought to be likely of oceanic origin[12]. Thus, the entire Qilian area, including the North Qilian orogen, the South-central Qilian craton, and the North Qaidam orogen, bears evidence of Neoproterozoic magmatism contemporaneous with the breakup of the supercontinent Rodinia. The existence of such oceanic protoliths led Yang et al.[12] to propose the existence of a paleo-ocean which opened and closed during the late Neoproterozoic in the Qilian area. Subsequently, these oceanic rocks became the protoliths of the ultrahigh-pressure metamorphism during the Caledonian orogeny. It is interesting to compare a pair of two parallel subduction zones in the Qilian area during the Caledonian period: the North Qilian subduction zone and the North Qaidam subduction zone. Both were Caledonian with the ages of 462±12 Ma[36] and ― 420―500 Ma[1 5] respectively, and both were subducted northward. The North Qilian subduction zone was of circum-Pacific type and was accompanied by LT/HP metamorphism[36], whereas the North Qaidam subduction was of Alpine type and was accompanied ― by ultrahigh-pressure metamorphism[2 5]. The ultrahigh-pres- sure metamorphic rocks have been found in the North Qaidam subduction complexes, but none in the North Qilian subduction complexes. The records of the Neoproterozoic magmatism have been found in the North Qaidam subduction complexes, but none in the North Qilian subduction complexes. Regional correlation between the North Qilian orogen and the Dabie-Sulu orogenic belt is very interesting. There have been abundant dates on the middle Neoproterozoic magmatism from the Dabie-Sulu orogenic belt[8, 9]. Zircon U-Pb geochronological studies showed that the ages of metaigneous protolith fall around 758±15 Ma[9]. Of special interest to the present study is the low-grade deformed granites from the Luzhenguan complex in the Beihuaiyang and Wulian zones[37,38], 968
which bears some resemblance to the two gneissic granitoids recording the Neoproterozoic magmatic events reported in the present study. Therefore, it is worthwhile to look into the possible correlation between the Beihuaiyang-Wulian zones and the North Qilian. Firstly, the granitiod in both localities has been deformed and suffered only low-grade metamorphism, not ultrahigh-pressure metamorphism. Secondly, the U-Pb geochronological studies on the zircons from the Beihuaiyang zone yielded a SHRIMP 206Pb/238U mean age of 783±22 Ma and a TIMS discordia intercept age of 762±16 Ma[9,37], both of which are close to the ages (776±10 Ma and 774±23 Ma) reported in the present study. Geotectonically speaking, the protoliths of the low-grade, deformed granitiods in the Beihuaiyang zone were originally the intrusive rocks emplaced in the Neoproterozoic rifting zones along the northern margin of the Yangtze craton. When the Yangtze craton subducted beneath the North China craton during the Triassic orogeny, these intrusions and their surrounding country rocks were scrapped off by the North China craton, accompanied by the low-grade metamorphism and deformation, to form the so-called passive continental margin accretionary wedge[37]. Such low-grade, deformed Neoproterozoic intrusive rocks in the passive continental margin accretionary wedge have frequently been reported from the Dabie-Sulu orogenic belt[37, 38]. In addition, the Xinyang Group of the Liuling unit in the Qinling Mountains was also considered part of passive continental margin accretionary wedge[37,39]. However, no such geotectonic structure has been reported from the Qilian Mountain in the western section of the Central Orogenic Belt. As mentioned earlier, the protolith ages of the ultrahigh-pressure metaigneous rocks from the North Qaidam orogen were 750―800 Ma[5,12], very similar to those ages from the Dabie-Sulu orogenic belt, though timing of the ultrahigh-pressure metamorphism is respectively Caledonian for the North Qaidam orogenic belt and Indosinian for the DabieSulu orogenic belt. The Niuxinshan gneissic granite and the Leigongshan gneissic tonalite bearing the signatures of the middle Neoproterozoic magmatism reported in the present study are also the low-grade, deformed intrusive rocks. When the South-central Qilian craton subducted beneath the Alax (North China) craton during the Caledonian orogeny, a passive continental margin accretionary wedge could have also formed in a manner like that in the Dabie-Sulu orogenic belt[37]. Then, could the Niuxinshan and the Leigongshan tecChinese Science Bulletin
Vol. 51 No. 8
April 2006
ARTICLES tonic masses possibly be the scrapped pieces of the South-central Qilian craton in the North Qilian orogenic belt? If so, to study the geological significance of those low-grade metaigneous rocks in the Qiling and Qilian orogen and a possible existence of the Caledonian version of passive continental margin accretionary wedge in the North Qilian orogen, the model for development of the passive continental margin accretionary wedge in the Dabie-Sulu orogenic belt could perhaps be adopted. 5
4 Song S G, Zhang L F, Niu Y L, et al. Geochronology of diamond-bearing zircons from garnet peridotite in the North Qaidam UHPM belt, Northern Tibetan Plateau: A record of complex histories from oceanic lithosphere subduction to continental collision. Earth and Planetary Science Letters, 2005, 234: 99―118 5 Zhang J X, Yang J S, Mattinson C G, et al. Two contrasting eclogite cooling histories, North Qaidam HP/UHP terrane, western China: Petrological and isotope constraints. Lithos, 2005, 84: 51―76 6 Chavagnac V, Jahn B M. Coesite-bearing eclogites from the Bixiling Complex, Dabie Mountains, China: Sm-Nd ages, geochemical characteristics and tectonic implications. Chem Geol, 1996, 133:
Conclusions
29―51
SHRIMP zircon U-Pb dating for gneissic granitoids in the Qilian orogenic belt suggests two important points with respect to the protolith origin and geotectonic evolution. (1) An episode of Neoproterozoic (~775 Ma) magmatism clearly occurred in the North Qilian orogen and was closely related to the magmatism associated with breakup of the supercontinent Rodinia. (2) The ~775 Ma magmatism reported in the present study and several 750―800 Ma magmatic activities reported by the others from the South-central Qilian craton and the North Qaidam orogen in West China imply that the precursor tectonics of the Qilian orogenic belt may be related to breakup of the supercontinent Rodinia. Acknowledgements The authors are very grateful to Profs. Zheng Yongfei and Dr. Li Xianhua as well as an anonymous reviewer for their constructive comments. Thanks are extended to Dr. Wang Yanbin for his guidance and help in the preparation of the manuscript. Special thanks are due to Profs. Zuo Guochao and Wu Hanquan for their help in the field work and to Zhang Yuhai and Tao Hua in the SHRIMP work. This work was supported by the Chinese Development Fund and the National Science Council (Grant Nos. 89-2116-M006-03, 91-2116-M-006-16, and 92-2116-M-006-010).
7 Rowley D B, Xue F, Tunker R D, et al. Ages of ultrahigh pressure metamorphism and protolith orthogneisses from the Central Dabie Shan: U/Pb zircon geochronology. Earth Planet Sci Lett, 1997, 151: 191―203 8 Zheng Y F, Fu B, Gong B, et al. Stable isotope geochemistry of ultrahigh pressure metamorphic rocks from the Dabie-Sulu orogen in China:
implications
for
geodynamics
and
fluid
regime.
Earth-Science Reviews, 2003, 62: 105―161 9 Zheng Y F, Wu Y B, Chen F K, et al. Zircon U-Pb and oxygen isotope evidence for a large-scale 18O depletion event in igneous rocks during the Neoproterozoic. Geochimica et Cosmochimica Acta, 2004, 68: 4145―4165 10 Wan Y S, Li R W, Wilde S A, et al. UHP metamorphism and exhumation of the Dabie Orogen, China: Evidence from SHRIMP dating of zircon and monazite from a UHP granitic gneiss cobble from the Hefei Basin. Geochimica et Cosmochimica Acta, 2005, 69: 4333―4348 11 Li X H, Su L, Song B, et al. SHRIMP U-Pb zircon age of the Jinchuan ultramafic intrusion and its geological significance. Chin Sci Bull, 2004, 49: 420―422 12 Yang J S, Zhang J X, Meng F C, et al. Ultrahigh pressure eclogites of the north Qaidam and Altun Mounains, NW China and their protoliths. Earth Science Frontiers (in Chinese with English abstract), 2003, 10: 291―314 13 Xu Z Q, Xu H F, Zhang J X, et al. The Zoulang Nanshan caledonian subduction complex in the Northern Qilian Mountains and its
References
dynamics. Acta Geologica Sinica (in Chinese with English abstract),
1 Yang J S, Xu Z Q, Dobrzhinetskaya L F, et al. Discovery of meta-
1994, 68: 225―241
morphic diamonds in central China: an indication of a > 4000-km-
14 Zhang Q, Chen Y, Zhou D J, et al. Geochemical characteristics and
long zone of deep subduction resulting from multiple continental
genesis of Dachadaban ophiolite in North Qilian area. Sci China
collisions. Terra Nova, 2003, 15: 370―379
Ser D-Earth Sci,1998, 41: 277―281
2 Yang J S, Xu Z Q, Zhang J X, et al. Early Palaeozoic North Qai-
15 Xia L Q, Xia Z C, Xu X Y. Magma genesis in the Ordovician
dam UHP metamorphic belt on the north-eastern Tibetan plateau
backarc basins of the Northern Qilian Mountains, China. Geologi-
and a paired subduction model. Terra Nova, 2002, 14: 397―404
cal Society of America Bulletin, 2003, 115: 1510―1522
3 Song S G, Yang J S, Liou J G, et al. Petrology, geochemistry and
16 Wang C Y, Zhang Q, Qian Q, et al. Geochemistry of the early Pa-
isotopic ages of eclogites from the Dulan UHPM terrane, the North
leozoic Baiyin volcanic rocks (NW China): Implications for the
Qaidam, NW China. Lithos, 2003, 70: 195―211
tectonic evolution of the North Qilian orogenic belt. The Journal of
www.scichina.com
www.springerlink.com
969
ARTICLES Geology, 2005, 113, 83―94 17 Zuo G C, Liu Y K, Zhang C. Tectono-stratigraphic characteristics of continent crustal remnants in central-western sector of the North Qilian orogen. Chinese Journal of Geology (in Chinese with English abstract), 2002, 37: 302―312 18 Feng Y M. Allochthones along west section of the North Qilian orogenic belt. Geological Review (in Chinese with English abstract), 1998, 44(4): 365―371 19 Lin T C. Geochemical and geochronological studies of the basement in central-western section of the North Qilian orogen. Master Thesis, National Cheng Kung University, Taiwan , 2003, 131 20 Cheng C K. Petrological and geochemical studies of the back-arc basinal granitoids——as exemplified by the plutonic rocks from Laohushan area, east section of the north Qilian fold belt. Master Thesis, National Taiwan University, Taiwan, 1999, 95 21 Willians I S. U-Th-Pb geochronology by ion microprobe. Rev Econ Geol, 1998, 7: 1―35
29 Hoffman P F. Did the breakup of Laurentia turn Gondwanalnad inside-out? Science, 1991, 252: 1409―1412 30 Moores E M. Southwest U.S.-East Antarctic (SWEAT) connection: a hypothesis. Geology, 1991, 19: 425―428 31 Park L K, Buchan K L, Harlan S S. A proposed giant radiating dyke swarm fragmented by the separation of Laurentia and Australia based on Paleomagnetism of ca. 780 Ma mafic intrusions in western North America. Earth Planet Sci Lett, 1995, 132: 129―139 32 Li Z X, Zhang L, Powell C M. South China in Rodinia: part of the missing link between Australia-East Antarctica and Laurentia? Geology, 1995, 23: 407―410 33 Li Z X, Zhang L, Powell C M. Positions of the East Asian Cratons in the Neoproterozoic supercontinent Rodinia. Aust J Earth Sci, 1996, 43: 593―604 34 Li Z X, Li X H, Kinny P D, et al. Geochronology of Neoproterozoic syn-rift magmatism in the Yangtze Craton, South China and
22 Black L P, Kamo S L, Allen C M, et al. TEMORA 1: A new zircon
correlations with other continents: evidence for a mantle super-
standard for Phanerozoic U-Pb geochronology. Chem Geol, 2003,
plume that broke up Rodinia. Precambrian Research, 2003, 122:
200: 155―170
85―109
23 Ludwig K R. Isoplot/Ex version 2.4. A geochronological toolkit for
35 Su J P, Hu N G, Zhang H F, et al. Single-zircon U-Pb dating and
Microsoft Excel. Berkeley Geochron Centre Spec Publ, 2000, 1―
geological significance of the Diaodaban granitic gneiss in the
56
western segment of North Qilian Mountains. Geological Science
24 Shi R D, Yang J S, Wu C L, et al. First SHRIMP dating for the formation of Yushigou ophiolite in late Sinian, North Qilian Mountains. Acta Geologica Sinica (in Chinese with English abstract), 2004, 78(5): 649―657 25 Wan Y S, Xu Z Q, Yang J S, et al. Ages and compositions of the Precambrian high-grade basement of the Qilian terrane and adjacent areas. Acta Geologica Sinica, 2001, 75(4): 375―384 26 Zhang J X, Wan Y S, Meng F C, et al. Geochemistry, Sm-Nd and U-Pb isotope study of gneisses (schists) enclosing eclogites in the North Qilian Mountains-deeply subducted Precambrian metamorphic basement? Acta Petrologica Sinica (in Chinese with English abstract), 2003, 19(3): 443―451 27 Lindsay J F, Korsch R J, Wilford J R. Timing the breakup of a Neoproterozoic supercontinent: Evidence from Australian intracratonic basins. Geology, 1987, 15: 1061―1064
and Technology Information (in Chinese with English abstract), 2004, 23(3): 11―14 36 Song S G, Zhang L F, Niu Y L, et al. Zircon U-Pb SHRIMP ages of eclogites from the North Qilian Mountains in NW China and their tectonic implication. Chin Sci Bull, 2004, 49: 848―852 37 Zheng Y F, Zhou J B, Wu Y B, et al. Low-grade metamorphic rocks in the Dabie-Sulu orogenic belt: A passive-margin accretionary wedge deformed during continent subuction. International Geology Review, 2005, 47: 851―871 38 Wu Y B, Zheng Y F, Zhou J B. Neoproterozoic granitoid in northwest Sulu and its bearing on the North China-South China Blocks boundary in east China. Geophysical Research Letters, 2004, 31, L07616, doi: 10.1029/2004GL019785 39 Ratschbacher L, Hacker B R, Calvert A, et al. Tectonic of the Qinling (central China): Tectonostratigraphy, geochronology, and
28 Meert J G. Torsvik T H. The making and unmaking of a supercontinent: Rodinia revisited. Tectonophysics, 2003, 375 (1-4): 261― 288
970
deformation history. Tectonophysics, 2003, 366: 1―53 (Received November 8, 2005; accepted January 24, 2006)
Chinese Science Bulletin
Vol. 51 No. 8
April 2006
