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Granulite xenoliths from Cenozoic basalts in SE China provide geochemical fingerprints to distinguish lower crust terranes from the North and South China tectonic blocks: comment
Lithos 73 (2004) 127 – 134 www.elsevier.com/locate/lithos
Discussion
Granulite xenoliths from Cenozoic basalts in SE China provide geochemical fingerprints to distinguish lower crust terranes from the North and South China tectonic blocks: comment Kai-Jun Zhang Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangzhou 510640, China Received 6 June 2003; accepted 6 October 2003
Abstract A careful examination of the geochemical data set for SE China granulite xenoliths in Cenozoic basalts shows differences between the magmatic and cumulate granulite xenoliths, but no distinction between the Nushan and the other South China magmatic granulite xenoliths. Nushan granulite xenoliths with Archean Nd model ages were most likely derived from the Archean basement of the Yangtze craton itself and overprinted by a Paleoproterozoic to Mesoproterozoic tectonothermal event that occurred in the South China block, including the northern margin of the Yangtze craton. The granulate xenoliths therefore cannot be used to distinguish the North China and South China lower crust. Further, the discovery of the UHP eclogite xenoliths west of the Tanlu fault zone, along with recent paleomagnetic, seismic profiling, and other geochemical studies, favors a deepseated, Tibetan-type, continental subduction of the Yangtze craton beneath North China along the Tanlu belt. D 2004 Elsevier B.V. All rights reserved. Keywords: Tanlu fault zone; North China block; South China block; Ultrahigh-pressure metamorphic rocks; Granulites; Lower crust; Tectonic settings
1. Introduction The North China –South China collision during early Mesozoic time formed the Dabie –Sulu ultrahigh-pressure (UHP) metamorphic belt in eastern China (Fig. 1), the largest UHP metamorphic belt on Earth. However, no consensus exists about the North and South China collision. Three main opin-
E-mail address: [email protected] (K.-J. Zhang). 0024-4937/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2003.10.005
ions about the North and South China plate boundary east of the Tanlu fault zone as well as the nature of this fault zone, based upon three sharply contrasting tectonic models, deal with the North and South China collision. Yin and Nie (1993) suggested that the North China block is separated from the South China block by the Tanlu –Sulu –Imjinging –Honam zones. Following this divide, furthermore, Li (1994) advocated a subsurface suture extending eastward approximately from Nanjing. In contrast, in earlier works (Zhang, 1997, 1999, 2000, 2002), I proposed that the
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Fig. 1. Sketch tectonic map of eastern China, revised after Zhang (2002). Note that NNE predominantly trending early Mesozoic contractional deformation in both the North and South China blocks marks a westward convergence and continental subduction between the North and South China blocks. Locations for UHP eclogite xenoliths west of the Tanlu fault zone (Wang et al., 2002; Xu et al., 2002): x = Xuzhou (northern Jiangsu Province), s = Suzhou (northern Anhui Province). Locations for dating in the northern margin of the Yangtze craton: 1 = Kongling (northern Hubei Province), where there is >3.2-Ga basement in an upper amphibolite to granulite terrane, and both the trondhjemites and metapelites contain f 1.9-Ga zircons, consistent with the intrusive age of the f 1.9-Ga Quanqitang granite (SHRIMP II U – Pb zircon; Qiu et al., 2000); 2 = Anqing (central Anhui Province), where the Dongling amphibolite – facies gneiss has been dated to be 1895 F 72 Ma (Sm – Nd isochron; Xing et al., 1993); 3 = Yueshan (central Anhui Province), where there are continental materials of 3330 F 180 Ma age in the granitoids (U – Pb zircon; Zhang et al., 1990); 4 = Zhangbaling (eastern Anhui Province), where there is 2493 F 19-Ma amphibolite – facies TTG gneiss (U – Pb zircon; Tu et al., 2001); and 5 = Longwangshan (ca. 50 km south of Nanjing city), where there are materials of 2403 – 2621 and 3232 Ma ages in the volcanic rocks (SHRIMP II U – Pb zircon; Zhang et al., 2003). N denotes the approximate location of the Nushan granulite xenoliths studied by Yu et al. (2003). The possible North – South China subsurface boundary is drawn in combination with the UHP eclogite xenoliths (Wang et al., 2002; Xu et al., 2002) and the Cenozoic basalts geochemical data (e.g., Cong et al., 2001), in the eastern part of the North China block west of the Tanlu fault zone. The locations for the diamond-bearing kimberlites of Early Paleozoic age in North China are after Lu et al. (1995) and Feng et al. (2000).
North and South China blocks are divided by the Tanlu – Sulu – Imjingang zones, and the whole of South Korea tectonically belongs to the South China block (Fig. 1). Recently, Yu et al. (2003) introduced a data set documenting the geochemistry and timing of granulite xenoliths from Cenozoic basalts in SE China. They are used to define the nature of the lower crust beneath
these regions and to define types of lithosphere. Yu et al. proposed a close affinity of the lower crust in an area near the Tanlu fault zone (Nushan) with events and deep-seated crustal rock types to other domains in the North China block and strong difference from those in the South China block. This supported Li’s hypothesis of a subsurface suture extending eastward approximately from Nanjing. However, a careful
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examination of their geochemical data set shows differences between the magmatic and cumulate granulite xenoliths, but no distinction between the Nushan and the other South China magmatic granulite xenoliths. The granulate xenoliths, therefore, cannot be used to distinguish the North China and South China lower crust, and thus, their conclusions are misleading. Moreover, recent discovery of UHP eclogite xenoliths from Late Mesozoic intrusions west of the Tanlu fault zone (Fig. 1; Xu et al., 2002; Wang et al., 2002) seriously undermines Li’s hypothesis that demands that the Sulu UHP terrane is superimposed on the North China lower crust.
2. No geochemical fingerprints of granulite xenoliths from SE China Cenozoic basalts provided to distinguish lower crust terranes from the North and South China blocks As Yu et al. (2003) pointed out, (1) the South China granulite xenoliths exhibit similar transition element relationships to the Nushan granulite xenoliths (lines 13 and 14, right column, p. 88); (2) the Nushan (magmatic) granulite xenoliths and the other South China magmatic granulite xenoliths have similar incompatible element (including rare-earth element) patterns (lines 32– 34, right column, p. 88; lines 28 –30, right column; p. 91); and (3) the differences in the patterns reinforce the distinction between the magmatic and cumulate granulite xenolith types (lines 12 –14, left column, p. 89) (Yu et al., 2003, Figs. 3 and 5 and Table 3). Revising their Fig. 2 according to their Table 3, with magmatic and cumulate granulite xenolith types clearly marked, the Nushan granulite xenoliths again cannot be distinguished from the other South China magmatic granulite xenoliths in major Fig. 2. Mg/(Mg + Fetotal) vs. major element variation diagrams of the mafic granulite xenoliths from SE China. Revised from Fig. 2 of Yu et al. (2003) in accordance with their Table 3, with magmatic and cumulate granulite xenolith types clearly marked. The sign ‘‘x’’ is added to some samples to represent cumulate granulite xenoliths, and the dashed lines with arrow are used to represent compositional differences between the magmatic and cumulate granulite xenolith types. All other elements are after Yu et al. (2003). These differences are clear, but the Nushan granulite xenoliths cannot be distinguished from the other South China magmatic granulite xenoliths in the diagrams.
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elements. Only the differences between the magmatic and cumulate granulite xenolith types are clearly demonstrated (Fig. 2). A similar conclusion can again easily be reached by revision of their Fig. 4 in accordance with their Table 3 for Mg/(Mg + Fetotal) vs. some incompatible element studies (Fig. 3). This
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Fig. 3. Mg/(Mg + Fetotal) vs. some incompatible element variation diagrams of the mafic granulite xenoliths from SE China. Revised from Fig. 4 of Yu et al. (2003) in accordance with their Table 3, with magmatic and cumulate granulite xenolith types clearly marked. The sign ‘‘x’’ is added to some samples to represent cumulate granulite xenoliths, and the dashed lines with arrow are used to represent compositional differences between the magmatic and cumulate granulite xenolith types. All other elements are after Yu et al. (2003). The same conclusion as in Fig. 2 can be drawn: only clear are the differences between the magmatic and cumulate granulite xenolith types, but the Nushan granulite xenoliths cannot be distinguished from the other South China magmatic granulite xenoliths in the diagrams.
conclusion is also true for the Sr –Nd isotopic studies. In a revision of their Fig. 6a in accordance with their Table 4 (Fig. 4), the cumulate granulite xenoliths fall in the MORB field, three South China magmatic granulite xenoliths are very near two Nushan samples and one South China magmatic granulite xenolith plots in the northern Dabieshan Mountains field, which is generally considered to be part of the South China block (e.g., Hacker et al., 1998). All of the abovementioned facts by Yu et al. provide no evidence that the Nushan granulite xenoliths can be geochemically distinguished from the other South China granulite xenoliths, much less lower crust terranes from the North and South China tectonic blocks as suggested by Yu et al. In contrast, I believe that there is no distinction between the Nushan and the other South China magmatic granulite xenoliths,
but the differences between the magmatic and cumulate granulite xenoliths are clear. Another key issue that Yu et al. use to support their view is that two Nushan samples have Archean Nd model ages of 3.1 – 2.8 Ga and underwent a possible Rb depletion event of roughly defined 1.8 –1.6 Ga. They believed that no Archean basement has been found in the South China block (first paragraph, right column, p. 98). This is not true. Take the isotopic dating studies in the northern margin of the Yangtze craton as examples. Qiu et al. (2000) reported the presence of >3.2 Ga basement in the Kongling upper amphibolite to granulite terrane on the northwest of the Dongting basin based on SHRIMP II U – Pb zircon analyses (Fig. 1, location 1). There are materials of 3330 F 180-Ma age in the granitoids in the Anqing area (Fig. 1, location 2) (U –Pb zircon, Zhang et al.,
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Fig. 4. Sr and Nd isotopic ratios of granulite xenoliths from SE China. Revised from Fig. 6a of Yu et al. (2003) in accordance with their Table 4, with magmatic and cumulate granulite xenolith types clearly marked. The sign ‘‘x’’ is added to some samples to represent cumulate granulite xenoliths. All other elements are after Yu et al. (2003). The cumulate granulite xenoliths fall in the MORB field, and three South China magmatic granulite xenoliths are very near two Nushan samples, one South China magmatic granulite xenoliths in the northern Dabieshan Mountains field, which is generally considered to be part of the South China block (e.g., Hacker et al., 1998). The differences between the magmatic and cumulate granulite xenolith types are clear, but the Nushan granulite xenoliths cannot be distinguished from the other South China magmatic granulite xenoliths.
1990), 2493 F 19-Ma amphibolite – facies TTG gneiss in the Zhangbaling area (Fig. 1, location 3) (U – Pb zircon, Tu et al., 2001). Both locations are just east of the Tanlu fault zone (Fig. 1). In the volcanic rocks ca. 50 km south of Nanjing city (Fig. 1, location 5), materials of 2403 – 2621 and 3232 Ma were also identified by SHRIMP II U – Pb zircon analyses (Zhang et al., 2003). Moreover, Gan et al. (1996), through U – Pb dating of 31 zircon grains, proposed that there are extensive Archean materials in Fujian, Guangxi, Hubei, Hunan, Jiangxi, and Zhejiang provinces of southern China (not shown in Fig. 1), with an oldest age of 3.1 Ga. Even in the western Qinling orogen, a granite that intruded the Yudongzi – Gelaoling metamorphic block (ca. 106j10 –15V, 33j16 – 18V, not shown in Fig. 1) is dated at 2688 F 9 Ma (U – Pb zircon), and the latter has Archean Nd model age of 3.0 Ga and similar Nd isotopic characteristics to the Kongling Group (Zhang et al., 2001). A Late Paleo-
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proterozoic to Mesoproterozoic tectonothermal event could have been widespread in South China, including the northern margin of the Yangtze craton. This is illustrated by a cluster of 1.7 –2.2 Ga of U –Pb zircon dating in South China (Gan et al., 1996, and references therein). Further, in the Kongling terrane (Fig. 1, location 1), both the trondhjemites and metapelites contain f 1.9-Ga zircons, consistent with the intrusive age of the f 1.9-Ga Quanqitang granite (Qiu et al., 2000), and in the Anqing area (location 2), the Dongling gneiss has been dated to be 1895 F 72 Ma (Sm –Nd isochron; Xing et al., 1993). Therefore, I conclude that, if only the aging data is considered, no necessity exists that the Nushan granulite xenoliths with Archean Nd model ages represent the North China basement as proposed by Yu et al. (2003), a more simple and reasonable interpretation is that they were just derived from the Archean Yangtze craton itself. The Rb depletion event documented in the Nushan samples should likewise not be correlated with the Paleoproterozoic to Mesoproterozoic amalgamation event within the North China block (Yu et al., 2003). Instead, it is better to relate this observation to a Paleoproterozoic to Mesoproterozoic tectonothermal event in (the northern margin of) the South China block.
3. Ultrahigh-pressure eclogite xenoliths from Late Mesozoic intrusions west of the Tanlu fault zone Notably, at least four monzodioritic porphyry complexes of Late Mesozoic age (130 F 2 Ma, SHRIMP dating of zircon. Xu Wenliang, 2003, personal communications) have recently been found to contain rich UHP eclogite xenoliths (inclusions). These monzodioritic porphyry complexes are located in northern Jiangsu Province (Liguo and Banjing monzodioritic porphyry complexes, within Xuzhou city) and northern Anhui Province (Caishan and Fengshan monzodioritic porphyry complexes, within Suzhou city) (Xu et al., 2002; Wang et al., 2002; Xu Wenliang, 2003, personal communications), approximately 120 –140 km west of the Tanlu fault zone (Fig. 1). This discovery favors a deep-seated continental subduction of the Yangtze craton along the Tanlu belt (Xu et al., 2002; Wang et al., 2002) over the model of Li (1994) that demands the Sulu UHP terrain to be exotic.
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Therefore, the discovery is a key to constraining the geometry and kinematics of the North China –South China collision, and I here take space to reiterate the observations mainly by Xu et al. (2002) and Wang et al. (2002). The xenoliths include eclogite and garnet pyroxenite. The former consists mainly of garnet, omphacite, quartz, and rutile (with titanite), with retrogressive amphiboles co-occurring with clinopyroxene and garnet. The latter is commonly composed of garnet + clinopyroxene + rutile, often with an amphibolite facies retrogressive metamorphic mineral assemblage (amphibole + plagioclase + quartz). The garnets are composed of pyrope (23 – 45 mol%), almandine (34 – 58 mol%), and grossular (15 – 22 mol%), with lesser amount of andradite (9 mol%) and spessartine (2 mol%). The clinopyroxenes in eclogites are omphacite (20 – 29 mol% jadeite), while those in garnet pyroxenite are augite varieties (2– 16 mol% jadeite). The amphiboles in eclogites are calcium amphiboles (pargasite or ferropargasite varieties rich in Al, Na and Mg), but those in garnet pyroxenites are edenitic hornblendes (rich in Fe and Si). The plagioclases in garnet pyroxenites are andesine and oligoclase, which constitute the symplectite with retrogressive metamorphic amphiboles together. Spectacular microstructures of abundant oriented quartz rods were found to be within clinopyroxenes (Wang et al., 2002), which implies preexistence of silica – oversaturated omphacite stable at ultrahighpressure conditions ( z 2.5 GPa) (Katayama et al., 2000; Schmadicke and Muller, 2000; Wang et al., 2002). The jadeite content in the omphacite also shows a >1.2 – 1.5-GPa pressure of the formation of eclogite. This and the co-occurrence of garnet core and clinopyroxene suggest eclogite facies metamorphic temperatures in the range of 709 –861 jC. The metamorphic temperatures of garnet pyroxenite in eclogite facies is estimated to be between 601 and 646 jC. Based on the Al barometry of hornblende and co-occurrence of garnet and amphibole, the amphibolite facies retrogressive metamorphic P –T conditions of eclogite and garnet pyroxenite were 0.83– 1.03 GPa, 679 – 738 jC and 0.70 –0.73 GPa, 666 – 671 jC, respectively. Thus, the eclogite is believed to have undergone UHP eclogite facies and subsequent (retrogressive) amphibolite facies metamorphism.
The Sm – Nd isochron age of whole rock and garnet in the eclogites is 219.4 Ma, which is consistent with the SHRIMP U – Pb zircon analyses of the eclogites (217 Ma), and can be compatible with the peak metamorphic age in the Dabie –Sulu UHP metamorphic belt (Xu et al., 2002; Xu Wenliang, 2003, personal communications). The eclogites and garnet pyroxenites in the region have the same Sm – Nd isotopic characteristics as those in the Dabie– Sulu UHP metamorphic belt (Xu et al., 2002). Therefore, the eclogite xenoliths in the Xuzhou –Suzhou region west of the Tanlu fault zone clearly are the equivalent of the Dabie – Sulu UHP eclogites (Xu et al., 2002; Wang et al., 2002).
4. Concluding remarks A careful examination of the geochemical data set for SE China granulite xenoliths in Cenozoic basalts shows differences between the magmatic and cumulate granulite xenoliths, but no distinction between the Nushan and the other South China magmatic granulite xenoliths. Nushan granulite xenoliths with Archean Nd model ages were most likely derived from the Archean basement of the Yangtze craton itself and overprinted by a Paleoproterozoic to Mesoproterozoic tectonothermal event that occurred in the South China block, including the northern margin of the Yangtze craton. The granulate xenoliths therefore cannot be used to distinguish the North China and South China lower crust, and thus, the conclusions by Yu et al. (2003) are incorrect and misleading, although their geochemical data set is excellent. Further, the discovery of the UHP eclogite xenoliths west of the Tanlu fault zone clearly discounts the crustal detachment model of Li (1994) that advocates a North China-type lower crust in the Subei region east of the Tanlu fault zone (Fig. 1) and demands a rootless Sulu UHP belt. A few other studies have also used the geochemical characteristics of the Cenozoic basalts in eastern China to decipher the geometry and kinematics of the North – South China collision (e.g., Zhang, 2000; Cong et al. 2001, and references therein). This is inspiring, but known important geologic, geochemical and geophysical facts should be taken to constrain any such attempt, because the geochemical data is still
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limited and can be multi-answered. Detailed studies of voluminous Cenozoic basalts from the boreholes for gas and oil in eastern China likewise indicate that the Cenozoic basalts from the Subei basin (Fig. 1) have prevalent EM2-type Sr – Nd – Pb isotopic signature (Cong et al., 2001), again proving this area cannot be structurally affiliated with the North China block, where the Cenozoic basalts were believed to have EM1-type isotopic signature (Zhang, 2000, and references therein). The UHP eclogite xenoliths west of the Tanlu fault zone indicate the Yangtze craton was underthrust westwards along the Tanlu fault zone (Xu et al., 2002), as proposed by Zhang (1997, 1999, 2000, 2002), which was also supported by recent paleomagnetic (e.g., Gilder et al., 1999) and seismic profiling (e.g., Xu et al., 2001; Yang, 2002) studies. A Tibetan-type continental subduction of the Yangtze craton could have perhaps developed beneath the North China block along the Tanlu fault zone. The important but difficult task ahead is to determine how far the Yangtze craton was subducted beneath the North China block, because the intensive lithospheric delamination of the North China block during Late Mesozoic time (e.g., Gao et al., 2002) could have destroyed most of the subducted continental lithosphere.
Acknowledgements I thank Dr. Xu Wenliang of Jilin University for helpful discussion and the kind offer of useful references. This work was supported by the CAS Hundred Talents Project.
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Discussion
Granulite xenoliths from Cenozoic basalts in SE China provide geochemical fingerprints to distinguish lower crust terranes from the North and South China tectonic blocks: comment Kai-Jun Zhang Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangzhou 510640, China Received 6 June 2003; accepted 6 October 2003
Abstract A careful examination of the geochemical data set for SE China granulite xenoliths in Cenozoic basalts shows differences between the magmatic and cumulate granulite xenoliths, but no distinction between the Nushan and the other South China magmatic granulite xenoliths. Nushan granulite xenoliths with Archean Nd model ages were most likely derived from the Archean basement of the Yangtze craton itself and overprinted by a Paleoproterozoic to Mesoproterozoic tectonothermal event that occurred in the South China block, including the northern margin of the Yangtze craton. The granulate xenoliths therefore cannot be used to distinguish the North China and South China lower crust. Further, the discovery of the UHP eclogite xenoliths west of the Tanlu fault zone, along with recent paleomagnetic, seismic profiling, and other geochemical studies, favors a deepseated, Tibetan-type, continental subduction of the Yangtze craton beneath North China along the Tanlu belt. D 2004 Elsevier B.V. All rights reserved. Keywords: Tanlu fault zone; North China block; South China block; Ultrahigh-pressure metamorphic rocks; Granulites; Lower crust; Tectonic settings
1. Introduction The North China –South China collision during early Mesozoic time formed the Dabie –Sulu ultrahigh-pressure (UHP) metamorphic belt in eastern China (Fig. 1), the largest UHP metamorphic belt on Earth. However, no consensus exists about the North and South China collision. Three main opin-
E-mail address: [email protected] (K.-J. Zhang). 0024-4937/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2003.10.005
ions about the North and South China plate boundary east of the Tanlu fault zone as well as the nature of this fault zone, based upon three sharply contrasting tectonic models, deal with the North and South China collision. Yin and Nie (1993) suggested that the North China block is separated from the South China block by the Tanlu –Sulu –Imjinging –Honam zones. Following this divide, furthermore, Li (1994) advocated a subsurface suture extending eastward approximately from Nanjing. In contrast, in earlier works (Zhang, 1997, 1999, 2000, 2002), I proposed that the
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Fig. 1. Sketch tectonic map of eastern China, revised after Zhang (2002). Note that NNE predominantly trending early Mesozoic contractional deformation in both the North and South China blocks marks a westward convergence and continental subduction between the North and South China blocks. Locations for UHP eclogite xenoliths west of the Tanlu fault zone (Wang et al., 2002; Xu et al., 2002): x = Xuzhou (northern Jiangsu Province), s = Suzhou (northern Anhui Province). Locations for dating in the northern margin of the Yangtze craton: 1 = Kongling (northern Hubei Province), where there is >3.2-Ga basement in an upper amphibolite to granulite terrane, and both the trondhjemites and metapelites contain f 1.9-Ga zircons, consistent with the intrusive age of the f 1.9-Ga Quanqitang granite (SHRIMP II U – Pb zircon; Qiu et al., 2000); 2 = Anqing (central Anhui Province), where the Dongling amphibolite – facies gneiss has been dated to be 1895 F 72 Ma (Sm – Nd isochron; Xing et al., 1993); 3 = Yueshan (central Anhui Province), where there are continental materials of 3330 F 180 Ma age in the granitoids (U – Pb zircon; Zhang et al., 1990); 4 = Zhangbaling (eastern Anhui Province), where there is 2493 F 19-Ma amphibolite – facies TTG gneiss (U – Pb zircon; Tu et al., 2001); and 5 = Longwangshan (ca. 50 km south of Nanjing city), where there are materials of 2403 – 2621 and 3232 Ma ages in the volcanic rocks (SHRIMP II U – Pb zircon; Zhang et al., 2003). N denotes the approximate location of the Nushan granulite xenoliths studied by Yu et al. (2003). The possible North – South China subsurface boundary is drawn in combination with the UHP eclogite xenoliths (Wang et al., 2002; Xu et al., 2002) and the Cenozoic basalts geochemical data (e.g., Cong et al., 2001), in the eastern part of the North China block west of the Tanlu fault zone. The locations for the diamond-bearing kimberlites of Early Paleozoic age in North China are after Lu et al. (1995) and Feng et al. (2000).
North and South China blocks are divided by the Tanlu – Sulu – Imjingang zones, and the whole of South Korea tectonically belongs to the South China block (Fig. 1). Recently, Yu et al. (2003) introduced a data set documenting the geochemistry and timing of granulite xenoliths from Cenozoic basalts in SE China. They are used to define the nature of the lower crust beneath
these regions and to define types of lithosphere. Yu et al. proposed a close affinity of the lower crust in an area near the Tanlu fault zone (Nushan) with events and deep-seated crustal rock types to other domains in the North China block and strong difference from those in the South China block. This supported Li’s hypothesis of a subsurface suture extending eastward approximately from Nanjing. However, a careful
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examination of their geochemical data set shows differences between the magmatic and cumulate granulite xenoliths, but no distinction between the Nushan and the other South China magmatic granulite xenoliths. The granulate xenoliths, therefore, cannot be used to distinguish the North China and South China lower crust, and thus, their conclusions are misleading. Moreover, recent discovery of UHP eclogite xenoliths from Late Mesozoic intrusions west of the Tanlu fault zone (Fig. 1; Xu et al., 2002; Wang et al., 2002) seriously undermines Li’s hypothesis that demands that the Sulu UHP terrane is superimposed on the North China lower crust.
2. No geochemical fingerprints of granulite xenoliths from SE China Cenozoic basalts provided to distinguish lower crust terranes from the North and South China blocks As Yu et al. (2003) pointed out, (1) the South China granulite xenoliths exhibit similar transition element relationships to the Nushan granulite xenoliths (lines 13 and 14, right column, p. 88); (2) the Nushan (magmatic) granulite xenoliths and the other South China magmatic granulite xenoliths have similar incompatible element (including rare-earth element) patterns (lines 32– 34, right column, p. 88; lines 28 –30, right column; p. 91); and (3) the differences in the patterns reinforce the distinction between the magmatic and cumulate granulite xenolith types (lines 12 –14, left column, p. 89) (Yu et al., 2003, Figs. 3 and 5 and Table 3). Revising their Fig. 2 according to their Table 3, with magmatic and cumulate granulite xenolith types clearly marked, the Nushan granulite xenoliths again cannot be distinguished from the other South China magmatic granulite xenoliths in major Fig. 2. Mg/(Mg + Fetotal) vs. major element variation diagrams of the mafic granulite xenoliths from SE China. Revised from Fig. 2 of Yu et al. (2003) in accordance with their Table 3, with magmatic and cumulate granulite xenolith types clearly marked. The sign ‘‘x’’ is added to some samples to represent cumulate granulite xenoliths, and the dashed lines with arrow are used to represent compositional differences between the magmatic and cumulate granulite xenolith types. All other elements are after Yu et al. (2003). These differences are clear, but the Nushan granulite xenoliths cannot be distinguished from the other South China magmatic granulite xenoliths in the diagrams.
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elements. Only the differences between the magmatic and cumulate granulite xenolith types are clearly demonstrated (Fig. 2). A similar conclusion can again easily be reached by revision of their Fig. 4 in accordance with their Table 3 for Mg/(Mg + Fetotal) vs. some incompatible element studies (Fig. 3). This
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Fig. 3. Mg/(Mg + Fetotal) vs. some incompatible element variation diagrams of the mafic granulite xenoliths from SE China. Revised from Fig. 4 of Yu et al. (2003) in accordance with their Table 3, with magmatic and cumulate granulite xenolith types clearly marked. The sign ‘‘x’’ is added to some samples to represent cumulate granulite xenoliths, and the dashed lines with arrow are used to represent compositional differences between the magmatic and cumulate granulite xenolith types. All other elements are after Yu et al. (2003). The same conclusion as in Fig. 2 can be drawn: only clear are the differences between the magmatic and cumulate granulite xenolith types, but the Nushan granulite xenoliths cannot be distinguished from the other South China magmatic granulite xenoliths in the diagrams.
conclusion is also true for the Sr –Nd isotopic studies. In a revision of their Fig. 6a in accordance with their Table 4 (Fig. 4), the cumulate granulite xenoliths fall in the MORB field, three South China magmatic granulite xenoliths are very near two Nushan samples and one South China magmatic granulite xenolith plots in the northern Dabieshan Mountains field, which is generally considered to be part of the South China block (e.g., Hacker et al., 1998). All of the abovementioned facts by Yu et al. provide no evidence that the Nushan granulite xenoliths can be geochemically distinguished from the other South China granulite xenoliths, much less lower crust terranes from the North and South China tectonic blocks as suggested by Yu et al. In contrast, I believe that there is no distinction between the Nushan and the other South China magmatic granulite xenoliths,
but the differences between the magmatic and cumulate granulite xenoliths are clear. Another key issue that Yu et al. use to support their view is that two Nushan samples have Archean Nd model ages of 3.1 – 2.8 Ga and underwent a possible Rb depletion event of roughly defined 1.8 –1.6 Ga. They believed that no Archean basement has been found in the South China block (first paragraph, right column, p. 98). This is not true. Take the isotopic dating studies in the northern margin of the Yangtze craton as examples. Qiu et al. (2000) reported the presence of >3.2 Ga basement in the Kongling upper amphibolite to granulite terrane on the northwest of the Dongting basin based on SHRIMP II U – Pb zircon analyses (Fig. 1, location 1). There are materials of 3330 F 180-Ma age in the granitoids in the Anqing area (Fig. 1, location 2) (U –Pb zircon, Zhang et al.,
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Fig. 4. Sr and Nd isotopic ratios of granulite xenoliths from SE China. Revised from Fig. 6a of Yu et al. (2003) in accordance with their Table 4, with magmatic and cumulate granulite xenolith types clearly marked. The sign ‘‘x’’ is added to some samples to represent cumulate granulite xenoliths. All other elements are after Yu et al. (2003). The cumulate granulite xenoliths fall in the MORB field, and three South China magmatic granulite xenoliths are very near two Nushan samples, one South China magmatic granulite xenoliths in the northern Dabieshan Mountains field, which is generally considered to be part of the South China block (e.g., Hacker et al., 1998). The differences between the magmatic and cumulate granulite xenolith types are clear, but the Nushan granulite xenoliths cannot be distinguished from the other South China magmatic granulite xenoliths.
1990), 2493 F 19-Ma amphibolite – facies TTG gneiss in the Zhangbaling area (Fig. 1, location 3) (U – Pb zircon, Tu et al., 2001). Both locations are just east of the Tanlu fault zone (Fig. 1). In the volcanic rocks ca. 50 km south of Nanjing city (Fig. 1, location 5), materials of 2403 – 2621 and 3232 Ma were also identified by SHRIMP II U – Pb zircon analyses (Zhang et al., 2003). Moreover, Gan et al. (1996), through U – Pb dating of 31 zircon grains, proposed that there are extensive Archean materials in Fujian, Guangxi, Hubei, Hunan, Jiangxi, and Zhejiang provinces of southern China (not shown in Fig. 1), with an oldest age of 3.1 Ga. Even in the western Qinling orogen, a granite that intruded the Yudongzi – Gelaoling metamorphic block (ca. 106j10 –15V, 33j16 – 18V, not shown in Fig. 1) is dated at 2688 F 9 Ma (U – Pb zircon), and the latter has Archean Nd model age of 3.0 Ga and similar Nd isotopic characteristics to the Kongling Group (Zhang et al., 2001). A Late Paleo-
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proterozoic to Mesoproterozoic tectonothermal event could have been widespread in South China, including the northern margin of the Yangtze craton. This is illustrated by a cluster of 1.7 –2.2 Ga of U –Pb zircon dating in South China (Gan et al., 1996, and references therein). Further, in the Kongling terrane (Fig. 1, location 1), both the trondhjemites and metapelites contain f 1.9-Ga zircons, consistent with the intrusive age of the f 1.9-Ga Quanqitang granite (Qiu et al., 2000), and in the Anqing area (location 2), the Dongling gneiss has been dated to be 1895 F 72 Ma (Sm –Nd isochron; Xing et al., 1993). Therefore, I conclude that, if only the aging data is considered, no necessity exists that the Nushan granulite xenoliths with Archean Nd model ages represent the North China basement as proposed by Yu et al. (2003), a more simple and reasonable interpretation is that they were just derived from the Archean Yangtze craton itself. The Rb depletion event documented in the Nushan samples should likewise not be correlated with the Paleoproterozoic to Mesoproterozoic amalgamation event within the North China block (Yu et al., 2003). Instead, it is better to relate this observation to a Paleoproterozoic to Mesoproterozoic tectonothermal event in (the northern margin of) the South China block.
3. Ultrahigh-pressure eclogite xenoliths from Late Mesozoic intrusions west of the Tanlu fault zone Notably, at least four monzodioritic porphyry complexes of Late Mesozoic age (130 F 2 Ma, SHRIMP dating of zircon. Xu Wenliang, 2003, personal communications) have recently been found to contain rich UHP eclogite xenoliths (inclusions). These monzodioritic porphyry complexes are located in northern Jiangsu Province (Liguo and Banjing monzodioritic porphyry complexes, within Xuzhou city) and northern Anhui Province (Caishan and Fengshan monzodioritic porphyry complexes, within Suzhou city) (Xu et al., 2002; Wang et al., 2002; Xu Wenliang, 2003, personal communications), approximately 120 –140 km west of the Tanlu fault zone (Fig. 1). This discovery favors a deep-seated continental subduction of the Yangtze craton along the Tanlu belt (Xu et al., 2002; Wang et al., 2002) over the model of Li (1994) that demands the Sulu UHP terrain to be exotic.
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Therefore, the discovery is a key to constraining the geometry and kinematics of the North China –South China collision, and I here take space to reiterate the observations mainly by Xu et al. (2002) and Wang et al. (2002). The xenoliths include eclogite and garnet pyroxenite. The former consists mainly of garnet, omphacite, quartz, and rutile (with titanite), with retrogressive amphiboles co-occurring with clinopyroxene and garnet. The latter is commonly composed of garnet + clinopyroxene + rutile, often with an amphibolite facies retrogressive metamorphic mineral assemblage (amphibole + plagioclase + quartz). The garnets are composed of pyrope (23 – 45 mol%), almandine (34 – 58 mol%), and grossular (15 – 22 mol%), with lesser amount of andradite (9 mol%) and spessartine (2 mol%). The clinopyroxenes in eclogites are omphacite (20 – 29 mol% jadeite), while those in garnet pyroxenite are augite varieties (2– 16 mol% jadeite). The amphiboles in eclogites are calcium amphiboles (pargasite or ferropargasite varieties rich in Al, Na and Mg), but those in garnet pyroxenites are edenitic hornblendes (rich in Fe and Si). The plagioclases in garnet pyroxenites are andesine and oligoclase, which constitute the symplectite with retrogressive metamorphic amphiboles together. Spectacular microstructures of abundant oriented quartz rods were found to be within clinopyroxenes (Wang et al., 2002), which implies preexistence of silica – oversaturated omphacite stable at ultrahighpressure conditions ( z 2.5 GPa) (Katayama et al., 2000; Schmadicke and Muller, 2000; Wang et al., 2002). The jadeite content in the omphacite also shows a >1.2 – 1.5-GPa pressure of the formation of eclogite. This and the co-occurrence of garnet core and clinopyroxene suggest eclogite facies metamorphic temperatures in the range of 709 –861 jC. The metamorphic temperatures of garnet pyroxenite in eclogite facies is estimated to be between 601 and 646 jC. Based on the Al barometry of hornblende and co-occurrence of garnet and amphibole, the amphibolite facies retrogressive metamorphic P –T conditions of eclogite and garnet pyroxenite were 0.83– 1.03 GPa, 679 – 738 jC and 0.70 –0.73 GPa, 666 – 671 jC, respectively. Thus, the eclogite is believed to have undergone UHP eclogite facies and subsequent (retrogressive) amphibolite facies metamorphism.
The Sm – Nd isochron age of whole rock and garnet in the eclogites is 219.4 Ma, which is consistent with the SHRIMP U – Pb zircon analyses of the eclogites (217 Ma), and can be compatible with the peak metamorphic age in the Dabie –Sulu UHP metamorphic belt (Xu et al., 2002; Xu Wenliang, 2003, personal communications). The eclogites and garnet pyroxenites in the region have the same Sm – Nd isotopic characteristics as those in the Dabie– Sulu UHP metamorphic belt (Xu et al., 2002). Therefore, the eclogite xenoliths in the Xuzhou –Suzhou region west of the Tanlu fault zone clearly are the equivalent of the Dabie – Sulu UHP eclogites (Xu et al., 2002; Wang et al., 2002).
4. Concluding remarks A careful examination of the geochemical data set for SE China granulite xenoliths in Cenozoic basalts shows differences between the magmatic and cumulate granulite xenoliths, but no distinction between the Nushan and the other South China magmatic granulite xenoliths. Nushan granulite xenoliths with Archean Nd model ages were most likely derived from the Archean basement of the Yangtze craton itself and overprinted by a Paleoproterozoic to Mesoproterozoic tectonothermal event that occurred in the South China block, including the northern margin of the Yangtze craton. The granulate xenoliths therefore cannot be used to distinguish the North China and South China lower crust, and thus, the conclusions by Yu et al. (2003) are incorrect and misleading, although their geochemical data set is excellent. Further, the discovery of the UHP eclogite xenoliths west of the Tanlu fault zone clearly discounts the crustal detachment model of Li (1994) that advocates a North China-type lower crust in the Subei region east of the Tanlu fault zone (Fig. 1) and demands a rootless Sulu UHP belt. A few other studies have also used the geochemical characteristics of the Cenozoic basalts in eastern China to decipher the geometry and kinematics of the North – South China collision (e.g., Zhang, 2000; Cong et al. 2001, and references therein). This is inspiring, but known important geologic, geochemical and geophysical facts should be taken to constrain any such attempt, because the geochemical data is still
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limited and can be multi-answered. Detailed studies of voluminous Cenozoic basalts from the boreholes for gas and oil in eastern China likewise indicate that the Cenozoic basalts from the Subei basin (Fig. 1) have prevalent EM2-type Sr – Nd – Pb isotopic signature (Cong et al., 2001), again proving this area cannot be structurally affiliated with the North China block, where the Cenozoic basalts were believed to have EM1-type isotopic signature (Zhang, 2000, and references therein). The UHP eclogite xenoliths west of the Tanlu fault zone indicate the Yangtze craton was underthrust westwards along the Tanlu fault zone (Xu et al., 2002), as proposed by Zhang (1997, 1999, 2000, 2002), which was also supported by recent paleomagnetic (e.g., Gilder et al., 1999) and seismic profiling (e.g., Xu et al., 2001; Yang, 2002) studies. A Tibetan-type continental subduction of the Yangtze craton could have perhaps developed beneath the North China block along the Tanlu fault zone. The important but difficult task ahead is to determine how far the Yangtze craton was subducted beneath the North China block, because the intensive lithospheric delamination of the North China block during Late Mesozoic time (e.g., Gao et al., 2002) could have destroyed most of the subducted continental lithosphere.
Acknowledgements I thank Dr. Xu Wenliang of Jilin University for helpful discussion and the kind offer of useful references. This work was supported by the CAS Hundred Talents Project.
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