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The South China piece in the Rodinian puzzle: A reply to the comment by Munteanu and Wilson
Precambrian Research 171 (2009) 77–79
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Reply
The South China piece in the Rodinian puzzle: A reply to the comment by Munteanu and Wilson Z.X. Li a,∗ , X.H. Li b , X.C. Wang b a b
Institute for Geoscience Research, Department of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, Western Australia 6845, Australia State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100027, China
a r t i c l e
i n f o
Article history: Received 4 January 2009 Accepted 26 January 2009 Keywords: Rodinia South China Neoproterozoic Mantle plume Continental rift Arc
a b s t r a c t Uncritical application of geochemical plots for granitic rocks, without full appreciation of the limitations of such plots, the broader geological constraints and information about coexisting other rock types, commonly lead to incorrect tectonic interpretations. We consider this to be the case for the arc interpretations of the 850–740 Ma igneous rocks in South China. In this Reply, we counter-argue all four lines of evidence used by Munteanu and Wilson [Munteanu, M., Wilson, A., this volume]. The South China piece in the Rodinian puzzle. A comment on “Assembly, configuration, and break-up history of Rodinia: a synthesis” by Li et al. (2008) [Precambrian Res. 160 (2008) 179–210]. Precambrian Res.] for an arc model in the western South China Block between 850 and 740 Ma. © 2009 Elsevier B.V. All rights reserved.
We welcome the comment by Munteanu and Wilson (this volume) on Li et al. (2008b). The Li et al. (2008b) paper was a global synthesis on the evolution of the supercontinent Rodinia involving a long list of co-authors from various continents and disciplines, whereas the comment of Munteanu and Wilson (this volume) focuses specifically on the Neoproterozoic geology of the western South China Block and implications for the position of South China within Rodinia. This Reply thus has a different authorship from the original paper to reflect the more regional focus of this discussion. Below, we address the comment of Munteanu and Wilson (this volume) point by point as they appear in their Comment. 1. Igneous petrographic association Munteanu and Wilson (this volume) propose a NS-trending, ca. 900–700 Ma magmatic arc along the western South China Block. They argue that because the magmatic belt consists of gabbros, diorites, tonalites, quartz diorites and granites, but lacks syenites and peralkaline granites, it must have been a continental arc. This ignores the fact that (1) 810–750 Ma volcanic rocks in this region are mostly bimodal in composition with predominant rhyolites and rhyodacites (ca. 90%) and subordinate basalts (ca. 10%; Li et al., 2002a). Further, a remnant of a 820–810 Ma basaltic large igneous province has recently been identified just north of the Kangdian
DOIs of original article: 10.1016/j.precamres.2007.04.021, DOI of comment: 10.1016/j.precamres.2009.01.009. ∗ Corresponding author. Tel.: +61 8 92662453; fax: +61 8 92663153. E-mail address: [email protected] (Z.X. Li). 0301-9268/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2009.01.010
region (Wang et al., 2008, 2009; marked as the “Bikou Terrane” in Figure 1 of Munteanu and Wilson, this volume); (2) most of the igneous rocks were formed at 825–740 Ma and are synchronous and spatially associated with the development of the Kangdian Rift which was totally filled by the late Neoproterozoic when the region was covered by platform carbonates again along with the rest of the Yangtze craton (Li et al., 2002a, 1999; Liu, 1991; Wang and Li, 2003). Two exceptions are the 1007 ± 14 Ma Huiqinggou gneissic granite (Li et al., 2002b) and the occurrence of minor amounts of >860 Ma basaltic rocks within the strongly deformed and metamorphosed Yanbian Group, which we interpret as being formed in a tectonic environment different from the younger rocks (e.g., Li et al., 2006); (3) andesite, the dominant rock type of continental volcanic arcs, has not been observed; (4) there is no continental margin sedimentary facies recorded in the region, suggesting that the present western Yangtze Block (to the east of the Longmenshan belt) was not a Neoproterozoic continental margin. Recent studies demonstrate that the Precambrian basement of the Yangtze Craton likely extended to the Songpan-Ganzi region, much further to the west than the presently exposed Precambrian outcrops in the Kangdian region (Zhang et al., 2006). Therefore, the ca. 825–740 Ma igneous rocks in the present western Yangtze Block were most likely formed through “within-plate” processes associated with intracontinental rifting.
2. Geochemistry Munteanu and Wilson (this volume) use some arc-like geochemical signatures of granitoid rocks to infer a subduction setting. We
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reiterate that geochemical signatures cannot be simply used as a diagnostic indicator of tectonic setting for granitoids, because the geochemical characteristics of granitoids reflect their sources as well as their melting and crystallization histories (e.g., Li et al., 2004, 2007a; Huang et al., 2008). On the other hand, basaltic rocks (basaltic lava and fine-grained mafic dikes) can provide much better constraints on tectonic settings, particularly for distinguishing continental arcs from mantle plumes, as shown by differences in the chemical structure and thermal states of their mantle sources (e.g., Arndt and Christensen, 1992; Turner and Hawkesworth, 1995; Wang et al., 2008, 2009). Significant to the current debate, ca. 810 Ma Suxiong alkaline basalts in the Kangdian Rift in the western Yangtze Block (Li et al., 2002a,b, 2005), and ca. 800–750 Ma mafic dike swarms in the Kangdian region (Li et al., 2003; Lin et al., 2007; Zhu et al., 2008), commonly show typical OIB-like geochemical and isotopic compositions, indicating a likely plume origin. In addition, abundant continental flood tholeiitic basalts have been reported from the western Yangtze Craton, such as the ca. 820–810 Ma Bikou basalts (Wang et al., 2008, 2009) and the 820 Ma Tiechuanshan tholeiitic basalts (Ling et al., 2003). It is generally considered that large quantities of tholeiitic basalts cannot be generated within continental arcs. Recently, ca. 825 Ma komatiitic basalts, high-temperature anhydrous basaltic lavas, have been identified from the central South China Block, indicating that its underlying mantle potential temperature was about 260 ◦ C higher than MORB-like mantle, thus supporting the plume model (Wang et al., 2007). Petrological, geochemical and isotopic studies demonstrate that these voluminous basalts represent the remnants of a 820–810 Ma continental flood basalt province (Ling et al., 2003; Wang et al., 2008, 2009), strongly favoring the plume model. A recent review of ca. 820–750 Ma basalts rocks throughout the South China Block indicates that most of these basaltic rocks have intraplate (or OIB-like) geochemical signatures, high mantle potential temperatures and recycled components, suggesting the presence of a mantle plume beneath the South China Block during the mid-Neoproterozoic (Wang et al., 2009). Despite depletion of Nb and Ta relative to La (so-called arc-like geochemical signatures) for some middle Neoproterozoic basaltic rocks (e.g., Li et al., 2008a; Lin et al., 2007; Zhou et al., 2007; Zhu et al., 2008), they have no visible depletion of Zr and Hf relative to Sm (a common feature of arc basalts) as in pre-860 Ma, genuine arc basalts in the region and elsewhere (Ling et al., 2003; Li et al., 2006, 2009). All these observations are at variance with the continental arc model for the post-860 Ma magmatic rocks in South China.
Hengdan Group as a constraint on the tectonics evolution of the western Yangtze Block at 825–740 Ma. We agree that the Yanbian Group, a volcano-sedimentary sequence that is strongly deformed with east-northeasterly trending structures and metamorphosed to greenschist facies, is an arc-related succession (Li et al., 2006), but its age is older than the rift-related successions and intrusions. The SHRIMP U-Pb age of ca. 860 Ma for the Guandaoshan pluton that intrudes the upper Yanbian Group constrains the Yanbian Group to be no younger than 860 Ma (most likely ca. 920–900 Ma; Li et al., 2006). Investigations of structural geology, igneous petrology, geochemistry and isotopes demonstrated that formation of the Yanbian Group was related to the Sibaoan orogeny (e.g., Zhou et al., 2006; Li et al., 2006). There appears to be a tectonic transition from the Sibao orogeny at ≥900 Ma to an intracontinental rift setting after ca. 860 Ma in the southwestern Yangtze Block. 4. Magmatism and deformation Munteanu and Wilson (this volume) use some features of deformation and metamorphism around the margins of certain 820–810 Ma plutons in the Yanbian region to support their arc interpretation. The deformation and metamorphism related to the two plutons concerned are limited to contact aureoles and no consistent internal deformation has been observed in those plutons. As pointed out by Li and Powell (2001) and Li et al. (2006), the regional deformation structures of the pre-860 Ma (most likely pre-900 Ma) rocks affected by the late Mesoproterozoic to earliest Neoproterozoic Sibao Orogeny (e.g., Li et al., 2002b, 2007b), including the Yanbian Group in western South China Block, are ENE to E-W trending, whereas a N-S trending arc as proposed by Zhou et al. (2002, 2006) and supported by Munteanu and Wilson (this volume) would require N-S trending structural trends. Post-860 Ma rocks in the western South China Block show neither consistent N-S trending structures (except for the Cenozoic structures related to Indian’s northward indentation) nor any sign of an active continental margin, both required by the arc model. In conclusion, while recognizing the depletion of Nb and Ta relative to La in some of the 825–740 Ma granitic rocks in western South China, the overall geological record of the region (deformation history, structural characteristics, basin history, etc.) and the geochemical characteristics of the basaltic rocks do not support the arc model for that time interval. Such arc-like features most likely reflect the source region of the anorogenic granitic melts. Acknowledgment
3. Stratigraphy Munteanu and Wilson (this volume) propose that two volcanosedimentary units (the so-called “Bikou terrane” and “Yanbian terrene”) were evidence for a subduction setting, and use the Hengdan turbiditic sequence in the “Bikou terrane” to support their subduction model. There are two major Precambrian rock units in the Bikou region, an integral part of the South China Block during the Neoproterozoic: the Bikou Group and the Hengdan Group. The Bikou Group consists predominantly of bimodal volcanics dated at ca. 820–780 Ma (Yan et al., 2003; Wang et al., 2008, 2009). Although the Hengdan Group contains volcaniclastic turbiditic flysch that could form in a fore-arc setting (Druschke et al., 2006), this interpretation is non-unique because volcaniclastic turbidite can also form in continental rift settings (e.g., Nelson et al., 1999). In addition, the Hengdan Group is not identical in age to the Bikou Group. SHRIMP U-Pb dating of detrital zircon grains from the Hengdan Group yield ages between 850 and 700 Ma, indicating that the maximum age of Hengdan Group is ≤700Ma. Thus, it is inappropriate to use the
We thank Simon Wilde for proofreading the draft Reply. This is TIGeR publication #174. References Arndt, N.T., Christensen, U.C., 1992. The role of the lithospheric mantle in generation of continental flood basalts. J. Geophys. Res. 97, 10967–10981. Druschke, P., Hanson, A.D., Yan, Q.R., Wang, Z.Q., Wang, T., 2006. Stratigraphic and UPbSHRIMP detrital zircon evidence for a Neoproterozoic continental arc, central China: Rodinia implications. J. Geol. 114 (5), 627–636. Huang, X.L., Xu, Y.G., Li, X.H., Li, W.X., Lan, W.B., Zhang, H.H., Liu, Y.S., Wang, Y.B., Li, H.Y., Luo, Z.Y., Yang, Q.J., 2008. Petrogenesis and tectonic implications of Neoproterozoic, highly fractionated A-type granites from Mianning, South China. Precambrian Res. 165, 190–204. Li, X.H., Li, W.X., Li, Z.X., Liu, Y., 2008a. 850-790 Ma bimodal volcanic and intrusive rocks in northern Zhejiang, South China: a major episode of continental rift magmatism during the breakup of Rodinia. Lithos 102, 341–357. Li, X.H., Li, Z.X., Ge, W., Zhou, H., Li, W., Liu, Y., Wingate, M.T.D., 2004. Reply to the comment: Mantle plume-, but not arc-related Neoproterozoic magmatism in South China. Precambrian Res. 132 (4), 405–407. Li, X.H., Li, Z.X., Sinclair, J.A., Li, W.X., Carter, G., 2006. Revisiting the “Yanbian Terrane”: implications for Neoproterozoic tectonic evolution of the western Yangtze Block, South China. Precambrian Res. 151, 14–30.
Z.X. Li et al. / Precambrian Research 171 (2009) 77–79 Li, X.H., Li, Z.X., Sinclair, J.A., Li, W.X., Carter, G., 2007a. Reply to the comment by Zhou et al. on: “Revisiting the ‘Yanbian Terrane’: Implications for Neoproterozoic tectonic evolution of the western Yangtze Block, South China” - [Precambrian Res. 151 (2006) 14–30] - [Precambrian Res. 154 (2007) 153–157]. Precambrian Res. 155(3–4), 318–323. Li, X.H., Li, Z.X., Zhou, H., Liu, Y., Kinny, P.D., 2002a. U-Pb zircon geochronology, geochemistry and Nd isotopic study of Neoproterozoic bimodal volcanic rocks in the Kangdian Rift of South China; implications for the initial rifting of Rodinia. Precambrian Res. 113 (1–2), 135–154. Li, X.H., Qi, C.S., Liu, Y., Liang, X.R., Tu, X.L., Xie, L.W., Yang, Y.H., 2005. Petrogenesis of the Neoproterozoic bimodal volcanic rocks along the western margin of the Yangtze Block: new constraints from Hf isotopes and Fe/Mn ratios. Chin. Sci. Bull. 50 (21), 2481–2486. Li, X.H., Li, W.X., Li, Z.X., Lo, C.H., Wang, J., Ye, M.F., Yang, Y.H., 2009. Amalgamation between the Yangtze and Cathaysia Blocks in South China: constraints from SHRIMP U-Pb zircon ages, geochemistry and Nd-Hf isotopes of the Shuangxiwu volcanic rocks. Precambrian Res. (under review). Li, Z.X., Bogdanova, S.V., Collins, A.S., Davidson, A., De Waele, B., Ernst, R.E., Fitzsimons, I.C.W., Fuck, R.A., Gladkochub, D.P., Jacobs, J., Karlstrom, K.E., Lu, S., Natapov, L.M., Pease, V., Pisarevsky, S.A., Thrane, K., Vernikovsky, V., 2008b. Assembly, configuration, and break-up history of Rodinia: a synthesis. Precambrian Res. 160 (1–2), 179–210. Li, Z.X., Li, X.H., Kinny, P.D., Wang, J., 1999. The breakup of Rodinia: did it start with a mantle plume beneath South China? Earth Planet. Sci. Lett. 173 (3), 171–181. Li, Z.X., Li, X.H., Kinny, P.D., Wang, J., Zhang, S., Zhou, H., 2003. Geochronology of Neoproterozoic syn-rift magmatism in the Yangtze Craton, South China and correlations with other continents: evidence for a mantle superplume that broke up Rodinia. Precambrian Res. 122 (1–4), 85–109. Li, Z.X., Li, X., Zhou, H., Kinny, P.D., 2002b. Grenvillian continental collision in South China: new SHRIMP U-Pb zircon results and implications for the configuration of Rodinia. Geology (Boulder) 30 (2), 163–166. Li, Z.X., Powell, C.M., 2001. An outline of the Palaeogeographic evolution of the Australasian region since the beginning of the Neoproterozoic. Earth-Sci. Rev. 53, 237–277. Li, Z.X., Wartho, J.A., Occhipinti, S., Zhang, C.L., Li, X.H., Wang, J., Bao, C.M., 2007b. Early history of the eastern Sibao Orogen (South China) during the assembly of Rodinia: New mica Ar40 /Ar39 dating and SHRIMP U-Pb detrital zircon provenance constraints. Precambrian Res. 159 (1–2), 79–94. Lin, G.C., Li, X.H., Li, W.X., 2007. SHRIMP U-Pb zircon age, geochemistry and Nd-Hf isotope of Neoproterozoic mafic dyke swarms in western Sichuan: Petrogenesis and tectonic significance. Sci. China Ser. D-Earth Sci. 50 (1), 1–16. Ling, W.L., Gao, S., Zhang, B.R., Li, H.M., Liu, Y., Cheng, J.P., 2003. Neoproterozoic tectonic evolution of the northwestern Yangtze craton, South China: implications for amalgamation and break-up of the Rodinia Supercontinent. Precambrian Res. 122 (1–4), 111–140.
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Liu, H., 1991. Chapter 4 Correlation of Sinian system. In: Liu, H. (Ed.), The Sinian System in China. Science Press, Beijing, pp. 126–170. Munteanu, M., Wilson, A., this volume. The South China piece in the Rodinian puzzle. A comment on “Assembly, configuration, and break-up history of Rodinia: a synthesis” by Li et al. (2008) [Precambrian Res. 160 (2008) 179–210]. Precambrian Res. Nelson, C.H., Karabanov, E.B., Colman, S.M., Escutia, C., 1999. Tectonic and sediment supply control of deep rift lake turbidite systems; Lake Baikal, Russia. Geology 27 (2), 163–166. Turner, S., Hawkesworth, C., 1995. The nature of the sub-continental mantle: constraints from the major-element composition of continental flood basalts. Chem. Geol. 120, 295–314. Wang, J., Li, Z.X., 2003. History of Neoproterozoic rift basins in South China: implications for Rodinia break-up. Precambrian Res. 122 (1–4), 141–158. Wang, X.C., Li, X.H., Li, W.X., Li, Z.X., 2007. Ca. 825 Ma komatiitic basalts in South China: First evidence for > 1500 ◦ C mantle melts by a Rodinian mantle plume. Geology 35 (12), 1103–1106. Wang, X.C., Li, X.H., Li, W.X., Li, Z.X., Liu, Y., Yang, Y.H., Liang, X.R., Tu, X.L., 2008. The Bikou basalts in the northwestern Yangtze block, South China: remnants of 820-810 Ma continental flood basalts. Geol. Soc. Am. Bull. 120 (11), 1478–1492. Wang, X.C., Li, X.H., Li, W.X., Li, Z.X., 2009. Variable involvements of mantle plumes in the genesis of mid-Neoproterozoic basaltic rocks in South China: a review. Gondwana Res., doi:10.1016/j.gr.2008.08.003. Yan, Q.R., Wang, Z.Q., Yan, Z., Hanson, A.D., Drushke, P.A., Liu, D.Y., Song, B., Jian, P., Wang, T., Jiang, C.F., 2003. SHRIMP age and Geochemistry of the Bikou volcanic terrane: implications for Neoproterozoic tectonics on the northern margin of the Yangtze craton. Acta Geol. Sin. 77, 497–508. Zhang, H.F., Zhang, L., Harris, N., Jin, L.L., Yuan, H.L., 2006. U–Pb zircon ages, geochemical and isotopic compositions of granitoids in Songpan-Ganze fold belt, eastern Tibetan Plateau: constraints on petrogenesis and tectonic evolution of the basement. Contrib. Miner. Petrol. 152, 75–88. Zhou, J.B., Li, X.-H., Ge, W., Li, Z.X., 2007. Age and origin of middle Neoproterozoic mafic magmatism in southern Yangtze Block and relevance to the break-up of Rodinia. Gondwana Res. 12 (1–2), 184–197. Zhou, M., Yan, D., Kennedy, A.K., Li, Y., Ding, J., 2002. SHRIMP U-Pb zircon geochronological and geochemical evidence for Neoproterozoic arc-magmatism along the western margin of the Yangtze Block, South China. Earth Planet. Sci. Lett. 196 (1–2), 51–67. Zhou, M.F., Ma, Y., Yan, D.P., Xia, X., Zhao, J.H., Sun, M., 2006. The Yanbian Terrane (Southern Sichuan Province, SW China): a Neoproterozoic arc assemblage in the western margin of the Yangtze Block. Precambrian Res. 144, 19–38. Zhu, W.G., Zhong, H., Li, X.H., Deng, H.L., He, D.F., Wu, K.W., Bai, Z.J., 2008. SHRIMP zircon U–Pb geochronology, elemental, and Nd isotopic geochemistry of the Neoproterozoic mafic dykes in the Yanbian area, SW China. Precambrian Res. 164, 66–85.
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Precambrian Research journal homepage: www.elsevier.com/locate/precamres
Reply
The South China piece in the Rodinian puzzle: A reply to the comment by Munteanu and Wilson Z.X. Li a,∗ , X.H. Li b , X.C. Wang b a b
Institute for Geoscience Research, Department of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, Western Australia 6845, Australia State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100027, China
a r t i c l e
i n f o
Article history: Received 4 January 2009 Accepted 26 January 2009 Keywords: Rodinia South China Neoproterozoic Mantle plume Continental rift Arc
a b s t r a c t Uncritical application of geochemical plots for granitic rocks, without full appreciation of the limitations of such plots, the broader geological constraints and information about coexisting other rock types, commonly lead to incorrect tectonic interpretations. We consider this to be the case for the arc interpretations of the 850–740 Ma igneous rocks in South China. In this Reply, we counter-argue all four lines of evidence used by Munteanu and Wilson [Munteanu, M., Wilson, A., this volume]. The South China piece in the Rodinian puzzle. A comment on “Assembly, configuration, and break-up history of Rodinia: a synthesis” by Li et al. (2008) [Precambrian Res. 160 (2008) 179–210]. Precambrian Res.] for an arc model in the western South China Block between 850 and 740 Ma. © 2009 Elsevier B.V. All rights reserved.
We welcome the comment by Munteanu and Wilson (this volume) on Li et al. (2008b). The Li et al. (2008b) paper was a global synthesis on the evolution of the supercontinent Rodinia involving a long list of co-authors from various continents and disciplines, whereas the comment of Munteanu and Wilson (this volume) focuses specifically on the Neoproterozoic geology of the western South China Block and implications for the position of South China within Rodinia. This Reply thus has a different authorship from the original paper to reflect the more regional focus of this discussion. Below, we address the comment of Munteanu and Wilson (this volume) point by point as they appear in their Comment. 1. Igneous petrographic association Munteanu and Wilson (this volume) propose a NS-trending, ca. 900–700 Ma magmatic arc along the western South China Block. They argue that because the magmatic belt consists of gabbros, diorites, tonalites, quartz diorites and granites, but lacks syenites and peralkaline granites, it must have been a continental arc. This ignores the fact that (1) 810–750 Ma volcanic rocks in this region are mostly bimodal in composition with predominant rhyolites and rhyodacites (ca. 90%) and subordinate basalts (ca. 10%; Li et al., 2002a). Further, a remnant of a 820–810 Ma basaltic large igneous province has recently been identified just north of the Kangdian
DOIs of original article: 10.1016/j.precamres.2007.04.021, DOI of comment: 10.1016/j.precamres.2009.01.009. ∗ Corresponding author. Tel.: +61 8 92662453; fax: +61 8 92663153. E-mail address: [email protected] (Z.X. Li). 0301-9268/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2009.01.010
region (Wang et al., 2008, 2009; marked as the “Bikou Terrane” in Figure 1 of Munteanu and Wilson, this volume); (2) most of the igneous rocks were formed at 825–740 Ma and are synchronous and spatially associated with the development of the Kangdian Rift which was totally filled by the late Neoproterozoic when the region was covered by platform carbonates again along with the rest of the Yangtze craton (Li et al., 2002a, 1999; Liu, 1991; Wang and Li, 2003). Two exceptions are the 1007 ± 14 Ma Huiqinggou gneissic granite (Li et al., 2002b) and the occurrence of minor amounts of >860 Ma basaltic rocks within the strongly deformed and metamorphosed Yanbian Group, which we interpret as being formed in a tectonic environment different from the younger rocks (e.g., Li et al., 2006); (3) andesite, the dominant rock type of continental volcanic arcs, has not been observed; (4) there is no continental margin sedimentary facies recorded in the region, suggesting that the present western Yangtze Block (to the east of the Longmenshan belt) was not a Neoproterozoic continental margin. Recent studies demonstrate that the Precambrian basement of the Yangtze Craton likely extended to the Songpan-Ganzi region, much further to the west than the presently exposed Precambrian outcrops in the Kangdian region (Zhang et al., 2006). Therefore, the ca. 825–740 Ma igneous rocks in the present western Yangtze Block were most likely formed through “within-plate” processes associated with intracontinental rifting.
2. Geochemistry Munteanu and Wilson (this volume) use some arc-like geochemical signatures of granitoid rocks to infer a subduction setting. We
78
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reiterate that geochemical signatures cannot be simply used as a diagnostic indicator of tectonic setting for granitoids, because the geochemical characteristics of granitoids reflect their sources as well as their melting and crystallization histories (e.g., Li et al., 2004, 2007a; Huang et al., 2008). On the other hand, basaltic rocks (basaltic lava and fine-grained mafic dikes) can provide much better constraints on tectonic settings, particularly for distinguishing continental arcs from mantle plumes, as shown by differences in the chemical structure and thermal states of their mantle sources (e.g., Arndt and Christensen, 1992; Turner and Hawkesworth, 1995; Wang et al., 2008, 2009). Significant to the current debate, ca. 810 Ma Suxiong alkaline basalts in the Kangdian Rift in the western Yangtze Block (Li et al., 2002a,b, 2005), and ca. 800–750 Ma mafic dike swarms in the Kangdian region (Li et al., 2003; Lin et al., 2007; Zhu et al., 2008), commonly show typical OIB-like geochemical and isotopic compositions, indicating a likely plume origin. In addition, abundant continental flood tholeiitic basalts have been reported from the western Yangtze Craton, such as the ca. 820–810 Ma Bikou basalts (Wang et al., 2008, 2009) and the 820 Ma Tiechuanshan tholeiitic basalts (Ling et al., 2003). It is generally considered that large quantities of tholeiitic basalts cannot be generated within continental arcs. Recently, ca. 825 Ma komatiitic basalts, high-temperature anhydrous basaltic lavas, have been identified from the central South China Block, indicating that its underlying mantle potential temperature was about 260 ◦ C higher than MORB-like mantle, thus supporting the plume model (Wang et al., 2007). Petrological, geochemical and isotopic studies demonstrate that these voluminous basalts represent the remnants of a 820–810 Ma continental flood basalt province (Ling et al., 2003; Wang et al., 2008, 2009), strongly favoring the plume model. A recent review of ca. 820–750 Ma basalts rocks throughout the South China Block indicates that most of these basaltic rocks have intraplate (or OIB-like) geochemical signatures, high mantle potential temperatures and recycled components, suggesting the presence of a mantle plume beneath the South China Block during the mid-Neoproterozoic (Wang et al., 2009). Despite depletion of Nb and Ta relative to La (so-called arc-like geochemical signatures) for some middle Neoproterozoic basaltic rocks (e.g., Li et al., 2008a; Lin et al., 2007; Zhou et al., 2007; Zhu et al., 2008), they have no visible depletion of Zr and Hf relative to Sm (a common feature of arc basalts) as in pre-860 Ma, genuine arc basalts in the region and elsewhere (Ling et al., 2003; Li et al., 2006, 2009). All these observations are at variance with the continental arc model for the post-860 Ma magmatic rocks in South China.
Hengdan Group as a constraint on the tectonics evolution of the western Yangtze Block at 825–740 Ma. We agree that the Yanbian Group, a volcano-sedimentary sequence that is strongly deformed with east-northeasterly trending structures and metamorphosed to greenschist facies, is an arc-related succession (Li et al., 2006), but its age is older than the rift-related successions and intrusions. The SHRIMP U-Pb age of ca. 860 Ma for the Guandaoshan pluton that intrudes the upper Yanbian Group constrains the Yanbian Group to be no younger than 860 Ma (most likely ca. 920–900 Ma; Li et al., 2006). Investigations of structural geology, igneous petrology, geochemistry and isotopes demonstrated that formation of the Yanbian Group was related to the Sibaoan orogeny (e.g., Zhou et al., 2006; Li et al., 2006). There appears to be a tectonic transition from the Sibao orogeny at ≥900 Ma to an intracontinental rift setting after ca. 860 Ma in the southwestern Yangtze Block. 4. Magmatism and deformation Munteanu and Wilson (this volume) use some features of deformation and metamorphism around the margins of certain 820–810 Ma plutons in the Yanbian region to support their arc interpretation. The deformation and metamorphism related to the two plutons concerned are limited to contact aureoles and no consistent internal deformation has been observed in those plutons. As pointed out by Li and Powell (2001) and Li et al. (2006), the regional deformation structures of the pre-860 Ma (most likely pre-900 Ma) rocks affected by the late Mesoproterozoic to earliest Neoproterozoic Sibao Orogeny (e.g., Li et al., 2002b, 2007b), including the Yanbian Group in western South China Block, are ENE to E-W trending, whereas a N-S trending arc as proposed by Zhou et al. (2002, 2006) and supported by Munteanu and Wilson (this volume) would require N-S trending structural trends. Post-860 Ma rocks in the western South China Block show neither consistent N-S trending structures (except for the Cenozoic structures related to Indian’s northward indentation) nor any sign of an active continental margin, both required by the arc model. In conclusion, while recognizing the depletion of Nb and Ta relative to La in some of the 825–740 Ma granitic rocks in western South China, the overall geological record of the region (deformation history, structural characteristics, basin history, etc.) and the geochemical characteristics of the basaltic rocks do not support the arc model for that time interval. Such arc-like features most likely reflect the source region of the anorogenic granitic melts. Acknowledgment
3. Stratigraphy Munteanu and Wilson (this volume) propose that two volcanosedimentary units (the so-called “Bikou terrane” and “Yanbian terrene”) were evidence for a subduction setting, and use the Hengdan turbiditic sequence in the “Bikou terrane” to support their subduction model. There are two major Precambrian rock units in the Bikou region, an integral part of the South China Block during the Neoproterozoic: the Bikou Group and the Hengdan Group. The Bikou Group consists predominantly of bimodal volcanics dated at ca. 820–780 Ma (Yan et al., 2003; Wang et al., 2008, 2009). Although the Hengdan Group contains volcaniclastic turbiditic flysch that could form in a fore-arc setting (Druschke et al., 2006), this interpretation is non-unique because volcaniclastic turbidite can also form in continental rift settings (e.g., Nelson et al., 1999). In addition, the Hengdan Group is not identical in age to the Bikou Group. SHRIMP U-Pb dating of detrital zircon grains from the Hengdan Group yield ages between 850 and 700 Ma, indicating that the maximum age of Hengdan Group is ≤700Ma. Thus, it is inappropriate to use the
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