May 2009 LIP of the Month

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860-750 Ma Large Igneous Provinces in South China: Record of Plume Events During the Breakup of the Supercontinent Rodinia

Zheng-Xiang Li1, Xian-Hua Li2,3, Xuan-Ce Wang2,3, Wu-Xian Li3, Qiang Wang3

  1. The Institute for Geoscience Research, Department of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia;
  2. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China
  3. Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China


Corresponds in part to event 67 in LIP record database

Neoproterozoic magmatism, including plutons, dyke swarms and volcanics, are widespread in South China (Fig. 1). However, their tectonic significance remains controversial. The traditional view was that the plutons signify the "cratonization" of the Yangtze Craton (e.g., (Wang and Mo, 1995). However, precise geochronology has demonstrated that a vast majority of the plutons are coeval with continental rifting events, predominantly between 825 Ma and 750 Ma (Liu, 1991; Li et al., 1999; Li et al., 2003a, 2003c; Wang and Li, 2003).

Li et al. (2003c) demonstrated that after a possible early start at ca. 860-850 Ma, Neoproterozoic magmatic events in South China exhibit four major peaks: ca. 825 Ma, ca. 800 Ma, ca. 780 Ma and ca. 750 Ma. More ages on all age groups have been reported in recent years. In particular, Li et al. (2003b, 2006, 2008b) demonstrated that anorogenic magmatism started at 860-850 Ma in South China. Although the anorogenic interpretation for the 860-750 Ma magmatism is not universally accepted (for alternative views see Wang et al., 2006 and Zhou et al., 2002b), here we present these rocks as products of plume-induced magmatism, particularly for those dated at 825-750 Ma because 860-850 Ma magmatism is not as widespread or well studied. Some of the magmatic events are also found in the southern Korean Peninsula, an extension of the South China Block.

Figure 1: A schematic diagram showing the distribution of the 825-750 Ma magmatic rocks and continental rift systems in South China (after Li et al., 2003a; Li et al., 1999; Li et al., 2003c).

Ca. 825 Ma magmatism: the Guibei LIP
The Guibei LIP covers products of a major bimodal magmatic event in South China. Mafic-ultramafic rocks include the 828 ± 7 Ma mafic-ultramafic dykes in Guibei (marked as #1 in Fig. 1; Li et al., 1999), the ca. 825-810 Ma Tongde-Gaojiacun complex (#2 in Fig. 1; Sinclair, 2001; Zhou et al., 2006; Zhu et al., 2006), the 821 ± 7 Ma to 811 ± 12 Ma Bikou Group basalts (#4 in Fig. 1; Wang et al., 2008), the 821 ± 7 Ma Tiechuanshan basalts (#4 in Fig. 1; Ling et al., 2003), the 820-810 Ma Wangjiangshan complex (#5 in Fig. 1; Zhou et al., 2002a), the 826 ± 3 Ma Yiyang komatiitic basalts (#6 in Fig. 1; Wang et al., 2009), the 827 ± 4 Ma basalts at Guangfeng (#7 in Fig. 1; Li et al., 2008a), and the 818 ± 9 Ma Mamianshan basalts (#8 in Fig. 1; Li et al., 2005). There are also numerous granitic intrusions of that age in both the interior and along the margins of the Yangtze Craton, and felsic volcanic and volcani-clastic rocks in the continental rift systems (Li et al., 2003a; Wang and Li, 2003).

The largest basaltic outcrop is the Bikou Group volcanics close to the present northwestern margin of the South China Block (#4 in Fig. 1). It covers an area of ~10,000 km2, with an estimated kilometres-thick of predominantly tholeiitic basalts (Wang et al., 2008). Well-preserved pillow structures are found at a number of successions (e.g., Fig. 2).

Figure 2: Pillow structures in the Yiyang komatiitic basalts (#6 in Fig. 1; Wang et al., 2007) and the Fanjingshan basalts (#9 in Fig. 1)

A mantle plume origin for this episode of bimodal magmatism was based on a number of geological, geochemical and geochronological observations, such as evidence for syn-magmatic doming, the bimodal and intraplate nature of the magmatism (see summary of geochemical characteristics below), and their associations with continental rifting (e.g., Li et al., 1999, 2003a, 2003c). More recent work by Wang et al. (2007, 2008) demonstrate that both the 826 ± 3 Ma Yiyang komatiitic basalts and the 811 ± 12 Ma upper Bikou basalts have mantle potential temperatures over 1500°C (Fig. 3).

Figure 3: Mantle potential temperatures (Tp) as a function of the MgO concentrations of  primary magmas (after Wang et al., 2009, modified from Herzberg et al., 2007).

Zhou et al. (2009) recently reported 822 ± 15 Ma basalts at Fanjingshan (#9 in Fig. 1; Fig. 2), but they interpret the rocks to be of arc origin. However, we notice that the basalts show no Zr and Hf depletion relative to Sm (as in Fig. 8c-d of Zhou et al., 2009), unlike typical arc basalts (e.g., McCulloch and Gamble, 1991). The Nb-Ta negative anomaly in the Fanjingshan basalts thus likely results from crustal or sub-continental lithospheric mantle contamination  (Li et al., 2007; Wang et al., 2009). Indeed, plume-related low-Ti continental flood basalts (CFBs) often show Nb-Ta negative anomalies (Turner and Hawkesworth, 1995; Hawkesworth et al., 2000; Puffer, 2001). The Fanjingshan basalts are also alkaline basalts (Fig. 6a), and therefore should not be classified as calc-alkiline basalts.

Ca. 800 Ma magmatism: the Suxiong-Xiaofeng LIP
The Suxiong-Xiaofeng LIP is represented by the 803 ± 12 Ma Suxiong-Kaijianqiao bimodal volcanics in the Kangdian Rift (#10 in Fig. 1; Li et al., 1999; Li et al., 2002; Wang and Li, 2003), the 802 ± 10 Ma Xiaofeng composite dykes (Li et al., 2004), the 794 ± 9 Daolinshan granite-diabase complex and the 792 ± 5 Ma Shangshu bimodal (basalt-rhyolite) volcanic rocks (Li et al., 2008b), and equivalent plutonic and volcanic rocks elsewhere. The Suxiong-Kaijianqiao bimodal volcanic succession is up to 10 km thick, and the Shangshu Group, dominately basalts, is up to 2.5 km thick.

Figure 4: The 802 ± 10 Ma Xiaofeng dykes intruding the 819 ± 7 Ma Huangling granite (Li et al., 2004).

Ca. 780 Ma magmatism: the Kangding LIP
Products of the Kangding events include the roughly E-W trending, 779 ± 6 Ma to 768 ± 7 Ma mafic dykes that intrude coeval granitic rocks in the Kangding region (#13 in Fig. 1, also Fig. 5; Li et al., 2003c; Lin et al., 2007), and numerous coeval granitic and volcanic units throughout the South China Block (Fig. 1).

Ca. 750 Ma magmatism: the Shaba LIP
The ca. 750 Ma event was widespread in South China, with mafic examples that include the Shaba gabbro (#14 in Fig. 1; Li et al., 2003c), the Sanmenjie spilites and gabbros, and the Guzhang mafic dykes (#14 in Fig. 1; Zhou et al., 2007). Both the Sanmenjie spilites and the Guzhang mafic dykes indicate high mantle potential temperatures (Fig. 3; Wang et al., 2009).

  Figure 5: Photos of 779 ± 6 Ma to 768 ± 7 Ma magma mingling (a), and ductile interaction between mafic dykes and the intruded granitoid when they both were in semi-solid states (b, c) in the Kangding region (Li et al., 2003c; Lin et al., 2007).

Geochemical characteristics of the 860-750 magmatic rocks in South China
The 860-750 Ma basaltic rocks are predominantly sub-alkaline series and subordinately alkaline series (Fig. 6a). The sub-alkaline basalts are dominantly of tholeiitic nature, and calc-alkaline basalts are rare (Fig. 6b). Fe-poor, Si-enriched and Nb-Ta-depleted characteristics of some basaltic rocks reflect contributions from SCLM. Nonetheless, the anhydrous high-temperature lavas and typical OIB-type basalts are believed to have sampled a plume mantle source.

Figure 6: (a) Nb/Y vs. Zr/TiO2×0.0001 diagram distinguishing sub-alkaline and alkaline basalts; (b) FeOT/MgO vs. SiO2 diagram distinguishing tholeiitic and calc- alkaline series.

Geodynamic model
Coeval Neoproterozoic plume events appear to be widespread across a number of continents during Rodinia time (e.g., Li et al., 2003c, 2008c; Ernst et al., 2008). Li et al. (2003c, 2008c) proposed that a mantle superplume beneath Rodinia was responsible for the widespread, and long-lasting (~100 Ma) magmatic events over Rodinia that eventually caused the breakup of the supercontinent. We use Figure 7 to illustrate a plume model for 825-750 Ma magmatic rocks in South China.

Figure 6: Geodynamic for early Neoproterozoic South China (modified after (Li et al., 1999; Wang et al., 2009)


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