January 2014 LIP of the Month

Potential superplume-related Neoproterozoic (850-820 Ma) LIPs in Central-Western China

Xin Xu and Shuguang Song

School of Earth and Space Sciences, Peking University, Beijing 100871, China.

Emails: xuxin19900815@163.com & sgsong@pku.edu.cn

1.    Introduction

Within-plate magmatism has been considered separate from that emplaced at sea-floor spreading ridges and convergent plate margins, and ranges in size up to that classified as Large Igneous Provinces (LIPs). LIPs are characterized by large volume (at least 100,000 km³), a short duration (or short duration pulses) and having geochemical and/or other affinities consistent with an intraplate tectonic setting (e.g., Coffin and Eldholm, 1994; Bryan and Ernst, 2008; Bryan and Ferrari, 2013; Ernst, 2014). A LIP consists of flood basalts, or their erosional/deformational remnants and a plumbing system of dykes, sills, and layered intrusions. A silicic component is often present. LIPs are critical for recognizing the episodes of continental breakup and supercontinent cycles.

Since its early recognition in the early 1990s (McMenamin and McMenamin, 1990; Dalziel et al., 1991; Moores et al., 1991; Hoffman et al., 1991), Rodinia, the Meso-Neoproterozoic supercontinent, has attracted much attention (e.g., Meert and Torsvik, 2003; Li et al. 2008b; Pisarevsky et al., 2008; Evans, 2013). The timing for the onset of the LIP magmatism contributing into the breakup of Rodinia has been controversial. On one hand, the breakup of Rodinia may have started at 870-850 Ma based on a small number of intrusions in South China, Africa, the Scottish promontory of Laurentia and the Scandinavian Caledonides (Li et al., 2003b; Li et al., 2010a; Li et al., 2010b; Paulsson and Adreasson, 2002; Dalziel and Soper, 2001; Ernst et al., 2008; Song et al., 2010) or even at ca. 930 Ma (North China, and West Africa) (Ernst et al., 2008; Peng et al., 2011). On the other hand, it is widely accepted that 830-720 Ma continental intraplate magmatism has particularly extensive distribution in Australia, Laurentia, South China, South Korea, India, Seychelles and Tarim and clearly represents a major phase of Rodinia breakup (Parrish and Scammell, 1988; Heaman et al., 1992; Su et al., 1994; Zhao et al., 1994; Fetter and Goldberg, 1995; Park et al., 1995; Lee et al., 1998, 2003; Li et al., 1999, 2003b; Preiss, 2000; Frimmel et al., 2001; Ashwal et al., 2002; Li et al., 2002, 2006; Ling et al., 2003; Shellnutt et al., 2004; Xu et al., 2005; Zhang et al., 2005; Wingate et al., 1998) and consists of multiple pulses (ca. 820, 800, 780 and 755 Ma; Ernst et al. 2008; Li et al., 2008b). The onset of this 820-740 Ma multi-pulse period of plume-related magmatism was first defined at ca. 825 Ma in South China in light of the ca. 825 Ma mafic dykes, komatiites, mafic-ultramafic intrusions, continental flood basalts (CFBs), anorogenic granitoids (Li et al., 2008a; Li et al., 2003b; Li et al., 2005b; Wang et al., 2007, 2008, 2009), which are also associated with the initiation of Nanhua rift basin (Li et al., 2003b). This plume related breakup is preceded by a period of magmatic quiescence (Li et al., 2003b; Li et al., 2006), which extends from 820 Ma back to ca. 1000 Ma, the timing of the Sibao Orogeny (marking the collision of the Yangtze and Cathaysian blocks).

More recent geochronological results from South China suggest that the earliest intraplate magmatism likely began at ca. 850 Ma (Shu et al., 2011; Li XH et al., 2002, 2010a). A large amount of within-plate magmatism of 850-740 Ma is also present in the Tarim and Qilian-Qaidam block (Song et al., 2010, 2013; Lu et al., 2008).

2.    Geological settings

The Qilian-Qaidam block, located to the southwest of the Yangtze block, has been widely accepted to have affinities with South China during the Neoproterozoic. In the North Qaidam ultra-high pressure metamorphic (UHPM) belt, the presence of ~850 Ma eclogites (flood basalt protolith) indicate that the Qaidam block is probably also a fragment of Rodinia with a volcanic-rifted passive margin (Song et al., 2010). Similarly, the protolith ages of UHM eclogites in North Qaidam and South Altun Blocks yield two ages: ~1000 Ma and 800-750 Ma corresponding to the assembly and fragmentation of Rodinia, respectively (Zhang et al., 2005).  

The Paleo-Qilian Ocean, often considered as the precursor of the North Qilian Orogen, opened at > 710 Ma as a consequence of rifting and continental lithospheric extension (Song et al., 2013). Similarities in age and tectonic environment of magma genesis in the Qilian and South China Blocks give strong support in favour of a connection between these two blocks in the Neoproterozoic (Tung et al., 2013). Based on the ~825 Ma age of igneous baddeleyites and zircons (Li et al., 2005a), the age of Jinchuan mafic-ultramafic intrusion (of the Alxa block) is comparable with similar mafic-ultramafic intrusions in the Qaidam block and North Qilian Orogenic Belt.

It has been proposed that the North Qaidam-Qilian basement terrane can be correlated with similar rocks in the South China block and is considered to belong to the latter (Wan et al., 2006; Song et al., 2012). On a broader scale, Lu et al. (2008) proposed a 820-740 Ma connection between various continental domains, including the Australian, Yangtze and Tarim Cratons as well as some smaller massifs (Qilian-Qaidam Block) from the latest Mesoproterozoic to the middle Neoproterozoic.

3.    Neoproterozoic plume-related (850-820 Ma) magmatism in Central-Western China

Because of the strong uplift and erosion of the North Tibetan Plateau, the magmatic record during the 850-820 Ma period is only sporadically preserved in central and western China, including Tarim, Qilian-Qaidam, and northwestern margin of South China blocks. Some of the better dated units are discussed below:

3.1     ~830 Ma Jinchuan Cu-Ni-bearing ultramafic to mafic intrusion

The ~830 Ma Jinchuan ultramafic intrusions of northwestern China hosts one of the world’s largest magmatic Ni-Cu sulfide deposits. The integrated mineralogical, petrological and geochemical data of the Jinchuan intrusion are consistent with a mantle plume with Tp > 1350 °C (site 1 in Fig.1; Li et al., 2005a).

3.2     The 850-820 Ma CFBs as protoliths of eclogite generated by continental subduction

The eclogites from the North Qaidam UHPM belt occur as blocks or layers of various sizes intercalated with granitic and pelitic gneisses and they have a protolith age of ca. 850-820 Ma; this UHPM belt extends discontinuously for a distance of ~400-km. The geochemical features of these eclogites are similar to enriched-type mid-ocean ridge basalts (E-MORB) and oceanic island basalts (OIB) and suggest that their protolith was CFBs with a mantle plume origin. It is noted that the protolith of the eclogites can be subdivided into a high-Ti group (Ti/Y > 500) and a low-Ti group (Ti/Y < 500). Many well-known CFB provinces such as Emeishan LIP also include high and low Ti geochemical groups (Xu et al., 2001) (site 2 in Fig. 1; Song et al., 2010, 2014).

If flood basalts are indeed the protolith for this eclogites, then it must be interpreted that these basalts (in a possible continental passive margin setting) would have been dragged down to depths of ~ 100 km during continental subduction in 440-420 Ma, before being later exhumed.

3.3     The 820 Ma mafic dyke swarms in Quanji massif

The Yingfeng diabase dyke swarm intruded into the Mid-Proterozoic basement exposed in the Quanji massif yielded a SHRIMP U-Pb age of 821 ± 11 Ma (site 3 in Fig.1; Lu et al., 2008).

3.4     The 825 Ma Bikou CFBs in Northwest of the Yangtze Craton

The Bikou basalts in northwestern South China are mainly tholeiitic in composition and erupted at 821~811 Ma. They are likely the remnants of CFBs derived from a ca. 825 Ma mantle plume with an anomalously hot asthenosphere source-160 °C hotter than the contemporary ambient MORB based on geochemical data and numerical modeling (Site 4 in Fig.1; Wang et al., 2008).

3.5     The 823 Ma Yiyang komatiitic basalts in Yangtze Craton

The ~823 Ma Yiyang komatiitic basalts are solid petrological evidence in favour of a proposed ca. 825 Ma mantle plume. The high MgO content in the primary magma implies the Yiyang komatiitic basalts were generated by melting of an anomalously hot mantle source with potential temperature (Tp) 260 ± 50 °C higher than the ambient MORB source mantle (site 5 in Fig.1; Wang et al., 2007).

Moreover, a large amount of plume-related magmatism has also been recognized at the eastern margin of Yangtze Block and the northeastern margin of Cathaysia Block (Li et al., 1999; Shu et al., 2011; Li XH et al., 2008a, 2010a; Wang et al., 2007; Li et al., 2010b). The abundant anorogenic igneous rocks, including komatiitic basalts, mafic to ultramafic dykes, sills and intrusions, bimodal volcanics and anorogenic granitoids, are consistent with a mantle plume origin.

4.    Tectonic implications for super-plume activity and the reconstruction of the supercontinent Rodinia

In general, a mantle plume which arrives at the base of the lithosphere can melt and/or cause the lithosphere to melt and produce a large amount of basaltic igneous rocks to form oceanic plateaus or continental LIPs. Three phases are recognized: precursor, main and syn-rifting stages (e.g., Bryan and Ernst, 2008). The earliest phase at 850-820 Ma marks a precursor with relatively low-volume transitional-alkaline basaltic eruptions. Specifically, the precursor magmatism during the period 850-820 Ma in Western and Southern China, consists of the 850 Ma CFB-like protolith of the North Qaidam UHPM eclogite (site 2 in Fig.1; Song et al., 2010), the ~850 Ma Shenwu dolerite dykes (Li et al., 2008a), the 850 Ma Gangbian alkaline complexes (Li et al., 2010a) and the 849 Ma Zhenzhushan bimodal volcanic rocks (Li et al., 2010b). We interpret that all these units represent the initial pulse of the LIP with low-volume magma which preceded the dominant CFB pulse between 830-820 Ma.

The main magmatic stage (830-820 Ma) is characterized by a dominantly eruptive pulse with rapid, large-volume magma, mainly tholeiitic flood basalts. The large and rapidly emplaced magmatic pulse also includes the 830 Ma Jinchuan ultramafic intrusions and associated dolerite dykes (site 1 in Fig. 1; Li et al., 2005a), the 821 Ma Yingfeng diabase dyke swarm (site 3 in Fig. 1; Lu et al., 2008), the 821 Ma Bikou basalts (site 4 in Fig. 1; Wang et al., 2008) and the 823 Ma Yiyang komatiitic basalts (site 5 in Fig. 1; Wang et al., 2007).

Finally, the syn-rifting phase (prior to the breakup of Rodinia) may be characterized by an episode of magmatism that peaks at the starting point of continental rifting-ca. 820 Ma and includes later multi-staged igneous rocks associated with the waning and protracted volcanism (Wang et al., 2009; Wang and Li, 2003; Li et al., 2003b; Li et al., 2006, 2008a; Lu et al., 2008).

The widespread distribution of the latest Mesoproterozoic to Neoproterozoic magmatism relevant to the evolution of Rodinia supercontinent in Tarim, Qilian-Qaidam and South China blocks was distinct from that of North China Craton which was covered with platform deposits during that period. The Qaidam-Qilian block is considered to be a fragment of Rodinia because it records both the 1000-900 Ma and 850-750 Ma magmatic events (Song et al., 2012; Tung et al., 2013), corresponding to the global-scale assembly (1300-900 Ma) and the breakup (825-740 Ma) of Rodinia. The South China craton shares these events: ca. 1.1-0.9 Ga Sibao Orogen and 850-740 Ma rifting event (Li et al., 2003a; Ling et al., 2003). So our conclusion is that the Tarim, Qilian-Qaidam and South China blocks share a ca. 1.0 Ga orogenic event and 850-740 Ma multi-pulse LIP record, and likely represent a single crustal block that was distinct from the North China craton.

By linking the 850-740 Ma multi-pulse LIP record between these blocks the scale of the individual LIP events is increased greatly. Work is in progress to further characterize the 850-740 Ma LIP magmatism of these blocks (consisting of Tarim, Qaidam, Alxa (Alashan), South China and perhaps Songpan blocks (Fig. 1) which have been collectively termed the“South-West China United Continent (SWCUC)” (Song et al. 2012).

5.    Reference