May 2022 LIP of the Month

Ca. 641-635 Ma magmatic events in northern South China, and link with the end of the Marinoan glaciation

Zhongwu Lan^1,2,3*, Magdalena H. Huyskens⁴, Guillaume Le Hir⁵, Ross N. Mitchell^1,6, Qing-Zhu Yin⁴, Gangyang Zhang⁷, and Xian-Hua Li^1,6

¹State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China. lzw1981@126.com

²State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, Jiangsu, China.

³State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, Hubei, China.

⁴Department of Earth and Planetary Sciences, University of California, Davis, CA 95616, USA.

⁵Institut de Physique du Globe de Paris, Université Paris, 75005 Paris, France.

⁶College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China.

⁷College of Earth Sciences, Chengdu University of Technology, Chengdu, Sichuan 610059, China.

Figures compiled from

Abundant late Cryogenian mafic-felsic volcanic rocks and intrusions and ultramafic intrusions (650-635 Ma) are present in South Qinling to the north of South China Block, with a spatial occurrence of ca. 30,000 km² (Figure 1). High-precision zircon geochronology with chemical abrasion–isotope-dilution isotope ratio mass spectrometry (CA-ID-IRMS) indicates that this extensive and thick sequence of rift-related magmatic rocks were erupted at 641-637 Ma, which is 2–6 Myr before the termination of the Marinoan glaciation. In combination with volcanism along the northern margin of South China Block that occurred from 637–635 Ma (Condon et al., 2005), new zircon U-Pb age data suggests the emplacement of an igneous province in central China lasted for 6 Myr (641–635 Ma) (Figure 2).

Geochemical analyses including major and trace element and Nd isotope composition analyses were conducted on these volcanic rocks, based on which two main subgroups were recognized including mafic and intermediate (basalts/trachyandesites/trachytes) and felsic (alkaline rhyolites) lithologies (Figure 3a). The basalts, trachyandesites, and trachytes mainly plot in the region of within-plate alkaline basalts in the 2Nb-Zr/4-Y ternary discrimination plot (Figure 3b). The alkaline rhyolites all plot in the anorogenic area in the Zr-10000*Ga/Al and Nb-Y-Ce plots (Figure 3c, d). Most basalts show depletion in Nb-Ta compared to La (i.e., Nb/La<1) and low abundance of Th (0.3–1.6 ppm) in the primitive mantle-normalized trace element spider diagram (Figure 4a), resembling alkali basalts formed in continental rifts. The alkaline rhyolites commonly show enriched trace element distribution patterns except for depletion in Sr and Eu due to fractional crystallization of feldspar (Figure 4b). They mostly display enrichment in Nb-Ta in comparison to La (Nb/La>1).

CO2 emission was estimated from the South Qinling rift. Faulting and fracturing systems offer pathways for CO₂ diffusion from the deep crust to the surface in continental extensional tectonics (Hunt et al., 2017). On the basis of an actualistic ad hoc point pattern analysis, magmatism-related CO₂ emissions are positively correlative with continental extensional tectonics (Tamburello et al., 2018). CO₂ degassing rate has been quantified in continental extensional tectonic regimes as 53,000 kg/km²/yr in the Himalayas, 250,000 kg/km²/yr in central Italy, and 117,000-412,000 kg/km²/yr in the Eastern Ethiopian rift (Tamburello et al., 2018). When applying this degassing rate to the continental rift in the South Qinling region, which has an outcrop area of ~200,000 km² and spans 6 Myr (641-635 Ma) it would have produced 63,600–494,400 Gt CO₂ (Lan et al., 2022).

We do a climate modelling using such an amount of CO₂ to see how it could have influenced the Marinoan glaciation. We firstly establish background CO₂ degassing before considering the additional CO₂ emissions from the South Qinling rift. Background CO₂ degassing includes CO₂ emissions from continental rifts, volcanic arcs, and mid-ocean ridges. In geological period, such background CO₂ degassing would never exceed modern levels, so we use 38% (deg0.38), 60% (deg0.6), 70% (deg0.7), and 80% (deg0.8) of modern levels for the simulations, respectively. With the lowest degassing rate (deg0.38), the melting threshold for the Marinoan glaciation (~0.05 bar) should be twice in comparison to that of the older 720-660 Ma Sturtian glaciation (0.09 bar) (Figure 5a). Considering their different durations, the Sturtian and Marinoan glaciations require different CO2 thresholds to melt. The difference value between the Sturtian/Marinoan thresholds increases with the enhancement of degassing, with the most significant rise plotting between deg0.6 (ΔpCO₂~0.07) and deg0.7 (ΔpCO₂~0.13) because of the non-linear rate of accumulation. Only a very massive carbon release from the South Qinling rift system and other volcanic provinces could shorten the ice age. Such additional CO₂ emissions from the South Qinling rift and its long-lasting volcanic activity could have shortened the Marinoan snowball by 2–5 Myr (Figure 5b).

Aside from the 641-635 Ma magmatism in northern South China, the end of the Marinoan is also typical of active rift-related volcanism in the Australia, Tarim, Siberia, and Congo cratons with an age range of ca. 643–635 Ma (Calver et al., 2013; Condon et al., 2005; Prave et al., 2016; Vernikovsky et al., 1999; Yarmolyuk et al., 2005; Zhu et al., 2008). Within analytical errors, these magmatic events are synchronous with the end-Marinoan magmatism in South Qinling and the northern margin of the Yangtze block (Wang et al., 2013). Of these, near end-Marinoan volcanic provinces were successively emplaced in South Australia (636.4 ± 0.45 Ma; Calver et al., 2013), then South China (635.2 ± 0.57 Ma; Condon et al., 2005) and Namibia (635.2 ± 0.59 Ma; Prave et al., 2016), where these coordinated rift-related volcanic provinces are related to final stages of the breakup of supercontinent Rodinia (Gernon et al., 2016; Mitchell et al., 2019). Considering uncertainties for CO₂ and dust emissions, multiple volcanic sources may be considered as a likely scenario, with a dominant source from the South Qinling region. The apparent discrepancy between the end of glaciation (635 Ma) and the prolonged magmatic activity (641–637 Ma) points to cumulative effect of CO₂ and dust emissions. Given all these volcanic activities occur in a rift setting their link with the Marinoan glaciation is likely, and further study is needed on these Marinoan-related LIPs.

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