Tracing the provenance of volcanic ash in Permian–Triassic boundary strata, South China: Constraints from inherited and syn-depositional magmatic zircons

https://doi.org/10.1016/j.palaeo.2018.12.002Get rights and content

Highlights

  • Investigation of source(s) of volcanic ash in Permian-Triassic Boundary beds of 4 deepwater sections across South China

  • Geochemical signatures of syn-depositional magmatic zircons indicate an arc-related/orogenic setting.

  • Zircon U–Pb age distributions consistent with ash sourced from South China or Indochina cratons but not the Siberian Craton

  • Spatial variation in total ash bed number and thickness and in zircon crystal size indicates ash sourced from paleo-south.

  • Most likely ash source was subduction-zone volcanism along the South China-Indochina convergent plate margin.

Abstract

The Permian-Triassic boundary (PTB) mass extinction, the most severe biocrisis in Earth's history, is thought to have been triggered by catastrophic volcanic activity. PTB sections in South China contain numerous volcanic ash beds, the source of which is inferred to have been either the Siberian Traps Large Igneous Province (STLIP) or intraregional subduction-zone arc volcanism. In this study, the provenance of these ash beds is determined through a comprehensive analysis based on the geochronological and geochemical signatures of zircons from four PTB sections (Dongpan, Xinmin, Ganxi and Shangsi). Inherited zircons yielded U–Pb ages of 250–3521 Ma with major age clusters in the Permian, Carboniferous to Silurian, early Cambrian to Neoproterozoic, early Neoproterozoic to late Mesoproterozoic, and early Paleoproterozoic. The age spectrum of these zircons is similar to those of detrital zircons from sedimentary and magmatic rocks of the South China and Indochina cratons but different from those of the Siberian Craton, providing evidence for an intraregional source of the volcanic material. The trace-element, εHf(t), and δ18O signatures of syn-depositional magmatic zircons are consistent with an arc-related/orogenic setting. PTB sections in South China can be assigned to sectors based on the number and cumulative thickness of ash beds and the length of zircon crystals. Sector I (South) includes the Dongpan and Xinmin sections, which have cumulative ash-bed thicknesses of 0.86–1.14 m and average zircon lengths of 151–217 μm. Sector II (North) includes the Ganxi, Shangsi and Daxiakou sections, which have cumulative ash-bed thicknesses of 0.17–0.33 m and average zircon lengths of 82–104 μm. Sector III (Northeast) includes the Niushan and Meishan sections, which have cumulative ash-bed thicknesses of 0.1–0.14 m and average zircon lengths of 71–73 μm. Systematic trends toward fewer and thinner ash beds and as well as smaller zircon sizes from Sector I to Sector III indicate a paleo-northward or -northeastward direction of ash transport. Collectively, our findings provide evidence that the source of volcanic ash in South China PTB sections was intraregional subduction-zone arc volcanism along the convergent margin between the South China and Indochina cratons.

Introduction

The Permian-Triassic boundary (PTB) represents the most severe mass extinction in Earth's history, when 90% of marine invertebrate species and 80% of terrestrial vertebrate families died out (Erwin et al., 2002; Irmis and Whiteside, 2012). Numerous volcanic ash beds are found close to the PTB in South China and have been invoked as evidence for a causative link between volcanic activity and the mass extinction (Xie et al., 2010; Shen et al., 2012). However, the origin of volcanic ash deposits in South China PTB sections remains contentious. Some researchers have proposed that they originated from the Siberian Traps Large Igneous Province (STLIP) (Xu et al., 2007; Shen et al., 2012), whereas others have inferred a source related to intraregional subduction-zone volcanic arcs, e.g., the Ailaoshan-Songma arc (Gao et al., 2013, Gao et al., 2015; Wang et al., 2018) or the Yidun arc in the Kunlun area (He et al., 2014). Although previous studies have used zircon U–Pb ages to show that PTB volcanic activity was synchronous with the mass extinction event (Shen et al., 2011; van de Schootbrugge and Wignall, 2016; Baresel et al., 2017), geochronological evidence alone is not conclusive because volcanism was prevalent on a global scale during the Permian-Triassic transition (Xiao et al., 2009; Metcalfe, 2013; Zi et al., 2013; Metcalfe et al., 2015; Gardiner et al., 2016).

Zircon is a common accessory mineral that is found in a wide variety of igneous rocks, and its refractory chemo-physical nature enables it to survive extreme temperature-pressure conditions and multiple cycles of weathering and erosion, preserving invaluable age and provenance information (Belousova et al., 2002; Grimes et al., 2015). Inherited zircons are exceedingly common in continental igneous rocks (Roddick and Bevier, 1995; Zeck and Williams, 2002) because residual zircons will not dissolve in magmas with a high Zr content (≥100 ppm) (Watson and Harrison, 1983). The age signatures recorded by inherited zircons provide evidence for the age distribution and evolutionary history of the crustal parent material (Zeck and Williams, 2002). Moreover, trace elements of zircons can provide insights into the nature, genesis and tectonic setting of the host magmas (Belousova et al., 2002; Grimes et al., 2015). PTB volcanic ash beds often contain zircons with inherited cores (Gao et al., 2013, Gao et al., 2015; Wang et al., 2018), but their significance has been insufficiently studied to date. In the present study, the age spectra of inherited zircons, as well as the ages and geochemical signatures of syn-depositional magmatic zircons, are integrated with other geological data to trace the provenance of the volcanic ash in PTB strata of South China.

Section snippets

Geological setting and study sections

The South China Craton consists of the Cathaysia Block in the southeast and the Yangtze Block in the northwest, which were assembled during the early Neoproterozoic Jiangnan Orogeny (Fig. 1; Cawood et al., 2017). The modern South China Craton is bordered by the North China Craton to the north (along the Qinling-Dabie orogenic belt), the Songpan-Ganzi Terrane to the west (along the Longmenshan Fault), and the Indochina Craton to the southwest (along the Ailaoshan-Song Ma Suture [ASSMS] Zone), as

Analytical methods

Zircon grains from the samples were separated by conventional magnetic and density separation procedures and then picked by hand under a binocular microscope. About 200 grains were randomly selected, mounted in epoxy and polished to expose the center of zircon crystals. Cathodoluminescence (CL) images were obtained using a JEOL JXA–8100 electron microscope to reveal the morphology and internal textures of the zircon grains. Zircon U–Pb dating and trace-element analyses were conducted at the

Zircon morphology and U–Pb dating

Zircons from volcanic ash beds of the four PTB study sections are mostly euhedral and prismatic, with crystal lengths varying mostly from 50 to 440 μm and aspect ratios from 1:1 to 3:1, and show well-developed core–rim textures. About 50% of the zircon crystals have relict cores, which are mostly round in shape and locally embayed (Fig. 3), reflecting partial dissolution. They are mostly 10–90 μm long and 5–80 μm wide, although their original sizes have been reduced by partial dissolution and

Sources of inherited zircons

Inherited zircons are incorporated into magmas during partial melting of preexisting continental crustal rocks (Watson and Harrison, 1983; Hill and Bickford, 2001). Therefore, the ages recorded by the inherited cores of zircons can be treated similarly to a population of detrital zircon grains and used to directly assess the age structure of basement rocks and other components of magma sources (Zeck and Williams, 2002). The abundant inherited zircons in the PTB volcanic ashes show a wide range

Conclusions

The inherited cores of zircon crystals from multiple volcanic ash layers in South China PTB strata yield an age spectrum highly similar to that of detrital zircons from the South China and Indochina cratons, but distinctly different from that of the Siberian Craton. Trace elements and O and Hf isotope signatures of the syn-depositional (ca. 251 Ma) zircons suggest a convergent margin setting for generation of the highly evolved source melts that produced Permian-Triassic ash layers on the South

Acknowledgments

We should like to thank Dr. Zhiyong Liao, Xiangdong Wang and Qiuling Gao for providing ash bed zircons images of NS, MS and DXK sections from South China. This work was jointly supported by the National Natural Science Foundation of China (41672222) and the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan (MSFGPMR201702). We thank Prof. Isabel Montanez for her editorial handling and Prof. Ian Metcalfe and an anonymous for constructive

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