Convergent continental margin volcanic source for ash beds at the Permian-Triassic boundary, South China: Constraints from trace elements and Hf-isotopes

Abstract

Volcanic activity around the time of the Permian-Triassic boundary (PTB) has been proposed as a trigger for the associated biocrisis. Multiple claystone beds are prominent near the PTB sections in South China. Twenty one PTB ash beds from three sections at Shangsi, Jianshi and Meishan in South China were sampled and analyzed. Volcanic ash geochemistry indicate dacite and rhyolite compositions. Zircons from the ash layers yield comparatively low Nb/Hf and high Th/Nb ratios, falling in the range of arc/orogenic-related settings. Zircon Hf-isotope compositions show that ε Hf (t) values vary from-11.7 to 1.8, indicating involvement of both juvenile and ancient crustal components. The ash beds (SS27a, JS129, JS130, JS133，MS25, MS26) near the biotic extinction horizon display a large variation in ε Hf (t) and relatively positive average values, indicating input of juvenile mantle or crust, and implying rapid transit through the older basement of the South China Craton. Spatial

Provenance of the Permian–Triassic boundary volcanic ash beds in South China

“…The XRD results show that the volcanic ash beds are composed of 50.6-95.8% clay minerals, 2.4-32.3% quartz, 0.3-5.9% plagioclase, and minor potassium feldspar, pyrite, and calcite (Table S1). These results are consistent with the findings on the mineralogical compositions of the volcanic ash beds by Wang et al (2019).…”

“…The succession through the PTB is well preserved at F I G U R E 1 Palaeogeography maps at the PTB (~252 Ma) (a) Global (after Scotese & Langford, 1995;Stampfli & Borel, 2002;Algeo et al, 2013), palaeogeography of world courtesy of Ron Blakey (http://jan.ucc.nau.edu/~rcb7/). (b) South China Craton (after Bao, 1998;Algeo et al, 2013; Mao'ershan section (Lyu et al, 2019;Wang et al, 2019;Zhang, Zhang, & Xia, 2007), and three volcanic ash beds with the thickness of 1 to 2 cm were identified and sampled (MES-11-1, MES-11-2, MES-13) (Figure 2a). The Late Permian succession at the Mao'ershan section is termed the Dalong Formation and consists of carbonaceous chertmudstone rhythms, and interbedded volcanic ash beds.…”

“…Although the eruptions of the Siberian Traps large igneous province (STLIP) has been considered as the main cause of PTB mass extinction, some volcanic ash beds in Upper Permian and Lower Triassic strata throughout South China are also associated with biotic extinction horizons and this volcanism has been recognized as part of the trigger mechanism for the PTME (Broadley, Barry, Ballentine, Taylor, & Burgess, 2018;Chen & Xu, 2019;Gao et al, 2013;He, Zhong, Xu, & Li, 2014;Shen et al, 2013;Shen et al, 2018;Wang et al, 2019;Yin et al, 1992).…”

“…A felsic nature, negative Eu anomalies, most zircons of age c. 255-248 Ma, and negative ε Hf (t) values, are the common characteristics of the volcanic ash beds (Gao et al, 2013(Gao et al, , 2015He et al, 2014;Hu et al, 2018;Wang et al, 2019;Zhao, Algeo, Feng, Zi, & Xu, 2019;. Convergent continental margins in the eastern Palaeotethys region (Wang et al, 2019), or rather, the convergent margin between the South China and Indochina blocks Zhao et al, 2019), the Simao-Indochina convergent continental margin (Zheng, Mou, Zhou, et al, 2020), the Palaeotethys continental arc magmatism in the Kunlun area (He et al, 2014), or the Qinling convergent continental margin (Zheng, Mou, Zhou, et al, 2020) all might be the possible source of the ash beds, based on major and trace element contents of the ashes, U-Pb ages, trace elements, and Hf isotopes of zircons.…”

Intensive Wildfire Associated With Volcanism Promoted the Vegetation Changeover in Southwest China During the Permian−Triassic Transition

“…Extensive volcanism was coincident with the EPME (Erwin, 1994;Jin et al, 2000), and the PTB sequences in southwestern China contain numerous ash beds (Fang et al, 2017;Xu et al, 2017;Hong et al, 2019). According to trace elements and Hf-isotope analyses, these ash beds were mainly deposited locally around the Tethys region (Wang et al, 2019). The volcanism derived from a continental volcanic arc could release a massive amount of CO 2 and lead to rapid changes of atmospheric pCO 2 level and δ 13 C. The magma or falling spark would induce widespread wildfires on land, and 13 C-depleted CO 2 can be generated by interactions between volcanic material and organic-rich sediments (Svensen et al, 2009;Korte et al, 2010;Grasby et al, 2011;Black et al, 2012).…”