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Ages, Trace Elements and Hf-Isotopic Journal of Earth Science, Vol. 26, No. 6, p. 872−882, December 2015 ISSN 1674-487X Printed in China DOI: 10.1007/s12583-015-0589-9 Ages, Trace Elements and Hf-Isotopic Compositions of Zircons from Claystones around the Permian-Triassic Boun- dary in the Zunyi Section, South China: Implications for Na- ture and Tectonic Setting of the Volcanism Qiuling Gao1, 2, 4, Zhong-Qiang Chen3, Ning Zhang*2, William L. Griffin4, Wenchen Xia2, Guoqing Wang2, Tengfei Jiang5, Xuefei Xia6, Suzanne Y. O’Reilly4 1. Exploration & Development Research Institute, Zhongyuan Oilfield Company, Puyang 457001, China 2. School of Earth Sciences, China University of Geosciences, Wuhan 430074, China 3. State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China 4. Australian Research Council Centre of Excellence for Core to Crust Fluid Systems and GEMOC National Key Centre, Department of Earth and Planetary Science, Macquarie University, NSW 2109 Sydney, Australia 5. Shandong Lunan Institute of Geo-Engineering Exploration, Jining 272100, China 6. Jianghan Oilfield Company, Qianjiang 433124, China ABSTRACT: A growing body of evidence shows that volcanism near the Permian-Triassic boun- dary (PTB) may be crucial in triggering the Permian–Triassic (P–Tr) mass extinction. Thus, the ash beds near the PTB in South China may carry information on this event. Three volcanic ash layers, altered to clay, outcropped in the PTB beds in Zunyi Section, Guizhou Province, Southwest China. The U-Pb ages, trace elements, and Hf-isotope compositions of zircon grains from these three ash beds were analyzed using LA-ICPMS and LA-MC-ICPMS. The zircons are mainly magmatic in origin (241−279 Ma) except for two inherited/xenocrystic zircons (939 and 2 325 Ma). The ages of these magmatic zircons indicate three episodes of magmatism which occurred around the Middle− Late Permian boundary (~261.5 Ma, MLPB), the Wuchiapingian-Changhsingian boundary (~254.5 Ma, WCB), and the PTB (~250.5 Ma), respectively. The first two episodes of magmatism near the MLPB and WCB may be attributed to magmatic inheritance or re-deposition. All magmatic zircons share similar trace-element and Hf-isotope compositions. They have Y, Hf, Th and U contents and Nb/Ta ratios are typical of zircons from silicic calc-alkaline magmas. These zircons also exhibit enriched Hf-isotope compositions with εHf(t) values of -11.4 to +0.2, which suggests that the three magmatic episodes involved melting of the continental crust. The more enriched Hf-isotope compo- sition (εHf(t)=-11.4− -4.8) of Bed ZY13 (~250.5 Ma) implies more input of ancient crustal material in the magma. Integration of the Hf-isotope and trace-element compositions of magmatic zircons sug- gest that these three episodes of magmatism may take place along the convergent continent margin in or near southwestern South China as a result of the closure of the Palaeo-Tethys Ocean. KEY WORDS: Permian-Triassic boundary, zircon, trace elements, Hf isotope, silicic volcanism, convergent continental margin, South China. 0 INTRODUCTION 2011; Shen S Z et al., 2011; Xie et al., 2010, 2007; Reichow et The causes of the Permian−Triassic mass extinction al., 2009; Isozaki et al., 2007), although other triggers, such as (PTME) have been disputed for decades (Erwin, 2006). In- sea-level fall, oceanic anoxia, bolide impact and global warm- creasing evidence shows that volcanism could be the most ing cannot be excluded (Jiang et al., 2014; Yin et al., 2014; plausible initial cause of the PTME (He et al., 2014; Shen J et Chen and Benton, 2012; Erwin, 2006). However, there are al., 2013a, 2012a, b; Chen and Benton, 2012; Luo et al., different viewpoints about the nature of the PTB volcanism. One suggests the eruption of the Siberian large igneous prov- *Corresponding author: [email protected] ince (SLIP) was responsible for the biocrisis (Zhao et al., © China University of Geosciences and Springer-Verlag Berlin 2013a; Shen et al., 2012a; Reichow et al., 2009; Payne and Heidelberg 2015 Kump, 2007; Kamo et al., 2003; Renne et al., 1995; Campbell et al., 1992). The other proposes that intense silicic volcanism Manuscript received June 22, 2014. along convergent continental margins was responsible for the Manuscript accepted July 16, 2015. PTME (He et al., 2014; Gao et al., 2013; Xie et al., 2010; Gao, Q. L., Chen, Z.-Q., Zhang, N., et al., 2015. Ages, Trace Elements and Hf-Isotopic Compositions of Zircons from Claystones around the Permian-Triassic Boundary in the Zunyi Section, South China: Implications for Nature and Tectonic Setting of the Volcanism. Journal of Earth Science, 26(6): 872–882. doi:10.1007/s12583-015-0589-9. http://en.earth-science.net Ages, Trace Elements and Hf-Isotopic Compositions of Zircons from Claystones around the Permian-Triassic Boundary 873 Isozaki et al., 2007; Yin et al., 1989). and the Yelang Formation of Induan age (Early Triassic). The In South China, numerous volcanic ash layers occur near PTB succession, 3.14 m thick, comprises mainly bioclastic the PTB, and some of which coincide with the extinction hori- packstones, bioclastic wackestones and mudstones with three zon (Song et al., 2013, 2009; Chen et al., 2009; Yin et al., 2007, interbedded claystone layers (Fig. 2). 1992; Xie et al., 2005), such as beds 25 and 28 in Meishan Preliminary studies of conodont biostratigraphy (Zhong, Section, the Global Stratotype of Section and Point (GSSP) for 2012) established three conodont zones (Fig. 2): Clarkina yini the PTB (Yin et al., 2001). Therefore, the understanding of (-C. meishanensis?), C. taylorae, and Hindeodus parvus zones. nature and origin of the PTB volcanism represented by those Among these, the C. taylorae zone was established from Bed ash layers is of great importance for recognition of the poten- 27a−27b and is just beneath the H. parvus zone (calibrated to tial trigger of the PTME. However, the volcanic source of Bed 27c) in the GSSP Meishan (Chen et al., 2015; Zhang et al., those ash layers still remains unclear. More recently, we found 2009; Jiang et al., 2007). The same conodont succession de- three volcanic ash layers near the PTB in Zunyi Section, fining the PTB with the C. taylorae and H. parvus zones below Guizhou Province, Southwest China (Fig. 1) and extracted and above, respectively, has also been recognized in the abundant zircon grains from them. We have analyzed U-Pb Daxiakou Section of the Three Gorges area, South China (Zhao ages, trace elements and Hf isotopes of zircons in order to et al., 2013b). The PTB, therefore, can be placed between the C. uncover the nature and tectonic setting of the PTB volcanism taylorae zone and H. parvus zone and was drawn in the middle in or near South China. part of Bed ZY5-2 in the Zunyi area, corresponding to the base of Bed 27c in Meishan (Yin et al., 2001). 1 GEOLOGICAL SETTING AND SAMPLING Three claystone layers were sampled continuously, and The Zunyi Section, GPS N27°44.54′, E106°56.34′, is ex- were labeled as beds ZY4, ZY6 and ZY13 in ascending order. posed in roadcuts, 3 km north of Zunyi City, Guizhou Province, Bed ZY4, 4 cm thick, is light greenish grey. Bed ZY6, 6 cm SW China (Fig. 1). During the Late Permian, the Zunyi area thick, is greenish grey. Bed ZY13, 1–2 cm thick, is off-white. was situated on a carbonate platform within the Yangtze Block The colors of three claystone layers are homogeneous and which represented continuous deposition of shallow-water similar to tuffs and ash layers in the Xinmin and Daxiakou carbonate rocks. The P–Tr succession is represented by the sections (Gao et al., 2013; Shen et al., 2013a, b), suggesting a Changxing Formation of Changhsingian age (Latest Permian) similar volcanic origin. Figure 1. Geological map of the Zunyi area, Guizhou Province, South China, showing the location of the Zunyi Section (after 1 : 200 000 Geological Map of Zunyi, 1977). Є. Cambrian terrain; O. Ordovician terrain; S. Silurian terrain; P1l+q. Lower Permian Liangshan and Qixia formations; P1m. Lower Permian Maokou Formation; P2l+c. Upper Permian Longtan and Changxing formations; T1y. Lower Triassic Yelang Formation; T1m. Lower Triassic Maocaopu Formation; T2s. Middle Triassic Songzikan Formation; T2sh. Middle Triassic Shizishan Formation; J1. Lower Jurassic terrain; J2. Middle Jurassic terrain. 874 Qiuling Gao, Zhong-Qiang Chen, Ning Zhang, William L. Griffin, Wenchen Xia, Guoqing Wang, and et al. Figure 2. Lithostratigraphy and biostratigraphy of the Permian-Triassic boundary succession in the Zunyi Section, showing correla- tions with the Global Standard Stratotype and Point for P-Tr boundary in the Meishan Section, South China. Conodont zones for Zunyi and Meishan are after Zhong (2012) and Jiang et al. (2007), respectively. Fr. Formation; Sys. System. 2 ANALYTICAL RESULTS ZY4-26, ZY6-8, ZY6-17, and ZY13-16), or a homogeneous 2.1 Zircon Morphology and Internal Structure centre with a thin or oscillatorily-zoned rim (i.e., ZY4-11, More than 98% of the zircon grains are colorless to yel- ZY4-13, ZY4-27, ZY6-2, ZY6-20, ZY13-2, ZY13-6, and lowish, transparent, and stubby to long prismatic (Fig. 3). The ZY13-7). Both the long-prismatic outline and light oscillatory grains are rather small. The lengths of stubby prismatic grains zoning or homogeneous internal structure indicate rapid crys- are mostly smaller than 120 μm. The widths of long prismatic tallization and cooling, and thus suggest a volcanic origin. grains are smaller than 50 μm, mostly smaller than 40 μm, A very few grains show irregular granular or ellipsoidal although their lengths may exceed 200 μm. These grains exhi- shapes and core-rim structure (i.e., ZY6-3), suggesting a dif- bit (light) oscillatory zoning (i.e., ZY4-5, ZY4-6, ZY4-19, ferent origin.
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