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Ages, Trace Elements and Hf-Isotopic

Ages, Trace Elements and Hf-Isotopic

Journal of 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 - 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 - 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 age (). 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. terrain; O. terrain; S. 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. Songzikan Formation; T2sh. Middle Triassic Shizishan Formation; J1. Lower terrain; J2. 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.

Ages, Trace Elements and Hf-Isotopic Compositions of Zircons from Claystones around the Permian-Triassic Boundary 875

Figure 3. Cathodoluminescence (CL) images of zircon grains from the Zunyi ash beds. The ages of three grains, ZY13-11, ZY13-14, and ZY13-18, are obviously younger than the PTB age and are strongly discordant. Therefore, 250 Ma (shown in the parentheses) was used for the εHf(t) calculations.

2.2 Zircon U-Pb Ages magmatic inheritance (Zhong et al., 2013; Mundil et al., 2004, A total of 74 zircon grains from three ash beds were ana- 2001). The radiometric ages derived from these three ash beds lyzed, and 63 concordant ages (concordance >90%) were ob- near the PTB indicate that three major episodes of magmatism tained (Table 1 and Appendix Table 1). Of these, 61 grains during the Late Permian took place near the MLPB (~261.5 show ages of 241−279 Ma (Figs. 4a−4c), and two grains are Ma), the WCB (~254.5 Ma), and the PTB (~250.5 Ma), re- dated as 939 and 2 325 Ma. The grains with ages of 241−279 spectively. Ma are prismatic and exhibit (light) oscillatory zoning or a homogeneous centre with a thin or an oscillatorily-zoned rim, 2.3 Zircon Trace Elements suggesting that these are magmatic zircons. The other two All of the magmatic zircons have similar trace-element grains with older ages are either granular or ellipsoidal, sug- compositions (Table 1 and Appendix Table 2). They show an gesting they are inherited or xenocrystic zircons. enrichment in heavy rare-earth elements (HREE), which is Twenty-seven concordant grains from Bed ZY4 (Fig. 4d) typical of magmatic zircons (Fig. 5). Rare-earth elements (REE) show an age peak of ~254.5 Ma, which is close to the radi- contents range from 562 ppm to 2 793 ppm. Positive Ce ano- ometric age of the Wuchiapingian-Changhsingian boundary malies are mostly >3, and negative Eu anomalies are <0.4. Y (WCB) (254.14±0.07 Ma, Shen S Z et al., 2013). Twenty-four contents vary from 856 ppm to 4 723 ppm, and Hf contents are concordant grains from Bed ZY6 define two age peaks, ~250.5 8 363 ppm−10 291 ppm. Th contents fluctuate in a range of 75 and ~261.5 Ma (Fig. 4e). The former date is close to the radi- ppm−551 ppm, and U contents are 142 ppm−741 ppm. U/Yb ometric age of the PTB (252.28±0.08 Ma, Shen et al., 2011; ratios are 0.37−1.06, Nb/Ta ratios are 1.38−3.96, and Th/U 252.4±0.3 Ma, Mundil et al., 2004; 251.4±0.3 Ma, Bowring et ratios are 0.45−0.99. al., 1998, for Bed 25 in Meishan), and the latter to the age of the Middle-Late Permian boundary (MLPB) (259.8±0.4 Ma, 2.4 Zircon Hf-Isotope Composition Shen S Z et al., 2013). In addition, 10 concordant grains from Magmatic zircons exhibit heterogenous Hf-isotope com-

ZY13 show an age peak of ~250.5 Ma (Fig. 4f) with a concor- positions (Table 1 and Appendix Table 3, Fig. 6). Their εHf(t) dia age of 251.9±1.8 Ma (Fig. 4c), consistent with the radi- values vary in the range of -11.4 to +0.2 with calculated ometric age of the PTB. The age peak of ~250.5 Ma from beds two- model ages (TDM2) of 1.27−1.99 Ga. One Early ZY6 and ZY13 represents volcanism near the PTB, while the Palaeoproterozoic grain (2 325 Ma) has an enriched Hf-isotope older age peaks of ~254.5 and ~261.5 Ma indicate two earlier composition with εHf(t) value of -7.5 and a TDM2 magmatic events, and might be attributed to re-deposition or age (3.32 Ga).

876 Qiuling Gao, Zhong-Qiang Chen, Ning Zhang, William L. Griffin, Wenchen Xia, Guoqing Wang, and et al.

Table 1 Summary of U-Pb age, Hf-isotope ratios and trace-element compositions of magmatic zircons from the Zunyi ash beds

206 238 Bed Concor- Pb/ U εHf(t) Mean TDM2 REE Eu ano- Ce ano- Y Mean Y2O3 a a No. dant age peak εHf(t) age maly maly Y 206Pb/238U age (Ma) (Ma) (Ga) (ppm) (ppm) (ppm) (ppm)

ZY13 244–264 ~250.5 -11.4 to -8.5 1.59– 1 216– 0.12– 2.6–18 1 960– 2 567 2 489– -4.8 1.99 1 923 0.28 3 178 4 036 ZY6 241–273 ~250.5 & ~261.5 -10.6 to -6.1 1.27– 961– 0.10– 2.9–37 1 473– 2 523 1 870– +0.2 1.94 2 793 0.39 4 723 5 998 ZY4 243–279 ~254.5 -8.1 to -4.9 1.34– 562– 0.12– 4.5–87 856– 1 786 1 087– -0.8 1.78 1 752 0.37 2 821 3 582

Bed Hf Mean Hf HfO2 Th U Th/U U/Yb Nb/Ta No. (ppm) (ppm) (ppm) (ppm) (ppm)

ZY13 8 363– 9 435 9 862– 148– 203– 0.71– 0.37– 1.63–3.19 10 131 11 947 349 463 0.98 0.60 ZY6 8 651– 9 517 10 202– 151– 229– 0.46– 0.41– 1.45–3.96 10 286 12 130 551 741 0.99 1.06 ZY4 8 964– 9 558 10 571– 75–347 142– 0.45– 0.43– 1.38–2.59 10 291 12 136 473 0.83 0.77

Note: a. the formula for calculating δEu and δCe: δEu=[Eu]/SQRT([Sm]×[Gd]); Ce=[Ce]/SQRT([La]×[Pr]). The normalized values of REE used in the Table are after McDoungh and Sun (1995).

Figure 4. U-Pb age diagrams of magmatic zircons from the Zunyi ash beds. (a), (b), (c) concordia diagrams. The data circles represent 1σ uncertainties. (d), (e), (f) 206Pb/238U age probability and histogram diagrams of concordant grains.

Ages, Trace Elements and Hf-Isotopic Compositions of Zircons from Claystones around the Permian-Triassic Boundary 877

Figure 5. REE patterns of magmatic zircons from the Zunyi ash beds chondrite-normalized data are after McDonough and Figure 6. Hf-isotope compositions of magmatic zircons from Sun (1995). the Zunyi ash beds. 1. Hf-isotope compositions of magmatic zircons from Daxiakou, Meishan and other sections (He et al., 2014; Gao et al., 2013); 2. Hf-isotope compositions of mag- Magmatic zircons from Bed ZY4 (~254.5 Ma) have εHf(t) values of -8.1− -0.8. Grains from Bed ZY6 (~250.5 and ~261.5 matic zircons from Siberian magmatism (Malitch et al., 2012); 3. Hf-isotope compositions of magmatic zircons from Ma) exhibit εHf(t) values of -10.6− +0.2, and there is no ob- vious variation between grains of different ages (Fig. 6). Emeishan felsic magmatism (Shellnutt et al., 2009; Xu et al., 2008); 4. Hf-isotope compositions of magmatic zircons from Grains from Bed ZY13 (~250.5 Ma) have εHf(t) values of -11.4− -4.8. Therefore, all magmatic grains from three ash beds claystones near the MLPB in the Penglaitan Section (Zhong et al., 2013). exhibit similar low εHf(t) , which suggests that all three epi- sodes of magmatism involved remelting of crustal sources (He et al., 2014; Gao et al., 2013; Zhong et al., 2013). Grains from subduction zones around the South China Block during the Middle−Late Permian and Early Triassic Bed ZY13 exhibit overall lower εHf(t) than grains from beds ZY4 and ZY6, suggesting more input of ancient crustal materi- (http://www.scotese.com/newpage5.htm, Fig. 8a). Subduction al in the magma (Gao et al., 2013). and collision among the South China, Indochina and Simao blocks took place during that time (Cai and Zhang, 2009; 3 DISCUSSION Owada et al., 2007; Lepvrier et al., 2004). Felsic calc-alkaline 3.1 Nature of the Three Magmatic Episodes and peraluminous magmatism was very active along the Zircon trace elements provide information on the compo- Songma, Ailaoshan and Jinshajiang collision zones (Zi et al., sition of the host magmas (Belousova et al., 2002; Pupin, 2000) 2012; Zhang et al., 2011; Hoa et al., 2008; Trung et al., 2007; and therefore were used to determine the nature of the three Fig. 8b). magmatic episodes recorded in the ash layers of the Zunyi Huge volumes of basaltic volcanism derived from Section. The similar trace-element compositions of all mag- mantle-plume activity, including the Siberian large igneous matic zircons (Table 1 and Appendix Table 2, Figs. 5 and 7) province (SLIP) (Zolotukhin and Al’Mukhamedov, 1988) and suggest a similar petrological character of the magmas of all the Emeishan large igneous province (ELIP) (Ali et al., 2010, three episodes. All magmatic zircons from the ash beds have 2005), also erupted during this period. They are composed identical Th and U contents, which are similar to those of zir- mainly of continental flood basalts with tholeiitic or alkaline cons from rhyolite and dacite (Fig. 7a). This finding is sup- affinity (Ali et al., 2010; Arndt et al., 1998; Zolotukhin and ported by their similar Y contents and Nb/Ta ratios (Fig. 7b). Al’Mukhamedov, 1988). Voluminous felsic volcanism was The means of Y and Hf contents of zircons from Beds ZY4 and also associated with the ELIP (Xu et al., 2010, 2008). In addi- ZY13 fall in the field of zircons from quartz-bearing interme- tion, a recent geochemical study (Zhong et al., 2013) suggests diate and felsic rocks and close to the field of zircons from that there was also ~260 Ma felsic volcanism with an arc source, which was derived from continental arcs during the felsic rocks with ‘high’ SiO2 content (Fig. 7c). Th, U, Y, and Hf contents and Nb/Ta ratios of magmatic zircons (Figs. 7a−7c) evolution of the Paleo-Tethys Ocean and unrelated to indicate that all three episodes involved felsic (silicic) magmas. the ELIP. The calc-alkaline silicic magmas are essentially different The Y2O3 and HfO2 contents of all grains project in the area of zircon from calc-alkaline rocks (Fig. 7d), suggesting a from the tholeiitic or alkaline basalts in the SLIP and ELIP. calc-alkaline affinity for the magmatism. The three magmatic The Hf contents of magmatic zircons from the three ash beds episodes therefore were silicic (rhyolitic-dacitic) in origin with (mostly >9 000 ppm) are obviously higher than and different calc-alkaline affinity, suggesting continuous silicic magmatism from those in felsic volcanic rocks with mantle-plume origins from the MLPB to PTB. (5 600 ppm−7 500 ppm, Chatterjee and Bhattacharji, 2004). Furthermore, the Hf-isotope compositions of zircon grains from 3.2 Tectonic Setting of the Late Permian Magmatisms the ash beds of the studied section are significantly different Recent tectonic studies indicate the activity of large from those from the SLIP and ELIP felsic magmatism, but

878 Qiuling Gao, Zhong-Qiang Chen, Ning Zhang, William L. Griffin, Wenchen Xia, Guoqing Wang, and et al.

Figure 7. Trace elements of magmatic zircons from the Zunyi ash beds, showing discrimination of potential sources. (a) U vs. Th

(Gao et al., 2013); (b) Y vs. Nb/Ta (Gao et al., 2013); (c) Hf vs. Y (Belousova et al., 2002; Shnukov et al., 1997); (d) Y2O3 vs. HfO2 (Pupin, 2000). In (a) and (b), green circles represent zircons from basalts; blue circles, zircons from trachytes and andesites; purple circles, zircons from rhyolites in post-collision extensional settings; red circles, zircons from Triassic dacites in the Qinling area, derived from convergent continental-margin volcanism before post-collision extension; orange circles, zircons from dacites in Italy emplaced during regional extension; grey circles, zircons from high-silicic rhyolites. (c) I. kimberlites; II. ultramafic, mafic and intermediate rocks; III. quartz-bearing intermediate and felsic rocks; IV. felsic rocks with ‘high’ SiO2 content; V. greisens; VI. alkaline rocks and alkaline metasomatites of alkaline complexes; VII. carbonatites. (d) Tholeiitic plagiogranites, 1a; hypersolvus alkaline granites/rhyolites; 1b-1c-1d-1e: silica over/under-saturated alkaline/peralkaline syenites/trachytes, 1c-1d-1e; gem zircon in hawaiites and alkali basalts; 1c: subsolvus alkaline granites/rhyolites; 1e, 2, 3a-3b-3c, 4a-4b-4c: basic to intermediate calc-alkaline rocks (gabbros, diorites, tonalites, quartz diorites and andesites-dacites); 4a-4b-4c, 5a-5b-5c, 6a-6b: calc-alkaline granites/rhyolites; 5a-5b-5c: high-K calc-alkaline, or Mg-K granites/rhyolites; 4a-4b, 5a-5b-5c: subalkaline, or Fe-K granites/rhyolites, 4c, 5a-5b-5c: peraluminous porphyritic granites/rhyolites; 3b-3c, 4b-4c, 5b-5c, 6a-6b: peraluminous leucogranites; 3c, 4c, 5c, 6a: autochthonous peraluminous granites and migmatites; 3c, 4c, 5c, 6a: grey area represents calc-alkaline intermediate rocks (diorite-tonalite/andesite-dacite). similar to those recorded in the Daxiakou Section and the other Penglaitan Section in South China (He et al., 2014; Gao et al., five PTB sections (Meishan, , Shangsi, Dongpan, and 2013; Zhong et al., 2013). The zircon trace-element composi- Rencunping) and the Penglaitan Section near the MLPB in tions are similar to trace element concentrations of zircons South China (Fig. 6). Thus it is unlikely that the three mag- from convergent continental margins before post-collision matic episodes recorded in the Zunyi Section were derived extension and post-collision extensional setting (Fig. 9), also from the SLIP or ELIP. indicating a convergent continental margin setting. Therefore, we suggest that the calc-alkaline silicic mag- The three episodes of silicic magmatism probably reflect matism recorded in the Zunyi ash beds probably occurred at the subduction and/or collision among the South China, Indo- convergent continental margins are related to subduction or china, and Simao blocks, in association with the closure of the collision, and have a similar tectonic setting with the silicic Paleo-Tethys Ocean and the assembly of the Pangea supercon- volcanism as the ash beds at the PTB in the Daxiakou and tinent (Cai and Zhang, 2009; Veevers and Tewari, 1995). other sections and the claystones near the MLPB in the

Ages, Trace Elements and Hf-Isotopic Compositions of Zircons from Claystones around the Permian-Triassic Boundary 879

Figure 8. Geological map around South China, showing subduction zones surrounding the South China Block. (a) 255 Ma ago (after http://www.scotese.com/newpage5.htm); (b) present (Lepvrier et al., 2004; Zhao et al., 2001).

Figure 9. Zircon Hf and Y contents vs. U/Yb (Gao et al., 2013; Grimes et al., 2007), distinguishing the possible tectonic setting of the volcanism that produced the zircons. Shadowed areas are from Grimes et al. (2007). Red circles (circles 1) represent zircons from felsic rocks at convergent continental margins before post-collision extension; yellow circles (circles 2), zircons from basalts and rhyolites in post-collision extensional setting; green circles (circles 3), zircons from meta-gabbros from divergent continental margin; blue circles (circles 4), zircons from basalts in intra-continental rifts. The legends are the same as Fig. 7.

4 CONCLUSION sure of the Paleo-Tethys Ocean. All magmatic zircons show

Magmatic zircons from three ash beds near the PTB in the Hf-isotope compositions (εHf(t)=-11.4− +0.2), consistent with Zunyi Section, South China, indicate that three episodes of derivation by melting of continental crust. The Hf-isotope magmatism occurred near the MLPB (~261.5 Ma), the WCB compositions (εHf(t)=-11.4− -4.8) of zircons from Bed ZY13 (~254.5 Ma), and the PTB (~250.5 Ma), respectively. Zircons (~250.5 Ma) indicate the input of more ancient crust material from the magmas of these three episodes share similar in the magmas. trace-element and Hf-isotope compositions. Zircon trace-element signatures indicate that the magmatism of all ACKNOWLEDGMENTS three episodes was rhyolitic-dacitic in composition with We are grateful to anonymous reviewers for their critical calc-alkaline affinity, typical of explosive volcanism in con- comments and constructive suggestionswhich have improved vergent continental-margin settings. The volcanism may have greatly the quality of the paper. We also thank Stephen Craven occurred in or near southwestern South China during the clo- for his help in sample preparation, Kevin Grant in CL imaging,

880 Qiuling Gao, Zhong-Qiang Chen, Ning Zhang, William L. Griffin, Wenchen Xia, Guoqing Wang, and et al. and both Yongsheng Liu and Wenxiu Tang in LA-ICPMS Chen, Z.-Q., Tong, J., Zhang, K., et al., 2009. Environmental analysis, and Wen Zhang in LA-MC-ICPMS analysis. This and Biotic Turnover across the Permian-Triassic study was supported by an aid grant from Chengdu Center, Boundary on a Shallow Carbonate Platform in Western China Geological Survey (No. 12120113049100-1), the Na- Zhejiang, South China. Australian Journal of Earth tional Natural Science Foundations (Nos. 40572068, 40839903 Sciences, 56(6): 775–797 and 41272044), the “111” Program (No. B08030), and an aid Chen, Z.-Q., Yang, H., Luo, M., et al., 2015. Complete Biotic grant (No. GBL11206) from the State Key Laboratory of Bio- and Sedimentary Records of the Permian-Triassic geology and Environmental Geology, China University of Transition from Meishan Section, South China: Geosciences (Wuhan), China. Some analytical data were ob- Ecologically Assessing Mass Extinction and Its Aftermath. tained using instruments funded by DEST Systematic Infra- Earth-Science Reviews, 149: 63–103 structure Grants, ARC LIEF, NCRIS, industry partners and Erwin, D. H., 2006. Extinction: How Life on Earth nearly Macquarie University. This is contribution 654 from the ARC Ended 250 Million Ago. Princeton University Press, Centre of Excellence for Core to Crust Fluid Systems Princeton (http://www.ccfs.mq.edu.au/) and 1025 in the GEMOC Key Gao, Q. L., Zhang, N., Xia, W. C., et al., 2013. Origin of Centre (http://www.gemoc.mq.edu.au/). This paper is also a Volcanic Ash Beds across the Permian-Triassic Boundary, contribution to the proceeds of geological survey and assess- Daxiakou, South China: Petrology and U-Pb Age, Trace ment of the Sanjiang mineralization belts, Southwest China. Elements and Hf-Isotope Composition of Zircon. 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