A temporal link between the Emeishan large igneous province(SW China) and the end-Guadalupian mass extinction

Mei-Fu Zhou a;*, John Malpas a, Xie-Yan Song a, Paul T. Robinson a,Min Sun a, Allen K. Kennedy b, C. Michael Lesher c, Reid R. Keays c

a Department of Earth Sciences, University of Hong Kong, Hong Kong, Chinab Department of Applied Physics, Curtin University of Technology, Bentley, WA, Australia

c Mineral Exploration Research Centre, Laurentian University, Sudbury, ON, Canada

Received 20 September 2001; received in revised form 11 December 2001; accepted 17 December 2001

Abstract

Previous studies have suggested that there were two mass extinction events in the Late Permian: one that occurred atthe Permo-Triassic (P/T) boundary (251 Ma) and a second, smaller mass extinction that occurred 5^8 Myr earlier at theend of the Guadalupian. Many workers have argued that there is a causal relationship between large-scale volcanicactivity and mass extinctions. The major mass extinction event at the P/T boundary coincides with the outpouring ofhuge quantities of lava that formed the Siberian flood basalt province in Russia. Courtillot et al. [Earth Planet. Sci.Lett. 166 (1999) 177^195] and Wignall [Earth Sci. Rev. 53 (2001) 1^33] suggested that the earlier Late Permian massextinction coincided with the eruption of the lavas that formed the Emeishan flood basalt (EFB) province in SW China.However, the age of eruption of the EFB lavas is poorly constrained. Using the Sensitive High-Resolution IonMicroprobe to analyze zircons, we have established the age of the Xinjie intrusion, believed to be a feeder to the mainphase of EFB volcanism, to be 259 þ 3 Ma. Hence, the formation of the EFB is coincident with a proposed extinctionevent at 256^259 Ma. This result supports a temporal link between the Emeishan large igneous province and the end-Guadalupian mass extinction. ß 2002 Elsevier Science B.V. All rights reserved.

Keywords: SHRIMP data; geochronology; Guadalupian; mass extinctions; Qinghai-Xizhang Plateau

1. Introduction

Volcanic eruptions associated with large igne-ous provinces (LIPs) are, in many cases, consid-ered synchronous with crises in global climate andwith mass extinctions [2^6]. It is well documented

that the Siberian traps basalts were erupted at theend of the Permian at 251 Ma [7,8]. Eruption ofthe voluminous Emeishan £ood basalts (EFB) inSW China was also previously thought to corre-late with the Permo-Triassic (P/T) boundary vol-canic layer in the Meishan section, Zhejiang prov-ince, China. Thus, the EFB, along with theSiberian traps, have traditionally been consideredto be products of synchronous mantle plumes thatcaused biological extinction at the P/T boundary[9,10]. However, the thin (10 cm) volcanic layer in

0012-821X / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved.PII: S 0 0 1 2 - 8 2 1 X ( 0 1 ) 0 0 6 0 8 - 2

* Corresponding author.Tel. : +86-852-2857-8251; Fax: +86-852-2517-6912.

E-mail address: mfzhou@hkucc.hku.hk (M.-F. Zhou).

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the Meishan section at the P/T boundary is inter-mediate-acidic in composition and is spread overa vast area including much of North China [9]and SE Siberia [11], whereas typical lithologiesof the EFB are basalt and basaltic andesite. Theprovenance of the volcanic layer needs to be re-interpreted, and the correlation of the EFB withthe P/T boundary extinction appears inappropri-ate [1,2]. A few geochronological studies (mainlyK^Ar) on the EFB have yielded ages rangingfrom 211 to 350 Ma [12,13]. The wide spread inages may re£ect post-eruption alteration or assim-ilation of older continental crust, but is still ques-tionable, as the ages have large analytical uncer-tainties. On the other hand, geologicalrelationships suggest that the EFB erupted atthe end-Guadalupian (i.e. 258 Ma) [14,15] and isthought to be contemporaneous and comparablewith the Panjal volcanics in NW India (see Re-views in [2]). Thus, precise dating of the EFB isimportant for interpreting the events that oc-curred in the Late Permian or at the P/T bound-ary. In an attempt to establish the age of theEmeishan LIP (ELIP), we have carried out preciseSensitive High-Resolution Ion Microprobe(SHRIMP) dating of zircon from the Xinjie intru-sion, which is believed to be contemporaneouswith the lavas. Our work shows that the mainphase of the EFB erupted signi¢cantly earlierthan the P/T boundary and that its age correlateswell with a proposed Late Permian mass ex-tinction at about 256^259 Ma [16]. We suggestthat eruption of the EFB was the cause ofthis extinction, and that this event predates thewell acknowledged P/T boundary extinction by5^8 Myr.

2. Geological background

The ELIP, which consists of massive £ood ba-salts and numerous contemporaneous ma¢c intru-sions, is exposed over a large part of SW Chinafrom the eastern margin of the Tibetan Plateau tothe western margin of the Yangtze Block (Fig. 1).This region was reworked during the collision be-tween the Indian and Eurasian continents [17]. Ona regional scale, the Xianshuihe and Red River

strike^slip faults demonstrate displacements (Fig.1).

Within the Yangtze Block, the Mesoproterozoicstrata are overlain by a thick sequence (s 9 km)of Sinian to Permian strata composed of clastic,carbonate, and meta-volcanic rocks [14]. Thereare abundant Sinian granites and Neoproterzoicarc plutonic^metamorphic assemblages along thewestern and northern margins of the YangtzeBlock. They range in age from 760 to 860 Maand suggest subduction of Rodinian oceanic litho-sphere toward the Yangtze Block during this timeperiod [18].

The Emeishan volcanic succession covers anarea of more than 5U105 km2 with thicknessesranging from several hundred meters up to 5 km[14,19] (Fig. 1). In the western part of the prov-ince, the volcanic succession was strongly de-formed, uplifted, and eroded as a result of theIndia^Eurasia collision during Cenozoic time.Several N^S trending strike^slip faults in theYuanmou^Xichang region have exposed Emeish-an dykes over a considerable range of crystalliza-tion depths. Although most of the £ood basalts inthis area have been eroded away, the many ma¢cfeeder dykes suggest that the lavas once extendedinto this region. The basalts have also been inter-sected in petroleum drill cores in the Sichuan ba-sin, suggesting that the Emeishan traps may haveinitially covered an area in excess of 200 000, andprobably as much as 500 000 km2.

The Emeishan volcanic succession comprisespredominantly basaltic £ows and pyroclastics,with minor amounts of picrite and basaltic ande-site. The succession overlies the Early PermianMaokou formation, which is composed of lime-stone, and is overlain by the Late Permian Xuan-wei formation, which is composed of sandstoneand mudstone with interbedded coal measures.The Xuanwei formation is, in turn, unconform-ably overlain by Triassic sedimentary rocks, theFeixianguan formation (detailed descriptions areavailable in [9,14,15]. The Yinkeng formation inMeishan, Changxiang county of Zhejiang prov-ince, an equivalent of the Feixuangun formation,overlies the Late Permian strata that includes theupper Changxiang and lower Longtan forma-tions. The Changxiang formation is divided into

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Fig. 1. Distribution of the EFBs and contemporaneous ma¢c intrusions within the ELIP (South China) (modi¢ed from [14]). Theupper right inset is the geological map of the Xinjie intrusion (according to the unpublished map of the Pan-Xi Geological Teamof Sichuan province and our own ¢eld observation). SGT = Songpan-Ganze Terrane; YB = Yangtze Block.

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the lower Baoqing and upper Meishan members(Fig. 2). On top of the Meishan member are in-termediate to acidic volcanic deposits, mainly sub-marine tu¡s typically altered to clay. In its typearea, this volcanic layer has been dated at 251 Maby the SHRIMP zircon U^Pb method [20,21] and

at 250 Ma by the Ar/Ar method on feldspar [7].Chung and Jahn [19] considered it to representthe last phase of the Emeishan volcanism. Thelayer is only about 10 cm thick on top of thelimestone, but it extends over 12 provinces inSouth China and North China [9] and is also

Fig. 2. Idealized column section of the P/T sedimentary sequence within the ELIP, showing that the EFBs lie below the LatePermian Xuanwei formation but above the Maokou formation (from [14] and our own ¢eld observation). Also shown is the P/Tboundary column section in Meishan, Zhejiang province, eastern China (after [9]).

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reported to occur in SE Siberia [11]. Although theprovenance of this P/T boundary layer is not yetclear, the eruption age of the main phase of theEFB needs to be determined.

Ultrama¢c/ma¢c intrusions within the ELIPhave been important mineral exploration targetsfor the Chinese, but their origin and ages of em-placement are poorly understood. In the centralpart of the province, from Yuanmou to Panzhi-hua and Xichang, several intrusions contain giantV^Ti^magnetite deposits and many contain Ni^Cu^(PGE) sul¢de deposits. We have selected theXinjie intusion (Fig. 1) for detailed age dating inorder to place constraints on the age of the asso-ciated lavas. This intrusion contains mineablemagmatic Ni^Cu^(PGE) sul¢de and V^Ti^mag-netite deposits, which were previously exploitedand are presently under re-evaluation.

3. Geology of the Xinjie intrusion

The Xinjie intrusion is a ma¢c/ultrama¢c sill,2 km long and 1^1.5 km wide, that intruded theEmeishan basalts (Fig. 1, inset). The sill exhibitsigneous layering and may be divided into threecyclic layers: I, II, and III. Layer I is composed,from the base upward, of peridotite, olivine py-roxenite, gabbro, and quartz gabbro. Layer IIcomprises olivine pyroxenite and ¢ne-grained gab-bro, whereas layer III comprises pyroxenite andquartz gabbro. The di¡erent rock types withinindividual layers are transitional. The base ofthe intrusion is marked by a marginal zone com-posed of gabbro with abundant blocks of felsicmaterial. This zone contains magnetite and Ni^Cu^(PGE) sul¢de ores.

4. SHRIMP zircon geochronology of the ELIP

4.1. Analytical methods

Zircons were separated from Xinjie gabbro us-ing conventional heavy liquid and magnetic tech-niques. The zircons were mounted in epoxy, pol-ished, and coated with gold. The mounts werethen photographed in transmitted and re£ected

light for identi¢cation of analyzed grains. Catho-doluminescence images were obtained on a PhilipsXL30 scanning electron microscope. The instru-mental techniques for isotopic analysis of zirconsusing the SHRIMP II at the Curtin University ofTechnology are similar to those described byCompston et al. [22].

All isotopic measurements were reduced by o¡-line computer programs using standard tech-niques. The calculation of 206Pb/238U ages is basedon the assumption that the bias in the measured206Pbþ/238Uþ ratio relative to the true ratio can bedescribed by the same power law relationship be-tween 206Pbþ/238Uþ and UOþ/Uþ for both the zir-con standard and sample [22]. Pb/U ages are nor-malized to a value of 564 Ma determined byconventional U^Pb analysis of the standard zirconCZ3. The 206Pb/238U and 207Pb/235U data havebeen corrected for uncertainties associated withthe measurement of the CZ3 standard. The uncer-tainties of 207Pb/206Pb ages are independent of thestandard analyses, but are sensitive to the commonPb correction in low U zircons that have beencalculated for zircons with ages less than 1000Ma. Because the 207Pb/206Pb ages are sensitive tothe common Pb correction, 206Pb/238U ages arenormally preferred. Common Pb was corrected us-ing the 204Pb method discussed by Compston et al.[22]. The analytical results are listed in Table 1.

4.2. Results

The analyzed sample x101, about 10 kg, wastaken from the gabbro zone of the Xinjie intru-sion. This sample contains hornblende gabbromatrix with abundant xenoliths and disseminatedsul¢des. The sul¢de ores are believed to representimmiscible sul¢de liquids formed by incorporationof wall rocks into the magma chamber. Twogroups of zircons have been identi¢ed in this sam-ple. The ¢rst population consists of clear, paleyellow^brown crystals with distinct euhedral ter-minations. The second group is morphologicallysimilar to the ¢rst, but consists of dark yellow^brown grains. Grains in the second group displaya range of isotopic compositions that exceeds theanalytical error for each of the U and Th^Pbisotopic systems.

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Tab

le1

U^T

h^P

bSH

RIM

Pan

alyt

ical

resu

lts

for

zirc

ons

from

the

Xin

jiein

trus

ion

Gra

inC

once

ntra

tion

Cal

cula

ted

rati

osC

alcu

late

dag

es(p

pm)

(in

Ma)

UT

hP

bT

h/U

207/

206

þ20

8/20

6þ

206/

238

þ20

7/23

5þ

206/

238

þ20

7/23

5þ

207/

206

þ

x101

-192

213

2449

1.43

712

0.05

1093

0.00

060.

4435

220.

003

0.04

170.

0007

0.29

360.

0064

263

426

15

245

26x1

01-2

1180

1878

631.

5918

0.05

1438

0.00

070.

5015

740.

003

0.04

0.00

070.

2839

0.00

6825

34

254

526

133

x101

-329

244

316

1.51

673

0.04

834

0.00

240.

4726

180.

007

0.04

20.

0008

0.27

990.

0153

265

525

112

116

111

x101

-412

9753

094

0.40

834

0.05

5475

0.00

050.

1260

410.

001

0.07

110.

0012

0.54

380.

0111

443

744

17

431

20x1

01-5

5051

31.

0166

40.

0519

290.

0095

0.31

4097

0.02

40.

0493

0.00

120.

3533

0.06

6231

07

307

5028

237

2x1

01-6

335

521

181.

5563

80.

0464

790.

0025

0.47

4084

0.00

70.

0406

0.00

070.

2605

0.01

5225

75

235

1234

111

x101

-747

036

325

0.77

093

0.05

2544

0.00

170.

2327

890.

004

0.04

840.

0009

0.35

040.

0134

305

530

510

309

72x1

01-8

363

237

190.

6534

50.

0527

890.

0009

0.20

2255

0.00

20.

0477

0.00

090.

347

0.00

9130

05

302

732

037

x101

-929

817

815

0.59

748

0.05

402

0.00

10.

1920

080.

003

0.04

810.

0009

0.35

840.

0097

303

531

17

372

41x1

01-1

015

2630

0792

1.97

075

0.05

0255

0.00

060.

6103

510.

003

0.04

190.

0007

0.29

040.

0064

265

425

95

207

28x1

01-1

115

914

09

0.87

811

0.05

0187

0.00

290.

2754

910.

008

0.05

120.

001

0.35

460.

0221

322

630

817

204

131

x101

-12

1090

400

760.

3674

60.

0566

840.

0006

0.11

197

0.00

20.

0694

0.00

130.

5427

0.01

2343

38

440

847

922

x101

-13

1847

462

126

0.25

0.05

5929

0.00

030.

0738

35E

-04

0.07

030.

0012

0.54

220.

0101

438

744

07

450

13x1

01-1

413

3571

686

0.53

654

0.05

483

0.00

120.

1629

530.

003

0.05

980.

001

0.45

240.

0136

375

637

910

405

51x1

01-1

534

417

212

70.

4997

70.

1188

330.

0005

0.13

4909

7E-0

40.

3419

0.00

65.

6012

0.10

3518

9629

1916

1619

398

x101

-16

266

463

151.

7397

80.

0470

820.

0031

0.53

5821

0.01

0.04

010.

0008

0.26

020.

0183

253

523

515

5814

4x1

01-1

742

811

6729

2.72

451

0.05

0674

0.00

160.

8637

420.

007

0.04

050.

0007

0.28

310.

0106

256

425

38

226

71x1

01-1

814

278

480.

5531

70.

1124

80.

0011

0.15

770.

002

0.30

620.

0055

4.74

830.

1011

1722

2717

7618

1840

17x1

01-1

920

938

112

1.82

307

0.05

1491

0.00

260.

5651

60.

009

0.04

070.

0008

0.28

890.

0165

257

525

813

263

117

x101

-20

234

108

170.

4616

10.

0543

610.

003

0.13

9912

0.00

70.

072

0.00

130.

5395

0.03

2644

88

438

2138

612

4x1

01-2

115

725

79

1.63

367

0.05

4253

0.00

140.

5080

780.

007

0.04

080.

0008

0.30

50.

0103

258

527

08

382

58x1

01-2

226

047

915

1.84

126

0.05

4483

0.00

110.

5917

960.

006

0.04

120.

0007

0.30

960.

0089

260

527

47

391

45

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All of the zircons in the ¢rst group (10 analyseson 10 grains) have high Th/U ratios (s 1.5) andyield an average age (206Pb/238U) of 259 þ 3 Ma(Fig. 3). The 207Pb/235U age is 259 þ 5 Ma, where-as the 207Pb/206U age is 249 þ 14 Ma.

Several di¡erent ages have been identi¢ed in thesecond group of zircon grains. Five analyses withTh/U ratios between 0.6 and 1.0 yielded an aver-age 206Pb/238U age of 307 þ 2.6 Ma. Four analyseswith even lower Th/U ratios (0.32^0.46) yieldedan average age of 440 þ 4 Ma. Two old, roundedand detrital zircon grains (one analysis of each)have ages of 1722 Ma and 1896 Ma and show lowTh/U ratios of 0.55 and 0.5, respectively. Oneanalysis of a small euhedral grain yielded an ageof 375 Ma (Fig. 3).

The Xinjie intrusion contains abundant olderzircon grains which are interpreted as being xeno-crystic. In ma¢c dykes, sills and plugs, and inparticular those that are feeder conduits, heat re-leased during cooling and crystallization oftencauses partial fusion of the wallrocks [23]. Thereare many descriptions of ma¢c lavas that containpartially fused crustal xenoliths. The di¡erentgroups of zircons have distinct Th/U ratios, at-tributable to di¡erent origins. Those of the ¢rstgroup are clearly higher than in the other zircongrains (Fig. 3), suggesting that they form a dis-tinct magmatic group. The detrital zircons wouldbe derived from sedimentary rocks, whereas zir-cons with ages of 440 Ma, 375 Ma and 307 Ma

are likely from granitic rocks, since granites withsimilar ages are known in the surrounding area[14]. Therefore, the average age for the ¢rstgroup, 259 þ 3 Ma, likely represents the crystalli-zation age of the Xinjie intrusion.

5. Discussion

5.1. Origin and timing of the ELIP igneous activity

Continental £ood basalt (CFB) magmatism hasbeen attributed to the ascent of mantle plumeheads, possibly originating close to the core^man-tle boundary [24,25], which resulted in short-lived,highly productive magmatic events (eruptive ratesof 0.1 to s 1.0 km3/yr) [26]. Chung et al. [10] (seealso [15]) have proposed that the EFB originatedfrom a mantle plume that reached the base of theSouth China block coincident with the P/Tboundary. Production of the basalts of the ELIPwas a rapid CFB event. Based on magnetostrato-graphic analysis, Huang and Opdyke [27] haveproposed that eruption of the entire EFB tookplace in less than 1 Myr.

The EFB has previously been dated by Chinesegeologists as Late Permian on the basis of strati-graphic correlation [9,15]. The volcanic succes-sions lie unconformably on the early Late Perm-ian Maokou formation (corresponding to theCapitanian/Kazanian stage) and are in places

Fig. 3. Concordia plots (left) and plots of Th versus U (right) of the SHRIMP zircon results for the Xinjie intrusion.

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overlain by the Late Permian Xuanwei formationof Dzhul¢an age [15] (Fig. 2). The Dzhul¢an im-mediately follows the Midian (and Wujiapingianfollows the Maokou), which are assumed to belateral equivalents of the Guadalupian. Recently,Jin and Shang [28] reported the presence of fusu-linid foraminifera from shallow marine limestonesinterbedded with basalt lava £ows, which un-equivocally indicate a Guadalupian age. There-fore, the stratigraphic data agree with the inter-pretation that the Emeishan traps were erupted atthe end of the Guadalupian, at ca. 258 Ma.

As pointed out by White [29], only about onequarter of the basaltic melt in a typical £ood ba-salt province is extruded as surface £ows, with theremainder being underplated or intruded into thecrust as dykes and sills. The Xinjie intrusion isbelieved to be consanguineous with and thereforesynchronous with the eruption of the EFB, so the259 þ 3 Ma age of the EFB is interpreted to be theeruption age of the EFB. These new SHRIMPzircon data therefore provide the ¢rst precisedate for the ELIP, which is remarkably consistentwith geological relationships.

5.2. Double extinctions in Late Permian

Links between mass extinctions and the erup-tion of CFBs have been postulated by several au-thors [3,5]. There is certainly a clear temporalrelationship between three of the major mass ex-tinctions and magmatic activity associated withLIPs. It has been proposed that the P/T biologicalcrisis was caused by the huge volcanic event thatproduced the Siberian traps [1,30,31]. Volcaniceruption of the Central Atlantic magmatic prov-ince at 200 Ma [32,33] is coeval with the end ofTriassic crisis [34], whereas the end of Cretaceousmass extinction occurred at 65 Ma during thevolcanic eruption of the Deccan traps [35]. Suchtemporal coincidences require consideration of apossible causal linkage.

The Late Permian appears to have two distinctextinction events. Stanley and Yang [16] ¢nd twopeaks in the rates of extinction, one at the end ofthe Guadalupian (V258 Ma) and another at theend of the Tatarian (V251 Ma), i.e. the P/Tboundary. Holser and Magaritz [36] pointed out

two sharp minima of sea level at these times.Based on their literature reviews, Courtillot etal. [1] proposed that the EFB were erupted atthe end of the Guadalupian, causing the earlierof these two phases of mass extinction. Our studyprovides the ¢rst direct evidence of a temporalrelationship between eruption of the EFB andthe mass extinction at 258 Ma and supports theidea of a causal relationship. Our results discountout a temporal coincidence between the EFB andthe mass extinction episode at the P/T boundary.

Sudden climate and other environmentalchanges associated with the EFB volcanism couldhave triggered the end-Guadalupian extinction.This event has only been identi¢ed in the lastfew years and primarily a¡ected equatorial marineinvertebrate taxa, notably fusulinid foraminifera[16]. The Siberian traps and the EFB were eruptedat very di¡erent latitudes, the former near 60^70‡N, the latter near the equator (see ¢gure 8 in[1]). Despite several parallels with the end-Perm-ian mass extinction (linked to £ood basalt erup-tion, climate warming, anoxia, and isotopictrends), the end-Guadalupian extinction is clearlyof a smaller magnitude. The smaller size of theEFB might explain its less severe e¡ect on globalbiota.

6. Conclusions

This study suggests that the ma¢c/ultrama¢cintrusions in the western margin of the YangtzeBlock are parts of the ELIP and are feeders to thelavas. The intrusions and lavas appear to be de-rived from melts produced by the same mantleplume at 259 Ma. There is a clear temporal rela-tionship between the ELIP and the end-Guadalu-pian extinction at about 259 Ma. The mass ex-tinction at this time may re£ect climatic andother environmental changes related to eruptionof the EFB. However, further study of its geo-chemical and ecological consequences is war-ranted. Detailed geochronologic studies, such as40Ar/39Ar techniques, which have been recentlyapplied extensively to the Deccan and Siberiantraps, would provide further constraints on thetiming of the EFB eruption.

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Acknowledgements

This study was fully supported by Grants fromthe Research Grant Council of the Hong KongSAR, China (HKU7301/99P and HKU7101/01P)to M.F.Z./J.M./C.M.L./R.R.K. Zircon analyseswere carried out on the Sensitive High ResolutionIon Micro Probe mass spectrometer (SHRIMP II)operated by a consortium consisting of CurtinUniversity of Technology, the University of West-ern Australia, and the Geological Survey of West-ern Australia, with the support of the AustralianResearch Council. We thank Professors V. Cour-tillot and Euan Nisbet for providing insightful re-views that helped to improve the manuscript. The¢rst author is grateful to Professor S.-S. Sun forstimulating discussions and comments on the sub-ject.[AC]

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