Age of the Emeishan £Ood Magmatism and Relations to Permian^Triassic Boundary Events

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Earth and Planetary Science Letters 198 (2002) 449^458 www.elsevier.com/locate/epsl

Age of the Emeishan £ood magmatism and relations to Permian^Triassic boundary events

Ching-Hua Lo a;Ã, Sun-Lin Chung a, Tung-Yi Lee b, Genyao Wu c

a Department of Geosciences, National Taiwan University, 245 Choushan Road, Taipei 106, Taiwan b Department of Earth Sciences, National Taiwan Normal University, Taipei, Taiwan c Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, PR China

Received 27 June 2001; received in revised form 12 February 2002; accepted 13 February 2002

Abstract

The Permian^Triassic (P^T) mass extinction, the greatest biological mortality event in the Earth’s history, was probably caused by dramatic and global forcing mechanisms such as the Siberian flood volcanism. Here we present the first set of high-precision 40Ar/39Ar dating results of volcanic and intrusive rocks from the Emeishan Traps, South China, which define a main stage of the flood magmatism at V251^253 Ma and a subordinate precursory activity at V255 Ma. This time span is generally coeval with, or slightly older than, the age of the P^T boundary estimated by the ash beds in the Meishan stratotype section and the main eruption of the Siberian Traps. Our data reinforces the notion that the eruption of the Emeishan Traps, rather than eruption of the Siberian Traps, accounted for the formation of the P^T boundary ash beds in South China. The Emeishan flood magmatism, which occurred in the continental margin comprising thick marine limestone formations, moreover, may have triggered rapid release of large volumes of methane and carbon dioxide that could have been responsible for the global N13C excursion and associated environmental crisis leading to the mass extinction at the P^T boundary. ß 2002 Elsevier Science B.V. All rights reserved.

Keywords: £ood basalts; South China Block; Permian^Triassic boundary; mass extinctions; Ar-40/Ar-39

1. Introduction this biologic great dying was slow, lasting at least several million years [1,2] and even occurred as Mass extinction at the Permian^Triassic (P^T) two or more separate events [3,4]. Causes for boundary was the most profound event in the his- such a prolonged extinction are therefore hy- tory of life on Earth. Nearly 90% of all species in pothesized to have been gradual processes such the ocean and 70% of vertebrate genera on the as sea level fall and climate change. To constrain continent vanished by the end of Paleozoic Era the tempo of the end-Permian extinction, Bowring [1,2]. Traditionally, paleontologists believe that et al. [5] conducted a detailed U^Pb zircon geochronological study of the P^T marine boundary sections in Meishan, South China, and placed the

* Corresponding author. Tel.: +886-2-2363-5880; P^T boundary at Bed 25 that was dated to be Fax: +886-2-2363-6095. 251.4 þ 0.3 Ma. Combining biostratigraphic infor- E-mail address: [email protected] (C.-H. Lo). mation [5^7] and the negative excursion in N13C

EPSL 6171 26-4-02 450 C.-H. Lo et al. / Earth and Planetary Science Letters 198 (2002) 449^458 across the boundary [5,7], these authors further- tracted numbers of recent investigations [15^20]. more suggested that the Changhsingian pulse of In this paper, we report the ¢rst set of high qual- the end-Permian extinction was rapid and lasted ity 40Ar/39Ar dating results of representative sam- less than 1 Myr. Such rapidity, however, has been ples from the Emeishan Traps, which de¢ne a recently questioned by Mundil et al. [8] based on main phase of the Emeishan £ood magmatism new U^Pb zircon data from the same boundary at V251^253 Ma and an earlier, but subordinate, layers. The age of Bed 25 has been rede¢ned to be phase at V255 Ma. These age data are generally V254 Ma and the P^T boundary moved to Bed coincident with, or slightly older, if considering all 27 (Fig. 1A), which should be slightly older than the potential systematic errors between the 40Ar/ Bed 28 dated as 252.5 þ 0.3 Ma [8]. Thus, the 39Ar and U^Pb methods, than the ages of the P^T main stage of the end-Permian extinction oc- boundary at Meishan and the Siberian Traps. curred at V253 Ma and the N13C excursion ap- This time sequence supports the hypothesis that pears to be signi¢cantly longer than previously the Emeishan eruption, in particular its later postulated (Fig. 1A). phase of felsic composition, could have accounted Among numerous scenarios proposed to ex- for the widespread deposition of the P^T bound- plain the P^T extinction [1^5], the Siberian £ood ary ash layers in South China. Given the fact that volcanism that represents the largest subaerial the Emeishan Traps were emplaced around the volcanic event in the Phanerozoic record has continental margin underlain by thick limestone been most widely accepted. U^Pb and 40Ar/39Ar formations, the vast eruption in a ‘marine’ envi- dating investigations [6,9^12] repeatedly argued ronment is proposed to have played a key role in that emplacement of the immense Siberian Traps triggering the profound biogeochemical changes ( s 2.5U106 km2 in area) took place at the P^T across the P^T boundary. boundary. Campbell et al. [9] proposed that eruption of the Siberian Traps injected vast volumes of volcanic dust and sulfate aerosols into the at- 2. The Emeishan Traps mosphere, a process that could have caused a short-lived volcanic winter. This volcanic winter, In South China, P^T boundary stratotype sec- according to the hypothesis by Renne et al. [11], tions that contain abundant interbedded volcanic was followed within several hundred thousand ash layers are widespread [5,6,21]. Owing essen- years by greenhouse conditions because the erup- tially to the synchronism between the Meishan tion would have also released large amounts of ash beds and the Siberian £ood volcanism, Camp- volcanogenic gases composed primarily of CO2. bell et al. [9] speculated that these boundary ash Such a cooling^warming climate cycle could layers are genetically linked to the Siberian erup- have resulted in an environmental crisis capable tion despite the fact that they were deposited sev- of causing the ¢erce mass extinction [6,9^12]. eral thousand kilometers in the south around the Being one of the large igneous provinces that equator (Fig. 1A) [16]. The P^T boundary ash occurred also around the P^T boundary [13,14], layers in South China are characterized by con- the eruption of the Emeishan Traps, South China, centrations of volcanogenic microspherules and and its relations to the boundary events have at- bipyramid quartz grains [21]. Based on geochem-

C Fig. 1. (A) At V253 Ma, the Siberian Traps were emplaced in the Arctic area, whereas the Emeishan Traps and Meishan P^T boundary ash beds (star) occurred in South China near the equator. The paleogeographic reconstruction is taken from Courtillot et al. [16] and inset for the Meishan section and relevant data is adopted from Mundil et al. [8]. (B) Outcrops of the Emeishan Traps in the western South China Block (black areas) and associated volcanic rocks in the Songpan-Ganze and Qiangtang terranes [14,15]. Insets illustrate distribution of major terranes in East Asia and two volcanic sections (Ertan and Bingchuan) of the Traps [23] from which samples were collected for this study. In the central part of the Emeishan Traps, close to the Ertan section, M and P indicate the locations of two large intrusions (Maomaogou and Panzhihua) from which the dated syenites were collected.

EPSL 6171 26-4-02 C.-H. Lo et al. / Earth and Planetary Science Letters 198 (2002) 449^458 451

EPSL 6171 26-4-02 452 C.-H. Lo et al. / Earth and Planetary Science Letters 198 (2002) 449^458 ical data, Zhou and Kyte [22] proposed that these associated with large V^Ti^Fe ore deposits ash layers resulted from massive silicic eruptions [15,24^26]. Gabbros and syenites from the Mao- from a nearby region, which have been ascribed maogou and Panzhihua complexes (Fig. 1B) pos- to £ood volcanism in the Emeishan (basalt) Traps sess geochemical and Sr^Nd isotopic composi- occurring in the western part of the South China tions comparable to those of the nearby basalts Block [15] (Fig. 1B). and trachytes^rhyolites, respectively, strongly sug- The Emeishan Traps, a large igneous province gesting a genetic link between the intrusive and formed by a mantle plume head [14], are generally volcanic rocks in the Emeishan Traps [15,23]. Re- referred to as erosion remnants of the P^T mas- cently, a SHRIMP U/Pb zircon dating study was sive volcanic successions which occurred predom- carried out for two of the gabbro-peridotite intru- inantly as ma¢c lava £ows and pyroclastics in the sions, yielding crystallization ages of 258.7 þ 1.5 western Yangtze (South China), Songpan-Ganze and 256.0 þ 1.0 Ma [17]. However, these are still and eastern Qiangtang terranes [15,18^20] (Fig. indirect age constraints inferred from the intrusive 1B). The volcanic successions, tilted and frag- rocks whose exact time relation with the main mented by complicated tectonic events during stage of the Emeishan eruption is unknown. Pre- Meso-Cenozoic times, are exposed in a rhombic cise dating directly on the volcanic successions is area of V2.5U105 km2 (500U500 km) in associ- hence highly desirable. ation with numerous intrusive bodies of ultrama¢c/ma¢c to felsic compositions. The volcanic sequence thickness ranges from V5kminthe 3. Analytical methods Bingchuan section (Fig. 1B) in the southwestern part of the Traps to several hundred meters in the Six whole-rock samples from basaltic £ows, eastern margin. The Emeishan £ood basalts show hornblende and biotite separates from a trachyte spatial and temporal variations. The basaltic lavas £ow and two syenite bodies, respectively, were can be divided into two major groups, i.e., the dated by 40Ar/39Ar step-heating and single-grain upper group with high-Ti (TiO2 s 3.5 wt%) and fusion methods using furnace and laser heating the lower group with low-Ti (1.5 s TiO2 s 2.5 techniques. Samples were ¢rst crushed and disin- wt%) compositions [23]. Lavas of the low-Ti tegrated. After sieving, mineral grains and rock group are con¢ned to the lower volcanic succes- chips in the range of 140^250 Wm were ultrasoni- sions in the western part of the Traps, whereas cally cleaned in distilled water and dried, and then lavas of the high-Ti group occur as the predom- handpicked to remove any visible contamination inant component in the upper succession in nearly before further mineral separation. The samples the entire region [23]. In localities, thick piles of were then irradiated together with the LP-6 bio- trachyte and/or rhyolite form an important mem- tite standard [27] in the VT-C position at the ber in the upper sequence. The successions uncon- THOR Reactor in Taiwan for 10 h (EM-90 ba- formably overlie the early Late Permian Maokou salt); 24 h (EM-86 hornblende, EM-PZH01 bio- Formation (composed mainly of marine lime- tite and EM-13 and EM-15 basalts); and 30 h (the stones corresponding to the Capitanian/Kazanian remaining samples). In order to monitor the neu- stage) and underlie the lower Triassic clastic sedi- tron £ux in the reactor, three aliquots of the LP-6 ments [15,18,26]. On the basis of magnetobiostra- standard, weighted in the range of 6V10 mg, tigraphic correlations, eruption of the traps was were stacked with samples in each irradiation traditionally accepted to have taken place in the package with a length of V9 cm. After irradia- Late Permian, despite that previous geochrono- tion, standards and samples were either incremen- logical data revealed a very scattered duration of tally heated or totally fused using a double-vac- V40^260 Ma [20,24^26]. There are numerous in- uum resistance furnace and/or a US LASER Nd- trusive bodies exposed in the Emeishan Traps. YAG laser operated in continuous mode, and the These intrusions range from ultrama¢c and ma¢c gas was measured by a VG3600 mass spectrome- to felsic compositions, some of the ma¢c bodies ter at the National Taiwan University. The J val-

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Fig. 2. Apparent age spectra of the step-heating analyses for: (A,B) two high-Ti basalts (whole rock), (C) a trachyte (hornblende), (D^F) three syenites (biotites) and (G,H) two low-Ti basalts from the Emeishan Traps. Plateau ages are calculated within arrows, which indicate gas fractions used for plateau age calculations. The vertical height of each step, shown as black hori- zontal bar, represents 2c external error. All errors shown are 1c which include uncertainties derived from the age of the irradiation standard. ues are calculated using argon compositions of the and 2 of the Background Data Set1, and the LP-6 biotite standard, with a 40Ar/39Ar age of data are plotted as age spectra and in isotope 128.4 þ 0.2 Ma, calibrated according to the age correlation diagrams in Figs. 2 and 3. of the Fish Canyon biotite by assuming that has the same age as the Fish Canyon sanidine (28.02 þ 0.28 Ma) [12,28]. Ages were calculated 4. Analytical results from Ar isotopic ratios measured after corrections made for mass discrimination, interfering nuclear For the basaltic rocks, two samples (EM-37 reactions, procedural blanks, and atmospheric Ar contamination. Detailed results of the 40Ar/39Ar experiments of this study are given in tables 1 1 http://www.elsevier.com/locate/epsl

EPSL 6171 26-4-02 454 C.-H. Lo et al. / Earth and Planetary Science Letters 198 (2002) 449^458

long to the high-Ti basalt group that represents the main eruption in the upper sequence of the Emeishan Traps. 40Ar/39Ar step-heating analyses of the whole-rock samples of EM-90 and EM-37 yielded £at age spectra with identical plateau ages of 251.5 þ 0.9 and 252.0 þ 1.3 Ma, respectively (Fig. 2A,B). In the 36Ar/40Ar versus 39Ar/40Ar correlation diagrams, the data of both samples de¢ne linear arrays with reasonable values of the mean square of the weighted deviates (MSWD; 2.501 for EM-90 and 0.998 for EM-37). The intercept ages and 40Ar/39Ar initial ratios, 251.2 þ 1.3 Ma and 299 þ 28 for EM-90 and 252.7 þ 1.6 Ma and 291.0 þ 5.3 for EM-37, are consistent with their respective plateau ages and the atmospheric 40Ar/36Ar ratio (295.5). Excess argon is not apparent despite that alteration might have occurred in sample EM-90, as re- £ected by some minor disturbances in the age spectrum. On the other hand, laser fusion experiments on the whole-rock sample of EM-52 gave apparent dates (n = 49) in the range of 249.0^ 255.6 Ma with a total gas age of 252.8 þ 1.3 Ma, and yielded an intercept age of 252.1 þ 1.4 Ma and 40Ar/36Ar initial value of 299 þ 3 (MSWD = 1.002) (Fig. 3A). This intercept age, in consideration with the data distribution and individual errors, is in good agreement with the two plateau dates obtained by the step-heating experiments. Moreover, a trachyte (EM-86) that underlies sample EM-90 from the Ertan section (Fig. 1B) was dated using hornblende separates and yielded a plateau age of 252.8 þ 1.3 Ma (Fig. 2C), and an intercept age and 40Ar/36Ar initial value of 252.6 þ 1.6 Ma and 285 þ 49, respectively. These dating results indicate that the main Fig. 3. 36Ar/40Ar^39Ar/40Ar isotope correlation diagrams for: eruption of the Emeishan Traps, constrained by (A) a whole-rock basalt, and (B,C) biotite separates from the upper Bingchuan (V3000 m thick) and Ertan two syenites, obtained by single-grain laser fusion experi- (V1000 m) sections, was of short duration be- ments. Individual data points are presented by solid circles, tween V251^253 Ma. with þ 1c error ellipses. Biotite separates from two large syenite intrusions in the Maomaogou and Panzhihua com- and EM-52) were from the upper part of the plexes, close to Ertan (Fig. 1B), were also dated Bingchuan section and one (EM-90) from the by the 40Ar/39Ar method. These include furnace top of the Ertan section; the latter sample was step-heating analyses of a Maomaogou sample collected from the £ow that underlies mid-Triassic (EM-MMG05) and two Panzhihua samples clastic formations and overlies a thick (V80 m) (EM-PZH01 and 11) (Fig. 2D^F) and single-grain trachyte £ow (Fig. 1B). These three samples be- laser fusion analyses of a biotite sample from each

EPSL 6171 26-4-02 C.-H. Lo et al. / Earth and Planetary Science Letters 198 (2002) 449^458 455 complex (EM-MMG01 and EM-PZH11) (Fig. [20]. Considering that these low-Ti samples are 3B,C). Sample EM-PZH11 was dated by both fresh in terms of petrography and geochemistry step-heating and total fusion methods. In the [23], we tentatively ascribe such serious distur- step-heating analyses, all three samples show £at bance features in the Ar systematics to the very 39 age spectra with s 87% of ArK released. The complicated Meso-Cenozoic tectonothermal his- Maomaogou sample (EM-MMG05) and one of tory of this area [15,20], which, in particular, in- the Panzhihua samples (EM-PZH11) yielded iden- cludes intensive crustal deformation and mag- tical plateau ages of 252.0 þ 1.3 and 251.6 þ 1.6 matic activity related to the India^Asia collision Ma (Fig. 2D,F), respectively, whereas another [29]. Therefore, the initiation age of the Emeishan Panzhihua sample (EM-PZH01) gave a slightly £ood volcanism has yet been accurately con- older plateau age of 254.6 þ 1.3 Ma (Fig. 2E). In strained and further works including careful and the isotope correlation diagrams, the intercept detailed sampling and high-precision dating of the ages and 40Ar/36Ar initial values are 251.4 þ 1.9 early phase of this volcanism are anticipated to be Ma and 312 þ 37 (EM-MMG05), 251.4 þ 1.2 Ma done. and 303 þ 5 (EM-PZH11), and 254.9 þ 1.5 Ma and 291 þ 33 (EM-PZH01), all in excellent agreement with their respective plateau ages and the atmo- 5. Discussion and conclusion spheric composition. The laser fusion analyses gave similar intercept ages of 251.9 þ 1.3 Ma The above results lead us to conclude that the (EM-MMG01) and 251.2 þ 1.2 Ma (EM- main stage of the Emeishan magmatism took PZH11), both with reasonable 40Ar/36Ar initial place in a short duration of V251.2^252.8 Ma ratios and MSWD values (Fig. 3B,C). The data (with errors in the range of þ 1.3^1.6 Ma). This of sample EM-PZH11 obtained by two indepen- age span appears to be slightly older than the dent procedures match fairly well, indicating good principal activity of the Siberian Traps, de¢ned reproducibility and accuracy. These biotite 40Ar/ by the recalculated 40Ar/39Ar ages of basaltic 39Ar dates support the conclusion reached by the £ows (249.9 þ 0.2 Ma) and the Norils’k intrusion whole-rock dating results that the major stage of (250.0 þ 0.2 Ma) [12]. Moreover, the older date the Emeishan magmatism took place in a short (254.6 þ 1.3 Ma) of the Panzhihua syenite (EM- period of V251^253 Ma. The relatively older PZH01) implies a precursory intrusion that may date of 254.6 þ 1.3 Ma for EM-PZH01 biotite, have accompanied the eruption of the lower vol- therefore, may imply an earlier intrusion that oc- canic sequence in the Bingchuan section. More curred V255 Ma. This appears to be consistent ma¢c intrusions may have occurred in the nearby with the notion by the SHRIMP U^Pb zircon region, as revealed by the U^Pb zircon data [17], dating results [17], which suggest the Emeishan- suggesting magmatic activity around 256V258 related magmatism to have started as early as Ma. Such precursory magmatism that forms as V256^258 Ma. However, the interval between the earliest manifestation of a mantle plume has these intrusions and the main phase of the been documented to have occurred several million Emeishan eruption remains unclear and requires years before the £ood volcanism in the Siberian more detailed investigations. Traps [30] and the end-Cretaceous Deccan Traps We also dated some low-Ti group basaltic lavas in India [31]. The precursory Siberian magmatism (e.g., EM-13 and 315; Fig. 1B) from the lower in the Maimecha-Kotui subprovince, dated at volcanic sequences. However, no statistically 253.3 þ 2.6 Ma [30], has been considered respon- meaningful 40Ar/39Ar dating results can be ob- sible for initiating the marked decrease in sea- tained. Both samples EM-13 and EM-15 yielded water 87Sr/86Sr (to values 6 0.707) that took place disturbed age spectra with wide ranges of 40Ar/ before the P^T boundary by assuming that the 39Ar apparent ages (Fig. 2G,H). Similar disturbed decrease in seawater 87Sr/86Sr re£ects an increas- age spectra have also been reported in a recent ing mantle-derived component and no evidence 40Ar/39Ar dating study on the Emeishan rocks exists for a heightened mid-ocean ridge basalt ac-

EPSL 6171 26-4-02 456 C.-H. Lo et al. / Earth and Planetary Science Letters 198 (2002) 449^458 tivity at this time [11]. In comparison to the with- argued for a causal link between the acidic erup- in-continent occurrence of the Siberian Traps, the tions in the Emeishan Traps and the P^T bound- Emeishan magmatism, which occurred in the ary ash beds widespread in South China. Addi- western continental margin of the South China tional 40Ar/39Ar and U^Pb data for the ash layers Block along the Panthalassan and Tethyan mar- in Meishan and other localities, as well as the gins can serve as an even better candidate for the Emeishan lavas, would certainly be required to seawater 87Sr/86Sr decrease. In the lower sequence further test such a hypothesis. in the Bingchuan section, basaltic £ows are Although the eruption of the Siberian Traps is marked by pillow structures indicative of a sub- widely accepted to have played a key role in caus- marine eruption environment [15,24,25]. ing the environmental changes across the P^T Compared to the P^T stratotype section at boundary, it can not be the only mechanism be- Meishan, the 40Ar/39Ar ages (V251^253 Ma) of cause the negative N13C excursion was too large to the main stage of the Emeishan magmatism are be explained by any single event [5]. Prominent slightly older than the 40Ar/39Ar age of Bed 25 and rapid excursions in N13C have been also re- dated as 250.0 þ 0.2 Ma [6] but seem to match ported at several periods in the geologic record, in face values with the P^T boundary age e.g., at V183 Ma in the Jurassic [32], V120 Ma (V253 Ma) newly de¢ned at Bed 27 (Fig. 1A) in the Cretaceous [33] and V55 Ma in the latest by the U^Pb dating study [8]. Note that the Paleocene [34,35], and these geochemical pertur- U^Pb age of Bed 25, the previously postulated bations have been interpreted as consequences of boundary layer dated as 251.4 þ 0.3 Ma [5], has massive release of methane stored in sedimentary been revised to be V254 Ma [8]. Such a signi¢- gas hydrates beneath the sea £oor [36]. According cant bias is likely to result from the e¡ect of Pb to the scenario of Dickens et al. [34], the methane loss combined with varying amounts and sources would be oxidized to CO2 and higher CO2 con- of inheritance [8]. The complexities of accurately centrations in the ocean and atmosphere would dating the Meishan boundary layers are further lead to environmental catastrophes. All the N13C enhanced if one compares the data obtained by excursions were associated with £ood basalt erup- the U^Pb and 40Ar/39Ar systems. For example, tions that occurred, respectively, in the Karoo- consideration of all the potential systematic un- Ferrar, Ontong-Java and Kerguelen, and North certainties would expand the 40Ar/39Ar age error Atlantic provinces [37]. The coincidences suggest from þ 0.2 to þ 4.6 Ma for the P^T samples a causal link between the two events and the gen- dated [8,12]. It is beyond the scope of this paper erally discussed model is that massive volcanism to settle the ‘inconsistencies’ between the two dat- in the marine realm warms and thus redirects sur- ing systems, however, in the light of 40Ar/39Ar face water to past some critical threshold which, results based on the same calibration standard, in turn, causes a circulation change, warming of our data suggest that the main eruption of the intermediate water mass, thermal propagation Emeishan Traps slightly predates the currently into slope sediments, and dissociation of gas hy- de¢ned P^T boundary at Meishan and the main drates [34,35,38]. This raises an intriguing issue of Siberian magmatism. The latter two are consid- whether a similar mechanism can be invoked at ered synchronous, despite the bias in U^Pb dates the P^T boundary [39]. An apparent drawback [8], because high-precision 40Ar/39Ar data from for the Siberian Traps is its within-continent oc- the two areas are indistinguishable [6]. In contrast currence. The Emeishan Traps, which were em- to the notion of extreme rapidity for the end-Per- placed in the western margin of the South China mian extinction [5,7], Mundil et al. [8] proposed Block where thick piles of marine limestone (the that the N13C excursion was much longer (V1^2 Maokou Formation) formed in association with a Myr instead of 165 kyr in [5]) and the formation continental breakup [15], therefore, may serve as of the Meishan ash layers began V257 Ma or an appropriate trigger required for the chain re- even earlier. In this sense, our data are consistent actions. with the hypothesis by Chung et al. [15], which It is further noted that the Emeishan magma-

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