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Age of the Emeishan £ood magmatism and relations to

Permian^Triassic boundary events

Ching-Hua Lo

a;

, 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-precision40Ar/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

Mass extinction at the Permian^Triassic (P^T) boundary was the most profound event in the his-tory of life on Earth. Nearly 90% of all species in the ocean and 70% of vertebrate genera on the continent vanished by the end of Paleozoic Era [1,2]. Traditionally, paleontologists believe that

this biologic great dying was slow, lasting at least several million years [1,2] and even occurred as two or more separate events [3,4]. Causes for such a prolonged extinction are therefore hy-pothesized to have been gradual processes such as sea level fall and climate change. To constrain the tempo of the end-Permian extinction, Bowring et al. [5] conducted a detailed U^Pb zircon geo-chronological study of the P^T marine boundary sections in Meishan, South China, and placed the P^T boundary at Bed 25 that was dated to be 251.4 þ 0.3 Ma. Combining biostratigraphic

infor-mation [5^7] and the negative excursion in N13C

* Corresponding author. Tel. : +886-2-2363-5880; Fax: +886-2-2363-6095.

E-mail address: loch@ccms.ntu.edu.tw (C.-H. Lo).

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across the boundary [5,7], these authors further-more suggested that the Changhsingian pulse of the end-Permian extinction was rapid and lasted less than 1 Myr. Such rapidity, however, has been recently questioned by Mundil et al. [8] based on new U^Pb zircon data from the same boundary layers. The age of Bed 25 has been rede¢ned to be

V254 Ma and the P^T boundary moved to Bed

27 (Fig. 1A), which should be slightly older than Bed 28 dated as 252.5 þ 0.3 Ma [8]. Thus, the main stage of the end-Permian extinction

oc-curred at V253 Ma and the N13C excursion

ap-pears to be signi¢cantly longer than previously postulated (Fig. 1A).

Among numerous scenarios proposed to ex-plain the P^T extinction [1^5], the Siberian £ood volcanism that represents the largest subaerial volcanic event in the Phanerozoic record has

been most widely accepted. U^Pb and 40Ar/39Ar

dating investigations [6,9^12] repeatedly argued that emplacement of the immense Siberian Traps

( s 2.5U106 km2 in area) took place at the P^T

boundary. Campbell et al. [9] proposed that erup-tion of the Siberian Traps injected vast volumes of volcanic dust and sulfate aerosols into the at-mosphere, a process that could have caused a short-lived volcanic winter. This volcanic winter, according to the hypothesis by Renne et al. [11], was followed within several hundred thousand years by greenhouse conditions because the erup-tion would have also released large amounts of

volcanogenic gases composed primarily of CO2.

Such a cooling^warming climate cycle could have resulted in an environmental crisis capable of causing the ¢erce mass extinction [6,9^12].

Being one of the large igneous provinces that occurred also around the P^T boundary [13,14], the eruption of the Emeishan Traps, South China, and its relations to the boundary events have

at-tracted numbers of recent investigations [15^20]. In this paper, we report the ¢rst set of high qual-ity40Ar/39Ar dating results of representative sam-ples from the Emeishan Traps, which de¢ne a main phase of the Emeishan £ood magmatism at V251^253 Ma and an earlier, but subordinate, phase at V255 Ma. These age data are generally coincident with, or slightly older, if considering all

the potential systematic errors between the 40Ar/

39Ar and U^Pb methods, than the ages of the P^T

boundary at Meishan and the Siberian Traps. This time sequence supports the hypothesis that the Emeishan eruption, in particular its later phase of felsic composition, could have accounted for the widespread deposition of the P^T bound-ary ash layers in South China. Given the fact that the Emeishan Traps were emplaced around the continental margin underlain by thick limestone formations, the vast eruption in a ‘marine’ envi-ronment is proposed to have played a key role in triggering the profound biogeochemical changes across the P^T boundary.

2. The Emeishan Traps

In South China, P^T boundary stratotype sec-tions that contain abundant interbedded volcanic ash layers are widespread [5,6,21]. Owing essen-tially to the synchronism between the Meishan ash beds and the Siberian £ood volcanism, Camp-bell et al. [9] speculated that these boundary ash layers are genetically linked to the Siberian erup-tion despite the fact that they were deposited sev-eral thousand kilometers in the south around the equator (Fig. 1A) [16]. The P^T boundary ash layers in South China are characterized by con-centrations of volcanogenic microspherules and 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 ter-ranes [14,15]. Insets illustrate distribution of major terter-ranes 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 sec-tion, M and P indicate the locations of two large intrusions (Maomaogou and Panzhihua) from which the dated syenites were collected.

C.-H. Lo et al. / Earth and Planetary Science Letters 198 (2002) 449^458 450

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ical data, Zhou and Kyte [22] proposed that these ash layers resulted from massive silicic eruptions from a nearby region, which have been ascribed to £ood volcanism in the Emeishan (basalt) Traps occurring in the western part of the South China Block [15] (Fig. 1B).

The Emeishan Traps, a large igneous province formed by a mantle plume head [14], are generally referred to as erosion remnants of the P^T mas-sive volcanic successions which occurred predom-inantly as ma¢c lava £ows and pyroclastics in the western Yangtze (South China), Songpan-Ganze and eastern Qiangtang terranes [15,18^20] (Fig. 1B). The volcanic successions, tilted and frag-mented by complicated tectonic events during Meso-Cenozoic times, are exposed in a rhombic

area of V2.5U105 km2 (500U500 km) in

associ-ation with numerous intrusive bodies of ultra-ma¢c/ma¢c to felsic compositions. The volcanic sequence thickness ranges from V5 km in the Bingchuan section (Fig. 1B) in the southwestern part of the Traps to several hundred meters in the eastern margin. The Emeishan £ood basalts show spatial and temporal variations. The basaltic lavas can be divided into two major groups, i.e., the

upper group with high-Ti (TiO2s 3.5 wt%) and

the lower group with low-Ti (1.5 s TiO2s 2.5

wt%) compositions [23]. Lavas of the low-Ti group are con¢ned to the lower volcanic succes-sions in the western part of the Traps, whereas lavas of the high-Ti group occur as the predom-inant component in the upper succession in nearly the entire region [23]. In localities, thick piles of trachyte and/or rhyolite form an important mem-ber in the upper sequence. The successions uncon-formably overlie the early Late Permian Maokou Formation (composed mainly of marine lime-stones corresponding to the Capitanian/Kazanian stage) and underlie the lower Triassic clastic sedi-ments [15,18,26]. On the basis of magnetobiostra-tigraphic correlations, eruption of the traps was traditionally accepted to have taken place in the Late Permian, despite that previous geochrono-logical data revealed a very scattered duration of

V40^260 Ma [20,24^26]. There are numerous

in-trusive bodies exposed in the Emeishan Traps. These intrusions range from ultrama¢c and ma¢c to felsic compositions, some of the ma¢c bodies

associated with large V^Ti^Fe ore deposits

[15,24^26]. Gabbros and syenites from the Mao-maogou and Panzhihua complexes (Fig. 1B) pos-sess geochemical and Sr^Nd isotopic composi-tions comparable to those of the nearby basalts and trachytes^rhyolites, respectively, strongly sug-gesting a genetic link between the intrusive and volcanic rocks in the Emeishan Traps [15,23]. Re-cently, a SHRIMP U/Pb zircon dating study was carried out for two of the gabbro-peridotite intru-sions, yielding crystallization ages of 258.7 þ 1.5 and 256.0 þ 1.0 Ma [17]. However, these are still indirect age constraints inferred from the intrusive rocks whose exact time relation with the main stage of the Emeishan eruption is unknown. Pre-cise dating directly on the volcanic successions is hence highly desirable.

3. Analytical methods

Six whole-rock samples from basaltic £ows, hornblende and biotite separates from a trachyte £ow and two syenite bodies, respectively, were

dated by 40Ar/39Ar step-heating and single-grain

fusion methods using furnace and laser heating techniques. Samples were ¢rst crushed and disin-tegrated. After sieving, mineral grains and rock chips in the range of 140^250 Wm were ultrasoni-cally cleaned in distilled water and dried, and then handpicked to remove any visible contamination before further mineral separation. The samples were then irradiated together with the LP-6 bio-tite standard [27] in the VT-C position at the THOR Reactor in Taiwan for 10 h (EM-90 ba-salt) ; 24 h (EM-86 hornblende, EM-PZH01 bio-tite and EM-13 and EM-15 basalts) ; and 30 h (the remaining samples). In order to monitor the neu-tron £ux in the reactor, three aliquots of the LP-6 standard, weighted in the range of 6V10 mg, were stacked with samples in each irradiation package with a length of V9 cm. After irradia-tion, standards and samples were either incremen-tally heated or toincremen-tally fused using a double-vac-uum resistance furnace and/or a US LASER Nd-YAG laser operated in continuous mode, and the gas was measured by a VG3600 mass spectrome-ter at the National Taiwan University. The J

val-C.-H. Lo et al. / Earth and Planetary Science Letters 198 (2002) 449^458 452

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ues are calculated using argon compositions of the

LP-6 biotite standard, with a 40Ar/39Ar age of

128.4 þ 0.2 Ma, calibrated according to the age 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 from Ar isotopic ratios measured after corrections made for mass discrimination, interfering nuclear reactions, procedural blanks, and atmospheric Ar

contamination. Detailed results of the 40Ar/39Ar

experiments of this study are given in tables 1

and 2 of the Background Data Set1, and the

data are plotted as age spectra and in isotope correlation diagrams in Figs. 2 and 3.

4. Analytical results

For the basaltic rocks, two samples (EM-37

Fig. 2. Apparent age spectra of the step-heating analyses for: (A,B) two high-Ti basalts (whole rock), (C) a trachyte (horn-blende), (D^F) three syenites (biotites) and (G,H) two low-Ti basalts from the Emeishan Traps. Plateau ages are calculated with-in arrows, which with-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.

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and EM-52) were from the upper part of the Bingchuan section and one (EM-90) from the top of the Ertan section ; the latter sample was collected from the £ow that underlies mid-Triassic clastic formations and overlies a thick (V80 m) trachyte £ow (Fig. 1B). These three samples

be-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

in-tercept 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 experi-ments 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 and40Ar/36Ar ini-tial value of 252.6 þ 1.6 Ma and 285 þ 49, respec-tively. These dating results indicate that the main eruption of the Emeishan Traps, constrained by the upper Bingchuan (V3000 m thick) and Ertan (V1000 m) sections, was of short duration be-tween V251^253 Ma.

Biotite separates from two large syenite intru-sions in the Maomaogou and Panzhihua com-plexes, close to Ertan (Fig. 1B), were also dated

by the 40Ar/39Ar method. These include furnace

step-heating analyses of a Maomaogou sample

(EM-MMG05) and two Panzhihua samples

(EM-PZH01 and 11) (Fig. 2D^F) and single-grain laser fusion analyses of a biotite sample from each

Fig. 3.36Ar/40Ar^39Ar/40Ar isotope correlation diagrams for:

(A) a whole-rock basalt, and (B,C) biotite separates from two syenites, obtained by single-grain laser fusion experi-ments. Individual data points are presented by solid circles, with þ 1c error ellipses.

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

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complex (EM-MMG01 and EM-PZH11) (Fig. 3B,C). Sample EM-PZH11 was dated by both step-heating and total fusion methods. In the step-heating analyses, all three samples show £at

age spectra with s 87% of 39Ar

K released. The

Maomaogou sample (EM-MMG05) and one of the Panzhihua samples (EM-PZH11) yielded iden-tical plateau ages of 252.0 þ 1.3 and 251.6 þ 1.6 Ma (Fig. 2D,F), respectively, whereas another Panzhihua sample (EM-PZH01) gave a slightly older plateau age of 254.6 þ 1.3 Ma (Fig. 2E). In the isotope correlation diagrams, the intercept ages and 40Ar/36Ar initial values are 251.4 þ 1.9 Ma and 312 þ 37 (EM-MMG05), 251.4 þ 1.2 Ma 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-spheric composition. The laser fusion analyses gave similar intercept ages of 251.9 þ 1.3 Ma

(EM-MMG01) and 251.2 þ 1.2 Ma

(EM-PZH11), both with reasonable 40Ar/36Ar initial

ratios and MSWD values (Fig. 3B,C). The data of sample EM-PZH11 obtained by two indepen-dent procedures match fairly well, indicating good

reproducibility and accuracy. These biotite 40Ar/

39Ar dates support the conclusion reached by the

whole-rock dating results that the major stage of the Emeishan magmatism took place in a short period of V251^253 Ma. The relatively older date of 254.6 þ 1.3 Ma for EM-PZH01 biotite, therefore, may imply an earlier intrusion that oc-curred V255 Ma. This appears to be consistent with the notion by the SHRIMP U^Pb zircon dating results [17], which suggest the Emeishan-related magmatism to have started as early as

V256^258 Ma. However, the interval between

these intrusions and the main phase of the Emeishan eruption remains unclear and requires more detailed investigations.

We also dated some low-Ti group basaltic lavas (e.g., EM-13 and 315; Fig. 1B) from the lower volcanic sequences. However, no statistically

meaningful 40Ar/39Ar dating results can be

ob-tained. Both samples EM-13 and EM-15 yielded

disturbed age spectra with wide ranges of 40Ar/

39Ar apparent ages (Fig. 2G,H). Similar disturbed

age spectra have also been reported in a recent

40Ar/39Ar dating study on the Emeishan rocks

[20]. Considering that these low-Ti samples are fresh in terms of petrography and geochemistry [23], we tentatively ascribe such serious distur-bance features in the Ar systematics to the very complicated Meso-Cenozoic tectonothermal his-tory of this area [15,20], which, in particular, in-cludes intensive crustal deformation and mag-matic activity related to the India^Asia collision [29]. Therefore, the initiation age of the Emeishan £ood volcanism has yet been accurately con-strained and further works including careful and detailed sampling and high-precision dating of the early phase of this volcanism are anticipated to be done.

5. Discussion and conclusion

The above results lead us to conclude that the main stage of the Emeishan magmatism took place in a short duration of V251.2^252.8 Ma (with errors in the range of þ 1.3^1.6 Ma). This age span appears to be slightly older than the principal activity of the Siberian Traps, de¢ned

by the recalculated 40Ar/39Ar ages of basaltic

£ows (249.9 þ 0.2 Ma) and the Norils’k intrusion (250.0 þ 0.2 Ma) [12]. Moreover, the older date (254.6 þ 1.3 Ma) of the Panzhihua syenite (EM-PZH01) implies a precursory intrusion that may have accompanied the eruption of the lower vol-canic sequence in the Bingchuan section. More ma¢c intrusions may have occurred in the nearby region, as revealed by the U^Pb zircon data [17], suggesting magmatic activity around 256V258 Ma. Such precursory magmatism that forms as the earliest manifestation of a mantle plume has been documented to have occurred several million years before the £ood volcanism in the Siberian Traps [30] and the end-Cretaceous Deccan Traps in India [31]. The precursory Siberian magmatism in the Maimecha-Kotui subprovince, dated at 253.3 þ 2.6 Ma [30], has been considered respon-sible for initiating the marked decrease in sea-water87Sr/86Sr (to values 6 0.707) that took place before the P^T boundary by assuming that the decrease in seawater 87Sr/86Sr re£ects an increas-ing mantle-derived component and no evidence exists for a heightened mid-ocean ridge basalt

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ac-tivity at this time [11]. In comparison to the with-in-continent occurrence of the Siberian Traps, the Emeishan magmatism, which occurred in the western continental margin of the South China Block along the Panthalassan and Tethyan mar-gins can serve as an even better candidate for the seawater87Sr/86Sr decrease. In the lower sequence in the Bingchuan section, basaltic £ows are marked by pillow structures indicative of a sub-marine eruption environment [15,24,25].

Compared to the P^T stratotype section at

Meishan, the 40Ar/39Ar ages (V251^253 Ma) of

the main stage of the Emeishan magmatism are

slightly older than the 40Ar/39Ar age of Bed 25

dated as 250.0 þ 0.2 Ma [6] but seem to match in face values with the P^T boundary age (V253 Ma) newly de¢ned at Bed 27 (Fig. 1A) by the U^Pb dating study [8]. Note that the U^Pb age of Bed 25, the previously postulated boundary layer dated as 251.4 þ 0.3 Ma [5], has been revised to be V254 Ma [8]. Such a signi¢-cant bias is likely to result from the e¡ect of Pb loss combined with varying amounts and sources of inheritance [8]. The complexities of accurately dating the Meishan boundary layers are further enhanced if one compares the data obtained by

the U^Pb and 40Ar/39Ar systems. For example,

consideration of all the potential systematic

un-certainties would expand the 40Ar/39Ar age error

from þ 0.2 to þ 4.6 Ma for the P^T samples dated [8,12]. It is beyond the scope of this paper to settle the ‘inconsistencies’ between the two

dat-ing systems, however, in the light of 40Ar/39Ar

results based on the same calibration standard, our data suggest that the main eruption of the Emeishan Traps slightly predates the currently de¢ned P^T boundary at Meishan and the main Siberian magmatism. The latter two are consid-ered synchronous, despite the bias in U^Pb dates

[8], because high-precision 40Ar/39Ar data from

the two areas are indistinguishable [6]. In contrast to the notion of extreme rapidity for the end-Per-mian extinction [5,7], Mundil et al. [8] proposed

that the N13C excursion was much longer (V1^2

Myr instead of 165 kyr in [5]) and the formation of the Meishan ash layers began V257 Ma or even earlier. In this sense, our data are consistent with the hypothesis by Chung et al. [15], which

argued for a causal link between the acidic erup-tions in the Emeishan Traps and the P^T bound-ary ash beds widespread in South China. Addi-tional40Ar/39Ar and U^Pb data for the ash layers in Meishan and other localities, as well as the Emeishan lavas, would certainly be required to further test such a hypothesis.

Although the eruption of the Siberian Traps is widely accepted to have played a key role in caus-ing the environmental changes across the P^T boundary, it can not be the only mechanism be-cause the negative N13C excursion was too large to be explained by any single event [5]. Prominent

and rapid excursions in N13C have been also

re-ported at several periods in the geologic record, e.g., at V183 Ma in the Jurassic [32], V120 Ma in the Cretaceous [33] and V55 Ma in the latest Paleocene [34,35], and these geochemical pertur-bations have been interpreted as consequences of massive release of methane stored in sedimentary gas hydrates beneath the sea £oor [36]. According to the scenario of Dickens et al. [34], the methane

would be oxidized to CO2 and higher CO2

con-centrations in the ocean and atmosphere would

lead to environmental catastrophes. All the N13C

excursions were associated with £ood basalt erup-tions that occurred, respectively, in the Karoo-Ferrar, Ontong-Java and Kerguelen, and North Atlantic provinces [37]. The coincidences suggest a causal link between the two events and the gen-erally discussed model is that massive volcanism in the marine realm warms and thus redirects sur-face water to past some critical threshold which, in turn, causes a circulation change, warming of intermediate water mass, thermal propagation into slope sediments, and dissociation of gas hy-drates [34,35,38]. This raises an intriguing issue of whether a similar mechanism can be invoked at the P^T boundary [39]. An apparent drawback for the Siberian Traps is its within-continent oc-currence. The Emeishan Traps, which were em-placed in the western margin of the South China Block where thick piles of marine limestone (the Maokou Formation) formed in association with a continental breakup [15], therefore, may serve as an appropriate trigger required for the chain re-actions.

It is further noted that the Emeishan

magma-C.-H. Lo et al. / Earth and Planetary Science Letters 198 (2002) 449^458 456

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tism, in comparison to two other possible mech-anisms (the Siberian eruption and a bolide im-pact) proposed for the P^T mass extinction and associated boundary events [5], better accommo-dates available geological data. Subaerial (within-continent) eruption of the Siberian Traps, which could have injected large amounts of volcanic aerosols and sulfates to produce global cooling and acid rain [6,9], has di⁄culties to form the boundary ashes in South China due to its distant

location and the N13C excursion resulting most

likely from a blast of gas hydrates in the sea. Impact of a bolide or asteroidal body, on the other hand, does not satisfy the recurrent deposi-tion of the volcanic ash beds. It is, moreover, not reconciled with the lack of iridium anomaly across the P^T boundary [22,40,41]. Given the fact that multiple interrelated events rather than a single mechanism would be necessary to inter-pret all the geological, geochemical and paleonto-logical data regarding the greatest dying, it is sug-gested that by the latest Permian the biota had already declined as a result of progressive envi-ronmental deterioration due to intensive

mag-matic activity along the Panthalassan and

Tethyan margins [42,43]. Under this circumstance, the beginning of the Emeishan magmatism may

have led to massive release of methane and CO2

from the sea that initiated environmental collapse. Then, the successive eruptions in the Emeishan and Siberian traps across the P^T boundary served combiningly as the ¢nal catalyst that pushed the most severe mass extinction in the his-tory of the Earth.

Acknowledgements

We thank Drs. James K.W. Lee and S.-s. Sun for useful discussions, and Dr. G.S. Odin for pro-viding the LP-6 biotite standard. Insightful and very constructive reviews provided by Drs. P.R. Renne, G. Dickens and an anonymous reviewer were of great help in clarifying a few points of this paper. This study bene¢tted from ¢nancial sup-ports by the NSC research Grants

(NSC89-2116-M002-017 and

NSC89-2116-M002-058).[BOY-LE]

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數據

Fig. 2. Apparent age spectra of the step-heating analyses for: (A,B) two high-Ti basalts (whole rock), (C) a trachyte (horn- (horn-blende), (D^F) three syenites (biotites) and (G,H) two low-Ti basalts from the Emeishan Traps
Fig. 3. 36 Ar/ 40 Ar^ 39 Ar/ 40 Ar isotope correlation diagrams for:

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