39Ar results for the 40 km Araguainha structure of central Brazil

Available online at www.sciencedirect.com

Geochimica et Cosmochimica Acta 86 (2012) 214–227 www.elsevier.com/locate/gca

Geochronological constraints on the age of a Permo–Triassic impact event: U–Pb and 40Ar/39Ar results for the 40 km Araguainha structure of central Brazil E. Tohver a,⇑, C. Lana b, P.A. Cawood a,c, I.R. Fletcher d, F. Jourdan d, S. Sherlock e, B. Rasmussen d, R.I.F. Trindade f, E. Yokoyama f, C.R. Souza Filho g, Y. Marangoni f a

School of Earth & Environment, University of Western Australia, Australia b Departamento de Geologia, Universidade Federal de Ouro Preto, Brazil c Department of Earth Sciences, St. Andrews University, United Kingdom d Department of Applied Geology, Curtin University of Technology, Australia e Department of Earth Sciences, Open University, United Kingdom f Instituto de Astronomia e Geofı´sica, Universidade de Sa˜o Paulo, Brazil g Instituto de Geocieˆncias, Universidade Estadual de Campinas, Brazil Received 22 July 2011; accepted in revised form 2 March 2012; available online 21 March 2012

Abstract Impact cratering has been a fundamental geological process in Earth history with major ramifications for the biosphere. The complexity of shocked and melted rocks within impact structures presents difficulties for accurate and precise radiogenic isotope age determination, hampering the assessment of the effects of an individual event in the geological record. We demonstrate the utility of a multi-chronometer approach in our study of samples from the 40 km diameter Araguainha impact structure of central Brazil. Samples of uplifted basement granite display abundant evidence of shock deformation, but U/Pb ages of shocked zircons and the 40Ar/39Ar ages of feldspar from the granite largely preserve the igneous crystallization and cooling history. Mixed results are obtained from in situ 40Ar/39Ar spot analyses of shocked igneous biotites in the granite, with deformation along kink-bands resulting in highly localized, partial resetting in these grains. Likewise, spot analyses of perlitic glass from pseudotachylitic breccia samples reflect a combination of argon inheritance from wall rock material, the age of the glass itself, and post-impact devitrification. The timing of crater formation is better assessed using samples of impactgenerated melt rock where isotopic resetting is associated with textural evidence of melting and in situ crystallization. Granular aggregates of neocrystallized zircon form a cluster of ten U–Pb ages that yield a “Concordia” age of 247.8 ± 3.8 Ma. The possibility of Pb loss from this population suggests that this is a minimum age for the impact event. The best evidence for the age of the impact comes from the U–Th–Pb dating of neocrystallized monazite and 40Ar/39Ar step heating of three separate populations of post-impact, inclusion-rich quartz grains that are derived from the infill of miarolitic cavities. The 206Pb/238U age of 254.5 ± 3.2 Ma (2r error) and 208Pb/232Th age of 255.2 ± 4.8 Ma (2r error) of monazite, together with the inverse, 18 point isochron age of 254 ± 10 Ma (MSWD = 0.52) for the inclusion-rich quartz grains yield a weighted mean age of 254.7 ± 2.5 Ma (0.99%, 2r error) for the impact event. The age of the Araguainha crater overlaps with the timing of the Permo–Triassic boundary, within error, but the calculated energy released by the Araguainha impact is insufficient to be a direct cause of the global mass extinction. However, the regional effects of the Araguainha impact event in the Parana´–Karoo Basin may have been substantial. Ó 2012 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +61 8 6488 2677.

E-mail address: [email protected] (E. Tohver). 0016-7037/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.gca.2012.03.005

E. Tohver et al. / Geochimica et Cosmochimica Acta 86 (2012) 214–227

1. INTRODUCTION The mass extinction that marks the end of the Paleozoic era is the largest biotic crisis in Earth history, marked by the disappearance of more than 80% of marine genera (Raup and Sepkoski, 1982) and similar reductions in terrestrial biodiversity (Ward et al., 2005). High precision U–Pb analyses of zircons from volcanic ashes have established the age of the Permo–Triassic biotic crisis in the marine realm to 252.6 ± 0.2 Ma (Mundil et al., 2004). According to Erwin (2002), the abruptness and severity of the end-Permian mass extinction are reminiscent of the effects of a massive bolide impact, although the absence of a large impact structure of Permo–Triassic age severely limits consideration of the impact hypothesis. Given the poor age constraints on most known impact structures (Jourdan et al., 2009), discussion of an impact mechanism for the Permo–Triassic mass extinction has remained speculative. The conflicting evidence for an impact event coincident with the Permian–Triassic boundary has generated controversy, most notably regarding the origin of a putative astrobleme, the Bedout structure of offshore Western Australia (Becker et al., 2004), which has been strongly contested by various workers (Glikson, 2004; Renne et al., 2004; Wignall et al., 2004). Examination of the Permo– Triassic stratigraphic boundary has allegedly yielded some evidence for an impact event, including the identification of meteorite fragments in a Permo–Triassic sedimentary sequence in Antarctica (Basu et al., 2003; cf. comment), the presence of shocked quartz and somewhat elevated levels of Ir in Permo–Triassic sections of southeastern Australia and Antarctica (Retallack et al., 1998), and reports of elevated Ir levels from the Meishan type-section of SE China (Xu et al., 1985). Here, too, there has been controversy: for example, Koeberl et al. (2004) raised the question of whether the chondritic fragments of Basu et al. (2003) represent ordinary terrestrial accumulation of micrometeorites; the identification of the Permo–Triassic boundary in Antarctica by Retallack et al. (1998) was questioned by Isbell et al. (1999); the alleged presence of shocked quartz in the Graphite Peak sequence was disproven by Langenhorst et al. (2005); and the Ir anomaly in the Meishan section of SE China was not reproduced in a subsequent investigation by Clark et al. (1986). Many stratigraphic sections, notably from Tethyan sites such as the Meishan region or the Austrian Alps, have not yielded any geochemical evidence of extraterrestrial input in the form of elevated levels of Ir or primordial 3He (Farley and Mukhopadhyay, 2001; Koeberl et al., 2004). In this contribution, we present new geochronological data for a known impact structure, the 40 km diameter Araguainha structure in central Brazil, and demonstrate that the age of this impact event overlaps with that of the critical Permo–Triassic boundary horizon. Though the Araguainha structure is too small to have been the sole cause of the global biotic crisis, the identification and accurate dating of this structure is the first step in assessing the stratigraphic record for impact events at the Permo– Triassic boundary. Furthermore, the location of the Araguainha crater within cratonic Gondwana is important in

215

delineating the possible effects of the impact and focuses attention on likely sedimentary repositories for impact ejecta from the Araguainha event. 2. GEOLOGY OF THE ARAGUAINHA STRUCTURE The Araguainha astrobleme is a 40 km diameter, multiring crater structure located in central Brazil (Fig. 1). The crater is located in the northern Parana´ Basin, the western arm of a 3  106 km2 seaway linked to the Karoo Basin of southern Africa (Milani and de Wit, 2007), prior to the opening of the Atlantic Ocean (Fig. 1). Based on the detailed stratigraphic and structural studies of the crater structure (Lana et al., 2007, 2008), the central uplift of the crater consists of shocked alkali granite basement cross-cut by numerous pseudotachylitic breccias with individual clasts of wall rock material encased by glassy melt. Isolated outcrops of massive melt rock are also observed structurally overlying the shocked granite, and constitute remnants of an eroded melt sheet. Sedimentary rocks that form a 2.5 km thick package overlying the granite basement prior to impact are variably exposed in the collar around the central uplift, together with a polymict mega-breccia (individual blocks >10 m), with significant displacements of the pre-impact stratigraphy also observed in the annular structures (Lana et al., 2007, 2008). The youngest target rocks exposed in the Araguainha impact structure belong to the late Permian Corumbataı´ Formation, which is dominated by mudstones deposited in a lagoonal to shallow-marine environment. The Guadalupian age of these rocks (Milani et al., 2007) provides a maximum age limit (ca. 260–270 Ma) for the impact event. This maximum age is more precisely constrained by the presence of a primary, dual-polarity magnetization in the

Fig. 1. Permo–Triassic paleogeographic location of the Araguainha crater (circle) with white outline of the Parana´ (P) and Karoo (K) basins. Concentric circles indicate the distance to prominent Permo–Triassic study sites.

216

E. Tohver et al. / Geochimica et Cosmochimica Acta 86 (2012) 214–227

Corumbataı´ Formation (Valencio et al., 1975) that indicates deposition after the end of the Kiaman Superchron at 262–265 Ma (Opdyke and Channell, 1996; Ogg et al., 2008). The regional stratigraphy of the northern Parana´ Basin is marked by an erosional unconformity between the Corumbataı´ Formation and the overlying aeolian deposits of the Piramboia Formation, the first post-impact, sedimentary deposits, but this formation is locally absent near the Araguainha structure (Lana et al., 2007). The Piramboia Formation is known to be early Triassic in age on the basis of Lystrosaurus fauna in the correlative formation in the southern Parana´ Basin, the Sanga do Cabral Formation (Milani et al., 2007). There are only two published geochronological studies of the Araguainha structure: a Rb–Sr mineral isochron age of 243 ± 19 Ma derived from alteration products of cordierite in the impact melt (Deutsch et al., 1992); and the 40Ar/39Ar analysis of biotite and whole rock samples by Hammerschmidt and von Engelhardt (1995). These latter results were re-examined by Jourdan et al. (2009), who recalculated an age of 246 ± 4 Ma (ca. 248 ± 4 Ma using the new K decay constant of Renne et al., 2010). Jourdan et al. (2009) cautioned that the disturbed degassing spectrum from the samples analyzed by Hammerschmidt and von Engelhardt (1995) signifies that the recalculated age should be taken as a minimum constraint on the age of the impact, noting also that the analyses were carried out using a neutron flux monitor that was not calibrated against international standards. In light of the uncertainty regarding the age of the Araguainha event, we have obtained more definitive constraints using both U–Pb and 40 Ar/39Ar isotopic analyses of multiple phases and samples from the shocked granite and impact-generated melt rocks. 3. SAMPLE DESCRIPTION The Araguainha granite, exposed in the uplifted core of the impact crater (Fig. 2a), is a pinkish, coarse-grained, monzo-syenogranite, with istotropic to locally aligned megacrysts of K-feldspar (P5 cm), as well as plagioclase, quartz, and biotite, as the major mineral constituents (Godoy et al., 2007). Common accessory phases include titanite, tourmaline, and zircon. Microgranular, hornblende-bearing enclaves are common, and occur at the centimeter to decimeter scale. Rare xenoliths of cordieritebearing schist and hornfels are also observed. Late, planar veins of black tourmaline are common – late-stage magmatic features that cross-cut all igneous facies. Shock features in the granite sample are chiefly brittle, grain-scale features, with occurrences of shatter cones (Fig 2b) restricted to the sedimentary rocks of the collar. Microstructural evidence of shock deformation in the granite includes extensive kink-banding in biotite, and planar deformation features (PDFs) in quartz. The kink bands in biotite are often decorated by iron oxides. Shock features in feldspar include extensive, millimeter-scale fracturing and mosaic extinction. Some intergranular, cataclastic veins are characterized by fine-grained material that may reflect highly comminuted material or the localized presence of a

melt phase; but they are not volumetrically abundant (Fig. 2c and d). Veins of pseudotachylitic breccia were also sampled for geochronology using a gasoline-powered Pomeroy drill. These veins are most commonly associated with outcrops of granite in the central uplift. The dark gray to reddish veins cross-cut the granite in a chaotic fashion, and individual veins vary in thickness on the 1–3 cm scale (Fig 2e). The reddish veins appear to be wholly devitrified, and are richly decorated with hematite and other iron oxides. Glass is preserved in only a few of the darker gray veins, but the veins are populated by numerous ovoid or rounded clasts of felsic wall rock material. Perlitic textures and sub-spherical extinction patterns in the glass matrix of the veins indicate that some devitrification has occurred (Fig. 2f). The impact melt rocks are gray to pink, and are characterized by a fine-grained groundmass of aphanitic to microcrystalline melt interspersed with brecciated clasts of feldspar, biotite, or lithic fragments that are typically derived from the granite basement. Some portions of the melt phase are marked by the presence of elongate, miarolitic vesicles that are rimmed by fine-grained biotite and infilled with polygonal, inclusion-rich quartz grains (Fig. 2g and h). The volume of melt is commonly 50 vol.% of the rock. 4. GEOCHRONOLOGICAL METHODS 4.1. U–Pb age determination by SHRIMP Zircon and monazite were separated by crushing and sieving of samples, followed by Wilfley table and heavy liquid separation. Grains were then handpicked using a binocular microscope, and then mounted in epoxy resin for polishing prior to scanning electron microscope and cathodoluminescence imaging. The shock and melt-related microstructures of some zircon grains were imaged using secondary electrons on the SEM prior to mounting of the grains in epoxy. The epoxy mounts were cleaned and gold-coated to have a uniform electrical conductivity for SHRIMP analysis. The zircon standard used was BR266 zircon (559 Ma, 903 ppm U). Monazite standards are French, QMa, and PD95 (Rasmussen et al., 2007) with the applied compositional matrix corrections as outlined in Fletcher et al. (2010). The isotopic composition of the minerals was determined using the SHRIMP II housed at Curtin University. Prior to spot analysis, rastering of the ion beam was carried out for 120–150 s to remove the gold coating and reduce the common Pb contaminant within the gold coating. For zircon, a primary ion beam of 2.5–3 nA with a diameter of 25 lm was focused onto the polished surface. A reduced spot size of 10–15 lm and weaker primary beam (1.2 nA) was used for monazite to reduce count rates for ThO2 on the ion collector. The common Pb correction was carried out using the measured amount of 204Pb. Isotope data were reduced using the SQUID2 software (Ludwig, 2009). Data were plotted on concordia diagrams using the Isoplot 3 software (Ludwig, 2008), with 2r error ellipses on the concordia plots and all ages reported at the 2r confidence level in the text. Data from SHRIMP

E. Tohver et al. / Geochimica et Cosmochimica Acta 86 (2012) 214–227

217

Fig. 2. (A) Photograph of the uplifted core of the Araguainha crater looking NE from ca. 4 km distance from the core. The topographic relief of hills forming the collar of sedimentary rocks, mega-breccia, and impact melt around the uplifted core is ca. 100 m. (B) Shatter cones within sandstone target rock with rock hammer for scale. (C) Hand sample of partly altered pseudotachylitic breccia in contact with shocked granite with corner of ca. 5 cm lens cap in back ground for scale. (D) Detail of vein-wall rock contact with perlitic textures in glass and clasts of wall rock material (cross-polarized light, field of view 2 mm). (E) Photomicrograph of shocked granite with kink-bands in biotite phenocrysts (plane polarized light, width of field of view is 5 mm) with white rectangle indicating area detailed in Fig. 2F. (F) Detail showing planar deformation features in quartz from box in E and a small, cataclastic vein (cross-polarized light, width of field of view is ca. 1 mm). (G) Miarolitic cavity in impact melt rock (plane polarized light, width of field of view is ca 2.5 mm) with detail in (H) showing inclusions in quartz and neoformed biotite grains (cross-polarized light, width of field of view is 0.5 mm).

analysis of zircon and monazite are presented in Supplementary Tables A1 and A2, respectively. 4.2.

40

Ar/39Ar dating techniques

Material for 40Ar/39Ar analysis was separated from crushed samples of the shocked granite and impact melt.

Thick polished sections of glassy material from the pseudotachylitic breccia veins were examined using a petrographic microscope before in situ laser 40Ar/39Ar analysis. Individual grains of feldspar and biotite were picked from the shocked granite sample, and populations of biotite and quartz were separated from the impact melt rocks using a binocular microscope. All argon age data for flux monitors

218

E. Tohver et al. / Geochimica et Cosmochimica Acta 86 (2012) 214–227

and unknowns were calculated using the original Steiger and Ja¨ger (1977) K-decay constant, with an age correction applied to reflect the new K decay constant of 5.5545  1010 a1 reported by Renne et al. (2010). All ages reported in the text are at the 2r confidence level. The in situ laser spot analyses were performed at the argon geochronology facility at Open University, UK using a Spectron Lasers Ltd. SL902 CW Nd-YAG 1064 nm laser with a manual shutter. Analyses were carried out by firing the laser in 30 ms shots, commonly one shot per analysis, resulting in pits of ca. 75 lm diameter, following analytical techniques documented in Sherlock et al. (2008). The IR laser was used on various sample materials, including a polished thick section of perlitic glass in an unaltered pseudotachylitic breccia vein, single grains of feldspar, and biotite. The IR laser yields a total gas fusion age for individual spots. These data are presented in Supplementary Table A3. Step heating 40Ar/39Ar age analysis of quartz populations was conducted at the Western Australian Argon Isotope Facility at Curtin University. Samples were loaded into one large well of one 1.9 cm diameter and 0.3 cm depth aluminum disc, bracketed by small wells that contained Fish Canyon sanidine (FCs) as a neutron flux monitor for (28.03 ± 0.08 Ma, Jourdan and Renne, 2007). The discs were Cd-shielded (to minimize undesirable nuclear interference reactions) and irradiated in position 5C for 25 h in the Hamilton McMaster University nuclear reactor, Canada. The mean J-value calculated for the quartz grain populations computed from standard grains within the small pits was 0.008700 ± 0.000023 (0.26%) determined as the average and standard deviation of J-values of the small wells for each irradiation disc. The quartz grains were subdivided into three populations and loaded into small Nb tubes that were sealed by crimping. The three aliquots were step-heated using a 110 W Spectron Laser Systems, with a continuous Nd-YAG (IR; 1064 nm) laser rastered over the sample for 1 min to ensure a homogenously distributed temperature. The gas was purified in a stainless steel extraction line using three SAES AP10 getters and a liquid nitrogen condensation trap. Ar isotopes were measured in static mode using a MAP 215-50 mass spectrometer (resolution of 600; sensitivity of 2  1014 mol/ V) with a Balzers SEV 217 electron multiplier mostly using 9–10 cycles of peak-hopping. Mass discrimination was monitored using an automated air pipette and correction factors applied for interfering isotopes were (39Ar/37Ar)Ca = 7.30  104 (±11%), (36Ar/37Ar)Ca = 2.82  104 40 39 4 (±1%) and ( Ar/ Ar)K = 6.76  10 (±32%). Blanks were monitored every 3–4 steps; typical 40Ar blanks range from 1  1016 to 2  1016 mol. Ar isotopic data corrected for blank, mass discrimination and radioactive decay are given in Supplementary Table A4 with individual errors given at the 1r level. Our criteria for the determination of plateaus are as follows: plateaus must include at least 70% of 39Ar with degassing distributed over a minimum of three consecutive steps agreeing at 95% confidence level and satisfying a probability of fit (P) of at least 0.05. Plateau ages are given at the 2r level and are calculated using the mean of all the plateau steps, each weighted by the inverse variance of their

individual analytical error. Integrated ages (2r) are calculated using the total gas released for each Ar isotope. Inverse isochrons include the maximum number of steps with a probability of fit P0.05. The uncertainties on the 40Ar*/39Ar ratios of the monitors are included in the calculation of the integrated and plateau age uncertainties, but not the errors on the age of the monitor or the decay constant. 5. GEOCHRONOLOGICAL RESULTS In the discussion that follows, a large number of isotopic data are presented, with the term “date” and “age” used to refer to distinct intervals of time. The term “date” or “apparent age” refers to the time elapsed according to the relevant parent–daughter ratios, although this may have an ambiguous geological significance, e.g. where inherited, igneous mineral phases cannot be isolated from younger, impact-related phases. The term “age” refers exclusively to a geologically significant number, i.e. the “crystallization age,” or “cooling age,” or the “impact age”. Similarly, the term “neocrystallization” is used to indicate a newly formed mineral phase that has crystallized directly from a melt, in lieu of “recrystallization,” commonly used to connote dynamic reorganization of grain boundaries during deformation or as a consequence of metamorphism. 5.1. SHRIMP dating of zircon Zircon grains were separated from samples of the shocked Araguainha granite (sample location 16°49.5580 S 53°0.0910 W) and the gray to pink impact melt (sample location 16°49.3290 S, 53°0.1500 W) from the uplifted core of the crater. Zircon grains from the shocked basement granite are euhedral and show no sign of melting (Fig. 3, grains i and ii). Although planar fractures (PFs) are abundant in zircons from the shocked granite, the internal structure revealed under cathodoluminescence is typically marked by igneous-type, oscillatory zonation with offsets across the PFs (Fig. 4d–f). The U–Pb isotopic ages from the shocked zircons from the granite sample are largely undisturbed and indicate a late Cambrian age (509.5 ± 12.0 Ma) for the Araguainha granite (Table A1), with a large proportion of 600– 1500 Ma grains inherited from the Meso-Neoproterozoic basement of the Paraguai and Brasilia belt (e.g., Dantas et al., 2007; Tohver et al., 2010, 2012). Generally, the morphology of zircon and their internal textures are more varied for the impact melt rocks, ranging from euhedral grains that are cross-cut by one or more sets of planar fractures to grains that appear “sugary” or turbid under the binocular optical microscope (Bohor et al., 1993; Corfu et al., 2003). Secondary electron images of these optically opaque zircons reveal a population of shocked zircons that are commonly coated with idiomorphic blasts of neoformed zircon (Fig. 3, grains iii and iv). Evidence for zircon melting is clear in the granular to vermicular surfaces of grains v and vi in Fig. 3. Cathodoluminescence images of the polished cross sections of granular zircons from the impact melt depict internal structures that are not concentrically zoned in the manner of the igneous grains (e.g., Fig 4a–c). Close inspection reveals this internal structure

E. Tohver et al. / Geochimica et Cosmochimica Acta 86 (2012) 214–227

219

Fig. 3. (A) SEM images of whole grains of zircons from the impact melt rock demonstrating a range of shock to melt textures increasing from top to bottom. Notice that shock deformation features such as planar fractures in grains i and ii give way to overgrowths of idiomorphic, neoformed zircon on a host grain of shocked zircon (grains iii and iv), and the truly granular morphologies of melt neocrystallized grains v and vi. White scale bar is 10 lm wide. (B) Detail of two sets of PFs from area in white rectangle in grain ii. (C) Detail of idiomorphic overgrowths on shocked zircon from white rectangle in grain iii. (D) Detail from white rectangle in grain iv showing partial development of neocrystallized granular aggregate as well as primary zircon remnant with PFs. (E) Detail from white rectangle in grain v depicting entirely granular surface of neocrystallized zircon.

to consist of microdomainal aggregates of new zircon grains, possibly the result of in situ crystallization of zircon from a congruent melt (Fig. 3, grains v and vi; Fig. 4a–c). Most of the zircon grains display intermediate characteristics between these two extremes, with the zircon host grain marked by both PFs and granular textures with idiomorphic zircon overgrowths (Fig. 3, grain iv, and inset 3d), suggesting that the shocked zircon grains served as a template for the nucleation of idiomorphic zircon overgrowths. SHRIMP analyses of individual zircons from the impact melt rocks form an array on the concordia diagram between the ca. 500 Ma crystallization age and the ca. 250 Ma impact age (Fig. 5b and d). Calculating the age of the youngest population, namely that of the neocrystallized grains, requires the evaluation of three factors: inheritance, i.e., the retained legacy of

radiogenic Pb generated by in situ decay of U after igneous crystallization; common Pb incorporated into the neocrystallized, microgranular aggregates of post-impact zircon and the resultant error introduced by using model Pb values to correct for this contamination (Stacey and Kramers, 1975); and Pb loss from grains that appear to be closed systems (i.e., are concordant within analytical error) but that yield ages that are younger than the zircon formation age (e.g., Mundil et al., 2001, 2004), in this case, the time of the impact event. The first issue of inheritance can be addressed by eliminating grains that are distributed along the upper end of the discordant array. The common Pb correction can be significant for recrystallised grains, with more than half of the recrystallised zircon grains not suitable for age calculations (>5% common Pb). This contaminant Pb may reside at the boundaries between the microgranular

220

E. Tohver et al. / Geochimica et Cosmochimica Acta 86 (2012) 214–227

Fig. 4. Paired secondary electron (SE) and cathodoluminescence images of mounted and polished zircon grains exhibiting internal textures associated with neocrystallization versus shock deformation. Note that the isotopically reset grains (A–C) are marked by a fine, granular structure in their interior, matched by internal homogenization, or the development of small, cathodoluminescent subgrains. This contrasts with the preservation of decorated planar fractures (white arrows in grains E and F) in grains that yield mixed dates, or magmatic ages. The cathodoluminescence of these shocked grains display oscillatory growth zoning that is typical of igneous zircon, and this zonation is offset across PFs.

domains in the neocrystallized zircon aggregates, as it is impervious to increased rastering with O2 ion beam prior to analysis or treatment with hot HCl and HNO3, and was presumably incorporated after impact-induced melting. Finally, a high proportion of analyses (34 of 99 analyses) are considered unreliable for geochronology purposes due to high discordance. Eliminating these three suspect populations; i.e., those grains that are most retentive of the igneous history, those that are most contaminated by common Pb, and those that are discordant, yields a subset of 10 concordant analyses from different zircon grains. A “Concordia” U–Pb age of 247.8 ± 3.8 Ma (MSWD 3.7) is calculated for this neocrystallized population of impact-generated zircon in the TeraWasserburg diagram (Fig 5f). The relatively high MSWD indicates that some heterogeneities exist within this population; selective pruning of data yields a smaller subset of

seven analyses with an apparently more precise age of 244.2 ± 3.0 Ma (MSWD = 0.54). However, given the higher common Pb in this younger subset and the possibility of undetected Pb loss even from apparently concordant analyses (e.g., Mundil et al., 2001), we regard the integrated age of 247.8 ± 3.8 Ma from the larger population as more robust. Still, the high surface area to volume ratio of these “grains”, which are better described as micro-granular aggregates rather than monocrystalline grains, suggests the strong possibility of Pb loss from this neocrystallized population. Therefore, this age is best regarded as a minimum age for the impact event. 5.2. SHRIMP dating of monazite The SEM images of monazite, like those for zircon, reveal the effects of shock metamorphism, i.e., planar

E. Tohver et al. / Geochimica et Cosmochimica Acta 86 (2012) 214–227

221

Fig. 5. U–Pb concordia plots for monazite (A) and zircon (B) from the impact melt rock sample showing array of concordant analyses ranging from the interpreted time of crystallization of the alkali granite to the time of the impact. White ovals are interpreted to reflect mixing with an inherited population, and black-shaded ovals are analyses from neocrystallized grains that are detailed in (E–F). (C and D) Histograms of monazite U–Th–Pb and zircon 238U/206Pb ages demonstrating the easily distinguishable, reset age results from monazite, versus the more continuous array of dates observed in zircon. (E and F) Tera-Wasserburg plots of the neocrystallized analyses of the monazite and zircon that are assigned mean ages in the text.

fractures (Fig 6a) and the development of new, granular aggregates of monazite (Fig. 6b). Compared to zircon, there are fewer monazite grains that are intermediate between the PF-laced texture of the pre-impact population and the granular texture of neocrystallized grains. All 53 analyses from 32 grains obtained during two analytical sessions are listed in an on-line data Supplement (Table A2). Eight

analyses with high common Pb were not considered reliable for geochronology. There is a wide range in both 206 Pb/238U and 208Pb/232Th ages, which are closely correlated (Fig. 7). However, many of the analyses at the older portion of the age spectrum show minor disagreement between the two decay schemes, i.e., t[208Pb/232Th] > t[ 206Pb/238U], reflected in a slight deviation from a line with

222

E. Tohver et al. / Geochimica et Cosmochimica Acta 86 (2012) 214–227

Fig. 7. Plot of U/Pb versus Th/Pb ages for individual monazite analyses demonstrating concordance of U–Pb and Th–Pb systems and the absence of excess 206Pb relative to 208Pb. This suggests that the young, wholly reset population of monazites does not require correction for initial Th–U disequilibrium at the time of recrystallization. The inset image shows the lower age range of these analyses, as indicated by black square on the main figure.

Fig. 6. Back-scattered electron images of individual monazite grains and SHRIMP spots from (A) shocked monazite with linear traces of two sets of PFs and (B) a granular aggregate grain with internal grain boundaries between individual subgrains.

slope = 1 at the upper end of the correlation trend in Fig. 7. It is unclear whether this is due to calibration limitations or to decoupling of the two decay systems during formation of the new monazite grains. For the youngest grains, the 206 Pb/238U and 208Pb/232U ages are closely correlated, implying that there is no chemical decoupling between Th and U upon formation of these grains. Furthermore, the trends are for t[206Pb/238U] < t[208Pb/232Th] and t[206Pb/ 238 U] < t[207Pb/235U] for this subgroup, so there is no evidence for excess 206Pb, and so no correction for initial disequilibrium between U and Th needs to be applied to the data for this group of youngest grains. For the six reset results (the only significant age-group), the mean-weighted 207 Pb/235U ages and 206Pb/238U ages are statistically indistinguishable, but the 207Pb/235U dates are more dispersed (MSWD = 4), and they are also more susceptible to the common Pb correction than 206Pb/238U by an order of magnitude. Monazite dates are spread unevenly between 500 and 250 Ma (Fig. 5a and c), an array that is interpreted to represent a variable, non-linear degree of resetting at 250 Ma. At the oldest end of this range, only two analyses from separate grains are concordant and self-consistent in age. These two grains yield a combined U–Th–Pb age of 498 ± 10 Ma, and this age is interpreted as the timing of monazite closure during cooling after the original emplacement of the Araguainha granite. The gap in monazite apparent “ages” between 350 and 280 Ma implies that the resetting is

highly non-linear, with precipitous breakdown and neocrystallization of monazite after >60% loss of radiogenic Pb. However, even among the youngest, neocrystallized population, some spread of dates is observed, with half of this youngest group of grains forming a statistically self-consistent subgroup defined by six concordant (2r error) analyses from five grains. A seventh analysis from another grain is similarly young but discordant. Concordant ages from this group are self-consistent in both the 206Pb/238U (MSWD = 1.2) and 208 Pb/232Th (MSWD = 0.43) systems. The six grains have weighted mean 206Pb/238U and 208Pb/232Th ages of 254.5 ± 3.2 Ma (Fig 5e) and 255.2 ± 4.8 Ma, respectively (95% confidence, including the uncertainty in standard calibration and decay constant errors). Since U–Pb and Th–Pb are measured independently, they can be combined into a U–Th–Pb age of 254.8 ± 3.1 Ma. If the discordant point is included, the combined result is 255.3 ± 3.6 Ma. Although minor amounts of igneous inheritance cannot be ruled out, the clear, ca. 80 Ma offset between the youngest grains and the partially- to wholly-igneous population suggests that monazite undergoes complete Pb-loss and recrystallization above some strain or temperature threshold. 5.3.

40

Ar/39Ar UV spot analysis

Laser spot analyses were carried out on five types of samples: feldspars from the shocked granite; optically isotropic “glass” in pseudotachylitic breccia veins (Fig. 2c and d); kink-banded biotites from the shocked granite (Fig. 2e); populations of small biotites separated from the melt rock (Fig. 2g and h); and vesicle-lining biotites. Complete isotope data for all analyses are found in an online data Supplement (Table A3). The distribution of apparent

E. Tohver et al. / Geochimica et Cosmochimica Acta 86 (2012) 214–227

223

“ages” varies according to the analyzed material. For example, total gas ages from the feldspar grains are in the 430– 480 Ma range, suggestive of a pre-impact cooling history following the intrusion of this late Cambrian granite. Spot data from kink-bands in shocked biotite demonstrate some localized resetting associated with mechanical deformation, but also partial retention by older igneous domains. Likewise, the spot analyses of “glass” in the pseudotachylitic breccia veins yield a mixture of “dates” that range from 180 Ma to ca. 350 Ma. The younger portion of this range probably reflects the effects of argon loss due to devitrification (cf. the perlitic textures in Fig. 2d) with the older dates reflecting the presence of clasts of wall rock material. Degassing of 2–3 grains of small biotites from mineral separates from the impact melt yielded the tightest range of dates, but the presence of inherited material, possibly small inherited grains in the melt matrix, is inferred for dates in the ca. 260–275 Ma range. Spot in situ analyses of small biotites lining the miarolitic cavities (Fig. 2e and f) give relatively consistent results, with a mean weighted date of 263.9 ± 4.3 Ma (MSWD = 0.63) calculated from the five youngest spot analyses (Fig. 8), but the possibility of Arloss or excess Ar cannot be assessed without step-heating. 5.4.

40

Ar/39Ar step-heating results

Step heating experiments were carried out on small, millimeter-sized chips of the glass from pseudotachylitic breccia veins, and the populations of inclusion-rich quartz grains from the impact melt rock. Degassing spectra from the glass chips of pseudotachylitic breccia are disturbed with no interpretable plateau age (Table A3). As with the spot analyses from these veins, it is likely that the degassing spectrum combines material from both the glass and wall rock, so the total gas “apparent ages” of ca. 320 Ma are significant only as maximum constraints for the age of the impact. Three populations of ca. 15 grains of quartz were separated (100–300 lm fraction) from the impact melt rock sample using a binocular microscope. These quartz grains are colorless and transparent, without discernible shock features, and represent either the fine-grained miarolitic, biotite-lined cavities in the impact melt or matrix grains (Fig. 2d and e). Colored inclusions (5–10 lm) are abundant in these quartz grains, with an optically isotropic character that suggests trapped inclusions of melt, although we cannot exclude the possibility of trapped, neoformed minerals. Step-heating of these multi-grain populations revealed uniform age spectra, throughout degassing, with two plateau ages obtained for three different populations (Table A4). The third population was degassed in a single heating step, yielding a total gas age. An inverse isochron age of 254 ± 10 Ma (MSWD = 0.52) was calculated from 18 of 20 individual gas fractions, and is taken as the time of formation of these trapped inclusions (Fig. 7). 6. DISCUSSION AND CONCLUSIONS The determination of accurate and precise ages for impact events requires mineral phases that were completely

Fig. 8. (A) Degassing spectra for 40Ar/39Ar age analysis of inclusions in quartz grains from the impact melt rock sample, demonstrating three different populations with a homogeneous distribution of radiogenic Ar for two populations with plateau ages (Tp). The total gas (Tg) age derives from a population that released its gas in a single step. (B) The inverse isochron age was calculated using isotopic analyses of individual gas aliquots from all three populations, excluding two of the 20 steps.

reset or newly formed by the impact event. Our results indicate the pervasive inheritance of the igneous crystallization and cooling history in all of the shocked, pre-impact mineral phases. The best results were obtained from neoformed minerals that grew in the impact melt itself. The pervasiveness of igneous inheritance is undoubtedly linked to the size of the Araguainha impact crater itself, for the larger volumes of melt generated by larger, >100 km craters provides more time for isotopic resetting in shocked minerals via thermally-enhanced diffusional loss, or, more likely, the growth of new minerals. Our geochronological results from the Araguainha crater illustrate the variable isotopic effects of shock deformation versus the more uniform results obtained from neoformed phases found in impact melts. 6.1. Summary of geochronological data The uplifted granite at the core of the Araguainha structure is characterized by shock metamorphism in the form of shatter cones, planar fractures in zircon and monazite, and planar deformation features in quartz. The microcataclasis of the feldspars and kink-banding in igneous biotite are also inferred to be caused by shock deformation, albeit not

224

E. Tohver et al. / Geochimica et Cosmochimica Acta 86 (2012) 214–227

uniquely diagnostic of an impact event. The igneous history is strongly retained in all minerals from the shocked granite, ranging from crystallization at 512.0 ± 11.0 Ma (U–Pb zircon U–Pb), and fast cooling through ca. 700° at 498 ± 10 (U–Pb monazite; Cherniak et al., 2004), to ca. 200° at 430–480 Ma (40Ar/39Ar total gas ages from feldspar; Lovera et al., 1997), respectively (Fig. 8). The application of in situ UV laser yielded mixed ages for pseudotachylitic breccias and shock-deformed biotite. These results demonstrate that shock deformation alone did not cause complete isotopic resetting (e.g., Deutsch and Scha¨rer, 1990) and that the melting and in situ neocrystallization of these minerals are essential for resetting of isotopic chronometers. Impact-generated melts yielded more informative constraints on the timing of impact events, although there are still clear examples of relict, igneous material amidst a population of neocrystallized, impactogenic phases. Zircon from the melt rock is particularly complex, with the apparent age of individual grains varying widely between these two end members (Fig. 5d). Impact-induced neocrystallization in zircon is associated with the development of vermicular to penetrative microgranular textures and significant incorporation of common Pb, presumably from the melt rock itself. Ten neocrystallized grains yield a U–Pb “Concordia” age of 247.8 ± 3.8 Ma (Fig 5f), interpreted as a minimum age for the impact, given the previously discussed possibility of Pb-loss from these microgranular grains. The large number of new, impactogenic zircons that are discordant is indicated by the large number of excluded analyses in Table A1. A lesser degree of inheritance is observed using 40 Ar/39Ar analysis of biotites from crushed samples of the impact melt with the five youngest grains yielding a mean-weighted value of 263.9 ± 4.3 Ma. The aforementioned “total gas age” produced by the UV laser spot technique makes the internal consistency of these dates difficult to evaluate. Furthermore, the individual dates from UV laser spots and biotite separates are distributed smoothly, and present no obvious means of discerning the impact age from mixed dates that are contaminated by undegassed, igneous material (Fig. 9). Thus, these youngest biotite grains provide a maximum age constraint on the time of the Araguainha impact. Inheritance is unlikely for the inclusion-rich quartz grains that were picked from the crushed melt rock sample. These quartz grains are easily recognizable from their transparent, shock-free lattices, and the large populations of randomly oriented inclusions that they host. The quartz grain itself may serve as a barrier to diffusional Ar loss from the small bodies it hosts, and step-heating confirms a homogeneous distribution of radiogenic Ar (Fig. 8a). However, the small volumes of these inclusions require relatively large, multi-grain aliquots. Individual gas fractions from three separate populations of inclusion-rich, post-impact quartz grains yield an inverse isochron age of 254 ± 10 Ma (Fig. 8). This age is interpreted as an accurate, albeit imprecise constraint on the timing of post-impact crystallization of the melt rock. The most precise age data derive from U–Th–Pb analysis of monazite. The clearly bimodal age distribution observed

in monazite allows for the less subjective discrimination of neocrystallized monazite ages, potentially making this a better chronometer than zircon. The U–Th–Pb age of 254.8 ± 3.1 Ma assigned for this population of monazite does not entirely exclude the possibility of igneous inheritance, so this may represent a maximum age for the impact event. Because of the faster, lower temperature annealing of monazite observed by many workers, Pb loss is of less concern than for zircon (cf. Mezger and Krogstad, 1997). The small time interval between the minimum age established by zircon and the maximum age established by monazite dating suggests that we are close to a directly measured age for the impact event itself. The 206Pb/238U and 208Pb/238Th ages for monazite, together with the isochron age for the inclusion-rich quartz grains (Fig. 9), yield a mean-weighted age of 254.7 ± 2.5 (95% confidence, MSWD = 0.039, probability = 0.96), and this is the best estimate for the age of the Araguainha impact. This new age assignment is in excellent agreement with the known post-265 Ma age of the Corumbataı´ Formation target rocks, and the lowermost Triassic age of the Lystrosaurus-bearing rocks of the overlying Piramboia Formation. The multiple chronometers employed allow for cross-checks between different isotope systems, i.e., using the newly calibrated decay constant for K as well as the different U and Th isotope decay schemes for different minerals. The 0.99% error of the integrated age assessment places the Araguainha crater in the company of the 16 other craters with isotopic age constraints known to better than 1%, out of 178 total recognized impacts (Jourdan et al., 2009). 6.2. Implications for the Permo–Triassic mass extinction and boundary sections The search for impact structures of Permo–Triassic age is obviously restricted by the resurfacing of roughly threequarters of the Earth by subduction and sea floor spreading over the past 250 Ma. So, the Araguainha position within an intracratonic basin is fortuitous from the standpoint of preservation. The location of the Araguainha structure also signifies that Gondwanan basins may prove more fertile locations for stratigraphic evidence of this impact. In contrast, the Euramerican and Tethyan realms lie on the other side of the equatorial tradewinds, paleogeographically distant from the crater location, suggesting that these regions are less likely to contain evidence for impact ejecta from the Araguainha event (Fig. 1). The negative assessments of evidence for a Permo–Triassic impact may be reinforced by the large number of studies from Tethyan sites. So, even though these locations preserve excellent Permo–Triassic marine sequences, the significance of the “absence of evidence” from these regions may be overstated in concluding that no large impact took place at this time. Although the newly established age of the Araguainha impact event overlaps with the known timing of the Permo–Triassic mass extinction within error, the question remains as to whether the Araguainha impact event played a direct role in the end Permian biotic crisis. According to Toon et al. (1997) catastrophic impacts can cause global mass extinctions through the “nuclear winter” scenario,

E. Tohver et al. / Geochimica et Cosmochimica Acta 86 (2012) 214–227

225

Fig. 9. (left) Summary of geochronological results from the impact melt rock (white background) depicting isotopic dates with height of bar signifying the magnitude of the 2r errors from each analysis and the shocked granite (gray background), with shaded bars indicating age results from quartz inclusions (M.I.), in situ UV laser spot ages of small biotites, large biotites in grain separates, monazite (Mz), and zircon (Z). For the samples from the impact melt rock, the black arrows below individual dates indicate analyses used to calculate the individual ages discussed in text. The gray shaded background indicates geochronological results from minerals and from pseudotachylitic breccia veins in the shocked granite. Numbers along the bottom axis give the fraction of impact-neocrystallized ages to the total number of analyses (n/N). (right) Grand mean age calculated from monazite and quartz inclusion analyses, with minimum age constraints imposed by zircon (Z-min) and UV spot analysis of biotite, respectively. Color-coding as in (A).

caused by the sun-blocking effects of impact ejecta in the upper atmosphere, widespread biomass burning, and ozone destruction. These authors also indicate that these effects are significant for impacts that release >106 Mt TNT equivalent of energy, which is estimated from crater size, e.g., the 180 km diameter Chicxulub crater released ca. 108 Mt TNT equivalent. Following the techniques of Collins et al. (2005), the Araguainha impact is found to have released 2  105  5  106 Mt TNT equivalent, at or below the threshold for globally catastrophic impacts. These calculations suggest that a “nuclear winter” created by ballistic ejecta from the Araguainha impact was probably not the leading cause of the end-Permian biotic crisis, barring the identification of one or more coeval crater(s), for example from a Shoemaker-Levy 9 type impact. At the regional scale, the Araguainha impact may have had sizeable effects throughout southern Gondwana, such as the Parana´–Karoo Basin of South America and southern Africa. We suggest that the crater formation was a prominent event in the sedimentary and biological history of southern and western Gondwana, and may have played some role in the demise of terrestrial vertebrates in the Karoo Basin (e.g., Smith, 1995; Ward et al., 2005) that might otherwise be unaffected by events in the marine realm. ACKNOWLEDGMENTS Support from the Australian Research Council (LP0991834 and DP110104818); the University of Western Australia; FAPESP, Brazil; and R. Van der Voo is acknowledged. SEM images were obtained at the UWA Centre for Microscopy and Microanalysis, an Australian Microscopy & Microanalysis Research Facility. U–Pb zircon analyses were performed on the sensitive high-resolution ion microprobes (SHRIMP II) located at the John de Laeter Centre of Mass Spectrometry, which is operated by Curtin University, the

University of Western Australia and the Geological Survey of Western Australia. Manuscript reviews by Klaus Mezger, Fernando Corfu, and Associate Editor Uwe Reimold, and of a previous version by Don Davis and an anonymous reviewer are gratefully acknowledged.

APPENDIX A. SUPPLEMENTARY DATA Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.gca.2012.03.005. REFERENCES Basu A. R., Petaev M. I., Poreda R. J., Jacobsen S. B. and Becker L. (2003) Chondritic meteorite fragments associated with the Permian–Triassic boundary in Antarctica. Science 302, 1388– 1391. Becker L., Poreda R. J., Basu A. R., Pope K. O., Harrison T. M., Nicholson C. and Iasky R. (2004) Bedout: a possible endPermian impact crater offshore of Northwestern Australia. Science 304, 1469–1476. Bohor B. F., Betterton W. J. and Krogh T. E. (1993) Impactshocked zircons: discovery of shock-induced textures reflecting increasing degrees of shock metamorphism. Earth Planet. Sci. Lett. 119, 419–424. Cherniak D. J., Watson E. B., Grove M. and Harrison T. M. (2004) Pb diffusion in monazite, a combined RBS/SIMS study. Geochim. Cosmochim. Acta 68, 829–840. Clark D. L., Yuan W. C., Orth C. J. and Gilmore J. S. (1986) Conodont survival and low iridium abundances across the Permian–Triassic boundary in South China. Science 233, 984–986. Collins G. S., Melosh J. H. and Marcus R. A. (2005) Earth impact effects program: a web-based computer program for calculating the regional environmental consequences of a meteoroid impact on earth. Meteorit. Planet. Sci. 40, 817–840.

226

E. Tohver et al. / Geochimica et Cosmochimica Acta 86 (2012) 214–227

Corfu F., Hanchar J. M., Hoskin P. W. O. and Kinny P. (2003) Atlas of zircon textures. Rev. Mineral. Geochem. 53, 469–500. Dantas E. L., Alvarenga C. J. S., Ventura R. and Pimentel M. M. (2007) Using Nd isotopes to understand the provenance of sedimentary rocks from a continental margin to a forelan basin in the Neoproterozoic Paraguay Belt, Central Brazil. Precambrian Res. 170, 1–12. Deutsch A. and Scha¨rer U. (1990) Isotope systematics and shockwave metamorphism: I. U–Pb in zircon, titanite, and monazite, shocked experimentally up to 59 GPa. Geochim. Cosmochim. Acta 54, 3427–3434. Deutsch A., Buhl D. and Langenhorst F. (1992) On the significance of crater ages: new ages for Dellen (Sweden) and Araguainha (Brazil). Tectonophysics 216, 205–218. Erwin D. H. (2002) Impact at the Permo–Triassic boundary: a critical evaluation. Astrobiology 3, 67–74. Farley K. A. and Mukhopadhyay S. (2001) An extraterrestrial impact at the Permian–Triassic boundary? Science 293, 2343. Fletcher I. R., McNaughton N. J., Davis W. J. and Rasmussen B. (2010) Matrix effects and calibration limitations in ion probe U–Pb and Th–Pb dating of monazite. Chem. Geol. 270, 31–44. Glikson A. (2004) Comment on “Bedout: a possible end-Permian impact crater offshore of Northwestern Australia”. Science 306, 613. Godoy A. M., Ruiz A. S., Manzano J. C. and Arau´jo-Ruiz L. M. B. (2007) Os Granito´ides Brasilianos da Faixa de Dobramentos Paraguai, MS e MT. Geol. USP Se´r. Cient. 7, 29–44. Hammerschmidt K. and von Engelhardt W. (1995) 40Ar/39Ar dating of the Araguainha impact structure, Mato Grosso, Brazil. Meteoritics 30, 227–233. Isbell J. L., Askin R. A. and Retallack G. J. (1999) Search for evidence of impact at the Permian–Triassic boundary in Antarctica and Australia: comment and reply. Geology 27, 859–860. Jourdan F. and Renne P. R. (2007) Age calibration of the Fish Canyon sanidine 40Ar/39Ar dating standard using primary K– Ar standards. Geochim. Cosmochim. Acta 71, 387–402. Jourdan F., Renne P. R. and Reimold W. U. (2009) An appraisal of the ages of terrestrial impact structures. Earth Planet. Sci. Lett. 286, 1–13. Koeberl C., Farley K. A., Peucker-Ehrenbrink B. and Sephton M. A. (2004) Geochemistry of the end-Permian extinction event in Austria and Italy: no evidence for an extraterrestrial component. Geology 32, 1053–1056. Lana C., Souza Filho C. R., Marangoni Y. R., Yokoyama E., Trindade R. I. F., Tohver E. and Reimold W. U. (2007) Insights into the morphology, geometry, and post-impact erosion of the Araguainha peak-ring structure, central Brazil. Geol. Soc. Am. Bull. 119, 1135–1150. Lana C., Souza Filho C. R. S., Marangoni Y. R., Yokoyama E., Trindade R. I. F., Tohver E. and Reimold W. U. (2008) Structural evolution of the 40 km wide Araguainha impact structure, central Brazil. Meteorit. Planet. Sci. 43, 701–716. Langenhorst F., Kyte F. T. and Retallack G. J. (2005) Reexamination of quartz grains from the Permian–Triassic boundary section at Graphite Peak, Antarctica. Lunar Planet. Sci. XXXVI, Abstract 2358. Lovera O. M., Grove M., Harrison T. M. and Mahon K. I. (1997) Systematic analysis of K-feldspar 40Ar/39Ar step-heating results: I. Significance of activation energy determinations. Geochim. Cosmochim. Acta 61, 3171–3192. Ludwig K. R. (2008) Isoplot 3.6; A Geochronology Toolkit for Microsoft Excel. Berkeley Geochronology Center, pp. 77. Ludwig K. R. (2009) Squid 2; A User’s Manual. Berkeley Geochronology Center, pp. 100.

Mezger K. and Krogstad E. J. (1997) Interpretation of discordant zircon data: an evaluation. J. Metamorph. Geol. 15, 127–140. Milani E.J. and de Wit M. J. (2007) Correlations between classic Parana and Cape-Karoo basins of South America and southern Africa and their basin infills flanking the Gondwanides: Du Toit revisited. In West Gondwana: Pre-Cenozoic Correlations Across the South Atlantic Region (eds. R. J. Pankhurst, R. A. J. Trouw, B. B. de Brito Neves and M. J. de Wit). Geol. Soc. Spec. Pub. vol. 294. pp. 319–342. Milani E. J., Goncßalves de Melo J. H., Souza P. A., Fernandes L. A. and Francßa A. B. (2007) Bacia do Parana´. Bull. Geoc. Petrobras 15, 265–287. Mundil R., Metcalfe I., Ludwig K. R., Renne P. R., Oberli F. and Nicoll R. S. (2001) Timing of the Permian–Triassic biotic crisis: implications from new zircon U/Pb age data (and their limitations). Earth Planet. Sci. Lett. 187, 131–145. Mundil R., Ludwig K. R., Metcalfe I. and Renne P. R. (2004) Age and timing of the Permian mass extinctions: U/Pb dating of closed-system zircons. Science 305, 1760–1763. Ogg J. G., Ogg G. and Gradstein F. M. (2008) The Concise Geologic Time Scale. Cambridge University Press, New York, pp. 177. Opdyke N. D. and Channell J. E. T. (1996) Magnetic Stratigraphy. Academic Press, San Diego, pp. 346. Rasmussen B., Fletcher I. R. and Muhling J. R. (2007) In situ U– Pb dating and element mapping of three generations of monazite: unravelling cryptic tectonothermal events in lowgrade terranes. Geochim. Cosmochim. Acta 71, 670–690. Raup D. M. and Sepkoski, Jr., J. J. (1982) Mass extinctions in the marine fossil record. Science 215, 1501. Renne P. R., Melosh H. J., Farley K. A., Reimold W. U., Koeberl C., Rampino M. R., Kelly S. P. and Ivanov B. A. (2004) Is Bedout an impact crater? Take 2. Science 306, 610–612. Renne P. R., Mundil R., Balco G., Min K. and Ludwig K. R. (2010) Joint determination of 40K decay constants and 40 Ar*/40K for the Fish Canyon sanindine standard, and improved accuracy for 40Ar/39Ar geochronology. Geochim. Cosmochim. Acta 74, 5349–5367. Retallack G. J., Seyedolali A., Krull E. S., Holser W. T., Ambers C. P. and Kyte F. T. (1998) Search for evidence of impact at the Permian–Triassic boundary in Antarctica and Australia. Geology 26, 979–982. Sherlock S. C., Jones K. A. and Park R. G. (2008) Grenville-age pseudotachylite in the Lewisian: laser probe 40Ar/39Ar ages from the Gairloch region of Scotland (UK). J. Geol. Soc. 165, 73–83. Smith R. M. H. (1995) Changing fluvial environments across the Permian–Triassic boundary in the Karoo Basin, South Africa and possible causes of tetrapod extinctions. Palaeogeogr. Palaeoclimatol. Palaeoecol. 117, 81–104. Stacey J. S. and Kramers J. D. (1975) Approximation of terrestrial lead isotope evolution by a two-stage model. Earth Planet. Sci. Lett. 26, 207–221. Steiger R. H. and Ja¨ger E. (1977) Subcommission on geochronology: convention on the use of decay constants in geo- and cosmochronology. Earth Planet. Sci. Lett. 36, 359–362. Tohver E., Trindade R. I. F., Solum J. G., Hall C. M., Riccomini C. and Nogueira A. C. (2010) Closing the Clymene Ocean and bending a Brasiliano belt, evidence for the Cambrian formation of Gondwana from SE Amazon craton. Geology 38, 267–270. Tohver E., Cawood P. A., Rossello E. A. and Jourdan F. (2012) Closure of the Clymene Ocean and formation of West Gondwana in the Cambrian: evidence from the Sierras Australes of the southernmost Rio de la Plata craton, Argentina. Gondwana Res. 21, 394–405.

E. Tohver et al. / Geochimica et Cosmochimica Acta 86 (2012) 214–227 Toon O. B., Zahnle K., Morrison D., Turco R. P. and Covey C. (1997) Environmental perturbation caused by the impact of asteroids and comets. Rev. Geophys. 35, 41–58. Valencio D. A., Rocha Campos A. C. and Pacca I. G. (1975) Paleomagnetism of some sedimentary rocks of the late Paleozoic Tubara˜o and Passa Dois Groups from the Parana´ Basin, Brazil. Rev. Bras. Geoc. 5, 186–197. Ward P. D., Botha J., Buick R., De Kock M. O., Erwin D. H., Garrison G. H., Kirschvink J. L. and Smith R. (2005) Abrupt and gradual extinction among late Permian land vertebrates in the Karoo Basin, South Africa. Science 307, 709–714.

227

Wignall P., Thomas B., Willink R. and Watling J. (2004) Is Bedout an impact crater? Take 1. Science 306, 609–610. Xu D. Y., Ma S. L., Chai Z. F., Mao X. Y., Sun Y. Y., Zhang Q. W. and Yang Z. Z. (1985) Abundance variation of iridium and trace elements at the Permian/Triassic boundary at Shangsi in China. Nature 314, 154–156. Associate editor: W. Uwe Reimold