Browsing by Author "Cawood, Peter Anthony"

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    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.
    (2012) Tohver, Eric; Lana, Cristiano de Carvalho; Cawood, Peter Anthony; Trindade, Ricardo Ivan Ferreira da; Yokoyama, Elder; Souza Filho, Carlos Roberto de; Marangoni, Yára Regina
    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 ± 10Ma (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.
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    Quantifying rates of dome-and-keel formation in the Barberton granitoid-greenstone belt, South Africa.
    (2010) Lana, Cristiano de Carvalho; Tohver, Eric; Cawood, Peter Anthony
    The Barberton granitoid-greenstone belt is a classic dome-and-keel province, characterized by kilometerscale gneiss domes and elongate keels of largely folded supracrustal rocks. Combined U–Pb SHRIMP data and structural mapping demonstrate that the geometry of the Barberton belt reflects events that occurred over ∼30 million year interval, from ca. 3230 and 3203 Ma. Early deformation with NW–SE shortening in the upper crust was accompanied by emplacement of tonalite-trondhjemite-granodiorite TTG magmas at 3234±12 and 3226±9 Ma. Much of the structural grain of the greenstone belt relates to a long episode of post-orogenic extension, with NE-directed extension in the lower crust leading to exhumation of high-grade gneisses in the southern Barberton terrane. Advective heat transfer during emplacement of kilometer-scale (TTG) plutons around the margins of the greenstone belt facilitated the infolding of the relatively denser and colder greenstone sequence. The end of this tectonic cycle is punctuated by the emplacement of the undeformed, 3203±7Ma Dalmein pluton, which sharply truncates not only anticlines and synclines in the greenstone belt but also the dominant fabric in the high-grade gneisses.
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    Shaking a methane fizz : seismicity from the Araguainha impact event and the Permian–Triassic global carbon isotope record.
    (2013) Tohver, Eric; Cawood, Peter Anthony; Riccomini, Claudio; Lana, Cristiano de Carvalho; Trindade, Ricardo Ivan Ferreira da
    The Late Permian and Early Triassic periods are marked by large fluctuations in the carbon isotope record, but the source(s) of the disturbance to the global carbon cycle and the link to the end-Permian mass extinction arewidely debated. This contribution explores the possible isotopic effects of an impact event into the hydrocarbon-rich rocks of the Paraná–Karoo Basin. Recent U–Pb and 40Ar/39Ar dating of the 40 km Araguainha impact structure of central Brazil reveals an age of 254.7 ± 2.5 Ma (2σ error) for this event. The calculated energy (10^5–10^6 MT of TNT equivalent) released by this impact is less than threshold values of 10^7–10^8 MT TNT equivalent for global mass extinctions. Thus, the Araguainha crater is unlikely to have been the cause of the end-Permian biotic crisis. However, the combined seismic effects from the impact itself and the post-impact collapse of the 20–25 km diameter transient crater to its present 40 km diameter would result in large magnitude earthquakes (Mw 9.3–10.5) and tsunamis in the shallow marine Paraná–Karoo Basin. Slope failure and sediment liquefaction are predicted to have occurredwithin a 700–3000 km radius of the crater, causing large-scale release of methane from organic-rich sediments of this basin, including the oil shale horizons of the Iratí Formation. New geological evidence for seismicity in the Paraná Basin at the time of impact is presented, together with a compilation of existing carbon isotope data from the Paraná Basin, which demonstrate a widespread pattern of disturbance consistent with the release of methane. These two datasets suggest that both seismicity and methane release took place within ca.1000 km of the impact site, with mass balance calculations suggesting ca. 1600 GT of methane were released into the atmosphere at this time. Methane release at this scale would have significant climate effects and would contribute to a sharp (<1 ka) negative shift in δ^13 C values at the time of the impact, which should be distinguishable from the more gradual shift over 0.5–1 Ma caused by contemporaneous intrusion of the Siberian traps.
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    Strain localization and fluid-assisted deformation in apatite and its influence on trace elements and U–Pb systematics.
    (2020) Ribeiro, Bruno Vieira; Lagoeiro, Leonardo Evangelista; Faleiros, Frederico Meira; Hunter, Nicholas J. R.; Queiroga, Gláucia Nascimento; Raveggi, M.; Cawood, Peter Anthony; Finch, M.; Campanha, Ginaldo Ademar da Cruz
    This paper presents electron backscatter diffraction (EBSD), trace element and U–Pb data of apatite grains from a granitic mylonite from the Taxaquara Shear Zone (SE Brazil). The mylonite recrystallized under upper-greenschist facies and presents two types of apatite with distinct microstructures. Type1 apatite appears in quartz-rich layers and does not exhibit any microstructural, crystallographic, or chemical evidence of deformation/recrystallization, and resembles the original igneous apatite. Type2 apatite appears in mica-rich layers and exhibits core-and-mantle microstructures, and intragranular subgrain development, suggesting that they have undergone dynamic recrystallization. Recrystallized tails of type-2 apatite grains exhibit a strong c-axis crystallographic preferred orientation parallel to the X-direction (stretching lineation), and lack evidence of dislocation density. This evidence from type-2 apatite grains, combined with REE depletion, high La and a negative Ce anomaly compared to type-1 grains, suggests that type-2 apatite tails underwent recrystallization via dissolution-precipitation creep, whereas parental grains underwent crystal-plastic deformation and subgrain formation through dynamic recrystallization. Phase-equilibrium modelling and quartz CPO opening-angle thermometry are consistent with recrystallization at ∼480 – 530◦C and 2.2 – 5.0 kbar. We were not able to determine precise deformation ages from type-2 apatite because fluid-assisted recrystallization appears to have substantially decreased the U/Pb ratio. We find that preferential fluid flow along high-strain, biotite-rich layers in the mylonite caused type-2 apatite to recrystallise, whereas type-1 apatite in low strain layers was unaffected and retained the characteristics of the protolith.