Browsing by Author "Tohver, Eric"

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    Genesis and evolution of a neoproterozoic magmatic arc : the cordilleran-type granitoids of the Araçuaí Belt, Brazil.
    (2018) Narduzzi, Francesco; Nalini Júnior, Hermínio Arias; Farina, Federico; Lana, Cristiano de Carvalho; Stevens, Gary; Nalini Júnior, Hermínio Arias; Tohver, Eric; Heilbron, Monica da Costa Pereira Lavalle; Queiroga, Gláucia Nascimento; Cipriano, Ricardo Augusto Scholz
    The Araçuaí orogen (SE Brazil) is one of the largest (350,000 km2 ) and long-lived (ca. 630 – 480 Ma) granitic province in the world. The peculiarity of this Orogen is its wide variety of granitoids recording mid- to lower crustal P - T conditions that allow direct investigation of petrological processes occurring in the deepest part of the continental crust. This study investigates the field, textural, geochemical, geochronological and isotopic evolution of the pre-collisional Galiléia Batholith outcropping in the central part of the Araçuaí orogen. This metaluminous to slightly metaluminous (ASI = 0.97– 1.07) batholith, is a large is (ca. 15,000 km2 ), Neoproterozoic (ca. 632–550 Ma) weakly foliated, calc-alkaline, granitoid body characterized by the widespread occurrence of grossular-rich magmatic garnet and magmatic epidote. This is a rare mineral association in Cordilleran-I-type granitoids and of special petrogenetic significance. Field, petrographic, and mineral chemistry evidence indicate that garnet, epidote, biotite as well as white mica crystals (low-Si phengite), are magmatic. There is no difference in bulk rock major and trace element composition between the Galiléia and other garnetfree cordilleran-type granitoids worldwide. Thus the uncommon garnet+epidote parageneses are related to the conditions of magma crystallization, such as pressure, temperature and water content. Comparison between the mineral assemblages and mineral compositions from this study and those recorded in crystallization experiments on metaluminous calc-alkaline magmas, as well as within garnet-bearing metaluminous volcanic rocks and granitoids, indicates that the supersolidus coexistence of grossularrich garnet, epidote and white mica is consistent with magma crystallization at pressures greater than 0.8 GPa (above 25 km depth). This indicates that the Galiléia batholith was assembled in the lower crust during the accretionary/collisional stages of the Neoproterozoic Brasiliano Orogeny. It is evident that this granitic body represents a natural laboratory for studying the history of a lower crustal magma reservoir. Indeed the lifetime of deep magma chambers and the duration of magmatic activity in them remains a puzzle, contrary to young upper crustal magmatic systems. Despite being homogeneous with respect to mineralogy/texture and major/trace elements, all samples from the central part of the batholith record extreme variability in U-Pb magmatic ages from ca. 630 to 555 Ma. Trace element geochemistry and Hf isotopes from the igneous zircons – here interpreted as autocrysts (ca. 555 – 560 Ma) and antecrysts (> 560 Ma) – are all consistent with an open-system crystallization, rather than a simple cooling following fractional crystallization at the level of magma emplacement. We interpret the age variability recorded by the Galiléia samples as the result of a long-lived, uninterrupted injection of magmas of similar composition during assembly of the batholith. Such continuous injections of magma took place in the lower crust, keeping the system above its solidus through the 80 Ma of zircon crystallization. Unradiogenic 176Hf/177Hf and 143Nd/144Nd isotopic values of the Galileía samples indicate no direct mixing with mantle-derived magmas during the assembly and growth of the Galiléia batholith. This explains the scarcity of mafic products in the region. All of these characteristics need a more suitable geodynamic scenario. Indeed mineral textural, geochronological and isotopic similarities with other younger and older granitic plutons constructed within accretionary / fore-arc settings, better explain the characteristics showed by the Galiléia granitoids. Thus it is suggested that this giant batholith was assembled in an accretionary prism during the Brasiliano Orogenic stages. Eventually, it is likely that during the Brasiliano/Pan-African orogeny, accretionary prism, fore- and back–arc setting were sites of voluminous silicic magmatism and commonplaces for the stabilization of continental crust and its differentiation.
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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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    Magnetic fabric of Araguainha complex impact structure (Central Brazil) : implications for deformation mechanisms and central uplift formation.
    (2012) Yokoyama, Elder; Trindade, Ricardo Ivan Ferreira da; Lana, Cristiano de Carvalho; Souza Filho, Carlos Roberto de; Baratoux, D.; Marangoni, Yára Regina; Tohver, Eric
    The weakening mechanisms involved in the collapse of compleximpact craters are controversial. The Araguainhaimpact crater, in Brazil, exposes a complexstructure of 40 km in diameter, and is an excellent object to address this issue. Its core is dominated by granite. In addition to microstructural observations, magnetic studies reveal its internal fabric acquired during the collapse phase. All granite samples exhibit impact-related planar deformation features (PDFs) and planar fractures (PFs), which were overprinted by cataclasis. Cataclastic deformation has evolved from incipient brittle fracturing to the development of discrete shear bands in the center of the structure. Fracture planes are systematically decorated by tiny grains (< 10 μm) of magnetite and hematite, and the orientation of magnetic lineation and magnetic foliation obtained by the anisotropies of magnetic susceptibility (AMS) and anhysteretic remanence (AAR) are perfectly coaxial in all studied sites. Therefore, we could track the orientation of deformation features which are decorated by iron oxides using the AMS and AAR. The magneticfabrics show a regular pattern at the borders of the central peak, with orientations consistent with the fabric of sediments at the crater's inner collar and complex in the center of the structure. Both the cataclastic flow revealed from microstructural observations and the structural pattern of the magnetic anisotropy match the predictions from numerical models of compleximpactstructures. The widespread occurrence of cataclasis in the central peak, and its orientations revealed by magnetic studies indicate that acoustic fluidization likely operates at all scales, including the mineral scales. The cataclastic flow made possible by acoustic fluidization results in an apparent plastic deformation at the macroscopic scale in the core.
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    A new appraisal of sri lankan bb zircon as a reference material for LA-ICP-MS U-Pb geochronology and Lu-Hf isotope tracing.
    (2017) Santos, Maristella Moreira; Lana, Cristiano de Carvalho; Buick, Ian S.; Schmitz, Mark D.; Kamo, Sandra L.; Gerdes, Axel; Corfu, Fernando; Tapster, Simon; Lancaster, Penelope; Storey, Craig Darryl; Basei, Miguel Ângelo Stipp; Tohver, Eric; Alkmim, Ana Ramalho; Nalini Júnior, Hermínio Arias; Krambrock, Klaus Wilhelm Heinrich; Leite, Cristiano Fantini; Wiedenbeck, Michael
    A potential zircon reference material (BB zircon) for laser abla tion-in ductively coupled plasma-mass spectrometry (LA-ICP-M S)U-Pb geochronology and Hf isotope geochemistry is described. A batch of twenty zircon megacrysts (0.5–1.5 cm3) from SriLanka was studied. Within-grain rare earth element (REE) compositions are largely homogeneous, albeit with somevariation seen between fractured and homogeneous domains. Excluding fractured cathodoluminescence bright domains,the variation in U content for all analysed crystals ranged from 227 to 368 lgg-1and the average Th/U ratios werebetween 0.20 and 0.47. The Hf isotope composition (0.56–0.84 g/100 g Hf) is homogeneous within and between thegrains – mean176Hf/177Hf of 0.281674 ± 0.000018 (2s). The calculated alpha dose of 0.59 3 1018g-1for a numberof BB grains falls within the trend of previously studied, untreated zircon samples from Sri Lanka. Aliquots of the samecrystal (analysed by ID-TIMS in four different laboratories) gave consistent U-Pb ages with excellent measurementreproducibility (0.1–0.4% RSD). Interlaboratory assessment (by LA-ICP-MS) from individual crystals returned results that arewithin uncertainty equivalent to the TIMS ages. Finally, we report on within- and between-grain homogeneity of theoxygen isotope systematic of four BB crystals (13.16‰ VSMOW).
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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.