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The South Atlantic Anomaly (SAA) is a region at Earth’s surface where the intensity of the magnetic field is particularly low. Accurate characterization of the SAA is important for both fundamental understanding of core dynamics and the geodynamo as well as societal issues such as the erosion of instruments at surface observatories and onboard spacecrafts. Here, we propose new measures to better characterize the SAA area and center, accounting for surface intensity changes outside the SAA region and shape anisotropy. Applying our characterization to a geomagnetic field model covering the historical era, we find that the SAA area and center are more time dependent, including episodes of steady area, eastward drift and rapid southward drift. We interpret these special events in terms of the secular vari‑ation of relevant large‑scale geomagnetic flux patches on the core–mantle boundary. Our characterization may be used as a constraint on Earth‑like numerical dynamo models.

The Moroccan Anti-Atlas orogenic belt encloses several Precambrian inliers comprising two major Neoproterozoic ophiolitic complexes: the Sirwa and Bou Azzer ophiolites. These ophiolites expose crustal and mantle units, thrusting over fragments of a long-lived intra-oceanic arc system. We present a detailed geochronological and petro-geochemical study of three mafic/ultramafic units of these two ophiolites: the Khzama sequence (Sirwa ophiolite) and the Northern and Southern Aït Ahmane sequences (Bou Azzer ophiolite). The crystallization of layered metagabbros from the Bou Azzer ophiolite (North Aït Ahmane sequence) has been dated here at 759 ± 2 Ma (U-Pb on zircons). This new age for the Bou Azzer ophiolite is similar to the formation of the Sirwa ophiolite (762 Ma) and suggests that both units formed during the same spreading event. Metabasalts of the three units show tholeiitic signature but with variable subduction-related imprints marked by LILE enrichments, HFSE depletions and variable Ti contents, similar to modern back-arc basin basalts (BABB). Their back-arc origin is also supported by the geochemical signature of ultramafic units showing very low contents in major and trace incompatible elements (Al2O3: 0.12–1.53 wt%, Ti: 3.5–64.2 ppm and Nb: 0.004–0.10 ppm), attesting of a highly refractory protolith. This is in agreement with the high Cr (0.44–0.81) and low to intermediate Mg (0.25–0.73) of their constitutive Cr-spinels. Dynamic melting models suggest that these serpentinites experienced intense and polyphased hydrous melting events, strongly influenced by supra-subduction zone SSZ-fluid influx and subduction-related melt percolation. Being particularly affected by these SSZ-melt/rock interactions and closer to arc units to the south, the Sirwa ophiolite and the South Aït Ahmane unit of the Bou Azzer ophiolite likely represent an early stage of the arc-back-arc system, which has been more influenced by the magmatic products of the arc activity compared to the North Aït Ahmane unit of the Bou Azzer ophiolite., SCOPUS: ar.j, info:eu-repo/semantics/published

In tropical Southern Hemisphere South America, late Quaternary marine core and speleothem records provide different proxies and accurate chronologies correlating millennial-scale intervals of increased precipitation with southward shifts of the Intertropical Convergence Zone (ITCZ). However, the climatic effect of these intervals on coastal landforms is poorly understood. Here we investigated the response of abrupt and long-lasting climate events during the Marine Isotope Stages 3 and 2 in a 13.8-m thick sand succession located in a large-scale coastal eolian system, the Lencois Maranhenses Dunefield, NE Brazil, where winds and precipitation are anti-phased and controlled by the single action of the ITCZ. A chronology of the sediment overburden determined by optically stimulated luminescence (OSL) dating of 14 sediment samples obtained at 1-m intervals showed ages between 132.2 ± 7.0 and 12.9 ± 0.6 thousand years ago in stratigraphic order. A multi-proxy approach based on grain size, surface grain texture, heavy minerals, thermoluminescence (TL) sensitivity, inorganic geochemistry, reflectance, and magnetic parameters from 268 sediment samples collected at 5-cm intervals indicated periods of dune building and stabilization. Significant stabilization periods are synchronous with Heinrich Stadials 6, 4, 3, 2, and 1, and Greenland Stadial 4, within age model uncertainties. Heavy mineral analysis indicated a steady sand source to the dune succession, while TL sensitivity analysis of quartz grains showed the input allochthonous sediments during stadials. The preservation of peak dune activity during the Last Glacial Maximum is attributed to the enduring precipitation brought by the subsequent Heinrich Stadial 1 event.

Made available in DSpace on 2022-04-28T19:47:15Z (GMT). No. of bitstreams: 0 Previous issue date: 2021-12-01 Global Neoproterozoic glaciations are related to extreme environmental changes and the reprise of iron formation in the rock record. However, the lack of narrow age constraints on Cryogenian successions bearing iron-formation deposits prevents correlation and understanding of these deposits on a global scale. Our new multiproxy data reveal a long Cryogenian record for the Jacadigo Group (Urucum District, Brazil) spanning the Sturtian and Marinoan ice ages. Deposition of the basal sequence of the Urucum Formation was influenced by Sturtian continental glaciation and was followed by a transgressive interglacial record of >600 m of carbonates that terminates in a glacioeustatic unconformity. Overlying this, there are up to 500 m of shale and sandstone interpreted as coeval to global Marinoan glacial advance. Glacial outwash delta deposits at the top of the formation correlate with diamictite-filled pa-leovalleys and are covered by massive Fe and Mn deposits of the Santa Cruz Formation and local carbonate. This second transgression is related to Marinoan deglaciation. Detrital zircon provenance supports glaciostatic control on Cryogenian sedimentary yield at the margins of the Amazon craton. These findings reveal the sedimentary response to two marked events of glacioeustatic incision and transgression, culminating in massive banded iron deposition during the Marinoan cryochron. Faculdade de Tecnologia Universidade de Campinas Departamento de Geología Universidade Federal de Ouro Preto Instituto de Geociências Instituto de Astronomía Geofísica e Ciências Atmosféricas Universidade de São Paulo Departamento de Geologia Universidade Estadual Paulista Departamento de Geologia Universidade Estadual Paulista

The sedimentary evolution of the Bambuí foreland basin system in the interior of West Gondwana is marked by periods of connection and isolation from the global ocean during the late Neoproterozoic and early Paleozoic. To understand the link between these periods and seawater redox conditions, we present an integrated study of trace-metal geochemistry and stratigraphy of the carbonate-siliciclastic rocks from the two lowermost second order transgressive-regressive sequences of the Bambuí Group, east Brazil. The basal 2nd-order sequence trace-metal pattern shows a progressive decrease of mass fractions of Co, Cr, Ni, Cu, Mo, U, V, Zn, and Cd, concomitant with a progressive decrease of Al and Fe contents and Mo/TOC ratios. Among all these elements, only Cd, Mo and U mass fractions seem to be less or not affected by detrital influence, so they can be used as reliable redox proxies for the paleoenvironmental analysis of the studied Bambuí strata. Moreover, normalization to aluminum shows a progressive increase of trace-metal enrichments for Cd and Mo in the order of 0.1–10 times and for U in the order of 1–100 times, accompanied by a progressive increase of organic carbon content upward section. These changes in sedimentary trace-metal patterns provide evidence for the chemical evolution of basinal deep-waters, whose conditions changed progressively from suboxic-anoxic to anoxic-euxinic at the basal transgressive-regressive sequence. We suggest that the paleomarine system represented by the basal Bambuí Group has probably evolved as an intracontinental silled basin recording changes in seawater chemistry associated with redox variations and restricted hydrographic conditions. Sedimentary trace-metal patterns indicate that Bambuí epeiric sea was initially in communication with open ocean followed by the marine restriction during the transgressive and regressive cycles, respectively. This resulted in a long deepwater residence time and chemical evolution of deep watermass as a response to tectonic pulses and consequent sea-level variations during the restricted stage. Under anoxic-euxinic conditions of seawater, trace metals scarcity and micronutrient fixation limitation would have impacted nitrate bioavailability, preventing the evolution of early benthic metazoans in the Bambuí paleomarine system during the late Ediacaran and early Cambrian.

Fil: Moncinhatto, Thiago R. Universidade de Sao Paulo. Instituto de Astronomia, Geofısica e Ciências Atmosféricas. São Paulo, Brazil. Fil: Haag, Maurício B. Universidade Federal do Rio Grande do Sul. Instituto de Geociências. Porto Alegre, Brazil. Fil: Hartmann, Gelvam A. Universidade Estadual de Campinas. Instituto de Geociências.Campinas. Brazil. Fil: Savian, Jairo F. Universidade Federal do Rio Grande do Sul. Instituto de Geociências. Porto Alegre, Brazil. Fil: Poletti, Wilbor. Universidade Federal dos Vales do Jequitinhonha e Mucuri. Instituto de Ciência e Tecnologia. Diamantina, Brazil. Fil: Sommer, Carlos A. Universidade Federal do Rio Grande do Sul. Instituto de Geociências. Porto Alegre, Brazil. Fil: Caselli, Alberto Tomás Universidad Nacional de Río Negro. Laboratorio de Estudio y Seguimiento de Volcanes Activos. Río Negro, Argentina. Fil: Trindade, Ricardo I. Universidade de Sao Paulo. Instituto de Astronomia, Geofısica e Ciências Atmosféricas. São Paulo, Brazil. Anisotropy of magnetic susceptibility is a petrofabric tool used to estimate the alignment of minerals at the site-scale, the imbrication between the magnetic foliation and the emplacement surface being an indicator of flow direction. However, despite numerous studies examining the flow direction in pyroclastic deposits and lava flows, the effect of magnetic mineralogy and the domain state of ferromagnetic phases on the magnetic fabric remains poorly understood. This paper describes the magnetic mineralogy and its influence on the magnetic fabric of Plio-Pleistocene lava flows and ignimbrites of the Caviahue-Copahue Volcanic Complex in the Andean Southern Volcanic Zone, Argentina. Rock magnetism, anisotropy of magnetic susceptibility and anhysteretic remanent magnetization and petrographic observations were performed on 30 sites of the volcanic complex. Results revealed the extrusive and pyroclastic rocks present varied magnetic mineralogy, formed in different stages of the magmatic evolu- tion. Magnetic mineralogy variations strongly affect the anisotropy of magnetic susceptibility data in volcanic rocks and associated ignimbrites, providing ‘scattered’ fabrics when late Ti- rich titanomagnetite phases dominate the fabric, and ‘inverse’ or ‘intermediate’ fabrics when single-domain grains are present. ‘Normal’ fabrics are typically found when early crystallized pure magnetite is present. Our results highlight the complexity in the interpretation of magnetic anisotropy data in volcanic rocks and ignimbrites. Anisotropy of magnetic susceptibility is a petrofabric tool used to estimate the alignment of minerals at the site-scale, the imbrication between the magnetic foliation and the emplacement surface being an indicator of flow direction. However, despite numerous studies examining the flow direction in pyroclastic deposits and lava flows, the effect of magnetic mineralogy and the domain state of ferromagnetic phases on the magnetic fabric remains poorly understood. This paper describes the magnetic mineralogy and its influence on the magnetic fabric of Plio-Pleistocene lava flows and ignimbrites of the Caviahue-Copahue Volcanic Complex in the Andean Southern Volcanic Zone, Argentina. Rock magnetism, anisotropy of magnetic susceptibility and anhysteretic remanent magnetization and petrographic observations were performed on 30 sites of the volcanic complex. Results revealed the extrusive and pyroclastic rocks present varied magnetic mineralogy, formed in different stages of the magmatic evolu- tion. Magnetic mineralogy variations strongly affect the anisotropy of magnetic susceptibility data in volcanic rocks and associated ignimbrites, providing ‘scattered’ fabrics when late Ti- rich titanomagnetite phases dominate the fabric, and ‘inverse’ or ‘intermediate’ fabrics when single-domain grains are present. ‘Normal’ fabrics are typically found when early crystallized pure magnetite is present. Our results highlight the complexity in the interpretation of magnetic anisotropy data in volcanic rocks and ignimbrites.

International audience; The paleogeography of the Meso-Neoproterozoic Rodinia supercontinent remains debated partly because many stable cratons still lack reliable paleomagnetic data for this period. A new geochronological and paleomagnetic study was conducted on the NNW-trending Manso dyke swarm of southern West Africa (Ghana) to clarify the position of this unconstrained continent in Rodinia. Two Usingle bondPb apatite ages of 857.2 ± 8.5 Ma and 855 ± 16 Ma agree with one previous baddeleyite age, indicating a ~ 860 Ma emplacement age for the Manso dykes. A characteristic remanent magnetization (ChRM) was isolated in stable single to pseudo-single domain (SD-PSD) magnetite. Well constrained site mean directions obtained for 13 dykes lead to a mean direction for the Manso dyke swarm of Dm = 181.9°, Im = −77.2° (N = 13, α95 = 7.6°, k = 30.6), yielding a paleomagnetic pole at 177.6°E, 28.3°S, (A95 = 12.7 °K = 11.6). Two directional clusters of opposite inclination pass a reversal test (C-class) and the primary origin is supported by a positive baked contact test, satisfying all the seven R-criteria to provide the first West African Tonian key paleomagnetic pole. This key pole indicates a high latitude for the West Africa Craton during the emplacement of the ~860 Manso dykes. A compilation of reliable paleomagnetic poles for West Africa, Baltica, Amazonia and Congo-São Francisco cratons suggests that these cratons were together between ~1200 and 800 Ma in a long-lived WABAMGO configuration. We suggest that the collision of this block with Laurentia along the Grenvillian-Sunsás orogens closed the external Nuna Ocean and formed Rodinia by extroversion.

Fe-bearing minerals are a tiny fraction of the composition of speleothems. They have their origin in the karst system or are transported from the drainage basin into the cave. Recent studies on the magnetism of speleothems focused on the variations of their magnetic mineralogy in specific time intervals and are usually limited to a single sample. In this study, we describe a database of environmental magnetism parameters built from 22 stalagmites from different caves located in Brazil (South America) at different latitudes, comprising different climates and biomes. The magnetic signal observed in these stalagmites is dominated by low-coercivity minerals (∼20 mT) whose magnetic properties resemble those of the magnetite formed in pedogenic environments. Also, a comparison with few samples from soils and the carbonate from cave’s walls shows a good agreement of the magnetic properties of speleothems with those of soil samples, reinforcing previous suggestions that in (sub-)tropical regimes, the dominant magnetic phase in speleothems is associated with the soil above the cave. Spearman’s rank correlation points to a positive strong correlation between magnetic concentration parameters (mass-normalized magnetic susceptibility, natural remanent magnetization, anhysteretic remanent magnetization, and isothermal remanent magnetization). This implies that ultrafine ferrimagnetic minerals are the dominant phase in these (sub-)tropical karst systems, which extend across a diverse range of biomes. Although the samples are concentrated in the savannah biome (Cerrado) (∼70%), comparison with other biomes shows a higher concentration of magnetic minerals in speleothem underlying savannahs and lower concentration in those underlying moist broadleaf forests (Atlantic and Amazon biome) and dry forests (Caatinga). Thus, rainfall, biome, and epikarst dynamics play an important role in the concentration of magnetic minerals in speleothems in (sub-)tropical sites and indicate they can be an important target for paleoenvironmental research in cave systems.

The West African Craton (WAC) is one of the major cratons in the Rodinia jigsaw puzzle (~1000–750 Ma). In the Rodinian models, the position of West Africa is mainly constrained by the assumption that it had been a partner of Amazonia since the Paleoproterozoic. Unfortunately, no paleomagnetic data are available for these cratons when the Rodina supercontinent is considered tectonically stable (~1000-750 Ma). Thus, every new reliable paleomagnetic pole for the West African Craton during the Neoproterozoic times is of paramount importance to constrain its position and testing the Rodinia models. In this study we present a combined paleomagnetic and geochronological investigation for the Manso dyke swarm in the Leo-Man Shield, southern West Africa (Ghana). The ~860 Ma emplacement age for the NNW-trending Manso dykes is thus well-constrained by two new U-Pb apatite ages of 857.2 ± 8.5 Ma and 855 ± 16 Ma, in agreement with baddeleyite data. Remanence of these coarse-to-fine grained dolerite dykes is carried by stable single to pseudo-single domain (SD-PSD) magnetite. A positive baked-contact test, associated to a positive reversal test (Class-C), support the primary remanence obtained for these dykes (13 sites). Moreover, our new paleomagnetic dataset satisfy all the seven R-criteria (R=7). The ~860 Ma Manso pole can thus be considered as the first key Tonian paleomagnetic pole for West Africa. We propose that the West Africa-Baltica-Amazonia-Congo-São Francisco were associated in a long-lived WABAMGO juxtaposition (~1100–800 Ma).Keywords: West Africa, Neoproterozoic, Tonian, Rodinia, paleomagnetism.

The Neoproterozoic-Paleozoic transition (~541 Ma) was a turning point in Earth’s history resulting in great biological changes between the microbial Precambrian life and the Ediacaran biotic revolution with the occupation of the sedimentary substrate, the dawn of biomineralization and the appearance of the earliest multicellular organisms. In parallel, this period is marked by a large plate reorganization leading to the assembly of Gondwana and by major climatic changes (extreme glacial events). Due in part to a poor paleomagnetic database for the different cratons in the Ediacarian-Cambrian times, the global paleogeography at that time remains controversial. In this study we present a new high-quality paleomagnetic pole (R = 7) for the Monteiro dyke swarm in the Borborema Province (NE Brazil) located at 18.2°S and 344.9°E (A95 = 11.7° K = 9.3). They are fine-grained hornblende dolerite dated by U-Pb on zircon at ~538 Ma. Rock magnetic data indicate that magnetite and pyrrhotite are the main remanence carriers. Positive baked-contact tests support the primary remanence obtained for these dykes (19 sites). A positive reversal test (classified C) was also obtained from the 14 sites with negative inclination and the 5 sites with positive inclination, indicating that the paleosecular variation was eliminated. Our new key pole is not consistent with the classical apparent polar wander path of the Gondwana which consists of a long track from a southern polar position at ~590 Ma to an equatorial position at ~520 Ma, and suggests instead rapid and small oscillations of the APW, after the end of the large IITPW at ca. 560 Ma. These TPWs are supposedly caused by changes in the inertia tensor of the Earth due to internal mass redistribution, related to rapid changes in subduction velocity. Links of these rapid oscillations and the timing of the Cambrian radiation could be crucial to understand the early history of animal life

The best-known Neoproterozoic glaciation-related iron deposits are the Rapitan-type banded iron formation (BIF) formed dominantly by chemical-sedimentary processes. We present a detailed study on a distinct type of glaciation-related iron deposit: the iron-rich metadiamictites of the Nova Aurora Iron District, comprising over 20 billion tons of iron oxide-rich rocks: the diamictitic iron formations (DIF >15 wt% Fe) and ferruginous metadiamictites (FD: 5–15 wt% Fe). Drill core logging, field mapping and analytical data revealed the protoliths, sediment sources, depositional environment, and ore enrichment processes on the iron district, and were used to discuss differences between the studied DIF and classic Rapitan-type BIF. U–Pb age spectra and Lu–Hf data for detrital zircon grains revealed a wide range of sediment sources and a maximum depositional age around 879 Ma. These data support correlations of the Nova Aurora iron deposits with the Cryogenian Macaubas rift, a basin filled by glaciomarine successions related to the Sturtian glaciation event. The matrices of prevailing hematite-rich metadiamictites comprise (in vol %): hematite (7–55), quartz (17–57), muscovite (2–40), carbonate ( 50 wt% Fe) occurs in local mylonitic shear zones and tightly crenulated bands after gangue minerals removal during deformation. Late hydrothermal alteration formed magnetite-rich metadiamictite in high-strain zones. From base to top, the stratigraphic type-section of the Nova Aurora Iron District shows barren metadiamictite with quartz-rich matrix and graded-bedded quartzite lenses, followed by hematite-rich-matrix metadiamictite, then passing to muscovite-rich-matrix metadiamictite gradually richer in magnetite and sulfide, enclosing metapelite lenses. This section presents a fining-up graded-bedded succession with clasts size decreasing upwards and recurrent load structures, typical of debris flows and turbiditic sedimentation. The hematite/mica, Fe/Al, Fe/Ti and Fe/REE ratios, as well as the biotite/muscovite and silicate/sulfide ratios, which accompany variations in the oxidizing conditions, decrease to the top. Accordingly, the early protoliths were quartz-rich diamictites that passed to iron-rich diamictites and then to clay-rich, and under more reducing conditions, sulfide-bearing diamictites. Sedimentological and geochemical evidences suggest ferrous-iron accumulation followed by iron oxide precipitation from seawater when the conditions changed from anoxic to oxidizing by the input of oxygen-rich waters from deglaciation, followed by increasing reducing conditions accompanying marine transgression. These processes took place in a relatively restricted sector of the glaciomarine basin, probably a graben of the Cryogenian Macaubas rift. Differences in relation to the classic Rapitan-type iron formations are the coarse-grained (ruditic) texture and the massive structure, the high-energy sedimentary environment, and the absence of true BIF in the Nova Aurora DIF deposits.

We present a compilation of reliable Precambrian paleomagnetic poles from three successive international workshops (in years 2009, 2014, 2017), comprising paleomagnetists specializing in Precambrian tectonic reconstructions. The working groups compiled lists of two global classes of poles, published through the end of 2017. “Grade-A” results are judged to provide essential constraints on tectonic reconstructions; “Grade-B” poles are judged to be suggestive of high-quality, but not yet demonstrated to be primary, or perhaps lacking precise geochronologic or other constraints. Our catalog documents a resurgence of high-quality data acquisition in recent years, and highlights specific cratons and time intervals that are most lacking in the data needed to reconstruct those blocks through supercontinental cycles.

Rhyolites compose an important record in the volcanic history of Earth, with significant occurrences in volcanic arcs, large igneous provinces and post-collisional terranes, often associated with explosive events. In several geologic provinces, rhyolites dominate as the most expressive geologic units (e.g., Silicic Large Igneous Provinces - SLIPs). Despite their importance, several aspects related to the emplacement of rhyolite flows are still enigmatic. Recent studies in modern rhyolite lavas suggest similar emplacement mechanisms to basaltic lavas, implying a more dynamic growth model for silicic flows, including outbreak lobes and outpour structures. Despite these advances, studies related to the recognition of these features in ancient flows are still rare. In this work we perform a multi-proxy study of an ancient (Neoproterozoic) rhyolitic lava flow combining fieldwork, petrography, geochemistry, rheology and magnetic fabric analysis. The Cerro do Perau outcrop (CP, southern Brazil) consists of a natural laboratory for the study of rhyolite lavas, presenting excellent exposure of a partially preserved flow with distinct flow features and folds. CP flow consists of a high-silica and low-crystal content rhyolite, suggesting its emplacement as an obsidian flow. Rheology data indicates high liquidus temperatures (>957 °C), with maximum viscosities of 108.5 Pa s and glass transition temperatures (Tg) of 750 °C. The absence of brittle features suggests little to none displacement below Tg. Structural analysis indicates the predominance of sub-vertical foliation planes, including axial planes of folds, indicative of proximal (near-vent) regions in rhyolite flows. The absence of lineations favors a predominantly planar accommodation of the flow-induced deformation, which is confirmed by the shape of the magnetic fabric ellipsoids. Several of these ellipsoids display a high degree of anisotropy, mostly related to an oblate fabric, indicative of the development of high-strain zones within the flow. Our data suggest that CP flow presents some similarities with recently proposed field-based emplacement models for rhyolitic flows, highlighting the significant data that can be extracted from a combination of magnetic fabrics and rheological analyses.

The early to mid-Aptian was punctuated by episodic phases of organic-carbon burial in various oceanographic settings, which are possibly related to massive volcanism associated with the emplacement of the Ontong Java, Manihiki, and Hikurangi oceanic plateaus in the southwestern Pacific Ocean, inferred to have formed a single plateau called Ontong Java Nui. Sedimentary osmium (Os) isotopic compositions are one of the best proxies for determining the timing of voluminous submarine volcanic episodes. However, available Os isotopic records during the age are limited to a narrow interval in the earliest Aptian, which is insufficient for the reconstruction of long-term hydrothermal activity. We document the early to mid-Aptian Os isotopic record using pelagic Tethyan sediments deposited in the Poggio le Guaine (Umbria-Marche Basin, Italy) to precisely constrain the timing of massive volcanic episodes and to assess their impact on the marine environment. Our new Os isotopic data reveal three shifts to unradiogenic values, two of which correspond to black shale horizons in the lower to mid-Aptian, namely the Wezel (herein named) and Fallot Levels. These Os isotopic excursions are ascribed to massive inputs of unradiogenic Os to the ocean through hydrothermal activity. Combining the new Os isotopic record with published data from the lowermost Aptian organic-rich interval in the Gorgo a Cerbara section of the Umbria-Marche Basin, it can be inferred that Ontong Java Nui volcanic eruptions persisted for ∼5 m.y. during the early to mid-Aptian.

International audience; Carbon isotope compositions of both sedimentary carbonate and organic matter can be used as key proxies of the global carbon cycle and of its evolution through time, as long as they are acquired from waters where the dissolved inorganic carbon (DIC) is in isotope equilibrium with the atmospheric CO2. However, in shallow water platforms and epeiric settings, the influence of local to regional parameters on carbon cycling may lead to DIC isotope variations unrelated to the global carbon cycle. This may be especially true for the terminal Neoproterozoic, when Gondwana assembly isolated waters masses from the global ocean, and extreme positive and negative carbon isotope excursions are recorded, potentially decoupled from global signals. To improve our understanding on the type of information recorded by these excursions, we investigate the paired δ13Ccarb and δ13Corg evolution for an increasingly restricted late Ediacaran-Cambrian foreland system in the West Gondwana interior: the basal Bambuí Group. This succession represents a 1st-order sedimentary sequence and records two major δ13Ccarb excursions in its two lowermost lower-rank sequences. The basal cap carbonate interval at the base of the first sequence, deposited when the basin was connected to the ocean, hosts antithetical negative and positive excursions for δ13Ccarb and δ13Corg, respectively, resulting in Δ13C values lower than 25‰. From the top of the basal sequence upwards, an extremely positive δ13Ccarb excursion is coupled to δ13Corg, reaching values of +14‰ and −14‰, respectively. This positive excursion represents a remarkable basin-wide carbon isotope feature of the Bambuí Group that occurs with only minor changes in Δ13C values, suggesting change in the DIC isotope composition. We argue that this regional isotopic excursion is related to a disconnection between the intrabasinal and the global carbon cycles. This extreme carbon isotope excursion may have been a product of a disequilibria between the basin DIC and atmospheric CO2 induced by an active methanogenesis, favored by the basin restriction. The drawdown of sulfate reservoir by microbial sulfate reduction in a poorly ventilated and dominantly anoxic basin would have triggered methanogenesis and ultimately methane escape to the atmosphere, resulting in a13C-enriched DIC influenced by methanogenic CO2. Isolated basins in the interior of the Gondwana supercontinent may have represented a significant source of methane inputs to the atmosphere, potentially affecting both the global carbon cycle and the climate.

The Borborema Province (NE-Brazil) experienced widespread intracontinental deformation associated with low-pressure metamorphism during the Neoproterozoic-Cambrian assembly of West Gondwana. The widespread intracontinental deformation was driven by tectonic stresses derived from two continental collisions reported in the literature at ca. 630–610 Ma and includes the development of the continental-scale Borborema Shear Zone System. In this study, we investigate metasedimentary rocks from the Serido Belt and eastern Patos Shear Zone, which occur in an intracontinental deformation zone at several hundred kilometers from the recognized collision zones. Using field data, quantitative compositional mapping, thermodynamic modeling, and monazite petrochronology, we constrained the metamorphic conditions and duration of intracontinental deformation in the inner Borborema Province. Monazite from leucosome-rich migmatite portions in the eastern Patos Shear Zone display 206Pb/238U dates spreading from 580 ± 13 to 512 ± 11 Ma (n = 125) with a cluster around ca. 570–550 Ma (n = 70). Staurolite-bearing schists from the western Serido Belt display a large spread of 206Pb/238U dates between 626 ± 13 and 519 ± 12 Ma (n = 50), with a well-defined cluster between ca. 570 and 550 Ma (n = 37). Garnet-andalusite-cordierite schists from the inner and eastern Serido Belt display monazite 206Pb/238U dates ranging from ca. 550 to 500 Ma (n = 126). Schists from the NNE-SSW striking Picui Shear Zone in the eastern Serido Belt exhibit Crd + And-rich nodules wrapped by the foliation, attesting the persistence of deformation after their generation. Monazite inclusions in cordierite and andalusite have REE patterns similar to matrix monazite and together they yield weighted average ages between ca. 530–525 Ma, interpreted as the age of metamorphism within the Picui Shear Zone. Iterative thermodynamic modeling using the local bulk composition of a Crd + And-rich nodule yielded optimal P-T conditions of ca. 590 °C and 3.8 kbar. The presented data indicate that intracontinental deformation in the inner Borborema Province was long-lived and lasted until at least 530–525 Ma. The deformation was associated with high geothermal gradient conditions that reduced lithospheric strength allowing the development and maintenance of the Borborema Shear Zone System.

Post-orogenic plutons are excellent recorders of the last stages of collapsing orogens. In the Neoproterozoic-Cambrian Aracuai orogen (Brazilian Southeast), one of the Brasiliano orogenies (Western Gondwana), voluminous amounts of Cambrian magmatism crop out. The Venda Nova pluton (VN) is located at the southern portion of the AO, exhibiting a norite/charnockitic west-ring in contact with a syenomonzonite that envelops an alkali-gabbro core with distinct geochemical signatures. To explore the tectonic significance of this post-orogenic pluton in the AO, anisotropy of magnetic susceptibility (AMS) together with a microstructural analysis were performed. AMS data reveals, majorly, a concentric magnetic foliation/lineation pattern, implying that buoyancy forces controlled the magma emplacement of syenomonzonites and gabbros, while a previous pulse of a charnockitic intrusion was deformed by their intrusion. Restrict solid-state deformation (following the regional trend) inside the pluton could result of the reactivation of an emplacement structure in the last stages of VN crystallization. The emplacement of inversely zoned post-orogenic plutons through deep conduits in the colder forelands of the AO, such as the VN, might record different stages of a large collapsing orogen in a single plutonic body.

Despite their importance in constraining the dynamics of Gondwana’s final phase of assembly, Cambrian paleomagnetic data from Gondwana are sparse. The Cambrian paleomagnetic dataset of Western Gondwana is especially poor, being defined by only a handful of poles. Here we contribute new Furongian paleomagnetic data from the ~500 Ma post-collisional, Santa Angelica and Venda Nova plutons from the southern portion of the Aracuai orogen in SE Brazil. The characteristic magnetization isolated from both plutons reveals two groups of directions that are demonstrated to be antipodal. On the basis of a thermal diffusion model, we attribute these antipodal directions to primary thermoremanent magnetizations acquired by cooling of the plutons in the presence of a reversing field. Together, paleomagnetic results from 35 sites distributed between the two plutons allows computation of a new ~500 Ma paleopole: 4.7° N, 332.2° E, A95 = 4.06 and K = 68.82. This pole does not resemble any younger paleomagnetic poles either from Gondwana or the independent South American plate after the demise of Pangea, but instead closely corresponds to the Miaolingian and Furongian sector of reference apparent polar wander paths for Gondwana. It also agrees well with the limited existing individual early Paleozoic poles from Western Gondwana. Our new result may thus be regarded as a reference pole for Western Gondwana in Furongian time. Considerations of the Cambrian paleomagnetic data from Eastern and Western Gondwana suggest that while the supercontinent was amalgamated by ~500 Ma, there was likely significant motion between Eastern and Western Gondwana in earlier Cambrian time.

Congo and Sao Francisco cratons are made of several Archean and Paleoproterozoic nuclei welded together by Paleoproterozoic belts. The pair is considered to be a stable lithospheric entity by ~2.0 Ga and hosts fissural mafic magmatism at 2.7–2.6, 1.71, 1.50, and 0.92 Ga; magmatic events at 1.38 and 1.10 Ga are restricted to Africa. Some of these dike swarms and also younger sedimentary units have been targeted for paleomagnetic studies, but the paleomagnetic data for the Congo−Sao Francisco craton is still scarce. Here we review the available data for both cratons. A total of 21 paleomagnetic poles was selected for paleogeographic reconstructions, 11 of them with Q-index higher than 4. Segments of apparent wander paths were drawn for 1.1–0.92, 0.79–0.74, and 0.57–0.52 Ga. Tentative reconstructions are proposed to place Congo−Sao Francisco into the context of supercontinents Columbia, Rodinia, and Gondwana.

The Ediacaran-Cambrian transition is marked by the diversification of metazoans in the marine realm. However, this is not recorded by the Ediacaran-Cambrian Bambuí Group of the São Francisco basin, Brazil. Containing the sedimentary record of a partially confined foreland basin system, the Bambuí strata bear rare metazoan remnants and a major carbon isotope positive excursion decoupled from the global record. This has been explained by changes in the paleogeography of the basin, which became a restricted epicontinental sea in the core of the Gondwana supercontinent, promoting episodes of shallow water anoxia. Here, we report new δ15Nbulk data from the two lowermost second-order transgressive-regressive sequences of the Bambuí Group. The results show a rise of δ15N values from +2 to +5‰ in the transgressive system tract of the basal sequence, which was deposited when the basin was connected to other marginal seas. Such excursion is interpreted as an oxygenation event in the Bambuí sea. Above, in the regressive systems tract, δ15N values vary from +2 to +5‰, pointing to instabilities in the N-cyle that are concomitant with the onset of basin restrictions, higher sedimentary supply/accommodation ratios, and the episodic anoxia. In the transgressive systems tract, the δ15N values stabilise at ∼+3.5‰, pointing to the establishment of an appreciable nitrate pool in shallow waters in spite of the basin full restriction as marked by the onset of a positive carbon isotope excursion. In sum, our data show that the N-cycle and its fluctuations were associated with variations in sedimentary supply/accommodation ratios induced by tectonically-related paleogeographic changes. The instability of the N-cycle and redox conditions plus the scarcity of nitrate along regression episodes might have hindered the development of early benthic metazoans within the Bambuí seawater and probably within other epicontinental seas during the late Ediacaran-Cambrian transition.

Fil: Haag, Mauricio Barcelos. Instituto de Geociências, Universidade Federal do Rio Grande do Sul. Brasil. Fil: Sommer, Carlos Augusto. Instituto de Geociências, Universidade Federal do Rio Grande do Sul. Brasil. Fil: Savian, Jairo Fransciso. Instituto de Geociências, Universidade Federal do Rio Grande do Sul. Brasil. Fil: Caselli, Alberto. Universidad Nacional de Río Negro. Laboratorio de Estudio y Seguimiento de Volcanes Activos. Argentina. Fil: Moncinhatto, Thiago Ribas. Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo. Brasil. Fil: Hartmannd, Gelvam André. Instituto de Geociências, Universidade Estadual de Campinas. Brasil. Fil: Ort, Michael. School of Earth and Sustainability. Estados Unidos. Fil: Poletti, Wilbor. Instituto de Ciência e Tecnologia, Universidade Federal dos Vales do Jequitinhonha e Mucuri. Brasil Fil: Ferreira da Trindade, Ricardo Ivan. Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo. Brasil. Pyroclastic deposits can cover significant areas and register major geological events. Despite their importance, understanding depositional dynamics of pyroclastic density currents (PDCs) and linking explosive deposits to their emission centers is still a challenge, especially in the case of non-welded, massive ignimbrites. Located in the Southern Andes, the Caviahue Copahue Volcanic Complex (CCVC) comprises one of the most active volcanic centers in the Andean Belt. This volcanic complex hosts massive ignimbrites with both source emplacement poorly constrained, currently grouped in the Riscos Bayos Ignimbrites (RBI). In this contribution, we perform a full magnetic characterization and anisotropy of magnetic susceptibility (AMS) study on the massive RBI of the CCVC. The magnetic characterization was performed using magnetic experiments including isothermal remanet magnetization, thermomagnetic curves, hysteresis loops, first-order reversal curves, and scanning electron mi croscopy. Magnetic experiments indicate primary, multi-domain, high Curie temperature titanomagnetites as the AMS carriers. Ellipsoids are predominately oblate, with a low degree of anisotropy and east-southeastward imbrication. This fabric arrangement is consistent with PDC sedimentary fabrics deposited under laminar flow conditions. Despite RBI massive structure AMS data reveals changes in transport capacity of the PDC and particle organization. These changes are marked by increasing AMS dispersion and decreasing degree of anisotropy up-section within flow units. Directional statistics of AMS data implies the Las Mellizas Caldera as the emission center of RBI. The reconstructed flow path also suggests the PDC overrun of the Caviahue Caldera topographic rim. This study highlights the application of AMS to the identification of emission centers of explosive deposits, featuring its application to massive ignimbrites.

International audience; Despite representing one of the largest cratons on Earth, the early geological evolution of the Amazonia Craton remains poorly known due to relatively poor exposure and because younger metamorphic and tectonic events have obscured initial information. In this study, we investigated the sedimentary archives of the Carajás Basin to unravel the early geological evolution of the southeastern Amazonia Craton. The Carajás Basin contains sedimentary rocks that were deposited throughout a long period spanning more than one billion years from the Mesoarchean to the Paleoproterozoic. The oldest archives preserved in this basin consist of a few ca. 3.6 Ga detrital zircon grains showing that the geological roots of the Amazonia Craton were already formed by the Eoarchean. During the Paleoarchean or the early Mesoarchean (

Geomagnetic field reversals remain as one of the most intriguing problems in geophysics and are regarded as chaotic processes resulting from a dynamo mechanism. In this article we use the polarity scale data of the last 170 Myrs collected from the ocean floor to provide robust evidence for an inverse relationship between the complexity of sequences of consecutive polarity intervals and the respective reversal rate. In particular, the variability of sequences of polarity intervals reaches minimum values in the mid-Jurassic when a maximum reversal rate is found, in the early Cretaceous preceding the Cretaceous Superchron, and twice in the last 20 Myrs. These facts raise the possibility of epochs of high regularity in the geomagnetic field reversals. To shed light on this process, we investigate the transition from regular to chaotic regime in a minimal model for geomagnetic reversals. We show that even in chaotic regimes, the system retains the signature of regular behavior near to transitions. We suggest that geomagnetic reversals have switched between different degrees of irregularity, with a dominant periodicity of ≈ 70 kyrs that results from the occurrence of “ghost” limit cycles (GLCs) or unstable periodic orbits (UPOs) immersed in a chaotic region of phase space.

Precambrian cellular remains frequently have simple morphologies, micrometric dimensions and are poorly preserved, imposing severe analytical and interpretational challenges, especially for irrefutable attestations of biogenicity. The 1.88 Ga Gunflint biota is a Precambrian microfossil assemblage with different types and qualities of preservation across its numerous geological localities and provides important insights into the Proterozoic biosphere and taphonomic processes. Here we use synchrotron-based ptychographic X-ray computed tomography to investigate well-preserved carbonaceous microfossils from the Schreiber Beach locality as well as poorly-preserved, iron-replaced fossil filaments from the Mink Mountain locality, Gunflint Formation. 3D nanoscale imaging with contrast based on electron density allowed us to assess the morphology and carbonaceous composition of different specimens and identify the minerals associated with their preservation based on retrieved mass densities. In the Mink Mountain filaments, the identification of mature kerogen and maghemite rather than the ubiquitously described hematite indicates an influence from biogenic organics on the local maturation of iron oxides through diagenesis. This non-destructive 3D approach to microfossil composition at the nanoscale within their geological context represents a powerful approach to assess the taphonomy and biogenicity of challenging or poorly preserved traces of early microbial life, and may be applied effectively to extraterrestrial samples returned from upcoming space missions.

The Caviahue-Copahue Volcanic Complex (CCVC, Argentina) composes one of the most active volcanic centers in the Southern Volcanic Zone (SVZ) of the Andes, characterized by the presence of voluminous explosive and effusive deposits. Despite its young age (< 5 Ma), CVCC deposits were strongly affected by two glaciations, leading to the removal of a considerable volume of the original deposits, requiring alternative techniques for the reconstruction of this volcanic center. The Riscos Bayos Ignimbrites (RBI) consist of a sequence of non-welded ignimbrites, located approximately 15 km southeast of the CVCC. This unit is commonly associated with the putative collapse of Caviahue caldera (15 x 20 km, 1 km deep) during the Pleistocene, although the source area and emplacement conditions of RBI still poorly constrained. In this work, we combine fieldwork, anisotropy of magnetic susceptibility (AMS, 23 sites) and rheological analyses (17 samples) in order to trace RBI source region and constrain its emplacement conditions, addressing its relevance to CVCC evolution. Rheological parameters, including viscosity, glass transition temperature, and liquidus temperatures were obtained using numerical models available from the literature, while AMS samples were measured using a Kappabridge MFK1-A (Agico) and the data processed using Anisoft5 (Agico). The magnetic mineralogy was characterized using several experiments, including isothermal remanet magnetization, thermomagnetic curves, hysteresis loops, first-order reversal curves and scanning electron microscopy. Our data indicate liquidus temperatures ranging from 969 to 1100 ºC, glass transition temperatures from 653 to 721 ºC, and viscosity (at liquidus temperature) from 3.4 to 7.3 log Pa.s. The absence of welding features in the samples implies RBI emplacement at temperatures below the glass transition temperature, suggesting a fast and effective cooling of the pyroclasts before their settling. The low crystal content of the samples suggests eruption temperatures close to the calculated liquidus temperature of the melt. AMS directional analyses indicate a consistent transport sense to SSE (Az of approximately 100º), implying the southern rim of the CVCC as the main source region of RBI. Magnetic experiments show primary, multi-domain, high curie temperature (580 ºC) titanomagnetites as the main carriers of the AMS signal. Most ellipsoids display oblate to triaxial geometry, with a low degree of anisotropy (< 5%) and magnetic susceptibility (1.0 x 10-2 SI). The low welding degree of RBI units and its geographic distribution outside the Caviahue depression contributes to the Caviahue caldera hypothesis in the region, suggesting its emplacement as an ‘extra-caldera’ pyroclastic unit.

Large, hot orogens are characterized by an orogenic plateau supported by a zone of weak ductile flow. During the collision phase, the magnitude of the belt and the temperature increase as radioactive crustal material is accreted, buried and heated. After convergence ends, no material is added to the orogenic system and the orogen undergo gravitational (or extensional) collapse that results from the lateral flow of the hot orogenic infrastructure. In the Araçuaí-West Congo orogen (AWO), the high temperatures, slow cooling, and excessive amount of melt in the hinterland, in the northern part of the belt, imply that a high temperature was maintained for a long time. Geochronologic results suggest that this internal domain was hot for a long time, cooling at < 3°/Myr since 600 Ma until 500 Ma, and cooling through the Ar/Ar retention temperature for biotite occurred around 470 Ma. In the south the collapse of the orogen is marked by the widespread intrusion of bimodal, composite plutons at ~500 Ma. Here we use the magnetic fabric (i.e. low-field anisotropy of magnetic susceptibility) of intrusions in the north and south sectors to track the kinematics and rheological changes across the belt. In the northern part of the AWO we studied the Padre Paraíso Charnockite and the southern part of the AWO we studied the Conceição de Muqui and Santa Angélica plutons. The Padre Paraíso charnockite has a coherent magnetic fabric, with magnetic foliations trending N-S, following the general structure of the belt in that sector. In turn, Conceição de Muqui and Santa Angélica plutons show a concentric distribution of foliations and lineations, in starking contrast with the general NE-SW trend of the belt in the south. This contrasting structural pattern for coeaval plutons along the AWO belt reveal the strain partitioning at the scale of the orogenic belt during the cooling of the AWO. At 500 Ma the hot northern sector remains warm enough to allow a coherent deformation of intrusions and host rocks. At the same time, more material was being added to the margins of the hot orogen, which already cold, with the diapire-like plutons structure being dominantly controlled by the forces of magma ascent and emplacement.

The Ediacaran Period (635-542 Ma) witnessed a series of extraordinary events. It arises with the end of the Marinoan Glaciation and deposition of worldwide enigmatic cap carbonate deposits. This abrupt shift in paleoclimatic conditions coincides with major fluctuations in the isotope ratios of carbon and sulfur, and with significant changes in the concentration of redox-sensitive elements in marine sediments. The Ediacaran is also a period marked by rapid changes in geomagnetic polarity. Magnetostratigraphy may therefore provide high-resolution correlation between Ediacaran successions worldwide. Here, we combine stratigraphy logs, carbon isotopes and magnetostratigraphy on the Avellaneda Formation (590-560 Ma) which at the Rio La Plata Craton, eastern Argentina. We investigated two drill cores (TSE-34 and TSE-7) with a 0.3-0.7 m resolution covering the entire Avellaneda Formation, corresponding to 98 standard specimens (25 mm in diameter). The basal contact of the Avellaneda Formation with the underlying mudstone rocks from Loma Negra Formation (~ 590 Ma) is present in both cores. The upper contact with the Alicia Formation, only observed in TSE-34 core, is transitional. The TSE-7 displays an erosional contact between Avellenda and Cerro Negro Formations (~ 560 Ma). After stepwise thermal demagnetization up to 600°C, almost all samples provided a characteristic magnetization between 350°C and 600°C, therefore Ti-poor magnetite or titanohematite is likely the main carrier of the stable remanence in these rocks. A high-temperature, dual-polarity component is persistent and coherent in the two drill cores. The base of the unit is marked by normal polarity, followed by a reverse interval, followed by persistent normal polarity across to the upper part of the Avellaneda Formation. This magnetostratigraphic framework, together with the carbon isotope signal, will be compared with results recently obtained for potentially coeval successions in China, Canada and United States.

The Neoproterozoic-Paleozoic transition (~541 Ma) was a turning point in Earth’s history resulting in great biological changes between the microbial Precambrian life and the Ediacaran biotic revolution with the occupation of the sedimentary substrate, the dawn of biomineralization and the appearance of the earliest multicellular organisms. In parallel, this period is marked by a large plate reorganization leading to the assembly of Gondwana and by major climatic changes (extreme glacial events). Due in part to a poor paleomagnetic database for the different cratons in the Ediacarian-Cambrian times, the global paleogeography at that time still remains controversial. In this study we present a new paleomagnetic pole (Q= 6) for the Monteiro dike swarms in the Borborema Province (NE Brazil). They are fine-grained hornblende dolerite dated by U-Pb on zircon at ~538 Ma. Rock magnetic data indicate that magnetite and pyrrhotite are the main remanence carriers. Positive baked-contact tests support the primary remanence obtained for these dikes (19 sites). A positive reversal test (classified C) was also obtained from the 14 sites with normal polarity and the 5 sites with reversed polarity, indicating that the secular variations was eliminated with our sampling. Our new key pole is not consistent with the classical Apparent Polar Wander Path of the West Gondwana which consists of a long track from a southern polar position at ~590 Ma to an equatorial position at ~520 Ma. The Monteiro paleomagnetic pole suggest instead rapid and small oscillations of the APW, or wobbles, after 560 Ma. These rapid oscillations may be related to inertial readjustments in response to true polar wander (TPW) of the spin axis. TPW events have been suggested from 615 to 590 and then from 575 to 565 Ma in previous works. These TPWs are supposedly caused by changes in the inertia tensor of the Earth due to internal mass redistribution, related to rapid changes in subduction velocity. Possible links between these events and life evolution will also be discussed.

Rare earth elements plus yttrium (REY) mass fractions of ancient carbonate rocks are used to track changes in chemistry of past seawater. Here we investigate REY patterns in two carbonate sections from the Ediacaran Bambui Group, Sao Francisco Basin (Brazil), which comprise its two lowermost transgressive-regressive second-order sedimentary sequences. Shale normalised distributions vary with the sequence stratigraphy framework. In the basal 2nd-order sequence, carbonate samples from the basal sequence transgressive systems tract display light REY (LREY) distributions slightly depleted to enriched that reflect input of freshwater, possibly in a post glacial episode. Upwards, carbonate rocks from the early highstand systems tract (EHST) yielded LREY enriched distributions, which progressively turns into LREY shale normalized depleted distributions on samples from the late highstand systems tract (LHST). This portion of the sequence also displays Y positive anomaly in some cases. Carbonate samples from the upper second-order sequence do not display coherent patterns. Ce/Ce* values > 1 in most samples throughout the two sections suggest permanent anoxia of seawater. The REY change from the EHST to LHST in the basal sequence marks an important paleoenvironmental overturn in the basin, with increasing alkalinity in seawater driving REY fractionation and LREY depletion. Confinement of the basin in the inner areas of West Gondwana due the uplift of marginal neoproterozoic orogens probably changed the weathering style of source areas to more congruent, thus delivering a higher ionic influx to a restricted setting, increasing alkalinity during LHST. Cloudina sp. fragments were reported in this stage of the Bambui Group and in carbonate rocks with high Sr mass fractions in other West Gondwana basins, supporting the hypothesis that the high alkalinity of seawater during late Ediacaran may have driven the appearance of the first biomineralizing organisms.

Anisotropy of magnetic susceptibility (AMS) and anhysteretic remanent magnetization (AARM) are tools to characterize flow structures and emplacement conditions of volcanic and subvolcanic bodies, directly related to magnetic fabrics. An AMS and AARM study was performed to contribute to the understanding of the processes involved in magma transportation and emplacement dynamics of the volcanic and subvolcanic rocks of Trindade Island, and their implications on the evolution of the volcanic field. Trindade Island is located in the South Atlantic Ocean, at 1260 km to the east of the Espirito Santo state coast in Brazil. The island is composed of lava flows, intrusions and pyroclastic rocks of alkaline, SiO2 undersaturated nature, forming five geological units aged 3.9–0.25 Ma. During fieldwork, 17 sampling sites were established in the phonolitic necks and melanephelinitic dyke of the oldest unit (Trindade Complex) and in the melanephelinitic ‘a'a flows of the youngest units (Morro Vermelho, Valado and Paredao Volcano formations), totaling 160 oriented mini-cores and 504 rock specimens. The analysis of petrofabric, magnetic mineralogy, AMS and AARM diagrams suggests the dominance of a low-coercivity magnetic mineral phase, represented by low-Ti titanomagnetite/titanomagnetite in the phonolitic necks and melanephelinitic dyke. These bodies display normal fabric (multi-domain and vortex grains) and sub-vertical/high-angle magnetic foliation and lineation. The melanephelinitic ‘a'a lavas present at least two magnetic mineral phases of high and low-coercivity (titanomagnetite, maghemite and hematite), with normal, intermediate or inverse fabrics (vortex or single domain grains). Mean magnetic foliation and lineation suggest that lavas flowed to NE. Data interpretation indicates that the subvolcanic bodies were emplaced vertically during the initial stages of island formation, followed by lavas flowing to the northeast. The orientation of the dyke and lava feeders along NNW-SSE leads to the conclusion that the tensional field during the evolution of volcanism remained the same, with NW direction for σ during approximately 4 My. This study is an example for the understanding of physical processes in the evolution of world oceanic islands.

International audience; Vase-shaped microfossils (VSMs) attributed to testate amoebae occur globally in diverse assemblages in Tonian rocks. These microfossils have thus been considered a potential biostratigraphic tool, especially for the interval between 789 and 729 Ma. Here we report a diverse and well-preserved in situ VSM assemblage, including several taxa previously considered as Tonian, within glacially influenced deposits for which sedimentological data support a Cryogenian age. However, the more robust recent multi-proxy correlation proposed by Freitas et al. (2021) indicates a Marinoan age for the studied succession. Detrital zircon data provide a maximum depositional weighted mean age of 749 ± 3 Ma for the VSM-bearing, organic-rich, fine-grained deposits within the Marinoan sequence in the Urucum Formation. Nine taxa are described from the fine-grained deposits in the upper Urucum Formation, Jacadigo Group, Brazil: Cycliocyrillium simplex, Bonniea dacruchares, Bonniea pytinaia, Bombycion micron, Limeta lageniformis, Palaeoarcella athanata, Trigonocyrillium horodyskii, Pakupaku kabin and cf. Taruma rata. The discovery of well-preserved in situ VSMs attributable to specific Tonian taxa within a Cryogenian succession challenges previous thinking that these organisms disappeared from marine ecosystems at the end of the Tonian.

The Cambrian Santa Angelica intrusive complex (SAIC - SE, Brazil), is composed of two lobes with concentric fabric and bull's eyes shapes, each lobe containing a mafic nuclei and granitic borders. It intruded during the post-collisional phase of the Aracuai orogen (AO). Although SAIC'S internal structure is described as exclusively magmatic, prominent solid-state deformation occurs at its borders, as well as an internal shear zone (ISZ). We studied the mechanisms that caused the different architectures displayed by the post-collisional bodies when emplaced in distinct crustal levels across the AO. We forward-modelled gravity data from the SAIC and its country rocks and performed structural analysis through anisotropy of magnetic susceptibility (AMS) and microstructural data of oriented thin sections in the SAIC border and the intrusion's country rocks. Magnetic measurements indicates that multidomain magnetite and pyrrhotite control the magnetic fabric of SAIC's country rocks. 2D gravity modelling shows that the northeast lobe outcrops its roots and its thickness is less than half of the southwest one. Three zones have been identified based on the occurrence of the solid-state microstructures: the country rocks and the border of the pluton display higher temperatures solid-state microstructures, overlapped by lower temperature ones, while the ISZ only shows high temperature structures. We suggest the SAIC experienced a reverse diapiric mechanism caused by the negative buoyancy of its mafic nuclei during crystallization. As the mafic cores sink through a ductile and hot halo in midcrust conditions, they deform the granitic rocks at the borders, as well as the country rocks, also generating the internal shear zone by the relative motion observed between the lobes. Furthermore, we infer that the buoyancy of the mafic and felsic magmas, together with crustal rheology, were the main constraints responsible to restrain the emplacement depth of the post-collisional plutons in the AO.

Knowledge about flow dynamics of volcanic sequences is fundamental for understanding their emplacement and consequently the evolution of the associated volcanic terrain. Despite this importance, studies that apply different approaches to ancient volcanic systems are still rare. In this paper, we study the case of silicic volcanic sequences in southernmost Brazil, contributing to the interpretation of the post-collisional Ediacaran volcanic settings of the Sul-riograndense shield. Rock magnetism analyses, anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM), were performed on 32 sites of silicic volcanic rocks integrated with fieldwork observations. Magnetic mineralogy data indicate that magnetite or Ti-poor magnetite and high-coercivity phases (e.g., hematite) are the main magnetic carriers for the studied volcanic deposits. AARM results reveal an inverse magnetic fabric when single-domain grains are present, strongly affecting the interpretation of flow directions of lavas and ignimbritic deposits. AMS scalar results integrated with ignimbrite lithofacies analyses showed different fabric imbrication styles between stratified lower units and rheomorphic upper ignimbrites, allowing their separation in the emplacement model. Flow directions based on AMS, AARM data and field observations show a potential correlation of these volcanic deposits with an intrusive complex located on the southeastern border of the ignimbritic plateau. The emplacement of pyroclastic flow deposits was probably associated with a complex fissure system, where discontinuities within the basement plateau border may have served as feed conduits for these deposits. Our results highlight the importance of applying a regionally distributed AMS sampling coupled with a strong mineralogical and field control to the study of ancient volcanic systems.

A recent hypothesis has suggested that Earth's inner core nucleated during the Mesoproterozoic, as evidenced by a rapid increase in the paleointensity (ancient geomagnetic field intensity) record; however, paleointensity data during the Paleoproterozoic and Mesoproterozoic period are limited. To address this problem, we have determined paleointensity from samples from three Paleoproterozoic Avanavero mafic sills (Amazonian Craton, Brazil): Cotingo, 1782 Ma, Puiuà 1788, and Pedra Preta, 1795 Ma. We adopted a multi-protocol approach for paleointensity estimates combining Thellier-type IZZI and LTD-IZZI methods, and the non-heating Preisach protocol. We obtained an average VDM value of 1.3 ± 0.7 × 1022Am2 (Cotingo) of 2.0 ± 0.4 × 1022Am2 (Puiuà) and 6 ± 4 × 1022Am2 (Pedra Preta); it is argued that the Cotingo estimate is the most robust. Our results are the first data from the upper Paleoproterozoic for South America and are comparable to data available from other regions and similar periods. The new data do not invalidate the hypothesis of that Earth's inner core nucleated during the Mesoproterozoic.

The understanding of the paleosecular variations (PSV) of the geomagnetic field in South America is still biased by the scarcity of data. Especially, the recent geomagnetic PSV is characterized by the large growth of the South Atlantic Magnetic Anomaly (SAMA) during the last centuries, first documented by the geomagnetic model gufm1 (Jackson et al., 2000). A large amount of data is required to understand the time and geographic distribution of this primary feature, and the Andean Pleistocene and Holocene volcanoes are an excellent recorder of instant local changes in SV. Here we present a preliminary study from 18 paleomagnetic samples collected during 2015 on what it was supposed to be the 1750 or the 1957–58 AD lava flow on the Llaima Volcano (38.692° S; 71.729° W), one of the most active centers of the Chilean Andes, in the Southern Volcanic Zone. A detailed paleomagnetic study was performed in order to recover the Declination and Inclination of the geomagnetic field, obtain the paleointensity and define the magnetic mineralogy. AF demagnetization until 1 T yielded an average vector at Dec/Inc 2.3°/-33.1° with α95 of 2.4°. This direction is carried by titanomagnetite grains with 40–45% ulvospinel as revealed by thermomagnetic curves. Paleointensity estimates were obtained following the IZZI-Thellier protocol. Seven specimens from 5 samples provided reliable results (success rate of 35%), giving an average paleointensity for these specimens of 30.88 ± 2.39 μT. The full magnetic vector obtained here was compared to archaeomagnetic reference curves and the IGRF suggest that the lava flow has the age of 1957–58 AD.

The Madeira granite is one of the Paleoproterozoic (1.82 Ga) A-type granite intrusions in the Amazonian Craton. It is elongated in the NE–SW direction and is composed of four facies. Classical structural techniques and the anisotropy of magnetic susceptibility (AMS) method were applied to the study of its internal fabric. Magnetic susceptibility measurements, thermomagnetic curves, remanent coercivity spectra, optical microscopy and SEM (scanning electron microscopy) analyses were carried out on the earlier and later facies of the Madeira granite: the rapakivi granite (RG) and the albite granite (AG) respectively. The last one is subdivided into the border albite granite (BAG) and the core albite granite (CAG) subfacies. AMS fabric pattern is controlled by pure magnetite in all facies, despite significant amounts of hematite in the BAG subfacies. Microstructural observations show that in almost all sites, magnetic fabric correlates to magmatic state fabrics that are defined by a weak NE–SW orientation of mafic and felsic silicates. However, strain mechanisms in both subfacies of AG also exhibit evidence for solid-state deformation at high to moderate temperatures. Pegmatite dyke, strike slip fault (SF A–B–C ), hydrothermal vein, normal fault (F 1–2 ) and joint (J) structures were observed and their orientation and kinematics is consistent with the magmatic and solid-state structures. Dykes, SF A–C and F 1 , are usually orientated along the N70°E/40°N plane, which is nearly parallel to the strike of AMS and magmatic foliations. In contrast, veins, SF B , F 2 and some J are oriented perpendicular to the N70°E trend. Kinematic analysis in these structures shows evidence for a dextral sense of movement in the system in the brittle regime. The coherent structural pattern for the three facies of Madeira granite suggests that the different facies form a nested pluton. The coherence in orientation and kinematics from magmatic to high-temperature solid-state, and into the brittle regime indicates the continuity in the stress regime from the last magmatic stages until the complete cooling of the pluton, likely along a NE–SW dextral corridor related to the regional deformation in the Uatuma–Anaua Domain of the Amazonian Craton.

International audience; We present a petrological, geochronological and geochemical study of Neoproterozoic ophiolitic units exposed in the Pan-African orogenic belt (Moroccan Anti-Atlas). These units comprise two main complexes: (i) the Khzama ophiolite (in the Sirwa window) to the west and (ii) the Aït Ahmane ophiolite to the east (in the Bou Azzer inlier). Both complexes mainly consist in serpentinized ultramafics associated with rare chromite pods and pyroxenites, which are in tectonic contact with mafic units made of isotropic and layered metagabbros and metabasalts dykes. Khzama ophiolite has been dated at 762 Ma (U-Pb zircon dating on plagiogranite dykes) and we dated Aït Ahmane ophiolite at 745.9 ± 7.4 Ma (U-Pb zircon dating on a layered metagabbro). The precursors of Aït Ahmane and Khzama serpentinites are spinel harzburgites associated with minor dunitic lenses, both entirely serpentinized. Bulk rock composition shows low contents in incompatible major and trace elements (Al2O3: 0.2-1.3 wt.% and Ti: 3-38 ppm) and in HREE ([Yb]N

Brazil is a natural laboratory for the application of isotope stratigraphy tools on the study of Precambrian sedimentary successions, with excellent registers of Archean, Paleo, Meso, and Neoproterozoic age. Those successions witnessed the most extreme and unidirectional events in Earth's history, such as the Paleoproterozoic and Neoproterozoic oxygenation events (GOE and NOE), the main stages of iron formation deposition, the global Cryogenian glaciations and the explosive diversification of metazoans near the Precambrian/Cambrian border. Chemical sedimentary rocks such as carbonates, iron formations and evaporites that can act as natural recorders of seawater composition are present in all of those successions and furnish important pieces of evidence in order to reconstruct the chemostratigraphic record. The field of isotope stratigraphy keeps expanding in Brazil and novel information from developing isotopic systems will surely add to the growing database and provide important information on Earth's ancient biogeochemical cycles in the years to come.

The Araguaia Belt encloses a poorly constrained Pan-African (Brasiliano Cycle) continental suture marked by a series of (~750 Ma) ophiolitic units which, when properly characterized, could provide important informations on its geological history, closely linked with the Rodinia demise and further western Gondwana amalgamation. We present new bulk-rock and mineral major and trace element compositions for these ultramafic and mafic units. They mainly consist in fully serpentinized harzburgite, scarce dunite lenses and chromite pods, tectonically overlain by basaltic pillow lavas. Low Al2O3/SiO2 ratios (0.01 to 0.06), rather high MgO concentrations (42.28 to 45.29 wt%) and spinels' Cr# and Mg# ratios comprised between 0.36 and 0.51 and 0.59 and 0.72, respectively, indicate a depleted oceanic-like protolith. MORB-peridotite interactions are evidenced both by pyroxenite, olivine gabbro and diabase occurrences in the serpentinites and by high TiO2 (up to 0.42 wt%) contents in spinels from some Serra do Quatipuru serpentinites. These observations support that the Araguaia Belt ophiolitic bodies are the remnants of the upper mantle section of a MOR or subcontinental lithosphere. The serpentinites whole-rock REE content can be modeled as resulting from a dry partial melting involving 14 to 24% of melt extraction, coupled with refertilization by fertile melts, generated deeper in the mantle. Such an oceanic-like setting is also supported by the N-MORB signature of Serra do Tapa and Morro do Agostinho pillow lavas basalts. All together, these results tend to infirm the supra-subduction zone (SSZ) setting previously proposed for these ophiolitic units. Important LILE, B and Li enrichments in the serpentinites likely result from a metasomatic event involving sediments-derived fluids that occurred during the obduction of the units on the Amazonian Craton. Our results combined with (1) the apparent scarcity of igneous crustal rocks, (2) the proximal nature of the metasedimentary rocks hosting the ophiolitic units, and (3) the occurrences of Amazonian Craton fragments eastward of the ophiolitic bodies, allow us to propose that the Araguaia Belt comprises a fossil ocean-continent transition (OCT) accreted on the eastern border of the Amazonian Craton.

In the terminal Neoproterozoic, drastic climate changes associated with biological innovations are coupled to isotope and elemental geochemical anomalies. However, lateral variability and local depositional controls may affect global geochemical signals, which can only be tracked through a proper stratigraphic/paleogeographic assessment. Here, we investigate the sequence stratigraphy and chemostratigraphy of the basal units of the Bambui Group, central-east Brazil. This stratigraphic unit records a foreland basin system developed during the Ediacaran-Cambrian West Gondwana assembly and represents a 1st-order sequence, in which the two lowermost 2nd-order sequences record major geochemical disturbances. The first 2nd-order sequence started with the deposition of a transgresive systems tract, possibly in a postglacial scenario, which accompanies a negative-to-positive δ13Ccarb excursion. The early highstand systems tract represents the establishment of a marine carbonate ramp throughout the basin. In terms of chemostratigraphy, it corresponds to a δ13Ccarb plateau close to 0‰ and Sr/Ca ratios around 0.001. The late highstand stage coincides with a remarkable increase in Sr content and Sr/Ca ratios at basinal scale. Occurrences of the Cloudina sp. late Ediacaran index fossil were reported in this stage. An erosional unconformity associated with a dolomitic interval, locally including subaerial exposure features, marks the top of the first 2nd-order sequence. This sequence boundary heralds an abrupt increase in δ13Ccarb values, up to +14‰. These extremely high δ13Ccarb values and high Sr/Ca ratios persist throughout the overlying sequence, as a result of progressive and enhanced restriction of the foreland basin system. Basin restriction at this stage has implications for the paleontological and chemostratigraphic record of epicontinental basins of the West Gondwana in the terminal Ediacaran. Late Ediacaran Sr-rich intervals in these basins show unusually non-radiogenic 87Sr/86Sr ratios, which may represent local depositional controls and deviations from the modern oceanographic models. Physiographic barriers and stressful conditions likely represented extreme environments for metazoan colonization.