Francisco Craton Research Articles

Geochemistry of BIF in the Quadrilátero ferrífero, Brazil, as a proxy to neoarchean paleoenvironmental and depositional conditions

Precambrian Algoma-type banded iron formations (BIFs) are biochemical metasedimentary rocks formed in the deep water environment relatively close to centers of volcanic expansion and can be characterized on the basis of their paleodepositional environment and the associated rock types. These rocks may contain geochemical signatures indicative of changes in the redox conditions of the Archean ocean. Such variations in the Meso- and Neoarchean are crucial in understanding the processes that led to the gradual increase of oxygen in the early atmosphere, culminating in the Great Oxidation Event (2.4–2.3 Ga). Oxide- and carbonate-facies BIFs were intercepted by drill cores in the Pilar gold deposit in the NE sector of the Quadrilátero Ferrífero (QF), Brazil, located in the Neoarchean Rio das Velhas greenstone belt. These rocks are interbedded with mafic and ultramafic metavolcanic rocks and carbonaceous phyllites. In light of the potential to reconstruct paleodepositional evidence indicative of the Earth's atmospheric redox history based on this stratigraphy, we have considered specific proxies from geological and geochemical observations. Pilar BIFs geochemistry exhibits positive anomalies of La, Eu, and Y, indicating characteristics inherited from seawater and high-temperature hydrothermal fluids. In addition, a superchondritic Y/Ho ratio further supports the influence of seawater. The signatures of the major elements Al2O3 and other immobile elements typical of detrital sediments corroborate the interpretation that these rocks were deposited in a deep marine environment with low clastic influence. Furthermore, negative Ce/Ce*SN anomalies in some sections of the Pilar BIF indicate limited oxygen availability which, together with Mo, U, V, and Zn trace-element enrichment factors, indicate deposition under suboxic/euxinic conditions. This research, together with other similar BIF data from the NW QF provide new insights into the paleoenvironmental conditions prevailing during the formation of the Archean volcano sedimentary basins that compose the southern São Francisco craton (SFC).

An oblique transpressive soft-collision records the amalgamation of the Siderian-Orosirian Mineiro belt (Brazil) and the São Francisco paleo-craton (Kenorland Supercontinent): Insights from the Bom Sucesso metasedimentary sequence

The southernmost São Francisco craton carries a long and polycyclic tectonic evolution from the Archean to the Proterozoic. The worldwide characterizations of rocks formed between these Eons, such as those of the Bom Sucesso metasedimentary sequence, is key to understand significant changes on Earth's geodynamic and atmosphere. This sequence is located between two important geotectonic domains in Brazil: the Archean São Francisco paleo-craton (a piece of the Kenorland Supercontinent) and the Siderian-Orosirian Mineiro belt. Although the Bom Sucesso metasedimentary sequence presents a strategic position to investigate the docking between different tectonic domains, longtime tectono-metamorphic episodes challenge the full understanding of the broad tectonic scenario. This work was performed aiming to unravel the links between the tectonic evolution of the Bom Sucesso metasedimentary sequence and their adjoining domains, represented by the Archean São Francisco paleo-craton and the Paleoproterozoic Mineiro belt. The original sedimentation of the Lagoa da Prata unit at the base of the Bom Sucesso metasedimentary sequence, the Bom Sucesso Banded Iron Formation and the Tabuãozinho unit developed in a passive margin setting with different Archean sources associated with nearby orthogneisses and metagranites of the São Francisco paleo-craton. This sequence yields an MDA of 2615 ± 4 Ma and is chronocorrelated to the Moeda Fm. at the base of the Minas Supergroup. After the deposition of the Bom Sucesso metasedimentary sequence in a passive margin setting, the amalgamation of the Paleoproterozoic Mineiro belt to the São Francisco paleo-craton between ca. 2.07 and 2.00 Ga by a soft-collision process produced two distinct compressional events. The first with N–S direction (σ1 = 181/16) originated 2 km-scale thrust faults, associated with a transcurrent movement between the Bom Sucesso metasedimentary sequence and both the adjoining Archean and Paleoproterozoic domains. The same compressional event also developed tectonic foliations under amphibolite facies conditions (2.8–6.3 kbar; 550–632 °C) and imprints a sillimanite-andalusite isograd along the Ibituruna and Bom Sucesso ridges. The second event has a NW-SE direction (σ1 = 333/18) and produced crosscutting structures under a ductile-brittle regime at lower temperatures been considered as retrogressive. A thermal influence of the Brasiliano orogeny in the region is suggested by new LA-ICPMS zircon Concordia age of 623 ± 4 Ma obtained in a garnet schist of the Lagoa da Prata unit, which is in agreement with Neoproterozoic Concordia Lower intercepts (probably Pb-loss) reported in zircon grains, and Ar–Ar ages (biotite) obtained along most of the Mineiro Belt area. This age may imply in a superposition of temporally distinct metamorphic events (Paleoproterozoic and Neoproterozoic) on the rocks of the Bom Sucesso metasedimentary sequence. The structural, metamorphic, and geochronologic data presented here brings new insights on structural and P-T-X conditions of the Paleoproterozoic geotectonic evolution of the southernmost São Francisco craton.

Unraveling Precambrian cratonic roots beneath South America: A contribution from surface wave tomography

We examine some aspects of the tectonic evolution of Precambrian cratonic roots beneath South America based on lithospheric distribution of shear-wave velocities. We derive our model by inverting 26,984 fundamental mode Rayleigh wave group velocity dispersion curves, at periods of 9–180 s. We first regionalize our measurements and then invert the result for a 3D S-wave velocity model extending to 200 km depth. Fast velocities beneath the Amazonian and São Francisco cratons, and beneath buried cratonic units in the Parnaíba and Paraná basins, are long wavelength features consistent with previous tomography studies. For the Amazonian craton at 150 km depth, we find an increase of velocities with province age, except for the Maroni-Itacaiúnas province, where we hypothesize that K'Mudku intraplate tectono-thermal events at the middle-late Mesoproterozoic and emplacement of a large igneous province following the breakup of Pangea could have altered at least partially its lithosphere. Our results are consistent with a São Francisco paleocontinent whose borders extend beyond the surface limits of the present São Francisco craton into the neighboring Araçuaí and Brasília belts. Based on slow shear-wave velocities in the upper mantle beneath the Borborema province, consistent with lithospheric thinning, we argue that a possible cratonic root of the São Francisco Paleocontinent beneath this province has likely been eroded away. This analysis is further corroborated by tectonic events that led to the alteration of the Borborema mantle, including hydration in the Paleoproterozoic, rifting in the early-middle Tonian, reworking during Neoproterozoic Brasiliano events, and lithospheric stretching during the breakup of Pangea. Finally, we also image a fast shear-wave velocity structure in the region of the Río de la Plata craton, consistent with magnetotelluric studies.

2.13 Ga Lawsonite/Barroisite-Bearing E-Morb Signature Metagabbro Associated with Spinel Metaperidotite from Itaguara (São Francisco Craton, Brazil): Oldest Blueschist-Facies Fragment of Oceanic Moho?

In close association with Paleoproterozoic retroeclogite and accretionary prism-related mica-quartz schist, a 2.13 Ga (metamorphic titanite U-Pb age) lawsonite/barroisite-bearing E-MORB signature metagabbro associated with spinel metaperidotite is found in the Itaguara Sequence from southern São Francisco craton, Brazil. Petrography and pressure-temperature equilibrium phase diagrams suggest that the metagabbro experienced blueschist-facies metamorphism, attaining peak metamorphic conditions at ~16 kbar and ~450 °C during subduction. The retrograde metamorphic path crossed epidote amphibolite-facies, in which the mineral assemblage found in metaperidotite (olivine, clinopyroxene, spinel, serpentine, chlorite, talc, and tremolite) was stable during a ca. 2.1 Ga continental collision-related exhumation that occurred between the Archean Campo Belo/Bonfim and Divinópolis complexes. This geological framework suggests that the metagabbro and adjacent spinel metaperidotite represent a subducted and exhumed blueschist-facies fragment of a Paleoproterozoic oceanic Mohorovičić (Moho) discontinuity, thus establishing the Itaguara metagabbro as the oldest-known occurrence of retrogressed blueschist and providing evidence for the activity of the modern-style plate tectonics more than 2 Gyr ago.

Geochemistry and U-Pb zircon geochronology of the Nkout banded iron formations and associated metasiliciclastic rocks, NW Congo craton, southern Cameroon

ABSTRACT The Nkout banded iron formations (BIFs)-hosted iron deposit is located in the Ntem Complex at the northwestern part of the Congo craton, southern Cameroon. We report geochronological and geochemical data of magnetite BIFs and interlayered mica schists from the Nkout iron ore deposit. The BIFs show typical features of chemical precipitates characterized by seawater-like shale-normalized rare earth elements plus yttrium spectra. Weak positive Eu anomalies indicate the contribution of high-temperature hydrothermal fluids to the BIFs, with low detrital input. The absence of true Ce anomalies indicates that the BIFs were likely to be deposited in anoxic seawater. LA-ICP-MS U-Pb detrital zircon data for the BIFs and associated metasiliciclasltic rocks constrained the maximum depositional age at ca. 2.8 Ga, suggesting that the Nkout BIFs are likely the oldest iron formations in the Ntem Complex. Our zircon U-Pb isotope data indicate that the Nkout succession experienced lower-amphibolite facies metamorphism at ca. 2.0 Ga, which correlates with the regional tectono-thermal event linked to the convergence and assembly of terranes between the Congo and São Francisco cratons during the Eburnean-Transamazonian orogeny.

Geochemistry of tourmaline-rich rocks in the Gavião Block, Northern São Francisco craton: implications for its formation

Geochemistry of tourmaline-rich rocks in the Gavião Block, Northern São Francisco craton: implications for its formation

The role of polyphase deformation in the formation of iron deposits and gold mineralization in the paleoproterozoic nyong complex greenstone belt (Southwestern Cameroon)

The Toko-Nlonkeng area is situated in the Nyong Complex of the northwestern Congo craton. In this study, airborne geophysical data (radiometric and magnetic) and structural data (outcrops and core samples) were used to investigate the relationship between structural evolution, iron formation (IFs)-hosted iron ore deposits, and gold mineralization in the Paleoproterozoic Nyong complex greenstone belt in southwestern Cameroon. Magnetic and radiometric trends, combined with field data, show general NE-SW, NW-SE, and E-W directions that correlate with the main gold-bearing structures. The study area has undergone four (04) deformation phases relating to polycyclic metamorphic and magmatic events. The D1 and D2 phases are the result of progressive ductile and transpressional deformation; D3 is a brittle-ductile deformation; and D4 is essentially brittle with various types of fracture networks late to post-orogenic. The D3 deformation phase is characterized by constrictive deformation with C3 shears and meso‑ to mega-P3-folds, and the main stress axes show φσ2 > φσ1> φσ3, reflecting a shearing tectonic regime with a maximum E-W shortening. This polyphase deformation is derived from the Eburnean/Trans-Amazonian deformation related to the collision between the Congo and São Francisco cratons. The main strain in the structural style of the Nyong Complex appears to be transpressive tectonics. This study reveals that iron ore and gold mineralization were mostly found on the hinges of folds and confirms that folding shear zones controlled the iron ore and gold target in the Nyong Complex and Northeast Brazil. The study proposes an approach to optimize future exploration activity based on the integration of the data.

Stabilization and evolution of the Brazilian subcontinental lithospheric mantle: Insights from garnet xenocrysts and peridotite xenoliths of Três Ranchos kimberlite (APIP, Brazil)

The thermal structure and chemical composition of the subcontinental lithospheric mantle beneath the Alto Paranaíba Igneous Province defined by the major and trace element compositions of garnet and clinopyroxene xenocrysts and mantle peridotite xenoliths entrained in the Cretaceous Três Ranchos kimberlite. Estimated temperatures and pressures indicate a typical cratonic geotherm, with a surface heat flow of ∼ 40 mW/m2 slightly hotter than that at the São Francisco Craton margin (35–36 mW/m2), suggesting lateral heterogeneities in the geothermal gradients. A deep mantle portion (160 km) is recorded by a lherzolite xenolith (5 GPa and 1200 °C). This portion has undergone pervasive interstitial melt percolation of an ultra-alkaline silicate melt and late injection of small silicate melt fractions such as the host carbonated kimberlite. This pervasive migration extended into shallower SCLM sectors at 124–100 km (3.9–3.1 GPa and ∼ 940–810 °C) as evidenced by metasomatized (±phlogopite) peridotite identified by sinusoidal REE garnet compositions. This portion is apparently intercalated at 120–110 km depth (3.7–3.3 GPa and ∼ 850 °C) with strongly depleted peridotites showing garnet and clinopyroxene compositions extremely impoverished in moderately incompatible elements, as a result of extreme ancient, melt extraction. Atop the metasomatized and depleted peridotites, at 100 km depth, a layer of fertile lherzolite is identified by garnet xenocrysts, with 'normal', HREE-enriched, steadily fractionated LREE-depleted patterns. Combining our data with those of the Osvaldo França SCLM points to the occurrence of a deeper sector (>100 km depth beneath Trȇs Ranchos) originally formed by extremely depleted peridotites, successively metasomatized by ultra-alkaline melts; and a shallower sector of the São Francisco craton composed of fertile lherzolites unrelated to ultra-alkaline magmatism. Similar fertile lherzolites are found in the uppermost sectors of the Siberian and Kaapval SCLM, suggesting that they may be a ubiquitous, primary feature, possibly related to the stabilization of the Archean cratons.

Petrology of the Cretaceous Riacho do Cordeiro dike swarm: Unraveling the giant low-Ti tholeiitic plumbing system of the Equatorial Atlantic Magmatic Province

The formation of the Early Cretaceous Equatorial Atlantic Magmatic Province (EQUAMP), northeastern Brazil, is related to the fragmentation of the West Gondwana supercontinent that culminated in the separation of South America and Africa, during the Lower Cretaceous. The EQUAMP is characterized by its intrusive nature, where mafic dike swarms intrude the Proterozoic basement of the Borborema Province. The other components of this province are represented by sill complexes hosted between the Paleozoic sediments in the Parnaíba Basin, described as the Sardinha Formation. The magmatism related to the EQUAMP is tholeiitic, with high-Ti, and low-Ti magmas compositions that with similar features that are observed in other Mesozoic igneous provinces of Gondwana. The Riacho do Cordeiro dike swarm (RCo) represents the southernmost manifestation of the EQUAMP, consisting of a series of igneous bodies aligned along the N45E trend, arranged in an en echelon pattern. This structure extends for about 350 km, cutting the Precambrian basement of the southern segment of the Borborema Province. The dikes have a composition similar to tholeiitic basalts and basaltic andesites, but they are distinguished from the other components of the EQUAMP by their exclusively low-Ti content (TiO2 = 1.7–1.2 wt%). Magmatic differentiation is the main petrogenetic mechanism of the low-Ti magmas, typified by the systematic fractionation of major elements with respect to the variation of MgO (6.4–2.4% wt%). Trace (including rare earth) element patterns show enrichment in incompatible elements and negative anomalies in NbTa, a pattern recognized in continental basalts derived from (or contaminated with) lithospheric mantle. The Sr (87Sr/86Sr(i) 0.7072 to 0.7091) and Pb (206Pb/204Pb 18.360 to 18.785) isotopic signatures, combined with negative values for εNd(i) (−4.8 to −6.1) and TDM model ages >1.2 Ga, confirm the involvement of reservoir(s) in the upper mantle possibly modified by ancient subduction processes. The first reported 40Ar/39Ar ages for the low-Ti tholeiites of the RCo indicate ages from 134.53 ± 3.10 to 132.19 ± 1.28 Ma. Geochemically, the low-Ti tholeiites of the RCo are equivalent to low-Ti dikes that constitute part of the Rio Ceará Mirim dike swarm, the main component of the EQUAMP. Besides that, in a scenario external to this province, the same compositional correspondence is found within the Cretaceous dikes of the Vitória-Colatina swarm, southeastern Brazil, and the low-Ti type lavas of the Paraná-Etendeka Province. This compositional and geochronological connection between the tholeiitic magmatism located north and south of the São Francisco craton supports a possible genetic connection between these two Early Cretaceous LIPs and serve as base to future investigations that deal with what would have been the (global) mechanisms that led to the rifting of the Atlantic, from the southern segment to the equatorial margin.

Wrapping a Craton: A Review of Neoproterozoic Fold Belts Surrounding the São Francisco Craton, Eastern Brazil

A synthesis of the evolution of the Neoproterozoic belts or orogens surrounding the São Francisco craton (SFC) in northeastern and southeastern Brazil is presented. Emphasis is placed on recognizing the superposition of sedimentary basins, from rift to passive margin to retroarc and foreland, as well as identifying three diachronic continental collisions in the formation of the SFC. The Tonian passive margin occurs in the southern Brasília Belt with the Vazante, Canastra, and Araxá Groups. During the Tonian, island magmatic arcs and basins developed in front and behind these arcs (fore- and back-arcs). Subsequently, in the Cryogenian–Ediacaran, a retroarc foreland basin developed with part of the Araxá Group and the Ibiá Group, and finally, a foreland basin developed, which was filled by the Bambuí Group. A tectonic structure of superimposed nappes, with subhorizontal S1–2 foliation, formed between 650 and 610 Ma, is striking. In the northern Brasília Belt, there is the Stenian passive margin of the Paranoá Group, the Tonian intrusion of the Mafic–Ultramafic Complexes, and the Mara Rosa Island magmatic arc, active since the Tonian, with limited volcanic–sedimentary basins associated with the arc. A thrust–fold belt structure is prominent, with S1 foliation and late transcurrent, transpressive tectonics characterized by the Transbrasiliano (TB) lineament. The Cryogenian–Ediacaran collision between the Paranapanema and São Francisco cratons is the first collisional orogenic event to the west. In the Rio Preto belt, on the northwestern margin of the São Francisco craton, the Cryogenian–Ediacaran Canabravinha rift basin is prominent, with gravitational sediments that represent the intracontinental termination of the passive margin that occurs further northeast. The rift basin was intensely deformed at the Ediacaran–Cambrian boundary, as was the Bambuí Group. On the northern and northeastern margins of the São Francisco craton, the Riacho do Pontal and Sergipano orogens stand out, showing a comparable evolution with Tonian and Cryogenian rifts (Brejo Seco, Miaba, and Canindé); Cryogenian–Ediacaran passive margin, where the Monte Orebe ophiolite is located; and Cordilleran magmatic arcs, which developed between 620 and 610 Ma. In the Sergipano fold belt, with a better-preserved outer domain, gravitational sedimentation occurs with glacial influence. A continental collision between the SFC and the PEAL (Pernambuco-Alagoas Massif) occurred between 610 and 540 Ma, with intense deformation of nappes and thrusts, with vergence to the south and accommodation by dextral transcurrent shear zones, such as the Pernambuco Lineament (PE). The Araçuaí belt or orogen was formed at the southeastern limit of the SFC by a Tonian intracontinental rift, later superimposed by a Cryogenian–Ediacaran rift–passive margin of the Macaúbas Group, with gravitational sedimentation and glacial influence, and distally by oceanic crust. It is overlain by a retroarc basin with syn-orogenic sedimentation of the Salinas Formation, partly derived from the Rio Doce cordilleran magmatic arc and associated basins, such as the Rio Doce and Nova Venécia Groups. A third continental collision event (SF and Congo cratons), at the end of the Ediacaran (580–530 Ma), developed a thrust–fold belt that deforms the sediments of the Araçuaí Belt and penetrates the Paramirim Corridor, transitioning to the south to a dextral strike-slip shear zone that characterizes the Ribeira Belt.

Emergence of the 2.1 Ga Francevillian biota was preceded by unprecedented hydrothermally driven seawater eutrophication

Recently, two independent studies suggest that the emergence of putative fossilized macro-eukaryotes in the Paleoproterozoic Francevillian Basin, ~2.1 billion years ago, may be related to a rise in seawater Zn bioavailability. This explanation is reliant on their extraordinary high Zn content and association with light Zn isotopes characteristic of eukaryotic enrichment. However, the trigger and origin of rising seawater Zn supply to the basin remains unknown. This study unravels a transient episode of intense submarine hydrothermal activity that triggered the weathering of a nutrient-rich oceanic crust reservoir, related to the collision of the Congo-São Francisco cratons during the Eburnean-Transamazonian orogeny, as the source of abundant seawater dissolved Zn, together with a suite of essential trace metals and phosphate to the continental margin waters. Surprisingly, the initiation of hydrothermal weathering coincided with the rapid onset of a rare Paleoproterozoic seawater eutrophication event. This transition is marked by basin-wide redox stratification, high sediment loading with organic carbon (Corg) and nitrogen, elevated C/N ratios, a steep negative Corg and positive bulk N isotope excursion, positive Ce anomalies, and low Mn/Fe ratios. Importantly, the transient eutrophication event ended with a reversal to lower seawater phosphate levels that coincided with rapid seawater ventilation and the appearance of macrofossil bearing sediments in Franceville. We suggest that these unexpected, localized conditions, set the stage for the emergence of the Francevillian biota.

GEOCHEMISTRY AND U-Pb GEOCHRONOLOGY OF THE NEOARCHEAN GNEISSES AND PALEOPROTEROZOIC GRANITES FROM JANUÁRIA HIGH: RECORDS OF JUVENILE AND YOUTHFUL EARTH IN THE SÃO FRANCISCO CRATON NUCLEUS (BRAZIL)

Within the proposal of secular evolution of the Earth system divided into seven phases: “Proto-Earth” (4.57–4.45 Ga); “Primordial Earth” (4.45–3.80 Ga); “Primitive Earth” (3.8–3.2 Ga); “Juvenile Earth” (3.2–2.5 Ga); “Youthful Earth” (2.5–1.8 Ga); “Middle Earth” (1.8–0.8 Ga); and “Contemporary Earth” (since 0.8 Ga), the 2.61-2.64 Ga High-K, meta- to peraluminous Januária High orthogneisses are inserted in the context of almost rigid behavior of the lithosphere, by the end of the Juvenile Earth. With protolith origin suggestive of intracrustal partial melting of local older TTG (tonalite-trondhjemite-granodiorite), these orthogneisses are the representatives in the São Francisco craton nucleus of the 2.8 Ga to 2.6 Ga potassic rocks found in archean cratons worldwide, and the occurrence of these rocks is a defining characteristic of the Juvenile Earth. Supported on evidence of the plate tectonic activity in Paleoproterozoic, the 2.14-2.19 Ga Januária High biotite granites are inserted in the context of Youthful Earth and they are the record of accretionary orogeny delineated by the edification of magmatic arc at that time. They are calc-alkaline, I-type rocks typical of subduction-related continental magmatic arcs. These biotite granites are probably derived from a High-K mafic source associated with tonalites.

KENGEDE MAFIC DYKE SWARM AND EXPANSION OF THE 1.50 Ga KUONAMKA LARGE IGNEOUS PROVINCE OF NORTHERN SIBERIA

Within the Anabar shield in the northern part of the Siberia, Late Precambrian mafic igneous units are widespread, which form dyke swarms of different ages of different trends. This paper presents new data on the composition, structure and U-Pb dating of the E-W trending Kengede dyke swarm. Three new U-Pb ID-TIMS baddeleyite ages (1496±7, 1494±3 and 1494±5 Ma) were obtained from three dykes, indicating that the Kengede swarm is part of the 1500 Ma Kuonamka large igneous province (LIP). The previously recognized Kuonamka Large Igneous Province (LIP) extends for 700 km from the Anabar shield to the Olenek uplift in the northern part of the Siberia and is potentially linked to coeval dykes and sills of the São Francisco craton and the Congo craton. The newly dated Kengede swarm is parallel to but offset by 50 km from the previously dated 1501±3 Ma Kuonamka swarm, and the identification of these two subparallel dyke subswarms of the Kuonamka LIP supports the earlier interpretation that mantle plume centre was located along the extrapolated trend of the dykes near the eastern or western margin of the Siberia. The paper examines features of sulfide Cu-Ni mineralization in dolerites of the Kengede and East Anabar dyke swarms and discusses potential Cu-Ni-sulfide mineralization linked to the Precambrian mafic dyke swarms of different ages in the north-east of the Siberia.

Unraveling the relationship between a tonalitic pluton and shear zones: Insights from magnetic fabrics and microstructures of the Alto Maranhão batholith, Mineiro belt, southern São Francisco craton

The Alto Maranhão batholith (SE Brazil) comprises granitic juvenile arc-related rocks within the Paleoproterozoic Mineiro Belt. The pluton is bounded by the NE-SW sinistral Jeceaba-Bom Sucesso (JBSZ) and NW-SE dextral Congonhas-Itaverava (CISZ) shear zones. Field, microstructural and magnetic fabric studies were employed in order to constrain its emplacement model and the spatial and temporal relationships between plutons and shear zones during Paleoproterozoic orogenic events. Combined meso- and microscale fabric analyses allowed us to individualize two different domains or lobes across the intrusion. The magmatic foliation, defined by biotite and hornblende, strikes NE-SW in the western lobe and rotates to NW-SE in the eastern lobe, moderately to steeply dipping, trending accordingly with the solid-state foliation exhibited by the country rocks. Field observations show that magmatic structures are preserved, such as elongated to rounded syn-magmatic enclaves and magmatic layering. The microstructures described for both domains are a direct record that deformation occurs locally and under magmatic conditions, with limited features of solid-state deformation such as undulose extinction in quartz and plagioclase on both tonalite and dioritic enclaves. AMS data exhibits dominantly steep magnetic lineations in the eastern lobe of the intrusion, except for sampling sites close to the CISZ, whose lineations are shallowly plunging. In both cases, the magnetic lineation trends NW-SE. Towards the western lobe, the magnetic lineation becomes horizontal and randomly oriented close to its westernmost tip. The magnetic and magmatic foliations follow the same orientation, therefore striking NW-SE in the eastern lobe and NE-SW in the west. Considering the batholith's heterogeneities highlighted by the AMS measurements, we suggest that the eastern lobe of the intrusion is characterized as a feeder zone in which magma was injected along the CISZ, being subsequently spread into the pluton's western lobe, until being stopped by a rheological barrier defined by JBSZ and country rocks, controlling its boomerang-like form. The proposed emplacement model suggests that the Alto Maranhão batholith was emplaced in a contractional site, by arc-normal dextral simple shear revealing that transcurrent deformation was taking place concurrently with the growth of the São Francisco paleocontinent, and addition of mantle-derived magmas to the continental crust.

Imaging exhumed continental and proto-oceanic crusts in the Camamu triple junction, Brazil

During the SALSA experiment, in 2014, twelve combined wide-angle refraction and coincident multi-channel seismic profiles were acquired in the Jequitinhonha-Almada-Camamu, Jacuípe, and Sergipe-Alagoas basins, NE Brazil. Profile SL09 images the Almada-Camamu basin and the São Francisco craton, with 18 four-channel ocean-bottom seismometers and 22 land stations. The datasets were forward modelled and combined with pre-stack depth migration to increase the horizontal resolution of the velocity models.Our results show that sediment thickness varies between 3.8 km in the oceanward part of the profile, 4.3 km in the Almada basin and 6.5 km in the Camamu basin. Crustal thickness at the north-western edge of the profile is of around 40 km, with velocity gradients indicating a continental origin.The Camamu basin, which corresponds to the triple junction between the aborted N–S oriented Tucano rift, the SW-NE oriented Jacuipe-Sergipe-Alagoas branch, and the N–S Jequitinhonha-Almada branch, presents two crustal layers: a very thin upper layer, about 1.5 km thick, which increases seawards to 3 km in the Almada basin, and an higher velocity (HV) layer (6.8–7.2 km/s) about 4 km thick. This lower layer gradually disappears in the Almada basin. At the south-eastern edge of the profile, the resolution is lower but the thickness of the crust seems to increase up to 5 km. Deep wide-angle reflections indicate upper mantle stratification.Crustal organisation and P-wave propagation velocities in the Almada and Camamu basins indicate a transitional crust domain of exhumed continental crust affinity. In the Camamu triple junction and beneath this thin exhumed continental crust, the HV layer may probably reflect intruded materials. No exhumed upper mantle is observed along the entire profile. The easternmost part of the profile may correspond to a proto-oceanic crust. Typical oceanic crust is never imaged along the 260 km-long offshore profile.

PALEOPROTEROZOIC GRAPHITIZATION IN THE CLÁUDIO SHEAR ZONE (SOUTHERN SÃO FRANCISCO CRATON - BRAZIL) INVESTIGATED BY RAMAN SPECTROSCOPY AND X-RAY DIFFRACTION

The southern São Francisco craton hosts some of the largest graphite mines in Brazil, located in the city of Itapecerica. This paper addresses graphite crystals that occur within the regional structure known as the Cláudio shear zone in terms of thermometry. Raman spectroscopy and X-ray diffraction data showed that, on average, graphite crystals of the Cláudio shear zone have high crystallinity and are similar to those from mines in Itapecerica. The average temperature found (702 ºC) in this study is close to the samples from Itapecerica. However, this temperature is still lower than the expected (~900 ºC) from previous studies. The hypothesis for this would be that the graphite devolved high crystallinity, but was modified during the evolution of the metamorphism, or during the post-collisional stage fluid percolation. Thus, the modified crystallinity parameters lead to the relatively low-temperature values found. The proximity and similarity of geological framework suggest correlation between the precursor basins that formed the Itapecerica and the Cláudio shear zone graphite occurrences. From a global perspective, the closure of precursor basins and formation of the Cláudio shear zone are part of the collisions of Archean and Paleoproterozoic crustal blocks that formed the Columbia supercontinent.

The polyphase evolution of the mafic rocks of the Juiz de Fora Complex: The record of two supercontinent cycles

The Juiz de Fora Complex (JFC) in the Araçuaí-Ribeira orogenic system, southeastern Brazil, is a basement unit displaying a variety of rock types that is tectonically imbricated with the supracrustal rocks of the Raposos Group in the Upper Thrust System of the Occidental Terrane, Ribeira Belt. Here we report new petrological, geochemical, and zircon U–Pb and Lu–Hf isotopic data on mafic garnet-granulites from the JFC with a view to evaluate the timing and P-T conditions of the collisional event as well as protolith formation. This work presents the most comprehensive metamorphic study on the Juiz de Fora Complex rocks to date. Metamorphic P-T estimates for the metamorphic peak conditions of 800–900 °C and 8–9 kbar in basic garnet-bearing granulites were obtained for the JFC, indicating that metamorphism took place at deeper crustal levels. Geochemical, zircon U–Pb geochronology, whole-rock Sm–Nd, Rb–Sr isotopes, and Lu–Hf isotopic data of mafic granulites yielded U–Pb ages of ca. 2201 Ma, and moderately juvenile εNd(t) = −0.5 and were obtained from the JFC marking the timing of volcanic arc magmatism during the Rhyacian period. This arc-related stage was followed by the post-collisional period during the Early Paleoproterozoic with ca. 1.94 Ga E-MORB signature granulites that shows evidence of magma sourced from Neoarchean (∼2.8 Ga) crust with addition of juvenile components. The high-grade metamorphism is possibly related to the final collision of the Congo and São Francisco Cratons at 590–560 Ma, which placed the Neoproterozoic metasediments and the JFC rocks at the same level. The integrated information sheds light on two collisional episodes where the oldest is represented by arc-related granulites during the Rhyacian Orogeny and the youngest is the higher-pressure Neoproterozoic overprint during the Brasiliano Orogeny.

KOMATIITE LAVAS FROM THE QUEBRA OSSO GROUP (RIO DAS VELHAS GREENSTONE BELT, SOUTHEAST BRAZIL): A FIELD GUIDE TO AN ARCHEAN FLOW FIELD

The Quebra Osso Group encompasses one of the best preserved Archean komatiite flows in the Quadrilátero Ferrífero region, southern São Francisco craton (Brazil). It is commonly associated with the base of the Rio das Velhas greenstone belt within the Santa Bárbara tectonostratigraphic block. The Quebra Osso rocks are well exposed along the valley of the homonymous river, near the Catas Altas city (Minas Gerais state), and in inactive serpentine quarries. In this field guide, we provide the descriptions of the key outcrops in this area, including a variety of komatiite flows, sedimentary rocks and pyroclastic rocks. Despite the pervasive alteration of primary mineral assemblages due to greenschist to amphibolite facies metamorphism, the ultramafic rocks exhibit diverse igneous textures and structures. Volcanic textures are recognizable due to the morphology of the original igneous olivine and pyroxene, which underwent pseudomorphosis by hydrous minerals – serpentine and chlorite. These features include distinctive spinifex and cumulate textures that provide clues of the conditions operating during emplacement and crystallization of the ultrabasic liquids. The predominance of thick-packages of massive flows suggests channelized lava flows, with subordinate thin marginal layered (spinifex-textured), pillowed, brecciated and pyroclastic flows. Volcanic breccias, pillow lavas and chemical sedimentary rocks indicate that these rocks erupted underwater. These multiple volcanic products characterize a unique komatiite flow field and their remarkable preservation distinguishes the Quebra Osso Group from the other ultramafic associations within the São Francisco Craton.

Quantitative methods for the analysis of comparative geological data: Large igneous province barcoding and supercontinent reconstruction

The reconstruction of Neoarchaean and Proterozoic continents relies on comparing geological characters of the appropriate age between cratons. We present a numerical approach to analysing large igneous province (LIP) barcoding patterns based on parsimony algorithms that minimise inconsistency within the data by constructing minimal-length trees. These algorithms were developed in biology to uncover natural biological groupings and to describe the relationships between them, and are applicable to any data with a hierarchical structure that indicates ’common ancestry’. We present a proof-of-concept analysis using data from 370 to 100 Ma which recovered Pangaea, Laurasia, and Gondwana. Our results support the supercraton model in the late Neoarchaean/ Palaeoproterozoic (2.6–1.87 Ga), with the recovery of four craton groupings including Superia and Sclavia, with the implication that a modern tectonic regime had not yet developed. In the Proterozoic, two craton groupings for the age ranges 1.8–1.2 Ga and 1–0.56 Ga were recovered. The older time period produced two groupings: African, European, North American, and East Antarctica cratons in one clade and Australian, South American, North China and the Congo-São Francisco cratons in the other. Both Nena and Nuna are found as natural groupings within the former clade, but the overall arrangement does not conform to any published reconstruction of Columbia. The younger time period also produced two groupings (Laurussia and ‘west Gondwana’ plus ‘east Gondwana’) and does not support the idea of a single supercontinent of Rodinia.

Corrigendum: Late Cryogenian and late Paleozoic ice ages on the São Francisco craton, east Brazil

Corrigendum: Late Cryogenian and late Paleozoic ice ages on the São Francisco craton, east Brazil