Neoarchean atmospheric chemistry and the preservation of S-MIF in sediments from the São Francisco Craton

Sulfur mass-independent fractionation (S-MIF) preserved in Archean sedimentary pyrite is interpreted to reflect atmospheric chemistry. Small ranges in Δ³³S that expanded into larger fractionations leading up to the Great Oxygenation Event (GOE; 2.45–2.2 Ga) are disproportionately represented by sequences from the Kaapvaal and Pilbara Cratons. These patterns of S-MIF attenuation and enhancement may differ from the timing and magnitude of minor sulfur isotope fractionations reported from other cratons, thus obscuring local for global sulfur cycling dynamics. By expanding the Δ³³S record to include the relatively underrepresented São Francisco Craton in Brazil, we suggest that marine biogeochemistry affected S-MIF preservation prior to the GOE. In an early Neoarchean sequence (2763–2730 Ma) from the Rio das Velhas Greenstone Belt, we propose that low δ¹³C_org (<?30‰) and dampened Δ³³S (0.4‰ to ?0.7‰) in banded iron formation reflect the marine diagenetic process of anaerobic methane oxidation. The overlying black shale (TOC up to 7.8%) with higher δ¹³C_org (?33.4‰ to ?19.2‰) and expanded Δ³³S (2.3‰ ± 0.8‰), recorded oxidative sulfur cycling that resulted in enhance preservation of S-MIF input from atmospheric sources of elemental sulfur. The sequence culminates in a metasandstone, where concomitant changes to more uniform δ¹³C_org (?30‰ to ?25‰), potentially associated with the RuBisCO I enzyme, and near-zero Δ³³S (?0.04‰ to 0.38‰) is mainly interpreted as evidence for local oxygen production. When placed in the context of other sequences worldwide, the Rio das Velhas helps differentiate the influences of global atmospheric chemistry and local marine diagenesis in Archean biogeochemical processes. Our data suggest that prokaryotic sulfur, iron, and methane cycles might have an underestimated role in pre-GOE sulfur minor isotope records.     

The development of a Meso- to Neoarchean rifting-convergence-collision-collapse cycle over an ancient thickened protocontinent in the south São Francisco craton,Brazil

The south São Francisco craton (SSFC) in Brazil is one of the few areas that are key to unveiling the Archean evolution of South America. Despite economic interest since the 18th century, the SSFC was not studied in detail until the beginning of the present decade. The two main greenstone belts in the SSFC, namely, the Rio das Velhas (RVGB) and the Pitangui (PGB), have traditionally been considered to represent a single Archean basin. Here, new geochemistry and geochronology data from both greenstone and TTG-granite rocks are integrated and suggest that these belts evolved as separate domains. These are marked by distinct basal komatiite geochemistry, indicating that the PGB evolved as a back-arc rift on a thick lithosphere section at 2.86 Ga, in contrast to the RVGB basin, which developed near an exotic juvenile TTG terrain. Approximately 100 Ma later, the PGB basin transitioned to a calc-alkaline dominated setting, coeval with the emplacement of two large TTG igneous bodies at the margins of a poly-recycled ancient terrain. This protracted recycling is marked by extremely negative ƐHf values, which are not common for Archean terrains. The dominant strong crustal signature of the SSFC Archean rocks implies the existence of an anomalous overthickened crust or a Meso- to Neoarchean protocontinent. This thickened crust developed by continuous magmatism, delamination and differentiation in a flat-subduction setting. These mechanisms suggest that modern-style plate tectonics or a similar process such as “dripduction” was operating in this area prior to 3.0 Ga, the time at which a thick continental crust regime and horizontal tectonics were thought to have been established. Late TTG magmatism in the PGB suggests the western SSFC experienced an episode of late crustal thickening at ~2.71 Ga in response to collision, while coeval shallower K-rich magmatism in the RVGB corroborates the diachronous evolution of both belts and the surrounding crystalline terrains.     

Isotopic signatures of Paleoproterozoic granitoids from the southern São Francisco Craton and implications for the evolution of the Transamazonian Orogeny

The southern São Francisco Craton, northeastern Brazil, consists of an Archean block surrounded by a Paleoproterozoic belt related to the Transamazonian Orogeny (ca. 2.0 Ga). A calc-alkaline plutonic arc developed within the belt and the granitoid plutons comprise two distinct groups. One group displays Archean T_DM ages (3.07–2.62 Ga), ε_Nd(t) values between −11.0 and −3.8 and high initial ⁸⁷Sr/⁸⁶Sr values, and it consists mainly of peraluminous granites. T_DM ages for the other group are Paleoproterozoic (2.43–2.27 Ga), and ε_Nd(t) values range between −2.8 and +1.3; the plutons are metaluminous tonalites (trondhjemites) to granodiorites. The Transamazonian granitoids can be related to contrasting source-regions, from mantle- to crust-derived ones. A number of them are probably derived from mixing of Paleoproterozoic juvenile material and variable proportions of Archean crust material. Magmatism related to deep faulting, during the compressional stages of the Transamazonian Orogeny, is a plausible model for granitoid generation. The contribution of mantle-derived material to the granitoid sources supports the idea that a significant episode of new crust formation occurred during the Transamazonian Orogeny.     

Tracking connection and restriction of West Gondwana São Francisco Basin through isotope chemostratigraphy

C and Sr isotope compositions of carbonate rocks from the intracontinental São Francisco basin can track ocean connections and restriction. The lower three formations of the Bambuí Group can be grouped into three chemostratigraphic intervals (CI), recording different evolution stages of the basin. Lowermost CI-1 comprises the basal cap carbonates of the Sete Lagoas Formation displaying an initial C negative excursion, followed by a coeval C and Sr positive excursions (δ¹³C values from − 5 to 0‰ and ⁸⁷Sr/⁸⁶Sr ratios from 0.7074 to 0.7082) in 10 m of stratigraphic record. It marks a change from a restricted shallow basin influenced by freshwater to a basin connected to external seawaters due to marine transgression. CI-2 comprises carbonates of the middle portion of the Sete Lagoas Formation with δ¹³C values around 0‰ and ⁸⁷Sr/⁸⁶Sr ratios around 0.7082 that matches those observed worldwide for the Late Ediacaran. It records the onset of a Gondwana sea pathway connecting several epicontinental basins, allowing migration of index-fossil Cloudina sp. Uppermost CI-3 starts after a major positive excursion in the δ¹³C values reaching + 16‰ and a steepened decrease of ⁸⁷Sr/⁸⁶Sr ratios to 0.7075 which are lower than those expected for the Ediacaran-Cambrian boundary. This interval comprises the upper Sete Lagoas, Serra de Santa Helena and Lagoa do Jacaré formations and records the end of the connection of the São Francisco basin to the Gondwana sea pathway setting a restricted epeiric sea. Restriction was probably caused by Late Ediacaran uplifting of orogenic belts surrounding the basin. Other West Gondwana Cloudina bearing units also display the same mismatch in the Sr isotope ratios, suggesting that the establishment of intracontinental basins inside large continental masses may challenge the use of isotope chemostratigraphy for interbasinal correlations.     

The Carrancas Formation,Bambuí Group: A record of pre-Marinoan sedimentation on the southern São Francisco craton,Brazil

The Carrancas Formation outcrops in east-central Brazil on the southern margin of the São Francisco craton where it comprises the base of the late Neoproterozoic Bambuí Group. It is overlain by the basal Ediacaran cap carbonate Sete Lagoas Formation and was for a long time considered to be glacially influenced and correlative with the glaciogenic Jequitaí Formation. New stratigraphic, isotopic and geochronologic data imply that the Carrancas Formation was instead formed by the shedding of debris from basement highs uplifted during an episode of minor continental rifting. Reddish dolostones in the upper Carrancas Formation have δ¹³C values ranging from +7.1 to +9.6‰, which is a unique C isotopic composition for the lowermost Bambuí Group but similar to values found in the Tijucuçu sequence, a pre-glacial unit in the Araçuaí fold belt on the eastern margin of the São Francisco craton. The stratigraphic position below basal Ediacaran cap carbonates and the highly positive δ¹³C values together indicate a Cryogenian interglacial age for the Carrancas Formation, with the high δ¹³C values representing the so-called Keele peak, which precedes the pre-Marinoan Trezona negative δ¹³C excursion in other well characterized Cryogenian sequences. Hence, The Carrancas Formation pre-dates de Marinoan Jequitaí Formation and represents an interval of Cryogenian stratigraphy not previously known to occur on the southern margin of São Francicso craton. Documentation of Cryogenian interglacial strata on the São Francisco craton reinforces recent revisions to the age of Bambuí Group strata and has implications for the development of the Bambuí basin.     

Detrital zircon record of the Paleoproterozoic to Mesoproterozoic cratonic basins in the São Francisco Craton

Paleoproterozoic to Mesoproterozoic cratonic basins in the São Francisco Craton are classified into three megasequences: Statherian, Calymmian–early Ectasian, and Stenian–early Tonian sequences. The location, depositional age, and provenance of these sequences are reflected in their detrital zircon U–Pb age distributions. A minimum of five detrital zircon age distributions are distinguishable: i) the Archean and Rhyacian distribution, which is restricted to the Calymmian interval at Chapada Diamantina; ii) the Rhyacian distribution, which occurs in the Statherian, Calymmian, and Stenian intervals and generally precedes the Proterozoic distribution; iii) the Proterozoic distribution, which always succeeds the Rhyacian distribution and appears in the Brasília Belt and Southern Espinhaço; iv) the mixed Archean and Proterozoic distribution, restricted to the Stenian interval at the Southern Brasília Belt; and v) the Statherian distribution, which is present in the Statherian interval at Espinhaço Basin and is recycled into the Calymmian and Stenian intervals at Chapada Diamantina. Based on the Hf isotope compositions of the detrital zircons, most of the sequences are predominantly sourced from Archean juvenile terrains and from Archean and Paleoproterozoic evolved terrains. The exception is the Paranoá Group, western São Francisco Craton, whose positive ɛHf(t) values suggest juvenile Paleoproterozoic sources such as the Goiás Massif. The Archean zircons from Chapada Diamantina have Hf depleted mantle model ages ranging from Paleoarchean to Neoarchean and are mainly derived from the Gavião Block within northeastern São Francisco Craton. The model unify a dataset spanning hundreds of millions of years of the depositional record over a large area of central and eastern Brazil. The records will assist researchers in unraveling the sedimentary and tectonic history of the Archean and Paleoproterozoic continents from new data collected regionally and in other continents.     

Neoproterozoic glacial deposits from the Araçuaí orogen,Brazil: Age,provenance and correlations with the São Francisco craton and West Congo belt

Glacigenic diamictite successions of the Macaúbas Group are widespread in the western domain of the Araçuaí orogen, east of the São Francisco craton (Brazil). Diamictites also occur on this craton and in the African counterpart of the Araçuaí orogen, the West Congo belt. Detrital zircon grains from the matrix of diamictites and sandstones from the Macaúbas Group were dated by the U–Pb SHRIMP technique. The geochronological study sets the maximum depositional age of the glacial diamictites at 900 Ma, and indicates multiple sources for the Macaúbas basin with ages ranging from 900 to 2800 Ma. Sm–Nd T_DM model ages, determined on whole rock samples, range from 1.8 Ga to 2.5 Ga and get older up-section. Comparison of our data with those from the cratonic area suggest that these glacial deposits can be correlated to the Jequitaí and Carrancas diamictites in the São Francisco craton, and to the Lower Mixtite Formation of the West Congolian Group, exposed in Africa. The 900–1000 Ma source is most probably represented by the Zadinian–Mayumbian volcanic rocks and related granites from the West Congo belt. However, one of the most voluminous sources, with ages in the 1.1–1.3 Ga interval, has not been detected in the São Francisco-Congo craton. Possible sources for these grains could occur elsewhere in Africa, or possibly from within the Brasília Belt in western central Brazil.     

Development of environmental and natural vulnerability maps for Brazilian coastal at São Sebastião in São Paulo State

In this study, environmental and natural vulnerability maps developed for a Brazilian coastal region at São Sebastião City in São Paulo State helped to address a better understanding of the ecosystem fragility. Intense anthropogenic activities take place in the site that are mainly related to the petroleum industry and involve the operation of refineries and storage/transport of gas/oil by pipelines. A Geographical Information System (GIS) allowed the maps elaboration and analysis, where the region vulnerability was classified into five indexes: very low, low, medium, high and very high. The results depicted well the most vulnerable areas in the site, which should receive special care for their preservation. They include the coastal zones, the fluvial marine plain, the mangrove and the urban area of São Sebastião City. The results reported herein also show that the approach utilized may provide the decision makers with tools to develop an integrated coastal zone management for preserving the environmental resources in São Sebastião municipality. The methodology adopted is of easy implementation and adaptation to other areas, only taking into account the decision rules fitting diverse criteria imposed by the new reality.     

Roaldite in the iron-meteorite São Julião de Moreira

Roaldite – Fe₄N – has been identified in the São Julião de Moreira iron meteorite using electron backscatter diffraction (EBSD) and simultaneously acquired energy-dispersive x-ray spectroscopy (EDS). Mean-periodic-number images derived from raw EBSD patterns confirm this phase by an even higher spatial resolution compared to EDS.Roaldite appears in the form of systematically and repetitively aligned plates. Despite the locally heavy plastic deformation, it is shown that the origin of the oriented precipitation of roaldite is linked to the orientation of the kamacite matrix. Roaldite can be considered to be precipitated from kamacite using an inverse Kurdjumov-Sachs (K-S) or Nishiyama-Wassermann (N-W) orientation relationship. A more accurate discrimination is impossible due to the accumulated shock deformation, which blurs the local reference orientation of kamacite. The habit plane of roaldite is found to be {112}_R, which is most likely parallel to {120}_K of kamacite. Some of the roaldite plates contain two orientation variants which repeatedly alternate. Their misorientation angle is about 12°.     

Significance of age periodicity in the continental crust record: The São Francisco Craton and adjacent Neoproterozoic orogens as a case study

The São Francisco Craton, in Brazil, together with adjacent orogenic systems formed during Gondwana assemblage, are well-suited for the study of crustal growth processes. The region's geological history is marked by a series of complete tectono-metamorphic cycles, from the Archean to late Neoproterozoic, comprising arc-related magmatism followed by continental collisions and ultimately post-tectonic igneous events and rifting. In this contribution, a comprehensive isotopic database was compiled from the literature, composed mainly of high-quality U-Pb magmatic and metamorphic ages (ca. 1000), together with Lu-Hf (ca. 1300) and Sm-Nd (ca. 300) data. Using this database, combined with a tectonic/geochemical synthesized review of the region, it is possible to test which of the available contending models can better explain the apparent periodicity in the formation of the continental crustal. Some interpreted the peaks and troughs in the crustal age record as periods of increased magmatic production, controlled by periodic mantellic events. Another hypothesis is that subduction-related rocks are shielded from tectonic erosion after continental amalgamation, the peaks thus reflecting enhanced preservation potential. The latter hypothesis is favored, as the variability regarding the timing of arc-related peak magmatic production (U-Pb age peaks) from different tectonic provinces around the globe and in the considered regions, coupled to the fact that peak arc-production is always closely followed in time by major continental amalgamations (supercontinent formation), precludes a unified global causation effect, such as mantellic overturns or slab avalanches, and supports the preservation bias hypothesis. Furthermore, the worldwide (including the São Francisco Craton) occurrence of plume-related magmatism is concentrated during the periods of supercontinent break-up (i.e. after major collisions), which better relates to a top-down control on mantle convection and opposes most of the models that advocate for the primary periodicity of magmatic production, which predict enhanced plume activity slightly prior or concomitant to supercontinent formation events.     

The Horto-Baratinha itabirite-hosted iron ore: A basal fragment of the Espinhaço basin in the eastern São Francisco Craton

The Horto-Baratinha (HBD) iron ore deposit is located at the eastern border of São Francisco Craton, comprising BIF-hosted high-grade bodies (>60 wt.% Fe) associated with polydeformed quartz-mica-schists, amphibole-schist of Statherian maximum deposition age, enclosed by Statherian granitoids of the Borrachudos Suite and Neoarchean gneiss. All the sequence is crosscut by undeformed dikes and sills of pegmatitic bodies probably formed during Late Ediacaran-Cambrian. The metasedimentary sequence is stratigraphically correlatable with the Orosirian-Statherian Serra da Serpentina and Serra de São José Groups that comprise the basal units of the Espinhaço Supergroup and was intensively segmented into distinct tectonic blocks. The sedimentary/diagenetic bedding of the metamorphosed BIF (itabirite) is generally transposed by an axial planar schistosity. The lamellar hematite from itabirite is the oldest iron oxide generation, which was formed during the syn-deformational stage, parallel-oriented to the rock foliation. The (keno)magnetite grains from itabirite, iron ore and pegmatite bodies developed as idioblasts that grew over the foliation formed during late and post-deformational stages. Magnetite oxidizes subsequently to martite and granular hematite. Coarse lamellar hematite crystals randomly oriented in the border of the pegmatitic bodies also formed during the post-deformational stage due to hydrothermal reaction with itabirite. The country rocks have undergone at least three stages of deformation developed during the syn-collisional and late-collisional (Ediacaran to early-Cambrian) phases of the Brasiliano Orogeny: stage 1 with the development of a pervasive foliation (S₁), parallel to axial plane to tight folds and transposition of all sedimentary structures; stage 2 with folding of S₁; stage 3 with refolding of S₁. Both fold systems interfere with each other making up a dome and basin refolding shape. During the late-collisional (Ediacaran to early-Cambrian) and post-collisional/gravitational collapse (Cambrian) the sequence was intruded by anatectic pegmatitic bodies, which are part of the Eastern Brazilian Pegmatite Province, one of the most significant pegmatitic regions worldwide. The fluid related with these intrusions could be related with the Si leaching, crystallization of magnetite and granular hematite, and consequent formation of high-grade iron bodies.     

Chemical-Abrasion U-Pb zircon geochronology reveals 150 Myr of partial melting events in the Archean crust of the São Francisco Craton

Field observations and CA-LA-ICP-MS U–Pb zircon ages and Hf isotope compositions obtained from migmatitic orthogneisses and granitoids from the Belo Horizonte Complex, southern São Francisco Craton, indicate a major period of partial melting and production of felsic rocks in the Neoarchean. Our observations show that the complex is an important site for studying partial melting processes of Archean crystalline crust. Much of the complex exposes fine-grained stromatic migmatites that are intruded by multiple leucogranitic veins and sheeted dikes. Both migmatites and leucogranite sheets are crosscut by several phases of granitoid batholiths and small granitic bodies; both of which are closely associated with the host banded gneisses. Chemical abrasion followed by detailed cathodoluminescence imaging revealed a wide variety of zircon textures that are consistent with a long-lived period of partial melting and crustal remobilization. Results of U-Pb and Hf isotopes disclose the complex as part of a much wider crustal segment, encompassing the entire southern part of the São Francisco Craton. Compilation of available U-Pb ages suggests that this crustal segment was consolidated sometime between 3000 Ma and 2900 Ma and that it experienced three main episodes of partial melting before stabilization at 2600 Ma. The partial melting episodes took place between 2750 Ma and 2600 Ma as a result of tectonic accretion and peeling off the lithospheric mantle and lower crust. This process is likely responsible for the emplacement of voluminous potassic granitoids across the entire São Francisco Craton. We believe that the partial melting of Meso-Archean crystalline crust and production of potassic granitoids are linked to a fundamental shift in the tectonics of the craton, which was also responsible for the widespread intrusion of large syenitic bodies in the northern part of the craton, and the construction of layered mafic–ultramafic intrusions to the south of the BHC.     

Thick chromitite of the Jacurici Complex (NE Craton São Francisco,Brazil): Cumulate chromite slurry in a conduit

The Jacurici Complex, located in the NE part of the São Francisco Craton, hosts the largest chromite deposit in Brazil. The mineralized intrusion is considered to be a single N-S elongated layered body, disrupted into many segments by subsequent deformation. The ore is hosted in a thick, massive layer. Two segments, Ipueira and Medrado, have been previously studied. We provide new geological information, and chromite composition results from the Monte Alegre Sul and Várzea do Macaco segments located farther north, and integrate these with previous results. The aim of this study is to determine and discuss the magma chamber process that could explain the formation of the thick chromitite layer. All segments exhibit similar stratigraphic successions with an ultramafic zone (250 m thick) hosting a 5–8 m thick main chromitite layer (MCL), and a mafic zone (40 m thick). The chromite composition of the MCL, Mg-numbers (0.48–0.72) and Cr-numbers (0.59–0.68), is similar to chromites from layered intrusions and other thick chromitites. Previous work concluded that the parental magma of the mineralized intrusion was very primitive based on olivine composition (up to Fo₉₃) and orthopyroxene composition (up to En₉₄) from harzburgite samples, and that it originated from an old subcontinental lithospheric mantle. We estimate that the melt from which the massive chromitite layer crystallized was similar to a boninite, or low siliceous high-Mg basalt, with a higher Fe/Mg ratio. The petrologic evidence from the mafic-ultramafic rocks suggests that a high volume of magma flowed through the sill, which acted as a dynamic conduit. Crustal contamination has previously been considered as the trigger for the chromite crystallization, based on isotope studies, as the more radiogenic signatures correlate with an increase in the volumetric percentage of amphibole (up to 20%). The abundant inclusions of hydrous silicate phases in the chromites from the massive ore suggest that the magma was hydrated during chromite crystallization. Fluids may have played an important role in the chromite formation and/or accumulation. However, the trigger for chromite crystallization remains debatable. The anomalous thickness of the chromitite is a difficult feature to explain. We suggest a combined model where chromite crystallized along the margins of the magma conduit, producing a semi-consolidated chromite slurry that slumped through the conduit forming a thick chromitite layer in the magma chamber where layered ultramafic rocks were previously formed. Subsequently, the conduit was obstructed and the resident magma fractionated to produce a more evolved composition.     

Heat flow–heat production relationship not found: what drives heat flow variability of the Western Canadian foreland basin?

International Journal of Earth Sciences - Heat flow high −80&nbsp;±&nbsp;10&nbsp;mW/m2 in the northern western parts of the Western Canadian foreland basin is in large...     

Petrology,geochemistry and Sm-Nd systematics of the Paleoproterozoic Itaguara retroeclogite from São Francisco/Congo Craton: One of the oldest records of the modern-style plate tectonics

Paleoproterozoic retrogressed eclogite (retroeclogite) occurs in the Itaguara Sequence included in the suture zone formed by collision between the Archean Divinópolis and Campo Belo/Bonfim Complexes in the southern São Francisco Craton, which represents the South American counterpart of the African Congo Craton. The Itaguara retroeclogite contains scarce omphacite and phengite but abundant garnet porphyroblasts embedded in a fine-grained, amphibole, biotite and quartz-bearing matrix. The 2.20 ± 0.05 Ga eclogitization event (garnet and whole rock Sm-Nd isochronic age) of the E-MORB protolith (T_DM ~ 2.47 Ga) is recorded by omphacite formation during high-pressure prograde stage in amphibole eclogite facies due to ~70 km depth subduction process. Amphibole eclogite facies metamorphic peak stage of 17–20 kbar and 600–700 °C occurred during ~2.1 Ga continental collision. Tectonic exhumation-related decompression during collision probably triggered partial melting of the eclogitic rock. Finally, decompression late stage estimated between 5 and 8 kbar and 550–650 °C under amphibolite facies overprint during orogenic collapse was responsible for appearance of kelyphitic reaction rims (symplectite) around garnet crystals. As its Paleoproterozoic contemporary analogues from Congo Craton, the Itaguara retroeclogite is one of the oldest records of the modern-style plate tectonics.     

The last 25,000 years in the Eastern Plateau of Southern Brazil according to Alpes de São Francisco record

The palaeoenvironmental history has been studied based on palynology of a sedimentary profile from the Alpes de São Francisco bog (29°29′35′′S, 50°37′18′′W), São Francisco de Paula municipality, Rio Grande do Sul eastern Plateau, extreme Southern Brazil. The results indicate a regional cold and dry climate between 25,000 and 12,500 yr BP, interpreted from the grassland vegetation, forest taxa were present in refuges and the shallow local lake began to fill in. Climatic conditions became more aride after 16,000 yr BP, when grassland became rare. From 12,500 yr BP onwards, the climate began to change and at 11,000–9700 yr BP a warm and moist climate permitted the slight migration of pioneer arboreal taxa from refuges and locally a marsh formation. Between 9700 and 6500 yr BP a warm and dry climate resulted in reduction of grassland, confined the forest in refuges, dried out the marsh. The gradual increase of humidity between 6500 and 4000 yr BP allowed migration of forests from refuges and a bog developed. Between 4000 and 2000 yr BP Araucaria forest spread, indicating moister climate. The local bog expanded. From 2000 yr BP onwards, humid but warmer climate seems to result in a lower reproductive capacity of Araucaria forest taxa limiting its expansion. The bog reached the present-day in a decline condition. The results are compared to previous records from Southern Brazil highlands and some places from Argentina in order to better elucidate the climatic and vegetational history of these important South America areas during the late Quaternary.     

Metallogeny of the Northern S?o Francisco Craton, Bahia, Brazil

Please refer to the attachment(s) for more details.     

Debris flow affecting the Cubatão Oil Refinery,Brazil

On February 6, 1994, a large debris flow developed because of intense rains in a 800-m-high mountain range called Serra do Cubatão, the local name for the Serra do Mar, located along the coast of the state of São Paulo, Brazil. It affected the Presidente Bernardes Refinery, owned by Petrobrás, in Cubatão. The damages amounted to about US $40 million because of the muck cleaning, repairs, and 3-week interruption of the operations. This prompted Petrobrás to conduct studies, carried out by the authors, to develop protection works, which were done at a cost of approximately US $12 million. The paper describes the studies conducted on debris flow mechanics. A new criteria to define rainfall intensities that trigger debris flows is presented, as well as a correlation of slipped area with soil porosity and rain intensity. Also presented are (a) an actual grain size distribution of a deposited material, determined by laboratory and a large-scale field test, and (b) the size distribution of large boulders along the river bed. Based on theory, empirical experience and back-analysis of the events, the main parameters as the front velocity, the peak discharge and the volume of the transported sediments were determined in a rational basis for the design of the protection works. Finally, the paper describes the set of the protection works built, emphasizing their concept and function. They also included some low-cost innovative works.     

Geophysical investigation using resistivity and GPR methods: a case study of a lubricant oil waste disposal area in the city of Ribeirão Preto,São Paulo,Brazil

Geophysics has been shown to be effective in identifying areas contaminated by waste disposal, contributing to the greater efficiency of soundings programs and the installation of monitoring wells. In the study area, four trenches were constructed with a total volume of about 25,000 m³. They were almost totally filled with re-refined lubricating oil waste for approximately 25 years. No protection liners were used in the bottoms and laterals of the disposal trenches. The purpose of this work is to evaluate the potential of the resistivity and ground penetrating radar (GPR) methods in characterizing the contamination of this lubricant oil waste disposal area in Ribeirão Preto, SP, situated on the geological domain of the basalt spills of the Serra Geral Formation and the sandstones of the Botucatu Formation. Geophysical results were shown in 2D profiles. The geophysical methods used enabled the identification of geophysical anomalies, which characterized the contamination produced by the trenches filled with lubricant oil waste. Conductive anomalies (smaller than 185 Ωm) immediately below the trenches suggest the action of bacteria in the hydrocarbons, as has been observed in several sites contaminated by hydrocarbons in previously reported cases in the literature. It was also possible to define the geometry of the trenches, as evidenced by the GPR method. Direct sampling (chemical analysis of the soil and the water in the monitoring well) confirmed the contamination. In the soil analysis, low concentrations of several polycyclic aromatic hydrocarbons (PAHs) were found, mainly naphthalene and phenanthrene. In the water samples, an analysis verified contamination of the groundwater by lead (Pb). The geophysical methods used in the investigation provided an excellent tool for environmental characterization in this study of a lubricant oil waste disposal area, and could be applied in the study of similar areas.     

A perspective on potassic and ultrapotassic rocks: Constraints on the Paleoproterozoic late to post-collisional event in the São Francisco paleocontinent

The late- to post-collisional stage in orogenic systems is characterized by the coeval existence of bimodal potassic to ultrapotassic magmatic activity related to partial melting of an enriched lithospheric mantle together with crustal derived melts. In this paper, we present new whole rock geochemical analyses combined with zircon and titanite U–Pb and zircon Hf isotopic data from potassic to ultrapotassic rocks from six plutons that occur within the Archean Itacambira-Monte Azul block (BIMA), to discuss their petrogenesis and the tectonic implications for the São Francisco paleocontinent. The new U–Pb ages range from ca. 2.06 Ga to 1.98 Ga and reveal long-lasting potassic magmatism within the BIMA, which is within the late- to- post-collisional stage of the São Francisco paleocontinent evolution. The ultrapotassic rocks are compatible with a fluid-related metasomatized mantle source enriched by previous subduction events, whereas the potassic rocks are bimodal and have a transitional shoshonitic to A-type affinity. These rocks have a hybrid nature, possible related to the mixing between the mafic potassic/ultrapotassic rocks and high temperature crustal melts of the Archean continental crust. Our results also show an increase of within-plate signature towards the younger potassic magmas. The participation of an important Archean crustal component in the genesis of these rocks is highlighted by the common and occasionally abundant occurrence of Archean inherited zircons. The Hf isotopic record shows that most of the zircon inheritance has dominantly subchondritic ε_Hf(t) values, which fits a crustal reworking derivation from a similar Eo- to Paleoarchean precursor crust. However, the presence of juvenile 2.36 Ga zircon inheritance in an ultrapotassic sample reveal the existence of a hidden reservoir that is somewhat similar to the described for the Mineiro Belt in southern São Francisco paleocontinent.