Mercury in sediments from gold and copper exploitation areas in the Camaquã River Basin,Southern Brazil

Mercury concentrations were determined in stream sediments from the Camaquã River Basin, located in the shield region of the state of Rio Grande do Sul, southern Brazil. The resulting geochemical data show that overbank floodplain deposits exhibit higher concentrations than sediments collected from the active channel bed. In addition, higher Hg concentrations were measured in the fine(<63 μm) sediment fraction of the samples. Total Hg concentrations in the fine fraction of active stream sediments from Lavras do Sul County, which have been influenced by past gold mining activities, have decreased during the last five years to values ≤142 ng g⁻¹. However, in a settling pond containing abandoned mine wastes, the Hg concentration of a bulk sample remained exceptionally high (5220 ng g⁻¹). Preliminary speciation results show that Hg⁰ is the predominant species in most of the samples. This was the form of Hg released by the gold amalgamation activities in the area, and appears to be relatively stable under the existing E_h and pH conditions.     

Ages (U–Pb SHRIMP and LA ICPMS) and stratigraphic evolution of the Neoproterozoic volcano-sedimentary successions from the extensional Camaquã Basin,Southern Brazil

During the Ediacaran, southern Brazil was the site of multiple episodes of volcanism and sedimentation, which are best preserved in the 3000 km² Camaquã Basin. The interlayered sedimentary and volcanic rocks record tectonic events and paleoenvironmental changes in a more than 10 km-thick succession. In this contribution, we report new U–Pb and Sm–Nd geochronological constraints for the 605 to 580 Ma Bom Jardim Group, the 570 Ma Acampamento Velho Formation, and a newly-recognized 544 Ma volcanism. Depositional patterns of these units reveal the transition from a restricted, fault-bounded basin into a wide, shallow basin. The expansion of the basin and diminished subsidence rates are demonstrated by increasing areal distribution and compressed isopachs and increasing onlap of sediments onto the basement to the west. The Sm–Nd isotopic composition of the volcanic rocks indicates mixed sources, including crustal rocks from the adjacent basement. Both Neoproterozoic and Paleoproterozoic sources are indicated for the western part of the basin, whereas only the older Paleoproterozoic signature can be discerned in the eastern part of the basin.     

Petrology of a Subduction-related Caldera and Post-Collisional,Extension-related Volcanic Cones from the Early Cambrian and Middle Ordovician (?) of the Camaquã Basin,Southern Brazil

The region studied, located in southern Rio Grande do Sul State, Brazil, 25 km to the northeast of Lavras do Sul, records two volcanic events. The first is the eruption of pyroclastic material and localized flows of alkali-basalt, trachyandesite and andesite composition, corresponding to the early Cambrian Hilário Formation. This event in the region, through tectonic reactivation and explosive expulsion of a large amount of material from the magmatic reservoir chamber, which later faulted and collapsed, has generated an elliptical caldera of 7.2 km × 3.0 km. The rocks in the caldera have derived from partial melting of a spinel lherzolitic or garnet lherzolitic mantle, in a typically orogenic, calc-alkaline environment. They were generated during the final phase of subduction of the Adamastor plate beneath the Rio de la Plata plate in the early Cambrian. The second event occurred in a post-collisional tectonic setting in the Middle Ordovician?, when alkaline magma was emplaced through deep fractures, generating four cones within the collapsed caldera. The rocks in the cones bear the geochemical signature of a more evolved magma when compared to the rocks in the caldera, having been formed through low fusion rates of a garnet-depleted source in the upper mantle. It represents the last phase of the Pan-African-Brasiliano Orogeny, which occurred right after the collision of the Rio de La Plata and Kalahari continental plates, in a retroarc setting.     

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°.     

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.     

Magnetostratigraphy of young pliocene volcanics at Santo Antão,Cape Verde Islands: The Escabecada and Chã de Morte Profile

Palaeomagnetic investigations of the lava flows in two profiles on the island of Sao Antão are presented. It is concluded that the ca. 290 m Cha de Morte-Profile is dominantly of reverse polarity, equivalent to a Lower Matuyama age (between 2.1 and 2.6 Ma). The slightly older Escabecada Profile, covering 15 lava flows, ca. 110m, shows reverse polarity in the top flows, and a dominant normal polarity in the lower flows, suggesting an age between 2.5 to 3 Ma, equivalent to the older part of Matuyama and the younger part of Gauss (marine chron C2An.1n). A minimum eruption frequency of the lava flows in the Escabecada Profile, equivalent to one flow every 15.0 ± ca. 5 Ka, is suggested.     

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.     

Multicusped crocodyliform teeth from the Upper Cretaceous (São José do Rio Preto Formation,Bauru Group) of São Paulo,Brazil

The six peculiar multicusped teeth described here were collected from sediments of the Upper Cretaceous of São José do Rio Preto Formation, near Ibirá (northeastern São Paulo, Brazil). Their bulbous crowns are slightly labio-lingual compressed, and bear a main plus two accessory cusps, which conceal a well developed cingulum. Wear facets are seen on the main and distal accessory cusps. Comparison to the known Crocodyliformes with multicusped teeth show that the new material is not referable to “protosuchians” or eusuchians, nor related to two unnamed forms from Morocco and “notosuchians” such as Uruguaysuchus, Chiamaerasuchus, and Simosuchus. On the other hand, possible affinities with Candidodon and Malawisuchus were maintained based on shared traits. This includes teeth with the main cusp and some accessory cusps arranged in more than one axis, a previously defined unambiguous apomorphy of the putative clade composed of Candidodon plus Malawisuchus. The term Candidodontidae can be applied to this group, and defined as all taxa closer to Candidodon itapecuruensis than to Notosuchus terrestris, Uruguaysuchus aznarezi, Comahuesuchus brachybuccalis, Sphagesaurus huenei, Baurusuchus pachecoi, and Crocodylus niloticus.     

Micro- and nano-inclusions in a superdeep diamond from São Luiz,Brazil

We report cloudy micro- and nano-inclusions in a superdeep diamond from São-Luiz, Brazil which contains inclusions of ferropericlase (Mg, Fe)O and former bridgmanite (Mg, Fe)SiO₃ and ringwoodite (Mg, Fe)₂SiO₄. Field emission-SEM and TEM observations showed that the cloudy inclusions were composed of euhedral micro-inclusions with grain sizes ranging from tens nanometers to submicrometers. Infrared absorption spectra of the cloudy inclusions showed that water, carbonate, and silicates were not major components of these micro- and nano-inclusions and suggested that the main constituent of the inclusions was infrared-inactive. Some inclusions were suggested to contain material with lower atomic numbers than that of carbon. Mineral phase of nano- and micro-inclusions is unclear at present. Microbeam X-ray fluorescence analysis clarified that the micro-inclusions contained transition metals (Cr, Mn, Fe, Co, Ni, Cu, Zn) possibly as metallic or sulfide phases. The cloudy inclusions provide an important information on the growth environment of superdeep diamonds in the transition zone or the lower mantle.     

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.     

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.     

GIS-based environmental risk assessment in the Ribeira Valley,São Paulo,Brazil

The Ribeira Valley region (State of Sao Paulo, Brazil) hosts the largest Brazilian Atlantic forest reserve. The region was an important mineral district during the ‘60s, from where several tons of lead and other metals were exploited. Residual materials produced by the mining are diffused in the environment until today. The area also encompasses a regional arsenium anomaly associated with unexplored gold deposits. The population presently living in this region performs basic farming activities, from which most of their food is yielded. These features coupled together imply in a complex relationship between natural and anthropogenic factors that are likely to affect the life and health of the local communities. The aim of this paper is to apply digital data integration techniques for environmental risk assessment in the Ribeira Valley using environmental geochemistry. Geochemical, digital elevation and remote sensing data (Landsat Thematic Mapper) were merged and analyzed using a geographical information system. The assumed model considered environmental mobilization through erosion and anomalous As–Pb areas. Data were analyzed through Boolean and fuzzy logic techniques. Fuzzy logic proved superior in this case study as it allowed not only the detection but also the distinction between low, moderate and high environmental risk areas.     

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.     

Weathering rates and anthropogenic influences in a sedimentary basin,São Paulo State,Brazil

The weathering rate of rocks and chemical dynamics of the Corumbataı́ River basin, São Paulo State, Brazil, were evaluated using major elements as natural tracers. This basin has serious environmental problems in terms of quality of surface and rainwater, which affect the determination of weathering rate. The Corumbataı́ River, downstream from Rio Claro City, receives several elements/compounds through anthropogenic activities, with only K, SO₄²⁻ and alkalinity yielding positive flux values. The negative flux of some anions/cations can be attributed to atmospheric loading mainly related to anthropogenic inputs, providing K a value of 16.7 ton/km⁻²a⁻¹ for the material removed by weathering in the Corumbataı́ River basin. This is equivalent to 26×10⁶ kg of rock being removed each year by the Corumbataı́ River. The instantaneous flux was found to be a function of discharge, with the majority of dry residue (dissolved load) being transported during the summer (wet) months. The removed material in Corumbataı́ River basin derives mainly from two sub-basins (Cabeças River and Passa Cinco River), where the sandstones weather more easily than siltstones and claystones in the basin.     

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.     

Estimates of heat flow and heat production and a thermal model of the São Francisco craton

An updated analysis of geothermal data from the highland area of eastern Brazil has been carried out and the characteristics of regional variations in geothermal gradients and heat flow examined. The database employed includes results of geothermal measurements at 45 localities. The results indicate that the Salvador craton and the adjacent metamorphic fold belts northeastern parts of the study area are characterized by geothermal gradients in the range of 6–17°C/km. The estimated heat flow values fall in the range of 28–53 mW/m², with low values in the cratonic area relative to the fold belts. On the other hand, the São Francisco craton and the intracratonic São Francisco sedimentary basin in the southwestern parts are characterized by relatively higher gradient values, in the range of 14–42°C/km, with the corresponding heat flow values falling in the range of 36–89 mW/m². Maps of regional variations indicate that high heat flow anomaly in the São Francisco craton is limited to areas of sedimentary cover, to the west of the Espinhaço mountain belt. Crustal thermal models have been developed to examine the implications of the observed intracratonic variations in heat flow. The thermal models take into consideration variation of thermal conductivity with temperature as well as change of radiogenic heat generation with depth. Vertical distributions of seismic velocities were used in obtaining estimates of radiogenic heat production in crustal layers. Crustal temperatures are calculated based on a procedure that makes simultaneous use of the Kirchoff and Generalized Integral Transforms, providing thereby analytical solutions in 2D and 3D geometry. The results point to temperature variations of up to 300°C at the Moho depth, between the northern Salvador and southern São Francisco cratons. There are indications that differences in rheological properties, related to thermal field, are responsible for the contrasting styles of deformation patterns in the adjacent metamorphic fold belts.     

Circulation and mixing due to tidal forcing in the Bertioga Channel, São Paulo, Brazil

Bertioga Channel is a partially mixed (type 2) tidal estuary on the coastal plain of São Paulo, Brazil. Hourly current and salinity measurements during neap and spring tides in July 1991 yielded information about the physical structure of the system. Peak along-channel velocities varied from 40 cm s^?1 to 60 cm s^?1 during flood tides and from 70 cm s^?1 to 100 cm s^?1 during ebb tides. Net vertical velocity profiles indicate that the net current reverses directions at a depth of 2.5–3.0 m in the halocline. Due to appreciable fortnightly tidal modulation, the estuary alternates from being highly stratified (type 2b) during neap tides, with advection and diffusion contributing equally to the net upstream salt flux, to being moderately stratified (type 2a) during spring tides, when 90% of the net upstream salt transport is the result of effective tidal diffusion. Decomposition of the salt flux indicates that the relative contribution to the upstream salt transport by gravitational circulation shear is greater than the oscillatory tidal flux by a factor of 2.6 during neap tides. The oscillatory tidal flux is generated by the correlation of the tidal components of the u-velocity and salinity and is responsible for approximately the same amount of upstream salt transport, during neap and spring tides. However, during spring tides, this oscillatory term is greater than the other salt flux terms by a factor of 1.4. The total salt transport, through a unit width of the section perpendicular to the flow, was within 2% of the sum of the seven major decomposed, advective and dispersive terms. On the assumption that the Bertioga Channel is laterally homogeneous, the results also indicate that the estuary is not in steady state with respect to salt flux.     

Bimodal magmatism of the Tucumã area,Carajás province: U-Pb geochronology,classification and processes

Geological mapping of the Tucumã area has enabled the identification of dike swarms intruded into an Archean basement. The disposition of these dikes is consistent with the well-defined NW-SE trending regional faults, and they can reach up to 20 km in length. They were divided into three main groups: (i) felsic dikes (70% of the dikes), composed exclusively of porphyritic rhyolite with euhedral phenocrysts of quartz and feldspars immersed in an aphyric felsite matrix; (ii) mafic dikes, with restricted occurrence, composed of basaltic andesite and subordinate basalt, with a mineralogical assembly consisting dominantly of plagioclase, clinopyroxene, orthopyroxene and olivine; and (iii) intermediate rocks, represented by andesite and dacite. Dacites are found in outcrops associated with felsic dikes, representing different degrees of hybridization or mixture of mafic and felsic magmas. This is evidenced by a large number of mafic enclaves in the felsic dikes and the frequent presence of embayment textures. SHRIMP U-Pb zircon dating of felsic dikes yielded an age of 1880.9 ± 3.3 Ma. The felsic dikes are peraluminous to slightly metaluminous and akin to A2, ferroan and reduced granites. The intermediate and mafic dikes are metaluminous and belong to the tholeiitic series. Geochemical modeling showed that mafic rocks evolved by pyroxene and plagioclase crystallization, while K-feldspar and biotite are the fractionate phases in felsic magma. A simple binary mixture model was used to determine the origin of intermediate rocks. It indicated that mixing 60% of rhyolite and 40% basaltic andesite melts could have generated the dacitic composition, while the andesite liquid could be produced by mixing of 60% and 40% basaltic andesite and rhyolite melts, respectively. The mixing of basaltic and andesitic magmas probably occurred during ascent and storage in the crust, where andesite dikes are likely produced by a more homogeneous mixture at high depths in the continental crust (mixing), while dacite dikes can be generated in the upper crust at a lower temperature, providing a less efficient mixing process (mingling). The affinities observed between the felsic to intermediate rocks of the Rio Maria and São Felix do Xingu areas and the bimodal magmatism of the Tucumã area reinforce the hypothesis that in the Paleoproterozoic the Carajás province was affected by processes involving thermal perturbations in the upper mantle, mafic underplating, and associated crustal extension or transtension. The 1.88 Ga fissure-controlled A-type magmatism of the Tucumã area was emplaced ∼1.0 to ∼0.65 Ga after stabilization of the Archean crust. Its origin is not related to subduction processes but to the disruption of the supercontinent at the end of the Paleoproterozoic.     

Petrology,isotope characteristics,and K-Ar ages of the Maranhão,northern Brazil,Mesozoic basalt province

Northern Brazil contains remnants of Mesozoic flood basalts and hypabyssal rocks that were apparently emplaced during tectonism related to opening of the Atlantic Ocean. Analyses and new K-Ar ages reveal that this 700x250 km Maranhão province (5°–8°S) has low-Ti basalts (1.1 wt% TiO₂) in the western part that range about 160 to 190 Ma, and high-Ti basalts (3.4–4.4 wt% TiO₂) in the eastern part about 115–122 Ma. Low-Ti basalt compositions are less evolved and have a smaller range, Mg# 62-56, than the high-Ti basalts, Mg# 44–33. General characteristics of the least evolved members of low- and high-Ti groups include, respectively, Zr 100 and 250 ppm, Sr 225 and 475 ppm, Ba 200 and 500 ppm, Nb 10 and 26 ppm, Y 29 and 36 ppm, La/Yb_(n) 4.2 and 8.8, where La_(n) is 30 and 90. Overall compositions resemble the low- and high-Ti basaltic rocks of the Mesozoic Serra Geral (Paraná) province in southern Brazil. The Maranhão low-Ti basalts have more radiogenic Sr and Pb and higher ¹⁸O than the high-Ti basalts. Respectively, low- vs high-Ti: _Sr26–54 vs 15–18; ²⁰⁶Pb/²⁰⁴Pb=18.25–.78 vs 18.22–.24; and ¹⁸O 8.9–12.6 vs 6.5–8.6. Nd isotopes overlap: _Nd–1.6 to –3.8 vs –2.1 to –3. Ages, compositions, and isotopes indicate that the low- and high-Ti groups had independent parentages from enriched subcontinental mantle. However, both groups can be modeled from one source composition if low-Ti basalt isotopes reflect crustal contamination, and if the parentages for each group were picritic liquids that represent either higher (for low-Ti) or lower (for high-Ti) percentages of melting of that single source. When comparing Pb isotopes of Maranhão and Serra Geral high-Ti basalts (uncontaminated) to evaluate the DUPAL anomaly, Maranhão has Pb 7/4=4.6–11, and Pb 8/4=72–87; Serra Geral has Pb 7/4=10–13, and Pb 8/4=95–125. The small difference is not enough to conform to DUPAL contours, and is inconsistent with large-scale isotopic heterogeneity of mantle beneath Brazil prior to rifting of South America from Africa. Maranhão low-Ti magmas probably relate to the opening of central North Atlantic, and high-Ti magmas to the opening of equatorial Atlantic. The proposed greater percentage of source melting for low-Ti basalts may reflect a Triassic-Jurassic hotspot, while lesser melting for high-Ti magmas may relate to Cretaceous decompressional (rifting) melting.