Alto Paranaiba Igneous Province Research Articles

Robust methods, tricky materials: Challenges in dating high common-Pb perovskite from Cretaceous Brazilian kimberlites

Kimberlites and related rocks are the focus of research and exploration in the Alto Paranaiba Igneous Province (APIP; Brazil) due to their potential correlation with adjacent diamond fields. New U-Pb perovskite ages were obtained using recommended protocols for error propagation and data interpretation. Despite the utility of perovskites for dating, their incorporation of significant common-Pb (common-Pb of new APIP data: 52–84 % for in situ analyses, 75–95 % for TIMS concentrate analyses) limits the age resolution of this dating approach. Consequently, the APIP U-Pb ages neither confirm nor discredit previously proposed geodynamic models that invoke age progression or distinct magmatic episodes. Comparing the new ID-TIMS and LA-ICP-MS ages of the same intrusions, the LA-ICP-MS data have lower uncertainties, largely due to accessing a greater spread in common-Pb, improving regressions, and tend to yield older ages. For perovskite grains collected from weathering profiles, we found evidence of Pb-loss. This process leads to higher uncertainties in individual analyses, increased data scatter, and consequently higher uncertainties in age estimates. For fresh samples with different degrees of crustal contamination from a single intrusion, we found an indication of some variability in 207Pb/206Pb and 238U/206Pb ratios of the perovskite data, with a tendency towards older ages for more contaminated sample. For almost fresh intrusions with geochemically and isotopically similar mantle sources, related to the same event and in a close geographic area, we prefer to use a unique local 207Pb/206Pb ratio as anchoring for each Tera-Wasserburg plot. This approach is particularly preferred when perovskite data of an intrusion are too clustered and the 207Pb/206Pb ratio from the unanchored T-W differs from that predicted from theoretical Pb-isotope evolution model. Despite the high-common Pb in APIP perovskite crystals, our new data using these strategies clearly indicate that APIP magmatism is restricted to the Cretaceous, with most cases crystallized in Upper Cretaceous, but some from Early Cretaceous, such as the diamondiferous occurrence of Catalão-1b (120.6 ± 6.7 Ma). Different triggers of APIP magmatism are related to both episodes, mainly attributed to supercontinent break-up stages.

U–Pb ages, geochemistry, C–O–Nd–Sr–Hf isotopes and petrogenesis of the Catalão II carbonatitic complex (Alto Paranaíba Igneous Province, Brazil): implications for regional-scale heterogeneities in the Brazilian carbonatite associations

The Catalao II carbonatitic complex is part of the Alto Paranaiba Igneous Province (APIP), central Brazil, close to the Catalao I complex. Drill-hole sampling and detailed mineralogical and geochemical study point out the existence of ultramafic lamprophyres (phlogopite-picrites), calciocarbonatites, ferrocarbonatites, magnetitites, apatitites, phlogopitites and fenites, most of them of cumulitic origin. U–Pb data have constrained the age of Catalao I carbonatitic complex between 78 ± 1 and 81 ± 4 Ma. The initial strontium, neodymium and hafnium isotopic data of Catalao II (87Sr/86Sri = 0.70503–0.70599; eNdi = −6.8 to −4.7; 176Hf/177Hf = 0.28248–0.28249; eHfi = −10.33 to −10.8) are similar to the isotopic composition of the Catalao I complex and fall within the field of APIP kimberlites, kamafugites and phlogopite-picrites, indicating the provenance from an old lithospheric mantle source. Carbon isotopic data for Catalao II carbonatites (δ13C = −6.35 to −5.68 ‰) confirm the mantle origin of the carbon for these rocks. The origin of Catalao II cumulitic rocks is thought to be caused by differential settling of the heavy phases (magnetite, apatite, pyrochlore and sulphides) in a magma chamber repeatedly filled by carbonatitic/ferrocarbonatitic liquids (s.l.). The Sr–Nd isotopic composition of the Catalao II rocks matches those of APIP rocks and is markedly different from the isotopic features of alkaline-carbonatitic complexes in the southernmost Brazil. The differences are also observed in the lithologies and the magmatic affinity of the igneous rocks found in the two areas, thus demonstrating the existence of regional-scale heterogeneity in the mantle sources underneath the Brazilian platform.

Stable (C, O, S) isotopes and whole-rock geochemistry of carbonatites from Alto Paranaíba Igneous Province, SE Brazil

ABSTRACT: The present work investigates the relationship between whole-rock geochemistry and stable isotope composition from carbonatites belonging to the Tapira, Araxá, Salitre, Serra Negra, Catalão I, and Catalão II alkaline-carbonatite complexes of the Alto Paranaiba Igneous Province (APIP), central Brazil and from the Jacupiranga Complex, of the Ponta Grossa Province, southeast Brazil. The APIP complexes are ultrapotassic, comprising bebedourites, phoscorites, nelsonites, and carbonatites, whereas Jacupiranga is a sodic complex composed of ijolite-series rocks, syenites, carbonatites, and alkaline gabbros. The geochemistry data allied to mineralogical constraints allowed us to classify the carbonatites into five groups, and to devise a chemical index (BaO/(BaO+SrO)) to gauge the magmatic evolution of the studied carbonatites.The APIP carbonatites evolve from apatite-rich calciocarbonatites toward Ba-, Sr-, and rare earth element (REE)-rich magnesiocarbonatites. This evolution is mostly driven by apatite, phlogopite, dolomite, and calcite fractionation and consequent enrichment in monazite, norsethite, and strontianite. Stable isotope data show a wide diversity of petrogenetic processes in play at the APIP, relatively to the Jacupiranga Complex, which is interpreted as a result of the shallower intrusion levels of the APIP complexes. Such shallower emplacement, at low lithostatic pressure, allowed for a complex interplay of fractional crystallization, liquid immiscibility, degassing, and interaction with hydrothermal and carbohydrothermal systems.

Sulfur isotopes from Brazilian alkaline carbonatite complexes

This work presents the first sulfur isotope data for the Tapira, Araxá, Salitre, Serra Negra, Catalão I and Catalão II alkaline–carbonatite complexes of the Alto Paranaiba Igneous Province (APIP) and the Jacupiranga complex of the Ponta Grossa Province, in central and southeast Brazil, respectively. The APIP complexes are composed of alkaline silicate rocks (bebedourite, alkali clinopyroxenite, minor syenite), phoscorites, nelsonites, and carbonatites. The Jacupiranga complex is composed of ijolite-series rocks, syenites, carbonatites and alkaline gabbros. Many of these rocks contain sulfides and the carbonatites contain both sulfides and sulfates.Although there is extensive overlap, the δ34S decreases in sulfides from ultramafic rocks through phoscorites to carbonatites in each complex. Initially, this variation may be controlled by the temperature dependence of the sulfur isotope fractionation between crystallizing sulfides and magma, but in later stages it is dominated by the increasing oxidation state of the magma and crystallization of 34S-enriched sulfate minerals.Core to rim isotopic variation in quenched individual pyrite crystals indicates that δ34S increases with degassing. Metasomatic sulfides have low δ34S, resulting from the interaction of late-stage carbonatite fluids with the host rock.On a regional scale, the APIP complexes have a wider sulfur isotopic range than Jacupiranga, suggesting a more extensive interplay between magma evolution, degassing and metasomatism, probably due to a shallower emplacement of the APIP intrusions and/or a less evolved character of the Jacupiranga magmas.Sulfur isotope ranges for the studied complexes are comparable with other worldwide carbonatite and alkaline provinces, but the APIP results extend to lower δ34S.

MAJOR- AND TRACE-ELEMENT COMPOSITION OF OLIVINE, PEROVSKITE, CLINOPYROXENE, Cr-Fe-Ti OXIDES, PHLOGOPITE AND HOST KAMAFUGITES AND KIMBERLITES, ALTO PARANAIBA, BRAZIL

An integrated whole-rock petrographic and geochemical study has been carried out on kamafugites and kimberlites of the Late Cretaceous Alto Paranaiba igneous province, in Brazil, and their main minerals, olivine, clinopyroxene, perovskite, phlogopite, spinels and ilmenite. Perovskite is by far the dominant repository for light lanthanides, Nb, Ta, Th and U, and occasionally other elements, reaching concentrations up to 3.4 × 104 chondrite values for light lanthanides and 105 chondrite for Th. A very strong fractionation between light and heavy lanthanides (chondrite-normalized La/Yb from ~175 to ~2000) is also observed. This is likely the first comprehensive dataset on natural perovskite. Clinopyroxene has variable trace-element contents, likely due to the different position of this phase in the crystallization sequence; Sc reaches values as high as 200 ppm, whereas the lanthanides show very variable enrichment in light over heavy REE, and commonly show a negative Eu anomaly. The olivine, phlogopite (and tetra-ferriphlogopite), Cr–Ti oxide and ilmenite are substantially barren minerals for lanthanides and most other trace elements, with the exception of Ba, Cs and Rb in mica, and V, Nb and Ta in ilmenite. Estimated mineral / whole-rock partition coefficients for lanthanides in perovskite are similar to previous determinations, though much higher than those calculated in experiments with synthetic compositions, testifying once more to the complex behavior of these elements in a natural environment. The enormous potential for exploitation of lanthanides, Th, U and high-field-strength elements in the Brazilian kamafugites, kimberlites and related rocks is clearly shown.

Tectonic insight into a pericratonic subcrustal lithosphere affected by anorogenic Cretaceous magmatism in central Brazil inferred from long-period Magnetotellurics

Abstract Long period magnetotelluric soundings are available in a 180-km long WSW–ENE profile across the Alto Paranaiba igneous province, a complex Cretaceous alkaline province situated mostly in the southern Neoproterozoic Brasilia fold and thrust belt in central Brazil. The data indicate 3D complexity at upper and mid-crust and a simpler 2D regional structure at lower crustal and upper mantle depths. A 2D inversion emphasizing long period data identifies a highly resistive block at the uppermost mantle below the central part of the profile, surrounded by a rapid decrease in resistivity with depth. Resistivities at the block are typical of dry olivine under upper mantle conditions and a deep cratonic lithosphere is defined for this region. It is proposed that the resistive block is a rheologically enduring structure preserved within a southwestward extension of the pericratonic lithosphere of the Archean–Early Proterozoic Sao Francisco craton that lies beneath the nappes of the Brasilia belt. Lower resistivities at shallower upper mantle depths beneath the Sanfranciscana basin at the northeastern end of the profile can be interpreted either as an increased conductivity within the lithosphere or as a localized thinned out lithosphere. The conductivity enhancement possibly arises from the addition of small amounts of water to mantle anhydrous minerals during previous metasomatic percolations.

PROVENIÊNCIA DE ARENITOS DAS FORMAÇÕES UBERABA E MARÍLIA (GRUPO BAURU) E DO GARIMPO DO BANDEIRA: IMPLICAÇÕES PARA A CONTROVÉRSIA SOBRE A FONTE DO DIAMANTE DO TRIÂNGULO MINEIRO

Clay minerals and REE provide important evidence on weathering, transport, and deposition of sediments, since they tend to reflect the mineralogical and chemical signatures of the source areas. Some Bauru Group sandstones in the Triângulo Mineiro region, particularly those of the Uberaba Formation, have a peculiar composition, given by large amounts of clastic material derived from alkaline igneous rocks. In this paper we combine chemical and mineralogical evidence from sandstones of the Uberaba and Marilia Formations, and from the sandy matrix of a diamond-bearing conglomerate (Garimpo do Bandeira), of unknown stratigraphic position, to investigate the provenance of their clastic load. Integrated analysis of clay mineralogy, and major, trace and REE, allowed the inference of three distinct sources for the studied sedimentary units. Clay-minerals of the Uberaba Formation are dominated by smectite and illite, indicating strong contribution of mafic rocks and incipient chemical weathering of the source area. Semi-quantitative modeling based on the absolute REE abundances indicates that at least 30% of a component similar to the alkaline rocks from the nearby Alto Paranaiba Igneous Province (APIP) must be added to the average continental upper crust, in order to generate the observed normalised REE ratio modeling indicates a contribution of at least 50-60 % of clasts derived from kamafugite and bebedourites of the APIP. In the Marilia Formation, palygorskite is the dominant clay-mineral, but it is probably associated with diagenetic carbonate. Semi-quantitative modeling indicated less than 5% of alkaline rocks contribution.The normalised REE ratios poit to the addition of 10-15% of APIP kamafugites or bebedourites to an average continental upper crust. Kaolinite is the dominant clay mineral in the matrix of the Garimpo do Bandeira conglomerate, suggesting an intense chemical weathering in the source area. These rocks do not fit consistently in the numerical models applied. The evidence presented in this paper indicates that the contribution of alkaline-rock clastic material is strong for the Uberaba Formation, minor for the Marilia Formation, and negligible for the Garimpo do Bandeira conglomerate, with important implications for the controversy on the origin of the Triângulo Mineiro diamond deposits. We suggest that a similar approach may be used in the study of other sedimentary sequences, where the compositional distinctions are less obvious.

A Província Alcalina de Goiás e a extensão do seu vulcanismo kamafugítico

The Late Cretaceous Goias Alkaline Province (GAP) is located along a 250 x 70 Km, NW-trending region, and consists of mafic ultramafic alkaline plutonic complexes in the north, sub-volcanic bodies in the central portion and kamafugitic lava flows in the south. This provice contains some of the largest known kamafugite exposures, including the Santo Antonio da Barra flows. The lava volume in these flows reaches at least 23 Km 3 , spread over an area of 371 Km 2 , and are the world’s largest known kamafugite lava accumulation. The calculated lava volume is consistent with magma chambers equivalent in size to the plutonic complexes in the north of the Province, or with the kamafugite-carbonatite intrusion in the nearby Alto Paranaiba Igneous Province (APIP).

CRYSTAL CHEMISTRY OF TRIOCTAHEDRAL MICAS-1M FROM THE ALTO PARANAIBA IGNEOUS PROVINCE, SOUTHEASTERN BRAZIL

The crystal structures of nine trioctahedral mica-1 M crystals (phlogopite and ferroan phlogopite containing Fe 3+ in tetrahedral position, ferroan tetra-ferriphlogopite, and titanian phlogopite), from ultramafic alkaline-silicate lavas and alkaline-silicate, silicate-carbonatite and carbonatite plutonic rocks occurring in the Alto Paranaiba Igneous Province (southeastern Brazil), were refined from single-crystal X-ray data in the space group C 2/ m to R values between 0.025 and 0.039. These mica crystals are characterized by low to very low Al content; therefore, Fe 3+ may be essential to fill the tetrahedral site. The octahedral position of all the crystals studied is characterized by variable amounts of Fe 2+ , Fe 3+ and Ti 4+ in substitution for Mg. The two independent octahedral sites, M 1 and M 2, show equivalent mean electron-density and mean bond-distances in most cases. Order at the octahedral sites is enhanced in the Ti-rich crystal from Presidente Olegario owing to the preference of Ti for the M 2 site. The lateral parameters a and b reflect variations in tetrahedron composition, whereas the periodicity along c depends on octahedron composition. The crystal-chemical features of these micas are controlled by the peculiar composition of the rocks, by the high f (O 2 ), a (H 2 O), and f (CO 2 ), and by metasomatic events that have affected the Alto Paranaiba Igneous Province.

THE KAMAFUGITE-CARBONATITE ASSOCIATION IN THE ALTO PARANAÍBA IGNEOUS PROVINCE (APIP) SOUTHEASTERN BRAZIL

The Late-Cretaceous Alto Paranaiba Igneous Province (APIP) comprises a variety of ultrapotassic rock-types, including kimberlites, lamproites and large volumes of kamafugites, in addition to a number of carbonatite-bearing plutonic alkaline complexes. Phlogopite-picrites are ultramafic fine-grained rocks typically composed of olivine phenocrysts set in a groundmass of phlogopite, carbonate, perovskite, apatite and chromite. They occur as dyke swarms in the carbonatite complexes, but are also scattered throughout the Province. The phlogopite-picrites represent the peralkaline, ultrapotassic, carbonate-rich, silicate magmas parental to the carbonatite-bearing complexes, and have strong chemical affinity with kamafugites. Together with petrographic similarities observed between silicate rocks from the carbonatite complexes and xenoliths occurring in APIP kamafugites, this provides a strong link between kamafugitic and carbonatitic magmatism in the Province.

Potassic and Sodic Igneous Rocks from Eastern Paraguay: their Origin from the Lithospheric Mantle and Genetic Relationships with the Associated Parana Flood Tholeiites

Eastern Paraguay represents the westernmost fringe of Early Cretaceous Parana flood tholeiites (Serra Geral Formation SGF). Besides the SGF, eastern Paraguay has been the site of alkaline magmatism since Mesozoic times: (1) Late Permian-Early Triassic sodic intrusions and lavas: (2) Early Cretaceous potassic igneous rocks and very scarce sodic lavas (3) Late Cretaceous–Oligocene sodic lavas. Two distinct magmatic events are dominant in the Asuncion–Sapucai graben (ASU) eastern Paraguay: (1) wide-spread potassic magmatism and SGF tholeiites (Early Cretaceous) (2) Asuncion sodic magmatism (Late Cretaceous–Oligocene). The potassic rocks form a compositional continuum from moderately to strongly potassic. Two potassic suites are proposed, i.e. basanite to phonolite and alkali basalt to trachyte and their intrusive analogues. The sodic rocks include ankaratites, nephelinites and phonolites. Two similar but distinct parental magmas have been inferred for the potassic suites, both characterized by strongly fractionated REE and negative ‘Ta–Nb–Ti anomalies’. Slight positive Ta and Nb anomalies distinguish the sodic rocks. Sr–Nd isotope data confirm the distinction of the potassic rocks, enriched in radiogenic Sr and low in radiogenic Nd, from the sodic rocks, close to bulk Earth. Crustal contamination does not appear to have been significant in the generation of the investigated rocks, supported by δ 18O data. The source of potassic rocks is constrained by high LILE, LREE, Th, U and K, relative to a primitive mantle composition, and a garnet peridotite is favoured as a possible mantle source for the investigated rocks. The close association of potassic and sodic rock suites in the ASU demands that their parental magmas derived from a heterogeneous subcontinental mantle, variously enriched in incompatible elements. Significant H2O, CO2 and F are expected in the mantle source(s) for the occurrence of coeval carbonatites. Any genetic hypothesis based on a ‘mantle plume’ system is constrained by strong lithospheric mantle characteristics. This does not preclude that thermal perturbations from the asthenosphere may have trggered magmatic activity in the lithospheric mantle. Model ages indicate that two distinct metasomatic events may have occurred during Late and Early–Middle Proterozoic as precursor to the genesis of tholeiitic and alkaline magmatism in the Parana basin. These metasomatic processes were chemically distinct, indicated by the strong differences in Ti, LILE and HFSE concentrations found in both alkaline provinces (e.g. potassic rocks from ASU vs Alto Paranaiba Igneous Province) and Parana tholeiites (low vs high Ti). Ingeneral, the relationships between the alkaline rocksfrom southeastern Brazil, i.e. AltoParanaiba, Ponta Grossa Arch, Serra do Mar, Lages and the flood basalts of the Parana Basin, support a common origin in the lithospheric mantle. Sr–Nd isotope and other geochemical data indicate that a significant role was played by a mantle component depleted in incompatible elements and with high Sm/Nd ratio. This component (N-MORB type) would be represented by the depleted portions of a mantle which was variously metasomatized during Proterozoic times. The isotopic and geochemical features of the modernTristan da Cunha plume are distinctly different from the component depleted in incompatible elements, and its contribution is not apparent in the compositions of the Parana tholeiites.