Leila Soares Marques

Paraná Magmatic Province-Tristan da Cunha plume system: fixed versus mobile plume, petrogenetic considerations and alternative heat sources

Abstract Paleomagnetic reconstructions demonstrate that the Tristan da Cunha (TC) plume, which is usually related to the genesis of the high- and low-Ti flood tholeiites of the Parana Magmatic Province (PMP), was located ∼1000 km south of the Parana Province at the time of the magma eruptions. Assuming plume mobility, and considering the low-velocity zone identified in the northern portion of the PMP as the TC ‘fossil’ plume (∼20° from the present TC position), the plume migrated southward from 133–132 (main volcanic phase) to 80 Ma at a rate of about 40 mm/yr. From 80 Ma to Present the plume remained virtually fixed, leaving a track (Walvis Ridge) compatible with the African plate movement. However, geochemical and Sr–Nd–Pb isotopic data do not support that the tholeiites from Walvis Ridge, Rio Grande Rise and Parana can result from mixing dominated by the TC plume and mid-ocean ridge basalt components. The similarity among the high-Ti basalts from Rio Grande Rise, part of Walvis Ridge (525A) and the Parana Province suggests that delaminated subcontinental lithospheric mantle must be considered in their genesis. Regional thermal anomalies in deep mantle mapped by geoid and seismic tomography data offer an alternative non-plume-related heat source for the generation of intracontinental magmatic provinces.

New time constraints on dyke swarms related to the Paraná-Etendeka magmatic province, and subsequent South Atlantic opening, southeastern Brazil

Abstract 40 Ar 39 Ar incremental heating analyses were carried out on tholeiitic dyke swarms of Santa Catalina (Florianopolis) and Rio de Janeiro and alkaline dykes from the proximity of Rio de Janeiro City, southeastern Brazil. Most of the plagioclases from the tholeiitic dykes display disturbed age spectra, with younger apparent ages most often clearly correlated with alteration phases. However, a calculated weighted mean age from the less disturbed released fractions of the NE-oriented Santa Catarina dyke swarm gives an age of 129.4 ± 0.3 Ma which is contemporaneous with the Ponta Grossa Dyke Swarm (PGDS) and at least the northern Parana Continental Flood Basalts (CFB). The Rio de Janeiro tholeiitic dykes display age spectra affected by alteration and probably excess argon, but with low-temperature ‘mini-plateau ages’ between 130.6 and 129.8 Ma, which are concordant with previous data obtained on the PGDS and Rio de Janeiro dyke swarms by Renne et al. (1996a) and Turner et al. (1994). These new geochronological data when added to the previous ones show that the initiation of Gondwanas breakup in the region was characterized by a volcano-tectonic system with a geometry more complicated than the rift-rift-rift system suggested by many authors. The different rift arms of this structural feature were operating during a brief time period, mainly from 131.5 to 129 Ma, and were contemporaneous at least with the northern part of the Parana-Etendeka Province (PEP). Single grain and bulk sample biotite from the alkaline, mainly coast-parallel dyke swarm near Rio de Janeiro City display plateau ages around 80 Ma (81.8 ± 1.8 and 78.8 ± 2.8 Ma) and 70 Ma (72.5 ± 0.5 and 69.7 ± 0.2 Ma). These magmatic intrusions refer to Late Cretaceous flexuring and vertical movements and rift basin formations (Almeida, 1976) due the northward progressing rifting motions separating South America and Africa during South Atlantic Ocean formation.

Continental Flood Volcanism From the Paraná Basin (Brazil)

In the past years study of the continental flood volcanism in the Parana basin had been mainly concentrated in the southern regions, where thick lava sequences are well exposed. Such research essentially concerned basic geology, petrography, radiometric dating and, subordinately, petrochemistry, isotope geochemistry and paleomagnetism (cf. Almeida, 1983; Amaral et al., 1966; Amaral and Crosta, 1983; Asmus and Baisch, 1983; Compston et al., 1968; Cordani et al., 1980; Creer et al., 1965; Ernesto, 1985; Ernesto et al., 1979; Fodor et al., 1985a, 1985b; Fulfaro et al., 1982; Halpern et al., 1974; Leinz et aL, 1966; McDougall and Ruegg, 1966; Mantovani et al., 1985a, 1985b; Marimon et al., 1983; Marques, 1983; Melfi, 1967; Minioli et al., 1971; Pacca and Hiodo, 1976; Pacca and Ernesto, 1982; Ruegg, 1976; Ruegg and Amaral, 1976; Sartori et al., 1975, 1982; Sartori and Gomes, 1980; Sartori and Filho, 1983; Souza, 1983).

The Central Atlantic Magmatic Province (CAMP) in Brazil: Petrology, Geochemistry, 40Ar/39Ar Ages, Paleomagnetism and Geodynamic Implications

The CAMP tholeiitic magmatism in Brazil (mean 40 Ar/ 39 Ar age of 199.0±2.4 Ma) occurs on the continental margin to ca. 2,000 km into the South American platform, near the boundary between the ancient terrains of the Amazonia craton and Proterozoic/Brazilian-cycle related mobile belts. Geological evidence indicates that this magmatism was preceded, in Permo-Triassic times, by continental sedimentation, indicating a possible regional uplift. The Brazilian CAMP tholeiites are generally evolved and characterized by a low TiO 2 concentration (less than 2 wt%). The Cassipore dykes, which are usually high in TiO 2 (more than 2wt%) are an exception. The Cassipore low- and high-TiO 2 basalts are characterized by a positive Nb anomaly and Sr-Nd isotopes that are parallel to typical mantle array. Except for one sample, all the other Brazilian CAMP tholeiites that are low in TiO 2 , show Sr-Nd isotopes trending towards crustal components. The latter isotopic characteristics could be related to crustal recycling ancient (Middle-Late Proterozoic) subductions, and/or low-pressure crustal interaction. All the Brazilian CAMP tholeiites show a decoupling between their Sr-Nd isotopic composition and Rb/Sr and Sm/Nd values, suggesting mantle metasomatism, and/or subduction-related crustal interaction before mantle melting. Notably, the chemical data show that tholeiites from specific Brazilian regions are related to mantle sources that reflect compositional mantle heterogeneity, including the lower mantle of the lithospheric thermal boundary layer. In general, paleomagnetic poles for CAMP rocks from South America, Africa and North America match an age of ca. 200 Ma, but also show a distribution pattern trending to younger ages (e.g. 190 Ma), especially for the South American poles relative to the CAMP magmatism of the continental edge. The Brazilian CAMP magmatism cannot be easily explained through plume head (active) models, being instead consistent with mantle geodynamic processes where the unstable buoyancy of the Pangea supercontinent played an essential role to approach isostatic stabilization. Therefore, it is proposed that the Brazilian CAMP magmatism was related to hot upper mantle incubation under thick continental lithosphere, and to edge-driven convection between lithospheric domains with different thickness.

Petrology, geochemistry and Sr–Nd isotopes of the Trindade and Martin Vaz volcanic rocks (Southern Atlantic Ocean)

Abstract Geochemical, mineralogical and Sr–Nd isotopic analyses were performed on the strongly undersaturated volcanic rocks of the Trindade and Martin Vaz islands, located in the South Atlantic Ocean (Brazil). The magmatic activity at Trindade was essentially bimodal, generating various ultrabasic rocks and mesocratic to leucocratic phonolites. The youngest volcanic events are made up exclusively of ultrabasic rocks, whereas the older episodes are characterized by abundant phonolites. Basic rocks (45

Uranium series disequilibria in ground waters from a fractured bedrock aquifer (Morungaba Granitoids--Southern Brazil): implications to the hydrochemical behavior of dissolved U and Ra.

Activity concentrations of dissolved (234)U, (238)U, (226)Ra and (228)Ra were determined in ground waters from two deep wells drilled in Morungaba Granitoids (Southern Brazil). Sampling was done monthly for little longer than 1 year. Significant disequilibrium between (238)U, (234)U and (226)Ra were observed in all samples. The variation of (238)U and (234)U activity concentrations and (234)U/(238)U activity ratios is related to seasonal changes. Although the distance between the two wells is short (about 900 m), systematic differences of activity concentrations of U isotopes, as well as of (234)U/(238)U, (226)Ra/(234)U and (228)Ra/(226)Ra activity ratios were noticed, indicating distinct host rock-water interactions. Slightly acidic ground water percolation through heterogeneous host rock, associated with different recharge processes, may explain uranium and radium isotope behavior.

Determination of uranium concentrations and activity ratios in silicates by alpha spectrometry: application to the volcanic rocks from the Trindade and Martin Vaz Islands (Brazil)

A detailed radiochemical procedure for alpha spectrometry measurements of uranium concentrations and of 234U/238U activity ratios in silicates is presented. The described chemical processing is characterized by relatively high uranium yields, in the range of 51-89%. The application of this chemical procedure to geological standards allowed estimating a reproducibility of the alpha spectrometry uranium measurements on the order of 3.5%. As an application example. the procedure was used in the analysis of quaternary volcanic rocks from the Trindade and Martin Vaz Islands, Brazil.

Paleomagnetic and geochemical constraints on the timing and duration of the CAMP activity in Northeastern Brazil

The northeastern border of the Maranhao basin (NE Brazil) is dominated by low-Ti tholeiites which show strong geochemical affinity with the low-Ti Early Jurassic Mosquito Formation from the western side of the basin. The low-Ti tholeiitic flows (Early Jurassic) from Lavras basin, and the low-Ti dikes outcropping in the Ceara State are also comparable to the Mosquito tholeiites. Compositional similarity also exists between the Early Cretaceous Sardinha intrusives (eastern Maranhao basin), the high-Ti dikes from the Ceara State, and the high-Ti rocks of the Rio Ceara-Mirim dike swarm (mainly subswarms I and III) easterwards. The high-Ti and low-Ti rocks show paleomagnetic remanences that allow the discrimination of the two groups. However, the characteristic remanent magnetization from the Rio Ceara-Mirim subswarm II differs significantly from the other two groups; these rocks can be also distinguished from the other high-Ti rocks by slight different chemical characteristics. On paleomagnetic basis, three age groups were identified: Early Cretaceous (Sardinha Formation, high-Ti Ceara dikes, and Ceara-Mirim subswarms I, III, and V), Early Jurassic (Mosquito Formation, low-Ti Ceara dikes, Lavras basin flow, and Ceara-Mirim subswarm IV), and Late Triassic (Ceara-Mirim subswarm II), and corresponding paleomagnetic poles were calculated. The Early Jurassic pole fits well with other CAMP poles from South America, extending easterwards the area in northern South America affected by that magmatic event. The Late Triassic age might represent a magmatic manifestation preceding by about 20-30 My the CAMP activity in NE Brazil.

Origin of the Rio Grande Rise–Walvis Ridge reviewed integrating palaeogeographic reconstruction, isotope geochemistry and flexural modelling

Abstract A palaeogeographical reconstruction of the South American and African continents back to anomaly C34 (84 Ma) brings together the Rio Grande Rise (RGR) and the central portion of the Walvis Ridge (WR), thus the RGR–WR aseismic ridges may have a common origin. If the construction of the RGR–WR basaltic plateau took place mainly between 89 and 78 Ma, as indicated by the ages of the basalts sampled by DSDP wells, then the basaltic magmas are the result of an ‘on-ridge’ volcanism. Once separated, the normal sea-floor spreading and thermal subsidence of the RGR and WR ridges continued until approximately 47 Ma when an Eocene magmatism took place in the RGR. In the WR, a younger volcanism is observed in the Guyot Province. The available geochemical and isotope data of the WR–RGR basalts do not indicate the participation of the continental crust melting component. Incompatible trace element ratios and isotope signatures of the basalts from the RGR–WR ridges are distinct from the present-day Tristan da Cunha alkaline rocks, and are nearly identical to the high-Ti Paraná Magmatic Province (PMP) tholeiites (133–132 Ma). Both the high-Ti PMP and the WR–RGR basalts are characterized by moderate initial 87Sr/86Sr and low 206Pb/204Pb isotope ratios [Enriched Mantle I (EMI) mantle component], suggesting melting from a common source, with significant participation of sub-continental lithospheric mantle (SCLM). A three-dimensional (3D) flexural modelling of the RGR and WR was conducted using ETOPO1 digital topography/bathymetry and EGM2008-derived free-air anomalies as a constraint. The best fit between the observed and calculated free-air anomalies was obtained for an elastic plate with elastic plate thickness (Te) of less than 5 km, consistent with an ‘on-ridge’ initial construction of the RGR–WR. The modelling of the crust–mantle interface depths indicates a total crustal thickness of up to 30 km in the RGR–WR. Flexural analysis reinforces the geological evidence that RGR was constructed during two main magmatic episodes, the tholeiitic basalts in the Santonian–Conician times and the alkaline magmatism in the Eocene. Geochemical and geophysical evidence, which rules out the classical deep-mantle plume model in explaining the generation of basalts of these volcanic provinces, is presented. Finally, three models to explain the geochemical and isotope signatures of RGR–WR basalts are reviewed: (1) thermal erosion of SCLM owing to edge-driven convection; (2) melting of fragmented or detached SCLM and lower crust; and (3) thermal erosion at the base of the SCLM with lateral transport of enriched components by mantle flow.

Nature and Evolution of the Archean Crust of the São Francisco Craton

We overview the Archean tectonic framework the Sao Francisco craton based on geologic constraints, integrated geochronologic interpretation and isotopic-geochemical evidence of basement rocks. U–Pb provenance studies of Archean and Paleoproterozoic supracrustal sequences are also used to provide additional inferences about the geodynamic scenario. The Archean rocks crop out mainly in two large areas in the southern and northern portions of the craton, surrounded and/or in tectonic contact with Paleoproterozoic orogenic belts. The ancient substratum is essentially composed of medium- to high-grade gneissic-migmatitic rocks including TTG suites and coeval granite-greenstone associations that collectively provide an isotopic record as old as 4.1 Ga. The combined U–Pb and Sm–Nd TDM age peaks coupled with U–Pb inherited ages in detrital zircons from the supracrustal sequences indicate that very ancient continental crust (˃3.5 Ga) exist, particularly in the northern portion of the craton. Mesoarchean events are episodic between 3.6–3.3 and 3.2–2.9 Ga, as for the Neoarchean (2.8–2.6 Ga) in both cratonic portions. This isotopic record indicates a protracted Archean history for the Sao Francisco craton, highlighted by peculiar tectonic-metamorphic histories of the basement rocks. From a tectonic point of view the compiled data concur with a diachronic evolution from Paleo- to Neoarchean times by means of juvenile accretion/differentiation events characterized by multiple TTG plutonism in genetic association with greenstone belts, coupled with partial melting events of earlier-formed material. All ancient basement complexes and/or continental blocks assembled diachronically during the Late Neoarchean by convergence-related processes akin to plate dynamics. Late-tectonic K-rich granitoids, mafic-ultramafic complexes and mafic dikes collectively mark the Neoarchean thickening and final cratonization of the continental crust.