Maria Irene Bartolomeu Raposo

Anisotropy of magnetic susceptibility in the Ponta Grossa dyke swarm (Brazil) and its relationship with magma flow direction

Abstract Measurements of anisotropy of magnetic susceptibility (AMS) in 95 mafic dykes (mainly tholeiites 10–200 m in width) from the Mesozoic Ponta Grossa swarm, Southern Brazil, revealed two main types of magnetic fabric. Type I fabric (plane K 1 - K 2 parallel to the dyke plane) represents magma flow within the dykes, whereas Type II (plane K 1 - K 3 parallel to the dyke plane) is compatible with a fabric pattern reflecting vertical compaction of the magma column. Fabric Type I dominates (51% of the dykes) within the swarm, whereas Type II (38% of the dykes) concentrates mainly in the western region where the dykes intrude sediments. Considering the dykes with Type I fabrics, it is concluded that 58% of the dykes were fed by horizontal or sub-horizontal ( K 1 inclinations less than 30°) magmatic flow, and 42% were fed by inclined to vertical ( K 1 inclinations more than 30°) magma flow. The latter are more frequent in the southeastern part of the swarm, suggesting a magma source close to this area, although there may have been other sources in other regions where dykes with inclined flow and distinct chemical characteristics are also found.

Paleomagnetism and 40Ar/39Ar dating of the early Cretaceous Florianópolis dike swarm (Santa Catarina Island), Southern Brazil

Abstract The Florianopolis (FL) basaltic dikes are well exposed on Santa Catarina Island, close to the present southeastern border of the Parana Magmatic Province in Southern Brazil, and their intrusion was related to the tectonic and magmatic evolution of rifting in the southern Atlantic. The dikes are vertical or subvertical, and cut the crystalline basement rocks, mainly Late Proterozoic granites. Most of the dikes exhibit NE trends, corresponding to structural trends of the crystalline basement. A few NW-trending dikes can also be observed, occasionally crosscutting the NE dikes. Samples from 73 dikes were collected for paleomagnetic work, and subjected to both alternating field and thermal demagnetization. Characteristic magnetizations of normal and reversed polarities interpreted as original thermoremanences are carried by Ti-poor magnetites showing remanent coercive forces in the range 18–45 mT. The corresponding paleomagnetic pole (FL pole) is located at 3.3°E, 89.1°S (N=65; α95=2.6°; k=47). Samples for 40 Ar/ 39 Ar dating were collected at the same paleomagnetic sites. Plagioclase samples from nine dikes were analyzed by stepwise heating; the apparent age spectra are strongly discordant due to excess 40 Ar contamination, but five samples yielded plateau ages ranging from 119.0±0.9 Ma to 128.3±0.5 Ma. One additional sample yielded an isochron age of 121.5±0.5 Ma. The inferred ages apparently define two modes at ∼119–122 Ma and ∼126–128 Ma, although the data set is probably insufficient to determine whether dike intrusion was continuous or episodic. The ages of 128–119 Ma are coeval with the final stages of the rifting at this latitude, suggesting that the emplacement of the FL dikes is related to extension of the continental crust just prior to the formation of oceanic crust. By comparing the FL pole with the existing paleomagnetic poles for the NW Ponta Grossa dike swarm (mainly 130.5 Ma) to the north, the Parana volcanics (mainly 133–132 Ma), and the Central Alkaline Province (127–130 Ma) on the western side of the Parana Province it is concluded that the differences in pole positions reflect plate displacements, and therefore the FL pole is probably dominated by the dikes with ages younger than 127 Ma.

An Early Cretaceous paleomagnetic pole from Ponta Grossa dikes (Brazil): Implications for the South American Mesozoic apparent polar wander path

Paleomagnetic results from the Early Cretaceous Ponta Grossa dike swarm are reported. These dikes crosscut sediments and basement rocks associated with the Ponta Grossa Arch, a tectonic feature in the eastern border of the Paleozoic-Mesozoic Parana Basin. These dikes are vertical or subvertical and trend mainly NW and NE. Samples from 127 dikes were submitted to both alternating field and thermal demagnetizations ; 121 of them yielded characteristic directions interpreted as original thermal remanent magnetizations. Both normal and reversed polarities as well as intermediate directions (6 dikes) are recorded. The paleomagnetic pole is located at 30.3°E, 82.4°S (N = 115 ; α 95 = 2.0° ; k = 43.8). This pole along with the available paleomagnetic poles for the Parana Basin volcanics (Serra Geral Formation), recalculated in this paper in the light of reported 40 Ar/ 39 Ar ages describe an apparent polar wander path segment corresponding to a clockwise rotation (∼6.9°) of the South American plate. This movement is compatible with the initial phase of the South Atlantic opening with rifting progressing from south to north. However, the calculated plate velocity is higher than the predicted velocities reported so far based on oceanic floor magnetic anomalies. The new data reported in this paper allowed the calculation of a mean Early Cretaceous pole (60.3°E, 83.8°S ; N = 7, α 95 = 2.9°) which can be discriminated from the Middle-Late Jurassic (191.3°E, 86.2°S ; N = 3, α 95 = 7.8°) and Late Cretaceous (346.5°E, 84.1°S ; N = 5, α 95 = 4.9°) mean poles for South America, indicating a more complex movement of the continent than the simple east-west drift generally proposed.

The effect of magnetic anisotropy on paleomagnetic directions in high-grade metamorphic rocks from the Juiz de Fora Complex, SE Brazil

The Juiz de Fora Complex is mainly composed of granulites and granodioritic–migmatite gneisses and is a cratonic basement of the Ribeira fold belt, which was formed during the last stage of Brasiliano orogeny. Samples widely distributed over the studied region (SE Brazil, Rio de Janeiro State) were collected for paleomagnetic and magnetic anisotropy determinations. After measurement of anisotropy of low-field magnetic susceptibility (AMS) the same specimens were submitted to both alternating field (for anisotropy of remanent magnetization, ARM, determinations) and thermal demagnetization for paleomagnetic analyses. Demagnetization processes allowed isolating an eastern, steep, positive-inclination characteristic remanent magnetization (ChRM) direction from the rocks. The studied specimens are strongly heterogeneous at sample scale regarding the amount of magnetic minerals even for specimens from the same core. Rock magnetism indicates that there is a complex mixture of magnetic minerals with both low and high coercivity which are associated with coarse-grained (titano)magnetite and either fine-grained (titano)magnetite or ‘titanohematite’ grains, respectively. These magnetic minerals are responsible for both remanent direction and magnetic anisotropies, even though for some specimens paramagnetic minerals are the carriers of AMS fabric. The ChRM direction found for the rocks should represent the cooling phase of the Brasiliano event in the area, and it was acquired after the magnetic fabric was formed. The AMS and ARM fabrics are coaxial and tectonic in origin, and compare favorably with the mesoscopic-scale fabrics in the adjacent areas. However, ARM measurements have reached only low-coercivity grains and it could be determined in few specimens. Then only an estimate of ARM effect on ChRMs correction is shown. The rocks are strongly magnetically anisotropic (>50%) and foliated. The paleomagnetic directions from each specimen within a site tend to approach its AMS or ARM foliation (Kmax–Kint or ARMmax–ARMmin plane). These directions were corrected, specimen-by-specimen from the sites, taking into account the pole of AMS or ARM foliation (Kmin or ARMmin) and the value of the degree of anisotropy (P). Results show that even for specimens with high P, the angular difference between corrected and uncorrected magnetization direction may be insignificant if the geomagnetic field at the time of acquired magnetization was approximately parallel to the magnetic foliation of the specimens. Our data set suggests that AMS is a powerful tool to correct the ChRM deviation from paleofields in rocks with complex magnetic mineralogy such as those from the Juiz de Fora Complex.

A systematic study of transfection efficiency and cytotoxicity in HeLa cells using iron oxide nanoparticles prepared with organic and inorganic bases.

Magnetic iron oxide nanoparticles (magnetite) (MNPs) were prepared using different organic and inorganic bases. Strong inorganic base (KOH) and organic bases (NH(4)OH and 1,4-diazabicyclo[2.2.2]octane (DABCO)) were used in the syntheses of the MNPs. The MNPs were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FT-IR) and magnetization measurements. MNPs prepared with strong inorganic base yielded an average size of 100 nm, whereas the average size of the MNPs prepared with the organic bases was 150 nm. The main competitive phase for MNPs prepared with the strong inorganic and organic bases was maghemite; however, syntheses with KOH yielded a pure magnetite phase. The transfection study performed with the MNPs revealed that the highest transfection rate was obtained with the MNPs prepared with KOH (74%). The correlation between the magnetic parameters and the transfection ratio without transfection agents indicated that MNPs prepared with KOH were a better vector for possible applications of these MNPs in biomedicine. HeLa cells incubated with MNP-KOH at 10 μg/mL for 24 and 48 h exhibited a decrease in population in comparison with the control cells and it was presumably related to the toxicity of the MNPs. However, the cells incubated with MNP-KOH at 50 and 100 μg/mL presented a very small difference in the viability between the cell populations studied at 24 and 48 h. These data illustrate the viability of HeLa cells treated with MNP-KOH and suggest the potential use of these MNPs in biomedical applications.

Magnetic Fabric Studies of High-Grade Metamorphic Rocks from the Juiz de Fora Complex, Ribeira Belt, Southeastern Brazil

Granulite from 66 sites along the Além-Paraiba dextral shear zone were collected for magnetic analyses. The rocks were affected by the Braziliano orogeny, which was responsible for the present structural pattern. Magnetic fabrics were determined applying anisotropy of low—field magnetic susceptibility (AMS, all sites) and anisotropy of remanence magnetization (ARM, in 21 sites). The ferromagnetic minerals are magnetite, titanohematite, and in some samples, minor pyrrhotite. Hysteresis curves show that both para— and ferromagnetic minerals are the carriers of AMS. Thus AMS is due to the preferred crystallographic orientation of paramagnetic matrix minerals and titanohematite, to the shape anisotropy of magnetite grains, or to a combination of all three. ARM was performed imposing both anhysteretic remanence (AAR) and isothermal remanence (AIRM). The AMS, AAR, and AIRM fabrics are coaxial and are tectonic in origin. Their parallelism indicates that both ferromagnetic and paramagnetic minerals recorded the same metamorphic event. A passive—marker model is suggested for ferromagnetic minerals at the outcrop scale. The magnetic foliation is very close to the strike of the Além Paraíba shear zone, suggesting that this generated the local rock fabrics during the Braziliano orogeny.

Paleomagnetic Study of the Juiz de Fora Complex, SE Brazil: Implications for Gondwana

The Juiz de Fora Complex is mainly composed of granulites, and granodioritic-migmatite gneisses and is a cratonic basement of the Ribeira belt. Paleomagnetic analysis on samples from 64 sites widely distributed along the Alem Paraiba dextral shear zone (SE Brazil, Rio de Janeiro State) yielded a northeastern, steep downward inclination direction (Dm=40.4°, Im=75.4, a95=6.0°, K=20.1) for 30 sites. The corresponding paleomagnetic pole (RB) is situated at 335.2°E; 0.6°S (a95=10.0°; K=7.9). Rock magnetism indicates that both (titano)magnetite and titanohematite are the main magnetic minerals responsible for this direction. Anisotropy of low-field magnetic susceptibility (AMS) measurements were used to correct the ChRM directions and consequently its corresponding paleomagnetic pole. This correction yielded a new mean ChRM (Dm = 2.9°, Im = 75.4°, a95 = 6.4°, K = 17.9) whose paleomagnetic pole RBc is located at 320.1°E, 4.2° N (a95=10.3°, K=7.5). Both mean ChRM and paleomagnetic pole obtained from uncorrected and corrected data are statistically different at the 95% confidence circle. Geological and geochronological data suggest that the age of the Juiz de Fora Complex pole is probably between 535–500 Ma, and paleomagnetic results permit further constraint on these ages to the interval 520–500 Ma by comparison with high quality paleomagnetic poles in the 560–500 Ma Gondwana APW path.

The inventory of geological heritage of the state of São Paulo, Brazil: Methodological basis, results and perspectives

An inventory of geological sites based on solid and clear criteria is a first step for any geoconservation strategy. This paper describes the method used in the geoheritage inventory of the State of São Paulo, Brazil, and presents its main results. This inventory developed by the geoscientific community aimed to identify geosites with scientific value in the whole state, using a systematic approach. All 142 geosites representative of 11 geological frameworks were characterised and quantitatively evaluated according to their scientific value and risk of degradation, in order to establish priorities for their future management. An online database of the inventory is under construction, which will be available to be easily consulted and updated by the geoscientific community. All data were made available to the State Geological Institute as the backbone for the implementation of a future state geoconservation strategy.

Alojamento do granito Lavras e a mineralização aurífera durante evolução de centro vulcano-plutônico pós-colisional, oeste do Escudo Sul-riograndense: dados geofísicos e estruturais

Based on the integration of original and available geological, geophysical and structural data, we assess the construction of the Lavras granite in an evolving volcanoplutonic center. This center encompasses the Lavras do Sul intrusive complex and the Hilario trachyandesitic sequence (604 - 590 Ma), both formed in the western foreland setting during the post-collisional period of the Dom Feliciano Orogeny, Sul-riograndense Shield. Brittle structures and magnetic lineaments indicate the volcanic activity starting near the orogenic collapse, since it was initially controlled by NW-SE and WNW-ESE-striking dextral transtensive systems that inverted to sinistral during tectonic relaxation. The intrusive complex was formed from the north to the south, with emplacement of the Tapera monzonite and the Lavras granite respectively, following slip change along the N70-75°W fault zone cutting it across. The granite has moderate dimensions (325 km³) and a tabular shape (length - L: thickness - T ≈ 3:1) tapering to the south, as deduced from gravity data. AMS petrofabric (anisotropy of magnetic susceptibility) reinforces two compositional-structural domains in the center and in the border of this granite body, represented by the magnesian terms (granodiorite and monzogranite) and the ferroan ones (syenogranite and alkali feldspar granite) respectively. These data together with brittle structures point to a multistage pluton constructed in two resurgent episodes involving: (1) the central laccolith through the emplacement of granodiorite beneath the monzogranite sill; and (2) the annular intrusions of ferroan granites, induced by expansion of the high-level reservoir due to recharge with lamprophyric mafic-ultramafic magmas. Its structural control, spatial distribution and association with lamprophyric dykes attest the link of gold mineralization with the last resurgent episode in a mature volcanoplutonic center.