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Maria Irene Bartolomeu Raposo Alexandre O. Chaves Paulo Lojkasek-Lima Manoel Souza D'Agrella-Filho Wilson Teixeira 《Tectonophysics》2004,378(1-2):43-63
Magnetic fabric and rock magnetism studies were performed on 32 mafic dikes of a Proterozoic dike swarm from the southern São Francisco Craton (SFC; Minas Gerais State, SE Brazil). Magnetic anisotropies were determined by applying anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of remanent magnetization (ARM). The latter was performed imposing both anhysteretic (total (AAR) and partial
pAAR)) and isothermal remanence magnetizations (AIRM). Partial anhysteretic remanence anisotropy was performed based on remanent coercivity spectra from a pilot specimen of each site. In most sites, AMS is dominantly carried by ferromagnetic minerals, however, in some sites, the paramagnetic contribution exceeds 70% of bulk susceptibility. Rock magnetism and thin section analysis allow classifying the dikes as non-hydrothermalized and hydrothermalized. Magnetic measurement shows that the mean magnetic susceptibility is usually lower than 5×10−3 (SI). Ti-poor titanomagnetites up to pure magnetite pseudo-single-domain (PSD) grain sizes carry the majority of magnetic fabrics for non-hydrothermalized dikes whereas coarse to fine grained Ti-poor titanomagnetites carry the majority of magnetic fabrics for hydrothermalized dikes.Three primary AMS fabrics are recognized which are coaxial with ARM fabric, except for two dikes, from both non-hydrothermalized and hydrothermalized dikes. Normal AMS fabric surprisingly is not dominant (31%). The parallelism between AMS, pAAR0–30, pAAR30–60 and pAAR60–90 fabrics in the hydrothermalized dikes indicates that magnetic grains formed due to late-stage crystallization or to remobilization of iron oxides due to hydrothermal alteration after dike emplacement have acquired a mimetic fabric coaxial with the primary fabric given by coarse-grained early crystallized Ti-poor titanomagnetites. This fabric is interpreted as magma flow in which the analysis of Kmax inclination permitted the inference that the dikes were fed by horizontal or subhorizontal fluxes (Kmax<30°). Intermediate AMS fabric is the most important (41%) in the investigated swarm. It is interpreted as due to vertical compaction of a static magma column with the minimum stress along the dike strike. ARM determinations for these sites also remained intermediate except for two dikes. In one of them, AIRM fabric resulted in normal AMS fabric while for the other AAR fabric resulted in inverse AMS fabric. A combination of AMS and ARM fabrics suggest that magmatic fabric for both dikes were overprinted by some late local event, probably related to Brasiliano orogenic processes after dike emplacement. InverseInverse AMS fabric is a minority (four dikes). ARM determinations also remained inverse suggesting a primary origin for inverse AMS fabric. 相似文献
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Ustinov Victor N. Feijo Bartolomeu Ana M. Zagainy Alexander K. Felix Joao T. Mikoev Igor I. Stegnitskiy Yury B. Lobkova Ludmila P. Kukui Irina M. Nikolaeva Evelina V. Antonov Sergei A. 《Mineralogy and Petrology》2018,112(2):383-396
Based on a comprehensive analysis of kimberlite pipes of Angola, including the near surface structural setting, deep lithospheric structure, pipe morphology and emplacement, mineralogical and petrographic features, diamond characteristics and locations of secondary deposits four geographical regions have been outlined within Angola representing four types of diamond bearing potential. These areas include high diamond bearing potential pipes, possible potential, no potential, and unclear potential areas. It was found that the depth of magmatism and diamond potential of kimberlites increases from the Atlantic coast in southwestern Angola into the continent in the north-easterly direction. Areas prospective for the discovery of new primary diamond deposits have been identified.
相似文献3.
We estimated phosphate uptake in an eutrophic and macrophyte-rich Pampean stream, in a basin where dominant land use was agricultural activity. Our hypothesis was that phosphate retention may be linked to macrophyte abundance, especially that of submerged plants. Four short-term constant-rate phosphate addition experiments were carried out in late spring, summer, autumn and winter under different discharge, background phosphate concentration and macrophyte abundance. Changes in added soluble reactive phosphorus (SRP) concentration were monitored along the studied reach and corrected for dilution to estimate phosphate uptake length (Sw) and other spiraling metrics (Vf or mass transfer coefficient, and U or nutrient uptake rate). All spiraling metrics indicated a higher SRP retention in spring than in autumn, despite macrophyte cover was similar in both sampling dates (39% and 51%, respectively). Phosphate retention was not detected neither in summer (when macrophyte biomass and especially that of submerged plants was the highest one) nor in winter. Ambient SRP concentrations in the stream were an order of magnitude higher than concentrations observed in pristine streams; nevertheless, under the varying conditions of discharge and phosphate levels observed in our study, we found no retention in summer and winter, a phosphate uptake comparable with those measured in impaired streams in autumn, and with those measured in pristine streams in late spring. 相似文献
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Maria Irene Bartolomeu Raposo 《Geophysical Journal International》1997,131(1):159-170
Magnetic fabric was determined by applying the anisotropy from the low-field magnetic susceptibility (AMS) technique in 62 mafic dykes from the Mesozoic Florianópolis (Santa Catarina Island) dyke swarm, southern Brazil. These dykes cut the crystalline basement rocks, which are mainly Proterozoic. They are vertical or subvertical in dip and trend mainly NE, although NW-trending dykes are also found. Dykes are tholeiitic in composition and are geochemically similar to those from the Ponta Grossa swarm. Thicknesses vary from 0.3 to 60 m. Polished sections show that titanomagnetites carry the AMS in these dykes. Hysteresis parameters show that the magnetic minerals fall in the PSD range. Two types of magnetic fabric are recognized. Type I is characterized by K 1 - K 2 parallel to the dyke wall, representing magma flow within the dykes; type II, with K 1 - K 3 parallel to the dyke wall, was found in four dykes. Type I is found in 94 per cent of the dykes, and approximately 20 per cent of these have K 1 inclinations of less than 30°, suggesting horizontal or subhorizontal flow. About 80 per cent have K 1 inclinations of greater than 30°, due to inclined to vertical flow. The comparison of AMS studies from both the Florianópolis and the Ponta Grossa dykes suggests a source position closer to Santa Catarina Island than the Ponta Grossa arch. 相似文献
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Manoel Souza D'Agrella-Filho Maria Irene Bartolomeu Raposo Marcos Egydio-Silva 《Gondwana Research》2004,7(1):103-113
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 Além Paraíba 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. 相似文献
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