HR: 0800h
AN: GP21A-0105    [Abstracts]
TI: Magnetic Studies of coastal Early-Late Cretaceous mafic dike swarms from São Paulo State (Brazil): Preliminary Results.
AU: * Raposo, M B
EM: irene@usp.br
AU: Malo, G K
EM: malo_gkm@yahoo.com.br
AB: The Mesozoic magmatism in Southern Brazil is represented mainly by the basaltic flows of the Serra Geral Formation, the dikes swarms from the Ponta Grossa Arch, Florianópolis, along the coast between São Paulo and Rio de Janeiro, and several alkaline complexes that lie along tectonic features associated with the evolution of the Paraná Basin. The emplacement of the dike swarms and the alkaline complexes is related to the processes of separation between South America and Africa and, consequently, with the opening of Atlantic Ocean. The studied swarms occur along the coast in the city of São Sebastião (NE of São Paulo State), and crosscut Archean and Proterozoic poilimetamorphosed rocks of the Costeiro Complex. The dikes are basaltic and alkaline (lamprophyric) in composition, and they crop out side by side in the beaches. They range from a few centimeters up to 2 m wide. Their trend is predominately N40-50E with vertical dips. Magnetic studies were performed in 32 dikes with both basaltic and alkaline compositions. Magnetic fabrics were determined by applying both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). Rock magnetism properties indicate that pseudo-single-domain grains of almost pure magnetite carry the magnetic fabrics. In addition, paleomagnetic directions are also being determined. The analysis at the individual-dike scale defines three fabrics for the dikes according to the eingenvector orientations with respect to the dike plane, called normal, intermediate and inverse fabrics. Normal AMS fabric acquired during magma flow is dominant, and its Kmax-Kint plane is parallel to the dike plane while the magnetic foliation pole (Kmin) is perpendicular to it. The analysis of the Kmax inclination permitted to infer that the dikes were fed by horizontal (Kmax < 30°), inclined (30° < Kmax< 60°), and vertical (Kmax > 60°) flows independent of their ages and composition. Intermediate and inverse AMS fabrics occur in a few dikes, and no direct relation with magnetic mineralogy and chemical composition of the dikes and the fabric type was observed.
DE: 1518 Magnetic fabrics and anisotropy
DE: 1540 Rock and mineral magnetism
DE: 8178 Tectonics and magmatism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting