HR: 0800h
AN: GP31A-02    [Abstracts]
TI: Gravity and Magnetic 3-D inversion of Morro do Engenho Alkaline Intrusion, Central Brazil
AU: * Marangoni, Y R
EM: yara@iag.usp.br
AF: Universidade de Sao Paulo, IAG-USP Rua do Matao 1226 Cidade Universitaria, Sao Paulo, SP 05508-090, Brazil
AU: Dutra, A C
EM: alanna@iag.usp.br
AF: Universidade de Sao Paulo, IAG-USP Rua do Matao 1226 Cidade Universitaria, Sao Paulo, SP 05508-090, Brazil
AB: The Goias Alkaline Province, in Central Brazil, is a suite of alkaline rocks whose composition varies from mafic- ultramafic alkaline complexes in the northern area, to subvolcanic alkaline rocks in the central part and volcanics (kamafugitic systems) in the south. In this Late Cretaceous Province some intrusives outcrop, including the Morro do Engenho Complex (ME), and some can be defined only using the aeromagnetic signal, like A2 anomaly, completely covered by Quaternary sediments. K-Ar radiometric ages are in the interval of 90-80 Ma for the intrusions. Geologic mapping of these two bodies is hampered by extensive alluvial sedimentation and strong weathering of the mafic/ultramafic rocks. This strong weathering is responsible for the Nickel enrichment of the region, increasing economic interest in the alkalines. The bodies are all marked by strong gravity and magnetic anomalies that are very effective in mapping them, so a 3-D inversion of gravity and magnetic data was carried out in order to estimate their subsurface geometry. In the 3D inversion the models are representative of the structures covering the place where ME outcrops. In this inversion, the maximum depth indicated by the density distribution was 10 km for ME and 6 km for A2. The recovered density model indicates a NE-SW preferential direction for the density contrast distribution in subsurface. Since the remanent magnetization is very important in the area, the 3-D inversion was accomplished considering three different possibilities: only induced field, only remanent field, and total field. Magnetic inversion resulted in narrower bodies that are almost 2 km deeper. These results are compatible with 2.5D direct modeling, where the magnetic susceptibility was more centralized along the body axis. It is clear that the adopted procedure of using a-priori information on magnetic remanesce before final inversion was very useful.
DE: 1219 Gravity anomalies and Earth structure (0920, 7205, 7240)
DE: 1295 Integrations of techniques
DE: 1517 Magnetic anomalies: modeling and interpretation
DE: 3642 Intrusive structures and rocks
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Joint Assembly