MODELO GRAVIMÉTRICO DO COMPLEXO ALCALINO DE PARIQUERA-AÇU (SP)

Abstract

A gravity survey was carried out in order to model the 3D geometry and physical characteristics of the Pariquera-Acu alkaline complex emplaced in the Ribeira Valley (SE - Brazil). As a result, gravity pattern, density distribution, 3D geometry and consequent volume and mass of the alkaline complex, were determined. The results have allowed the definition of the present geological interpretation and, in particular, to establish the tectonic nature of the alkaline complex emplacement. To solve the problem, geophysical and numerical methods for determining the Bouguer residual field were used. Mathematical tools as Butterworth filter, least squares and robust polynomial were selected, taking into account the available data, including those from magnetic and radiometric surveys. Qualitative and quantitative analyses were used to establish the polynomial degree for which the gravity regional field would be better represented. The best fit worth a 6th degree robust polynomial, being the difference with the observed gravity field always lower than a mGal. Comparing three models (2.5 D, prismatic 3 D, and irregular 3 D) using the measured geophysical parameters and the boundary conditions inferred from geological information, the 3D irregular shape gravity model is preferred. The best fit 3D model gives as result an average density of 3,14 g/cm 3 for the alkaline complex, its density contrast to the wall rock being 0,46 g/cm 3 , its maximum depth 8,93 km, its surface area 6 km 2 and a volume of 8,26 km 3 . The modeled geometry results in a total mass for the alkaline complex of 0,38 x 10 10 t.