Balanço de massa de granitos

Revista Brasileira de Geociências

João Henrique Larizzatti & Sonia Maria Barros de Oliveira

35(2):273-284, junho de 2005

EVOLUÇÃO GEOQUÍMICA E BALANÇO DE MASSA NA FORMAÇÃO E
DEGRADAÇÃO DE PERFIS LATERÍTICOS ENCOURAÇADOS NA ÁREA DA
FAZENDA PISON, VALE DO RIO TAPAJÓS, AMAZÔNIA CENTRAL
JOÃO HENRIQUE LARIZZATTI 1& SONIA MARIA BARROS DE OLIVEIRA 2
Abstract GEOCHEMICAL EVOLUTION AND MASS BALANCE IN THE FORMATION AND DEGRADATION OF DURICRUST
LATERITIC PROFILES IN THE FAZENDA PISON AREA, TAPAJÓS RIVER VALLEY, CENTRAL AMAZON
In the Tapajós
Province, situated in the south-central part of the Amazon craton, the landscape consists of an undulating relief which resulted from the incision of the Velhas Surface. During the Velhas geomorphological cycle (Neogene), under contrasted tropical climate, an iron duricrust lateritic mantle developed upon Proterozoic igneous rocks. Subsequent more humid climatic conditions led to the degradation of the duricrust into latosols containing dispersed nodules and pisolites. In a totally preserved profile (from 20 to 30 m deep), five main horizons can be distinguished, namely, from base to top: (1) a saprolitic horizon, (2) a mottled clay horizon, (3) a discontinuous ferruginous duricrust, (4) a red latosol horizon, and (5) a yellow latosol horizon. Factor analysis carried out on chemical data related to 26 samples and 24 variables reveals 3 groups of elements with distinct behavior. The first group, Si-Ca-Na-Mg-K-Mn-Ba-Cu-ZnRb, is characteristic of the saprolite, and represents the residual primary minerals and the manganesiferous concretions which are common at the base of the profile. The second, Fe-V-Cr-S-Ga, corresponds to the geochemical signature of the iron duricrust, and the third, Al-Ce-La-Nb-Zr-Sr-Ti-P-Pb, is related to kaolinite, and marks either the mottled clay layer and the latosols. Mass balance has been calculated assuming the immobility of Ti. A volumetric reduction of approximately - 50% was found in