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

Abstract

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-Zn-Rb, 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 the mottled clay layer and in the overlying horizons, compared to the saprolite. Besides, a strong depletion of Ca, Na, K and Mg (-80 to -100 %), and a moderate depletion of Si (-65 to -80 %) and Al (-45 to -65 %) were observed along the profile. Iron is moderately enriched in the mottled clay horizon (+50 to +60 %), and strongly enriched in the ferricrete (+330 to +700 %), while in the latosols it is slightly depleted (-20 to -38 %). The absolute accumulation of iron in the ferricrete is likely due to the geochemical vertical transport of this element from the upper horizons of a previous lateritic mantle which underwent aggressive weathering. During the geochemical degradation of the ferricrete into red and yellow latosols important amounts of Fe have been leached out from the profile, and Si and Al have been concentrated. However, the latosols derive not only from the degradation of the ferricrete. Part of their silica content must be due to the upward transfer of material from the mottled clay horizon, by termite activity.