Intemperismo na região da Serra do Tucano-RR

Abstract

The objective of this dissertation was to investigate the genesis and evolution of the lateritic lithotypes, resulting from the weathering in savannah areas in the region of Tucano ridge. The structural and textual characteristics were studied and the mineralogy, the chemical composition of the chemical elements Al, Ca, Si, Fe, K, Mg, Na, P, Ti, PF, trace elements (As, Au, B, Ba, Be, Bi, Cd, Co, Cr, Cu, Cs, Ga, Ge, Hf, Hg, Li, Mn, Mo, Nb, Ni, Pb, Pd, Rb, Re, Sb, S, Se,Sc, Sn, Sr, Ta, Te, Th, Tl, U, V, W, Y, Zn, Zr and ETR) and Pb isotopes was analyzed. In the region there are from thin, thick sandstones to rusty conglomeratic (Fe oxidation), vermiform crusts, protopisolitic, pisolitic, loosed concretionary material and clay and sandy soils consisting primarily of quartz, goethite-hematite, kaolinite, anatase, rutile and gibbsite. In the southwestern (SW) of Tucano ridge there is Redondo Hill constituted of diabase. The Si is high in the diabase, in the sandstones and in the soils while Fe is higher in the crusts and in the concretionary whereas Al and Ti are high in the soils. The alkalis and P are below the limit of the lateritic lithotypes detection and they are higher in the diabase. The Ba, Co, Cr, Cu, Ni, Rb, Sc, Sr, W and Zn are more concentrated in the diabase. The Ag, As, Au, Bi, Ga, Hg, Sb, Se, Sn, Pb, Tl, Th and V are in the crusts and concretionary. The Hf, Nb and Zr are in the soils and the Mn in the conglomeratic and thick sandstones, in the pisolitic crust, in the concretionary and in the clay soils. The REE are more concentrated in the conglomeratic sandstone, in the pisolitic crust, in the concretionary 2 and in the soils where there are positive anomalies of Ce/Ce* and soft and negative ones of Eu. In the isotopic studies, higher rations of 207Pb/206Pb in the sandstones conglomeratic and clay soils and the rations of 208Pb/206Pb are higher especially in the diabase, in the thick sandstones to rusty conglomeratic, in the concretionary and in the clay soils. No estudo isotópico as razões do 207Pb/206Pb são elevadas no arenito conglomerático e solos argilosos e as razões do 208Pb/206Pb são mais altas no diabásio, arenitos conglomerático e grosso, no concrecionário e nos solo argiloso. The geochemistry allowed three associations that define the lithotypes signatures: 1) Fe and PF representing the crusts; 2) P, Ba, Co, Mn, V and Zn represent the diabase; and the 3) Al, Si, Ti, Cs, Hf, Nb, Ta, Y, Zr and REE represent the soils. The lateritic crusts found in the region of Tucano ridge possibly had their development after the Miocene, with the individualization of the vermiform facies, protopisolitic and pisolitic towards the quaternary in similar conditions to the present climate.