As crostas lateríticas da região central de Roraima - RR

Abstract

The lateritic crusts, which represent the relict of paleoclimatic changes especially between the Cenozoic and the Recent, allow inferring the conditions and the evolution of weathering in the intertropical region of the Earth. At the edge of the Guianas Shield, in the central region of Roraima, where occur lateritic crusts outcropping, has developed the Graben of the Tacutu. It`s reactivation in the Pliocene caused severe erosion what the region tectonically unstable. With the objective to determine the implications of that environment in the weathering of the basement rocks from the Guiana Shield, the region of Mucajai was selected to where it is situated the Serra da Prata and to a less extent the Serra da Moça. These relief features are inserted in the Piedmont Rio Branco-Rio Negro, and its surroundings lowered portion on the Dissected Plateau North of the Amazonas. Rocks, crusts and soils of the region were studied on basis of field survey of the lithotypes and their textural, mineralogical and geochemical characterization. The major oxides, the trace elements (Mn, Zr, V, Ni, Cu, Zn, Sc, Co, Ga, Cr, Y, Th, Hf, Nb, Ta, Ba, Mo, Sr, Rb, Cs, U, W, As, Au, Ag, Hg, Ge, In, Li, Sb, Se, Te, Tl and ETR) and the Pb isotopes were analyzed. The lateritic lithotypes developed from charnockites, gabbro, granites and gneisses. Crusts of protopisoliticas and pisoliticas types were formed at the top of the Serra da Prata and vermiform in the lower portions of the landscape which are cover by clay-sandy reddish brown soils. The crusts are immature, friables with abundant clay matrix, composed mainly of goethite and hematite, and low content of gibbsite. The rocks are distinguished by SiO2, Na2O, K2O, Ba, Rb, Tl, Y, Mo, REE (La-and Eu) association, while the lateritic rocks by Fe2O3, PF, Co, Cr, Ga, Zn, Mn, W . There is a greater geochemical affinity between the crusts and soils with granite, charnockite and gneiss than with the gabbro, as they are the most abundant rocks and also because they have little chemical variation; however, the Pb isotopic ratios indicate provenance from the charnockite. The development of these crusts marks the Quaternary landscape evolution of the central portion of Roraima after the tectonic reactivation in the Miocene.