Aspectos mineral?gicos, micromorfol?gicos e texturais da pedog?nese no sul do Complexo Ba??o, Quadril?tero Ferr?fero, MG.

Programa de P?s-Gradua??o em Evolu??o Crustal e Recursos Naturais. Departamento de Geologia, Escola de Minas, Universidade Federal de Ouro Preto.Alguns aspectos dos processos de altera??o superficial e seus reflexos na pedogeomorfologia Quatern?ria na sub-bacia do alto Ribeir?o Maracuj?, no Complexo Ba??o, Quadril?tero Ferr?fero, MG, s?o aqui caracterizados. S?o investigados caracteres relativos ? din?mica evolutiva de tr?s vertentes (topossequ?ncias 1, 2 e 3), situadas sobre gn?isses, mas com graus de erodibilidade diferenciados. Nos segmentos de alta vertente das topossequ?ncias 1 e 2, os perfis de solos s?o pouco desenvolvidos e aut?ctones. Na topossequ?ncia 3, no mesmo segmento, ocorre um latossolo com fei??es de aloctonia. Nos segmentos de meia vertente das tr?s topossequ?ncias ocorrem perfis latoss?licos espessos, desenvolvidos ? partir de sedimentos coluviais provindos da alta vertente. Nos segmentos de baixa vertente h? ruptura nos processos de transporte e deposi??o dos sedimentos, sendo o perfil de solo da topossequ?ncia 2 um Latossolo-c?mbico aut?ctone e, nas topossequ?ncias 1 e 3, perfis desenvolvidos ? partir de materiais al?vio-coluviais depositados sobre o saprolito gn?issico. Apesar da din?mica evolutiva das tr?s vertentes corroborar os modelos geomorfol?gicos tropicais at? o segmento de meia vertente, ? jusante, a ruptura do coluvionamento na baixa vertente, evidencia um recente desequil?brio morfodin?mico. Al?m disso, as evid?ncias micromorfol?gicas e mineral?gicas n?o sinalizam correla??o com susceptibilidade erosiva diferenciada verificada na ?rea. Varia??es na porosidade de solos em diferentes est?gios de intemperismo s?o tamb?m investigadas no presente trabalho. O material analisado em laborat?rio nesta fase consistiu em agregados in natura com cerca de 5 mm de di?metro, submetidos ?s an?lises pelos m?todos de sor??o de nitrog?nio a 77 K (N2 BET) e porosimetria por intrus?o de merc?rio. A combina??o dessas t?cnicas porosim?tricas mostrou que as amostras oriundas dos horizontes de altera??o mais desenvolvidos (latossolos) cujos constituintes materiais s?o de natureza al?ctone al?m de deterem uma maior ?rea de superf?cie apresentam maiores volumes adsorvidos e de intrus?o, refletindo uma maior porosidade, e, uma ampla distribui??o de classes porais na faixa de microporos, mesoporos e macroporos. As amostras de perfis menos desenvolvidos (cambissolos) mostram uma menor ?rea poral, onde predominam os mesoporos de natureza textural. A terceira e ?ltima fase do trabalho consistiu na investiga??o mineral?gica dos ?xidos de ferro de horizontes B. Foram utilizadas tr?s amostras, coletadas em tr?s distintos perfis, nos segmentos de alta, meia e baixa vertente na topossequ?ncia 2 (VH). No laborat?rio, assim como na primeira fase, foram realizadas an?lises f?sica (granulometria), qu?mica (composi??o dos elementos maiores por fluoresc?ncia de raios X - FRX) e mineral?gica (difratometria de raios X - DRX e espectroscopia M?ssbauer). Com base nos resultados anal?ticos de FRX e DRX, as amostras revelaram uma composi??o mineral?gica similar semiquantitativa, constitu?da pela sequ?ncia quartzo >> gibbsita > caulinita > feldspatos > goethita. Os resultados M?ssbauer a 4,2 K confirmaram somente a coexist?ncia de goethita (majorit?ria) e hematita. Com base no conte?do de alum?nio isomorficamente substituinte estimado a partir dos campos hiperfinos, foram alocadas as f?rmulas qu?micas para as goethitas: ?Fe0,79Al0,21OOH (alta vertente); ?Fe0,75Al0,25OOH (meia vertente) e ?Fe0,78Al0,22OOH (baixa vertente). Essas goethitas cont?m n?veis distintos de substitui??o isom?rfica por alum?nio. H? evid?ncias de que a din?mica dos ?xidos de ferro nos horizontes B dos respectivos segmentos de vertente sirva de testemunhos de paleoflutua??es clim?ticas na ?rea: goethitas mais aluminosas indicariam um ambiente ?mido durante seu processo de forma??o, enquanto que as menos aluminosas indicariam um ambiente mais seco. Portanto, o presente trabalho procurou caracterizar a evolu??o pedogen?tica local, correlacionando-a com as transforma??es ambientais naturais ocorridas provavelmente no Neopleistoceno.This work defines some aspects of the processes of superficial alteration and its reflexes in the Quaternary pedogeomorphology in the sub-basin of upper Maracuj? River, in the Complexo Ba??o, Quadril?tero Ferr?fero, MG. Characters related to the dynamic evolution of three toposequences (1, 2 and 3) developed on gneiss, but with a different grade of erosion, were investigated. In the up slope segments from the 1 and 2 toposequences, the soil profiles are little developed and show an in situ evolution. In the 3 toposequence, the soil profile from the up slope segment is a well developed Latosol with allochthonous features. Similarly, in the mid slope segments from the three toposequences the soil profiles are thick, latosolic and originated from the colluvial sediments transported from the up slope. However, in the down slope segments occurs a rupture of the transport and deposition process, and the soil profiles from the 2 toposequence are autochthonous CambicLatosol, and, those from the 1 and 3 toposequences, were developed from the alluvial - colluvial materials deposited on the gneiss saprolite basement. In spite of the dynamic evolution of the up and mid slope segments from the three toposequences corroborating the tropical geomorphological models, the sequential rupture to the down slope segments suggests a recent morphodynamic imbalance. Besides, the micromorphological and mineralogical features do not show correlation with the differentiated erosive susceptibility verified in the area. Variations in the porosity of soils in different weathering stages are also investigated in the present work. The soil porosity was investigated on agregates "in natura" by using N2 (77 K) BET porosimetry and mercury intrusion porosimetry. The combination of the porosimetric techniques shows that the samples from the well developed soil profiles (Latosols), which are constituted by allocthonous material, have not only highest surface area, but also highest values of adsorved volumes that reflect a highest porosity and a large distribution of pores sizes in the range of micropores, mesopores and macropores. The samples from the poorly developed soil profiles (cambic horizon) show smallest values of adsorved volumes that is related to a lower porosity where the mesopores of textural origin are predominant. The third and last phase of the work consisted of mineralogical investigation of the iron oxides. The earth fine fractions (mean diameter, ? = 2 mm) were separated for all samples. The chemical composition of the major elements was determined with the X-ray fluorescence technique; the mineralogical analysis was performed with powder X-ray diffractrometry and M?ssbauer spectroscopy. All samples have similar mineralogical composition, with a general occurrence corresponding to the sequence quartz >> gibbsite > kaolinite > feldspars > goethite. From the 4.2 KM?ssbauer results, it is confirmed the coexistence of goethite (major) and hematite. The isomorphical aluminum contents as they were deduced from the hyperfine fields led to the following chemical formulas for goethites: ?Fe0,79Al0,21OOH (upslope), ?Fe0,75Al0,25OOH (midslope) and ?Fe0,78Al0,22OOH (downslope). Some evidences indicate that the iron oxides transformation dynamics in B horizons along the slope is an useful indicator of the paleo-climatic oscillations in this area: aluminous goethites is an indicator of humid paleo-environments, whereas aluminous-poorer goethite reveals drier pedogenic conditions. Therefore, the present work tried to characterize the pedogenetic evolution these soils, correlating with the natural environmental transformations probably happened in Neopleistocene