J. J. G. Quéméneur

MICROPEDOLOGIA DA ALTERAÇÃO EM PERFIS DE SOLOS COM B TEXTURAL NA REGIÃO DE LAVRAS, MINAS GERAIS

In the region of Lavras, Minas Gerais, Brazil, soils profiles with argillic horizons were selected to evaluate the alterations which occurred during the process of soil formation and to investigate pedogenetic characteristics related to the parent material. Within this region, these soils occur on sloping and hilly landscapes. They were selected for this study because they preserve characteristics inherited from the original bedrocks. Three profiles formed from geologic material with different chemical composition were selected: Profile 1 - Red-Yellow Argisol developed from granitic rocks (Granito de Itutinga); Profile 2 - Red Argisol formed from dioritic rocks (Diorito do Rosario); and Profile 3 - Argiluvic Chernosol formed from basic rocks of the Greenstone-Belt of Lavras. Throughout the layers of each profile, from the consolidated bedrock to the solum, studies were carried out (petrographic analyses on the consolidated and weathered bedrock, micromorphological analyses of C, BC horizons and solum) to investigate the processes of soil formation by evaluating weathering reactions and textural features, with the aid of X-ray diffraction analyses to characterize clay mineralogy. Based on the results obtained, the relationships between parent material and pedogenetic processes were assessed and the order of destruction of primary minerals established. The evolution of the alteration processes along the profiles was found to be directly linked to the texture, chemical and mineralogical composition of the parent materials, revealed in the micromorphological characteristics and mineralogical composition of the horizons constituents of each profile, particularly in the argillic horizon.

Transformações mineralógicas ao longo de perfis de alteração pedogenética na região de Lavras (MG)

Na regiao de Lavras (MG), realizou-se a avaliacao das transformacoes mineralogicas ao longo da alteracao pedogenetica em perfis de solos, discriminados pela composicao geoquimica acida, intermediaria e basica do material de origem. Os solos com horizonte B textural foram selecionados para tal estudo, sendo escolhidas as seguintes classes destes solos: Perfil 1 - Argissolo Vermelho-Amarelo distrofico (PVAd), originado por alteracao de rochas do Dominio geologico Granito de Itutinga; Perfil 2 - Argissolo Vermelho eutrofico (PVe), desenvolvido a partir de rochas do Dominio geologico Diorito do Rosario, e Perfil 3 - Chernossolo Argiluvico ferrico (MTf), evoluido a partir de rochas do Dominio geologico Greenstone-belt de Lavras. Ao longo de cada perfil, desde a rocha fresca ate o solum, realizaram-se analises microscopicas para avaliar os processos de alteracao, mediante evolucao das reacoes de transformacao mineralogica, com auxilio da difracao de raios-X na identificacao da mineralogia da fracao argila. O estudo permitiu elaborar a ordem de destruicao intemperica dos minerais primarios e consequente formacao da mineralogia do solo, estabelecendo-se reacoes de transformacoes mineralogicas e caracterizacao dos processos atuantes ao longo da evolucao da alteracao pedogenetica, assim como suas relacoes com o material de origem. As reacoes de transformacao mineralogica esquematizadas indicam que, nos Argissolos estudados, os minerais primarios foram alterados para caulinita, gibbsita e sesquioxidos de ferro, acrescentando-se sesquioxidos de titânio e vermiculita com hidroxi entre camadas no caso do PVe. Persistiram como residuos o quartzo e o feldspato potassico, este ultimo apenas no PVAd. Ja no Chernossolo avaliado, as alteracoes originaram talco, esmectita, caulinita e sesquioxidos de ferro e titânio, nao havendo minerais primarios residuais.

COMPARATIVE STUDY OF TWO PEGMATITIC FIELDS FROM MINAS GERAIS, BRAZIL, USING THE Rb AND Cs CONTENTS OF MICAS AND FELDSPARS

The distribution of alkali elements and F was investigated in minerals of pegmatites and their contact aureole of metasomatism, in two groups of pegmatites from the State of Minas Gerais, southeast Brazil: the Transamazonian-cycle age (1,930 Ma) Volta Grande pegmatites near Sao Joao del Rei and the Brasiliano-cycle age (525 Ma) pegmatites of Itinga. The Volta Grande pegmatites are albite and spodumene bearing and their contact aureole of metasomatism, in the amphibolite host rock, are characterized by the presence of holmquistite and zinnwaldite. The Itinga pegmatites correspond to complex lepidolite and pet alite-bearing pegmatites with tourmaline, zinnwaldite and muscovite in their contact aureole developed in quartz-biotite-muscovite-schists. The K-bearing minerals of the Volta Grande pegmatite are exceptionally rich in Rb. Using the experimentally determined partition coefficients between minerals and fluids, the atomic ratio K/Rb of the pegmatitic fluid may be evaluated. This ratio shows homogenous values, around 4. Similar values are found in the metasomatic aureole, at the contact, which increase as a function of the distance from the contact, due to the crystallization of trioctahedral micas that deplete the fluid in Rb. The K/Rb ratios of the fluid could represent the ratio of the initial pegmatitic magma before differentiation. Rb content in the Itinga pegmatites is much lower and strongly differs from border to core. The presence of K-bearing minerals in the wall rocks contributes for the increasing the K/Rb ratios of the neoformed minerals at the contact zone. It is then more difficult to evaluate the K/Rb ratio of the initial magma. Considering the composition of the minerals at the border of each pegmatite, it varies from 25 at Urubu to 50 at Jose de Linto and Xuxa localities. The Itinga pegmatites are clearly poorer in Rb than the Volta Grande pegmatites. Geochemically, a relationship between the Rb and F contents in the micas is clearly demonstrated in each group of pegmatites. The situation is more complex for Cs because crystallization of all minerals concentrates Cs in the fluid. Urubu pegmatite is the only one where pollucite was found in the core. Cs content in K-bearing minerals is higher in the lepidolite of the core and in zinnwaldite of the aureole. Cs2O in the Volta Grande pegmatite can be as high as 2%, at 1 meter from the contact, as metasomatic fluids become enriched in Cs while the metasomatism proceeds. This element could be used as guide for exploration of the alkali pegmatites.

Pedogeoquímica em perfis de alteração na região de Lavras (MG). II - elementos menores e elementos das terras raras

The geochemical behavior of trace elements (TE) and rare earth elements (REE) was evaluated in the region of Lavras (MG), during the pedogenetic evolution in soil profiles with argillic horizons differentiated by acid, intermediate and basic compositions of the parent material.Chemical analyses of TE and REE were performed along the profiles and gains and losses were evaluated through mass chemical balance.The study allowed to group the TE into two classes: (a)moderate to high mobility: loss from the consolidated rocks to the solum; and (b)moderate mobility: relative and occasionally real enrichments in the solum.The REE showed a great mobility throughout the evolution of the alteration profiles.

Mapeamento preliminar de solos com horizonte B textural e B nítico na região de Lavras, MG

Estudos de caracterizacao de solos e estabelecimento de relacoes entre pedologia, geologia e geomorfologia permitem a elaboracao de modelos preditivos da distribuicao dos solos na paisagem. Objetivou-se, com o presente trabalho, estudar estas relacoes, com a finalidade de representar a distribuicao de solos com horizonte B textural e B nitico na paisagem da regiao de Lavras, MG, por intermedio da geracao de mapa preliminar de solos, utilizando tecnicas de geoprocessamento. Os estudos foram realizados em solos com horizonte B textural e B nitico, uma vez que esses solos apresentam caracteristicas relacionadas a variacao da composicao geoquimica das rochas igneas de origem, refletindo-se em varios atributos dos solos, enfatizando a cor e o teor de Fe2O3. Foram realizadas campanhas de campo com avaliacoes morfologicas, fisicas e quimicas de perfis de solos representativos, desenvolvidos a partir de unidades geologicas de composicoes geoquimicas distintas, estabelecendo as relacoes pedogeologicas. Verificou-se, tambem, a distribuicao dos solos na paisagem, com o estabelecimento das relacoes pedogeomorfologicas. Foi constatado que tais solos sao desenvolvidos na regiao em relevo caracterizado por classes de declividade de 12 a 45%, originando diferentes classes de solos com horizonte B textural e B nitico, diretamente relacionados a variacao do material de origem. Os criterios estabelecidos nas relacoes entre pedologia, geologia e geomorfologia permitiram avaliar a distribuicao dos solos na paisagem estudada e a geracao do mapa preliminar de solos, com individualizacao de solos com horizonte B textural e B nitico.

Pedogeoquímica em perfis de alteração na região de Lavras (MG). I- elementos maiores - óxidos constituintes

The geochemical behavior in soil alteration profiles in Lavras (MG), Brazil, was evaluated during the process of soil formation leading to the formation of argillic horizons, in parent material ranging from acid, intermediate to mafic geochemical composition.Along each profile, from the consolidated bedrock to the solum, geochemical analyses of major elements-main oxides were performed, calculating the gains and losses through a mass chemical balance.The results showed a moderate loss of SiO2, bases leaching, and a slight relative enrichment of Al, Fe and Ti.The leaching order of these elements from the alterites was also established.