Navegando por Orientadores "RONCAL, Juan Rolando Zuleta"

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Contribuição à petrologia do granito central da serra dos Carajás
(Universidade Federal do Pará, 1980-09-02) ALMEIDA, Regina Célia Cunha; RONCAL, Juan Rolando Zuleta
This study was carried out in the central region of the Serra dos Carajás in the South of the State of Pará. The area referred to is underlain by a granite batholith, surrounded by basic volcanic rocks in the north-east and slightly metamorphised sedimentary clastic rocks. In order to characterize the petrogenetic nature of the granitic body, a study was realized of the petrography and petrochemistry of the various facies of the granite and of the country rocks. Petrographic and geochemical evidence suggests a magmatic origin for the granite of Carajás. During its consolidation the granitic magma underwent a slight differentiation in the formation of various facies. The intrusive character of the body is e4denced by the presence of contact metamorphism of the hornblende hornfels in the country rocks in the proximity of the contacts. The observed mineralogical association (orthoclase and plagioclase) indicates that the granitic body may be included in the SUBSOLVUS group in the classification of the Tuttle and Bowen (1958). The magma was probably originated by partial melting of older crustal rocks.
Acesso aberto (Open Access)
Dispersão geoquímica dos elementos Si, Al, Fe, Mn, Na, K, Cu e Zn nos solos e sua aplicação na caracterização de áreas geoquimicamente homogêneas
(Universidade Federal do Pará, 1980-05-13) SILVA, Waldise Rossycléa Lima da; RONCAL, Juan Rolando Zuleta
Variations in the chemical composition of soils are used to characterize sub-areas geochemitally homogenous. The aplication of this methodology in a tropical humid region of accentuated topography constitute the principal objective of the present research. Samples of red latosols (Horizon B) developed over gravite, sandstone and basalt occurring in the Central Granite Region of the Serra dos Carajás, Pará State, Brasil were analized for the elements Si, AL., Fe, 40Mn, Na, K, Cu e Zn, by atomic absorption spectrophotometry. Based on the criterion of similarity in the chemical composition (Cluster Analysis, Factor Analysis) the soils were separeted in to different groups. The geographical distribution of the different groups permit the establishment of a Glose relationship between the different parent lithologies and their corresponding soils.
Acesso aberto (Open Access)
Distribuição dos elementos Si, Al, Fe, Ca, Mg, Mn, Ti, Cu, K e Na em solos desenvolvidos na região do granito central da Serra dos Carajás - sul do estado do Pará
(Universidade Federal do Pará, 1981-03-13) DAMOUS, Nina Rosa Leal; RONCAL, Juan Rolando Zuleta
The distribution of elements in the weathered pro rocks is effected mainly by climate, geomorphology and parent material. The present study deals with the distribution of Si, Al., Fe, Ca, Mg, Mn, Ti, Cu, K and Na in the weathered products of the Central Gravite at Serra dos Carajas - Para, Brazil, which occurs in an area which is characterized by ondulated relief and humid tropical climate. Samples representing two selected weathering profiles were collected to depths that did not exceed 6 m. The analytical treatment consisted of poten/biometric determinations of pH values, the determination of the chemical and mineralogical compositions of the sand, silt and clay fractions, and the determination of the chemical composition of the oxide fraction. The two profiles reveal an intensa process of feldspar removal from the parent granite and the formation of a sand-clayey lato-soil. In this process quartz has been preserved and clay minerals and aluminium and iron hidroxides have been formed constituting the dominant assemblage of the soil. Ca, Mg, Mn, Na and K from the parental rock were nearly completely leached away to the extent that these elements are only found in small amounts in the remaining feldspars or adsorbed on clay minerals. On the other hand, Ti present principally in resistant minerals, practically maintains its' original concentration. Cu is sligthly enriched in the profiles due to adsorption in the clay minerals. The presence of only very small amounts of feldspar along the profiles shows the great intensity of the weathering process acti.ng on the granitic rocks. The direct transformation of feldspar to clay minerals or aluminium oxides is related to the climatic conditions frainy and dry seasons) prevalent in the area.
Acesso aberto (Open Access)
Intemperismo químico de rochas graníticas na zona bragantina nordeste do Pará
(Universidade Federal do Pará, 1980-05-13) GOULART, Antonio Taranto; RONCAL, Juan Rolando Zuleta
Profiles of residual soils derived from granitic rocks which occur in the Bragantina Region, Northeastern Pará, were studied chemically and mineralogically. These soils were developed in an ares of gentle topography and good drainage under tropical conditions with well defined "dry" and rainy seasons. The mineral composition of these soils consists essentially of quartz and with decreasing abundantes also appear kaolinite, muscovite, secondary oxides, feldspar and heavy minerals. This mineral assemblage resulted from the chemical weathering of the parent granitic rocks wh.ose original minerals were decomposed in the following sequence: biotite, feldspars, muscovite, quartz and heavy minerals. The soils are dominantly sandy from which significant amounts of kaolinite, produced during the earlier stages of weathering, were lost in consequente of the intense leaching that is favored by the high pluvial precipitation oves the region. The great stability of quartz under tropical conditions together with the intense leaching of the regolith has led to the development of texturally homogeneous soils in the Bragantina Region.