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Master's Dissertation
DOI
10.11606/D.44.1984.tde-12082015-143655
Document
Author
Full name
Ana Maria Dreher
Institute/School/College
Knowledge Area
Date of Defense
Published
São Paulo, 1984
Supervisor
Committee
Girardi, Vicente Antonio Vitorio (President)
Oliveira, Marcos Aurelio Farias de
Qualifik, Paul
Title in Portuguese
Petrologia dos rodingitos da mina de Cana Brava, Goiás
Keywords in Portuguese
Petrologia
Abstract in Portuguese
Corpos de rodingitos ocorrem em meio a serpentinitos mineralizados a amianto da mina de Cana Brava, situada na extremidade sudeste do complexo de Cana BRava, Goiás. São derivados da alteração metassomática de metaleucogabros que constituem provavelmente camadas de diferenciação dentro da unidade ultramáfica do maciço. Os rodingitos são constituídos essencialmente por granadas da série grossulária-andradita, vesuvianitas zonadas e clinopiroxênios bastante cálcicos. Clorita, zoisita e apatita são minerais subordinados. Pectolita e xenotlita desenvolvem-se em veios. Os metaleucogabros residuais encontram-se albitizados. Os serpentinitos associados aos rodingitos são derivados de harzburgitos, e compostos por antigorita e lizardita, contendo veios tardios de crisotilo; apresentam-se cloritizados no contato com os rodingitos. a unidade ultramáfica contém ainda metapiroxenitos parcialmente serpentinizados e rochas talco-carbonáticas. Quimicamente os rodingitos de Cana Brava são comparáveis a outros descritos na literatura, embora ligeiramente mais cálcicos. Com relação aos leocogabros originais, mostram-se enriquecidos em Ca, e mais pobres em Si e Al, sendo sendo particamente isentos de álcalis. Uma relação genética entre a serpentinização e a formação dos rodingitos foi estabelecida. Fluidos ricos em Ca liberado pela alteração das ultramáficas migraram até os corpos básicos, possivelmente por força de um gradiente de pressão, provocando reações e a substituição das fases originais por uma assembléia calco-silicática. Os álcalis deslocaram-se para além da zona de reação reoingítica, causando a albitização dos leucogabros. O Al sofreu difusão para os serpentinitos, originando as zonas cloríticas. Os processos de rodingitização e serpentinização ocorreram durante a fase inicial de um episódio metamórfico regressivo que afetou as rochas marginais e encaixantes do complexo de Cana Brava. Temperaturas de 400-500 ) 'GRAUS'C e pressões ('P IND. 'H IND. 2'O''APROXIMADAMENTE IGUAL A'Pt) baixas, inferiores a 5 Kb, foram estimadas para esta fase. Num segundo evento de serpentinização, formaram-se veios de amianto nos serpentinos e pectolita e xenotlita nos rodingitos. Falhas, fraturas e deformações acompanham esta fase, para a qual temperaturas da ordem de 200-300 'GRAUS'C foram admitidas. Rochas talco-carbonáticas e veios de carbonato que cortam ultramafitos e rodingitos desenvolveram-se num último estágio, durante o qual C'O IND. 2' teve acesso às rochas através de falhas e fraturas. Os rodingitos de Cana Brava diferem da maior parte dos rodingitos descritos na literatura por não se encontrarem associados a um complexo do tipo ofiolítico, e por terem-se formado a temperaturas mais elevadas, associados a serpentinos portadores de antigorita.
Title in English
Not available.
Keywords in English
Not available.
Abstract in English
Rodingites have been found in association with asbestos-bearing serpentinites of the Cana Brava mine, which is located in the southeastern edge of the Cana Brava Complex, Goiás. The rodingites were formed by the metasomatic alteration of leucocratic metagabbros which probably constitute differentiated layers of the ultramafic unit of the complex. The rodingites are essentially made up of garnets of the grossular-andradites series, zoned vesuvianites and strongly calcic pyroxenes. Additional minerals include chlorite, zoisite and apatite. Pectolite and xonotlite occur in associated veins. The residual leucocratic metagabbros are albitized. The serpentinites associated with the rodingites are derived from harzburgites. They are composed by antigorite and lizardite as well as chrysotile in late formed veins, and are chloritized along the contact with the rodingites. The ultramafic unit also contains partially serpentinized metapyroxenites as well as talc-carbonate rocks. The rodingites of Cana Brava are chemically comparable to but slightly more calcic than similar rocks described in the literature. With respect to the original leucogabbros, they are enriched in Ca, depleted in Si and Al, and almost devoid of alkalis. A genetic relationship between serpentinization and generation of the rodingites has been established. Ca-rich fluids released by the alteration of the ultramafics migrated towards the basic layers, possibly in response to a pressure gradient, causing reactions and the replacement of the original minerals by a calc-silicate assemblage. The alkalis moved to a distance from the rodingitic reaction zone and provoked the albitization of the leucogabbros. Al was diffused towards the serpentinites and produced the chloritic borders. The processes of rodingitization and serpentinization took place during the initial stage of a retrogressive metamorphic episode which affected the margins and country rocks of the Cana Brava Complex. Temperatures of 400-500°C and pressures ('P IND. 'H IND. 2'O''APROXIMADAMENTE IGUAL A'Pt) below 5 kb were estimated for this phase. In a second serpentinization event, crysotile asbestos veins, and pectolite and xonotlite were formed in the serpentinites and rodingites, respectively. Folding, fracturing, and deformation occurred during this phase, for which temperatures around 200-300°C were assumed. Talc-carbonate rocks, along with carbonate veins that cut the ultramafics and rodingites, were formed at a later stage during which C'O IND. 2' - bearing fluids percolated the rocks through faults and fractures. The Cana Brava rodingites differ from the bulk of similar rocks reported in the literature in the sense that they are not related to an ophiolitic-type complex, and that they were formed at somewhat higher temperatures, in association with antigorite-bearing serpentinites.
 
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2015-08-17
 
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