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Master's Dissertation
DOI
10.11606/D.44.1990.tde-26082015-140109
Document
Author
Full name
Hendrik Herman Ens
Institute/School/College
Knowledge Area
Date of Defense
Published
São Paulo, 1990
Supervisor
Committee
Coutinho, Jose Moacyr Vianna (President)
Candia, Maria Angela Fornoni
Oliveira, Marcos Aurelio Farias de
Title in Portuguese
Petrogênese dos escarnitos de Itaoca - Vale do Ribeira - SP
Keywords in Portuguese
Petrologia
Abstract in Portuguese
Os escarnitos estudados ocorrem no interior e bordas de enclaves e possíveis tetos pendentes metassedimentares que afloram na porção central do Maciço Granítico de Itaoca. Pela ação térmica e emissão de fluidos aquosos do corpo magmático as rochas metassedimentares dos enclaves, assim como as encaixantes, originaram hornfels diversos, mármores e escarnitos. Foram identificados na área dois tipos distintos de escarnitos, denominados respectivamente de granada-salita escarnitos e granada-wollastonita escarnitos. Os granada-salita escarnitos ocorrem em zonas métricas ao longo dos contatos entre granitóides e mármores, constituindo-se essencialmente de clinopiroxênios (salita-ferrosalita) com bandas e veios granatíferos (grossulária-andradita). Fases minerais tardias (epídoto, albita, calcita) ocorrem substituindo principalmente as granadas. Os granada-wollastonita escarnitos desenvolveram-se como espessos pacotes entre os mármores e hornfels pelíticos: constituem-se de wollastonita, grossulária e algum diopsídio. Localmente as granadas apresentam-se bastante albitizadas e vesuvianitizadas. O metamorfismo de contato atingiu temperaturas superiores a 650°C e pressão em torno de 2 Kbar. Formaram-se inicialmente, por bimetassomatismo (difusão), ainda a temperaturas elevadas, da ordem de 600°C, em condições redutoras, salita/ferrosalita escarnitos. Com o progressivo abaixamento da temperatura os fluidos infiltrantes tornaram-se oxidantes e enriquecidos em alumina e sílica, promovendo a oxidação dos piroxênios e formação de bandas granatíferas. Nesta fase, a temperatura entre 450 e 550°C, formaram-se os granada-wollastonita escarnitos, em ambiente distal, por ação dos fluidos já empobrecidos em ferro. Durante a fase retrógrada formaram-se albita, epídoto e vesuvianita pela destruição da granada e ocorreu a deposição de sulfetos. Por último, na fase hidrotermal final, os fluidos tornaram se enriquecidos em CO2, propiciando a carbonatação dos minerais formados anteriormente e a formação de veios e vênulas de apofilita e prehnita.
Title in English
Not available.
Keywords in English
Not available.
Abstract in English
The skarns under consideration can be found inside or at the rims of meta-sedimentary inclusions and, conceivably, roof pendants, which outcrop in the central part of the Itaoca granite pluton. Such skarns, together with a variety of hornfels and marbles were formed by thermal metamorphism and outward diffusion of aqueous fluids from the magmatic body. Garnet-salite and garnet-wollastonite are the two main assemblages making up Itaoca skarns. Garnet-salite skarns occur in zones of metric thickness along the granitoid-marble contact and consist of essential clinopiroxene (salite-ferrosalite) and garnet-rich (grossular-andradite) bands and veins. Late mineral phases (epidote, albite, calcite) may replace earlier ones (garnet mainly).Garnet-wollastonite skarn occur as thick bodies between marble and politic hornfels and consist of essential wollastonite and grossular with some diopside. Garnet will, locally, exibit a high degree of replacement by albite and vesuvianite. Contact metamorphism reached its peak at temperatures above 650°C and pressures around 2 Kbar. Salite/ferrosalite skarns were formed by bidirectional metasomatism at the still elevated temperatures of about 600°C under reducing conditions. In the progressively cooling environment the infiltrating fluids became more and more oxidizing and enriched in alumina and silica, a fact which resulted in the oxidation of pyroxenes and formation of garnet bands. At this stage and temperatures between 450 and 550°C, distal garnet-wollastonite skarns were formed by metasomatic processes linked to iron depleted fluids. In a retrograde stage, garnet was destroyed and replaced by albite, epidote and vesuvianite, sulphides being deposited. Finally, during the last hydrothermal stage, CO2- rich fluids furthered the carbonation of early minerals and the formation of apophyllite-prehnite venules.
 
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