The Gawler Craton in South Australia consists of an Archaean to Palaeoproterozoic core surrounded and intruded by a series of Palaeo- to Mesoproterozoic metasediments and igneous suites. The region has experienced a protracted c. 1700 Myr tectonic history from the Archaean through to the Mesoproterozoic, experiencing numerous cycles of deformation, magmatism and basin development. Despite hosting a number of mineral deposits, including the immense Olympic Dam iron oxide-copper-gold deposit, the tectonothermal evolution of the Gawler Craton remains poorly constrained. A significant ambiguity in our current understanding of the geological framework of the Gawler Craton revolves around the timing and spatial distribution of the tectonic events within the craton and their metamorphic evolution. This study addresses some of this ambiguity by unravelling the timing and tectonothermal evolution of the reworked southern Gawler Craton, using a combination of structural and metamorphic analysis, coupled with targeted geochronology. These methods have been applied to three locations representing different lithologies across the southern Gawler Craton. Putting absolute time into structural and metamorphic analysis is a vital tool for unravelling the development of ancient and modern orogenic systems. Electron Probe Micro-Analysis (EPMA) chemical dating of monazite provides a useful method of obtaining good precision age data from monazite bearing assemblages. This technique was developed at the University of Adelaide in order to constrain the timing of reworked assemblages from the southern Gawler Craton. EPMA measurements carried out on samples of known age, from Palaeoproterozoic to Ordovician, produce ages which are within error of the isotopically determined ages, indicating the validity of the developed setup. This technique, together with SHRIMP monazite and titanite and garnet Sm-Nd geochronology, was used on selected samples from the southern Gawler Craton to determine the timing of high-grade metamorphism and deformation. The results show that the Sleaford Complex records evidence of an early D₁event during the c. 2450 Ma Sleaford Orogeny recorded within structural boudins. The majority of the data indicates that the region underwent subsequent reworking and thorough overprinting during the 1725–1690 Ma Kimban Orogeny. In the Coffin Bay region, Palaeoproterozoic peraluminous granites of the Dutton Suite are reworked by a series of migmatitic and mylonitic shear zones during the Kimban Orogeny. Peak metamorphic conditions recorded in mafic assemblages indicate conditions of 10 kbar at 730°C. The post-peak evolution is constrained by partial to complete replacement of garnet – clinopyroxene bearing mafic assemblages by hornblende – plagioclase symplectites, which record conditions of c. 6 kbar at 700°C, implying a steeply decompressional exhumation path. The Shoal Point region consists of a series of reworked granulite-facies metapelitic and metaigneous units which belong to the late Archaean Sleaford Complex. Structural evidence indicates three phases of fabric development with D₁retained within boudins, D₂consisting of a series upright open to isoclinal folds producing an axial planar fabric and D₃, a highly planar vertical high-strain fabric which overprints the D₂ fabric. Geochronology constrains the D₁ event to the c. 2450 Ma Sleafordian Orogeny while the D₂the D₃events are constrained to the 1730–1690 Ma Kimban Orogeny. P-T pseudosections constrain the metamorphic conditions for the Sleafordian Orogeny to between 4.5–6 kbar and 750–780 °C. Subsequent Kimban-aged reworking reached peak metamorphic conditions of 8–9 kbar at 820–850 °C during the D₂ event. Followed by near isothermal decompression to metamorphic conditions <6 kbar and 790–850 °C associated with the development of the D₃high-strain fabric. The Pt Neill and Mine Creek regions are located in the core and on the flank of the crustal scale Kalinjala Shear Zone, which forms the main structural element of the poorly exposed Kimban Orogen. Samples record a similar structural development with a dextrally transpressive system resulting in a layer parallel migmatitic gneissic to mylonitic KS₁ fabric which was subsequently deformed and reworked by upright folds and discrete KD₂ east-side-down sub-solidus mylonitic shear zones during east-west compression. Geochronology constrains the timing of deformation and metamorphism to the Kimban Orogeny between 1720 and 1700 Ma. Metamorphic P-T analysis and pseudosections constrain the peak M₁ conditions in the core of the shear zone to 10–11 kbar at c. 800 °C reflecting lower crustal conditions at depths of up to 30 km. On the flank of the shear zone the M₁ conditions reached 6–7 kbar at 750 °C followed by sub-solidus reworking during KD₂ at conditions of 3–4 kbar at 600–660 °C, suggesting a maximum burial of <24 km. Cooling rates suggest that the core of the shear zone cooled at rates in excess of 40–80 °CMa⁻¹ while the flank underwent much slower cooling at < 10°CMa⁻¹. The rapid cooling and inferred decompression in the core of the shear zone reflects rapid burial and exhumation of lower-crustal material into the mid-crust along the Kalinjala Shear Zone. The absence of evidence for extension indicates that differential exhumation and the extrusion of lower-crustal material into the mid-crust was driven by transpression along the shear zone and highlights the role of transpression in creating large variations in vertical exhumation over relatively short lateral extents. Garnet is a vital mineral for determining constrained P-T-t paths as it can give both the P-T and t information directly. However, estimates of the closure temperature of the Sm-Nd system in garnet vary considerably leading to significant uncertainties in the timing of peak conditions. Five igneous garnets of varying size from an undeformed 2414 ± 6 Ma garnet – cordierite bearing s-type granite from the Coffin Bay region, that were subjected to high-T reworking during the Kimban Orogeny, have been dated to examine their diffusional behaviour in the Sm-Nd system. Garnets were compositionally profiled and then dated. A direct correlation exists between grain size and amount of resetting highlighting the effect of grain size on closure temperature. Major element and REE traverses reveal homogonous major element profiles and relict igneous REE profiles. The retention of REE zoning and homogenisation of major element zoning suggests that diffusion rates of REE’s are considerably slower than that of the major cations, in disagreement with recent experimental determinations of the diffusion rates of REE in garnet. The retention of REE zoning and the lack of resetting in the largest grains suggests that Sm-Nd closure temperature in garnet is a function of grain-size, thermal history and REE zoning in garnet. The findings of this study provide the first temporally constrained tectonothermal model of the evolution of the southern Gawler Craton. The P-T conditions obtained from the earliest D₁ fabric provide the first quantitative constraints on the P-T conditions of the southern Sleafordian Orogeny. The P-T-t evolution determined for the 1725–1690 Ma Kimban Orogeny indicate it developed along a clockwise P-T path, and dominates the structural and metamorphic character of the southern Gawler Craton. The large variations in exhumation over short lateral extents reflect the exhumation of lower crustal rocks during the Kimban Orogeny driven by transpression during the development of a regional transpressional ‘flower structure’. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1372052 / Thesis (Ph.D.) - University of Adelaide, School of Earth and Environmental Sciences, 2009

The Gawler Craton in South Australia consists of an Archaean to Palaeoproterozoic core surrounded and intruded by a series of Palaeo- to Mesoproterozoic metasediments and igneous suites. The region has experienced a protracted c. 1700 Myr tectonic history from the Archaean through to the Mesoproterozoic, experiencing numerous cycles of deformation, magmatism and basin development. Despite hosting a number of mineral deposits, including the immense Olympic Dam iron oxide-copper-gold deposit, the tectonothermal evolution of the Gawler Craton remains poorly constrained. A significant ambiguity in our current understanding of the geological framework of the Gawler Craton revolves around the timing and spatial distribution of the tectonic events within the craton and their metamorphic evolution. This study addresses some of this ambiguity by unravelling the timing and tectonothermal evolution of the reworked southern Gawler Craton, using a combination of structural and metamorphic analysis, coupled with targeted geochronology. These methods have been applied to three locations representing different lithologies across the southern Gawler Craton. Putting absolute time into structural and metamorphic analysis is a vital tool for unravelling the development of ancient and modern orogenic systems. Electron Probe Micro-Analysis (EPMA) chemical dating of monazite provides a useful method of obtaining good precision age data from monazite bearing assemblages. This technique was developed at the University of Adelaide in order to constrain the timing of reworked assemblages from the southern Gawler Craton. EPMA measurements carried out on samples of known age, from Palaeoproterozoic to Ordovician, produce ages which are within error of the isotopically determined ages, indicating the validity of the developed setup. This technique, together with SHRIMP monazite and titanite and garnet Sm-Nd geochronology, was used on selected samples from the southern Gawler Craton to determine the timing of high-grade metamorphism and deformation. The results show that the Sleaford Complex records evidence of an early D₁event during the c. 2450 Ma Sleaford Orogeny recorded within structural boudins. The majority of the data indicates that the region underwent subsequent reworking and thorough overprinting during the 1725–1690 Ma Kimban Orogeny. In the Coffin Bay region, Palaeoproterozoic peraluminous granites of the Dutton Suite are reworked by a series of migmatitic and mylonitic shear zones during the Kimban Orogeny. Peak metamorphic conditions recorded in mafic assemblages indicate conditions of 10 kbar at 730°C. The post-peak evolution is constrained by partial to complete replacement of garnet – clinopyroxene bearing mafic assemblages by hornblende – plagioclase symplectites, which record conditions of c. 6 kbar at 700°C, implying a steeply decompressional exhumation path. The Shoal Point region consists of a series of reworked granulite-facies metapelitic and metaigneous units which belong to the late Archaean Sleaford Complex. Structural evidence indicates three phases of fabric development with D₁retained within boudins, D₂consisting of a series upright open to isoclinal folds producing an axial planar fabric and D₃, a highly planar vertical high-strain fabric which overprints the D₂ fabric. Geochronology constrains the D₁ event to the c. 2450 Ma Sleafordian Orogeny while the D₂the D₃events are constrained to the 1730–1690 Ma Kimban Orogeny. P-T pseudosections constrain the metamorphic conditions for the Sleafordian Orogeny to between 4.5–6 kbar and 750–780 °C. Subsequent Kimban-aged reworking reached peak metamorphic conditions of 8–9 kbar at 820–850 °C during the D₂ event. Followed by near isothermal decompression to metamorphic conditions <6 kbar and 790–850 °C associated with the development of the D₃high-strain fabric. The Pt Neill and Mine Creek regions are located in the core and on the flank of the crustal scale Kalinjala Shear Zone, which forms the main structural element of the poorly exposed Kimban Orogen. Samples record a similar structural development with a dextrally transpressive system resulting in a layer parallel migmatitic gneissic to mylonitic KS₁ fabric which was subsequently deformed and reworked by upright folds and discrete KD₂ east-side-down sub-solidus mylonitic shear zones during east-west compression. Geochronology constrains the timing of deformation and metamorphism to the Kimban Orogeny between 1720 and 1700 Ma. Metamorphic P-T analysis and pseudosections constrain the peak M₁ conditions in the core of the shear zone to 10–11 kbar at c. 800 °C reflecting lower crustal conditions at depths of up to 30 km. On the flank of the shear zone the M₁ conditions reached 6–7 kbar at 750 °C followed by sub-solidus reworking during KD₂ at conditions of 3–4 kbar at 600–660 °C, suggesting a maximum burial of <24 km. Cooling rates suggest that the core of the shear zone cooled at rates in excess of 40–80 °CMa⁻¹ while the flank underwent much slower cooling at < 10°CMa⁻¹. The rapid cooling and inferred decompression in the core of the shear zone reflects rapid burial and exhumation of lower-crustal material into the mid-crust along the Kalinjala Shear Zone. The absence of evidence for extension indicates that differential exhumation and the extrusion of lower-crustal material into the mid-crust was driven by transpression along the shear zone and highlights the role of transpression in creating large variations in vertical exhumation over relatively short lateral extents. Garnet is a vital mineral for determining constrained P-T-t paths as it can give both the P-T and t information directly. However, estimates of the closure temperature of the Sm-Nd system in garnet vary considerably leading to significant uncertainties in the timing of peak conditions. Five igneous garnets of varying size from an undeformed 2414 ± 6 Ma garnet – cordierite bearing s-type granite from the Coffin Bay region, that were subjected to high-T reworking during the Kimban Orogeny, have been dated to examine their diffusional behaviour in the Sm-Nd system. Garnets were compositionally profiled and then dated. A direct correlation exists between grain size and amount of resetting highlighting the effect of grain size on closure temperature. Major element and REE traverses reveal homogonous major element profiles and relict igneous REE profiles. The retention of REE zoning and homogenisation of major element zoning suggests that diffusion rates of REE’s are considerably slower than that of the major cations, in disagreement with recent experimental determinations of the diffusion rates of REE in garnet. The retention of REE zoning and the lack of resetting in the largest grains suggests that Sm-Nd closure temperature in garnet is a function of grain-size, thermal history and REE zoning in garnet. The findings of this study provide the first temporally constrained tectonothermal model of the evolution of the southern Gawler Craton. The P-T conditions obtained from the earliest D₁ fabric provide the first quantitative constraints on the P-T conditions of the southern Sleafordian Orogeny. The P-T-t evolution determined for the 1725–1690 Ma Kimban Orogeny indicate it developed along a clockwise P-T path, and dominates the structural and metamorphic character of the southern Gawler Craton. The large variations in exhumation over short lateral extents reflect the exhumation of lower crustal rocks during the Kimban Orogeny driven by transpression during the development of a regional transpressional ‘flower structure’. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1372052 / Thesis (Ph.D.) - University of Adelaide, School of Earth and Environmental Sciences, 2009

Conselho Nacional de Desenvolvimento Científico e Tecnológico / A região entre Bom Jesus de Itabapoana e Italva tem seu contexto geológico situado no Terreno Oriental (Domínios Cambuci, Costeiro e Italva) do Setor Central da Faixa Ribeira. Esta Faixa foi formada durante a Orogenia Brasiliana (Neoproterozóico-Cambriano), na borda sul/sudeste do Cráton São Francisco. A partir de mapas publicados na área e coletas de amostra em dois perfis estruturais, desenvolveu-se o presente trabalho, cuja meta foi a análise química de mineral para a investigação do metamorfismo dos Domínios Cambuci, Costeiro (Unidades Angelim e São Fidélis) e Italva. Para tal, foi objeto de estudo os metapelitos e metabasitos dos domínios supracitados. A identificação de associações minerais, a discriminação de diferentes equilíbrios metamórficos e a determinação de valores absolutos de pressão e temperatura (geotermobarometria) para cada um dos Domínios Tectônicos do Terreno Oriental foram os parâmetros fundamentais para a caracterização da evolução metamórfica da área de estudo. Estes dados indicaram diferentes condições e evoluções metamórficas, com distintos níveis crustais, para os diferentes domínios do Terreno Oriental. As rochas do Domínio Cambuci registram dois eventos térmicos distintos, ambos sob condições de alta temperatura, sendo o primeiro formado sob condições de pressão mais baixas que o segundo. Os litotipos do Domínio Costeiro registram pico metamórfico sob condições de facies granulito de pressão intermediária e os valores de pressão e temperatura obtidos foram os mais altos dentre os três domínios estudados. As rochas do Domínio Italva fornecem valores de temperatura correspondentes às condições de facies anfibolito superior a granulito, sob pressão baixa a intermediária. / The region between Bom Jesus de Itabapoana and Italva consists of a high-grade metamorphic terrane situated within the so-called Oriental Terrane of the Central Sector of the Ribeira Belt. This belt, of neoproterozóic-cambrian age, was formed during the Brasiliano Orogeny, in the south / southeast margin of the São Francisco Craton. The present work was developed on the basis of preexisting geological maps and collection of samples along two structural sections of the study area. The aim of this work is the investigation of the metamorphism of the Cambuci, Costeiro and Italva Domains of the Oriental Terrane. This investigation was based on the acquisition and interpretation of chemical analyses of mineral phases from metapelites and metabasites of the previously quoted domains. The identification of mineral parageneses, the discrimination of different metamorphic equilibriums and the determination of absolute values of pressure and temperature (geothermobarometry) for each one of the study tectonic domains were the basic tools used for the characterization of the metamorphic evolution of the area. The obtained data indicated different conditions and metamorphic evolutions, including different crustal levels, for the different tectonic domains of the Oriental Terrane. The rocks of the Cambuci Domain register two different thermal events, both under high temperature and medium pressure conditions. The first event, not inequivocaly metamorphic, was developed under lower pressures than the second one. The rocks of the Costeiro Domain record metamorphic peak under conditions of medium pressure granulite facies. The obtained pressure and temperature values were the highest among the three studied domains. The rocks of the Italva Domain yield temperature values of the transition zone between upper anfibolite and granulite facies conditions. Low to medium pressure values were obtained for this domain.

Conselho Nacional de Desenvolvimento Científico e Tecnológico / A região entre Bom Jesus de Itabapoana e Italva tem seu contexto geológico situado no Terreno Oriental (Domínios Cambuci, Costeiro e Italva) do Setor Central da Faixa Ribeira. Esta Faixa foi formada durante a Orogenia Brasiliana (Neoproterozóico-Cambriano), na borda sul/sudeste do Cráton São Francisco. A partir de mapas publicados na área e coletas de amostra em dois perfis estruturais, desenvolveu-se o presente trabalho, cuja meta foi a análise química de mineral para a investigação do metamorfismo dos Domínios Cambuci, Costeiro (Unidades Angelim e São Fidélis) e Italva. Para tal, foi objeto de estudo os metapelitos e metabasitos dos domínios supracitados. A identificação de associações minerais, a discriminação de diferentes equilíbrios metamórficos e a determinação de valores absolutos de pressão e temperatura (geotermobarometria) para cada um dos Domínios Tectônicos do Terreno Oriental foram os parâmetros fundamentais para a caracterização da evolução metamórfica da área de estudo. Estes dados indicaram diferentes condições e evoluções metamórficas, com distintos níveis crustais, para os diferentes domínios do Terreno Oriental. As rochas do Domínio Cambuci registram dois eventos térmicos distintos, ambos sob condições de alta temperatura, sendo o primeiro formado sob condições de pressão mais baixas que o segundo. Os litotipos do Domínio Costeiro registram pico metamórfico sob condições de facies granulito de pressão intermediária e os valores de pressão e temperatura obtidos foram os mais altos dentre os três domínios estudados. As rochas do Domínio Italva fornecem valores de temperatura correspondentes às condições de facies anfibolito superior a granulito, sob pressão baixa a intermediária. / The region between Bom Jesus de Itabapoana and Italva consists of a high-grade metamorphic terrane situated within the so-called Oriental Terrane of the Central Sector of the Ribeira Belt. This belt, of neoproterozóic-cambrian age, was formed during the Brasiliano Orogeny, in the south / southeast margin of the São Francisco Craton. The present work was developed on the basis of preexisting geological maps and collection of samples along two structural sections of the study area. The aim of this work is the investigation of the metamorphism of the Cambuci, Costeiro and Italva Domains of the Oriental Terrane. This investigation was based on the acquisition and interpretation of chemical analyses of mineral phases from metapelites and metabasites of the previously quoted domains. The identification of mineral parageneses, the discrimination of different metamorphic equilibriums and the determination of absolute values of pressure and temperature (geothermobarometry) for each one of the study tectonic domains were the basic tools used for the characterization of the metamorphic evolution of the area. The obtained data indicated different conditions and metamorphic evolutions, including different crustal levels, for the different tectonic domains of the Oriental Terrane. The rocks of the Cambuci Domain register two different thermal events, both under high temperature and medium pressure conditions. The first event, not inequivocaly metamorphic, was developed under lower pressures than the second one. The rocks of the Costeiro Domain record metamorphic peak under conditions of medium pressure granulite facies. The obtained pressure and temperature values were the highest among the three studied domains. The rocks of the Italva Domain yield temperature values of the transition zone between upper anfibolite and granulite facies conditions. Low to medium pressure values were obtained for this domain.

Este trabalho propõe a geração e fechamento de um proto-oceano e consequente formação de um arco de ilhas durante o Neoproterozóico no Complexo Metamórfico Passo Feio na região de Caçapava do Sul, RS. O Complexo Metamórfico Passo Feio, constituinte do Terreno São Gabriel, consiste em uma sequência metavulcanossedimentar supracrustal neoproterozóica intrudida pelo Granito Caçapava. O estudo utiliza petrografia, química de rocha total e química mineral por microssonda para classificar e identificar a afinidade geoquímica e ambiente tectônico de geração de anfibolitos contidos na sequência metavulcanossedimentar do Complexo Metamórfico Passo Feio. Os resultados são comparados com basaltos e pillow lavas da Formação Arroio Mudador, além de diques intermediários, básicos e ultrabásicos, intrudidos em rochas vulcanogênicas da Formação Hilário. A petrografia com auxílio da geotermometria da clorita, composição dos anfibólios e sua relação núcleo-borda permitiu a classificação da fácies metamórfica em prehnita-pumpeleíta para os basaltos e pillow lavas, xisto verde para os diques e fácies anfibolito inferior para os anfibolitos Os diagramas discriminantes de geoquímica de rocha total e de química mineral indicam afinidade desde toleítica a alcalina para os diques da Formação Hilário. Os basaltos, pillow lavas e anfibolitos possuem composição basáltica a basáltico-andesítica e afinidade toleítica, por vezes transicionando para calci-alcalina, interpretados como derivados de um mesmo protólito basáltico. As diferenças mineralógicas devem-se aos diferentes graus metamórficos. Os diagramas discriminantes de ambientes geotectônicos indicam em sua maioria basaltos de arco vulcânico (e toleítos de arco de ilha que se incluem nessa classificação). Os dados obtidos combinados com dados da literatura permitiram a interpretação de um modelo de evolução onde há uma abertura de um proto-oceano através do rifteamento do Arco da Bossoroca em cerca de 760 Ma, onde houve a formação de um arco de ilha (rochas do Complexo Passo Feio), a precipitação de carbonatos e margas neste proto-oceano, subsequente fechamento, dois eventos de metamorfismo e por fim a intrusão do Granito Caçapava a 562 Ma. / This paper proposes the generation and closure of a proto-ocean and consequently formation of an island arc in the Neoproterozoic Passo Feio Metamorphic Complex, Caçapava do Sul region, southern Brazil. The Passo Feio Metamorphic Complex, São Gabriel Terrane, consists of a Neoproterozoic supracrustal metavolcano-sedimentary sequence intruded by the Caçapava Granite. The study uses petrography, whole-rock geochemistry and mineral chemistry by microprobe to classify and identify the geochemical affinity and tectonic environment of the amphibolites contained in the metavolcano-sedimentary sequence of Passo Feio Metamorphic Complex. The results are compared with the basalts and pillow lavas of Arroio Mudador Formation, besides intermediate, basic and ultrabasic dykes, intruded in nearby volcanogenic rocks of Hilário Formation The petrography with aid of chlorite geothermometry, amphibole composition and its core-rim relation allow the metamorphic facies classification of prehnite-pumpellyite for the basalts and pillow lavas, greenschist for the dykes and lower amphibolite facies for the amphibolites. The whole-rock geochemistry and mineral chemistry discriminant diagrams indicate from tholeiitic to alkaline affinities for the Hilário Formation dykes. The basalts, pillow lavas and amphibolites have basaltic to basaltic-andesite composition and tholeiitic affinity, occasionally transitioning to calc-alkaline, and are interpreted as derived by the same basaltic protolith. The mineralogical differences are due to the different metamorphic grades. The geotectonic environment discriminant diagrams mostly indicate volcanic arc basalts (and island arc tholeiites, included in this classification). The present results combined with literature data allow an interpretation and propose a geotectonic evolution model for the region, starting with a proto-ocean opening through the Bossoroca Arc rifting in about 760 Ma, where occurred the formation of an island arc (Passo Feio Complex rocks), the precipitation of carbonates and marls in this proto-ocean, subsequent closure, two metamorphism events and at last the Caçapava Granite intrusion at ca. 562 Ma.

Este trabalho propõe a geração e fechamento de um proto-oceano e consequente formação de um arco de ilhas durante o Neoproterozóico no Complexo Metamórfico Passo Feio na região de Caçapava do Sul, RS. O Complexo Metamórfico Passo Feio, constituinte do Terreno São Gabriel, consiste em uma sequência metavulcanossedimentar supracrustal neoproterozóica intrudida pelo Granito Caçapava. O estudo utiliza petrografia, química de rocha total e química mineral por microssonda para classificar e identificar a afinidade geoquímica e ambiente tectônico de geração de anfibolitos contidos na sequência metavulcanossedimentar do Complexo Metamórfico Passo Feio. Os resultados são comparados com basaltos e pillow lavas da Formação Arroio Mudador, além de diques intermediários, básicos e ultrabásicos, intrudidos em rochas vulcanogênicas da Formação Hilário. A petrografia com auxílio da geotermometria da clorita, composição dos anfibólios e sua relação núcleo-borda permitiu a classificação da fácies metamórfica em prehnita-pumpeleíta para os basaltos e pillow lavas, xisto verde para os diques e fácies anfibolito inferior para os anfibolitos Os diagramas discriminantes de geoquímica de rocha total e de química mineral indicam afinidade desde toleítica a alcalina para os diques da Formação Hilário. Os basaltos, pillow lavas e anfibolitos possuem composição basáltica a basáltico-andesítica e afinidade toleítica, por vezes transicionando para calci-alcalina, interpretados como derivados de um mesmo protólito basáltico. As diferenças mineralógicas devem-se aos diferentes graus metamórficos. Os diagramas discriminantes de ambientes geotectônicos indicam em sua maioria basaltos de arco vulcânico (e toleítos de arco de ilha que se incluem nessa classificação). Os dados obtidos combinados com dados da literatura permitiram a interpretação de um modelo de evolução onde há uma abertura de um proto-oceano através do rifteamento do Arco da Bossoroca em cerca de 760 Ma, onde houve a formação de um arco de ilha (rochas do Complexo Passo Feio), a precipitação de carbonatos e margas neste proto-oceano, subsequente fechamento, dois eventos de metamorfismo e por fim a intrusão do Granito Caçapava a 562 Ma. / This paper proposes the generation and closure of a proto-ocean and consequently formation of an island arc in the Neoproterozoic Passo Feio Metamorphic Complex, Caçapava do Sul region, southern Brazil. The Passo Feio Metamorphic Complex, São Gabriel Terrane, consists of a Neoproterozoic supracrustal metavolcano-sedimentary sequence intruded by the Caçapava Granite. The study uses petrography, whole-rock geochemistry and mineral chemistry by microprobe to classify and identify the geochemical affinity and tectonic environment of the amphibolites contained in the metavolcano-sedimentary sequence of Passo Feio Metamorphic Complex. The results are compared with the basalts and pillow lavas of Arroio Mudador Formation, besides intermediate, basic and ultrabasic dykes, intruded in nearby volcanogenic rocks of Hilário Formation The petrography with aid of chlorite geothermometry, amphibole composition and its core-rim relation allow the metamorphic facies classification of prehnite-pumpellyite for the basalts and pillow lavas, greenschist for the dykes and lower amphibolite facies for the amphibolites. The whole-rock geochemistry and mineral chemistry discriminant diagrams indicate from tholeiitic to alkaline affinities for the Hilário Formation dykes. The basalts, pillow lavas and amphibolites have basaltic to basaltic-andesite composition and tholeiitic affinity, occasionally transitioning to calc-alkaline, and are interpreted as derived by the same basaltic protolith. The mineralogical differences are due to the different metamorphic grades. The geotectonic environment discriminant diagrams mostly indicate volcanic arc basalts (and island arc tholeiites, included in this classification). The present results combined with literature data allow an interpretation and propose a geotectonic evolution model for the region, starting with a proto-ocean opening through the Bossoroca Arc rifting in about 760 Ma, where occurred the formation of an island arc (Passo Feio Complex rocks), the precipitation of carbonates and marls in this proto-ocean, subsequent closure, two metamorphism events and at last the Caçapava Granite intrusion at ca. 562 Ma.

Este trabalho propõe a geração e fechamento de um proto-oceano e consequente formação de um arco de ilhas durante o Neoproterozóico no Complexo Metamórfico Passo Feio na região de Caçapava do Sul, RS. O Complexo Metamórfico Passo Feio, constituinte do Terreno São Gabriel, consiste em uma sequência metavulcanossedimentar supracrustal neoproterozóica intrudida pelo Granito Caçapava. O estudo utiliza petrografia, química de rocha total e química mineral por microssonda para classificar e identificar a afinidade geoquímica e ambiente tectônico de geração de anfibolitos contidos na sequência metavulcanossedimentar do Complexo Metamórfico Passo Feio. Os resultados são comparados com basaltos e pillow lavas da Formação Arroio Mudador, além de diques intermediários, básicos e ultrabásicos, intrudidos em rochas vulcanogênicas da Formação Hilário. A petrografia com auxílio da geotermometria da clorita, composição dos anfibólios e sua relação núcleo-borda permitiu a classificação da fácies metamórfica em prehnita-pumpeleíta para os basaltos e pillow lavas, xisto verde para os diques e fácies anfibolito inferior para os anfibolitos Os diagramas discriminantes de geoquímica de rocha total e de química mineral indicam afinidade desde toleítica a alcalina para os diques da Formação Hilário. Os basaltos, pillow lavas e anfibolitos possuem composição basáltica a basáltico-andesítica e afinidade toleítica, por vezes transicionando para calci-alcalina, interpretados como derivados de um mesmo protólito basáltico. As diferenças mineralógicas devem-se aos diferentes graus metamórficos. Os diagramas discriminantes de ambientes geotectônicos indicam em sua maioria basaltos de arco vulcânico (e toleítos de arco de ilha que se incluem nessa classificação). Os dados obtidos combinados com dados da literatura permitiram a interpretação de um modelo de evolução onde há uma abertura de um proto-oceano através do rifteamento do Arco da Bossoroca em cerca de 760 Ma, onde houve a formação de um arco de ilha (rochas do Complexo Passo Feio), a precipitação de carbonatos e margas neste proto-oceano, subsequente fechamento, dois eventos de metamorfismo e por fim a intrusão do Granito Caçapava a 562 Ma. / This paper proposes the generation and closure of a proto-ocean and consequently formation of an island arc in the Neoproterozoic Passo Feio Metamorphic Complex, Caçapava do Sul region, southern Brazil. The Passo Feio Metamorphic Complex, São Gabriel Terrane, consists of a Neoproterozoic supracrustal metavolcano-sedimentary sequence intruded by the Caçapava Granite. The study uses petrography, whole-rock geochemistry and mineral chemistry by microprobe to classify and identify the geochemical affinity and tectonic environment of the amphibolites contained in the metavolcano-sedimentary sequence of Passo Feio Metamorphic Complex. The results are compared with the basalts and pillow lavas of Arroio Mudador Formation, besides intermediate, basic and ultrabasic dykes, intruded in nearby volcanogenic rocks of Hilário Formation The petrography with aid of chlorite geothermometry, amphibole composition and its core-rim relation allow the metamorphic facies classification of prehnite-pumpellyite for the basalts and pillow lavas, greenschist for the dykes and lower amphibolite facies for the amphibolites. The whole-rock geochemistry and mineral chemistry discriminant diagrams indicate from tholeiitic to alkaline affinities for the Hilário Formation dykes. The basalts, pillow lavas and amphibolites have basaltic to basaltic-andesite composition and tholeiitic affinity, occasionally transitioning to calc-alkaline, and are interpreted as derived by the same basaltic protolith. The mineralogical differences are due to the different metamorphic grades. The geotectonic environment discriminant diagrams mostly indicate volcanic arc basalts (and island arc tholeiites, included in this classification). The present results combined with literature data allow an interpretation and propose a geotectonic evolution model for the region, starting with a proto-ocean opening through the Bossoroca Arc rifting in about 760 Ma, where occurred the formation of an island arc (Passo Feio Complex rocks), the precipitation of carbonates and marls in this proto-ocean, subsequent closure, two metamorphism events and at last the Caçapava Granite intrusion at ca. 562 Ma.