Petrologia e termocronologia de granulitos no sector central da Faixa Ribeira: (região de São Fidélis, Rio de Janeiro, Brasil)

Tese de doutoramento em Geologia (Geoquímica), apresentada à Universidade de Lisboa através da Faculdade de Ciências, 2008A região de São Fidelis localiza-se a norte do Rio de Janeiro (SE do Brasil) e pertence à zona central da Faixa Ribeira, uma faixa móvel de idade Neoproterozóica Ordovícica. Esta tese apresenta dados referentes a diversas metodologias com o objectivo de caracterizar a evolução dos granulitos presentes na área e, desta forma, discriminar a evolução geodinâmica da Faixa Ribeira, formada no final do Ciclo Brasiliano por colisão dos crátons de São Francisco e Congo no contexto da aglutinação do mega-continente Gondwana Ocidental. Os resultados indicam que após a colisão há 630-610Ma, o metamorfismo granulítico (T~850ºC; P~8kbar), contemporâneo de cavalgamentos de alta temperatura (no contexto da formação de uma mega-estrutura em flor), extensa migmatização de rochas supracrustais e geração de granitóides, foi atingido há ~565Ma, tendo o arrefecimento que se seguiu sido processado mais rapidamente nas rochas próximas da zona central da estrutura em flor (3 10ºC/Ma) que nas presentes nas rampas laterais (~1ºC/Ma). Uma anomalia térmica, provavelmente resultante de upwelling da astenosfera e underplating (num contexto de extensão pós-colisional), terá permitido, auxiliada pelo longo período de deformação transpressiva sub-horizontal que se seguiu, um arrefecimento muito lento e manutenção de T 650ºC para alguns granulitos durante ~100Ma. Após o longo período transpressivo (há 510 480Ma), as taxas de arrefecimento aumentaram rapidamente (8 30ºC/Ma) por exumação mais rápida relacionada com o relaxamento termo-tectónico do colapso do orógeno. A evolução termocronológica da área estudada sugere a existência de um longo período com manutenção de condições térmicas elevadas que terá condicionado tanto a evolução das condições físicas (T, P, O2, aH2O) deste segmento crustal como a formação das feições petrológicas e geoquímicas que destas dependem, sendo que a manutenção da anomalia térmica por ~100Ma terá levado a elevada desidratação da crosta inferior e produção generalizada de charnockitos na Faixa Ribeira.The São Fidelis region is located north of Rio de Janeiro (SE Brazil) and is part of the central zone of Ribeira Belt, a Neoproterozoic Ordovicic mobile belt. This thesis presents data obtained with several techniques with the purpose of characterizing the evolution of the granulites present in the area and, in this way, to constrain the geodynamic evolution of the Ribeira Belt, formed in the final stages of the Braziliano Cycle as the result of the collision between the São Francisco and Congo cratons in the Western Gondwana mega-continent amalgamation context. Results show that after the collision at 630-610Ma, granulitic metamorphism (T~850ºC; P~8kbar), coeval with high-temperature thrusts (as the result of the formation of a mega flower structure), widespread migmatization of supracrustal rocks and granitoid production, was attained at ~565Ma, and the cooling that followed was faster for the rocks closer to the centre of the flower structure (3 10ºC/Ma) than for those present in the lateral ramps (~1ºC/Ma). A thermal anomaly, probably resultant of asthenospheric upwelling and underplating (in a post-collisional extension context), allowed, aided by the long-term sub-horizontal transpressive deformation period that followed, a very slow cooling and preservation of T>650ºC for some granulites during ~100Ma. After the long-term transpressive period (at 510 480Ma), cooling rates increased rapidly (8 30ºC/Ma) by faster exhumation due to thermo-tectonic relaxation and orogenic collapse. The thermochronologic evolution of the studied area suggests a long-term maintenance of high-thermal flux, that conditioned the evolution of this crustal segment physical conditions (T, P, O2, aH2O), as well as, the petrological and geochemical features dependent. The long-term thermal anomaly that lasted ~100Ma may have led to extensive lower crust dehydration and widespread formation of charnockites in the Ribeira Belt

Zona de cisalhamento de Juzbado - Penalva do Castelo no sector de Figueira de Castelo Rodrigo: implicações para a exumação de rochas metamórficas de alto grau

A Zona de Cisalhamento Juzbado-Penalva do Castelo (Zona Centro Ibérica) separa unidades com distintas características estruturais, metamórficas e litoestratigráficas, revelando-se como um acidente tectónico de primeira ordem durante a evolução da Cadeia Varisca Ibérica. A realização de cartografia geológica e estrutural detalhada em sectores escolhidos e a sua extrapolação regional, colocou em evidência o carácter progressivo da deformação durante a evolução desta zona de cisalhamento. O novo padrão cartográfico proposto é discutido no contexto da evolução desta zona de cisalhamento em regime transcorrente esquerdo e no quadro do seu papel na exumação do Complexo Anatéctico de Figueira de Castelo Rodrigo-Lumbrales

The Juzbado-Penalva do Castelo wrench ductile shear zone : a major structure oblique to the main Iberian Variscan trend

The ENE-WSW Juzbado-Penalva do Castelo Shear Zone (JPCSZ), 200 km long and 5-15 km wide, is a first order structure of the Iberian Variscides with a sinistral component emphasized by a 65 km ductile reject of the major D1 structures (Iglesias & Ribeiro, 1981); the NW-SE regional trend (e.g. Marão and Tamames structures) changes to E-W (e.g. Moncorvo and Poiares synclines) when approaching the JPCSZ and even to ENE-WSW along it (e.g. Marofa syncline). Although this sigmoidal pattern clearly post-dated the early structures of the first and main Variscan event (D1), some evidence show that the JPCSZ should have been active since the early Variscan collision (e.g. D1 kinematics changes in both sides of the shear zone), or even earlier (e.g. it is the most plausible boundary between two major pre-Ordovician lithostratigraphic domains - the Beiras and Douro Groups). We present preliminary structural and petrological data for the Figueira de Castelo Rodrigo area (a key region to study the E-W to ENE-WNW transition) in order to constrain its Variscan evolution. In the northern part of the studied area, several syn-tectonic granitoids crop out, whereas to the south migmatites, probably part of the Pre-Cambrian to Cambrian Excomungada Formation (Ribeiro, 2001), predominate. These granulite-facies migmatites (T ≥ 800 ºC) contact, to the south, with Ordovician lowgrade (biotite zone) metapelites and metaquartzites, materializing a “temperature jump” of at least 400 :C, which, considering a barrovian-type geothermal gradient of about 25 :C.km-1, suggests a vertical offset of 16 km (or 8 km for a 50 :C.km-1 geothermal gradient). In the autochthon of the Central Iberian Zone, it is solely along the JPCSZ that high grade metamorphic rocks are exhumed. At the Figueira de Castelo Rodrigo area two segments were identified in the Marofa Syncline. The western segment has a clear ENE-WSW direction trend coincident with the JPCSZ, whereas at the eastern segment, E-W trends predominate. The deformation is much more intense in the western segment, giving rise to a pervasive sub-vertical slightly wavy fold axis associated with a sub-horizontal to slightly northeast dipping stretching lineation; this deformation is coeval with significant recrystallization and the development of a mylonitic foliation. These strong fabrics, better observed in the metapelites and metapsamites interlayered in the Ordovician metaquartzite sequence, show widespread sinistral shear criteria structures (e.g. C-S shear bands, distorted/rotated porphyroclast and consistent asymmetrical folds). This shear sense, also observed in the diatexite lenses inside the JPCSZ, is congruent with the inferred movement from the analysis of the regional structures. The eastern segment is characterized by a less intense deformation, where folds and other related mesoscopic structures are almost absent. Nevertheless, a stretching lineation is often found, ranging from low dip to 50: towards SW. This behavior shows that the eastern segment (E-W trend) has had a predominant thrusting with a sinistral component. The kinematics preserved in this segment was the result of the reworking of previous structures oblique to the main sinistral ENE-WSW shear (JPCSZ), with the consequent induction of a restraining zone along the EW trend

Figueira de Castelo Rodrigo-Lumbrales anatectic complex (Central Iberian Zone): new geothermobarometric data

In the Central Iberian Zone (Iberian Massif) there is a significant change in the Variscan D1 trend from the dominant NW–SE to E–W (Marofa and Ahigal de los Aceiteros synclines). This change is due to the 65 to 100 km sinistral movement of the Juzbado–Penalva do Castelo shear zone (JPCSZ), a composite structure where several shear bands formed as a result of progressive deformation (Pereira et al., 2013). Despite some uncertainties, the JPCSZ has been considered to have been active during the Variscan D3, syn- to-post regional metamorphic peak (Villar et al., 2000). The JPCSZ was responsible for the exhumation of the Figueira de Castelo Rodrigo–Lumbrales anatectic complex, resulting in its juxtaposition to low-grade (350–450 ºC) metasediments. The complex is essentially composed of metatexites, diatexites and twomica anatectic granites. We present new geothermobarometric data in order to better constrain the regional metamorphic climax, as well as the evolution of this first-order Variscan tectonic feature. Mineral analyses were conducted on a JEOL JXA-8200 electron microprobe and P-T estimates were obtained using THERMOCALC software (Holland and Powell, 1998), version 3.33. Due to lack of garnet in the migmatites, peak metamorphism calculations were made using interlayered calc-silicate rocks with a mineral assemblage of plagioclase + biotite + amphibole + clinopyroxene + garnet ± titanite ± apatite ± zircon ± oxides. These minerals are chemically unzoned, displaying flat rim-core-rim chemical profiles. The garnet is grossular-rich (XAlm = 0.51; XPy = 0.05; XGr = 0.31; XSpss = 0.13), whereas clinopyroxene is essentially hedenbergitic (XEn = 0.25; XFs = 0.28; XWo = 0.47). Peak paragenesis is also composed of plagioclase (XAn = 0.95) and ferrohornblende. P-T estimates using garnet-pyroxene-amphibole-plagioclase equilibrium reactions establish the metamorphic peak at 725±50 ºC and 5.4±1 kbar. These new results are compatible with a geothermal gradient of 36 ºC.km-1, slightly higher than that prevailing in typical Barrovian-type conditions, implying relatively shallow depths for crustal anatexis during Variscan orogenic events. These new results provide an explanation for the absence of garnet in the anatectic pelites, whereas the absence of mineral zoning in the peak parageneses suggests fast exhumation of the Figueira de Castelo Rodrigo–Lumbrakes complex induced by the JPCSZ. If the low-grade rocks were formed under the same geothermal gradient, the vertical exhumation of migmatitic rocks from their level of generation to the final juxtaposition with greenschist facies rocks is estimated to be over 12 km

Exhumation of a migmatite complex along a transpressive shear zone: inferences from the Variscan Juzbado-Penalva do Castelo Shear Zone (Central Iberian Zone)

High-grade metamorphic rocks associated with S-type granites are recorded in the Central Iberian Zone, Iberian Variscides. Though most of these occur as inferred metamorphic core complexes affiliated with detachment faults, others, such as the Figueira de Castelo Rodrigo–Lumbrales Anatectic Complex, crop out between low-grade metamorphic rocks separated by steeply-dipping strike-slip shear zones, such as the Juzbado–Penalva do Castelo Shear Zone. Our structural analysis has been able to constrain two major deformation stages during the Variscan D3: (a) D3a ductile deformation event, with clear sinistral kinematic criteria; and (b) D3b thrusting ductile–brittle deformation event. The petrological investigation confirmed the jump in metamorphic grade between the host rocks and the anatectic complex. P–T estimates on calc-silicate rocks interlayered with the metapelites of the anatectic complex provided minimum metamorphic peak conditions of T = 761 ± 50°C and P = 5.0 ± 1.0 kbar. However, petrological modelling results show that P–T conditions must have exceeded T > 800°C. Both structural and geothermobarometric data support a two-step model for the exhumation of the Anatectic Complex, including a 5 – 8 km vertical exhumation along a 65 – 100 km horizontal displacement due to a simple shear-dominated transpression mechanism during the Variscan D3 events

Garnet-biotite diffusion mechanisms in complex high-grade orogenic belts : understanding and constraining petrological cooling rates in granulites from Ribeira Fold Belt (SE Brazil)

Cooling rates based on the retrograde diffusion of Fe2+ and Mg between garnet and biotite inclusions commonly show two contrasting scenarios: a) narrow closure temperature range with apparent absence of retrograde diffusion; or b) high result dispersion due to compositional variations in garnet and biotite. Cooling rates from migmatites, felsic and mafic granulites from Ribeira Fold Belt (SE Brazil) also show these two scenarios. Although the former can be explained by very fast cooling, the latter is often the result of open-system behaviour caused by deformation. Retrogressive cooling during the exhumation of granulite-facies rocks is often processed by thrusting and shearing which may cause plastic deformation, fractures and cracks in the garnet megablasts, allowing chemical diffusion outside the garnet megablast – biotite inclusion system. However, a careful use of garnets and biotites with large Fe/Mg variation and software that reduces result dispersion provides a good correlation between closure temperatures and the size of biotite inclusions which are mostly due to diffusion and compositional readjustment to thermal evolution during retrogression. Results show that felsic and mafic granulites have low cooling rates (1–2 °C/Ma) at higher temperatures and high cooling rates (~100 °C/Ma) at lower temperatures, suggesting a two-step cooling/exhumation process, whereas migmatites show a small decrease in cooling rates during cooling (from 2.0 to 0.5 °C/Ma). These results agree with previously obtained thermochronological data, which indicates that this method is a valid tool to obtain meaningful petrological cooling rates in complex high-grade orogenic belts, such as the Ribeira Fold Belt

Inspecting zircon populations of the Iberian Pyrite Belt: tracking the Cadomian record of the South Portuguese Zone

AIMS OF THE MEETING: The scientific sessions will be focused on the Pan-African and Cadomian Orogenies recorded in North Africa and western Europe across the Ediacaran Cambrian transition and its bearing in the assembly and demise of Pannotia. Contributions dealing with structural, magmatic, provenance sources, palaeomagnetic, sedimentary, chronostratigraphic and radiometric constraints are particularly welcome. ORGANIZING COMMITTEE: J. Javier Álvaro, Instituto de Geociencias (CSIC-UCM), Spain Martim Chichorro, Universidade Nova de Lisboa, Portugal Gabriel Gutiérrez-Alonso, Universidad de Salamanca.ABSTRACT: The palaeogeographic location of the southernmost zone of the Iberian Massif, the South Portuguese Zone (SPZ), prior to the amalgamation of Pangaea is still a matter of debate. In this work, we attempt to track its palaeogeographic setting during the final stages of the Cadomian Cycle.info:eu-repo/semantics/publishedVersio

Complexo anatético de Figueira de Castelo Rodrigo - Lumbrales, Zona Centro Ibérica: análise preliminar do metamorfismo e geoquímica

Na região de Figueira de Castelo Rodrigo – Lumbrales ocorrem espacialmente associadas rochas de alto grau metamórfico e diferentes tipos de granitóides. A análise geoquímica preliminar revela uma provável relação genética entre os migmatitos e alguns dos granitóides analisados que, nesta perspectiva, são considerados anatécticos e sin-orogénicos. No entanto na região ocorrem alguns granitóides sem aparente relação genética com os migmatitos aflorantes, que poderão ter sido gerados em período tardi-colisional