Prasinophyte bloom and putative fungi abundance near the Kačák event (Middle Devonian) from the Odivelas Limestone, Southwest Iberia

The Kačák Event is one of the several prominent Devonian climatic and biotic perturbations with a supra-regional to global extent. Its record can be traced in the litho- and biostratigraphy of uppermost Eifelian-lowermostGivetian strata and with multiple geochemical and geophysical proxies. In southwest Iberia, within southwestern Ossa-Morena Zone domains, there are rare, scattered Early-Middle Devonian limestone occurrences. One of these occurrences—the Odivelas Limestone type locality—is revisited in terms of conodont biostratigraphy and palynology and the results compared with previous data on reef macrofauna and magnetic susceptibility stratigraphy. The new data show that this locality is probably within the Polygnathus hemiansatus zone, i.e. of the earliest Givetian age. The palynological content of a ca. 2 m of black, organic-rich, fine grained limestone section is dominated, in varied proportions, by amorphous organic matter, putative fungal hyphae, and prasinophycean algae. The results are interpreted as an indication of high organic productivity and deposition in hypoxic to anoxic settings and probable biotic crisis, in both marine and terrestrial realms, connected with the Middle Devonian Kačák Event. The similarity of this record with other Devonian events is discussed

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

Assumptions and problems with Fe-Mg garnet - biotite diffusion based petrological cooling rates : a case study in granulites and migmatites from central Ribeira Fold Belt, SE Brazil

O sector São Fidelis - Santo António de Pádua, pertencente à zona central da Faixa Ribeira, é fundamentalmente composto por migmatites, granulitos e blasmilonitos. A aplicação de diferentes metodologias com o objectivo de obetr taxas de arrefecimento baseadas na difusão Fe-Mg entre granada e respectivas inclusões de biotite revelou-se infrutífera. Tal deve-se fundamentalmente a: a) elevada dispersão dos resultados causada pela existência de um sistema aberto; b) estreita variação das temperaturas de fecho do sistema causada por re-homogeneização da granada e altas temperaturas seguida de arrefecimento muito rápido. Os resultados obtidos são fundamentalmente qualitativos, mas estão de acordo com resultados termocronológicos previamente obtidos baseados na integração de múltiplos sistemas isotópicos

Pseudosection and thermobarometry constraints on the P-T-t evolution of Ribeira Belt, SE Brazil

Geochemica et Cosmochimica Acta, Vol. 72, issue 12, Suppl. 1, A66

C-O-H isotopic evidences for fluid sources of granulites in Ribeira Belt, SE Brazil

Publicado em: Geochemica et Cosmochimica Acta, Vol. 72, issue 12, Suppl. 1, A7

Whole-rock geochemistry and Sr-Nd isotopic evidences for sources and processes of granulite formation in Ribeira Belt, Brazil

Geochemica et Cosmochimica Acta, Vol. 72, issue 12, Suppl. 1, A93

Problemas e soluções das taxas de arrefecimento petrológicas baseadas em difusão mineral em granulitos da Faixa Ribeira, Brasil : poderá a difusão granada-biotite ser usada para algo? = Pitfalls and breakthroughs of petrological cooling rates based on mineral diffusion from granulites in Ribeira Belt, Brazil : can garnet-biotite diffusion mechanisms be used for anything?

O uso de software capaz de remover a dispersão nas taxas de arrefecimento petrológicas em migmatitos e granulitos da Faixa Ribeira (Brasil) revela padrões de arrefecimento confruentes com resultados termocronológicos baseados na integração de múltiplos sistemas isotópicos; a) os migmatitos arrefecem rapidamente a partir de altas temperaturas, diminuindo as taxas de arrefecimento com o tempo; b) os granulitos experimentaram taxas de arrefecimento baixas e altas temperaturas, seguindo-se um período curto de arrefecimento muito rápido, aumentando as taxas de arrefecimento durante a retrogradação