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

摘要

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.