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A história Natural do Grupo Serra Geral desde o Cretáceo até o Recente

The natural history of the Serra Geral Group comprises a series of processes, starting with the partial melting of the asthenosphere, continuing with the upwelling and modification of magmas and reaching the effusion of the basalt lavas. It also includes interaction of these magmas with the surrounding crust, especially causing heating of the Guarani aquifer and pyrometamorphism of the sedimentary units of the Parana Basin. This evolution culminates with the hydrothermal alteration and weathering of silica gossans over amethyst geode zones, recording in the gossans’ mud the local evolution of life and climate in the last 10 thousand years. Besides the well-studied magmatic processes, the distinctive formation of paralavas related with methane combustion was identified, including re-melting of basalts at around 1.800 o C. The re-melting may generate Au-EGP-Cu ore deposits in stratiform complexes. Hydrothermal alteration caused the filling of amygdales with zeolites and smectite, allowing the use of such basalts in the ‘rock for crops’ practice. Gem-quality amethyst and agate deposits are also hydrothermal in origin, as well as native copper (first hydrothermal event) and sand injections (second hydrothermal event). The natural history of the volcanic group is thus better understood; its study demands the description of many aspects of the planet’s dynamics, both in its interior and at the surface.