brasil geologia ecoturismo geoturismo turismo geológico Magmatismo alcalino brasil geología geoturismo turismo geológico Magmatismo alcalino brasilien geologie reise reisen urlaub alkalischer alkaliner Magamatismus

Geological Travel

Alkaline Magmatism

Hiking Tours / Brazil – In Southeast Brazil, between longitudes 42° and 47° W, a linear alignment of alkaline igneous intrusive complexes (Poços de Caldas, Ilhabela, Passa Quatro / Itatiaia, Mendanha, Tinguá, Soarinho, Cabo Frio), with decreasing ages in eastward direction (85 - 55 Ma), occurs. Over a length of 500 km, they were emplaced into the northern segment of the Neoproterozoic Ribeira belt. With 800 sqkm, Poços de Caldas is the largest alkaline complex in Brazil and one of the largest in the world. The smaller Itatiaia / Passo Quatro complexes with peaks up to 2.800 m represent the highest elevations on the Atlantic side of South America.

magmatism

These complexes most probably represent the roots of extinct Cretaceous volcanoes which at that time might have been comparable to today's Mts Kenya (5.199) or Kilimanjaro (5.895 m) along the East African Rift. Over time the volcanic superstructures have eroded and just the plutons remained. There is some evidence that these intrusive rocks were originated by the deflected melts of the Trindade Mantle Plume.

The plume probably impacted lithosphere about 85 Ma beneath Brasília, today's capital of Brazil. Since then, the South American Plate drifted slowly westwards (3 - 6 cm/year) passing over the essentially fixed - position mantle plume. From 85 to 55 Ma the plume tail passed beneath the Archaean São Francisco Craton and from 55 Ma until today beneath the seafloor of the South Atlantic. Today, it is supposed to be located beneath the volcanic island group Trindade - Martin Vaz which was built by the plume and its associated magmatism during the last 3 million years.

Physical evidence for the plume track on the oceanic part of the south american plate is the Vitória seamount chain which over a distance of 1.200 km connects Trindade - Martin Vaz with the Brazilian mainland. The landward end of the chain is formed by the archipelago of Abrolhos which has been originated by the plume between 52 and 42 Ma. On the continental part of the south american plate it is believed that the deep (200 - 250 km) lithospheric roots of the São Francisco Craton deflected the hot (1.300 - 1.600° C) upwelling mantle to regions with thinner lithosphere in the south, where it decompressed, melted and finally erupted through the alkaline volcanoes described above – Geological Tours / Brazil.