Metamorphic evidences from the Monchique pluton (South Portugal): Contact metamorphism vs regional metamorphism under very low-grade conditions

1. Isabel Abad ¹
2. Fernando Nieto ²
3. Nicolas Velilla ²
4. Isabel Suárez-Ruiz ³

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More publications in: Revista de la Sociedad Geológica de España

Abstract

During the Cretaceous, the Monchique pluton was emplaced in metapelite and greywacke host rocks, producing a thermal aureole with hornfels containing cordierite, andalusite and biotite. Apparently, thermal effects persist more than 3 km from this syenitic igneous body, evidenced by vitrinite reflectance values higher than 4% and natural coke with a mosaic optical texture. Nevertheless, the host rocks not transformed to hornfels show the same textural, chemical, mineralogical and crystalchemical characteristics as the equivalent clastic rocks from the distant Brejeira and Mira Formations. This difference between the reaction progress of inorganic and organic materials can be related to the slow reaction rates of the clay minerals and the absence of pervasive tectonic effects during the thermal processes, which are one of the most important driving forces of their reaction progress.

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