Petrographic features of contact metamorphism associated to the Penamacor-Monsanto granite pluton [Aspectos petrográficos do metamorfismo de contacto associado ao plutão granítico de Penamacor-Monsanto]
Ribeiro da Costa I.,University of Lisbon |
Ribeiro da Costa I.,CREMINER LARSyS Laboratory Associado |
Antunes I.M.,Polytechnic Institute of Castelo Branco |
Ramos J.F.,CREMINER LARSyS Laboratory Associado |
And 6 more authors.
Comunicacoes Geologicas | Year: 2013
Detailed petrography of the marginal granites and contact zone of the Penamacor-Monsanto pluton (Central Iberian Zone) has revealed textural and mineralogical features relevant to the interpretation of the effects of the granitic intrusion on the host Schist-Greywacke Complex (SGC). Among sampled marginal granites, muscovite-biotite monzogranites with accessory tourmaline predominate, mostly medium-to-coarse grained, though finer-grained and even aplitic facies occur. Argillization or sericitization of plagioclase and biotite replacement by chlorite are incipient to moderate. The narrow metamorphic aureole surrounding this pluton comprises a thin, probably discontinuous, aureole of hornfels, along the contact with the intrusion, and a wider rim of spotted-schists, which eventually give way to the monotonous, fine-grained phyllites and greywackes of the SGC, affected by Variscan regional metamorphism. Thermal and compositional variations during the emplacement of the marginal granites, deduced from the heterogeneities referred above, and the lithological gradations within the host SGC itself are the most likely causes for the textural and mineralogical variations found in the narrow contact aureole which envelops this pluton. Most hornfels are very fine-grained, compact rocks, made of quartz and feldspars, with some proportion of mica, and rare detrital tourmaline grains. However, a few hornfels present a high proportion of coarse porphyroblasts (mostly altered to fine-grained quartz-sericite material), often aligned along a preferred orientation, thus conferring a semblance of foliation and suggesting these rocks may be transitional to the spotted-schists of the outer rim of the contact zone. Spotted-schists are much altered: both porphyroblasts and matrix are invariably reduced to very fine-grained quartz-sericitic material, occasionally mixed with fine-grained Fe oxi-hydroxides attesting to the intense meteoric alteration these rocks have gone through. The predominant oval shape of these ghost porphyroblasts and the abundance of microinclusions suggest they may replace cordierite, rather than andaluzite. Cordierite-schists are also common in granitic contact aureoles hosted in Al-rich host-rocks. Detrital tourmaline is much rarer in the spotted-schists than in the hornfels. Boron-enriched late magmatic fluids are likely to be responsible, not only for the presence of tourmaline in most marginal granites, but also for the local occurrence of quartz(± mica)-tourmaline rocks, as sampled in the Medelim area, and of thorough replacement of biotite by tourmaline in finegrained biotite-quartz schists from several places around the intrusion. We hope the petrographic detail this study has afforded us will be a useful contribution to the overall geological study of the Penamacor-Monsanto region and of similar granite-related metamorphic aureoles in Portugal. © 2013 LNEG - Laboratório Nacional de Geologia e Energia IP.