Study of the volcano-sedimentary nature of the Serro formations  Silurian terranes of Serra de Arga - Minho (Northern Portugal)

Patrícia A. Dias (NIG-R, Department of Earth Sciences, University of Minho, Gualtar, 4710-057 Braga, Portugal)
Carlos Leal Gomes (same address as P.A. Dias)

volume 5 - nº 9 - 2008

Key-words: metavolcanic rocks, exhalites, Silurian Terranes, iron sulfides, polimodal volcanism.

Abstract: Petrography and paragenetic analysis of some phyllitic Silurian formations from the North of Serra de Arga (Minho – Northern Portugal) revealed an abnormal modal percentage of iron sulfides in a complex set of lithologies that include amphibolic members and tourmaline-garnet rich horizons as well. Excluding the geochemical influence and expression of superposed metamorphism and metasomatism, whole rock analysis shows conspicuous high Fe and Al percentages and low contents of incompatible elements in some facies. Significant Zn, Mn, Sc, Ba and Sr may be related to the presence of accessory minerals suggesting an exhalitic affinity.  Pyrrotite and pyrite are the main sulfides but sphalerite, galena and molybdenite are also present in low quantities. These petrographic and geochemical characteristics and the presence of textural aspects such as clastic and/or porphyric relict geometries support the establishment of some correspondences between the observed rock types and few possible volcano-sedimentary or exhalitic ancestors.
Whole rock geochemistry also suggests the manifestation of a protolithic polimodal volcanism with a main expression in the felsitic domain with several complex primordial textures indicating that the primitive rock types could be tuffaceous/volcanoclastic to porphyric.

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