IJUP08 - Universidade do Porto

THE STUDY OF DE FÂNZERES-GONDOMAR GRANITES: A
MULTIDISCIPLINARY APROACH
M. Alves 1
1 Dep.Geologia, Fac. Ciências, Univ. Porto, Rua do Campo Alegre, 687 4169-007 Porto, Portugal
(c0770094@alunos.fc.up.pt).
The granites from Fânzeres-Gondomar area («granito gnaissico de Fânzeres» and «granito
de Gondomar») are located in the Central Iberian Zone. Multidisciplinary studies, namely
Anisotropy of Magnetic Susceptibility (AMS), geometry of fluid inclusions plans (FIPs)
studies and microthermometric fluid characterization, were carried out. The Gondomar
granite presents an average magnetic susceptibility of 29.06 µSI, AMS indicates a
magnetic foliation (N19º; 69ºSE) which is the result of the magmatic flow (magmatic
foliation). The most representative FIPs trending NNE-SSW, are parallel to the magnetic
foliation, which indicate that they enhanced a pre-existent magmatic anisotropy. The fluids
that circulate within these micro-cracks are aqueous with high salinities (19.9 to 26.4
mass% eq. NaCl) and minimum temperature of entrapment between 90 and 120ºC. This
type of fluids has been referred to as being a result of a contribution of meteoric fluids
from late to post-Variscan cover. The Fânzeres granite presents a gneissic foliation, which
is visible on the field and on the microscope. The average magnetic susceptibility is 40.97
µSI and the magnetic foliation (N26º; 58ºSE) is parallel to the gneissic foliation. FIPs
trending NW-SE are the most representative. They are characterized by aqueous fluids of
low salinity (0.2 to 1.2 mass% eq. NaCl) and minimum temperature of entrapment between
130 and 200ºC. These FIPs are perpendicular to the gneissic and magnetic foliation, which
agrees with the process of the formation of FIPs. We assume that the FIPs correspond to a
stress field where the minimum stress, σ3, is normal to the FIPs that materialize the
vertical plane σ1 σ2.
Keywords: granites, anisotropy of magnetic susceptibility, fluid inclusions planes.
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