Abstract: Microthermometric characteristics of metamorphic to hydrothermal fluids and microfracturing were studied in a contact zone between metamorphic series and peraluminous granites, located in the southern part of the Mont Lozère pluton (Massif Central, France). Four major stages of fluid production or migration have been recognized: (1) N2-CH4 (±CO2)-rich fluids related to the metamorphism of the C-bearing shales, occurring as fluid inclusion along the quartz grain boundaries; (2) CO2-CH4-H2O vapours or liquids, with homogenization temperatures of 400 ± 20 and 350 ± 50°C respectively, related to the first hydrothermal stage produced by the late peraluminuous intrusions; (3) aqueous fluids having low salinities and Th in the range 150–330°C; (4) low-temperature aqueous fluids.
It is shown that the percolation of hydrothermal fluids occurs through a dense set of microfissures on a microscopic scale. The different stages of fluid percolation have been investigated by relating the deformational events to the observed fracturing. The nature of the hydrothermal fluid has been deduced by studying the trails of fluid inclusions. Analysis of the relationships of the fluid inclusion trails (F.I.T.) with structures associated with plastic deformation show that their propagation is independent of the intracrystalline anisotropies. Combined studies of their orientation in space and their microthermometric characteristics show that: (1) according to the direction, several generations of fluids are distinguished within each sample on the basis of their physical-chemical characteristics; they correspond to different stages of the hydrothermal activity and to different directions of micro-crack opening; (2) in bulk isotropic media (granite), fluid inclusion trails are essentially mode I cracks which can be used as excellent markers of paleostress fields; however, in bulk anisotropic media (quartz lenses in mica schists) the migration directions of the fluids are mostly dependent on the local reorientations of the stress fields.
The study of the contact zone between granites and a metamorphic series submitted to local abnormal heat flows shows that fluid characteristics are significantly different in the two environments. Migration of carbonic fluids from mica schists towards granites occurred but is relatively limited, whilst aqueous fluids mixed in variable amounts with carbonic fluids in the metamorphic zone.