The Silvretta thrust sheet, located in the eastern Alps, is one of the upper Austroalpine nappes and represents part of the old polymetamorphic basement of the Alps (Maggetti and Flisch, 1993, and references therein).
The alpine metamorphic overprint is at sub-greenschist facies conditions. Variscan metamorphism occurred under amphibolite facies conditions and peak metamorphic temperature was in the sillimanite+staurolite+muscovite stability field (ca. 550 °C). After temperature climax, pressure increased quasi-isothermally to ca. 0.5-0.6 GPa (constrained by phase relations and fluid inclusion data). During the retrograde path, the Silvretta entered from
the kyanite into the andalusite stability field. The petro-
structural evolution and fluid inclusion data can be explained by an anticlockwise P-T path that involved a period of heating coeval with compressional deformation followed by loading. At the end of the Variscan cycle extensional structures coincided with exhumation. These structures may be related either to large scale extension tectonics or to more local events.
Direct dating of core, rim, and bulk staurolite fractions yielded, within error, identical single-mineral Pb-Pb and U-Pb ages of ca. 310±10 Ma (Frei et al., 1995). This age was interpreted as a staurolite formation age dating the prograde part of a Variscan path. The staurolite age is a few Ma older than white-mica Rb-Sr and K-Ar ages, it suggests that the whole evolution was fast (ca. 20 Ma).
Two generations of quartz-andalusite±kyanite veins formed during the Variscan amphibolite facies metamorphism both in metapelite and paragneiss rock types. The first generation of quartz-andalusite veins is coeval with the syn-(ductile) compressive deformational phase. Extensive structures, formation of quartz-andalusite-muscovite aggregates and the last generation of quartz-andalusite veins characterise the decompression path.
Fluid inclusion study (microthermometric data at CRPG-CNRS, Vandoeuvre-lès-Nancy, and at Mineralogisch-Petrographisches Institut, Bern; Raman Spectrometry analysis at CREGU, Vandoeuvre-lès-Nancy) combined with microstructural observation has been performed on the quartz from quartz-andalusite veins. According to microstructural observation and chemical data two main generations of fluid inclusions can be distinguished: - CH4-N2 (Vcn) fluid inclusions have variable size, are abundant, organised in clusters (Vcn1), subparallel (Vcn2) and perpendicular (Vcn3) to the main Variscan schistosity. The temperature of homogenisation ranges from -105.4 °C to -155.2 °C for approximately constant composition of CH4 = 87-82 % mol-1 and N2 = 18-13 % mol-1. A variation of molar volume from 54.5 to 40 cm3/mol was calculated. According to re-equilibration textures (annular shape) and geometric relationships, the older fluid inclusions (Vcn1 and Vcn2) recorded an increase in pressure of ca. 0.2-0.3 GPa consistent with petrologic results.
- saline water (Lw-s) fluid inclusions are small and trapped in late planes subperpendicular (Lw-s1) or at 45° oblique (Lw-s2) to the main Variscan schistosity. The ice melting temperature (Tm) varies between -17.4 and -31.0 C.
The country rocks of the veins are metasomatised sediments characterised by large porphyroblasts of oligoclase. According to microstructural observation, oligoclase is synkinematic, partially replaced Ms, and is associated to the fluid infiltration episode responsible for vein formation. We modelled, under isobaric conditions, a fluid-rock reaction flow path characterised by a increase and decrease in temperature. As a result, along the prograde fluid path Pl replaces Ms and/or K-fs and Qtz dissolves, but along the cooling path from the same fluid Qtz, Ms and/or K-fs precipitate and Pl dissolves.
Combination of field and microstructural observation with petrologic, isotopic, fluid inclusion investigation provide powerful tools to unravel the complex evolution of a polymetamorphic basement.
Frei, R., Biino, G.G. & Prospert, C., Geology (1995, in press).
Maggetti, M. & Flisch, M., In Pre-Mesozoic geology in the Alps. (von Raumer, J.F. & Neubauer, F., eds.) 469-484. (Springer-Verlag, Berlin and Heidelberg, 1993).