Fluid inclusions related to Variscan and Alpine metamorphism in the Austroalpine Otztal Basement, Eastern Alps

The Austroalpine Otztal Basement (AOB) was affected by three metamorphic ev ents during the Caledonian (ca. 450 Ma), Variscan (ca. 320 Ma) and Eo-Alpin e (ca. 90 Ma) orogeny. The Variscan and Alpine events are clearly distingui shed in terms of pressure and temperature evolution. Significant difference s were observed between textures and densities of fluid inclusions of Varis can and Alpine age. Microthermometric and Raman spectroscopic investigation s were done on fluid inclusions entrapped in garnet, apatite and quartz. Ap atite hosts fluid inclusions of Variscan age which re-equilibrated during t he Alpine overprint. Variscan garnets from metasedimentary rocks contain fl uid inclusions, which were entrapped during isothermal decompression after the Variscan amphibolite facies temperature peak. The crystallinity of grap hite coatings on the walls of fluid inclusions in Variscan garnets from ort hogneisses as determined by Raman spectrosocopy indicates formation tempera tures in excess of 550 degrees C. Superdense CO2 inclusions in quartz indic ate isobaric cooling after the Alpine metamorphic temperature peak. The tex tures and the high density of late aqueous inclusions suggest their entrapp ment during the Alpine event. Raman spectra from hydrohalite and antarctici te support the presence of Na and Ca in the Variscan metamorphic fluid. Met asomatic reactions involving feldspars lead to the enrichment of these two elements in the fluid phase. The similarities of the chemical compositions of the metamorphic fluids within Variscan and Alpine inclusions suggest min or compositional changes with time and corroborate efficient rock buffering of the fluids. The combination of data from fluid inclusions, petrographic observations and published geothermobarometric data yields an improved and more detailed picture of the Variscan and Alpine metamorphic P-T evolution .