Helmholtz Centre Potsdam
GFZ German Research Centre for Geosciences
Abstract (EDOC: 12057)
A detailed microscopic study of the amphiboles crystallized during the late Alpine metamorphism in the hornblende garbenschists of the western Tauern Window reveals several stages of crystallization (Cr1-Cr3) and deformation (D1-D3). During the first stage of crystallization (Cr1) poikiloblastic green hornblende of tschermakitic composition overgrew a matrix showing a strong s1 foliation. A slight tendency towards increasing Tschermak's and edenite exchange, with glaucophane component decreasing towards the rim, suggests amphibole growth during heating and decompression. This prograde metamorphism leads to simultaneous formation of cummingtonite and hornblende in 3 out of 16 studied samples. The cummingtonite-forming reactions are discussed. Both hornblende and cummingtonite show microcracks forming s2, which is oblique to s1. s2 is missing in quartz and feldspar and also in some cases in the rims of the hornblende, but is traceable through trails of fluid inclusions. The last deformational event (D3) is a tensional one producing s3 as large cracks in the amphiboles. Correlation of deformation and exchange components with published P-T paths suggests growth of amphibole cores at amphibolite-facies conditions. Hornblende and cummingtonite rims formed at decreasing pressures but still rising temperatures. s2 microcracks are due to tectonic deformation (D2) while the s3 extensional cracks were formed by ductile thinning (D3) at rising temperatures, as revealed by the mineral parageneses found in the cracks. This paper presents the petrological framework for detailed geochronological investigations giving the time calibration of the P-T path of the Alpine metamorphism of the Western Tauern Window.
(1989): Crystallization and deformation of amphiboles, and cummingtonite-forming reactions in the garbenschists of the Western Tauern Window (Eastern Alps, northeast Italy-western Austria). Lithos, 23, 3, 163-177.