Microstructural, petrological and petrochronological study of the graphitic schists ("Furtschaglschiefer") of Passo di Vizze (SW Tauern Window, Eastern Alps, Italy)

Furtschaglschiefer is a thick layer of graphitic schists belonging to the Greiner unit in the western Tauern Window. The precise tectonostratigraphic assignment of these rocks is debated, but they are considered the post-Variscan cover of the Zillertal-Riffl nappe, which was metamorphosed only during the Alpine orogeny.

These schists display a peculiar structure, where biotite adds to garnet to form porphyroblasts up to 1 cm long, defining a marked lineation. The texturally well-equilibrated mineral assemblage of the schists comprises quartz, plagioclase, garnet, biotite, ilmenite, muscovite, graphite ± chlorite ± epidote ± staurolite.

Garnet occurs as euhedral rhombic dodecahedral porphyroblasts 2-3 mm in diameter. Biotite occurs in three textural generations, of which the most abundant and coarser is represented by microboudinaged porphyroblasts with cleavage at high angle to the foliation, showing evidence of repeated crystals opening at cleavage planes and sealing by new biotite and in places by quartz. The evidence of this process is given by the variable distribution of graphite inclusions within biotite. The abundant graphite is disseminated in layers defining the main foliation of the rock, probably an isoclinally folded sedimentary bedding. The resulting "accordion-like" biotite porphyroblasts display an average total strain of 175%. Like garnet and ilmenite, these porphyroblasts show symmetric synkinematic features (rotation of cleavage and internal foliations) and strain shadows. When observed, sinistral and dextral shear sense indicators are equally recorded. The 2D analysis performed on thin sections was complemented by a 3D µCT study on two samples. Microboudinaged biotites show a prolate strain, with the long axis parallel to the rock lineation. Many coin-shaped ilmenite porphyroblasts are oriented parallel to the main foliation. The microboudinaged biotite of the Furtschaglschiefer probably formed in a two-stage process including 1) random static growth followed by 2) pure shear, intense microboudinage and precipitation of new biotite (and quartz) with very little rotation.

Continuous biotite and garnet growth occurred during and after deformation and foliation development, under almost constant PT conditions as evidenced by the constant chemical composition of synkinematic phases like biotite.

Phase equilibrium modelling and thermometry based on Raman spectroscopy of carbonaceous matter and titanium in biotite converge to indicate metamorphic temperatures of c. 550 °C, in agreement with previous studies. Constraints on metamorphic pressure are still poor, and at present there is no evidence for significant decompression from higher-P conditions predating the development of the main Grt-Bt-Ilm±Chl assemblage.

Preliminary results of in situ U-Pb dating by LA-MC-ICPMS provide an age 24.0 ± 2.6 Ma for garnet and of 26.1 ± 3.3 Ma for ilmenite, confirming the Oligocene age of amphibolite-facies metamorphism in this part of the Tauern Window.