Evoluzione metamorfica delle unità paraderivate nella Finestra dei Tauri occidentale

The western Tauern Window is a key area for the reconstruction of the tectonic evolution of the Eastern Alps. Here basement units derived from the European margin are stacked together with their parautochthonous post-Variscan cover in a south-vergent crustal scale duplex that also involves remnants of the Penninic ocean (Glockner nappe). The major lithostratigraphic units comprise: i) a polymetamorphic basement consisting of mainly amphibolites, migmatites, orthogneisses, and locally pre-Mesozoic ultramafic to mafic bodies; ii) the Tux and Zillertal orthogneiss that intruded unit (i) during early Permian times; iii) the parautochthonous metasedimentary cover including meta-conglomerates, calcitic and dolomitic marbles, calcareous micaschists, quartzites, and subordinate greenschists. iv) the Glockner Nappe, km-thick sequences of calcschists with interbeddings of prasinites, amphibolites and ultramafic bodies. Along the tectonic contact between the northern and southern duplex antiforms a slice (about 1km-thick, wedging out westward) of graphite-rich garnet micaschists with minor quartzites and calcschists (“Greiner schists” Auct.) occurs. A peculiar feature of the parautochthonous metasedimentary cover is the occurrence of static recrystallization of amphibole, micas, garnet, and staurolite (“Tauerncrystallisation” Auct.) crosscutting the main foliation, D2; this late metamorphic stage is controversial in the Glockner nappe, as poorly visible on the field, but possibly characterized at a microscopic scale by a secondary stage of growth of biotite and garnet in micaschists. Garnet and chloritoid were found in structural units of the Glockner nappe close to metasedimentary cover. Garnet amphibolites were recovered in units stacked to the Zillertal gneiss only. Two phases of roughly coaxial isoclinal folding, the first one of these responsible for the regional foliation, have been recognized in the post-Variscan cover of the “Zentralgneiss”. In the frame of the of the South Tyrol Province CARG project, devoted to 1:10000 mapping of the Vipiteno sheet, the goal of this work is to provide the constraints to identify tectonometamorphic units with coherent structural and metamorphic evolution. Preliminary results display significant differences among the reconstructed P-T paths of the Glockner schists and metasedimentary rocks of the post-Variscan cover of the Tux and Zillertal gneisses. Systematics of mineral chemistry has highlighted that garnets grown in micaschists synchronous to regional foliation in the Glockner nappe show Ca-Mg rich rims to a relatively Ca-Mg-poor core, with a zonation pattern revealing two stages of growth. On the contrary, in the metasedimentary rocks the Ca content in the garnet tends to decrease from core to rim, as Mg increases. A set of 49 whole rock analyses, both on metapelites and on calcschists, provide us the constraints to infer the features of protolith, and to compute pseudosections for thermobarometric estimates.