Caratteri del metamorfismo ercinico nella fillade sudalpina ad ovest di Bressanone.

The aim of this paper is to contribute to the solution of the numerous problems concerning the South-Alpine crystalline basement of the Eastern Alps. An area located west of Bressanone (Brixen) (Fig. 1) was considered and the petrographic and microtextural features of its metapelites analyzed. The main rock-types are quartz-phyllites and phyllitic schists, within which metarhyolites ("porphyroids"), quartzites and metabasites are interlayered. The distinction between the upper ("M. Cavallino Formation", "M. Cane Phyllites") and the lower ("Bressanone Phyllites") phyllitic complex is only possible according to mesostructural criteria: the metapelites making up the two complexes do display identical micro- and mesoscopic features. In particular, the almandine-biotite phyllites, which display the most complicated tectono-metamorphic history, have identical mineral assemblages in the two rock complexes, identical chemistry of the main mineral phases (Tables 1-4) and identical microtextural relationships between crystallization and deformation. The tectono-metamorphic history of the considered metapelites was reconstructed. It consists of five crystallization stages, including two thermal climaxes which correspond to the higher-temperature part of the greenschists facies. The mineral compatibilities of each crystallization stage were also ascertained (Figs. 8-11). Geothermometric estimations were deduced from the prevailing mineral assemblages (Fig. 13). Geobarometric estimations were based on the b value of the potassic white micas: the mean b value (8.774A) indicates low-pressure conditions of about 3 Kb. Considering the pattern of the b isopleths in the P-T field, an approximate value of 40°C/Km was assigned to the metamorphic thermal gradient of the Hercynian event.