{"type":"FeatureCollection","features":[{"type":"Feature","properties":{"id":7488,"name":"Migmatites of the Gesso-Stura-Vesubie Compelx","description":"
In the first outcrop (fig. 1a) migmatites of the GSV, formed at the expense of metasedimentary rocks, can be observed. These rocks, located outside the FMSZ, are medium-grained migmatitic gneiss with a quartz-plagioclase-biotite-fibrolitic sillimanite-cordierite assemblage with few K-feldspar and muscovite. Relict kyanite and garnet rarely included in plagioclase suggest that these rocks reached P-T conditions of the upper amphibolite-facies (650\u2013700 \u00b0C; 0.6\u20130.8 GPa) and subsequently suffered a decompression from the kyanite to the sillimanite stability field. From the structural point of view the migmatites shows a gneissic foliation (fig. 1b) that is affected by open to tight symmetric folds (fig. 1a, b). The folding event occurred in a strain regime compatible with the shearing deformation in the FMSZ and contributed to accomodate a nearly horizontal shortening.<\/p>\n
Coarse grained protomylonitic gneiss (fig. 2a) with deformed leucosomes of K-feldspar, plagioclase and quartz in a medium grained biotite and sillimanite matrix with sometimes white mica. The amount of matrix is variable from a maximum of nearly 50% to a minimum of 30% – 40%. S-C fabric and asimmetric and stretched leucosomes are recognizable (fig. 2a), All the kinematic indicators point to a top-to-the SE sense of shear (fig. 2b).<\/p>\n
Mylonitic migmatitic gneiss of the FMSZ (fig. 3a). These rocks present sheared leucosomes and K-feldspar, plagioclase and quartz porphyloclasts in a fine grained biotite and white mica matrix. The amount of matrix in this lithotype is about 50 \u2013 60%. Asymmetric and stretched leucosomes are recognizable and points to a dextral top-to-the SW sense of shear (fig. 3a). In these outcrops it is also possible to observe post-mylonitic gentle folds with sub-horizontal axial planes affecting the mylonitic foliation (fig. 3b). The folds occur also in the other lithotypes of the shear zone and are responsible of variations of the orientation of the mylonitic foliation. They are not associated to a ductile pervasive axial plane foliation and they have been related to a post-shearing tectonic collapse as they are compatible with a nearly vertical shortening direction.<\/p>\n
The most sheared rocks of the FMSZ crops out in the core of the shear zone, the outcrop showed here is located along the crest of the Costabella del Piz. They are ultramylonites (fig. 5a) with an amount of matrix over 90%. Foliation is marked by white mica and chlorite. This mineral assemblage is indicative of a greenschist facies metamorphism. S-C-C\u2019 fabric is recognizable both at the outcrop scale (fig. 5a) and at the microscale (fig. 5b) and is indicative of a top-to-the SE sense of shear.<\/p>\n