The grade of metamorphism in the metapelites of the Internal Liguride units (Northern Apennines, Italy)

The metamorphic grade of the Internal Liguride Units (Northern Apennines, Italy) is investigated in metapelites through mineral assemblages, "crystallinity", polytypism and "b0" of illite, as well as the "crystallinity" of chlorite. These units, known as Cravasco/Voltaggio, Mt. Figogna, Mt. Gottero, Colli/Tavarone and Bracco/Val Graveglia, normally consist of well preserved ophiolite sequences with their associated sedimentary cover, which are interpreted as remnants of the Western Tethys oceanic lithosphere. In the course of a pre-Oligocene orogeny, this oceanic lithosphere was involved in subduction processes and deformed and tectonically disrupted into several units. During the deformation phase corresponding to the metamorphic peak, these units were juxtaposed from the top to the bottom of the tectonic pile in the following order: Bracco/Val Graveglia, Colli/Tavarone, Mt. Gottero, Mt. Figogna and Cravasco/Voltaggio. All metamorphic indicators point to very low-grade conditions for all the Internal Liguride Units. Within these overall conditions, however, an increase of metamorphic grade from the highest to the lowest tectonic unit is clearly recognized. Utilising an empirical approach to define temperature conditions, the following approximate values are obtained: 160-210°C (Bracco/Val Graveglia Unit), 200-250°C (Colli/Tavarone Unit), 235-285°C (Mt. Gottero Unit), 270-320°C (Mt. Figogna Unit), and 300-350°C (Cravasco/Voltaggio Unit). The pressure conditions, estimated approximately through the illite "b0" parameter and calibrated with respect to pressure data from the literature, range from 2-3 kbar (Bracco/Val Graveglia and Colli/Tavarone Units), 4 kbar (Mt. Gottero Unit), 6 kbar (Mt. Figogna Unit) to ca. 7 kbar (Cravasco/Voltaggio Unit). These values are in good agreement with those estimated from the metamorphic parageneses of the associated metabasites (Cravasco/Voltaggio, Mt. Figogna and Bracco/Val Graveglia Units) and are consistent with the tectonic setting proposed by recent geological models (Mt. Gottero and Colli/Tavarone Units). The present study supports the reliability of illite "crystallinity" as a sensitive indicator of thermal conditions over the entire very low-grade zone, while illite polytype content and chlorite "crystallinity" appear to be less sensitive monitors of temperature variations.