Internal liguride units from central Liguria, Italy: New constraints to the tectonic setting from white mica and chlorite studies

The Internal Liguride Units (ILU) of the Northern Apennines are interpreted as remnants of the Western Tethys oceanic lithosphere. In Late Cretaceous-Paleocene time, this lithosphere was underthrusted, underplated and later exhumed in an accretionary wedge related to an east-dipping, low-rate subduction zone. Until recently, only five IL units resulting from disruption of this lithosphere (namely Cravasco/Voltaggio, Figogna, Gottero, Colli/Tavarone and Bracco/Val Graveglia) have been recognized. A recent structural survey of the Valpolcevera-Valle Scrivia area (Central Liguria) revealed the existence of four more tectonic units: Bric Montaldo, Serra, Vallecalda and Ciaè units. Mineral assemblages, "crystallinity", polytypism and b cell parameter of illite, as well as the "crystallinity" of chlorite in metapelites from these units indicate that they have attained diagenetic to middle anchizonal grade and have experienced baric conditions approximating the intermediate-pressure facies series of Miyashiro (1961). Overall, the data on the metamorphic grade, together with the deformation character, indicate that the Valpolcevera-Valle Scrivia units belong to the ILU, but they were underplated at shallower levels than those reached by the westernmost, highest-grade units (Cravasco/Voltaggio and Figogna); their features point to metamorphic conditions similar to those of the easternmost units (Gottero, Colli/Tavarone and Bracco/Val Graveglia). This study shows the reliability of illite "crystallinity" as a tool to estimate thermal conditions over the entire very low-grade metamorphic zone; it appears sensitive enough to reveal even small differences in grade between different tectonic units.