Titanite from metacarbonates: a (unique) petrological and geochronological tool. An example from the Valle Strona di Omegna (Ivrea-Verbano Zone, Western Alps, Italy)

Abstract

The Ivrea-Verbano Zone (IVZ) is a well-exposed middle-lower continental crustal section located in the southern Alps of northwest Italy. This crustal section consists mainly of middle-high grade metapelites and metabasites (Kinzigite Formation) with numerous marble lenses. In particular, the Kinzigite Formation is best exposed in the Valle Strona di Omegna, showing progressively higher T conditions, from amphibolite to granulite facies, with increasing crustal depth. Several previous studies focused on the P-T conditions of the metapelites and metabasites (e.g., Kunz and White 2019 and references therein). We present new petrological and geochronological data for metacarbonate rocks occurring at the different crustal levels. We focus on titanite (CaTiSiO5), which is a common and almost unique accessory mineral in the metacarbonates for the lack of other petro-chronological tools (e.g. zircon, monazite and rutile). Titanite may contain numerous important geochemical elements, such as REE, Pb, U, Zr and Nb that are useful for petrological and geochronological reconstruction (Kohn, 2017). Major and trace elements were determined for titanite from metacarbonates with different mineral assemblages along the crustal profile. The chemistry of titanite shows interesting correlations with lithologies, mineral assemblages and titanite modal abundance. Zr-in titanite thermometer (Hayden et al 2008) provided temperature estimates ranging from 700 to 900 °C, coherently with the high temperature conditions recorded by surrounding metapelites/metabasites at Carboniferous-Permian interval (Kunz and White, 2019; Redler et al., 2012). The titanite U concentration ranges from about 20 to 300 ppm; U–Pb dating was only performed for titanite grains from granulite facies metacarbonate, containing more than 100 ppm of U. Preliminary U–Pb dates range from Middle Triassic to Lower Jurassic, suggesting a decoupling between Zr-in titanite geothermometer and U–Pb isotopic results.