Reconstruction of metamorphic gradients and thinning in the lower continental crust

Lower crustal terranes exposed at the Earth’s surface offer unique insights into the metamorphic conditions prevailing in the deep crust and at the crust-mantle boundary. In particular, the reconstruction of time-resolved pressure-temperature (P-T) gradients across terranes is essential for characterizing different tectonic settings and heat sources during metamorphism, with direct geodynamic implications. Additionally, physicochemical gradients along depth, such as variations in bulk rock composition and density, are also fundamental parameters intrinsically linked to crustal formation and evolution. However, a quantitative understanding of the deep Earth is strongly limited by the incomplete mineral record and inherent uncertainties in thermobarometry estimates. In practice, a standard approach involves sampling along a field gradient to retrieve punctual pressure-temperature-time information.

This contribution presents a natural case study of a lower continental crustal section, the Ivrea Verbano Zone (IVZ), northern Italy, where a metamorphic field gradient from amphibolite to granulite facies is exposed along the Ossola Valley. U-Pb dating of zircon from different lithologies and crustal depths constrains the high-temperature history and associated melting between 285–260 Ma. Multiple thermobarometers have been applied on mafic and felsic rocks along the section, including thermodynamic phase equilibria modelling and Zr-in-rutile thermometry. The Zr-in-garnet temperature dependence was also applied as a thermometer, revealing good agreement with Zr-in-rutile temperatures and successfully retaining peak temperatures in granulite facies metasedimentary rocks. The metamorphic gradient continuously evolves from ~ 5 kbar, 600 °C to 11 kbar, 1000 °C, and defines a present-day geobaric gradient of ~0.79 kbar/km, significantly higher than what is expected in a steady-state lower crust (0.28-0.3 kbar/km). Paleodepth reconstructions based on barometry and measured densities reveal that the lower crustal section experienced significant degrees of thinning (thinning factor β ~2.7). This result indicates that syn-to-post metamorphic extension has led to the modification of the geobaric gradient. Furthermore, it complements previous studies from the region, indicating that there is a lateral gradient in β along the axis of the current IVZ.

Lithological proportions and associated measured bulk rock compositions continuously evolve upgrade and define two distinct crustal endmembers. The amphibolite facies lower crust is volumetrically dominated by felsic metasediments and compositionally resembles typical upper continental crust, relatively enriched in heat producing elements. In contrast, the granulite facies lower crust is dominated by mafic lithologies, and its composition more closely resembles typical lower continental crust (Rudnick and Gao, 2014). Measured densities show significant variabilities (± 250 kg/m³, 2SD) within both felsic and mafic lithologies, with a linear increase from ~ 2750 to 3150 kg/m³ at the base of the section. Overall, our results reveal that the change in lithological proportions with paleodepth and high-temperature metamorphism play a primary role in controlling the physicochemical properties of the lower continental crust and its evolution.