Tracing anatexis and bottom-up crustal homogenisation with in situ Sm-Nd isotopes

Whole rock Nd isotopes are commonly used to assess mantle-crust contributions in magma sources and to constrain the timing of magmatic and metamorphic events. However, such measurements provide limited insights into deep crustal roots, where complex processes (e.g., open system melting, magma hybridisation) may occur. Here, we combine whole rock and in situ Sm-Nd isotopic analyses across a 25–30 km thick crustal section in Calabria (Italy). This section exposes lower crustal granulites and migmatites overlain by mid-crustal post-collisional granitoids, forming a 13 km thick batholith. The lower crust is strongly heterogeneous (whole rock ɛNd(i) = −10.5 to +1.7) with isotopic variability evident from outcrop to grain scale. By contrast, the mid-crustal igneous rocks display remarkable homogeneity with consistent crustal signatures (ɛNd(i) ≈ −7). Our results indicate efficient isotopic homogenisation from a 1–2 km thick transition zone at the lower-middle crust boundary, where hybridisation between mafic and felsic magmas is evidenced at the grain scale using Sm-Nd isotopic analyses. A minor mantle contribution was likely involved in the batholith genesis but largely obscured by processes like crustal assimilation and cannot be resolved in the granitoids using the Sm-Nd system.