Contrasting signatures of Phanerozoic I-type magmas proxy non-oceanic subduction crustal formation at the Archean–Proterozoic transition

Active margins are widely recognised as the main contributors to continental growth via I-type magmatism. The two constituent suites, namely subduction and post-collisional magmas, record the exchange of mass between the crust and the mantle through geological time. However, their geochemical similarities and the existing controversies over their igneous sources hinder their usage to track crustal evolution, as well as proxies for early continental crust formation during the Archean-Proterozoic transition. In spite of their geochemical similarities, compositional data from a global compilation of I-type magmas indicate consistent differences in their major and trace element compositions (CaO, MgO, K₂O, La/Sm and Ba/Th). Subduction-related magmas are characterised by higher CaO and lower K₂O, consistent with derivation from an ocean–mantle source. In contrast, post-collisional magmas display higher K₂O and elevated incompatible element abundances, reflecting a hybrid continental–mantle lithospheric source. Comparisons with Archean sanukitoid magmas highlight striking geochemical similarities to Phanerozoic post-collisional magmas, pointing to analogous igneous sources. Together with their resemblance of the bulk composition of the continental crust, this suggests that crust–mantle hybridisation, rather than oceanic subduction, was the dominant process driving crustal evolution at the Neoarchean–Proterozoic transition.