Hafnium and silicon isotope evidence for multiple unsilicified basaltic sources at the origin of Rhyacian TTG magmas in the Mineiro Belt (Brazil)

Tonalite-trondhjemite-granodiorite (TTGs) are silica-rich plutonic rocks that correspond to the dominant lithologies of Archean basements. Understanding their formation is essential to unravel the early Earth tectonic regime and constrain continental crust formation and evolution. Archean TTGs commonly exhibit heavy-silicon isotope compositions attributed to ocean-derived silica in their sources, thereby indicating an interaction with the surface. The sedimentary record shows that the dissolved silica in seawater (DSi) has decreased secularly from the Archean onward. Despite the general disappearance of TTGs after the Archean, some granitoids of TTG affinity can still be found in Paleoproterozoic crustal domains that are tectonically well-constrained. Comparing the geochemical characteristics of Archean and Proterozoic TTGs, as well as their sources, therefore appears important for feeding debates on the evolution of continental crust and tectonic settings. To shed new light on these questions, the present study examines the magmatic record of three Paleoproterozoic TTG units in the Mineiro Belt (Brazil) by focusing on whole-rock petrography and geochemistry alongside zircon U-Pb geochronology, elemental, and isotopic (Hf and Si) geochemistry. These new data indicate episodic formation of TTG magmas during the Rhyacian (2.15–2.10 Ga), mainly driven by melting of Rb- and Ba-enriched low-K mafic sources with variably long crustal residence times (∼0.2–0.8 Ga). Studied Paleoproterozoic TTGs exhibit lower silicon isotopic signatures than their Archean counterparts. This difference is likely linked to variations in source pre-melting histories, such that they may not have been hydrothermally silicified, seawater may have had a distinct Si isotope signature, or the mechanisms of Si enrichment may not have been the same as for Archean TTG sources. Finally, a limited, though detectable, influence of pre-existing crust is nevertheless visible in these magmas through zircon inheritance, which could also account for some of the low Si isotope signatures observed. Our study also provides an alternative explanation for the apparent homogenization of Si isotope signatures in the upper-continental crust over time, since Proterozoic TTGs can exhibit relatively low Si isotope signatures compared to their Archean equivalents.