Coupled S-Si isotopes reveal supracrustal origin of Archean continental crusts

The Archean continental crust, dominated by tonalite–trondhjemite–granodiorite (TTG) suites, is widely interpreted to have formed through partial melting of a hydrous mafic protolith. However, the nature of this protolith remains highly debated. Proposed sources include hydrothermally altered supracrustal basalts recycled to melting depths, as well as unaltered, mantle-derived gabbros emplaced into the lower crust by mantle plumes. Silicon and quadruple sulfur isotopes are powerful discriminants between these scenarios because they directly trace the relative supracrustal contributions to felsic continental crust. Here, we integrate whole-rock silicon and quadruple sulfur isotopes data from Neoarchean granitoids in the North China Craton to constrain the origin and evolution of their mafic protoliths. These granitoids display non-chondritic Δ³³S (to ~−0.06‰) and elevated δ³⁰Si (−0.09‰ to −0.05‰), which indicate supracrustal origin and contrast with previously reported mantle-like zircon δ¹⁸O values. A global compilation of Δ³³S and δ³⁰Si data shows that granitoids formed after 3.8 Ga consistently exhibit enriched δ³⁰Si and non-zero Δ³³S. Together, these observations indicate that Archean continental crust was generated by partial melting of supracrustal basalt rather than unmodified mafic cumulates.