Open-system fractional melting of Archean basalts: implications for tonalite-trondhjemite-granodiorite (TTG) magma genesis

Tonalite-trondhjemite-granodiorite (TTG) gneiss forms a major component of Archean continental crust. Resolving the origin of TTG gneisses and the secular change in their compositions is critical for better understanding how the continental crust has evolved and when a global plate tectonic regime began on Earth. Archean TTGs were generated by partial melting of hydrous basalts, although the geodynamic setting in which this occurred is still debated. In this study, an integrated modeling including thermodynamic modeling, accessory mineral solubility modeling, and trace element modeling is conducted on Coucal basalts from Pilbara craton and averaged Archean arc-like basalts along various thermal gradients for both closed and melt-drained systems. The results show the amount and composition of melts would be primarily controlled by source compositions, although geothermal gradient and the ability for melt to leave the system also contribute. Of both bulk compositions, averaged Archean arc-like basalts generate more melts with a medium- to low-pressure signature by fluid-absent melting during intracrustal loading. High-pressure TTGs may be generated by mixing residua and melts derived by fluid-absent melting or just fluid-present melting in a hot subduction zone. Most TTGs produced before the Neoarchean were likely generated at the lower levels of thickened basaltic crust; however, rare Eoarchean and Mesoarchean TTGs with high-pressure signatures suggest subduction occurred on the early Earth in isolated localities. We interpret a secular increase in the proportion of high-pressure TTGs atc. 3.0-2.5 Ga as recording a transition from localized subduction to a global plate tectonic regime.