TITANIUM ISOTOPES CONSTRAIN A MAGMATIC TRANSITION AT THE HADEAN-ARCHEAN BOUNDARY IN THE ACASTA GNEISS COMPLEX

Plate subduction greatly influences the physical and chemical characteristics of Earth's surface and deep interior, yet the timing of its initiation is debated because of the lack of exposed rocks from Earth's early history. We show that the stable titanium isotopic composition of orthogneisses from the Acasta Gneiss Complex spanning the transition from the Hadean to the Eoarchean falls on two distinct magmatic differentiation trends. Hadean tonalitic gneisses show titanium isotopic compositions comparable to modern evolved tholeiitic magmas, formed by differentiation of dry parental magmas in plume settings. Younger Eoarchean granitoid gneisses have titanium isotopic compositions comparable to modern calc-alkaline magmas produced in convergent arcs. Our data therefore document a shift from tholeiitic to calc-alkaline-style magmatism between 4.02 and 3.75 billion years (Ga) in the Slave craton.