The Onset of Modern-Style Subduction Recorded by Late-Archean Sanukitoids: Insights from Thermodynamic Modeling

Abstract:

The timing of the initiation of modern-style plate tectonics on Earth is controversial. The Archean geological record has long been scrutinized for evidence of subduction, typically with focus on the ubiquitous tonalite-trondhjemite-granodiorite (TTG) suites, yielding ambiguous results. The less common Archean sanukitoids have received less attention but may be more suitable than TTGs as indicators of subduction. Sanukitoid suites comprise plutonic rocks with elevated concentrations of incompatible elements as well as elevated Mg#, Ni, and Cr, which has been interpreted to indicate a metasomatized mantle source, possibly the lithospheric mantle. They are typically Neoarchean in age, representing the final stages of Archean craton development and stabilization. The geochemistry and timing of these rocks have led to speculation that sanukitoids represent a shift in tectonic regime, potentially toward modern-style processes. Here, we test this hypothesis by modeling sanukitoid genesis using the newest thermodynamic models to calculate both major and trace elements of model melt generated from metasomatized depleted mantle. The main goals of the modeling are to determine the suitability of TTG and sediment-derived melt as metasomatic agents in the mantle source and to estimate possible P–T conditions of mantle partial melting. Major and trace element trends of sanukitoids from a global geochemical dataset are best matched by the modeled melt when melt fraction is high (30%) and the residue has no garnet. Sediment-derived melt may have been necessary to produce the incompatible element enrichment in sanukitoids, though some TTG melt input may be feasible. Modeled melt with the best fit to natural sanukitoids requires temperatures >1300°C at 2.5 GPa. In recent numerical models of Archean subduction, melting of metasomatized lithospheric mantle at such conditions can be achieved during modern-style subduction of a steeply dipping slab; additional processes such as slab breakoff are not required. The paucity of sanukitoids prior to the Neoarchean may indicate that modern-style subduction zones did not exist or were unstable and short-lived for most of the Archean Eon.