Continent Formation in the Archean and Chemical Evolution of the Cratonic Lithosphere: Melt-Rock Reaction Experiments at 3-4 GPa and Petrogenesis of Archean Mg-Diorites (Sanukitoids)

To better understand the genetic relationship between granitoid rocks of the Archean cratons and the underlying 'keel' of subcratonic lithospheric mantle (SCLM), we have conducted two types of experiments in a multi-anvil apparatus at 3-4 GPa: (1) peridotite assimilation experiments, in which natural, hydrous TTG (trondhjemite-tonalite-granodiorite) melts are reacted with mantle peridotite at relatively high melt-rock ratios; (2) liquidus saturation experiments on hydrous Mg-rich diorite (sanukitoid) melts. Our results demonstrate that liquids similar to typically late- to post-tectonic sanukitoid intrusions can form by hybridization of initial TTG melts by assimilation of olivine-bearing peridotite, and that these primitive granitoid melts are in equilibrium with reaction residues consisting of olivine-free garnet websterite or garnet pyroxenite. The experimental melts retain the distinctive trace element signature of TTGs, overprinted by a 'primitive' mantle signature (i.e. high Mg-number, elevated Cr and Ni abundances), whereas the various phases of the crystalline residues acquire trace element signatures reflecting equilibration with Mg-rich granitoid melts. At low melt:rock ratios, metasomatism by TTG melts may be responsible for the silica enrichment and high modal orthopyroxene content of some cratonic peridotites and cryptic trace element overprints in garnet, clinopyroxene, and orthopyroxene. Our results demonstrate that the lithospheric keel of Archean cratons represents the product of reaction between TTG melts and previously depleted mantle peridotite at relatively low melt:rock ratios, as evidenced by the trace element signature in garnet pyroxenite and orthopyroxene-enriched garnet peridotite xenoliths, whereas Late Archean sanukitoids represent the products of these same reactions at relatively high melt:rock ratios.