OCEANIC PLATEAUS: NUCLEI FOR ARCHEAN CRATONS

Tonalite-Trondjhemite-Granodiorite (TTG) rocks of Archean cratons represent the earliest known felsic continental crust. The formation and stabilization of continental crust is therefore intimately linked to the origin of these magmatic rocks. Rare-earth-element signatures of Archean TTGs requires derivation at depths where garnet makes up a substantial (>20 vol. %) fraction of the residue. Our experimental results on a typical MORB basalt composition indicate that pressure >18 kbar (depths >50 km) are needed to stabilize the required amount of garnet. This has important implications for the tectonic settings of Archean TTG formation. Proposed non-subduction models involving the melting of mafic rocks in the lower crust of oceanic plateaus cannot adequately explain the abundance of TTGs in the early Archean record because the crust in such settings rarely (never?) attains the thickness required for generating the typical strongly-depleted HREE patterns of Archean TTGs. Our results favor a subduction setting for the formation of early Archean TTGs. We suggest that TTG formation occurred during convergence of an oceanic plate and an oceanic plateau. Lateral compositional contrasts between these converging entities would favor subduction of the oceanic plate beneath the plateau (Niu et al., 2003). In such geodynamic settings, TTGs are formed by melting of the subducted oceanic crust and the resulting magmas are emplaced in the plateau crust. Oceanic plateaus, therefore, may have been the sites of subduction in Archean and served as nucleii for stabilization of felsic continental crust. Oceanic plateaus were previously considered subduction zone ‘chokers' or potential accretionary materials in a convergent tectonic setting. We suggest that oceanic plateaus were the ‘birth zones' of subduction in a hotter Archean earth. If this is true, the Archean cratons as we see them today are remnants of ancient oceanic plateaus modified by TTG magmatism and later terrain accretion.