The origin of early Archean banded iron formations and of continental crust, Isua, southern West Greenland

The high-temperature hydrothermal alteration of the oceanic crust produced significant hydrothermal discharge with large quantities of Fe necessary to contribute to the deposition of the Isua BIFs. The hydrothermal alteration may have resulted from the opening of an asthenospheric window developed as a consequence of ridge subduction beneath an early Archean arc-forearc region. The ridge subduction model can also explain the origin of the contemporaneous tonalite-trondhjemite-granodiorite (TTG) intrusions in the Isua region. Partial melting of laterally accreted and thickened oceanic crust under amphibolite to eclogite metamorphic conditions by upwelling of asthenospheric windows may have produced TTG melts. The diapirically upwelling TTG melts formed the early Archean continental crust and the complementary eclogitic residues contributed to the sub-continental lithospheric mantle.