Abstract
High temperatures in the interior of the young Earth led to the initiation of melting at great depths in the mantle (> 150 km) and the production of large volumes of magma that erupted to form a >40 km thick basaltic crust. The ubiquitous presence of this crust dominated Archaean tectonics. Granitoid rocks, the source for >4 Ga Western Australian zircons, formed by partial melting at the base of the crust during the first 600 Ma of Earth history, but were always subordinate to basalt. The lunar record provides evidence that both the Moon and Earth experienced major impacting during this period. The impacts mixed granitoid with mantle-derived basalt to produce a composite granitoid-basalt layer with isotopic compositions close to bulk-Earth values. No record of its existence was retained in the oldest extant continents which formed - 3.9 Ga ago, after major impacting had ceased.
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