An open-system fractional crystallization model for very early continental crust formation

A model is discussed which seeks to explain the origin of Archaean tonalitic-trondhjemitic-granodioritic (TTG) crust by an open-system fractional crystallization mechanism. Following the assumption that in the pre- and early Archaean the melt which formed above sites of large-scale mantle upwelling was too voluminous and too hot to solidify, an almost global near-surface partially molten zone could form. The magma in this layer would converge above mantle sinks and undergo fractional crystallization. Recharged fractionating reservoirs would form at sites of convergence; the major element composition of the magma in such reservoirs would evolve towards one resembling that of TTG rocks. Provided that amphibole was a significant fractionating phase from such reservoirs, their incompatible as well as compatible trace element abundances would also become similar to those of TTG terrains. It is proposed that such terrains resulted from the consolidation of these recharged fractionating reservoirs. It is shown that the reservoirs would remain indistinguishable from the primitive mantle in terms of radiogenic isotopes, so that model ages cannot be greater than the age of their consolidation. Consideration of the main aspects of greenstone belts and Archaean high grade terrains indicates no contradictions with the model. The intrusion of late Archaean adamellites and granites is interpreted as an indirect effect of pressure-induced breakdown of amphibole in the cumulate underlying the TTG crust, which in turn resulted from crustal shortening. Cratonization is seen as the result of consolidation not only of the crust but also of the underlying cumulate as subcontinental lithosphere.