Abstract

Archean granulite terrains are generally dominated by charnockite-enderbite suites. These rocks are geochemically similar to the tonalite-trondhjemite-granodiorite (TTG) gneisses of Archean cratons. Following an open-system fractional crystallization model for formation of TTG crust through solidification from a shallow-level tonalitic magma ocean, we propose that such granulites could have formed by crystallization at depth at the base of the same magma layer, which is undergoing convection. Increasing solubilities with depth of fluids in a magma mean that the base of a thick convecting magma layer will be undersaturated with respect to H2O and CO2. This allows the formation of charnockitic assemblages. Crystallization at depth can involve the reverse of dehydration-melting reactions; e.g., producing biotite and quartz at the expense of K-feldspar orthopyroxene, and liquid. Such reactions buffer the activity of water but not of CO2 during crystallization; hence, fluids exsolved during progressive solidification are likely to be CO2-rich.

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