The evolution and tectonic consequences of a tonalitic magma layer within Archean continents

Abstract

The petrological and tectonic consequences of early continent formation by localized open-system fractional crystallization of an almost global shallow-level tholeiitic magma layer are investigated. A near-surface tonalitic–trondhjemitic–granodioritic (TTG) magma "ocean", evolved by such a process near mantle sinks, would rapidly crystallize a stably floating solid upper crust several kilometres thick and could therefore be a long-lived feature of Archean geology. Given likely mantle heat fluxes, crystallization of the molten layer resulting from secular cooling and movement from mantle-upwelling to -downwelling regions would normally be from the top downward. In areas of extremely low mantle heat flow, crystallization from the base upward is also possible.The partially molten layer will convect with cells on a much smaller scale than those in the underlying mantle. This convection will influence tectonic patterns in the overlying solid crust, the history of which will be largely independent of specific mantle convection patterns. Small-amplitude surface topography is predicted. Intrusion of mafic or ultramafic magmas to high levels would not be possible through tonalite with a melt proportion greater than about 40% and is likely therefore to be concentrated at cooler, downwelling zones of the small-scale convection system. These two predictions together suggest that Archean greenstone belts form over local downwarping zones of TTG crust. Early, shelf-facies sedimentation is related to convection-induced topographic relief, whereas later, higher energy sedimentation is related to gravitational instability resulting from localized loading of the crust.So long as a continental magma layer is continually replenished by tholeiite melt, its composition remains essentially tonalitic. When it ceases to be replenished, fractionation will give rise to granitic melts that through their lower density will increase the gravitational instability of the Archean crust.