The processes that control crustal differentiation: perspectives from the Limpopo Belt anatectites, South Africa

Differentiation of the crust is the result of complex interaction between a large number of processes, which govern partial melting of the deep crust, magma formation and segregation, and magma ascent to significantly higher crustal levels.

The middle and lower crustal lithologies exposed in the Pietersburg block (northern edge of the Kaapval Craton) represent a unique laboratory where the processes, which operate in the crustal column can be directly observed in the field. The formation of migmatites occured via protolith dehydration along the prograde path followed by fluid-absent biotite incongruent melting reactions, which produced cm- to m-scale, K2O-poor garnet-bearing stromatic leucosomes, with high Ca/Ca+Na ratios relative to their source rocks. Importantly, previous studies have demonstrated that the leucosomes formed and solidified during the granulite facies prograde evolution of the terrane and indicate that the formation of the leucosomes involved crystallization of plagioclase and quartz from the melt at close to peak metamorphic conditions.

Field observation combined with phase equilibrium modelling designed to investigate disequilibrium during partial melting, show that the outcrop displays different steps in the evolution of the leucosomes. Results drawn from this study identify three major mechanisms as important in the behaviour of a partial molten crust: (1) The degree of disequilibrium is controlled by the amount of plagioclase available to the melting reaction, which produce H2O-richer melts than equilibrium melting; (2) Equilibration of the segregated magma with a subordinate volume of the bounding residuum operates via H2O from the melt. The consequence is the crystallization of 5 to 10% peak metamorphic biotite in the residuum and the crystallization of plagioclase, quartz and garnet HT felsic cumulate (a.k.a. leucosomes); (3) The entrainment of peritectic garnet exerts a strong control on the chemistry of the leucosome, the residuum and the resulting upper crustal granitoid intrusion . These hypotheses have been tested using thermodynamically constrained software and provide a complete scenario for the evolution of the crustal column in an orogenic setting within the frame of the source-leucosome-granite (SLG) system.

The formation of such leucosomes reveals that the melting of the lower crust is likely to be controlled by the heterogeneity of the source. In particular, plagioclase crystal size and abundance is likely to play a key role in determining whether or not disequilibrium partial melting occurs. These findings have important implications for crustal differentiation and demonstrate that in some migmatitic granulites, the leucosomes constitute a type of felsic refractory residuum, not evidence of failed crustal differentiation.