Simulations of crustal anatexis: Implications for the growth and differentiation of continental crust

Abstract

There is overwhelming and incontrovertible petrological and geophysical evidence for the significant role played by mantle‐derived mafic magma in the generation and growth of continental crust. Likewise, intrusion of mafic magmas beneath or into continental crust is very likely the major source of enthalpy that drives intracrustal differentiation. A simple dynamical model has been constructed to examine the critical factors that govern the evolution and time‐scale of crustal anatexic events when driven by basaltic magma underplating or injection into crust. Critical factors include the intensity of the enthalpy input (i.e., the power dissipated) and the rheological properties, bulk composition and compositional structure of the source rocks undergoing partial fusion. We have performed numerical simulations to evaluate these factors using phase equilibria and thermochemical and transport property data applicable to the binary eutectic system CaAl2Si2O8‐CaMgSi2O6 as a rough analog to study anatexis of mafic lower crust. The role of enthalpy power is tested by varying the enthalpy (or temperature) along the base of the crustal block while maintaining a fixed temperature at the top. As ratio Tbot/Ttop increases modestly from 1.05 to 1.15, the average fraction of melt at steady state in the anatexic region increases from 50% to 75%. Time to attain steady state scales inversely with Tbot/Ttop with an increase by a factor of 2 for a 10% decrease in Tbot/Ttop. Typical anatexic timescales are in the range 103 to 105 years for length scales in range 102 to 103 m. The consequences of different rheological models, especially the importance of Darcy percolative flow relative to en masse flow within the partial melt region was also investigated. A value of the solid fraction called the critical value (ƒscrit) is set such that for 0<ƒs<ƒscrit no relative motion is allowed between solid and melt. The relevant viscosity is the viscosity of the magmatic suspension which is a function of the local value of ƒs. In contrast, for values of ƒs such that ƒscrit<ƒs<1, momentum transport is accomplished by Darcy flow so that solid is considered immobile and melt is free to percolate through the solid matrix of varying composition. Numerical experiments have been performed for ƒscrit of 0.7, 0.5, 0.3 and 0.0 to assess the importance of the rheological model. At high values of ƒscrit (e.g., >0.5) relatively large volumes of nearly homogeneous melt are rapidly generated. In distinction, for ƒscrit small, more enthalpy is stored in the solid and less melt is produced. However, the compositional spectrum of liquids generated within the anatexic region is significantly larger for small ƒscrit. Because of the sensitive dependence of solidi and liquidi on bulk composition, melt productivity is related to bulk composition given a particular enthalpy power input and rheological model. There is a factor of 2 difference in volume of melt generated when the bulk composition of the source is 10 modal percent less refractory. Compositionally zoned melts form by melting of either homogeneous or layered sources, although composition‐frequency relations are sensitive to the initial (subsolidus) compositional structure. The effects of anatexic events are to fundamentally reorganize the pattern of compositional structure within the crust. Intracrustal differentiation is a long‐term inevitable process associated with the underplating or injection of mafic magma into preexisting crust.