MECHANICS OF STRUCTURES AND MELT SEGREGATION DURING MIGMATITE DEFORMATION

A close correspondence exists between melt segregation by Darcy flow and metamorphic differentiation by dissolution-diffusion-precipitation. We treat a foliated rock containing a mobile phase, such as a melt or a soluble mineral, as an anisotropic poroviscous fluid with viscosity in foliation-parallel shortening h_n and in foliation-parallel shear h_s, and in dilatation h_n/n. For Darcy flow of a melt, with permeability K, melt viscosity, h_f, using k = K/h_f, the melt flow during low-amplitude (A) growth of folds of wavelength L = 2p/l, is from limbs to hinges, and the local dilatation rate is:

d/dt(DV/V) = {-4kh_nl²/[1+4kh_nl²(1+2n)/2n)]}4(h_n/h_s-1)(lA)²D_xxcos2lx

where D_xx is the basic-state rate of shortening. As the fold wavelength decreases, the rate of dilatation increases as the transport length decreases, towards a limiting rate determined by the resistance to dilatation alone. A corresponding relation, replacing Darcy flow with intergranular diffusion, applies in metamorphic differentiation.

Results for this and other structures (layer folding, boudinage, melt segregation about stiff inclusions) will be discussed in the context of estimating parameters and dimensionless groups from observations of structure and melt segregation in migmatites.

RCF was supported by a Center of Excellence grant from the Norwegian Research Council to PGP.