FABRIC HOMOGENEITY OF MIGMATITES REVEALED BY ANISOTROPY OF MAGNETIC SUSCEPTIBILITY: IMPLICATIONS FOR FLOW IN A PARTIALLY MOLTEN CRUST

Migmatite massifs form by partial melting in the lower-middle continental crust. Regardless of tectonic setting, anatexis results in decrease in crustal strength. It is therefore important to better understand the feedback relationships between deformation, partial melting and flow in the lower-middle continental crust. Migmatites have for long been regarded as structurally complex due to their compositional heterogeneity. Recent research on several migmatite terranes suggests that the structural complexity of migmatites often masks a remarkable fabric homogeneity.

The source of magnetic susceptibility (K) is magnetite or mafic silicates. The AMS results from either 1) the shape anisotropy of magnetite, when large magnetite grains are present, or 2) the lattice preferred orientation (LPO) of mafic silicates, when magnetite is absent. In case #1, magnetite is deformed plastically during melting and its shape anisotropy increases with the intensity of rock fabric. If deformation continues upon cooling, magnetite grains can be deformed by micro-boudinage which may lead to the development of a distribution anisotropy between neighboring grains. Hence the degree of magnetic anisotropy (P) can be taken as a strain marker as long as deformation remains plastic. The pre-melting anisotropy of magnetite grains may also affect strain analysis as can be seen from migmatites derived from different protoliths. In case #2, the degree of magnetic anisotropy reflects the intensity of LPO up to a saturation level determined by the most magnetically anisotropic silicate present in the rock (e.g., P ≤ 1.38 for biotite). Both leucosome and mesosome record the same plastic flow. The interpretation of AMS in the melanosome requires to determine its metamorphic origin either as an unmolten component or as a melting residue.

The most remarkable characteristics of migmatites is their fabric homogeneity at various scales (cm to km). This suggests that, in spite of their (obvious) macroscopic compositional heterogeneity, migmatites flow in bulk and that flow fabrics are frozen upon cooling. By analogy with most granite plutons, in which emplacement fabrics are recorded over a relatively short period of time (<10⁵ yr), we suggest that, after peak metamorphism, migmatite massifs also record short lived deformation.