AXIAL-SURFACE LEUCOSOMES IN ANATECTIC MIGMATITES

Abstract

The generation of layers of magma in planes perpendicular to directions of active contraction, whether millimetre-scale leucosomes or multi-kilometre-scale batholiths, remains an intriguing dilemma. It is important because timing of melt generation and orientations of principal stresses during deformation are often inferred from such bodies.Several mechanisms can form leucosomes approximately parallel to axial surfaces of folds in anatectic migmatites. Mechanisms for magma injection include: (1) fluid pressure overcoming σ1 plus the tensile strength of the rock parallel to σ1 which can occur during folding if the plane perpendicular to σ1 has a prominent foliation and σ1-σ3 is small, (2) magma filling transient dilation sites or being forced into axial-surface folia during temporary relaxation of folding, which may involve temporary reversals of local stress, (3) magma filling transient, dilatant jogs opened during relative movement along the axial surfaces of strongly asymmetrical folds or local shear zones without stress reversals on the fold scale, (4) injection of leucosome into extensional shear zones in fold limbs with an oblique extensional crenulation-foliation, (5) boudinage of stronger layers, allowing leucosome to form vein-like bodies parallel to the axial surface, and (6) early injected leucosomes localizing fold hinges.Mechanisms for in situ formation of anatectic leucosomes include: (7) fluid-promoted melting along an axial surface foliation during similar conditions to those of mechanism 2, (8) syn-folding, in situ, fluid-promoted melting preferentially along axial-surface folia, owing to preferential access of fluids by diffusion parallel to the folia or preferential dehydration-melting reactions promoted by strain in these folia, and (9) segregation of leucosome around pre-existing large grains of strong minerals, becoming elongate because of growth parallel to a foliation or other potentially dilatant surface.