Melt segragation, pervasive melt migration and magma mobility in the continental crust: the structural record from pores to orogens

Abstract

Structural analysis of migmatites based on the distribution and proportion of the granitic fraction at the outcrop scale is taken as a guide to decipher the behavior of partially molten rocks during orogenesis. This approach evaluates the various mechanisms that control melt segregation and magma mobility from pores to orogens. During partial melting, the first liquid appears at grain interfaces but such textures are rarely preserved in igneous rocks. Mechanisms of melt movement are scale-dependent and it is important to distinguish melt segregation at grain-scale from melt migration which occurs over larger distances. Melt segregation is controlled by melt connectivity and ubiquitous localization of granites in structurally-controlled dilatant sites provides evidence of the efficiency of melt segregation at the outcrop scale, probably achieved by porous flow at the grain scale. Magma mobility is controlled by the continuity of the solid framework which controls the rheologic threshold at the transition from solid-dominated metatexites to liquid-dominated diatexites. The presence of laccoliths of homogeneous leucogranite, emplaced at higher structural levels far from their source, indicates the efficiency of melt migration beyond the grain scale. The transition zone between diatexites (melt source) and granitic laccoliths (melt sink) is characterized by a network of granitic veins centimeter- to meter-thick. The geometric characteristics of this network suggest that, depending on structural level and the competency contrast between liquid and solid, veins propagate by either channeled porous flow, ductile deformation or fracturing. The main driving forces for upward melt migration appear to be buoyancy and dilatancy; the characteristics of local and regional deformation control the patterns of the granitic vein networks. Partial melting and redistribution of melt and magma from segregation by percolation at the grain scale relayed by pervasive migration through vein networks, is associated with chemical differentiation and generation of new rheological layering of the orogenic crust.