Migmatites, granites and orogeny: Flow modes of partially-molten rocks and magmas associated with melt/solid segregation in orogenic belts

Abstract

This paper presents a model for the genesis of migmatites and granites during orogenic evolution based on the analysis of several Phanerozoic crustal segments and a review of the physical properties of partially-molten rocks and magmas. This model inventories the modes of bulk flow of partially-molten rocks and magmas and the mechanisms of melt/solid segregation for each mode. Partial melting of rocks is associated with a strength decrease of two to three orders of magnitude leading to strain partitioning expressed by channel flow driven either by forces related to plate tectonics (vertical channel flow) or by the gravity force associated with lateral variations of crustal thicknesses (horizontal channel flow). Although the structural characteristics of migmatites indicate that deformation plays a role in melt migration, the emplacement of laccoliths of leucogranites above migmatites attests to the efficiency of the buoyancy force. Another decrease in apparent strength of about ten orders of magnitude is associated with the loss of continuity of the solid framework marking the transition from partially-molten rocks to magmas. The breakdown of the solid framework also allows for settling of the solid in suspension increasing the buoyancy of the remaining magma. Accordingly, domes cored by diatexites (former heterogeneous magmas) and mantled by metatexites (former partially-molten rocks) are interpreted as gravitational instabilities driven by the relative buoyancy of the magma and permitted by the weakness of the partially-molten rocks. This model provides a first order framework to elucidate the development of a crustal-scale horizontal layering during the evolution of orogenic belts that are affected by partial melting. In this case, the middle crust is dominated by migmatites with domes cored by diatexites and mantled by metatexites that correspond to a partially-molten and magmatic zone, respectively. The granitic dikes and sills and the associated laccoliths of leucogranites correspond to an intrusive zone overlying the partially-molten zone. The refractory lower crust is potentially in part formed by accumulation of solids segregated from the overlying heterogeneous magmas.