Demystifying migmatites: Introduction for field-based geologist

Abstract

Migmatites are heterogeneous, medium- to high-grade metamorphic rocks with at least one component formed by partial melting. As migmatites form under P–T conditions common in the middle and lower crust, they are widespread and likely to be encountered in the field. They are complex rocks that can be confusing, and the present paper provides a background for the field-based geologist. Migmatites comprise several main components. Neosome is newly formed by partial melting and includes leucosome and residuum (or melanosome). These essentially represent the liquid and solid components of the melt reaction. Palaeosome is the part of the migmatite that did not undergo partial melting and is typically of a more refractory composition. Migmatites can be divided into two main types. Metatexites have a lower melt fraction and preserve coherent pre-partial melting textures in the palaeosome. Diatexites are dominated by the neosome, and coherent pre-partial melting structures are absent. These can be further subdivided, with the different metatexites based on where the melt has ponded to form the leucosomes, while diatexites are subdivided based on the geometry of the palaeosome and/or residuum or its absence. The different types of migmatites provide information about the proportion and distribution of melt and the type and degree of strain. Facing direction can be obtained from migmatites, using criteria such as cauliflower structures and vein orientation and distribution. The leucosome margins can provide information about the rheological contrasts between the leucosomes and their host and their timing relative to anatexis and deformation. Consequently, these complex rocks can be important tools in understanding deformation within the middle to lower crust.