Igneous microstructures in migmatites

Abstract

Clear differences between the microstructures of leucosomes and mesosomes occur in some nebulitic migmatites in the Proterozoic Arunta block, central Australia. The leucosomes show crystal faces of K-feldspar (microcline-microperthite), cordierite, andalusite, and plagio clase against quartz, indicating crystallization of a melt. In contrast, the mesosomes show predominantly polygonal grain shapes, modified by (001) crystal faces of biotite and rare crystal faces in some porphyroblasts of garnet, cordierite, and orthopyroxene, indicating re-crystallization in the solid state. The occurrence of abundant crystal faces in minerals such as feldspar, cordierite, and andalusite (which typically grow crystal faces in magmas, but generally not in metamorphic rocks) is evidence of former melt. Therefore, igneous microstructures can be preserved in leucosomes that have not undergone penetrative subsolidus deformation and recrystallization. The cores of many of the crystals of cordierite and K-feldspar have small, locally oriented inclusions similar to those in the same minerals in the mesosomes. These indicate that the cores remained solid during crystallization of the leucosome, which produced inclusion-free, euhedral rims. These crystals provide criteria for recognizing possible restite in granitoid magmas.